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application note AN4107 design of power factor correction using fan7527 www.fairchildsemi.com ?2001 fairchild semiconductor corporation rev. a, may 2000 1. introduction the fan7527 is an active power factor correction(pfc) controller for boost pfc application which operates in the critical conduction mode. it turns on mosfet when the inductor current reaches zero and turns off mosfet when the inductor current meets the desired input current reference voltage as shown in fig. 1. in this way, the input current waveform follows that of the input voltage, therefore a good power factor is obtained. 1-1. internal block diagram it contains following blocks. ? error amplifier (e/a) ? zero current detection (idet) ? switch current sensing (cs) ? input voltage sensing (mult) ? switch drive (out) rev. 1.0.2 . figure 2. block diagram of the fan7527 inductor peak current inductor current inductor average current gating signal figure 1. inductor current waveform inv 1 2 vea(-) error amp vref + + + + ? ? ? ? ovp current detector isovp=30ua idovp=40ua + + + + ? ? ? ? vref vm2 vref~vref+2.5v 2.25v static ovp + + + + ? ? ? ? ea_out 6 gnd ) ( vref 2 vm 1 vm vmo k ? ? ? ? ? ?? ? = = = = multiplier + + + + ? ? ? ? vm1 vmo 0 ~ 3.8v 8pf 40k + + + + ? ? ? ? + + + + ? ? ? ? 2.5v ref internal bias timer r 7 vcc 5 4 3 8 vcc idet cs m ult ou t drive output 0.25v veao(l)=2.25v r q s uvlo 6.5v zero current detector current sense comparator 1.8v 2v 1.5v 12v 9v
AN4107 application note 2 ?2001 fairchild semiconductor corporation 2. device block description 2-1. error amplifier and over voltage protection block the sensed and divided output voltage is fedback to the error amplifier inverting input(inv) to regulate the output voltage. the non-inverting input is internally biased at 2.5v. the error amp output(ea_out) is internally connected to the multiplier and is pinned out for the loop compensation. generally, the control loop bandwidth of pfc converter is set below 20hz to get a good power factor. in this application, a capacitor is connected between inv and ea_out. however, in case of over voltage condition, the e/a must be saturated low as soon as possible, but the narrow e/a bandwidth slows down the response. to make the over voltage protection fast, the soft ovp and dynamic ovp is added. the fan7527 monitors the current flowing into the ea_out pin. if the monitored current reaches about 30ua, the output of multiplier is forced to be decreased, thus reducing the input current drawn from the mains(soft ovp). if the monitored current exceeds 40ua, the ovp protection is triggered(dynamic ovp), then the external power transistor is switched off until the current falls below about 10ua. in this case, it disables some internal blocks reducing the quiescent current of the chip to 2ma. however, if the over voltage lasts so long that the output of e/a goes below 2.25v, then the protection is activated(static ovp) keeping the output stage and the external power switch turned off. the operation of the device is re-enabled as the e/a output goes back into its linear region. o v 1 r 2 r inv 1 2 vea(-) error amp vref ovp current detector isovp=30ua idovp=40ua vref vm2 vref~vref+2.5v 2.25v static ovp output stage multiplier 2-2. multiplier a single quadrant, two input multiplier is the critical element that enables this device to get power factor correction. one input of multiplier(pin 3) is connected to an external resistor divider which monitors the rectified ac line voltage. the other input is internally driven by a dc voltage which is the difference between error amplifier output (pin 2) and reference voltage, vref. the multiplier is designed to have an extremely linear transfer curve over a wide dynamic range, 0v to 3.8v for pin 3, and 2.25v to 6v for error amplifier output under all line and load conditions. the multiplier output controls the current sense comparator threshold voltage as the ac voltage traverses sinusoidally from zero to peak line. this allows the inductor peak current to follow the ac line thus forcing the average input current to be sinusoidal. in other words, this has the effect of forcing the mosfet on-time to track the input line voltage, resulting in a fixed drive output on-time, thus making the pre-converter load appear to be resistive to the ac line. the equation below describes the relationship between multiplier output and its inputs. vmo = k vm1 (vm2 - vref) k : multiplier gain vm1: voltage at pin 3 vm2: error amp output voltage vmo: multiplier output voltage figure 3. error amplifier and ovp block
application note AN4107 3 ?2001 fairchild semiconductor corporation figure 5. current sense circuit vmo 8pf 40k 4 cs current sense comparator 1.8v rsense 2-3. current sense comparator the current sense comparator adopts the rs latch configuration to ensure that only a single pulse appears at the drive output during a given cycle. mosfet drain current is sensed using an external sense resistor in series with the external mosfet. when the sensed voltage exceeds the threshold set by the multiplier output, the current sense comparator turns off the mosfet and resets the pwm latch. the latch insures that the output remains in a low state after the mosfet drain current falls back to zero. the peak inductor current under the normal operating condition is controlled by the multiplier output, vmo. the abnormal operating condition occurs during pre-converter start-up at extremely high line or as output voltage sensing is lost. under these conditions, the multiplier output and current sense threshold will be internally clamped to 1.8v. therefore, the maximum peak switch current is limited to: ipk(max) = 1.8v / rsense in the fan7527, an internal r/c filter has been included to attenuate any high frequency noise that may be present on the current waveform. this circuit block eliminates the need for an external r/c filter otherwise required for proper operation of the circuit. inv 1 2 vea(-) error amp vref ovp current detector isovp=30ua idovp=40ua vref vm2 vref~vref+2.5v ea_out ) ( vref 2 vm 1 vm vmo k multiplier vm1 vmo 0 ~ 3.8v mult 0.25v veao(l)=2.25v current sense comparator 1.8v ovp 3 figure 4. multiplier block
AN4107 application note 4 ?2001 fairchild semiconductor corporation 2-4. zero current detector fan7527 operates as a critical conduction current mode controller. the zero current detector switches on the external mosfet as the voltage across the boost inductor reverses, just after the current through the boost inductor has gone to zero. the slope of the inductor current is indirectly detected by monitoring the voltage across an auxiliary winding and connecting it to the zero current detector pin 5. once the inductor current reaches ground level, the polarity of the voltage across the winding is reversed. when the idet input falls below 1.5v, the comparator output is triggered to the low state. to prevent false tripping, 0.5v hysteresis is provided. the zero current detector input is protected internally by two clamps. the upper 6.5v clamp prevents input over voltage breakdown while the lower 0.75v clamp prevents substrate injection. an internal current limit resistor protects the lower clamp transistor in case the idet pin is shorted to ground accidentally. a watchdog timer function is added to the ic to eliminate the need for an external oscillator when used in stand-alone applications. the timer provides a means to start or restart the pre-converter automatically if the drive output has been off for more than 500us after the inductor current reached zero. figure 6. zero current detector block 5 idet 6.5v zero current detector 2v 1.5v to f/f vin 2-5. output drive the fan7527 contains a single totem-pole output stage designed specifically for a direct drive of power mosfet. the drive output is capable of up to 500ma peak current with a typical rise and fall time of 130ns, 50ns respectively with a 1.0nf load. additional circuitry has been added to keep the drive output in a sinking mode whenever the uvlo is active. this characteristic eliminates the need for an external gate pull-down resistor. internal voltage clamping ensures that the output driver is always lower than 14v when supply voltage exceeds the rated vgs of the external mosfet. this eliminates an external zener diode and extra power dissipation associated with it that otherwise is required for the reliable circuit operation. 3. circuit components design 3-1. power stage design 1) boost inductor design the boost inductor value is determined by the minimum switching frequency limitation. the minimum switching frequency has to be above the audio frequency. the switching period is maximum when the input voltage is highest at maximum load condition. t s(max) is a function of v in(peak) and v o . it can have maximum value at highest line or at lowest line according to v o . check t s(max) at v in(peak_min) and v in(peak_max) , then take the higher value for the maximum switching period. the boost inductor value can be obtained by (5) t on l i l peak t v in peak t sin ---------------------------------------------- l 2i in peak t sin v in peak t sin ----------------------------------------------- (1) == l 2i in peak v in peak --------------------------- - =
application note AN4107 5 ?2001 fairchild semiconductor corporation 2) auxiliary winding design the auxiliary winding voltage is lowest at the highest line. so the number of auxiliary winding can be obtained by (7). a small resistor is connected to the auxiliary winding to suppress the high frequency ringing voltage. 3) input capacitor design the voltage ripple of the input capacitor is maximum when the line is lowest and the load is heaviest. if f sw(min) >> f ac , the input current can be assumed to be constant during a switching period. t off l i l peak t v o t sin ? -------------------------------- l 2i in peak t sin v o v in peak t sin ? ------------------------------------------------------- (2) == i in peak 2v o i o v in peak ------------------------------ (3) = t s t on t off + = 2li in peak 1 v in peak ---------------------------- - t sin v o v in peak t sin ? --------------------------------------------------------------- - +
(4) = 4lv o i omax ---------------------------------------- - 1 v 2 in peak -------------------------------- 1 v in peak v o v in peak ? ----------------------------------------------------------------------------- - +
= t smax 4lv o i omax ---------------------------------------- - 1 v 2 in peak -------------------------------- 1 v in peak v o v in peak ? ----------------------------------------------------------------------------- - +
(5) = l 4f sw min v o i omax 1 v 2 in peak -------------------------------- 1 v in peak v o v in peak ? ----------------------------------------------------------------------------- - +
------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------- - (6) = n aux v cc n p v o 22 ---------- -v in hl ?
------------------------------------------------- = 7 max) _ ( peak in i max) _ ( 2 peak in i 2 / on t on t input current inductor current the input capacitor must be larger than the value calculated by (8). and the maximum input capacitance is limited by the input displacement factor(idf), defined as idf cos . therefore the input capacitor must be smaller than c in(max) calculated by (12). c in 2 v in max ------------------------- 0 ton 2 -------- i in peak_max 2i in peak_max t on ------------------------------------- - t ?
dt t on i in peak_max 2 v in max ----------------------------------------- - 8 li 2 omax v 2 o v in max v 3 in peak_max ----------------------------------------------------------------- v a v a v in peak t cos == 9 i a i a t cos = i a i a i c i a t cos c in v in peak t sin ? = + =10 tan 1 ? c in v in peak i a ----------------------------------- =11 c in max i a v in peak -------------------------- - cos 1 ? idf tan = 2v o i o v 2 in peak_max --------------------------------------- - cos 1 ? idf tan =12 figure 7. input current and inductor current waveform during a switching cycle
AN4107 application note 6 ?2001 fairchild semiconductor corporation figure 8. input voltage and current displacement due to input filter capacitance in l in c pfc circuit a i a i a v a v input filter c i re im a i c i a i a v 4) output capacitor design the output capacitor is determined by the relation between the input power and the output power. as shown in fig. 10, the minimum output capacitance is determined by (14). figure 9. pfc configuration figure 10. diode current and output voltage waveform 5) mosfet and diode selection maximum mosfet rms current is obtained by (15) and the conduction loss of the mosfet is calculated by (16). when mosfet turns on the mosfet current rises slowly so the turn on loss is negligible. mosfet turn off loss and mosfet discharge loss are obtained by (17) and (18) respectively. the switching frequency of the critical conduction mode boost pfc converter varies according to the line condition and load condition. therefore the switching frequency is the average value during a line period. the total mosfet loss can be calculate by (19) and then a mosfet can be selected considering mosfet thermal characteristic. and the mosfet gate drive resistor is determined by (20). pfc load o c d i o i o v in v in i p in i in rms v in rms 1 cos ?2 t i d v o == i d i in rms v in rms v o -------------------------------------- 1 cos ?2 t = i o 1 cos ?2 t =13 )) 2 cos( 1 ( ) ( t i i o avg d o i o o o c i v o v c omin i omax 2 f ac v omax ------------------------------------------ - 14 i qrms i l peak_max 1 6 -- - 42v in ll 9 v o ----------------------------- ? = 22v o i omax v in ll ----------------------------------------- 1 6 -- - 42v in ll 9 v o ----------------------------- ? =15 p on i 2 qrms r dson =16 p turn off ? 1 6 -- - v o i l peak_max t f f sw = 2 3 ------ - v 2 o i omax v in ll --------------------------------- t f f sw =17 p disch e arg 4 3 -- - c oss.vo v 2 o f sw =18 p mosfet p on p turn-off p disch e arg ++ =19 r g v omax i omax ---------------- - 16v 500ma ------------------ 3 2 == 20
application note AN4107 7 ?2001 fairchild semiconductor corporation diode average current can be calculated by (21). the total diode loss can be calculated by (22) and then a diode can be selected considering diode thermal characteristic. 3-2. control circuit design 1) output voltage sensing resistor and feedback loop design r 1 is determined by the maximum output over voltage, v ovp and r 2 is determined by (23). the feedback loop bandwidth must be narrower than 20hz for the pfc application. therefore a capacitor is connected between inv and ea_out to eliminate the 120hz ripple voltage by 40db. the error amp compensation capacitor can be calculated by (24). to improve the power factor, c comp must be increased than the calculated value. and to improve the system response, c comp must be lowered than the calculated value. 2) zero current detection resistor design idet current should be less than 3ma, therefore zero current detection resistor is determined by (25). 3) start-up circuit design to start up the fan7527, the start-up current must be supplied through a start-up resistor. the resistor value is calculated by (26) and (27). the start-up capacitor must supply ic operating current before the auxiliary winding supplies ic operating current maintaining vcc voltage higher than the uvlo voltage. therefore the start up capacitor is designed by (28). 4) line voltage sense resistor and current sense resistor design the maximum line voltage sensing gain is determined by (29) at the highest line. calculate the pin 3 voltage at the lowest line using g in(max) by (30). then the current sense resistor is determined by (31), (32) and (34). once the current sense resistor is determined, then the minimum line voltage sensing gain, g in(max) is determined by (31). and attach 1nf capacitor in parallel with r2 to reduce the switching ripple voltage. 4. design example a 100w converter is designed to illustrate the design proce- dure. the system parameters are as follows. ? maximum output power : 100w ? input voltage range : 85vrms~265vrms ? output voltage : 400v ? ac line frequency : 60hz ? pfc efficiency : 90% ? minimum switching frequency : 34khz ? input displacement factor(idf) : 0.98 ? input capacitor ripple voltage : 24v ? output voltage ripple : 8v ? ovp set voltage : 450v 4-1. inductor design the boost inductor is determined by (6). calculate it at both the lowest line and the highest line and choose the lower value. the calculated value is 586uh. to get the calculate inductor value, ei3026 core is used and the primary winding is 62 turns. the air gap is 0.586mm at both legs of the ei core. the auxiliary winding is determined by (7) and the auxiliary winding is 5 turns. 4-2. input capacitor design the minimum input capacitance is determined by the input voltage ripple specification. the calculated minimum input i davg i omax =21 p diode v f i davg =22 r 1 r 2 ------ v o 2.5 ? 2.5 -------------------- - r 1 v ovp 40 a ------------------ = , = r 2 2.5r 1 v o 2.5 ? -------------------- - = ,23 c comp 1 0.01 2 120hz r 1 ----------------------------------------------------- =24 r idet n aux v o n p 3ma ------------------------ - r st v in peak_min v th st max ? i stmax --------------------------------------------------------------- 26 p rst v 2 in rms_max r st --------------------------------- =0.5w 27 c st i dcc 2 f ac hy st min ------------------------------------------------- 28 v pin3 v in peak_max r in2 r in1 r in2 + --------------------------- - = v in peak_max g in max 3.8v =29 v om kv in peak_min r in2 r in1 r in2 + ------------------------------- - vm2 max (30) = r semse v om i l peak_max ------------------------------------- k v in peak_min r in2 r in1 r in2 + ------------------------------- - = 2.5 v v in peak_min 4 v o i omax -------------------------------------------- (31) r sense 1.8v i l peak_max ------------------------------------- 1 . 8 v v in peak_min 4 v o i omax -------------------------------------------- = 32 p rsense 2 v o i omax v in peak_min --------------------------------------------
2 r sense =1w 33 r sense 1w 2 -------- - v in peak_min v o i omax --------------------------------------------
2 34
AN4107 application note 8 ?2001 fairchild semiconductor corporation capacitor value is 0.56uf. and the maximum input capacitance is restricted by idf. the calculated value is 0.76uf. the selected value is 0.67uf for the input capacitor(sum of all capacitors connected to the input). 4-3. output capacitor design the minimum output capacitor is determined by (14) and the calculated value is 83uf. the selected value is 100uf capacitor. 4-4. mosfet and diode selection by (15)~(19), 500v/6a mosfet fqp6n50 is selected and by (21)~(23), and 1000v/1a diode byv26e is selected by (21)~(22). 4-5. output voltage sense resistor and feedback loop design the upper output voltage sense resistor is 1.2m and the bottom output voltage sense resistor is 7k plus 10k variable resistor. a variable resistor is used to adjust the output voltage. the error amp compensation capacitance must be larger than 0.11uf by (24). therefore 0.33uf capacitor is used. 4-6. zero current detection resistor design the calculate value is 430 and the selected value is 22k . 4-7. start-up circuit design the maximum start-up resistor is 1 m and the minimum is 140k by (26)~(27). our selection is 150k . and the start-up capacitance must be larger than 10.6uf by (28). the selected value is 47uf. 4-8. line voltage sense resistor and current sense resistor design the maximum input voltage sensing gain is determined by (29). using the calculated value, the current sense resistance is determined by (31), (32) and (34). the maximum current sense resistance is 0.48 and the selected value is 0.2 . then the minimum input voltage sensing gain is determined by (30). if we choose the input voltage sense bottom resistor to be 18k then the maximum input voltage sense upper resistance and the minimum input voltage sense upper resistance can be obtained from g in(min) and g in(max) . the selected value is 2.7m . fig. 11 shows the designed application circuit diagram and table 1 shows the 100w demo board components list. figure 11. application circuit diagram f1 ac input 5 6 7 8 out vcc gnd idet inv ea_out mult cs fan7527 1 2 3 4 v1 c1 c3 c4 lf1 c2 ntc bd1 c5 r1 r2 c9 c6 r3 r4 r5 t1 d6 c7 r7 r10 r6 r8 d2 r9 vr1 c8 q1
application note AN4107 9 ?2001 fairchild semiconductor corporation table 1: 100w demo board part list part# value note part# value note fuse capacitor f1 250v/3a - c1 47nf 275vac varistor c2 150nf 275vac v1 471 - c3,c4 2200pf 3000v ntc c5 0.47nf 630v rt1 10d-9 - c6 47nf 35v resistor c7 0.33nf mlcc r1 2.7m 1/4w c8 100nf 450v r2 18k 1/4w c9 102 ceramic r3 150k 1w diode r4 100 1/4w bd1 660(600v/6a) bridge diode r5 22k 1/4w d1 1n4148 - r6 47 1/4w d2 byv26e 1000v/1a r7 0.2 1w line filter r8 1.2m 1/4w lf1 45mh - r9 7k 1/4w inductor r10 500k 1/4w t1 590uh(62t : 5t) ei3026 vr1 103 - mosfet ic q1 fqpf6n50 500v/6a ic1 fan7527 ----
AN4107 application note 10 ?2001 fairchild semiconductor corporation table 2: 150w demo board part list part# value note part# value note fuse capacitor f1 250v/3a - c1 330nf 275vac varistor c2 330nf 275vac v1 471 - c3,c4 2200pf 3000v ntc c5 0.68nf 630v rt1 10d-9 - c6 47nf 35v resistor c7 1nf mlcc r1 2.2m 1/4w c8 150nf 450v r2 20k 1/4w c9 102 ceramic r3 150k 1w diode r4 100 1/4w bd1 660(600v/6a) bridge diode r5 22k 1/4w d1 1n4148 - r6 47 1/4w d2 suf15j 600v/1.5a r7 0.2 1w line filter r8 1.2m 1/4w lf1 45mh - r9 7k 1/4w inductor r10 500k 1/4w t1 500uh(83t:5t) mpp core vr1 103 - mosfet ic q1 fqa9n50 500v/9a ic1 fan7527 - - - -
application note AN4107 11 ?2001 fairchild semiconductor corporation table 3: 200w demo board part list part# value note part# value note fuse capacitor f1 250v/3a - c1 330nf 275vac varistor c2 330nf 275vac v1 471 - c3,c4 2200pf 3000v ntc c5 0.68nf 630v rt1 10d-9 - c6 47nf 35v resistor c7 1nf mlcc r1 2.2m 1/4w c8 220nf 450v r2 22k 1/4w c9 102 ceramic r3 150k 1w diode r4 100 1/4w bd1 660(600v/6a) bridge diode r5 22k 1/4w d1 1n4148 - r6 47 1/4w d2 suf15j 600v/1.5a r7 0.1 1w line filter r8 1.2m 1/4w lf1 45mh - r9 7k 1/4w inductor r10 500k 1/4w t1 400uh(74t:5t) mpp core vr1 103 - mosfet ic q1 fqa13n50 500v/13a ic1 fan7527 - - - -
AN4107 application note 12 ?2001 fairchild semiconductor corporation nomenclature i l(peak) (t) : inductor current peak value during one switching cycle i l(peak) : inductor current peak value during one ac line cycle i l(peak_max) : maximum inductor current peak value i l (t) : inductor current i d : boost diode current i in (t) : input current i in (peak) : input current peak value i in (peak_max) : maximum of the input current peak value i in (rms) : input current rms value i qrms : mosfet rms current i drms : diode rms current i davg : diode average current i o : output current i o (max) : maximum output current v in (t) : input voltage v in (max) : maximum input voltage ripple v in (peak) : input voltage peak value v in (peak_max) : maximum input voltage peak value v in (peak_min) : minimum input voltage peak value v in (rms) : input voltage rms value v in (rms_max) : maximum input voltage rms value v in (rms_min) : minimum input voltage rms value v in (ll) : low line rms input voltage v in (hl) : high line rms input voltage v o : output voltage v o (max) : maximum output voltage ripple v ovp : maximum output over voltage p o : output power p o(max) : maximum output power p in : input power : converter efficiency t on : switch on time t off : switch off time t f : mosfet current falling time t s : switching period f ac : ac line frequency : ac line angular frequency f sw : switching frequency f sw(max) : maximum switching frequency f sw(min) : minimum switching frequency l : boost inductance c o : output capacitance c in : input capacitance : converter efficiency n aux : auxiliary winding turn number n p : boost inductor turn number c comp : compensation capacitance r idet : zero current detection resistance r st : start up resistance r 1 : output voltage divider top resistance r 2 : output voltage divider bottom resistance r in1 : input voltage divider top resistance r in2 : input voltage divider bottom resistance r sense : current sense resistance i stmax : maximum start up supply current c st : start up capacitance hy (st)min : minimum uvlo hysteresis k : multiplier gain g in (min) : minimum input voltage sense gain g in (max) : maximum input voltage sense gain
application note AN4107 13 ?2001 fairchild semiconductor corporation
AN4107 application note 10/11/01 0.0m 002 stock#anxxxxxxxxx 2001 fairchild semiconductor corporation disclaimer fairchild semiconductor reserves the right to make changes without further notice to any products herein to improve reliability, function or design. fairchild does not assume any liability arising out of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights, nor the rights of others. life support policy fairchild?s products are not authorized for use as critical components in life support devices or systems without the express written approval of the president of fairchild semiconductor corproation. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. a critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com

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