What should be the deadtime for Hbridge circuit??? - diyAudio
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Old 3rd December 2012, 06:07 PM   #1
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Default What should be the deadtime for Hbridge circuit???

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hi,
i am building a Hbridge SMPS with SG3525 and IR2110, the MOSFETS are IRFP460 i need to know what will be the sufficient deadtime for 50khz frequency???

Because MOSFETs are exploding with 880ns deadtime, is deadtime the problem because it works fine with 60Vdc as input but when i apply 310Vdc booooom

Last edited by Mooly; 4th December 2012 at 05:09 PM. Reason: Safety note added
 
Old 3rd December 2012, 06:24 PM   #2
RJM1 is offline RJM1  United States
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#1 depends on what MOSFETs you are using and their turn off delay. A schematic would be useful? The turn on delay for the IRFP460 is 18uS, you probably want to be a bit longer than that.
#2 depends on your gate drive circuit.

Last edited by RJM1; 3rd December 2012 at 06:41 PM.
 
Old 3rd December 2012, 07:28 PM   #3
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What kind of monster are u building?
Drain voltage squared is a factor in switching losses, so u dont need to increase the voltage before it goes too far. You need to increase the voltage slower and observe when the devices temp gets critical.
Can you place current transformers in series with drai and gate to check currents? You'd be able to tweak DT that way. Assuming you have a good o-scope.
 
Old 4th December 2012, 06:59 AM   #4
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More possibilities:
Could it be spikes becoming too high as the voltage is increased killing the MOSFET?
 
Old 4th December 2012, 07:55 AM   #5
MarianB is offline MarianB  Romania
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IRFP460 are quite sturdy MOSfet's but they have a rather large Ciss witch if not charged fast enough it causes slow turn on and so the power dissipated on the mosfets increases with increasing voltage and they cand be destroyed by this, so you need to be sure you have enough gate current for them. Anyway a schematic is the only way for us to determin what is your problem, so do show us what it is.
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Old 4th December 2012, 08:01 AM   #6
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Or:
Is your flux walking? How did you prevent that in your circuit?

[QUOTE=RJM1;3268831]A schematic would be useful?[QUOTE]
 
Old 4th December 2012, 09:27 AM   #7
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Originally Posted by MarianB View Post
IRFP460 are quite sturdy MOSfet's but they have a rather large Ciss witch if not charged fast enough it causes slow turn on and so the power dissipated on the mosfets increases with increasing voltage and they cand be destroyed by this, so you need to be sure you have enough gate current for them. Anyway a schematic is the only way for us to determin what is your problem, so do show us what it is.
yep. perhaps the duty cycle is becoming too short as the voltage is increased, so short it is not turning the FET on properly....?
 
Old 4th December 2012, 10:04 AM   #8
MarianB is offline MarianB  Romania
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That too could be the reason for failing, but fet's are verry fast devices and 50Khz does not seam that high a freq, if they would have enough gate current deadtime should not be a problem. But until we do not see the exact schematic used all we can do is suppose things.
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Old 4th December 2012, 03:29 PM   #9
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hi all,
here is the schematic
Attached Images
File Type: jpg 50khz smps_1.JPG (70.0 KB, 73 views)
 
Old 4th December 2012, 03:59 PM   #10
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Are you actually using MUR1560 at the output?
Are you sure you have not blown those already and that is the cause of problems?
 

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