# Bootstrap instead of any driver for mosfet

#### rabiuls

Joined Jan 18, 2012
8
Hello,
I want to make a buck converter for lead acid battery charging. I want to use IRF540 N Channel mosfet. And I don't want to use any driver IC to drive the mosfet gate. I shall send pwm signal from 5v microcontroller. Can I use the following bootstrap circuit (found on net) instead of using any driver IC for my purpose. Or is there any better method to drive the mosfet without a mosfet driver IC? PAD1 of the circuit would be connected to microcontroller's PWM pin.

Best regards.

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#### wayneh

Joined Sep 9, 2010
17,189
The answer depends on the frequency of operation. Which is?

At high frequency, supplying and removing the gate charge adds up to a significant current. At "low" frequency, it's too small to matter.

#### rabiuls

Joined Jan 18, 2012
8
The frequency can be 100kHz to 500kHz. Which frequency will work best for the circuit?

Thanks.

#### wayneh

Joined Sep 9, 2010
17,189
This is out of my range, but I believe the capacitance of the IRF540 is 2nF, or 20nC at 10V. Moving that charge in and out at 500kHz is a current of 10mC/s = 10mA (or is it double that, the sum of in+out?).

That doesn't sound so bad. Maybe inductance increases the problem.

Anyway, we'd better see if someone with more experience can help you here.

#### crutschow

Joined Mar 14, 2008
31,091
This is out of my range, but I believe the capacitance of the IRF540 is 2nF, or 20nC at 10V. Moving that charge in and out at 500kHz is a current of 10mC/s = 10mA (or is it double that, the sum of in+out?).

That doesn't sound so bad. Maybe inductance increases the problem.
It's a little worse than that.

Per the data sheet the IRF540 has a total gate switching charge of 71nC. At 500kHz the switch period is 2μs. Assuming you want a switch rise and fall time of no more than 10% of that or 0.2μs gives a switching current of 71nC/0.2μs = 355mA. That certainly requires a much lower resistor value than the 1kΩ pull-up shown in the schematic. A push-pull or totem-pole type driver would likely be required.

#12

#### wayneh

Joined Sep 9, 2010
17,189
I knew 500kHz wouldn't be that easy. Thanks for setting us right.

#### SgtWookie

Joined Jul 17, 2007
22,227
50kHz is about as fast as you'd want to run that. You might could go somewhat faster depending on the value of the inductor that you would use with it.

You're showing +12v for your supply, which won't be adequate for charging a 12v lead-acid battery. You would need ~13.7v just to float-maintain it (standby mode). If you're charging it after being partially drained, you should charge it at a constant current until you hit ~14v to ~14.5v before dropping back to the "float" value.

#### rabiuls

Joined Jan 18, 2012
8
Hello all,
By reading the replies I am not confident with the circuit. Can I use a NPN transistor like TIP-122 instead of mosfet? Since it does not have special gate voltage requirement like N Channel mosfet.

#### Tim Woolaver

Joined May 15, 2015
2
Just though you would like to know

I built this and it works and I am using it in a product design for a 30A PWM controlled voltage source.

Values are different by it is the same schematically.

And I am a real live electrical engineer who designs circuits for a living.

Thanks

#### ebeowulf17

Joined Aug 12, 2014
3,307
Just though you would like to know

I built this and it works and I am using it in a product design for a 30A PWM controlled voltage source.

Values are different by it is the same schematically.

And I am a real live electrical engineer who designs circuits for a living.

Thanks
At what frequency?

#### Tim Woolaver

Joined May 15, 2015
2
100KHz pwm driven from a microcontroller

#### crutschow

Joined Mar 14, 2008
31,091
.................
I built this and it works and I am using it in a product design for a 30A PWM controlled voltage source.

Values are different by it is the same schematically.
.........................
So what are these circuit "different values"?

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#### crutschow

Joined Mar 14, 2008
31,091
The frequency can be 100kHz to 500kHz. Which frequency will work best for the circuit?
Why such a high frequency?

You cannot use that circuit to charge a battery without some limiting impedance in series with the battery (resistance or inductance).