Power dissipation in a Darlington built with an optocoupler and NPN

Thread Starter

Salus Valverde Forcadell

Joined Feb 28, 2017
12
--SOLVED--
I have implemented a PWM control over a relay from an Arduino. To isolate both circuits I have used an optocoupler 4N35 from Vishay. Since it cannot provide enough current flow to drive the relay, I have used a BC548B transistor creating a Darlington structure with the internal output transistor of the 4N35.


The problem is that I have simulated the circuit but then, when I try to calculate the currents in the circuit I am unable to achieve the simulated values. I don't know how to achieve the \(V_CE(OK3)\) neither \(I_F\).

\(I_{K1} = \frac{+12V_{PS2}-V_{CE(Q4)}}{K1}\)

\(I_{K1}=\frac{I_{C(Q4)}}{\beta _{Q4}}\frac{\beta _{OK3}}{(\beta _{OK3}+1)}+I_{C(Q4)}\)

\(V_{CE(Q4)}=V_{CE(OK3)}+V_{BE(Q4)}\)

\(P_{OK3}=I_FV_D+I_{C(OK3)}V_{CE(OK3)}\)

\(P_{Q4}=I_{C(Q4)}V_{CE(Q4)}\)

Even if I've checked the 4N35 datasheet, I am unable to calculate the values for my circuit. The \(V_F\) for the input diode is only expressed by a fixed current of 10 mA. Then, I am unable to know what will be the \(V_CE(OK3)\) and therefore I cannot achieve the \(V_CE(Q4)\). Summarizing, I have nothing.

I hope you can guide me.

Salus

Moderators note: removed the plains from the post
 
Last edited:

Thread Starter

Salus Valverde Forcadell

Joined Feb 28, 2017
12
Do you need an exact expression for the power dissipated, or just an engineering approximation?
It depends on how approximated would be the expression ;-) The problem is for example, that I cannot determine the I_F because it depends on the R7 and also on the V_D. Well, I know the value of the R7 but how can I know the current through it? In the same way, I don't know how to determine the V_CE(OK3), if I achieve these parameters, then the power dissipation will be easy to calculate.
 

dl324

Joined Mar 30, 2015
10,712
Reformatted tex commands so they can be read...
The problem is that I have simulated the circuit but then, when I try to calculate the currents in the circuit I am unable to achieve the simulated values. I don't know how to achieve the \( \small V_CE(OK3)\)
neither
\( \small I_F\).

\( \small I_{K1} = \frac{+12V_{PS2}-V_{CE(Q4)}}{K1}
I_{K1}=\frac{I_{C(Q4)}}{\beta _{Q4}}\frac{\beta _{OK3}}{(\beta _{OK3}+1)}+I_{C(Q4)}
V_{CE(Q4)}=V_{CE(OK3)}+V_{BE(Q4)}
P_{OK3}=I_FV_D+I_{C(OK3)}V_{CE(OK3)}
P_{Q4}=I_{C(Q4)}V_{CE(Q4)}\)

Even if I've checked the 4N35 datasheet, I am unable to calculate the values for my circuit. The \( \small V_F\) for the input diode is only expressed by a fixed current of 10 mA. Then, I am unable to know what will be the \( \small V_CE(OK3)\) and therefore I cannot achieve the \( \small V_CE(Q4)\). Summarizing, I have nothing.
 
Last edited:

dl324

Joined Mar 30, 2015
10,712
Because I have to justify the power dissipated by this circuit.
Given the voltage driving the opto emitter, you can calculate dissipation in the current limit resistor and the emitter. Knowing the CTR of the optocoupler, you can estimate collector currents in the transistors.

You can decrease dissipation in the external transistor if you connect the optocoupler transistor as a split darlington instead of darlington.
 

Thread Starter

Salus Valverde Forcadell

Joined Feb 28, 2017
12
Given the voltage driving the opto emitter, you can calculate dissipation in the current limit resistor and the emitter. Knowing the CTR of the optocoupler, you can estimate collector currents in the transistors.

You can decrease dissipation in the external transistor if you connect the optocoupler transistor as a split darlington instead of darlington.
But the voltage on the Diode depends on the current and vice versa. How can I determine both?
By the way, is it normal that the page doesn't allow me to edit the first post?
 

dl324

Joined Mar 30, 2015
10,712
But the voltage on the Diode depends on the current and vice versa. How can I determine both?
Iterative calculations, and associated circuit changes, are sometimes required. When you're designing the circuit, you use an approximate voltage for the LED and select a resistor. If you require a more precise current, you modify the current limit resistor value.
By the way, is it normal that the page doesn't allow me to edit the first post?
I don't know. I've only started a few posts and didn't need to make changes.
 

Thread Starter

Salus Valverde Forcadell

Joined Feb 28, 2017
12
Iterative calculations, and associated circuit changes, are sometimes required. When you're designing the circuit, you use an approximate voltage for the LED and select a resistor. If you require a more precise current, you modify the current limit resistor value.
I don't know. I've only started a few posts and didn't need to make changes.
Thanks a milion!
 

Thread Starter

Salus Valverde Forcadell

Joined Feb 28, 2017
12
Forward voltage and current transfer characteristics for 4N35:
View attachment 121517

View attachment 121518
The second graph is not aplicable in my case, I think. There is no 10V as \(V_{CE}\).
I know in my case the 4N35 doesn't allow the Q4 to saturate at 0.2V which is a normal value. I expected the 4N35 to saturate at 0'2V but in the LTspice simulation it doesn't take this values.
 
Top