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Simple and low cost MOSFET gate driver.

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LAN_403

Storken

Mon Nov 18 2013, 12:16AM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
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Posts: 48

Hello. I'm looking at driving a N-MOSFET for load connect/disconnect (as a relay). This is going into a battery monitoring system.

I'm looking at a NMOS inverter as a possible solution. I want the application to be "no power usage in idle", the NMOS inverter is "no power usage in active".

I want to control the gate with a 3V3 signal, so a PMOS inverter is out of the question. Gate voltage out is ~ 9V to 13V.

I know I can use additional discretes to make this circuit do what I want it to, but I want to keep the total parts count to a minimum. If there is an easy way to do this, please help me out :)

Billybobjoe

Mon Nov 18 2013, 01:11AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

What are you trying to switch on/off? For most loads, you can just replace the resistor in your diagram with your load.

Ben Solon

Mon Nov 18 2013, 02:17AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

yes, the picture you have linked is a logic inverter(or buffer with a swap of devices). In order to use the mosfet as a load switch it has to be in series with it. Then when the mosfet is active the load is active.

Storken

Mon Nov 18 2013, 06:07AM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
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Posts: 48

Billybobjoe wrote ...

What are you trying to switch on/off? For most loads, you can just replace the resistor in your diagram with your load.

I'm trying to switch on/off a "huge" N-MOSFET, this is a safety disconnect from too deep discharge - for the battery.

Ben Solon wrote ...

yes, the picture you have linked is a logic inverter(or buffer with a swap of devices). In order to use the mosfet as a load switch it has to be in series with it. Then when the mosfet is active the load is active.

The load is not resistive, it is a MOSFET gate. This load is capacitive, and active high. This is fine. But I'm looking to replace the logic inverter with something - so that power consumption during "cutoff" is nonexistent.

I can do this with some extra npn's and resistors, to make the PMOS inverter work - but is there a more elegant solution?

Dr. Slack

Mon Nov 18 2013, 07:44AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Why are you using an NMOS inverter? A CMOS inverter would be 'no power either active or inactive'.

If you're using NMOS because you have to, then increasing the value of the pullup resistor will decrease your power consumption, at the cost of taking longer to charge the gate capacitance.

Storken

Mon Nov 18 2013, 10:18AM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
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Posts: 48

My 3V3 levels isn't enough for CMOS logic. Additional components are needed.

I'm open to different implementations, as I definitely see the shortcomings of the NMOS inverter. Currently tho; the NMOS is the only one that can give a relatively high voltage out, for a small control voltage in.

Dr. Dark Current

Mon Nov 18 2013, 02:55PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

What about two transistors, NPN and PNP, cascaded in common emitter configuration? The PNP will have a pull down resistor on its output.
If the input will be 0 volts, the output will be 0 volts as well, with zero current draw (well maybe some microamps of leakage current)

Storken

Mon Nov 18 2013, 07:44PM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48

Dr. Dark Current wrote ...

What about two transistors, NPN and PNP, cascaded in common emitter configuration? The PNP will have a pull down resistor on its output.
If the input will be 0 volts, the output will be 0 volts as well, with zero current draw (well maybe some microamps of leakage current)

I think you've solved my problems. This seems to do exactly what I want to do. To make sure we are on the same page: A bipolar totem pole MOSFET driver is suitable for this application.

This is the schematic, to the right.
PushPull 5v

I will test this driver soon.

PS: Informative page about MOSFET drivers

First time I go for discretes, I've used IC MOSFET drivers a lot :P

Thanks :)

Shrad

Mon Nov 18 2013, 09:15PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

there are existing n-mos switch devices...

Dr. Dark Current

Mon Nov 18 2013, 09:18PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

This is what I meant:

If you want a high current turn-OFF capability, you can add one PNP transistor and a diode on the output. Tell me if you're not sure how.

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