High Primary Leakage inductance
I have designed a few power supplies using PI Expert. The devices are TNY280PN, TOP261EN, TOP247YN and a few more. In most cases I observed that the maximum primary leakage inductance allowed by the design was crossed. Sometimes it went to two to three times recommended. I saw an option in the "Key Parameters" to change the Primary Leakage Inductance. If I feed the actual measured Primary Leakage Inductance value in the PI-Expert, it changes the values of clamping R & C. If I adopt those values will the design work reliably? Does it make substantial difference to the power consumption and noise?
Kiran
Comments
Many thanks for the response. I will check the Vds and revert back to you. It seems from your reply that the high primary leakage inductance transformers can well be used with the change in the clamp rather than scrapping those.
Kiran
I tried to measure the Vds on my Oscilloscope however even if I use 10X probe the maximum displayed voltage window on the scope is 400V. I have 60MHz Textronix digital storage scope. Can I make use of the same and be able to measure Vds? Or is there any other method to measure the Vds?
Regards,
Kiran
You will need a 100x probe. The voltage you are measuring can easily reach 600V to 650V
Hi Kiran.
You should always check if the clamp is acceptable by measuring the voltage between drain and source (check the application note 43 for more details). Depending on your application you may want to see a maximum Vds of around 650V.
It is difficult to accurately estimate the ideal clamp for a given circuit even if you have the correct value of the stray inductance. A particular example is if you use slow diodes. In this case you will tend to recover part of the energy in the stray inductance back into the PSU which means that your clamp does not need to dissipate the whole energy coming from the stray inductance.
What will happen according to my experience is that the PIExpert will usually be conservative in the selection of the clamp, meaning that you won't have any issues with Vds, but you will lose a bit in terms of efficiency. For more details on how PIExpert calculates the clamp, check this document from PI University (starting at page 11). It is good to start with conservative values on the bench, but usually you will want to optimize on your prototype in order to improve efficiency and even EMI.