Turning on and off the irfz44n Mosfet on high side

jpanhalt

Joined Jan 18, 2008
11,088
The problem is the diode between S and D. You have the source attached to the Vcc rail. If you simply reverse that connection, then it will not turn on with that circuit.

With that mosfet,you need to connect the + side of load load to solar, the drain to the negative-side of load, and the source to ground.
 

DickCappels

Joined Aug 21, 2008
7,424
The gate voltage needs to go almost all the way up to the voltage to which the source is connected. As shown not there will be nearly 20 volts holding the gate on.

One solution is to use an NPN transistor to allow the signal to switch up to the 24V supply. Otherwise you might need to use a gate driver chip to switch the gate over the correct voltage. Be careful -your MOSFET might be damaged with 23 volts of gate drive.
 

Ian0

Joined Aug 7, 2020
2,178
A normal high-side driver isn't going to work unless it is switching at more than a few tens of Hz.
I presume that this circuit doesn't switch very often, so would need one with a separate charge pump.

How about switching the negative side?
 

jpanhalt

Joined Jan 18, 2008
11,088
This is the circuit am using
Yes, I saw that. Here's the problem:
1609501754748.png

There is a diode between "S" and "D" that is forward biased. It will not turn off, regardless of the gate voltage. If you reverse those connections, then the mosfet will be off and can be turned on with a gate voltage about 8V to 10V GREATER than the solar voltage.
 

Thread Starter

CarltonDranko

Joined Dec 30, 2020
19
Yes, I saw that. Here's the problem:
View attachment 226459

There is a diode between "S" and "D" that is forward biased. It will not turn off, regardless of the gate voltage. If you reverse those connections, then the mosfet will be off and can be turned on with a gate voltage about 8V to 10V GREATER than the solar voltage.
Meaning that if solar voltage is 20V then gate voltage should be around (28-30).....Sir please explain am not understanding what you saying
 

jpanhalt

Joined Jan 18, 2008
11,088
Here Are two links that discuss N-channel mosfets (N-mosfet):

https://toshiba.semicon-storage.com/info/docget.jsp?did=59460
https://www.infineon.com/dgdl/an-936.pdf?fileId=5546d462533600a40153559e997e1180

In brief, an N-mosfet needs its source at ground or needs a high-side driver. If you use a P-mosfet, as recommended earlier, then you do not need the high side driver.

Why is that so? The voltage drop across a mosfet that is "on" is usually very low. Thus, when the N-mosfet in your circuit (if connected properly) turns on, the voltage at the source will be the same as the voltage at the drain. The gate voltage needs to be about 10V more than that to keep it on.

As for putting the mosfet in backwards, I posted a schematic to show that problem. Doing that is the same as replacing the mosfet with a diode, except for the small diode drop that doesn't exist when it is turned on. It will still require about 10V to turn on completely.
 

DickCappels

Joined Aug 21, 2008
7,424
If you are going to switch the positive supply the MOSFET really needs to be a p-channel unless you transformer couple the drive or make a power supply that you can float your gate drive using an isolated driver.
 
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