DER-19

TOP245P Multiple Output Set Top Box

March 30, 2004

Page 7 of 25

Power Integrations, Inc.

Tel: +1 408 414 9200 Fax: +1 408 414 9201

www.powerint.com

4

Circuit Description

This power supply uses the latest generation TOPSwitch in a DIL08 package to minimize

heatsink requirements. It is designed for 32W continuous operation in a 50°C ambient

with magnetics designed to allow short peak power levels of up to 50W.

4.1

Input EMI Filtering

Due to the frequency jittering function of TOPSwitch, the input EMI filtering is minimal,

consisting of a 15mH common-mode choke and 220nF x-capacitor. Protection is

provided by a 1A, 250V antisurge fuse. Inrush limiting is provided by a thermistor. Surge

protection is provided by a VDR on the input. If only 4kV surge is required, this part can

be removed since the TOP245P incorporates over-voltage shutdown giving additional

protection.

4.2

TOPSwitch Primary

On the primary side of the supply, the TOPSwitch integrates a number of functions:-

Frequency jitter which reduces the QP and AV EMI levels by up to 10dB

Soft-Start which prevents transformer saturation during start-up. This increases

long term reliability

Line UV and OV detection to give additional differential surge withstand capability

Regulation to zero load without pre-load due to very low minimum duty cycle

capability

Line feed forward which improves 100Hz ripple rejection

Hysteretic thermal and short circuit protection to increase long term reliability

A 47uF input capacitor has been used to provide the high peak power capability. If peak

power operation is not required, this can be reduced to 33uF, 400V which will save

further cost.

4.3

Output Rectification

A fully AC stacked design has been used to give good cross-regulation. A snubber is

placed across D7 to reduce high frequency common-mode EMI emissions. Post filters

are used on all outputs to meet noise and ripple requirements.

4.4

Output Feedback

Full PWM feedback has been implemented using a TL431 reference and opto-coupler.

Feedback is split over the 3V3 and 5V rails, each giving equal influence to the feedback

network. D10, R10 and C11 provide a soft-finish function, ensuring a monotonic rise in

the output voltages with zero overshoot.