# Optocoupler for high frequency

Joined Sep 28, 2021
266

I got this PS2501 optocoupler datasheet at here. In the image shown, the rise time and fall time is microseconds and I am afraid it's not suited for my needs as it will produce some switching losses in H-bridge circuit running at 100kHz minimum.

Are there any very-high speed optocoupler suited running at 300kHz with very little switching loss in the modern marker nowadays?

And, regarding the graph of switching time vs load resistance, it worries me because when RL is large, the switching time will increase a lot and it would mean that if i attach a mosfet as RL as a switch (Resistance of GS very high), the switching time will be extremely large which makes my H-bridge suffer from switching losses ?

#### LowQCab

Joined Nov 6, 2012
4,075
The first thing You need to do is to make a comprehensive statement regarding
exactly what it is that your project will accomplish.

Anything running at ~300khz is going to be very tricky.
Do You have to have a Frequency that high ?
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#### Papabravo

Joined Feb 24, 2006
21,227
HCPL-7710

#### crutschow

Joined Mar 14, 2008
34,466
it worries me because when RL is large, the switching time will increase a lot and it would mean that if i attach a mosfet as RL as a switch (Resistance of GS very high), the switching time will be extremely large which makes my H-bridge suffer from switching losses ?
No, since you would add a load resistor of a low value to provide the switching voltage to the gate.

#### MisterBill2

Joined Jan 23, 2018
18,590
300KHz is only medium speed for an opto, there are many that are much faster.

#### ronsimpson

Joined Oct 7, 2019
3,047
Look at "digital isolators" like HCPL-7723.

Joined Sep 28, 2021
266
No, since you would add a load resistor of a low value to provide the switching voltage to the gate.
Do you mean attaching the RL=100Ohm parallel to the GS junction of mosfet?

#### crutschow

Joined Mar 14, 2008
34,466
Do you mean attaching the RL=100Ohm parallel to the GS junction of mosfet?
No.
Post a circuit diagram of what circuit configuration you are referring to and I will try to explain further.

Joined Sep 28, 2021
266
No.
Post a circuit diagram of what circuit configuration you are referring to and I will try to explain further.
Something like this in datasheet. Can you label in the picture where the mosfet should be placed? Should be between Vout and Ground right?

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#### MisterBill2

Joined Jan 23, 2018
18,590
I suggest looking at the applications notes for the oppto device, because those always show how to obtain the published performance of the device.

Joined Sep 28, 2021
266
I suggest looking at the applications notes for the oppto device, because those always show how to obtain the published performance of the device.
Hi, from the diagram, lets say my pulse input (square wave) is 9V or 0V, and my Vcc is 4V, what should the resistor values be on the diode and RL? I tried this schematic on a low frequency square wave which works, but when it becomes high frequency(100kHz) , the output at collector of optocoupler gives me 0V (no output). Anyone can provide some clues whether this optocoupler is rated at such high frequency to produce good square wave signal at collector output?

Thank you and have a great new year

#### MisterBill2

Joined Jan 23, 2018
18,590
It sort of sounds like a power availability issue more than a opto-coupler problem. If the LED power source is not able to provide all the required current at the square wave frequency then the LED output will lag, and at some frequency not deliver enough light for the photo transistor.So if you can put the scope input across the opto LED that will tell if that is the problem

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Joined Sep 28, 2021
266
op
I sort of sounds like a power availability issue more than a opto-coupler problem. If the LED power source is not able to provide all the required current at the square wave frequency then the LED output will lag, and at some frequency not deliver enough light for the photo transistor.So if you can put the scope input across the opto LED that will tell if that is the problem
optoled has good square wave on my scope, output (transistor collector ) has problem

#### MisterBill2

Joined Jan 23, 2018
18,590
op

optoled has good square wave on my scope, output (transistor collector ) has problem
That was a possibility, and if that is not the problem, then it may have to do with the operating point. Spec sheets and the test conditions to get that claimed performance are subject to the limitations that are not always spelled out. In this case, a specific voltage, current, and load resistance are given. So with different conditions the results may not be as good.
It might be that you need a faster device.