Changing MOSFET makes circuit stop working

Discussion in 'General Electronics Chat' started by gumattos, Jan 16, 2017.

  1. gumattos

    Thread Starter New Member

    Jan 16, 2017
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    Hello everyone, I am facing a very interesting problem here. I already worked with MOSFETs before, but never seen something like that. I have a circuit using TL494 oscillating at ~80KHz and it works fine, I am driving the MOSFET normally, and it is a IRF9540. However, if I replace the IRF9540 for a IRFR5305, the output goes to Vcc and not the regulated output, any ideas? I am using a SB360 schottky diode with an input of 24V (the same for 48V input). Anybody have any idea why the circuit works with one MOSFET and not the other?

    The original circuit is 48V and it works great with the IRF9540 but the same problem happens with the IRFR5305.

    I couldn't find the model for TL494 on TINA so I just build this schematic for representation.

    I tested the IRFR5305 with the multimeter and it seems to be working correctly, I have 5 of them, all new, one of them I haven't used yet in case I am doing something very wrong so I have a brand new for testing, the other four have the same problem. Sorry for the bad english.

    tina forum.PNG
     
    Last edited by a moderator: Jan 16, 2017
  2. wayneh

    Expert

    Sep 9, 2010
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    Check the data sheets. I'm guessing there is a difference in the threshold voltage such that the second MOSFET never shuts off.
     
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Having reviewed the datasheets, I see no obvious indication of incompatibility...
    http://www.vishay.com/docs/91078/91078.pdf
    http://www.infineon.com/dgdl/irfr5305pbf.pdf?fileId=5546d462533600a401535632522820ff

    So...
    To begin - please verify the signal at the gate of the IRFR5305, if as expected please check the signal at the cathode of SD2 (C3 disconnected)...

    Best regards and good luck!
    HP:)
     
    Last edited: Jan 16, 2017
  4. gumattos

    Thread Starter New Member

    Jan 16, 2017
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    Hello, thank you for the reply, I will try to access a oscilloscope and take a look at the waveforms. Another thing, as the SB360 is 60V and the MOSFET is 55V, maybe the the spikes created by the indutor could be affecting the MOSFET, what do you think?
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    When T2 turns on there is the full input voltage of 24 V across gate to source on a device only rated for 20V.

    Exceeding Vgs is so critical you don't even want to come close since it instantly destroys the device.
     
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  6. crutschow

    Expert

    Mar 14, 2008
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    As ErnieM noted, your circuit exceeds the Vgs 20V maximum rating of the MOSFETs.
    That's a no-no.
     
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  7. gumattos

    Thread Starter New Member

    Jan 16, 2017
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    Oh, my bad, the schematic is wrong, the real circuit is 100 Ohm and 220 Ohm, so the divider sets 48V and 33V sorry.
     
  8. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    I missed that:oops: -- @gumattos Thus it seems the '9540 was 'cutting you a break' whereas the '5305 merely 'broke':( --- FWIW, Having first corrected the circuit, it may be interesting to try another '5305 so as to 'clinch' the diagnosis, as it were - FWIW it may be possible to detect a punctured gate via a DMM resistance reading - otherwise static bias analysis will readily reveal such damage...

    Best regards
    HP:)
     
  9. Hypatia's Protege

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    If I'm 'following you', each 'T2-on' state is still grounding the MOSFET's gate? (and, hence, imposing 24V (V2) across the the latter's G-S "junction")

    Best regards
    HP
     
    Last edited: Jan 16, 2017
  10. gumattos

    Thread Starter New Member

    Jan 16, 2017
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    My intention in this circuit is the gate oscillating between 48V and 33V, like this simulation and circuit, with I used a BD137 as the transistor on the TL494 and the oscillator as the square wave generator, and the result is what I wanted, and it's inside the 'safe zone' I guess.

    Capturar.PNG

    2.PNG

    The green (yellow?) wave is the voltage on the gate and the red is the oscillator that turns the BD137 that is the internal transistor on TL494 on and off.
     
  11. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Allright - Let's 'walk through' this...

    1) The Source of the MOSFET is connected to the positive rail of V2
    2) The Gate of the MOSFET is connected to the emitter of T2
    3) the collector of T2 is connected to the negative rail of V2
    4) When T2 enters (or approaches) saturation, V2 (48V) is placed across the MOSFET's G-S "junction"
    5) The MOSFET dies!

    But then there must be 'a hole in there somewhere' inasmuch as the IRF9540 would not survive either:confused::confused::confused:

    Best regards
    HP:)
     
    Last edited: Jan 21, 2017
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  12. Alec_t

    Expert

    Sep 17, 2013
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    Do you have a pull-up to 24V on the TL494 collectors?
     
  13. Hypatia's Protege

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    Indeed! -- A complete schematic would be most helpful! @gumattos I understand that it is your assertion that the gate driver transistors are operating far from saturation - howbeit, as shown, such is neither guaranteed nor likely...

    Best regards
    HP
     
  14. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    At least it won't be a slow and painful death... but rather quick, painless and humane :confused:
     
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  15. gumattos

    Thread Starter New Member

    Jan 16, 2017
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    Here is the complete circuit

    o.PNG
     
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  16. Hypatia's Protege

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    Thanks! -- Please perform the initial troubleshooting procedure outlined below - then report your results to this thread:)

    Please note
    - In the interests of perspicuity and eschewal of an all too common source of error/misapprehension fairly plaguing discussion of positive channel/junction device operation, I have elected to represent quantities merely as 'magnitudes' in the following -- sans introduction of ambiguity! Should you find such 'treatment' condescending or otherwise 'irksome' please merely 'read-over' same with my apologies!:oops:

    1) Please remove the MOSFET from the circuit then observe the signal at the emitters of Q1/Q2 with attention to the peak 'excursion' from the positive rail of V1

    2) Should the magnitude of said excursion fall within operational limits (i.e. well below 20V yet greater than 8V) please install a known good IRFR5305 then have another 'look' at said signal (it's barely possible the gate capacitance is producing unexpected operation).

    Note Re: step 2 (above): Gate-Source signal magnitudes in excess of 20V will damage the gate insulation whereas Gate-Source magnitudes much less than 10V will likely prove insufficient to drive the device into saturation...

    3) Should the gate signal remain within limits following installation of the IRFR5305 - please observe the signal at the MOSFET's Drain...

    Best regards and good luck!
    HP:)
     
  17. Alec_t

    Expert

    Sep 17, 2013
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    The circuit in post #15 shows a TL493 and a 48V supply. Are you now using a TL494?
    You apparently (see below re R12/13) have no pull-up resistor from the C1 and C2 outputs of the '494 to either the 48V rail or to Vcc (33V?), so Q1 cannot turn on. However, the rated maximum voltage for the C1 and C2 pins is 41V so a 48V rail would kill them. Failure to switch Q1 on results in the FET being permanently on.
    Are R12/R13 connected together or not (the pic isn't clear). If not, there is nothing to pull the base of Q2 down.
    Vgs is excessive (as HP has noted).
     
    Last edited: Jan 17, 2017
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  18. Hypatia's Protege

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    FWIW a 'connection dot' is perceptible when the image is viewed 'full size' -- That had me 'goin' for a moment as well:eek:

    Best regards
    HP:cool:
     
    Last edited: Jan 21, 2017
  19. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I would also suggest redrawing it, and placing the positive rail on top, and ground at the bottom. And the components placed in such way that currents could be followed vertically, and not sideways. That would make the diagram far easier to read.
     
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  20. Alec_t

    Expert

    Sep 17, 2013
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    Simulation shows that breakdown of the output transistors in the '494 at around 41V would prevent the FET from turning off if it has a low Vgs(thr).
     
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