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    IGBT Short Circuit Detection using Current sensing

    Hi friends

    In an inverter I want to cut the gate signal in short circuits
    i using STM32F103

    The problem I have is the short-circuit tolerance time is about 5-10 microseconds

    How can I quickly detect the current ?

    High-speed sensors are very expensive

    if i using HCPL-7800 and shunt resistor
    How can I calculate how fast can I read the current?

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    Re: IGBT Short Circuit Detection using Current sensing

    First your diagram shows a different (and faster, 1.6uS) isolation amplifier than you mention in the text.

    You need to do a timing stackup and I'd guess isolation amplifier bandwidth and delay plus your digital controller response are the big contributors (gate drive isolation is typically nS).


    It would be common in this case to have a separate hardware loop on your isolated secondary which bypasses the digital control to shut down the gate drivers. A common circuit uses the 0.7V Vbe threshold of a BJT as a fast and simple comparator. If you can't tolerate 0.7V drop comparators with <1uS propagation delay aren't too hard to find and could have a lower threshold (too low and noise will become an issue). A high bandwidth shunt amplifier may benefit some of these options (INA106)


    If you need digitally managed shutdown hall effect sensors have bandwidth up to 1Mhz and may be a better alternative to the shunt+isolation amplifier shown (see Allegro and LEM).


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    Re: IGBT Short Circuit Detection using Current sensing

    Hi,

    Is all the isolation really necessary?
    It's more fast without optocouplers.

    And it's more fast when you don't feed the overcurrent signal through the microcontroller.

    Klaus
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    Re: IGBT Short Circuit Detection using Current sensing

    There are special IGBT driver chips that you can buy which automatically shut off the igbt in case of short circuit......i think its called "DESAT detect" or something


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    Re: IGBT Short Circuit Detection using Current sensing

    thank you

    Quote Originally Posted by asdf44 View Post
    First your diagram shows a different (and faster, 1.6uS) isolation amplifier than you mention in the text.

    You need to do a timing stackup and I'd guess isolation amplifier bandwidth and delay plus your digital controller response are the big contributors (gate drive isolation is typically nS).


    It would be common in this case to have a separate hardware loop on your isolated secondary which bypasses the digital control to shut down the gate drivers. A common circuit uses the 0.7V Vbe threshold of a BJT as a fast and simple comparator. If you can't tolerate 0.7V drop comparators with <1uS propagation delay aren't too hard to find and could have a lower threshold (too low and noise will become an issue). A high bandwidth shunt amplifier may benefit some of these options (INA106)


    If you need digitally managed shutdown hall effect sensors have bandwidth up to 1Mhz and may be a better alternative to the shunt+isolation amplifier shown (see Allegro and LEM).
    So with a quick opamp. I need to detect overcurrent and and turn off IGBTs ( Regardless soft turn off )

    One question is, how do I determine if the spike is passing or is the short circuit ?

    We only have shunt resistor opamp and a Vref for detect over Current



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    Re: IGBT Short Circuit Detection using Current sensing

    Hi,

    So with a quick opamp. I need to detect overcurrent and and turn off IGBTs
    An OPAMP is an amplifier, it has an analog output.
    To detect "= logic output" you should use a comparator.
    (Although OPAMP and comparator share the same schematic symbol, they are designed for different operating conditions. It has been discussed several times here in ths forum)
    I'd say for your overcurrent system a comparator is essential, an Opamp only is needed if the analog signal is too low for the comparator to work correctly.

    What exactly do you mean with "soft turn off"? .. in a discussion about "short circuit detection".

    One question is, how do I determine if the spike is passing or is the short circuit ?
    I don't understand what you mean with "passing".

    "Short circuit" usually is meant when the outputs of a device are connected with (too) low impedance. This usually causes higher current than at normal operation. You (the designer) has to determine the behaviour of current vs time at worst case normal operating conditions.
    Then you need to determine worst case short circuit conditions.
    In my eyes a "short circuit" system needs to safely run at normal conditions (with headroom) but needs to switch OFF the output at overcurrent condition. Overcurrent threshold as well as timing needs to be determined (by you) in a way, that it protects your circuit from being destroyed.

    For fast reaction I recommend to use a flipflop to trigger and hold the overcurrent condition. It should be cleared by software.

    Klaus
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    Re: IGBT Short Circuit Detection using Current sensing

    Regardless soft turn off
    Implemented to avoid overvoltage when cutting high short circuit currents. The feature must be supported by gate drivers. ACPL-332x doesn't, so it's no option here.

    One question is, how do I determine if the spike is passing or is the short circuit?
    You should be aware of current peaks caused by cable and winding capacitances. Respectively the overcurrent detection should implement a certain delay or low-pass filter.


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    Re: IGBT Short Circuit Detection using Current sensing

    Fastest response mean to remain in analog zone.
    High current may be detected by looking at derivative... need fast circuits.
    May use few high speed comparators and an precision timer(s) to looking at derivative and if is about a short-circuit, this must send turn-off signal as fast possible.


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    Re: IGBT Short Circuit Detection using Current sensing

    Hi friends a question

    Usually the shunt resistor has an inductance value
    I want to identify the short circuit by shunt resistance
    How can I ignore the inductance effect of shunt resistance ?



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    Re: IGBT Short Circuit Detection using Current sensing

    Hi,

    Use a shunt with low inductance.
    No wirewound one. Metal strip resistors are good.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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