DER-34

13W Integrated Digital TV PSU

April 1, 2004

Page 6 of 19

Power Integrations

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4

Circuit Description

This schematic presented above details an isolated flyback power supply using TOP242P

from Power Integrations. This IC integrates the following features to minimize PSU cost:-

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

for cheaper EMI filter components

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

to increase reliability

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

DIP08 package which requires no additional heatsink, minimizing BOM and

manufacturing cost.

4.1

Design Architecture

The approach used with this design is to provide the 2.5V output via a diode drop from

the 3V3 rail. This technique is a good way to cost effectively produce 2.5V without

significant efficiency penalty. If tighter regulation is required on 2.5V, a feedback resistor

from 2.5V to the TL431 can be added.

4.2

Input Stage

Mains input protection is provided by fuse F1 and a VDR, R14. Common-mode and

differential mode EMI filtering is provided by C7, L5 and C3. AC rectification and bulk

storage is provided by D4, D5, D8, D9 and C10.

4.3

Primary Side Clamp

Primary side clamping is provided by D7, R4, R2 and C4. D7 is specified as 1N4007GP

and it is important that the GP version is used. The GP version has a specified reverse

recovery behavior, which recycles some of the clamp energy and aid efficiency.

4.4

Output Rectification, Filtering and Feedback

Schottky diodes are used on both the 3V3 and 5V outputs to give good voltage centering

and high efficiency. The 2V5 rail is provided using a UF5400 diode drop from the 3V3 rail.

All rails use LC post filters to give very low output ripple and the 32V rail is provided by a

charge pump driven from the AC side of the 32V winding.

Feedback is derived from the 3V3 and 5V windings, which will give +/-5% accuracy on

both rails. If tighter regulation is required on 2.5V, a feedback resistor from 2.5V to the

TL431 can be added.