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Old Jun 21, 2006, 06:43 PM
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GI, Michigan
Joined Jun 2004
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Help!
H-bridge PWM frequancy????

I stumbled upon some chips very similar to the Motorola MPM3002 chip. Iv got the chips working fine except they get extremely hot only after a few seconds of running. Im using PWM signals to drive the chip and Iv tested them with only 20% duty cycles, as in 50ms low pauses 10ms high. If I raise or lower the frequancy so the thing will cool off? Im running a standard 300 size motor with no load and 12volts. The chip is rated to 8amps continuous and 100v at 62watts of power. There is no reason why the chip should be getting hot with such little power.
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Old Jun 21, 2006, 07:13 PM
"Simplify, then add lightness"
Raleigh,NC
Joined Nov 2000
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You won't be able to drive a 300 sized motor with that chip without an extremely large heat sink and even then it would be questionable. The MPM3002 has an on resistance of 0.4 ohms. That is 20 to 100 times higher than the mosfets used in most ESC's. At 8 amps, that would be 25 watts disipated by the chip. Without a heat sink, the MPM3002 has a Tja of 35 deg C per watt. If you tried to run 8 amps through it with no heat sink, the junction would try to get to 875 deg C, but would burn up long before that.

PS I just looked at the spec sheet again, and its even worse. The .4 ohms was only the P-channel. The N-channel is .15 ohms so the total for the bridge would be .55 ohms.
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Last edited by jeffs555; Jun 21, 2006 at 07:21 PM.
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Old Jun 21, 2006, 07:24 PM
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GI, Michigan
Joined Jun 2004
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The motors I plan on driving are 280 size and draw .750amps when they are locked. Thats only 9 watts of power to be drawing. Ill give the heatsink a try and see what happens.
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Old Jun 21, 2006, 08:14 PM
"Simplify, then add lightness"
Raleigh,NC
Joined Nov 2000
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Actually, at .75 amps and .55 ohms that would only be 0.31 watts dissipated in the IC, and should be fine without a heat sink. However, that sounds like extremely low current for a 280 sized motor.
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Old Jun 21, 2006, 08:54 PM
Dimension Engineering
Akron, Ohio
Joined Jan 2002
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What sort of circuit are you using to drive the high side Pfets? You can't just connect them to the microcontroller or they'll alternate between "on" and "sorta on" which will get the chip toasty really quick. You need to use some sort of level shifter to get the control signals to 12v and 0v.
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Old Jun 21, 2006, 09:34 PM
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Im driving the chip directly from the MC. What sort "switching" set up should I use?
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Old Jun 21, 2006, 10:11 PM
Dimension Engineering
Akron, Ohio
Joined Jan 2002
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An NPN transistor with a 1k ohm resistor between the collector and +12v, and a 1k ohm resistor between base and the microcontroller would work. The gate of the pfet goes to the collector of the NPN.
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Old Jun 22, 2006, 06:42 AM
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GI, Michigan
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Thanks for the help Ill give a shot, I just gotta see if I can find the NPN transistors now at work.

EDIT: Where does the emitter side go to??? I take it what you ment was the emitter goes to the Pfet. If you ment somthing different then Ill figure it out pretty soon!
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