Mosfet gates

Discussion in 'Electronics Resources' started by bytraper, Dec 20, 2011.

  1. bytraper

    Thread Starter Active Member

    Sep 28, 2010
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    Hi Guys,

    I've noticed a lot that in a lot of instances, when a mosfet blows up or dies, it manages to die with a dead short between the gate and source.

    Of course this all goes back to the driver and manages to blow that up too which then blows a whole lot of different things on the control side of things.

    I tried using zeners, but I've found that zeners will also mostly die shorted.

    Can anyone think of a way to protect the the driver from the dead mosfet?
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    If there are a LOT of blown MOSFETs then there is something wrong with the layout or the voltages/currents do not meet the MOSFET's specs.

    It's difficult to protect the driver itself, however isolated gate drivers exist and even though the may blow they protect the whole rest of the control logic.
     
  3. bytraper

    Thread Starter Active Member

    Sep 28, 2010
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    Its a generic motor controller, it can be used for a variety of different applications. But sometimes they get overloaded because of incorrect motor ratings, or sometimes a user shorts the outputs or really, any one of a hundred different scenarios.

    When I repair them though, as I mentioned in the first post I've noticed most go dead short between gate to source which then blows the driver ic, the regulator and other associated ic's.

    Im trying to find a way that when this happens, something on the way to the gate blows up with an open short which will save the driver and isolate the dead fet.
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    In this case it would be best to include an overcurrent or short circuit protection.

    Does it have an inductor at the output or are the MOSFETs directly connected to the load? If there is no inductor it will be difficult to have a short circuit protection though.
    Can you post the schematic of the output section of this product?

    Isolated gate drivers are one of the best solutions but they require another power supply .
     
  5. bytraper

    Thread Starter Active Member

    Sep 28, 2010
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    I have over current sensors on a more advanced model, this is just a basic model using the least amount of parts for the smallest footprint.

    The mosfets are connected directly to the load. This is one leg of the bridge.

    [​IMG]

    The diodes are SD10A and the resistors are 150 Ohm
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Running 3 MOSFETs in parallel to handle the load?

    What is the part number of the MOSFETs?

    What is the load? (Voltage, Current, type)

    Do all 3 MOSFETs blow, or just one of them?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    I would be very tempted to lower the power rating of those 22 Ohm resistors and the diodes. They'll act like cheap fuses then.
     
  8. bytraper

    Thread Starter Active Member

    Sep 28, 2010
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    That picture just shows 1 leg of the H bridge. The voltage could be 12v-36v and the current can be up to 100a on the bridge. The fets are IRF1405's.

    If one fet blows, The the short goes through to the other gates on that leg of the bridge, which kills them, and creates a shoot through condition which takes out the other legs of the bridge as well, so 12 fets dead. Then the driver tries to source more current which burns out the driver, then when the driver dies, the current draw is massive and blows the 5A regulator to smithereens. This makes repairs expensive and time consuming.

    Its a pretty colossal failure state.

    Sarge that's a really good idea, really low power resistors would blow open circuit!

    Do you think a "0201" size smd resistor (0.05w) would be suitable for driving the gates? What would you recommend as a suitable diode?
     
    Last edited: Dec 21, 2011
  9. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Is this a finished product or one that is still in development?
    How is the layout? Are the MOSFETs sharing equally the total current? The To-220 has a 75A rating (package limit). 33A for each of them is quite a lot. Are they mounted on an appropriate heatsink?
    I find the TO-220 package quite cumbersome to work with especially when they have to be isolated from the heatsink. (The mounting hole is not in the middle, as for other packages too, which causes unequal pressure and can easily cut the isolator.)

    Yes, lower power ratings for the resistors could help, but that's not a safe way to protect you from all possible faults. IMHO the device should be made so it can handle short circuits and overloads without getting destroyed.

    That means one would need to make a list of failures that could occur and then find a solution for each of them.

    If you want to go the gate resistor "protection way" then I'd recommend to build a test fixture where you can try out different power ratings and resistance values, A test fixture with easily accessible MOSFETs, since you will blow up some of them while testing.
     
  10. bytraper

    Thread Starter Active Member

    Sep 28, 2010
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    Both, its an ongoing development.

    When it's not overloaded or abused its a good reliable controller. It's only when it's overloaded or shorted, there's problems. The drain off each of the mosfets is pretty equal (within about 300mA) so there's no big problems there. Heat sink is the case, the case is 2kg of aluminium fins.

    "I find the TO-220 package quite cumbersome to work with especially when they have to be isolated from the heatsink."

    Amen to that.

    I found this on 4QD's website, which stacks up with what sarge said.

    "When MOSFETs fail they often go short-circuit drain to gate. This can put the drain voltage back onto the gate where of course if feeds (via the gate resistors) into the drive circuitry, maybe blowing that. It also will get to any other paralleled MOSFET gates, blowing them. So - if the MOSFETs are deceased, check the drivers as well! This is probably the best reason for adding a source-gate zener: zeners fail short circuit and a properly connected zener can limit the damage in a failure! 4QD also use sub-miniature gate resistors - which tend to fail open circuit under this overload, disconnecting the dud MOSFET's gate."
     
    Last edited: Dec 21, 2011
  11. SgtWookie

    Expert

    Jul 17, 2007
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    I mis-wrote 22 Ohms; I meant 150 Ohms starting with R22.

    I don't know what you are using to drive the IRF1405's. I don't know how fast you're driving them, nor what your load should be.
     
  12. bytraper

    Thread Starter Active Member

    Sep 28, 2010
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    Sarge,
    The output is identical to the OSMC output.
    Driver is a HIP4081A, Frequency 25khz.
     
  13. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Assuming 260nC worst case total gate charge for the MOSFET, with 3 in parallel and a 150R gate resistor the total gate charge power dissipation would be almost 300mW, since 150R is quite big most of it is dissipated in the gate resistor.

    Source: http://www.ti.com/lit/ml/slup169/slup169.pdf page 8.

    The gate charge depends on Vds and Vgs. What are their values?
     
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