MOSFET gate voltage spikes

Discussion in 'The Projects Forum' started by cade007, Apr 28, 2006.

  1. cade007

    Thread Starter Member

    Apr 14, 2006
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    I am powering a half bridge with IRF740 MOSFETS using an IR2111 driver. The input voltage needs to get up to 200 V, but there is significant spiking in the voltage at the high side gate driver. The spiking get worse as I turn the voltage up. At about 40 V the voltage spike goes up to 110 V. If I turn the voltage up any higher than about 60 V the MOSFETS will blow up. Is there some sort of snubber configuration I could use?
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    Could you post the schematic please? And what kind of load are you driving with the half bridge? Putting a snubber on the gate would certainly help with the spike, however it would also cripple the gate drive and might cause other problems.
     
  3. dbwgwee

    Member

    Mar 30, 2006
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    Using a snubber config in this case likely equates to putting a bandaid on an arterial bleed. Please post the schematic so the length of time before it bleeds out can be determined.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    The presence of a spike on the gate of a mosfet which is essentially a big capacitor suggests the presence of an inductance. How far is the mosfet physically from the driver. Could there be enough trace inductance to cause this behavior? I still think you should post the schematic.
     
  5. cade007

    Thread Starter Member

    Apr 14, 2006
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    The schematic is actually already posted, under my original topic, which is what I am still working on.

    Schematic:
    https://webfiles.colorado.edu/penney/index.mht

    Overall Project:
    http://forum.allaboutcircuits.com/index.php?showtopic=3801

    There needs to be a wire going from pin 6 of the IR2111 to the source of each high side MOSFET. Also a wire from pin 3 of the IR2111 to the source of the low side drivers. I just made a full bridge out of using two half bridge drivers and inverted the PWM signal. The load on the output is about 144 Ohms. The cap across the output is about 10 uF and the inductor is about 2.8 mH. The bootstrap caps for the bridge drivers are about 1 uF. I thought my voltage spiking could be due to these bootstrap capacitors but I specifically solved for a bootstrap capacitor of about 960 nF. The bootstrap capacitor has to be bigger than this so I used a 1 uF.

    The MOSFET is actually about a foot or so away and I have twised pairs connecting them, so I could see some inductance being present there, but would it be enough to cause significant voltage spikes on a 100 kHz signal?
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    Your diagram is using the symbol for a jfet and we can't tell which is the source and which is the drain. If you used the proper symbol it would be obvious.

    Second neither your mosfets, nor the voltage source Vg are ground referenced, but the gates presumably are via the IR2111. Is this on purpose? How can you know if anything is on if you cant establish what Vgs is?
     
  7. cade007

    Thread Starter Member

    Apr 14, 2006
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    Sorry this drawing is not that great, but its the only onw I have. They are NMOS, so the source of the high side MOSFET is connected to the drain of the low side MOSFET on each side.

    I am not sure what you mean in your second question. There is a return path for Vg, which is the ground reference, vg is in reference to the wire right below the MOSFETS. Everything has worked at low voltages. I get a 60 Hz sine wave at the output but I cannot go up to higher DC voltages at the input since there is significant spiking at the gates of the MOSFETS, which will fry the MOSFETs.
     
  8. Papabravo

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    Feb 24, 2006
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    You may think it is the ground reference for the mosfets and Vg, but it is most certainly not the same ground reference as seen by the IR2111, according to the diagram.
     
  9. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    Where are the freewheeling diodes across the MOSFETs? If you don't have them, you would get spikes on MOSFETs sources which are transferred to gates through the miller capacitance, or more likely through the Vs pin of IR2111.
     
  10. dragan733

    Senior Member

    Dec 12, 2004
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    The fault is maybe choosing wrong driver IC for the Mosfets. Choose another driver IC where the GND of the supply HV and the GND of the signal are differents, not connected together. Or as I said in the past use gate drive transformers, that are not arrived yet at me.
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    Why is the IR2111, the wrong driver IC. Do you know something the rest of us don't?
     
  12. cade007

    Thread Starter Member

    Apr 14, 2006
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    Sorry I need to get a better schematic online. There is a wire connected form pin 6 (before the cap) to the midpoint (inbetween the two transistors where the inductor is) and from pin 3 to the ground of the power stage below at the source of the MOSFETS. After I added slightly larger resistors at the gates of the transistors I think that the voltage spikes are not a problem.

    How much current does the IR2111 source from the PWM signal? I have a TC427 driver driving my PWM signal which is then going to the half bridge driver. AS I turn the voltage up on the input the TC427 driver gets really hot and blows up. I was looking at the high side gate driver signals as I was turning up the voltage and the voltage suddenly dropped and the current on the input shot up. For the IR2111 I am using a bootstrap cap of 1 uF and a 1N4004 diode. Is there too much current going through the diode as I turn it up to 200 V? I solved an equation for the bootstrap cap so I think it is ok. My main question is how much current is the half bridge driver going to pull from my TC427 driver? Do I need a driver that can allow for more current? And is the diode going to see more current than 1 A? Thanks
     
  13. cade007

    Thread Starter Member

    Apr 14, 2006
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    There is a pn juction built into the MOSFET which are used as the frewwheeling diodes. How much current will these diodes see?
     
  14. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    Don't rely on the internal diodes, put external ones and then see if the spikes are still there.
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Cade007,

    Could you post your corrected schematic?

    Typical circuits are included in the datasheets, so if you haven't viewed the IR2111 datasheet from IRF, just click the link.

    Looking forward to seeing your revisied schematic.
     
  16. cade007

    Thread Starter Member

    Apr 14, 2006
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    https://webfiles.colorado.edu/penney/www/Doc1.html.mht

    here is my revised schematic. I actually discovered today that the problem was the probe. I was probing the high side gates and attaching the probe ground to the ground of the power stage, which when the input saw higher voltages the probe disrupted the gate driver signals. I tried turning up the voltage without the probes on the gate drives and it was fine, so the spikes are not a problem anymore.

    The problem I am having now is the noise on the output. Also it is very hard to measure the output waveform (across the output cap and resistor). I take a probe on each output them I add them together, but the output is hard to see since there are other noise issues.

    Another question: is there a way to allow my circuit not to buck as much as it is? My problem is actually in the PWM chip, since I made this circuit a buck converter using a 200 V input and a 120rms output. This required the duty cycle to vary from 7.5% to 92.5%, but my PWM can only go up to about 80%. Therefore it can only go down to 20% (in order to avoid a DC offset at the output). Could I add a cap somewhere or mess with my switching frequency? I want to avoid getting a different PWM chip since I need to be done with this very soon.

    Here is what I am seeing on the output with a 200 V input:

    https://webfiles.colorado.edu/penney/www/FF...he%20output.mht

    I did a FFT on the output and the largest frequency is at 60 Hz, but there are also higher harmonics that I would like to get rid of. That output voltage is not entirely correct as you can see it is not bucking down to the correct voltage. The duty cycle is not entirely correct from the PWM so I need to adjust that which should help with the output. But I cannot get the PWM chip up to 97% duty cycle so is there some way around this?


    The other thing I forgot on my schematic again is there is a resistor from the gate to source of every MOSFET of about 1.2kohm
     
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