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[SOLVED] mosfet driver problem (IRS2104 &IRF740)

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kappa_am

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Hi all,
I am testing my designed circuit all things is alright, except the drivers.
the driver circuit is shown in below picture. it is ok when MOSFET is not connected to the source (second picture). but when I connect drain of the mosfet to the positive rail of the source (power section), the output waveform of the driver turn like the third picture. it seems fairly acceptable. but when the load is connected everything go worse (fourth picture) and even under 1A load mosfet gets hot. what's wrong in your opinion?

Thank you for your help



P.S: in the fourth picture the output voltage (yellow waveform) is inverted. third and fourth waveform are x10.
 

iamthegospel

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Grounds of driver ICs and Switches are given separately.
 

montyw47

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What the schematic look like? the PCB layout does not tell us much.
 

Orson Cart

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You've got a lot of hash (noise) coming from somewhere else... all grounds should be solidly tied together, you need lots of de-coupling close to all the chips 1uF - 10uF, and tight twisted leads if you have interconnects.... try and report back...
 

kappa_am

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Thanks;
I solved it by adding an 1nF capacitor to the output of the optocoupler.
 

Orson Cart

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So a lot of common mode induced hash solved by adding C across the C-E of the opto...?
 

kappa_am

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yes, the opto was very noise susceptible. even with touching its body ( without touching pins) the output was affected.
 

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