Abstract: Introduced how the power factor adjustment control circuit and the power main transfer network’s principle and does choose the primary device and the parameter.
Key word: Power factor adjustment; Electromagnetic interference; Pressure-rise transformation; Soft switch
Introduction
Along with the computer and so on some communication facility’s day-by-day popularization, the user unceasingly is also growing to power source’s demand, requests the power source manufacturer to be able to produce is more highly effective, the higher quality green power source, reduces the electrical energy consumption, lightens the electrical network burden. This must to power source product like UPS, the high frequency switch eliminator supply and so on input circuit carry on the active power factor to adjust, by maximum limit reduces the harmonic current. When actual surveys rectification load and so on computer PF=0.7, input current’s total overtone degree of distortion nearly 80%, namely the idle current is the wattful current 80%. UPS GB code (GB7286-87) stipulated that input always relative harmonic content ≤10%, rectifier product country profession standing operating procedure power input factor >0.9, therefore, how designs the outstanding PFC electric circuit is the very essential technology, the correct PFC circuit design technology is mainly composed of the following several parts: Control circuit, power main circuit, primary device choice and parameter design.
1 control circuit
In the early-1990s, because the PFC control chip has not gone on the market, we under correlation theories’s instruction, in domestic took the lead in 1992 to develop by establish separately the control circuit which the primary device was composed, principle like chart in 1 dashed line frame show.
Make in the laboratory and the small batch in the 48V/50A rectifier product, first level PFC electric circuit’s PF is about 0.98, η=93% (AC/DC, VDC=395V, Po=2000W). The above control circuit principle and UC Corporation’s PFC control principle (at the end of 1994 promoted UC3854) was consistent, but because the electric circuit was by establishes separately the primary device to be composed, antijamming ability was bad, the craft was complex, the debugging process was very long, therefore, had not utilized in the mass product. Controls IC like UC3854, UC3854A, UC3855 along with UC Corporation extrusion, by establishes separately the control circuit which the primary device is composed then special-purpose to control IC to substitute.
2 PFC power main circuit
The power main circuit selects relates the entire PFC electric circuit’s conversion efficiency as well as the EMI size, is the circuit design key technologies. Early main circuit as shown in Figure 2.
This is the typical Boost electric circuit, the principle is simple, but hard switching circuit, because had not considered that the switch component’s objective parameter, the high-pressured rectification diode restores the characteristic reverse, the main switch power tube’s switching loss characteristic, causes switch component’s dv/dt and di/dt is very high, enlarges correspondingly to the component stress request. Diode characteristic as shown in Figure 3, id is the diode current waveform, vd is the diode voltage waveform, when the switching valve S breakover, diode D restores the reverse recovery current which electric charge Qrr forms to lose nearly completely reverse on the main switch tube, has increased switching valve’s switching loss, in ta~tc time, diode D pressure drop, when is the switching valve S drain electrode voltage is Vo, had negative reverse recovery current to wind through switching valve S, in tc~tb time diode D di/dt>0, then the diode D end place prompt generation time negative voltage value, on the electric circuit will present big EMI, because distributed parameterThe existence, the conduction which and the radiated interference serious influence overall system’s stability will produce in the switching process.
In order to overcome the above insufficiency, then had the improvement PFC electric circuit, as shown in Figure 4. Increased the main switch diode’s stand-by circuit, its principle has used the L1 linear area and the misalignment area fully, when main switch tube breakover resumed the energy storage reverse the rectification diode to inductance L1, did not increase the main switch tube’s clear loss, when the main switch tube shut off inductance L1 memory energy by heat energy form consumption in resistance. Because the saturated inductance L1 existence, dv/dt and di/dt reduce the approximately nearly 1 magnitude, the main switch component switch stress sharp decline, EMI reduced greatly. This kind of electric circuit’s PF is about 0.99 (AC/DC, VDC=395V, Po=2500W), efficiency about η=94%.
To further raise the efficiency, forms the energy full use which diode’s memory electric charge on the stored energy and resistance R consumes then to develop as shown in Figure 5 the electric circuit.
This is one kind of passive lossless cushion structural circuit, its principle is: When the S breakover, takes diode’s cushion inductance by L1, resumes the energy storage reverse the diode to small inductance L1, simultaneously the C1 electric discharge, the C2 charge, shifts into the C1 stored energy C2; When the S shutdown the L1 stored energy shifts to the C1 charge and through diode D1, D2, D3 the stored energy to C, by now C2 also to the C electric discharge, through adjusted L1, C1, C2 the parameter and the coordinated S turn-on frequency, because the electric capacity (leaks - source distributed capacity CDS or collecting electrode - emitter electrode distributed capacity CCE and C1 by main switch tube is composed) on voltage not to be able to change suddenly, when S shuts off instantaneous VC1 approximately to be equal to zero, S may realize the zero potential shutdown. Because the inductance (is composed on electric current by L1 and line various thoughts) not to be able to change suddenly, when S breakover instantaneous, iL1 approximately is equal to zero, S may realize zero electric current breakover.
This electric circuit’s PF is about 0.99, (AC/DC, VDC=395V, Po=2500W), the efficiency η=96%~97%, the input end does not have EMI, the target definitely to be able nearly to achieve and to surpass the VDEA level standard. This kind of passive soft switch boosted circuit performance is outstanding, the reliability surpasses UC3855 the composition the active soft switch PFC electric circuit, is intelligent high frequency UPS and the high frequency switch eliminator supply ideal input level electric circuit, has the very high application value.
3 main primary device’s choices
3.1 Boost inductance magnetic material choice
The early time, the Boost inductance magnetic material is the ferrites magnetic core generally, like EE or EI and so on, through add the air gap Delta to adjust Mu the value, thus the adjustment inductance quantity, this method’s cost is relatively low, but the L value’s temperature characteristic relatively slightly misses, moreover the air gap leakage will increase the electromagnetic interference. Now, uses the metal magnetic powder core generally, like powdered iron core, hard nickel magnetism links and so on powder core, molybdenum permalloy, hard aluminum silicon, crystalless alloy. Each material has respective good and bad points, like the powdered iron core cost is low, but the Q value, Mu value each characteristic, like the temperature, linear and so on is relatively bad, the hard nickel powder core next best, the hard aluminum silicon, the molybdenum permalloy is relatively good, but price expensive, therefore, the PFC inductance magnetic material uses the hard aluminum silicon magnetism link to be good.
3.2 inductance L value computation
The power factor adjusts the prerequisite is causes in the input inductance the electric current maintains the continual condition, namely the ripple electric current δi must be smaller than smallest input alternating current peak value two times. Then takes inductance L≥ threshold inductance Lmin. But Lmin (mH) is
In the formula: Vmin(p) is the smallest input sine wave voltage peak value (V);
Vo is the output DC voltage (V);
f is switch modulation frequency (Hz);
Po is output cocurrent power (W);
Vmin is the smallest input sine wave voltage effective value.
Magnetic elements magnetism link (material quality for powdered iron or hard aluminum silicon) the choice (3) calculates through the type.
L=4μN2 (S/D)×10-6 (3)
In the formula: L is the inductance quantity (mH);
Mu is the magnetic core effective magnetic conductivity;
N is the coil number of windings;
S is the magnetic core leads magnetism cross-sectional area (cm2);
D is the magnetic core average magnetism link diameter (cm).
3.3 electric capacity’s choices
The electric capacity must use the low loss generally, the high ripple current mode’s electrolysis electric capacity, Rong Zhic is
C=Po/(2ωo×Vo×ΔVo)
In the formula: ωo is the city electrical angle frequency;
δVo for permission output cocurrent ripple voltage (V).
3.4 diode’s choices
Chooses trr to be small, the forward voltage drop small and restores (soft well) softly the characteristic good diode.
3.5 switch component’s choice
Chooses MOS or IGBT. Because the IGBT shutdown has a spot trailing phenomenon, when then turn-on frequency >20kHz, must choose MOS. Mainly cared what to MOS is the breakover loses, elected breakover resistance RDS is small; Mainly cared what to IGBT is the switching loss, elected switch characteristic good IGBT. Certainly, what is most ideal is acts according to IGBT and MOS the respective frequency characteristic direct parallel control signal to do according to the respective characteristic the corresponding succession adjustment.
4 conclusions
This article summarizes through the practice, designs one kind of outstanding soft switch PFC electric circuit, and uses the UC3854 chip to realize the technical production. This kind of PFC electric circuit is intelligent high frequency UPS and the high frequency switch eliminator supply input level electric circuit’s ideal solution. Meanwhile has made primary device’s characteristic the careful analysis, the optimization.
