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Calculating losses in a DC-DC converter.

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jrz126

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I'm working on a DC-DC buck converter for work (so I cant post a schematic) and I had some questions about calculating the losses.

Its a 20-80V input and an isolated 12V,5A output (60W). It will typically be running at 74VDC in. When starting the engine, the voltage will drop to 25-35V while cranking.
When running at 80V in, 5A out, I am putting in 133W and getting 65W out. I'm losing 68W somewhere, and I'd like to figure out why.

First off, what is the typical efficiency of a buck converter? It seems to me that 50% is pretty low, concidering my boss was talking about reaching 90%, but we could be wrong.
It is more efficent at lower input voltages too, at 35Vin 5A out, I'm putting in 87W and getting 65W out. thats about 73%.

Also, how significant are the switching losses? Its currently switching at 225kHz.

Finally, is there an easy way to calculate the power lost in the fets and diode?
 
if something in there is chewing up most of the 68 watt difference then its probably getting real hot in the process. What component is getting excessively hot? that should tell you where your problem is.

i'm not an expert but i think you can get a very rough estimate of losses by calculating the various non-idealities in the components versus their operating conditions.

The diode has recovery losses and series conduction losses. If you're using a very fast diode you can consider the conduction losses the dominant part of the loss so just multiply the forward drop by the output current.

you're inductor has DC resistance so calculate the power loss for that from the current your putting into it. I think the equation is "P = I^2 * R" if i remember correctly.

same thing with your fets and their Rds(on), Although the switching losses will also be an issue. Assuming you're hard switching your fets the equivalent drain- source capacitance is being shorted at the switching rate. So you multiply the equivalent energy in the capacitance by the switching rate to give you the power loss in the switching losses of the fet.

Oh well, i'm probably wrong somewhere in the above assumptions so hopefully some more knowledgable member can correct me.


Have you hooked up your circuit to an oscilioscope and checked whether your fets are being driven into saturation fast enough? One time i had a similar problem with my DC DC converter and the fets were heating up quite a bit. i then checked with oscillioscope and saw that instead of square wave across the drain and source i was getting alot of distortion because my gate driver wasn't driving them hard enough fast enough. i fixed that and the efficiency went straight up.
 
The "lost" power will warm things up so check around to see what's warm and what isn't. You might pay particular attention to the FETs and if they get warmer as efficiency drops then you'll have at least one point of focus.

It's my understanding that the transistion time between off and on can have a profound effect on losses. That might be something you could work on if you find the losses are in the FETs.
 
The fets are definitly disipating alot of power, and I have mainly been focusing on them

I have posted a shot of a gate pulse below, and I think it may be the cause. The trace is at 5V/div and 2uS/div. The lip thingy at the end is roughly 3.5 volts and lasts for 800nS. The Vgs threshold is typically 4V +/- a volt. Is the Fet in linear mode during this time? Is this being caused by my driver chip being too weak? I think it has a max of 500mA per output.

I also found out that the supply is a push pull type topology if that helps.
 

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