Spice Modelling for Opto Isolators

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fastbike

Joined Dec 29, 2020
85
I have a design that requires a PWM input to be isolated from the output. The input is driven from a mcu (3.3V), the output has a 15V DC power supply and requires the PWM to be 4V.
I have modelled this in LT Spice with a basic 4N25 and can get the concept to work OK at 1kHz. I require resolution of 40 discrete values for the PWM so 6 bit accuracy is fine. Resistors R2/R3 drop the supply voltage down to the required 4V output.
1667072860345.png
Now the 4N25 is a pretty basic low speed part which won't work as I'm looking to get the PWM frequency up to a minimum of 100kHz.
At my required resolution of 40 discrete values this would required a turn on/turn off time of 250nS.


I've looked at parts with active outputs to get the speed up, the constraint being the 15V DC power supply (most active devices run to ~6V). I've found two parts that run to 30V supply:
- Vishay VOM453, which has an open collector output so could be plugged into my circuit. The speed though is a bit slow with turn off and turn on running to 1,000 nS.
- Broadcom ACPL-M484, which has a push/pull totem pole output. I think this will require an additional N channel driver for my P channel output but has faster turn on/turn off times (~200nS)

I'm having trouble finding LT Spice compatible models for these two parts.
How would I modify an existing model ?
 

Ian0

Joined Aug 7, 2020
6,677
Use a H11L1 which has a built-in Schmitt trigger. It's good enough for data up to 250 kbaud.
It will operate at 16V, so all you need to buffer it is a CMOS gate such as a CD4050.
 
Last edited:

Papabravo

Joined Feb 24, 2006
19,570
I have a design that requires a PWM input to be isolated from the output. The input is driven from a mcu (3.3V), the output has a 15V DC power supply and requires the PWM to be 4V.
I have modelled this in LT Spice with a basic 4N25 and can get the concept to work OK at 1kHz. I require resolution of 40 discrete values for the PWM so 6 bit accuracy is fine. Resistors R2/R3 drop the supply voltage down to the required 4V output.
View attachment 279551
Now the 4N25 is a pretty basic low speed part which won't work as I'm looking to get the PWM frequency up to a minimum of 100kHz.
At my required resolution of 40 discrete values this would required a turn on/turn off time of 250nS.


I've looked at parts with active outputs to get the speed up, the constraint being the 15V DC power supply (most active devices run to ~6V). I've found two parts that run to 30V supply:
- Vishay VOM453, which has an open collector output so could be plugged into my circuit. The speed though is a bit slow with turn off and turn on running to 1,000 nS.
- Broadcom ACPL-M484, which has a push/pull totem pole output. I think this will require an additional N channel driver for my P channel output but has faster turn on/turn off times (~200nS)

I'm having trouble finding LT Spice compatible models for these two parts.
How would I modify an existing model ?
The subcircuit for the 4N25 is just a voltage dependent current source. (a "G" device), with a variable gain to set the current transfer ratio and a generic NPN transistor and a diode. It should be possible to adapt that subcircuit to your needs.

* Copyright © Linear Technology Corp. 1998, 1999, 2000. All rights reserved.
*
.subckt 4N25 1 2 3 4 5
R1 N003 2 2
D1 1 N003 LD
G1 3 5 N003 2 .876m
C1 1 2 18p
Q1 3 5 4 [4] NP
.model LD D(Is=1e-20 Cjo=18p)
.model NP NPN(Bf=610 Vaf=140 Ikf=15m Rc=1 Cjc=19p Cje=7p Cjs=7p C2=1e-15)
.ends 4N25
 

eetech00

Joined Jun 8, 2013
3,418
I have a design that requires a PWM input to be isolated from the output. The input is driven from a mcu (3.3V), the output has a 15V DC power supply and requires the PWM to be 4V.
I have modelled this in LT Spice with a basic 4N25 and can get the concept to work OK at 1kHz. I require resolution of 40 discrete values for the PWM so 6 bit accuracy is fine. Resistors R2/R3 drop the supply voltage down to the required 4V output.
View attachment 279551
Now the 4N25 is a pretty basic low speed part which won't work as I'm looking to get the PWM frequency up to a minimum of 100kHz.
At my required resolution of 40 discrete values this would required a turn on/turn off time of 250nS.


I've looked at parts with active outputs to get the speed up, the constraint being the 15V DC power supply (most active devices run to ~6V). I've found two parts that run to 30V supply:
- Vishay VOM453, which has an open collector output so could be plugged into my circuit. The speed though is a bit slow with turn off and turn on running to 1,000 nS.
- Broadcom ACPL-M484, which has a push/pull totem pole output. I think this will require an additional N channel driver for my P channel output but has faster turn on/turn off times (~200nS)

I'm having trouble finding LT Spice compatible models for these two parts.
How would I modify an existing model ?
Model files and symbols for LTspice
 

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