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DC-DC desing with same LDRV and HDRV transistors

Other Parts Discussed in Thread: CSD18509Q5B, CSD17303Q5

Hello to all,

Today I was looking for a setting in WeBench, want to enable automatic pic of same transistor for both, High and Low side transistors. Here is the sample schematics:

It uses two N-Chan NexFET transistors CSD17303Q5 and CSD18509Q5B. From the money perspective it might be cheaper to put different transistors, when manufacturing really big quantities, but if we plan to make lets say 100 pcs of DC-DC converters - it would be more reasonable to have similar transistors (this would me a bigger quantity for the part). But in WeBech I found that I can only replace transistors in pairs, not by one of pair. Is there any technical issue with using same transistor on both sides ?

Another thing that I am missing in WeBench - to select the size of SMD capacitors/resistors/coils, so all of them would be one size (for example 0805). This is needed in case if a designer has a limited machinery, which can only pic and place 0805 or different size parts. Now I had to pick every part, just to have the right size, for big schematics it can be very time consuming. But I guess this should be forwarded to some other part of this forum...

Best regards
Dainius

  • Yes I will forward this to the Power and lighting Tools Forum...
  • Thanks for forward, John and hello to all ! Could anyone confirm, that using same transistors for both sides is wrong ?

    Best regards

    Dainius

  • I do not directly support any of those types of controllers but I can make a general comment.

    In many cases you may be bucking a relatively high input voltage down to a relatively low bus voltage say 12 V input to 1.2 V output. In that case, the duty cycle will be about 10 %. So it makes sense to use a low side FET that is lower in RDSon than the high side FET as there is much more power dissipated in the low side. If you must, there is no reason I can think of why you could not use the same FET, but generally you would prefer to use a different FET for high and low side based on the duty cycle involved.
  • Thank you for your reply. As it is only one to this moment I am glad that it repeats what I am thinking. I intend to use LOW side FETs on both sides, so the minimum RDS would be maintained anyway. And the gate charge is even lower than High side FET has, so it will be easier to switch. Well, I'll wait for couple of days, if there will be no opinions against - I will let the production process. 

    Best regards

    Dainius

    P.S. So WeBench could be updated to let select same FETs for both sides, checking if everything is OK with RDS.

  • Hi Dainius, Regarding your request to have an option in WEBENCH Power Designer to pair the top and bottom FETs in synchronous controller designs - This feature is already available!

    To find it, when you are in WEBENCH Power Designer Visualizer, look at the "Change Inputs" panel. There is a link to "Use Advanced Options >> ". Click that, and in the popup there is a section on the right for Component Selection. Click the box for "Synchronous FET Pairing (Prefer)." This will set up the synchronous controller designs to pick matched FETs, if possible.

  • Hi Wanda, thanks for your comment. It really helped! Though I understood, that there is no way to change already generated schematics... Anyway, with the TPS chip that I want to use it does not generate schematics with synchronous FETs even with selection marked. Maybe WeBench thinks this is not possible for some reason (it could be shown after the schematics is opened). But I found another very useful thing there - smallest part size limit! And this works perfectly.

    Best regards

    Dainius