Fairchild FPS Series Design Manual Download Page 23

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FPS

™

Design Guide

The circuit shown consists of a PFC stage built around the FAN7527B/FQP13N50C/EGP30J circuit and the quasi-resonant PWM
stage built around the KA5Q1265RF/T1 circuit. This circuit is suited for input voltages in the range from around 195V to 265V.

The transition mode PFC stage generates a DC bus voltage of around 400V. The purpose of the stage is to reduce the harmonic con-
tent of the input current drawn from the AC supply as required by the EN61000-3-2 standard. An additional benefit is that the input
power factor is very high.

The KA5Q1265RF circuit generates the required output voltages using a multiple output flyback configuration. The device operates in
discontinuous mode and detects the point where the secondary current has dropped to zero. The device then switches on after a
delay set by the circuit around C105. As the delay is chosen to be at the first minimum of the primary side voltage ring as it
changes from Vin + nVo to Vin - nVo the device is switched on at a low voltage, which reduces the switching loss. The switching
frequency is therefore asynchronous and varies with the load. This reduces the visible effect of switching noise on the television
screen. Fixed frequency switching noise would be seen as diagonal lines on the screen. The turns ratio is chosen to be unusually low
for a standard flyback because the output voltage on the main winding is exceptionally high. This keeps the reflected voltage nV

low.

If the load on a quasi-resonant flyback circuit is reduced, the switching frequency increases which causes a reduction in efficiency.

The KA5Q series has a burst mode of operation. In normal operation the High/Low signal is High. When this signal which is typically
supplied by a microcontroller is Low, the current increases through the optocoupler, the feedback voltage goes to ground and the
device enters burst mode. In this case the output voltages drop until the voltage supplied to the chip through the auxiliary winding
drops to around 12V. The device remains in hysteretic burst mode until the feedback voltage increases. In this low power mode, the
PFC chip is deactivated via D304. In normal operation, the auxiliary winding voltage is around 24V, so there is sufficient voltage to
power up the PFC chip. In burst mode, the FPS™ voltage is between 11V and 12V, so the FAN7527B chip is deactivated, as its supply
voltage is around 8V lower than this.

• Complete PFC and PWM solution for a color television power supply

– High efficiency (typically 90% at full load)
– High power factor and low input current harmonics

• Quasi-resonant mode ideal for TV applications

– High efficiency due to lower voltage switching
– Asynchronous switching is not at constant frequency
– Slower dV/dt causes lower internal radiated interference

• Supports low power standby

– Hysteretic burst mode for KA5Q1265RF device
– FAN7527B PFC controller deactivated at low power

Fairchild Devices

Description

KA5Q1265RF

Fairchild Power Switch (8A/quasi resonant)

FAN7527B

Transition mode PFC controller

FQP13N50C

High Voltage MOSFET (13A/500V)

EGP30J

Fast Recovery Diode (3A/600V)