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help needed in selecting bias resistor for power mosfet

Hi all, I want to develop an miswiring protection ckt for my regulator that was used inside my application board.Please go through the below CKT.

(MTP23p06VDS) S D -------------------------->Mosfet-------------- O/P | | | G | | R1= ? | -------^^^^^^---------------| | | | | | ^ 24vdc ^ | R2=? ^ | ^ | | | | GND -----------------------------|------------------->

The mosfet(MTP23p06VDS) that was used in the above ckt has the following specifications.

Drain?to?Source Voltage--------------------- VDSS 60 Vdc Drain?to?Gate Voltage (RGS = 1.0 MW)-------- VDGR 60 Vdc Gate?to?Source Voltage ? Continuous ----------± 15 Vdc ? Non?repetitive-------± 25 Vpk

Drain Current ? Continuous @ 25°C ---------23A

The output from mosfet drain and the GND are going to my regulator input pins.My question is here ,how can i calculate here the resistor values R1 and R2.Please Give me your valuable suggestions.

With Regards, Bhanu Prakash.

Reply to
banu
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  • What miswiring are you attempting to protect against?
  • How is this circuit supposed to do it?
  • I'm lazy: is that a P-channel or an N-channel MOSFET?
  • What's the source of the 24V?
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply to
Tim Wescott

If your intent is to limit the O/P current, this really won't work. Drain current varies radically with gate voltage, temperature, and between parts, so the short-circuit current won't be predictable. For a one-off, you might adjust Vg with a pot, but that's still nasty.

The best thing might be to put a small resistor in the source driving a comparator. When the voltage drop across the resistor exceeds some target value, turn off the fet.

I confess that I once made a heated microphone, using a PNP power transistor as the heater, and limited heater current with a selected base resistor, thus committing the ultimate sin of beta-based design.

John

Reply to
John Larkin

What is a 'miswiring protection circuit' ?

Dream on !

Graham

Reply to
Pooh Bear

That's a p-channel FET, and the circuit will work properly, acting as a low-resistance passthrough for the correct input polarity and as an open circuit for the wrong polarity. Provided, as you say, the gate voltage is correctly applied. Given that your 24V source may sag, and it certainly isn't 24V during startup, it's preferable to employ a zener in the gate-drive circuit, IMHO.

We prefer to think of power and current flow from left to right, source to load, like this.

.. P-channel POLARITY-PROTECTION SWITCH .. mosfet .. + (O)------- D S ---+---+---> + The FET comes on early, .. G _|_ | shorting out the substrate .. 24V power | /_ 22k diode that otherwise .. source |______|___| carries the current in .. | 12V to the desired direction. .. 3.3k zener LOAD .. | .. - (O)----------+--------------> -

The 22k resistor insures 0V on the gate during power off.

Normally a p-type FET conducts in the other direction, but this circuit works because of the principle that a power MOSFET can carry current in either direction when it's turned on.

--
 Thanks,
    - Win
Reply to
Winfield Hill

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