Problems with DC/DC converters in isolated H-Bridge design

Discussion in 'General Electronics Chat' started by stube40, Apr 1, 2010.

  1. stube40

    Thread Starter Member

    Feb 3, 2010
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    I am pulling my hair out since I keep destroying the Murata NMC1215SC DC/DC converters in my CPU controlled H-Bridge design. There is no obvious signs of damage such as heat or smell, they simply stop outputting +/- 15V. Testing out of circuit against a reference device shows they truly are dead.

    The H-bridge design uses the 4x Murata device to provide an 4x isolated power rails for the 4x Fairchild IGBTs and the Vishay VO3120 opto-isolated MOSFET drivers.

    I have attached a pictures that shows the general idea of the schematic, as well as a detailed schematic with every component.

    The design is for switching a 150V / 40A high-voltage power source, but so far I've only been bench testing with 12V into a 27 Ohm resistor.

    I suppose I must be damaging the Murata's though inadequate protection, although so far I haven't been able to catch any suspicious spikes on the oscilloscope. At $20 a pop though from Farnell, these parts aint cheap and it's proving to be an expensive detective project.

    My only other thought is that I'm over-loading the output of the Muratas which are only rated to 1W.

    I'm beginning to run out of ideas - can anyone suggest something to try?
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
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    I have two suggestions.

    1. Watch out for the four 220uH inductor in series with the DC/DC output VOH supply line. I know you want to "filter" the output but the configuration easily become a "boost" SMPS configuration and high voltage could be generated via the inductor when the VO3120 switches. Try short circuited the four inductors and see if the problem go away.

    2. Try out similar cheaper priced DC/DC converters until you have debugged the circuit.
     
  3. stube40

    Thread Starter Member

    Feb 3, 2010
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    Great advice, especially the first tip. Thanks!
     
  4. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Is it any chance that you have upper and lower transistors (of the same side) conducting at the same time even for a short period?
     
  5. stube40

    Thread Starter Member

    Feb 3, 2010
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    In general, when switching the state of the 4x IGBTs the firmware of the CPU turns off all 4x IGBTs first of all then in the next CPU instruction cycle turns on the 2x IGBTs required with a single operation.

    Hence, in theory the answer to your question is no, it's not possible that the upper and lower IGBTs on the same side can ever be on at the same time because the firmware in the CPU prevents it. However, that is in theory - in practice it may be that the CPU is faster than the gate turnoff time and hence maybe the delay between turning off the 4x IGBTs and then turning on the 2x IGBTs is too small, meaning that the 2x IGBTs on the same side ARE on at the same time. I will put a scope on this to try and find out.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I'm no expert but, Like when you use a ir2110 driver you need a diode between the Vcc and the capacitor on the high side switch. If not when the high side conducts it puts the emitter voltage back into the Vcc supply. And in this/your case blow the DC-DC converter. The capacitor from pin 5 to pin 8 is to keep pin 8 floating on the emitter voltage.
     
  7. stube40

    Thread Starter Member

    Feb 3, 2010
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    I have modified the schematic to add a diode (JPG attached). Is this what you mean?
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    As far as your modification- No D1 should go to the other side of C22(+ side) and it should be the cathode to the capacitor.

    Like I said in the first post I'm not a expert, but a h-bridge is what brought me to this forum and I figured out a lot since then.

    Some questions for you,
    1. What are you switching? Motor?

    2. Why are you using IGBT for 150V 40A? Mosfets are cheaper and faster to turn off and on and and at that voltage there is no need for IGBT.

    3. Do you know that the driver you chose is only good to 32V max? With your 150V + 15V = 165V they will be the next to let out smoke!

    4. Why the inductor and zeners in the drive circuits? Never scene that in any of the circuits.

    Probably why your blowing the DC-DC converters is that on U11 the 0V pin is tied to the Vee pin on U10. When U12 starts to turn on there is emitter voltage going into a pin that should be a ground(0V)

    If you were to change your drivers (V03120), to using IR2110 you could go from 4 ICs to using 2 ICs and eliminate the NMA1215 ICs also. And the IR2110 chip is rated to 600V on the gate volts. The IR2110 will also work to switch the IGBT's if they're still good.

    This is by far the most elaborate H-bridge I've seen.

    Not trying to tell you what to do, just what I see here. I'll help if you want?
     
  9. stube40

    Thread Starter Member

    Feb 3, 2010
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    I am switching an inductive load of 45mH

    Cost is not the predominant factor, hence IGBTs seemed like the most robust and simple solution.

    The 32V max is not a problem because each driver is floating using the galvanically isolated Murata NMA1215SC DC/DC converter, which has 0V referenced to the IGBT emitter.

    The inductor L4 and capacitor C22 combination is specifically recommended in the NMA1215SC data sheet to reduce output ripple current of the DC/DC convertor. The Zeners D7 and D8 between the IGBT gate/emitter are a commonly used way of protecting the device. The TranZorb D9 and Zener D10 were suggested by another electronics expert who thought they would offer added protection.

    Thanks again for your advice, if you agree or disagree with anything I am saying then please let me know. I am certainly not an expert!!
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I put your quotes in ( ) and my Q/A after.


    (I am switching an inductive load of 45mH)--- Then why a H-bridge, wouldn't a simple IGBT switch do? And to make it simpler use a low side switch.

    (The 32V max is not a problem because each driver is floating using the galvanically isolated Murata NMA1215SC DC/DC converter, which has 0V referenced to the IGBT emitter) ----- But in a high side switch/driver, the gate voltage HAS to be higher, than the emitter on voltage. So in your case it will be 165V in the working circuit. A Mosfet will turn on hard at 12V(above the source voltage), which you already have available in your circuit. That eliminates the need for the NMA1215SC.


    (The Zeners D7 and D8 between the IGBT gate/emitter are a commonly used way of protecting the device)---- Usually Zeners aren't used for protection and NOT between the gate/emitter. There is NO electrical connection internally between the gate and emitter in a IGBT, it is just like a capacitor, it uses a electrical field to turn on the IGBT.

    The protection Diodes are used between the COLLECTOR/EMITTER on a IGBT. And then they would be a diode, not a Zener.


    (Cost is not the predominant factor, hence IGBTs seemed like the most robust and simple solution.)---- While at first glance it might seem so, there are some things on IGBTs that are bad in some circuits, something called tail current.


    Would you elaborate more on the project your doing? It would help to understand the big picture of this. The way I was always told/taught about things was, simpler is better.
     
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