MOSFET High Voltage driver (High Side) problem

Discussion in 'The Projects Forum' started by technogamma, Nov 3, 2008.

  1. technogamma

    Thread Starter New Member

    Oct 22, 2008
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    Hello everyone,

    I'm having strange problem with the design of a high voltage MOSFET driver. I'm using 4 N-channel MOSFETs in H-bridge setup, so the two high side FETs are controlled with a bootstrapping (please have a look at the attached circuit of the high side driver).

    The problem is as I increase the Vin (the voltage switched by the FET) the driver stops working. At 80V of Vin it's all working allright and having 200V makes the gate signal blurry on the high side (I believe the FET can't really be turned on) by unknown reason. I'm testing only the high side FET, using a 10K capacitor connected from its source to ground.

    Some details on the parts I'm using:
    - FET - IRF740
    - Bootstrap capacitor - 0.1 uF
    - Vcc - 15V
    - Control signal - 8kHz 50% duty

    All help and tips are highly appreciated.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    See the resistor right below Dbst?

    As the voltage on Cbst has to go higher and higher in order to switch the gate of the MOSFET, the current through that resistor also increases. After a certain point, the current through the resistor bleeds off too much of the voltage from Cbst, so the MOSFET gate isn't getting turned fully on.

    What's the high-side MOSFET driver IC that you're using? Or is it merely a PWM controller?
     
  3. technogamma

    Thread Starter New Member

    Oct 22, 2008
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    Thanks for the tip sergeant. Unfortunately that's not the problem as I tried increasing the bootstrap capacitor before posting the thread and I tried it now (values up to 5K) but no luck at all.

    Actually I'm not using an IC driver as I had some trouble with them in the past so currently I'm using circuit with discrete elements as shown on the picture. The control signal is transferred via optocoupler.

    If you have any more ideas in the stack they are more than welcome.
    Thanks.
     
  4. technogamma

    Thread Starter New Member

    Oct 22, 2008
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    Can anyone tell me what's the idea behind the usage of Qls transistor in the circuit I've attached - not controlling its gate but its source?

    Thanks.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I edited your schematic to add reference designators (R1, C1, etc).

    It would help us help you if you can provide the values or part numbers where appropriate. We will then be able to simulate the circuit so that we can figure out what is going wrong.
     
  6. technogamma

    Thread Starter New Member

    Oct 22, 2008
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    Here is a detailed circuit of all the components used (this is a modified version of the original base circuit because I had various problems with the one I used as a guide). Please have in mind that the generator is actually an optocoupler.

    Thanks for the effort.
     
  7. di11igaff

    New Member

    Nov 20, 2008
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    To answer your question about driving the FET from the source rather than the gate you need to research cascode amplifiers. The main advantages in this case are negating the miller capacitance of the FET and allowing for greater isolation from the drive circuit. If you look up cascode in Wikipedia there is a fairly good explanation of the cascode amplifier.
    If your looking for a simple solution to this problem I'd suggest looking into the IRF2110 family of high low side drivers, but be forewarned. The reason I found this website is because I'm looking for a replacement for that part. IR recently modified the die and change the part number to an IRFS part and it's no longer compatible. We're running into premature failure problems that we're researching now for a solution.
     
    Last edited: Nov 20, 2008
  8. feline694

    New Member

    May 19, 2009
    1
    0
    Hi all

    I am facing the same problem. While using IR2110 I've blown one highside FET. Now I am trying to use discrete Gate drive, again face the problem like it works fine at low voltage (80V) but above that the Fuse blown (as i put a fuse of 2A to save the fet to be blown).

    Need a solution.
    Transformer drive may solve this problem?

    BR/fel
     
  9. jr235711

    New Member

    Oct 8, 2009
    1
    0
    di11igaf - did you find the solution to your problem with the IR2110 part? I am looking at using this part for a Vin > 100V.

    feline694 - did you find a descreet solution to this high voltage gate drive?
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    technogamma ... where did you get the original diagram?
     
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