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H bridge gone wild - Need a little help. — Parallax Forums

H bridge gone wild - Need a little help.

Zap-oZap-o Posts: 452
edited 2011-01-26 16:30 in General Discussion
I am stumped on this h-bridge.

I apply the following truth table

0V at [ A ]
12V at [ B ]
5V at [ C ]
0V at [ D ]

Current flows at approximately 7amps but the Mosfet [ C ] begins to get very hot and very quickly. And as expected if I reverse the truth table Mosfet [ A ] gets very hot. Is there any thing you can bring to light any thing I am missing?

Thanks in advance
H bridge.jpg
H bridge.jpg
561 x 750 - 33K

Comments

  • LeonLeon Posts: 7,620
    edited 2011-01-26 09:26
    What is the load?
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2011-01-26 09:34
    Are you sure you don't have A and C swapped?

    In that configuration, A should be getting hot. Notice I said that A should be getting hot, The drive strength that you have for your N-Mos transistors is adequate at 12V, but the drive strength that you have for your P-Mos transistors is only 5V... in order to match the P-Mos drive to the N-Mos drive you need to supply a -7V instead of 0V.

    What's the voltage across the S/D of the N-Mos compared to the voltage across the S/D of the P-Mos ?
  • PliersPliers Posts: 280
    edited 2011-01-26 10:27
    Assuming your load is where the meter is shown, the active FETs would be A and B or C and D.
    If turn on B and C at the same time they will short and burn up.
    Same with A and D gated at the same time.
  • Zap-oZap-o Posts: 452
    edited 2011-01-26 10:29
    The load is a resistor that I run at ~5V 10A ~50Watts.

    I cant imagine why any mosfet would be getting hot. Turning a mosfet completely on or completely off dissipates little to no heat (power) according to page 67 of the process and control book. You think I have not enough negative voltage on the Pmosfets?

    ** Edited - I did take out half of the bridge and it works flawlessly, so negative voltage cant be the issue can it?
  • PliersPliers Posts: 280
    edited 2011-01-26 11:26
    Perhaps the 10 Meg pull downs should be 10K?
    The FETs that are conducting may not be fully on.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2011-01-26 16:30
    Zap-o,

    "I cant imagine why any mosfet would be getting hot. Turning a mosfet completely on or completely off dissipates little to no heat (power)" ... absolutely right, but your P-Mos transistors aren't saturated at the current you are trying to demand of them....

    Your driving the N-Mos transistors with a voltage potential between the Source and Gate of 12V, that's perfect. However your driving the P-Mos transistors with only a 5 V potential across the Source and Gate. <-- This is your weak link. Since your supply to the H-Bridge is only 5V, you need -7 Volts to come up with the same amount of drive across the P-Mos (Gate and Source) to create a total voltage potential of 12V.

    Measure the voltage across the Source and Drain, and I'll bet you find that you have quite a bit of loss across the P-Mos transistors.
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