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Old 9th May 2008, 07:41 PM   #1
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Question Disabling MOSFET body diode...

Hello guys.

I'm currently designing the power stage for a Class D amplifer. My primary choice at the moment are the IRFB4020pbf power MOSFET.

Concerning this device, even when internal body diode specs are relatively good, they obviously look poor when compared to discrete ultrafast soft-recovery diodes or schottky rectifiers.

I was wondering about disabling the internal diode by means of a series and an antiparallel diode. I'm aware that conduction loses would be increased and also strait inductances but it could pay off in term of relaxing MOSFET current stress and minimizing switching loses... at least at first sight. I saw a couple of designs around the web and it called my attention that the antiparallel diode is usually a ultrafast diode and not a schottky as I would have expected (because of they not presenting reverse recovery issues and having smaller Vf). Would it be because there is not too much availability of schottky rectifiers with high reverse voltage capability?

Anyway, if any of you have tried this topology, comments on what resulted would be very welcome!

Thanks!

Pablo
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Old 9th May 2008, 08:52 PM   #2
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All you have to do is place an even faster diode in parallel with the fet, then the internal diode wont do anything.
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Old 9th May 2008, 09:22 PM   #3
Pafi is offline Pafi  Hungary
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Quote:
the internal diode wont do anything.
Are you sure? Check forward voltages of a (hot) body diode and an ultrafast or schottky diode!

The voltage drop across FET must be increased. Series diode or resistor, then FRED parallel with them.
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Old 9th May 2008, 10:05 PM   #4
Dave is offline Dave  New Zealand
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I agree with Tekko, the usual practice is a schottky diode across the FET. ie. in parallel with and pointing in the same direction as the body diode.
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Old 10th May 2008, 07:51 AM   #5
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I have used seires schottky and anti parallel Fred with mosfets to get rid of body diode issues which are posing reliability problems.

Losses are not that high, only slightly more, but again what you get with this approach is bullet proof reliability from the drastic effects of freewheeling current.

Have a look at this combo device

http://ixdev.ixys.com/DataSheet/16fd...c143100898.pdf
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Old 10th May 2008, 02:44 PM   #6
JohnG is offline JohnG  United States
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It will be hard to find a Si Schottky with a 200V rating, and it is likely that its voltage drop will be comparable to the MOSFET. You can look at SiC or GaAs Schottky diodes, but their voltage drop is likely to be even higher.

The voltage drop of the external anti-parallel diode should be substantially less than that of the internal body diode.
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Old 11th May 2008, 11:13 AM   #7
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Workhorse: Hi nice to see you again. Do you have any estimate on the impact on the natural THD of the output stage, from your circuit, as compared to a direct connected MOSFET?
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Old 12th May 2008, 06:53 AM   #8
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Quote:
Originally posted by Lars Clausen
Workhorse: Hi nice to see you again. Do you have any estimate on the impact on the natural THD of the output stage, from your circuit, as compared to a direct connected MOSFET?
Nice to see you too, Lars !!

No, I never tried that open-loop test, in my close-loop design the THD is not effected by using series schottkys, but one thing I have achieved is added reliability with this scheme.

My amp runs with rails @ +/- 185VDC and target application is professional use..........so i always focus for reliability first, i donot care whether the THD is 0.1% or 0.001%, because it hardly matters in sound reinforcement systems..........but Yes, it does matter in fancy amps
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Old 12th May 2008, 10:14 AM   #9
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+-185V

No we don't go that high on fancy amps
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Old 12th May 2008, 02:24 PM   #10
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Thanks Workhorse for your comments. The amplifier I am designing will work in +- 75. Then the impact of freewheeling currents will be quite smaller than in your case, but it would be quite interesting to compare both performances with and without series schottkys. Sadly I dont have the time to build both prototypes so I guess I'll have to pick one in advance. I'll run simulations as accurate as possible and decide on this basis...
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