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  1. #1
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    inverter design

    I'm designing a mosfet based single phase inverter using h-bridge

    I'm using PIC controller to control the mosfets

    The problem is that even for low duty cycle pulses, the mosfet keeps conducting for a much longer period than it should and at high frequencies (close to 40Hz), one of the high side transistors starts heating up... so.. i think that the problem is with turn-off (transistor is taking too long to turn-off)... so, i want to employ an RC snubber circuit in parallel or something else... can anyone help me by telling me what values i should use or whether or not there is a better alternative

    •   Alt25th June 2009, 13:41

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    FvM
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    mosfet based inverter

    A snubber shouldn't be supposed to reduce the said issues. It rather sounds like bad gate drive.


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    •   Alt25th June 2009, 23:06

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    mosfet inverters

    What kind of gate drive circuit do you use ? for PWM inverters you must use a dedicated IC like IR2110, 2113 which can charge/discharge MOS-FET gate very fast. if you used a simple circuit like in modified sine inverter, with just one transistor and resistor for chare/discharge, this is the result.
    there are other options too, like fast optocoupler followed by a gate drive, or gate transformer, but the ideea is to be able to source/sink current fast enough, in terms of tents of nS. also the gate resistor must have the value b etween 10-47 ohms, and frequently is connected in parallel with a diode which have the role to dischae the gate faster than it charge.



    •   Alt26th June 2009, 07:41

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    inverter mosfet

    Thanx for the reply

    The diode helped

    It's better now

    But still, it tends to get a little hot

    I was using an optocoupler, connected to BS170 (n channel mosfet) connected to buk438 (power mosfet)

    Though the heating has reduced significantly, the voltages haven't dropped to the value they would've had the actual on-time of the mosfet been equal to that of the controller's port pin

    I'll try the IR2110

    Thanx



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    inverters design

    i was using an optocoupler hcpl 3120 and hcpl 4504, they have high speed and high CMR, you should to use faster components.good luck.



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    hcpl-3120 inverter

    will do

    thanx

    :)



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    inverter with mosfet

    Hi adilmahsan,
    Use optocoupler hcpl 3120 to interface Microcontroller with IR2110/2113 to drive the Mosfets in High Side and Low side Mosfets can be driven directly without the use of IR2110. Since for 2 High side you require to use 2 IR2110 and those are High and Low side driver,you can use those 2 to drive both High and Low side Mosfets. One more suggestion, Please provide 1k resistor each for each Mosfets connected to source/ground from gate to offset the Miller Capacitance of the Mosfets. Thanks.



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