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N-Channel Mosfet switch

Discussion in 'General Electronics Discussion' started by Lord_grezington, Sep 27, 2017.

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  1. Lord_grezington

    Lord_grezington

    124
    2
    May 3, 2013
    Hello

    I have this circuit built into a PCB, and its a little strange. The fact is it works some times and not others and I am unsure of why...

    Power_Button+ & Power_Button- to go a simple low power switch that is designed to switch the power to the rest of the circuit. And it does work (sometimes). The mosfet is a CSD19503KCS with a Vth of 2.8V. When I take a scope to the gate pin it says 0V even when I flick the switch on or off. But, the power is still on current is flowing through the circuit @ 0V on gate!! I did not think there were any min values for the drain/source voltage but maybe I am wrong... It has currently decided not to work so I cant get the voltages when its working OK.

    D2 is a 45V TVS

    Thanks

    upload_2017-9-27_14-38-58.png
     
  2. Alec_t

    Alec_t

    2,891
    775
    Jul 7, 2015
    What is the battery voltage?
    Why are R4/5 such high values and why use both?
    There may not be enough current through the switch to 'wet' the contacts.
     
  3. Lord_grezington

    Lord_grezington

    124
    2
    May 3, 2013
    Battery voltage is Nominal 36V however as its lithium Ion it goes up to around 42.6V i think.
    R4 & R5 I wanted to keep fairly high to reduce the charge time of the gate capacitance. I did not want the Mosfet to charge too quickly. They are two 100K's because I already used loads of 100K's in the design and no 200K's.
    I could probably try and switch the 100K for 10K's to increase the current. But hte issue is not the Mosfet switching on, its the Mosfet not turning off even though the gate Voltage is @ 0V.
     
  4. Alec_t

    Alec_t

    2,891
    775
    Jul 7, 2015
    Where did you have the meter probes? The FET source is not grounded.
     
  5. Lord_grezington

    Lord_grezington

    124
    2
    May 3, 2013
    Morning...

    In this case the probe GND clip is on the Bat-

    I have made a spice simulation and that seems to work fine (as long as there is enough load to drain the capacitors - not shown in the schematic). But I did the same simulation before designing the PCB as well.

    I need to try and diagnose the PCB as there must be a hardware problem somewhere... Thanks
     
  6. Lord_grezington

    Lord_grezington

    124
    2
    May 3, 2013
    Resolved my own problem... when I measured the resistance between Power_Button- and Bat- I was expecting 100K (R14) but instead i was seeing just 140R. There is a dead short between mosfet D & S. My mosfet is somehow blown...

    Spice Model show that on power up the capacitors surge at a peak of 248A for around a ns. This has to be the cause of the failure - to resolve this I am adding a 10u cap in parallel to R14 to slow the gate voltage rise time. Spice says with a 10u Cap peak current only spikes to 1.8A which is more than acceptable and the circuit Voltage rise time extends to around 40ms.

    Thanks
     
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