Mosfet design questions

Discussion in 'The Projects Forum' started by Jorj, Oct 2, 2015.

  1. Jorj

    Thread Starter New Member

    Sep 30, 2015
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    Hey folks I have quite a few questions about Mosfet transistor design. I've put all in one schematic, frankenstein style.

    The Mosfet maximum voltage Vgs is +20V/-10V. Recommended Vgs is +15V/-5V.
    Diodes D1 and D2 should protect the Mosfet from higher than the recommended, still lower than the maximum ratings. (Is this okay)?

    R1 should help to reduce "ringing" effect, Since the gate driver is split (High output / Low output) there's no reverse diode connected in parallel with R1.
    (Is this okay)?

    Since I want to drive with positive and negative voltage, +15V and -5V are used to power the gate driver IC (UCC27532)

    R2 is placed for faster discharge, reducing "Miller" effect (Is this okay)?

    R3 + R4 - I don't know what they are for. I've seen this in one design (HV flyback Auxiliary power supply - CREE power device reference design).
    Maybe is for filtering (Is this okay)?

    C1, C2, C3 - not sure if they do a good job.. or not needed (Is this okay)?

    D3, D4 - protection diodes (flyback). Not sure if I need the both, just D3 or D4. Is the polarity correct? What type and what voltage they should be - right now there are 900V (same rating as the mosfet - schottky diode) (Is this okay)?

    I would appreciate your criticism and comments.
     
  2. Jorj

    Thread Starter New Member

    Sep 30, 2015
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  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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    OK

    Yes, it is ok, but I would tie both driver outputs together then the 10 ohm so it reduces ringing in both directions.

    ok

    While it may help turn off a little I think it's main purpose is to keep the FET from turning on if the driver is not connected.

    I see no use for them in this circuit.

    They should go to ground, but if you remove the 2 resistors they will. :D

    I don't think D3 is needed.
    Since the power supply voltage is only 12 volts there is no need for the FET or the diode to be rated for say 2X that voltage. The current depends on the maximum current expected in the inductor.
     
  4. Jorj

    Thread Starter New Member

    Sep 30, 2015
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    Sounds great, I've changed the schematic according to your suggestions.
    The current peak is expected to be around 2A-2.5A, peak voltage will be 900V because of the protection diode.
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    There is very little difference between a schottky and a zener diode symbol. Are you confusing the two?
     
  6. Jorj

    Thread Starter New Member

    Sep 30, 2015
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    Yes you are right, D1 and D2 are zener diodes (on the schematic there they are drawn with shottky symbol).

    Btw R3 + R4 I think they provide RC tank and limit the current that is used to charge the capacitors C2 + C3 with lower constant current (range 100-500mA), without them the capacitors current ripple will be higher, something like 1-2 amps
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    R3 + R4 are for current sensing, which you apparently don´t need in your design.
    Anyway, what is this circuit supposed to do? I don´t really see any obvious function in the way the output is connected.
    What is R5 for? Why D4?
     
  8. Jorj

    Thread Starter New Member

    Sep 30, 2015
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    It's for PI metal detector. R5 is voltage damping resistor. D4 is not needed btw.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Ahh, a little different then.
    You could use D$ as a zener then, but you would need to turn it around. However; If the FET is avalanche rated you can just let it clamp the inductive kick.
    R3 and R4 could be used for current sense if you use it - only you know that. But if you do the big decoupling caps need to go to ground.
    Is your coil really only 500 uh.?
     
  10. Jorj

    Thread Starter New Member

    Sep 30, 2015
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    The transistor isn't avalache rated but I can pick another one that is, in order to simplify the layout and probably be cheaper if we consider extra board space, additional heatsinks, etc.
    I found very useful this article (Figure 17. Implementation of Fast-Recovery Diode for Power MOSFET)
    http://www.allaboutcircuits.com/technical-articles/a-review-on-power-semiconductor-devices/

    Yes the coil is 450-550uH, it is typical number for such devices (usually start at 300uH up to 800uH).
     
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