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FAN6920

Integrated Critical-Mode PFC and Quasi-Resonant Current-Mode PWM Controller

Fairchild Semiconductor 

the DET pin voltage and flows out current IDET. Since the

current IDET is in accordance with VAUX voltage,

FAN6920MR depends on this current during tON to

regulate the current limit level of the PWM switch to

perform high / low line over-power compensation.

As the input voltage increases, the reflected voltage on

the auxiliary winding VAUX becomes higher as well as

the current IDET and the controller regulates the VLIMIT to

a lower level.

The RDET resistor is connected from auxiliary winding to

the DET pin. Engineers can adjust this RDET resistor to

get proper VLIMIT voltage to fit power system needs. The

characteristic curve of IDET current vs. VLIMIT voltage on

CSPWM pin is shown in Figure 44.

  IDET  VIN  NA NP  RDET

(1)

where VIN is input voltage; NA is turn number of auxiliary

winding; and NP is turn number of primary winding.

VAUX

VDET

OPWM

Figure 43. Relationship between VAUX and VIN

in and a small RC filter (e.g. 100Ω, 470pF) is

recommended between the CSPWM pin and GND.

Protection for PWM Stage

VDD Pin Over-Voltage Protection (OVP)

VDD over-voltage protection prevents device damage

once VDD voltage is higher than device stress rating

voltage. In the case of VDD OVP, the controller stops all

switching operation immediately and enters auto-

recovery protection.

Adjustable Over-Temperature Protection and

Externally Protection Triggering (RT Pin)

Figure 45 is a typical application circuit with an internal

block of RT pin. As shown, a constant current IRT flows

out from the RT pin, so the voltage VRT on the RT pin

can be obtained as IRT current multiplied by the resistor,

which consists of NTC resistor and RA resistor. If the RT

pin voltage is lower than 0.8V and lasts for a debounce

time, auto-recovery protection is activated and stops all

PFC and PWM switching.

RT pin is usually used to achieve over-temperature

protection with a NTC resistor and provides external

protection triggering for additional protection. Engineers

can use an external triggering circuit (e.g. transistor) to

pull the RT pin low and activate controller auto-recovery

protection.

Generally, the external protection triggering needs to

activate rapidly since it is usually used to protect the

power system from abnormal conditions. Therefore, the

protection debounce time of the RT pin is set to around

110µs once the RT pin voltage is lower than 0.5V.

For over-temperature protection, because the

temperature does not change immediately; the RT pin

voltage is reduced slowly as well. The debounce time

for adjustable OTP should not need a fast reaction. To

prevent improper protection triggering on the RT pin due

to exacting test condition (e.g. lightning test); when the

RT pin triggering voltage is higher than 0.5V, the

protection debounce time is set to around 10ms. To

avoid improper triggering on the RT pin, add a small

value capacitor (e.g. 1000pF) paralleled with NTC and

the RA resistor.

Figure 44. IDET Current vs. VLIMIT Voltage

Characteristic Curve

Leading-Edge Blanking (LEB)

When the PFC or PWM switches are turned on, a

voltage spike is induced on the current-sense resistor

due to the reciprocal effect by reverse-recovery energy

of the output diode and COSS of power MOSFET. To

prevent this spike, a leading-edge blanking time is built-

© 2010 Fairchild Semiconductor Corporation

FAN6920MR • Rev. 1.0.3

Figure 45. Adjustable Over-Temperature Protection

www.fairchildsemi.com

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