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Old 30th May 2008, 02:15 PM   #1
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Default problem with heat of MOSFET

hi, all
i am in try to design class d power amplifier with output power 100W. in the design i use max 626 IC driver for driving gate MOSFET. design method to build class d amplifier i use self oscillating method. Switching Frequency i use 400000 Hertz and half bridge configuration power mosfet i used. The output voltage from halfbridge can give out rail to rail from suply voltage. and i happy about that result. But i have a problem with power MOSFET that is heat in power MOSFET cannot be tolerated and can previously damaged in the mosfet. I think this happened because of dead time problem. But i am in out of idea to design it from diskrit circuit.
Can all of you give me some suggestion or simple circuit, schematics to do that, to build power stage with MOSFET power with low disipation ( heat in mosfet can be tolerated or litlle heat in mosfet ) with this driver IC ??????????????????
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Old 30th May 2008, 06:17 PM   #2
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You cannot use this ic for a half/full bridge. You need a proper halfbridge gate driver such as IR2010.

The 626 is a dual gate driver but it does not contain a high side driver. You need a high side gate driver for the high side fet with a bootsrap circuit, see the diy class d schemas on the forum.
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Old 18th June 2008, 02:03 PM   #3
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i use this IC and can give output rail to rail from power suply ( this is my aim in design class d especially in power stage blok), and i sample this IC so I use it....
i think it doesn't matter with high side problem because the output of MOSFET can give output rail to rail to power suplly. ...I think the dead time problem.... It is true?????????
if it is true how i can design it???
thanx
by the way, i try to sample that IC ( ir2010) and i can get it..... because IRF does not give a samples....
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Old 18th June 2008, 09:11 PM   #4
s2 is offline s2  United Kingdom
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I put small inductors in the drains with back emf diodes to stop problem.
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Old 19th June 2008, 07:41 AM   #5
m-tech is offline m-tech  Serbia
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Yes, you can use MAX626 as half bridge driver ( upper fet must be P-channel type ).

Did you make something like this ?
If you are, then look ( and read design description ) on this schematic.
R43,R44,D27,D28,C19 and C20 are dead time generators.
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File Type: zip amp30.zip (48.7 KB, 169 views)
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Old 8th July 2008, 09:57 PM   #6
Tahmid is offline Tahmid  Bangladesh
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Hi,
Be careful when using P-channel MOSFET at voltage >20v. If input voltage is >20v, it would blow out the p channel MOSFET gate. In order to prevent this from happening, you HAVE to use level-shifting method to raise the gate above ground level to -15v in reference to source.
This is because gate is always driven by the Vgs.
So, if you are new to this level shifting method, i would recommend you to stick to n channel MOSFETs and use IR2181/84 or IR2010-2114 which have isolated high side gate drive circuitry.
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Old 9th July 2008, 01:18 AM   #7
Pafi is offline Pafi  Hungary
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Thamid!

There is level shifter in this circuit, just it's done by a capacitor and a clamping zener (and a BJT, but it's only for DC). The gates are in safety!
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Old 23rd July 2008, 10:11 AM   #8
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i had been try that circuit ( amp30e) and i have the problem :
1. Max 626 must give suplly >9V - 12V, but this driver being hot and very hot and may be max 626 can suffer seriously damaged.
2. i try that circuit but just can generates self - oscillating frequencies about 60Hz. In my design i must generates self oscillating frequencies 400000Hz. I think the problem being complex with characteristics respon device with this frequencies.

can any give solution????
I deal with IR2XXX to drive gate mosfet
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Old 9th February 2009, 03:11 PM   #9
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wow finnaly i could do it ( bias the Mosfet with fixed bias method )...i use level shifter ( Resistance and Capacitance ) with connect to Vss and other via connect to input signal..


thanx....thanx for idea
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