LNK603 design, EMC and efficiency

Posted by: cooperman on 11/21/2012

Hi I have just taken over responsibility for a design using an LNK603 which is about to go into volume production. The design was done by someone else.

It is intended to supply 1 W max from a half wave rectified full range input. It has two 5v 100mA outputs from separate windings on the transformer. I have problems regarding efficiency. I am measuring only 50% efficiency at low power.

I have not designed using these devices before but I have tried to use the PIXI software to analyse what I have and the results I get from the software lead me to beleive the design I have may be unreliable in the long term as well as being inefficient.

Please can I send a PI engineer a pdf of the design I have for comment?

I am also concerned about emc.

This is urgent as the design is about to go into manufacture in volume.

Thanks you.

Yours

Dave Cooper

Comments

And advise why you think the design is not reliable, also let me know any warnings from PIXI.

regards

Hi Here is the schematic

I am worried about the FET voltage being exceeded in the LNK603. I have been using PI Expert with the LNK603 substituted for a LNK623 as PI Expert would not let me choose a LNK603.

The circuit design I have does not have a resistor R1 in parallel with the 47pF snubber capacitor C3. I have set R1 to 9000K in pi Expert to approximate to having no resistor. PI warns me that the voltage may go beyond 1KV if I set the snubber resistor to infinite.

Thanks very much in advance for taking the time to check the design.

Based on your schematic and PIXLS result, the follwoing three problems need tobe solved.

1. c3 use 4.7nF -10Nf, and parallel a 100k resistor with C3. Test the Vds when the board is availabe, and trim the 100k ressitor. Vds too high, then reduce the resistor value.

2. Use same rectifier as the design tool suggested. Do not use the haf wave input rectifier.

3. Reduce Vor to solve low Dcon time issue.

Regards

Hi, Thanks for the help. I can not change to full wave rectifier as the pcb is designed and tested. What are the consequences of staying with a half wave rectifier please ?

Dave

All the calculation of the PI design tool is based on the bridge rectifier, and thus the input voltage is much higher than the input of the real circuit. That may casue problems such as lower load capability, higher output ripple and even more, since the condition of the design tool is not match with the real circuit.