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3 Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized application, Buyer shall indemnify and hold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that ON Semiconductor was negligent regarding the design or manufacture of the part . ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any Note AN4107 design of Power Factor Correction Using FAN7527 2001 Fairchild Semiconductor CorporationRev.

4 A, May 20001. IntroductionThe FAN7527 is an active power factor correction(PFC)controller for boost PFC application which operates in thecritical conduction mode. It turns on MOSFET when theinductor current reaches zero and turns off MOSFET whenthe inductor current meets the desired input current referencevoltage as shown in Fig. 1. In this way, the input currentwaveform follows that of the input voltage, therefore a goodpower factor is Internal Block DiagramIt contains following blocks. Error amplifier (E/A) Zero current detection (Idet) Switch current sensing (CS) Input voltage sensing (MULT) Switch drive (OUT) Rev. Figure 2.

5 Block diagram of the FAN7527 Inductor Peak CurrentInductor CurrentInductor Average CurrentGatingSignalFigure 1. Inductor Current WaveformINV12 Vea(-)Error AmpVref++++ OVPC urrentDetectorIsovp=30uAIdovp=40uA++++ VrefVm2 Vref~Vref+ OVP++++ EA_OUT6 GND)(Vref2Vm1 VmVmoK ====Multiplier++++ Vm1 Vmo0 ~ ++++ ++++ RefInternalBiasTimer (L)= CurrentDetectorCurrent NOTE2 2001 Fairchild Semiconductor Corporation2. Device Block Description2-1. Error Amplifier and Over Voltage Protection Block The sensed and divided output voltage is fedback to the erroramplifier inverting input(INV) to regulate the output voltage. The non-inverting input is internally biased at error amp output(EA_OUT) is internally connected tothe multiplier and is pinned out for the loop , the control loop bandwidth of PFC converter isset below 20Hz to get a good power factor.

6 In this application, a capacitor is connected between INV andEA_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 makethe over voltage protection fast, the soft OVP and dynamicOVP is added. The FAN7527 monitors the current flowinginto the EA_OUT pin. If the monitored current reaches about30uA, the output of multiplier is forced to be decreased, thusreducing the input current drawn from the mains(soft OVP).If the monitored current exceeds 40uA, the OVP protectionis triggered(dynamic OVP), then the external power transistor is switched off until the current falls below about10uA. In this case, it disables some internal blocks reducingthe quiescent current of the chip to 2mA.

7 However, if theover voltage lasts so long that the output of E/A goes , then the protection is activated(static OVP) keepingthe output stage and the external power switch turned operation of the device is re-enabled as the E/A outputgoes back into its linear ++++ Vea(-)Error AmpVref++++ OVPC urrentDetectorIsovp=30uAIdovp=40uA++++ VrefVm2 Vref~Vref+ OVPO utputStageMultiplier2-2. MultiplierA single quadrant, two input multiplier is the critical elementthat enables this device to get power factor correction. Oneinput of multiplier(Pin 3) is connected to an external resistordivider which monitors the rectified ac line voltage. Theother input is internally driven by a DC voltage which is thedifference between error amplifier output (Pin 2) and reference voltage, Vref.

8 The multiplier is designed to have anextremely linear transfer curve over a wide dynamic range,0V to for Pin 3, and to 6V for error amplifieroutput under all line and load multiplier output controls the current sense comparatorthreshold voltage as the ac voltage traverses sinusoidallyfrom zero to peak line. This allows the inductor peak currentto follow the ac line thus forcing the average input current tobe sinusoidal. In other words, this has the effect of forcingthe 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 equation below describes the relationship between multiplier output and its = K Vm1 (Vm2 - Vref)K : Multiplier gainVm1: Voltage at Pin 3Vm2: Error amp output voltageVmo: Multiplier output voltage Figure 3.

9 Error Amplifier and OVP BlockAPPLICATION NOTEAN41073 2001 Fairchild Semiconductor CorporationFigure 5. Current Sense Circuit ++++ Vmo8pF40k4 CSCurrent Current Sense ComparatorThe current sense comparator adopts the RS latch configuration to ensure that only a single pulse appears at thedrive output during a given cycle. MOSFET drain current issensed using an external sense resistor in series with theexternal MOSFET. When the sensed voltage exceeds thethreshold set by the multiplier output, the current sense comparator turns off the MOSFET and resets the PWMlatch. The latch insures that the output remains in a low stateafter the MOSFET drain current falls back to peak inductor current under the normal operating condition is controlled by the multiplier output, Vmo.

10 Theabnormal operating condition occurs during pre-converterstart-up at extremely high line or as output voltage sensing islost. Under these conditions, the multiplier output and current sense threshold will be internally clamped to , the maximum peak switch current is limited to:Ipk(max) = / RsenseIn the FAN7527, an internal R/C filter has been included toattenuate any high frequency noise that may be present onthe current waveform. This circuit block eliminates the needfor an external R/C filter otherwise required for proper operation of the (-)Error AmpVrefOVPC urrentDetectorIsovp=30uAIdovp=40uA++++ VrefVm2 Vref~Vref+ ++++ EA_OUT)(Vref2Vm1 VmVmoK ====MultiplierVm1 Vmo0 ~ (L)= 4. Multiplier blockAN4107 APPLICATION NOTE4 2001 Fairchild Semiconductor Corporation2-4.