Active PFC for Electronic Power Supplies

1 The proliferation of Electronic loads onpower distribution systems has led toinefficient and unsafe conditions dueto the typically poor Power factor ofelectronic Power conversion equip-ment. Waveform distortion and theoverheating of transformers andneutral conductors in three-phasesystems are just a few of the , economic and safetyconcerns along with new regulationsdesigned to maintain the integrity ofpower distribution systems havecreated an acute interest in powerfactor correction (PFC) choosing a strategy for PFC, itis essential to recognize that the poorpower factor occurring in electronicpower conversion equipment isentirely different from the traditionalpoor Power factor seen with inductivemotor loads, and requires a differentcorrective Sources of Poor PFIn its simplest form, poor Power factorcaused by reactive linear circuitelements results as the current eitherleads or lags the voltage, depending onwhether the load looks capacitive orinductive (Figure 1a).

2 This type ofpoor Power factor is easily correctedby adding a reactive component ofopposite sign in parallel with load tocancel the reactive term (Figure 1b).On the other hand, less than acceptablepower factor typically associated withelectronic Power conversion equip-ment is caused by nonlinear circuitelements. In most off-line powersupplies, the AC-DC front end consistsof a bridge rectifier followed by a largefilter capacitor (Figure 2b). With thiscircuit, current is drawn from the lineonly when the peak voltage on the lineexceeds the voltage on the filtercapacitor (Figure 2a). Since the rate ofrise and fall of the current is greaterthan that of the line voltage, and thecurrent flows discontinuously, a seriesof predominantly odd harmonics isgenerated third, fifth, seventh, etc.

3 (Figure 2c). It is these harmonics thatApplication NoteVICOR CORPORATION 25 Frontage Road Andover, MA 01810 TEL: 800-735-6200 FAX: 978-475-6715 5/95 Active PFC for Electronic Power Supplies (continued)Figure 1 Traditional poor Power factor the current either leads or lags the for improved Power factorInductive Load1a1bFigure 2 Entirely different from the traditional type, the poor Power factor occurring in Electronic loads generates odd DC-DC ConverterVBUS +L1L2 Holdup CapacitorFigure 3 Correcting the poor Power factor associated with Electronic Power Supplies requires an Active approach in which acontrol circuit adjusts a boost voltage to maintain a sinusoidal input Voltage (B) Rectified Line Voltage (A) Boost Voltage (B A) Line Current (I) Bus Voltage (B)

4 Holdup CapacitorL1L2 Control CircuitLine CurrentRectified Line VoltageOutput VoltageAIBoost ConverterTo DC-DC ConverterTypical Input Current Spectrum of an Electronic Load 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 CurrentHarmonic Number2a2b2c3a3bcause the problems with the powerdistribution Power factor of the system shownin Figure 2 can be improved slightlyby either adding series inductance withthe line or decreasing the value of theholdup capacitor, which will lengthenthe conduction angle. However, bothof these solutions severely limit theamount of Power that can be drawnfrom the Active Approach to PFCIt is generally accepted that the mosteffective way to correct the poor powerfactor of Electronic Power Supplies isto take an Active the operation of an Active powerfactor correction circuit (Figure 3b),the incoming line voltage passesthrough a bridge rectifier, whichproduces a full wave rectified output(Figure 3a A).

5 Since the peak value ofthe line is less than the bus voltage, nocurrent will flow into the holdupcapacitor unless the line voltage isboosted above the voltage present onthe holdup capacitor. This allows thecontrol circuit to adjust the boostvoltage (3a B-A) to maintain asinusoidal input maintain a sinusoidal input current,the control circuit uses the inputvoltage waveform as a template. Thecontrol circuit measures the inputcurrent, compares it to the input volt-age waveform, and adjusts the boostvoltage to produce an input currentwaveform of the same shape (3a I). Atthe same time, the control circuitmonitors the bus voltage and adjuststhe boost voltage to maintain acoarsely regulated DC output (3a B).Since the primary function of thecontrol circuit is to maintain asinusoidal input current, the DC busvoltage is allowed to vary is important to remember that a welldesigned Power factor correction cir-cuit will faithfully replicate distortionpresent in the incoming line voltage, soit is essential to use a low distortionvoltage source when evaluating powerfactor correcting 3b illustrates the approach topower factor correction taken with theVicor VI-HAM Harmonic AttenuatorModule, a component-level AC frontend that, when used with VI-26x or VI-J6x DC-DC converters, provides auniversal input, near-unity powerfactor.

6 Off-line switching Power supplythat meets IEC use of an Active Power factorcorrecting circuit results in fewdiscontinuities in the input current andconsequently low distortion andharmonic content of the input currentbeing drawn from the line. Forassistance in designing a componentpower solution with Power factorcorrection, call Vicor s applicationengineering NoteVICOR CORPORATION 25 Frontage Road Andover, MA 01810 TEL: 800-735-6200 FAX: 978-475-6715 5/95 Active PFC(page 2)