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Does anyone know how to increase voltage from optocoupler?

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nirza

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Hi!
How to increase the voltage from optocoupler 4n35? i had connected the input pin of the optocoupler to the output pin of the microcontroller. the other pin is connected to the groud. for the other side of the optocoupler, for emitter i connect it to ground. collector is to the vcc and to the mosfet gate.

however, the the voltage input to the mosfet seems to be a constant value of 12V. i thought when i already isolate it from the microcontroller, the pwm signal will follow. but it seems that no pwm signal into the mosfet.

can anyone give suggestion or modification for my circuit?? :-:)-:)-(

Untitled.png
 

Hi,

the gate voltage is 12V because you connected the gate to 12V.

I assume the collector signal of the optocoupler output is correct.

--> correct your wiring.

Klaus
 

Thank you KlausST. But how should i make the signal into the mosfet gate similar to the one i generated from the microcontroller? i thought the optocoupler had transferred the pwm signal, isnt it? hmm
 

With the gate-source short you have made in your schematic, the opto coupler will surely not drive the gate.

This might be just a drawing error, you need however a low impedance push-pull driver to control the gate, a general purpose opto coupler isn't suited for this purpose. A dedicated opto isolated gate driver like TLP250 has been suggested in your previous thread.

The shown circuit doesn't involve isolation, both sides of the coupler are referenced to the same ground. A simple NPN transistor could replace 4N35 in this circuit. With two more transistors, you get a push-pull driver.

Some ideas discussed in this previous thread https://www.edaboard.com/showthread.php?t=325976, I know there are much more on Edaboard
 

Hi,

yes, the optocoupler most probably will have created the correct signal. (--> use the scope to make it visible)

Then you just need to connect the Mosfet_Gate to this signal (gate --> OptoC_collector) instead of connecting the gate to 12V.

****
I assume you additionally need to use the correct MOSFET with the correct connection:
P-CH MOSFET
* D to 12V
* G to collector of optocoupler
* S to load.

****Some improvements:

You used high side switching. This is more difficult than low side switching. I wonder why.
***
You used an optocoupler. This is a good device for galvanically isolate the input from the output.
But you connected input as well as output to the same GND. I wonder why you choose an optocoupler.
A simple NPN could do the same job, usually with improvements in cost, speed and gain.

****
Klaus

added:
..FvM was faster...
 

FvM said:
With the gate-source short you have made in your schematic, the opto coupler will surely not drive the gate.

This might be just a drawing error, you need however a low impedance push-pull driver to control the gate, a general purpose opto coupler isn't suited for this purpose. A dedicated opto isolated gate driver like TLP250 has been suggested in your previous thread.

The shown circuit doesn't involve isolation, both sides of the coupler are referenced to the same ground. A simple NPN transistor could replace 4N35 in this circuit. With two more transistors, you get a push-pull driver.

Some ideas discussed in this previous thread https://www.edaboard.com/showthread.php?t=325976, I know there are much more on Edaboard
Click to expand...


Thank you FvM. Yes, in the previous thread someone had suggested TLP250 for my project but i couldnt find any at my place. Because of the very short time, i tried another alternatives suggested by my lecturers and friends such as bootstrap capacitor circuit, push pull circuit but it seems that none of them are working. And they told me that i need an isolated gate driver which is the optocoupler. For the circuit without isolation, I guess it might be because of my connection is wrong. I will try again from your suggestion. Thank you very much. :thumbsup:
 

KlausST said:
Hi,

yes, the optocoupler most probably will have created the correct signal. (--> use the scope to make it visible)

Then you just need to connect the Mosfet_Gate to this signal (gate --> OptoC_collector) instead of connecting the gate to 12V.

****
I assume you additionally need to use the correct MOSFET with the correct connection:
P-CH MOSFET
* D to 12V
* G to collector of optocoupler
* S to load.

****Some improvements:

You used high side switching. This is more difficult than low side switching. I wonder why.
***
You used an optocoupler. This is a good device for galvanically isolate the input from the output.
But you connected input as well as output to the same GND. I wonder why you choose an optocoupler.
A simple NPN could do the same job, usually with improvements in cost, speed and gain.

****
Klaus

added:
..FvM was faster...
Click to expand...

What do you mean that high side switching is more difficult? So, I should change to the low side configuration instead of high side? Can you explain in detail how the simple npn can help me?

I'm very sorryyyyyyyy for asking toooooo muchhhhh :cry: :cry:
 

i do not know if my circuit is correct or not. but the voltage at the gate can have the highest pulse value at 7V only which is not satisfy the Vgs,th = 10V. My lecturer told me that i need 15V to drive the gate. Please help me.. :bang::bang::cry::cry:
 

schmitt trigger said:
Are you attempting an inverting topology?
Click to expand...

the project is about the inverting buck boost converter. means that the output voltage polarity is different from input voltage. just a simple buck boost converter basic topology actually. the problem is that when i use an optocoupler, the 5V output from microcontroller is isolated from the gate driver circuit but, the generated pwm signal is not transferred to other part of the optocoupler. i have no idea on how to make the voltage into the gate of the mosfet becomes greater than 12V with the desired pwm signal.
 

nirza said:
i have no idea on how to make the voltage into the gate of the mosfet becomes greater than 12V with the desired pwm signal.
Click to expand...
Did you follow post #5 and recommended thread in post #4 ?
 

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