optocoupler with mosfet

[mshh]

I used optocoupler pc817 to drive mosfet using pwm generated by Microcontroller but the output on the optocoupler wasn't PWM it was continuous on the mosfet gate and i couldn't control it I don't know why , could you help please ?
https://obrazki.elektroda.pl/5987439400_1447873098.jpg

 

[jamespond]

Hi! The IRF need more higher gate voltage in this scheme than +24v. Or need rearrange the loads and the FET in the circuit. I think this circuit will work if the FET source will be on the gnd and the load between the drain and +24v. And need adjust the voltage of pulse on the gate.

 

[treez]

Looks like you try to pass too much current through opto...also, what is frequency, optos cant do high frequency.

 

[mshh]

Looks like you try to pass too much current through opto...also, what is frequency, optos cant do high frequency.

I don't need much current in the opto. the collector current is around 24 mA safe. the frequency was 500 Hz when displayed on oscilloscope however in proteus simulation it is 5Khz> in datasheet of the opto it has cuttoff frequency 80 Khz

 

[treez]

ok what is voltage across gate source when you pulse the opto on?...because it looks to me you have load in the source connection of mosfet...so it is putting voltage there, which is meaning your gate-source voltage is not sufficient to turn on your fet...because of the volts across that stuff in the source connection

 

[mshh]

voltage on gate was around 20 volt continuous and there was a current passing in the load and the max current was 0.4 amp no more .

 

[schmitt trigger]

You have to read the datasheet very carefully. It says:

Cut-off frequency fc at VCE= 5V, IC= 2mA, RL= 100Ω, - 3dB, 80Khz.

This means that that cutoff frequency is ONLY ACHIEVABLE if the coupler is operated in linear mode, and with a very small collector current. Definitively you are not operating that way.

There are also other issues. The first is attempting to use a Mosfet as a source follower without taking into account that as the gate voltage rises, so does the source voltage rises, which restricts the ability to enhance the gate. Do you really require a high side drive? If so, then use either a P-channel or a proper N-channel high side driver.

The last one is that you do not mention what is the microcontroller's supply voltage. If you are operating it at 3.3 volts, the current supplied to drive the optocoupler would be (3.3 - 1.2)/210 = 10 mA, which may not be sufficient if the optocoupler's CTR is in the low side.

 

[mshh]

it doesn't matter for me to use the mosfet as high side or low side i just need it to switch.
the microcontroller's supply is 5 volt