driving a power mosfet with an op-amp

Thread Starter

robert.guttormson

Joined Oct 3, 2009
26
I am trying to charge a battery when I detect it is discharged. For this I can use a DC controlled DC relay or I can use a power mosfet that will switch on based on a signal from a GPIO(I need to realize SPST not anything fancier). Here I have a doubt regarding the gate driver to be used for the mosfet. The mosfet I use is irfiz48n. Frequency of relay toggle is 20-30Hz


My question is can I use an opamp (one of the inputs from the microcontroller and the other at 1.5v and positive source connected to 15v) to drive the gate of the MOSFET? Or do i need an elaborate gate driver.
 

tom66

Joined May 9, 2009
2,591
Well, what is your supply voltage? Most MOSFETs require 10V to conduct properly, but you can buy logic level MOSFETs which require 4V or sometimes even less to conduct. Since you're operating at a low frequency then you don't need to worry too much about gate charge current specifications on any driver.
 

Thread Starter

robert.guttormson

Joined Oct 3, 2009
26
the irfiz48n needs 10v(max Vgs = 20) to operate, and i have a power rail that can deliver 15v so i plan to provide that as positive supply to the opamp. On a side note why cant opamps be used to drive mosfets in general. While they cant provide isolation by opto means they sure can drive the mosfet until the switching frequency is 10 or 20 KHz. Is my understanding correct ?
 

tom66

Joined May 9, 2009
2,591
the irfiz48n needs 10v(max Vgs = 20) to operate, and i have a power rail that can deliver 15v so i plan to provide that as positive supply to the opamp. On a side note why cant opamps be used to drive mosfets in general. While they cant provide isolation by opto means they sure can drive the mosfet until the switching frequency is 10 or 20 KHz. Is my understanding correct ?
They can drive a MOSFET, they are just poor choices because there are much cheaper ways to accomplish the task. For example, a single NPN transistor and two resistors, as below, will replace an entire op-amp:



The output is inverted though: a logic high input turns the MOSFET off and a logic low input turns the MOSFET on.

Click this link to simulate the circuit.
 

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Thread Starter

robert.guttormson

Joined Oct 3, 2009
26
Thanks Tom, I have a couple of LT opamps in hand and I was able to drive a mosfet(not the irf one mentioned above) on LTspice using these opamps, but since i am getting started I thought i will ask around and find out if this works in real world
 

SgtWookie

Joined Jul 17, 2007
22,201
Robert,
Please attach your LTSpice .asc file so that we can see what you're attempting.

Trying to charge a battery from a fixed 20v source without proper voltage monitoring and current limiting will likely result in disaster.

89nC is a rather large gate charge. While 30cps is pretty slow, you could still wind up with a large amount of power dissipation. The IRLU8721 is a physically smaller MOSFET with a lower Vdss rating, but it is logic level with less than 1/10th the gate charge and a lower Rds(on). It could be driven directly by your GPIO with only a resistor in between to suppress gate "ringing".
 
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