I tested the following circuit .... The problem is it doesn't switch off it remains on ON state .....wat is the problem

 

Because you have an N-channel MOSFET is a circuit that needs a P-Channel MOSFET.

 

hi CD.
Welcome to AAC.
Have you considered the inbuilt diode.?
E

 

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.

 

This is the circuit am using

 

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.

 

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?

 

Hi,
There is an internal diode in the MOSFET.
E

 

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?

Am using a 50KHz signal ....by the way it is a buck converter

 

This is the circuit am using

Yes, I saw that. Here's the problem:


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.

 

Hi,
There is an internal diode in the MOSFET.
E

Had first connected the Mosfet in that way (vice versing the drain and source terminals) but the voltage at the source will be 0V so will be the one at the gate so it will remain at off state

 

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

 

Am using a 50KHz signal ....by the way it is a buck converter

Isn't 50kHz a bit of a challenge for a PC817?

 

Isn't 50kHz a bit of a challenge for a PC817?

I think it can handle that

 

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.

 

I think it can handle that

Rise time for 1k load = 20us. Fall time for 1k load = 20us. That's already 40us. You're much more optimistic than I am!

 

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.