Converter technology
Introduction
In recent decades, the amount of electronics used in homes, offices and factories has increased dramatically, and most devices use switching power supplies. Such sources generate harmonic and non-linear current distortions that adversely affect the wiring of the electrical network and electrical appliances connected to it. This influence is expressed not only in various interference affecting the operation of sensitive devices, but also in overheating of the neutral line. When currents flow in loads with significant harmonic components that are out of phase with the voltage, the current in the neutral wire (which, when symmetrical load, practically, zero) can increase to a critical value.
The International Electrotechnical Commission (IEC) and the European Organization for Electrotechnical Standards (CENELEC) have adopted the IEC555 and EN60555 standards, which set limits on the harmonic content of input current secondary power supplies, electronic loads fluorescent lamps, motor drivers direct current and similar devices.
One of the effective ways to solve this problem is to use power factor correctors PFC ( power factor correction). In practice, this means that the input circuit of almost any electronic device with pulse converters, it is necessary to include a special PFC circuit that provides a reduction or complete suppression of current harmonics.
Power factor correction
A typical switching power supply consists of a mains rectifier, a smoothing capacitor, and a voltage converter. Such a source consumes power only at those moments when the voltage supplied from the rectifier to the smoothing capacitor is higher than the voltage on it (capacitor), which occurs for about a quarter of a period. The rest of the time, the source does not consume power from the network, since the load is powered by a capacitor. This leads to the fact that the power is taken by the load only at the voltage peak, the consumed current has the form of a short pulse and contains a set of harmonic components (see Fig. 1).
The secondary power supply, which has power factor correction, consumes current with low harmonic distortion, draws power from the network more evenly, has a crest factor (ratio amplitude value current to its rms value) is lower than that of an uncorrected source. Power factor correction reduces the RMS current drawn, allowing more devices to be connected to the same power outlet without overcurrent (see Figure 2).
Power factor
Power factor (PF) - a parameter that characterizes the distortion created by the load (in our case, the secondary power supply) in the AC network. There are two types of distortion - harmonic and non-linear. Harmonic distortion is caused by a reactive load and is a phase shift between current and voltage. Nonlinear distortions are introduced into the network by "nonlinear" loads. These distortions are expressed in the deviation of the current or voltage waveform from a sinusoid. When harmonic distortion The power factor is considered to be the cosine of the phase difference between current and voltage or the ratio of active power to the apparent power consumed from the network. For non-linear distortion the power factor is equal to the proportion of the power of the first harmonic component of the current in the total power consumed by the device. It can be considered an indicator of how evenly the device consumes power from the mains.
In general power factor is the product of the cosine of the angle of the phase difference between voltage and current and the cosine of the angle between the fundamental vector and the vector full current. The reasoning given below leads to this definition. The effective current flowing in an active load has the form:
I 2 eff \u003d I 2 0 + I 2 1 eff + SI 2 neff,
where I 2 neff is the constant component (in the case of a sinusoidal voltage it is equal to zero), I 2 1eff is the fundamental harmonic, and under the sum sign are the lower harmonics. When working on a reactive load, a reactive component appears in this expression, and it takes the form:
I 2 eff \u003d I 2 0 + (I 2 1 eff (P) + I 2 1 eff (Q)) + SI 2 n eff. Active power is the average value of the power allocated to the active load over the period.
It can be represented as a product of the effective voltage and the active component of the current P \u003d U eff H I 1eff (P) . Physically, this is the energy released in the form of heat per unit time per active resistance. Reactive power is understood as the product of the effective voltage and the reactive component of the current: Q \u003d U eff H I 1eff (Q) . The physical meaning is the energy that is pumped twice per period from the generator to the load and twice - from the load to the generator. Apparent power is the product of the effective voltage and the total effective current: S=U eff H I eff(gen) . On the complex plane, it can be represented as the sum of the vectors P and Q, from which the dependence I 2 \u003d I 1eff (total) cos j is visible, where j is the angle between the vectors P and Q, which also characterizes the phase difference between current and voltage in the circuit.
Based on the above, we derive the definition for the power factor:
PF=P/S=(I 1eff cos j)/(I eff(gen)).
It is worth noting that the ratio (I 1eff)/(I eff(gen)) is the cosine of the angle between the vectors corresponding to effective value total current and the effective value of its first harmonic. If this angle is denoted by q, then the expression for the power factor becomes: PF=cos j × cos q. The task of power factor correction is to bring the angle of the phase difference j between voltage and current to zero, as well as the angle q of harmonic distortion of the consumed current (or, in other words, to bring the shape of the current curve as close as possible to a sinusoid and to compensate for the phase shift as much as possible).
Power factor is expressed as a decimal fraction, the value of which lies between 0 and 1. Its ideal value is one (for comparison, a typical switching power supply without correction has a power factor value of about 0.65), 0.95 is a good value; 0.9 - satisfactory; 0.8 - unsatisfactory. Applying power factor correction can increase the device's power factor from 0.65 to 0.95. Values in the range of 0.97 ... 0.99 are also quite real. Ideally, when the power factor equal to one, the device consumes a sinusoidal current from the network with zero phase shift relative to the voltage (which corresponds to a fully resistive load with a linear current-voltage characteristic).
Passive power factor correction
The passive correction method is most often used in low-cost low-power devices (where there is no strict requirements to the intensity of the lower harmonics of the current). Passive correction makes it possible to achieve a power factor of about 0.9. This is convenient when the power supply has already been designed, it remains only to create a suitable filter and include it in the input circuit.
Passive power factor correction consists in filtering the consumed current using a bandpass LC filter. This method has several limitations. An LC filter can only be effective as a power factor corrector if voltage, frequency, and load vary within a narrow range.. Since the filter must work in the area low frequencies(50/60 Hz), its components are large in size, weight and low quality factor(which is not always acceptable). Firstly, the number of components in the passive approach is much less and, therefore, the time between failures is greater, and secondly, with passive correction, less electromagnetic and contact interference is created than with active correction.
Active power factor correction
An active power factor corrector must satisfy three conditions:
1) The form of the consumed current should be as close to sinusoidal as possible and - "in phase" with the voltage. The instantaneous value of the current consumed from the source must be proportional to the instantaneous voltage of the network.
2) The power taken from the source must remain constant even if the mains voltage changes. This means that when the mains voltage decreases, the load current must be increased, and vice versa.
3) The voltage at the output of the PFC-corrector should not depend on the magnitude of the load. When the voltage on the load decreases, the current through it must increase, and vice versa.
There are several schemes with which you can implement active power factor correction. The most popular at present is the “boost converter circuit”. This scheme satisfies all the requirements for modern sources nutrition. Firstly, it allows you to work in networks with different meanings supply voltage (from 85 to 270 V) without restrictions and any additional adjustments. Secondly, it is less susceptible to deviations in the electrical parameters of the network (power surges or short-term power outages). Another advantage of this scheme is that simple implementation surge protection. A simplified diagram of the “boost converter” is shown in fig. 3.
Principle of operation
The standard power factor corrector is a Pulse Width Modulated (PWM) AD/DC converter. The modulator controls a powerful (usually MOSFET) switch, which converts a constant or rectified mains voltage into a sequence of pulses, after rectification of which a constant voltage is obtained at the output.
Timing diagrams of corrector operation are shown in fig. 4. When the MOSFET switch is on, the current in the inductor increases linearly - while the diode is locked, and capacitor C2 is discharged to the load. Then, when the transistor turns off, the voltage across the inductor "opens" the diode and the energy stored in the inductor charges the capacitor C2 (and simultaneously powers the load). In the above circuit (unlike the source without correction), the capacitor C1 has a small capacitance and serves to filter high frequency interference. The conversion frequency is 50...100 kHz. In the simplest case, the circuit operates with a constant duty cycle. There are ways to increase the efficiency of correction by dynamically changing the duty cycle (matching the cycle with the voltage envelope from the mains rectifier).
The "boost converter" circuit can operate in three modes: continuous , discrete and the so-called critical conduction mode". AT discrete mode during each period, the inductor current has time to "fall" to zero and after a while starts to increase again, and in continuous- the current, not having time to reach zero, again begins to increase. Mode critical conductivity less frequently used than the previous two. It is more difficult to implement. Its meaning is that the MOSFET opens at the moment when the inductor current reaches zero value. When operating in this mode, it is easier to adjust the output voltage.
The choice of mode depends on the required output power of the power supply. In devices with a power of more than 400 W, continuous mode is used, and in low-power ones, discrete mode. Active power factor correction allows reaching values of 0.97...0.99 with THD (Total Harmonic Distortion) within 0.04...0.08.
Linear and switching power supplies
Let's start with the basics. The power supply in the computer performs three functions. First, alternating current from the household power supply must be converted to direct current. The second task of the PSU is to reduce the voltage of 110-230 V, which is redundant for computer electronics, to the standard values required by power converters. individual components PC, - 12 V, 5 V and 3.3 V (as well as negative voltages, which we will talk about a little later). Finally, the PSU plays the role of a voltage stabilizer.
There are two main types of power supplies that perform these functions - linear and switching. The simplest linear PSU is based on a transformer, on which the AC voltage is reduced to the required value, and then the current is rectified by a diode bridge.
However, the PSU is also required to stabilize the output voltage, which is due to both the instability of the voltage in the household network and the voltage drop in response to an increase in current in the load.
To compensate for the voltage drop, in a linear power supply, the transformer is dimensioned to provide excess power. Then, at a high current in the load, the required voltage will be observed. However, overvoltage, which will occur without any means of compensation at low current in the payload, is also unacceptable. Excessive voltage is eliminated by including a non-useful load in the circuit. In the simplest case, this is a resistor or transistor connected via a Zener diode. In a more advanced one, the transistor is controlled by a microcircuit with a comparator. Be that as it may, excess power is simply dissipated in the form of heat, which negatively affects the efficiency of the device.
In the switching power supply circuit, another variable appears, on which the output voltage depends, in addition to the two already available: the input voltage and the load resistance. In series with the load there is a key (which in the case of interest to us is a transistor), controlled by the microcontroller in the mode pulse width modulation(PWM). The higher the duration of the open states of the transistor in relation to their period (this parameter is called the duty cycle, in Russian terminology the inverse value is used - the duty cycle), the higher the output voltage. Due to the presence of a key, a switching power supply is also called Switched-Mode Power Supply (SMPS).
No current flows through a closed transistor, and the resistance of an open transistor is ideally negligible. In reality, an open transistor has resistance and dissipates some of the power in the form of heat. Also, the transition between transistor states is not perfectly discrete. And yet, the efficiency of a pulsed current source can exceed 90%, while the efficiency of a linear PSU with a stabilizer in best case reaches 50%.
Another advantage of switching power supplies is a radical reduction in the size and weight of the transformer compared to linear power supplies of the same power. It is known that the higher the frequency of alternating current in the primary winding of the transformer, the smaller the required core size and the number of turns of the winding. Therefore, the key transistor in the circuit is placed not after, but before the transformer and, in addition to voltage stabilization, is used to produce alternating current high frequency(for computer PSUs, this is from 30 to 100 kHz and higher, and as a rule - about 60 kHz). A transformer operating at a frequency of 50-60 Hz, for the power required by a standard computer, would be ten times more massive.
Linear PSUs today are mainly used in low power applications where the relatively complex electronics required for a switching power supply is a more costly expense than a transformer. These are, for example, 9 V power supplies that are used for guitar effects pedals, and once for game consoles and so on. But chargers for smartphones are already entirely pulsed - here the costs are justified. Due to the significantly lower amplitude of the voltage ripple at the output, linear power supplies are also used in areas where this quality is in demand.
⇡ The general scheme of the ATX standard power supply
BP desktop computer is a switching power supply, the input of which is supplied with the voltage of a household power supply with parameters of 110/230 V, 50-60 Hz, and at the output there are a number of DC lines, the main of which have a rating of 12, 5 and 3.3 V. In addition , the PSU provides the -12V, and sometime also the -5V voltage needed for the ISA bus. But the latter was at some point excluded from the ATX standard due to the termination of support for ISA itself.
In the simplified diagram of a standard switching power supply presented above, four main stages can be distinguished. In the same order, we consider the components of power supplies in the reviews, namely:
- EMI filter - electromagnetic interference (RFI filter);
- primary circuit - input rectifier (rectifier), key transistors (switcher) that create high-frequency alternating current on the primary winding of the transformer;
- main transformer;
- secondary circuit - current rectifiers from the secondary winding of the transformer (rectifiers), smoothing filters at the output (filtering).
⇡ EMI filter
The filter at the PSU input serves to suppress two types of electromagnetic interference: differential (differential-mode) - when the interference current flows into different sides in power lines, and common mode (common-mode) - when the current flows in one direction.
Differential noise is suppressed by a CX capacitor (large yellow film capacitor in the photo above) connected in parallel with the load. Sometimes a choke is additionally hung on each wire, which performs the same function (not in the diagram).
The common mode filter is formed by CY capacitors (blue teardrop-shaped ceramic capacitors in the photo), at a common point connecting the power lines to ground, and the so-called. common mode choke (common-mode choke, LF1 in the diagram), the current in the two windings of which flows in the same direction, which creates resistance to common mode noise.
In cheap models, they install minimum set filter parts, in the more expensive described circuits form repeating (in whole or in part) links. In the past, it was not uncommon to see PSUs without an EMI filter at all. Now this is rather a curious exception, although when buying a very cheap PSU, you can still run into such a surprise. As a result, not only and not so much the computer itself will suffer, but other equipment included in the household network - pulsed power supplies are powerful source interference.
In the area of \u200b\u200bthe filter of a good PSU, you can find several details that protect the device itself or its owner from damage. There is almost always a simple fuse to protect against short circuit(F1 on the diagram). Note that when the fuse blows, the protected object is no longer the power supply. If a short circuit has occurred, then it means that the key transistors have already broken through, and it is important to at least prevent the ignition of the electrical wiring. If a fuse suddenly blows in the PSU, then it is most likely pointless to change it to a new one.
Separately, protection against short-term voltage surges using a varistor (MOV - Metal Oxide Varistor). But there are no means of protection against a prolonged increase in voltage in computer power supplies. This function is performed by external stabilizers with their own transformer inside.
The capacitor in the PFC circuit after the rectifier can retain a significant charge after being disconnected from the power supply. So that a careless person who puts his finger into the power connector is not shocked, a high-value discharge resistor (bleeder resistor) is installed between the wires. In a more sophisticated version - along with a control circuit that prevents the charge from leaking when the device is in operation.
By the way, the presence of a filter in the PC power supply (and in the monitor power supply and almost any computer technology it is also there) means that buying a separate "surge protector" instead of a conventional extension cord, in general, is useless. He has the same inside. The only condition in any case is normal three-pin wiring with grounding. Otherwise, the CY capacitors connected to ground will simply not be able to perform their function.
⇡ Input rectifier
After the filter, the alternating current is converted to direct current using a diode bridge - usually in the form of an assembly in a common housing. A separate radiator for cooling the bridge is strongly welcomed. A bridge assembled from four discrete diodes is an attribute of cheap power supplies. You can also ask what current the bridge is designed to determine if it matches the power of the PSU itself. Although this parameter, as a rule, there is a good margin.
⇡ Active PFC block
In an AC circuit with a linear load (such as an incandescent lamp or electric stove), the flowing current follows the same sinusoid as the voltage. But this is not the case with devices that have an input rectifier, such as switching power supplies. The power supply passes current in short pulses, roughly coinciding in time with the peaks of the voltage sine wave (i.e., the maximum instantaneous voltage), when the rectifier smoothing capacitor is recharged.
The distorted current signal is decomposed into several harmonic oscillations in total with a sinusoid of a given amplitude (an ideal signal that would occur with a linear load).
Power used to commit useful work(which, in fact, is the heating of PC components), is indicated in the characteristics of the PSU and is called active. The rest of the power generated harmonic vibrations current is called reactive. It does no useful work, but heats up wires and puts a strain on transformers and other power equipment.
The vector sum of reactive and active power is called apparent power. And the ratio of active power to full power is called the power factor (power factor) - not to be confused with efficiency!
A switching PSU has a rather low power factor initially - about 0.7. For a private consumer, reactive power is not a problem (fortunately, it is not taken into account by electricity meters), unless he uses a UPS. It just falls on the uninterruptible full power loads. On the scale of an office or a city network, the excess reactive power generated by switching power supplies already significantly reduces the quality of power supply and causes costs, so it is being actively combated.
In particular, the vast majority of computer power supplies are equipped with circuits active correction power factor (Active PFC). The unit with active PFC is easily identified by the single large capacitor and inductor installed after the rectifier. In essence, Active PFC is another switching converter that maintains a constant charge on the capacitor with a voltage of about 400 V. In this case, the current from the mains is consumed by short pulses, the width of which is chosen so that the signal is approximated by a sinusoid - which is required to simulate a linear load . To synchronize the current demand signal with the voltage sine wave, the PFC controller has special logic.
The active PFC circuit contains one or two key transistors and a powerful diode, which are placed on the same radiator with the key transistors of the main PSU converter. As a rule, the PWM controller of the main converter key and the Active PFC key are one chip (PWM/PFC Combo).
The power factor of switching power supplies with active PFC reaches 0.95 and higher. In addition, they have one additional advantage - they do not require a 110/230 V mains switch and a corresponding voltage doubler inside the PSU. Most PFC circuits digest voltages from 85 to 265 V. In addition, the PSU's sensitivity to short-term voltage dips is reduced.
By the way, in addition to active PFC correction, there is also a passive one, which involves installing a high inductance inductor in series with the load. Its effectiveness is low, and you are unlikely to find this in a modern PSU.
⇡ Main transducer
The general principle of operation for all pulsed power supplies of an isolated topology (with a transformer) is the same: the key transistor (or transistors) creates an alternating current on the primary winding of the transformer, and the PWM controller controls the duty cycle of their switching. Specific circuits, however, differ both in the number of key transistors and other elements, and in quality characteristics: efficiency, waveform, interference, etc. But here too much depends on the specific implementation to be worth focusing on. For those interested, we present a set of diagrams and a table that will allow them to be identified in specific devices by the composition of parts.
| transistors | Diodes | Capacitors | Legs of the primary winding of the transformer | |
| Single Transistor Forward | 1 | 1 | 1 | 4 |
| 2 | 2 | 0 | 2 | |
| 2 | 0 | 2 | 2 | |
| 4 | 0 | 0 | 2 | |
| 2 | 0 | 0 | 3 |
In addition to the above topologies, in expensive PSUs there are resonant (resonant) versions of Half Bridge, which are easy to identify by an additional large inductor (or two) and a capacitor forming an oscillatory circuit.
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Single Transistor Forward |
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⇡ Secondary circuit
The secondary circuit is everything that is after the secondary winding of the transformer. In most modern power supplies, the transformer has two windings: 12 V is removed from one of them, and 5 V is removed from the other. The current is first rectified using an assembly of two Schottky diodes - one or more per bus (on the most heavily loaded bus - 12 V - there are four assemblies in powerful power supplies). More efficient in terms of efficiency are synchronous rectifiers, which use field-effect transistors instead of diodes. But this is the prerogative of truly advanced and expensive PSUs that claim the 80 PLUS Platinum certificate.
The 3.3V rail is typically derived from the same winding as the 5V rail, only the voltage is stepped down with a saturable choke (Mag Amp). A special winding on a 3.3 V transformer is an exotic option. Of the negative voltages in the current ATX standard, only -12 V remains, which is removed from the secondary winding under the 12 V bus through separate low-current diodes.
PWM key control of the converter changes the voltage on the primary winding of the transformer, and therefore on all the secondary windings at once. At the same time, the current consumption by the computer is by no means evenly distributed between the PSU buses. In modern hardware, the most loaded bus is 12-V.
Additional measures are required for separate voltage stabilization on different buses. Classic way involves the use of a choke group stabilization. Three main tires are passed through its windings, and as a result, if the current increases on one bus, then the voltage drops on the others. Let's say the current increased on the 12 V bus, and in order to prevent a voltage drop, the PWM controller reduced the duty cycle of the key transistors. As a result, the voltage on the 5 V bus could go beyond the permissible limits, but was suppressed by the group stabilization inductor.
The 3.3V rail voltage is additionally regulated by another saturable choke.
In a more advanced version, separate stabilization of the 5 and 12 V buses is provided due to saturable chokes, but now this design in expensive high-quality PSUs has given way to DC-DC converters. AT last case the transformer has a single secondary winding with a voltage of 12 V, and the voltages of 5 V and 3.3 V are obtained through DC converters. This method is most favorable for voltage stability.
Output filter
The final stage on each bus is a filter that smooths out the voltage ripple caused by the key transistors. In addition, pulsations of the input rectifier, whose frequency is equal to twice the frequency of the mains, break through to the secondary circuit of the PSU to one degree or another.
The ripple filter includes a choke and capacitors large capacity. High-quality power supplies are characterized by a capacitance of at least 2,000 microfarads, but manufacturers of cheap models have a reserve for savings when they install capacitors, for example, of half the value, which inevitably affects the ripple amplitude.
⇡ Standby power supply +5VSB
A description of the components of the power supply would be incomplete without mentioning the standby voltage of 5 V, which makes it possible to sleep the PC and ensures the operation of all devices that must be turned on all the time. "Duty room" is powered by a separate pulse converter with a low-power transformer. In some power supplies, there is also a third transformer used in the circuit feedback to isolate the PWM controller from the primary circuit of the main converter. In other cases, this function is performed by optocouplers (LED and phototransistor in one package).
⇡ Power supply testing methodology
One of the main parameters of the PSU is voltage stability, which is reflected in the so-called. cross-load characteristic. KNKH is a diagram in which the current or power on the 12 V bus is plotted on one axis, and the total current or power on the 3.3 and 5 V buses is plotted on the other. At the intersection points, for different values of both variables, the voltage deviation from the nominal by one tire or another. Accordingly, we publish two different KNX - for the 12 V bus and for the 5 / 3.3 V bus.
The color of the dot means the deviation percentage:
- green: ≤ 1%;
- light green: ≤ 2%;
- yellow: ≤ 3%;
- orange: ≤ 4%;
- red: ≤ 5%.
- white: > 5% (not allowed by the ATX standard).
To obtain CNC, a custom-made power supply test bench is used, which creates a load due to heat dissipation on powerful field-effect transistors.
Another equally important test is to determine the range of ripples at the PSU output. The ATX standard allows ripples within 120 mV for a 12 V bus and 50 mV for a 5 V bus. There are high-frequency ripples (at twice the frequency of the main converter key) and low-frequency ones (at twice the mains frequency).
We measure this parameter using the Hantek DSO-6022BE USB oscilloscope at the maximum load on the power supply unit specified by the specifications. In the oscillogram below, the green graph corresponds to a 12 V bus, yellow - 5 V. It can be seen that the ripples are within normal limits, and even with a margin.
For comparison, we present a picture of ripples at the output of the PSU of an old computer. This block wasn't great initially, but clearly hasn't gotten any better over time. Judging by the range of low-frequency ripples (note that the voltage base division is increased to 50 mV to fit the oscillations on the screen), the smoothing capacitor at the input has already become unusable. High-frequency ripple on the 5 V bus is on the verge of an acceptable 50 mV.
The following test determines the efficiency of the unit at a load of 10 to 100% of rated power(by comparing the output power with the input power measured with a household wattmeter). For comparison, the graph shows the criteria for different categories of 80 PLUS. However, it does not arouse much interest these days. The graph shows the results of the top Corsair PSU in comparison with the very cheap Antec, and the difference is not that very big.
A more pressing issue for the user is the noise from the built-in fan. It is impossible to directly measure it near the roaring power supply test bench, so we measure the speed of rotation of the impeller with a laser tachometer - also at power from 10 to 100%. In the graph below, you can see that at low load on this PSU, the 135mm fan maintains a low RPM and is hardly audible at all. At maximum load, the noise can already be distinguished, but the level is still quite acceptable.
The pros and cons of an active PFC power supply
The stable operation of a computer directly depends on the quality voltage that we feed it. Since many of us are not able to control the quality of the voltage in the network, but we can, with the help of a good power supply, insure us against unwanted problems.
So, modern multi-core processors, video cards (it has become fashionable to put them in pairs), various USB devices (often powered by a computer) force us to purchase more and more powerful power supplies (PSUs). Meanwhile, almost all modern PSUs of respected brands with a power of 450 W or more are equipped with power factor correction devices ( PFC - Power Factor Correction).
What is PFC and what do we get from it?
Passive RFC
It is the simplest and most common, and is a conventional high-capacity inductor (and size), connected in series with the power supply. I must say that he practically does not solve the problem, and takes up a lot of space.
Active PFC
It is another switching power supply, moreover, increasing the voltage. The resulting power factor of such a unit can reach 0.95...0.98 when operating at full load.
In addition to the fact that active PFC provides close to ideal power factor, it also improves the operation of the power supply - it additionally stabilizes the input voltage of the main regulator of the unit: the unit becomes noticeably less sensitive to reduced mains voltage.
Also, when using an active PFC, blocks with a universal power supply of 110 ... 230V are quite easily developed that do not require manual switching of the mains voltage.
Also, the use of an active PFC improves the response of the power supply during short-term (fractions of a second) mains voltage dips - at such moments, the unit operates due to the energy of the capacitors of the high-voltage rectifier. Another advantage of using active PFC is more low level high-frequency interference on the output lines, that is, such power supplies are recommended for use in a PC with peripherals designed to work with analog audio / video material.
In a word, everything speaks in favor of using a power supply unit with active PFC - it is he who will provide that high-quality gasoline for our computers!
A hidden problem that we didn’t know about: UPS for PSUs with active PFC
So, you bought a computer - you did not spare money for the power supply and all that. Work, play, everything is in order - the soul rejoices. Unfortunately, not everything is as easy and simple as we would like, since our network is not ideal, we will deal with surges and failures of electricity.
Well, everything is simple, you say. Buy a UPS (Uninterruptible Power Supply - uninterruptible power supply), stick a monitor and a system unit into it, and you will always have time to Shut Down your Windows. The main thing is that the power of the UPS (aka UPS - Uninterruptible Power Supply) matches the power of the computer's power supply plus the power consumption of the monitor.
But the fact is that the operation of a PSU with an active PFC in conjunction with cheap UPSs that issue a stepped signal when running on batteries can lead to computer failures, so manufacturers recommend using a UPS in such cases. smart class, which always outputs a sinusoidal signal.
There is one more nuance. All UPSs are roughly divided into redundant, line-interactive and continuous operation (OnLine). For the first two, the power switching time from external network on batteries is a few milliseconds, and this is enough in the case of conventional power supplies. But a power supply unit with active PFC in the event of a power failure instantly and dramatically increases electricity consumption by several times. At the same time, your uninterruptible power supply either turns off or burns out, and the computer is de-energized in an emergency with all the ensuing hardware, software and financial consequences.
There are 4 options for getting out of this situation:
Since you have purchased a cool power supply with active power compensation, and your electricity often disappears or just jumps (as elsewhere in our country, where power grids are not designed for universal computerization), and you cannot call existence without an uninterruptible power supply joyful, then choose a solution problems themselves.
1. Cheapest(but not always acceptable). Change the PSU to another one without an active PFC.
2. Do without UPS. This is fraught with the fact that the motherboard may burn out (financial costs), the system may fly off (time spent on reinstalling it), but the worst thing is that a screw may be covered, and all your work may be covered with a copper basin right before delivery to the customer.
3. The surest way out(not cheap, costs - from 300 USD). Purchase of continuous UPS (OnLine). In such sources uninterruptible power supply double conversion technology is used, which ensures excellent protection, how conventional computers, and servers.
The mechanism of double voltage conversion allows you to eliminate all interference that occurs in the power supply network. Rectifier converts AC voltage mains to permanent. DC voltage is used to charge the batteries and power the inverter. The inverter converts the DC voltage into AC (with a sinusoidal waveform), which continuously powers the computer.
In the absence of mains voltage, the inverter is powered by batteries, so the computer will not be left without electricity even for a moment!
4. Also an exit. Not cheaper than the previous one, but more cumbersome - this is the purchase of a line-interactive Smart type UPS (with a sine wave at the output) with a power reserve of 3-5 times (this is a prerequisite!). It will cost in the same range as OnLine, but it will weigh much more! And the fan in it will be more powerful (and louder).
These are the mines that the world of computers laid in the wallets of naive users :))) Maybe you, dear reader, think that we are exaggerating the problem? - Not at all. So on the websites of reputable UPS manufacturers (for example, APC) they write about this - they say that backup and line-interactive UPSs with active PFCs do not work!
Good afternoon friends!
Surely many of you have seen the mysterious letters "PFC" on the computer power supply. Let's say right away that most likely there will not be these letters on the cheapest blocks. Do you want me to tell you this terrible secret? Pay attention!
What is PFC?
PFC is an acronym for Power Factor Correction. Before deciphering this term, let's remember what types of power there are.
Active and reactive power
Even in the school physics course, we were told that power can be active and reactive.
Active power does useful work, in particular, being released in the form of heat.
Classical examples are the iron and the incandescent lamp. An iron and a light bulb are almost a purely active load, the voltage and current on such a load are in phase.
But there is also a load with reactivity - inductive (electric motors) and capacitive (capacitors). In reactive circuits, there is a phase shift between current and voltage, the so-called cosine φ (Phi).
The current can lag the voltage (in an inductive load) or lead it (in a capacitive load).
Reactive power does not produce useful work, but only dangles from the generator to the load and vice versa, uselessly heating wires
.
This means that the wiring must have a margin.
The greater the phase shift between current and voltage, the more power is wasted on the wires.
Reactive power in the power supply
In a computer, after the rectifier bridge, there are capacitors of a sufficiently large capacity. Thus, there is a reactive power component. If the computer is used at home, then usually there are no problems. Reactive power is not recorded by a conventional household electricity meter.But in a building where a hundred or a thousand computers are installed, reactive power must be taken into account!
The typical value of the cosine Phi for computer power supplies without correction is about 0.7, i.e. the wiring must be calculated with a 30% power margin.
However, the matter is not limited to excessive load on the wires!
In the power supply itself, the current flows through the input high-voltage in the form of short pulses. The width and amplitude of these pulses can vary depending on the load.
A large current amplitude adversely affects high-voltage capacitors and diodes, reducing their service life. If the rectifier diodes are chosen "back to back" (which is often the case in cheap models), then the reliability of the entire power supply is further reduced.
How is power factor correction performed?
To combat all these phenomena, devices that increase the power factor are used.
They are divided into active and passive.
The passive PFC circuit is a choke connected between the rectifier and high voltage capacitors.
A choke is an inductance that has a reactive (more precisely, complex) resistance.
The nature of its reactivity is opposite capacitance capacitors, so some compensation occurs. The inductance of the inductor prevents the current from rising, the current pulses are slightly stretched, their amplitude decreases.
However, the cosine φ increases insignificantly and there is no large gain in reactive power.
For more significant compensation, apply active PFC circuits.
The active circuit raises the cosine φ to 0.95 and higher. The active circuit contains a boost converter based on an inductor (choke) and power switching elements, which are controlled by a separate controller. The inductor periodically stores energy, then gives it away.
At the output of the PFC, there is a filtering electrolytic capacitor, but of a smaller capacity. A power supply with active PFC is less sensitive to short-term “dips” in the supply voltage i, which is an advantage. However, the application active circuit increases the cost of construction.
In conclusion, we note that the presence of PFC in a particular supply unit can be identified by the letters “PFC” or “Active PFC”. However, there may be cases where the inscriptions are not true.
You can unambiguously judge the presence of a passive circuit by the presence of a fairly weighty throttle, and an active one by the presence of another radiator with power elements (there should be three in total).
That's it, friends! The computer power supply is cunning, isn't it?
All the best!
See you on the blog!
Hello again!..Unfortunately, my article was delayed, because. an urgent work project arose, and there were also interesting difficulties when implementing the power factor corrector ( further KKM). And they were caused by the following - in our production, to control the KKM, we use a “custom” microcircuit, which Austria, especially in 1941, friendly, especially in 1941, produces for our tasks and, accordingly, cannot be found on sale. Therefore, the task arose to remake this module for an accessible elementary base, and my choice fell on a PWM controller chip - L6561.
Why is she? Banal accessibility, or rather found it in "Chip & Dip", I read the datasheet - I liked it. I ordered 50 pieces at once, because. cheaper and in my amateur projects I already have a few tasks for her.
Now about the main thing:
in this article I will tell you how almost from scratch I recalled the design of single-cycle converters ( it would seem what are they doing here), why he killed a dozen keys and how you can avoid it. This part will tell the theory and what happens if you neglect it. The practical implementation will be released in the next part, as I promised along with charger, because they are essentially one module and they need to be tested together.
Looking ahead, I’ll say that for the next part I have already prepared a couple of dozen photos and videos, where my memory is not for long "retrained" first in welding machine and then to the power supply for "goat". Those who work in production will understand what kind of animal it is and how much it consumes to keep us warm)))
And now to our sheep...
Why do we need this KKM at all?
The main thing misfortune The "classic" rectifier with storage capacitors (this is the thing that turns 220V AC into + 308V DC), which operates from a sinusoidal current, is that this very capacitor is charged (takes energy from the network) only at the moments when the voltage is applied more to him than to himself.
In human language, faint of heart and with scientific degrees, do not read
As we know electricity completely refuses to go if there is no potential difference. The direction of current flow will also depend on the sign of this difference! If you freaked out and decided to try to charge your mobile with a voltage of 2V, where the Li-ion battery is designed for 3.7V, then nothing will come of it. Because the current will be given by the source that has a higher potential, and the one with the lower potential will receive energy.
Everything is like in life! You weigh 60 kg, and the guy on the street who came up to ask you to call is 120 kg - it's clear that he will distribute pizdyuly, and you will receive them. So here - a battery with its 60 kg 2V will not be able to give current to a battery with 120 kg 3.7V. With a capacitor in the same way, if it has + 310V and you apply + 200V to it, then it will refuse to receive current and will not charge.
It is also worth noting that, based on the “rule” described above, the time allotted for the capacitor to charge will be very small. In our case, the current changes according to a sinusoidal law, which means the required voltage will be only at the peaks of the sinusoid! But the capacitor needs to work, so it gets nervous and tries to recharge. He knows the laws of physics, unlike some, and “understands” that time is short and therefore begins to consume just a huge current at these very moments when the voltage is at its peak. After all, it should be enough to operate the device until the next peak.
A little about these "peaks":
Figure 1 - Peaks in which the capacitor is charged
As we can see, the piece of the period in which the EMF takes on a sufficient value for the charge (figuratively 280-310V) is about 10% of the full period in the AC network. It turns out that instead of constantly taking energy smoothly from the network, we pull it out only in small episodes, thereby we “overload” the network. With a power of 1 kW and an inductive load, the current at the time of such “peaks” can calmly reach values of 60-80A.
Therefore, our task is to ensure a uniform selection of energy from the network so as not to overload the network! It is KKM that will allow us to realize this task on practice.
Who is this KKM of yours?
Power corrector- This is a conventional step-up voltage converter, most often it is single-ended. Because we use PWM modulation, then at the moment public key the voltage across the capacitor is constant. If we stabilize the output voltage, then the current taken from the network is proportional to the input voltage, that is, it changes smoothly according to a sinusoidal law without the previously described consumption peaks and jumps.Circuitry of our KKM
Then I decided not to change my principles and also relied on the datasheet of the controller I chose - L6561. Company engineers STMicroelectronics have already done everything for me, and more specifically, they have already developed the ideal circuitry for their product.Yes, I can count everything from scratch and spend a day or two on this business, that is, all my own and so rare weekends, but I ask why? To prove to myself that I can, this stage, fortunately, has long been passed)) Here I recall a bearded anecdote about the area of red balls, they say a mathematician applies a formula, and an engineer takes out a table with the area of red balls .... So it is in this case.
I advise you to immediately pay attention to the fact that the circuit in the datasheet is designed for 120 W, which means we should follow it adapt to our 3 kW and extreme operating stresses.
Now some documentation for the above:
Datasheet on L6561
If we look at page 6, we will see several diagrams, we are interested in a diagram with a signature Wide Range Mains, which means from Basurman "for operation in a wide range of supply voltage" . It was this “mode” that I had in mind when speaking of extreme stresses. The device is considered universal, that is, it can work from any standard network(for example, in the states 110V) with a voltage range of 85 - 265V.
This solution allows us to provide our UPS with the function of a voltage stabilizer! For many, this range will seem redundant and then they can make this module taking into account the supply voltage of 220V + - 15%. This is considered the norm and 90% of devices in price category up to 40 thousand rubles are completely deprived of CCM, and 10% use it only with the calculation of deviations of no more than 15%. This undoubtedly allows you to somewhat reduce the cost and dimensions, but if you have not forgotten, then we are making a device that must compete with ARS!
Therefore, for myself, I decided to choose the most correct option and make an indestructible tank that can be pulled out even in the country, where there is a welding machine or a pump in the well 100V in the network: 
Figure 2 - Standard circuit solution offered by ST
Adaptation of standard circuitry to our tasks
a) When I look at this scheme from LH, the first thing that comes to mind - you need to add a common mode filter! And rightly so, because at high power, they will begin to "crazy" electronics. For currents of 15 A or more, it will have a more complicated look than many are used to seeing in the same computer PSUs, where there are only 500-600 watts. Therefore, this revision will be a separate item.B) We see the capacitor C1, you can take a tricky formula and calculate the required capacitance, and I advise those who want to delve into it to do it, remembering electrical engineering 2 courses from any polytechnic in one. But I will not do this, because. according to my own observations from old calculations, I remember that up to 10 kW this capacity grows almost linearly relative to the increase in power. That is, taking into account 1 uF per 100 W, we get that for 3000 W we need 30 uF. This container is easily collected from 7 film capacitors of 4.7 uF and 400V each. Even a little with a margin, because The capacitance of a capacitor is highly dependent on the applied voltage.
C) We need a serious power transistor, because. the current consumed from the network will be calculated as follows: 
Figure 3 - Calculation of the rated current for PFC
We got 41.83A. Now we honestly admit that we will not be able to keep the temperature of the transistor crystal in the region of 20-25 ° C. Rather, we can overpower it, but it will be expensive for such power. After 750 kW, the cost of cooling with freon or liquid oxygen is eroded, but so far this is far from it))) Therefore, we need to find a transistor that can give 45-50A at a temperature of 55-60 o C.
Given that there is inductance in the circuit, then I prefer IGBT transistor, for the most tenacious. The limiting current must be chosen for the search, first about 100A, because this is the current at 25 ° C, with increasing temperature, the limiting switched current of the transistor decreases.
A little about Cree FET
I literally received on January 9 a parcel from the States from my friend with a bunch of different transistors for a test, this miracle is called - CREE FET. I won't say it's new mega technology, in fact, silicon carbide-based transistors were made back in the 80s, they just brought it to mind why only now. I, as the original materials scientist and composite engineer, am generally scrupulous about this industry, so I was very interested in this product, especially since 1200V was declared at tens and hundreds of amperes. I couldn’t buy them in Russia, so I turned to my former classmate and he kindly sent me a bunch of samples and a test board with forward.
I can say one thing - it was my most expensive fireworks!
8 keys flickered so that I was upset for a long time ... In fact, 1200V is a theoretical figure for the technology, the declared 65A turned out to be only a pulsed current, although the nominal one was clearly written in the documentation. Apparently there was a "nominal impulse current"Well, or whatever else the Chinese come up with. In general, it's still bullshit, but there is one BUT!
When I did it all CMF10120D corrector for 300 W, it turned out that on the same radiator and circuit it had a temperature of 32 ° C versus 43 for IGBT, and this is very significant!
Conclusion on CREE: the technology is raw, but it is promising and it definitely SHOULD be.
As a result, after looking through the catalogs from the exhibitions I visited (a handy thing, by the way, ala parametric search), I chose two keys, they became - IRG7PH50 and IRGPS60B120. Both are at 1200V, both are at 100+A, but upon opening the datasheet, the first key was eliminated immediately - it is able to switch a current of 100A only at a frequency of 1 kHz, which is disastrous for our task. The second key is 120A and a frequency of 40 kHz, which is quite suitable. See the datasheet at the link below and look for a graph with the dependence of current on temperature:
Figure 4.1 - Graph with the dependence of the maximum current on the switching frequency for the IRG7PH50, let's leave it to the frequency converter
Figure 4.2 - Graph with operating current at a given temperature for IRGPS60B120
Here we observe the cherished figures that show us that at 125 ° C both the transistor and the diode can easily handle currents slightly more than 60A, while we can implement the conversion at a frequency of 25 kHz without any problems and restrictions.
D) Diode D1, we need to choose a diode with an operating voltage of at least 600V and a current rated for our load, that is 45A. I decided to use those diodes that I had at hand (not long ago I bought them to develop a welder under the "oblique bridge") this is - VS-60EPF12. As can be seen from the marking, it is 60A and 1200V. I put everything with a margin, because. this prototype is made for myself and I feel so much calmer.
You can actually put a 50-60A and 600V diode, but there is no price between the 600 and 1200V version.
E) Capacitor C5, everything is the same as in the case of C1 - it is enough to increase the value from the datasheet in proportion to the power. It’s only worth considering that if you are planning a powerful inductive load or a dynamic one with rapid power increases (like a 2 kW concert amp), then it’s better not to save on this item.
I will put in my version 10 electrolytes at 330 uF and 450V, if you plan to power a couple of computers, routers and other trifles, then you can limit yourself to 4 electrolytes of 330 microfarads and 450V.
E) R6 - it is also a current shunt, it will save us from crooked hands and accidental errors, it also protects the circuit from short circuits and overload. The thing is definitely useful, but if we act like engineers from ST, then at currents of 40A we will get an ordinary boiler. There are 2 options here: a current transformer or a factory shunt with a drop of 75mV + op amp ala LM358.
The first option is simpler and provides galvanic isolation given node scheme. How to calculate the current transformer I gave in a previous article, it is important to remember that the protection will work when the voltage on leg 4 rises to 2.5V (in reality, up to 2.34V).
Knowing this circuit voltage and current, using the formulas from parts 5 you can easily count the current transformer.
G) And the last point is the power choke. About him a little lower.
Power choke and its calculation
If someone carefully read my articles and he has an excellent memory, then he should remember article 2 and photo no. 5, it shows 3 hank elements that we use. I'll show you again:
Figure 5 - Frames and core for power winding products
In this module, we will again use our favorite pulverized iron toroidal rings, only this time not one, but 10 at once! And how did you want? 3 kW is not Chinese crafts for you ...
We have initial data:
1) Current - 45A + 30-40% for the amplitude in the inductor, total 58.5A
2) Output voltage 390-400V
3) input voltage 85-265V AC
4) Core - material -52, D46
5) Clearance - distributed
Figure 6 - And again, dear Starichok51 saves us time and considers it a program CaclPFC
I think the calculation showed everyone how serious it would be)) 4 rings, yes a radiator, diode bridge, yes IGBT - horror!
Winding rules can be read in the article "Part 2". The secondary winding on the rings is wound in the amount - 1 turn.
Throttle summary:
1) as you can see, the number of rings is already 10 pieces! This is expensive, each ring costs about 140 rubles, but what will we get in return in the following paragraphs
2) the operating temperature is 60-70 ° C - this is absolutely ideal, because many lay the operating temperature at 125 ° C. In our production, we lay 85 ° C. Why this was done - for a restful sleep, I calmly leave home for a week and I know that nothing will flare up, everything icy will not burn out. I think the price for this at 1500r is not so deadly, is it?
3) I set the current density to a meager 4 A / mm 2, this will affect both heat and insulation and, accordingly, reliability.
4) As you can see, according to the calculation, the capacitance after the inductor is recommended to be almost 3000 microfarads, so my choice with 10 electrolytes of 330 microfarads fits perfectly here. The capacitance of the capacitor C1 turned out to be 15 microfarads, we have a double margin - you can reduce it to 4 film conders, you can leave 7 pieces and it will be better.
Important! The number of rings in the main inductor can be reduced to 4-5, simultaneously increasing the current density to 7-8 A / mm 2. This will save a lot of money, but the current amplitude will increase somewhat, and most importantly, the temperature will rise to at least 135 ° C. I consider this a good solution for welding inverter with 60% duty cycle, but not for a UPS that runs around the clock and probably in a rather limited space.
What can I say - we have a growing monster)))
Common mode filter
To understand how the circuits for this filter differ for currents of 3A (the computer PSU mentioned above) and for currents of 20A, you can compare the schematic from Google on ATX with the following:
Figure 7 - circuit diagram common mode filter
A few features:
1) C29 is a capacitor for filtering electromagnetic interference, it is marked "X1". Its value should be in the range of 0.001 - 0.5 mF.
2) The throttle is wound on on the core E42/21/20.
3) Two chokes on rings DR7 and DR9 are wound on any core from a spray and with a diameter of more than 20 mm. I wound on all the same D46 from -52 material before filling in 2 layers. There is practically no noise in the network even at rated power, but this is actually redundant even in my understanding.
4) Capacitors C28 and C31 of 0.047 uF and 1 kV and they must be set to class Y2.
According to the calculation of the inductance of the chokes:
1) Common mode inductance should be 3.2-3.5mH
2) Inductance for differential chokes is calculated by the formula: 
Figure 8 - Calculation of the inductance of differential chokes without magnetic coupling
Epilogue
Using the competent and professional developments of ST engineers, I managed to produce, at minimal cost, if not perfect, then simply excellent active power factor correction with better parameters than any Schneider. The only thing you should definitely remember is how much you need it? And based on this, adjust the parameters for yourself.My goal in this article was just to show the calculation process with the possibility of correcting the initial data, so that everyone, having decided on the parameters for their tasks, would already calculate and manufacture the module. I hope I managed to show this and in the next article I will demonstrate joint work KKM and charger from part 5.
