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Old 22nd January 2005, 09:01 PM   #1
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Default How much dead time?

Hi,

I'm new to this forum so I don't know if this subject has been already been dealt with in this forum, but I couldn't find any threads with search. My guestion is that how should I calculate the amount of dead time needed in a full bridge class d amp? I'm using IRF 543 mosfets.

Thanks,

Jyrgen80
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Old 23rd January 2005, 03:53 PM   #2
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Once you have established enough dead time to keep the circuit from destroying components, dead time will effect THD. You will have to experimentally determine the 'best' amount of dead time for a specific amp by changing it and measuring the amp's performance. Search for threads about 'over shoot' and 'shoot through'. I was reading a thread about it just a few days ago
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Old 23rd January 2005, 05:09 PM   #3
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Yeah, I know pretty much about shoot through and all that. And I'm not trying to get the smallest amount of dead time, but I would like to hear your way of determining the approximate amount. Do you have a "rule of thumb" for that?

Thanks,

Jyrgen80
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Old 23rd January 2005, 08:37 PM   #4
ekaerin is offline ekaerin  Sweden
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Jyrgen,
If you have a scope you can look at the voltage waveform as you
narrower the deadtime and also watch the current consumption.
However this just gives you an idea. Better is to monitor the drain current of the lower or upper FETs. Or, I have not tried, look
at the return current in the bridge ground.
Current probes are handy but expensive. There are ways of doing
your own from a small toroid core with a winding terminated with
a 50 Ohm resistor giving you I/V conversion to feed your scope.

/ Mattias
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Old 23rd January 2005, 10:40 PM   #5
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What I was trying to say is that there is no calculation to give you the optimum time. You must build the amp and experiment. Once the amplifier is working solidly, meaure THD or FFT if you don't have anything to measure THD directly, and then adjust dead time until THD is the lowest. Because tiny parasitics on the board and even small component tolerences can effect dead time, and thus THD, it must be adjusted in-circuit with measurement equipment to find an optimum value.
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