gate resistors on these mosfets, there are two types, 10 ohm and 4.7k

Discussion in 'General Electronics Chat' started by sdowney717, Apr 22, 2015.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    The 4.7k ohm smd resistors, many are significantly out of tolerance. Some do read 4700 ohms, but many read 2300 ohms.

    These 4700 ohm resistors are draining - grounding the gates, I assume to make sure the mosfets turn off?

    Question is should I replace them or leave them alone?
    for example R84 in this pic is one of these 4.7k, all the top row are the 4.7k
    The 10 ohm resistors turn the mosfet on and are driven by the mosfet driver.
    [​IMG]
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    You cannot measure a resistor in circuit and expect to get accurate readings. Leave them in, or take them out, is up to you if you think you know what you are doing. How can you expect us to express an opinion by giving us a picture? Do you think it conveys any meaningful information? We don't know what the application is and we don't know what kind of parts we are talking about. Where is the SCHEMATIC diagram?
     
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  3. dl324

    Distinguished Member

    Mar 30, 2015
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    Need a much more detailed description to give a meaningful answer. MOSFETs come in P and N and then there are depletion and enhancement mode. A schematic would be easier to follow...
     
  4. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    I decided to replace them. One end is out of circuit as the n channel mosfets are removed from the board
     
  5. kyu9971

    Member

    Mar 28, 2015
    57
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    In realtà, le foto senza schema e 'non molto utile.
    si tratta di è un inverter?
     
  6. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Please post in English. This is a English only forum. Did you mean to say something like "In fact , the photos without schema and ' not very useful . is is an inverter ?". The Google translation is not very clear.
     
  7. kyu9971

    Member

    Mar 28, 2015
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    i am sorry For my reply in italian language.

    I just wanted to say that without diagram is not clear what it is this.
    maybe is it an inverter dc/ ac?
     
  8. GentleRV

    New Member

    Dec 18, 2010
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    Short remark. Gate resistors are between 1 Ohm and 100 Ohm never higher.
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You will see low value resitors between the driver and the gate of the mosfet.
    The high value resistors are connected between the gate and source of the mosfet in order to discharge the gate capacitance more quickly.
    This will enhance the turn off speed of the mosfet.

    Bertus
     
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  10. Roderick Young

    Member

    Feb 22, 2015
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    The 4.7k may have no function in the circuit, other than to insure that the transistor is off at startup, when the driver might be floating. That's a good enough reason to have something there, though. I guess what I'm saying is that the value is not so critical, and if it burned but is still a resistor, it would probably still work.

    In my small experience with switching supplies, I've never seen a pull-down resistor on the gate of a mosfet used to enhance turn-off speed. We do want the transistor to turn off as quickly as possible in most cases, but that is done by driving the other side of the 10 ohm resistor to ground, or possibly something like -10 volts.

    BUT, good move replacing them anyway. When a MOSFET burns, invariably I've seen drain and gate fused together to some extent. The gate becomes no longer insulated, and whatever voltage was on the drain can appear on the gate. If there was 160 volts on the drain, it suddenly appears on the gate, which is what cooks the 4.7k resistor. Note that it likely burned out the driver on the other side of the 10 ohm resistor, too, so check that.
     
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  11. sdowney717

    Thread Starter Member

    Jul 18, 2012
    395
    13
    Yes, the mosfets drivers seem to always burn out on this inverter when the mosfets burn out.

    I found an easy way to test them. Simply use a really small 3 amp hour battery from a small lawnmower. Momentarily (maybe a half second to a second) power up the inverter with it all apart. It will error with a red light and out of 16 input board mosfets, several mosfets will get hot fast. it only takes a few seconds to find out is not working,

    I did replace the 4.7k. The 10 ohm I replaced with metallic resistors.
    I moved all these gate resistors to the other side of the board. The layout and fit now is better than it was.

    For the mosfet drivers, they are an 8 pin dip chip. so this time I ordered 3 amp gold plate dip sockets round pin.
    I plan to put a little grease on the DIP pins and then they can be more easily changed vs soldered directly to the board.
    http://www.digikey.com/product-search/en?KeyWords=ED90032-ND&WT.z_header=search_go

    I am still waiting on the 10 dip chip order for the RX3152 mosfet drivers.
    These dual non inverting mosfet driver I found a working description of here
    http://www.egr.msu.edu/classes/ece480/capstone/fall14/group08/Documents/How to Implement a MOSFET with a Gate Driver-1.pdf

    I thought also about going to a T0247 packaged 200 amp mosfet instead of these IRF3205 100 amp mosfets, but the price is way higher, although I wonder if they would survive the surge better?
    http://www.irf.com/product-info/datasheets/data/irfp3077pbf.pdf
    They have a significantly higher input capacitance, which you would expect more current is needed to turn them on. The mosfet drivers are 1.6 amp rated.
    Question is would doubling from 100 to 200 amp ratings prevent the inverter from frying itself on a large inductive load? And why is the inverter frying itself on the heat pump motor , why is it not shutting itself off? Does the heat pump motor create a back fe

    I put a relay to lockout the 16k btu cruisair heat pump since both times I tried to run it, it sizzled the inverter. So if inverter turns on, it energizes the coil of a 4PDT relay. The relay as the arm swings open circuits the heat pump and closes the inverter circuit.
     
    Last edited: May 9, 2015
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The amp rating might help but I think you were suffering from a MOSFET with too much gate capacitance.

    - the high gate capacitance caused stress on the driver as it is only driving a 10 ohm load (until the gate is charged) but still, many times and you may have stress.

    - the high gate capacitance was causing the MOSFET to rise slowly and overheat.

    At least that is my theory based on the limited information given.

    Solution, select a MOSFET and driver that will perform at the rated amperage AT THE DESIGNED FREQUENCY the of the gate, (not the frequency of the 60Hz output).
     
    Last edited: May 9, 2015
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