# SHUNT resistances value

1. ## SHUNT resistances value

Hi,

I'm trying to create a Driver circuit for a laser diode stack (see attached diagram)

To adjust the power of the laser stack, on pin 3 of the operational IC3, using a voltage ranging from 0 to 4V, I adjust the maximum current that the driver can supply.

With the R1 trimmer I carry out the calibration.

+ Vo1 and + Vo2 = + 12Vdc
-Vo1 and -Vo2 = -12Vdc
On V + and V- there are 24Vdc
On LD + and LD- the laser diode stack is connected, it can have a maximum absorption of 60 amps for a few milliseconds e.g. 30ms.

If you kindly give me some advice on the value of the SHUNT resistances (R25, R24, R23, R22) to use ...

is 0.1 ohm 10Watt ok?

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2. ## Re: SHUNT resistances value

I'm not going to analyze your entire circuit, but consider this: the resistors will have an instantaneous power of 360W. 30mS only provides half the information; what's the duty cycle? THAT will tell you the average power the resistors have to dissipate. You also need to verify that the chosen resistors can handle 60 AMP peaks.

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3. ## Re: SHUNT resistances value

You have 15A per branch or 22.5 W per 0.1 ohm resistor. Sounds conservative.

I'm not aware of any laser diode application with ms pulse width, thus I don't want comment the application and circuit in general.

How did you arrive at the current control loop design? Did you check its stability? Also some other circuit details like capacitor C9 look dubious, may be anything is working by chance.

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4. ## Re: SHUNT resistances value

Originally Posted by FvM
You have 15A per branch or 22.5 W per 0.1 ohm resistor. Sounds conservative.

I'm not aware of any laser diode application with ms pulse width, thus I don't want comment the application and circuit in general.

How did you arrive at the current control loop design? Did you check its stability? Also some other circuit details like capacitor C9 look dubious, may be anything is working by chance.
Ah, right, I didn't notice that these were 4 paralleled MOSFETs (figured it was an H-Bridge). I told you I didn't analyze the circuit.

5. ## Re: SHUNT resistances value

Originally Posted by FvM
You have 15A per branch or 22.5 W per 0.1 ohm resistor. Sounds conservative.

I'm not aware of any laser diode application with ms pulse width, thus I don't want comment the application and circuit in general.

How did you arrive at the current control loop design? Did you check its stability? Also some other circuit details like capacitor C9 look dubious, may be anything is working by chance.
Thanks for your answer...
then what do you think of the following reasoning:

1. The voltage drop across the shunt resistor must be approximately 1% of the supply voltage, therefore:

Vs = 1% of V + = 1% of 24 V = 0.24 V (volts eliminated through the shunt resistor at maximum current)

2. shunt resistor value Rs = Vs / I = 0.24 V / 15A (60A max current divided by 4 because it is distributed over 4 MOSFETs) = 0.016 Ohm

3. Power dissipation required for Rs
P = I x I x Rs = 15Ax15Ax0.016Ohm = 3.6Watt

What do you think about it?

P.s: I attach the complete scheme to better understand the initial operational …

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