Finding L & C for Circuit Protection: dv/dt & di/dt Values

In summary: If the cap is resonating with the inductor, then the current through the inductor will be a harmonic of the voltage across the cap, which would be bad. In summary, a cct is a circuit that is used to protect a device from high rates of anode-to-cathode voltage and anode current. It consists of a thyristor (scr) and a snubber. The thyristor is modeled as having a threshold voltage and on resistance. The snubber is used to protect the device from large rates of voltage and current.
  • #1
Johnie
3
0
I'm trying to use this protection cct. What I do know is that (dv/dt)max = 50 V/us and (di/dt) = 10A/us (us = microseconds)
R = 1 ohm, R = 5 ohm, Vcc = 120 V
Can anyone give me an idea how to find the right L and C ?

Thanks
 

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  • #2
What's a cct? Is that a zener? What is the application of this circuit/what are you trying to protect and where is it connected?
 
  • #3
snubber

cct = circuit

That is a thyristor (scr) (not a zener)

A snubber is used mainly to protect a device from large rates of anode-to-cathode voltage and anode current. If dv/dt for the thyristor is too large, the device will begin to conduct without a gate signal present. If di/dt is too large during turn-on, localized heating will result from the high current density in the region of the gate connection as the current spreads out over the whole junction.

This circuit can be applied to a lot of things -> ie. a buck converter

I know that I can follow the loop from Vcc to ground and determine what (dv/dt)max is using Laplace Transform. To when I'm determining C and L, do have have to pick a value for one (C or L) and then determine the other one ? and then check to ensure di/dt is ok ?
 
  • #4
OK, so we're talking about across and through the thyristor, I take it. I am considering the circuit now.



Johnie said:
I know that I can follow the loop from Vcc to ground and determine what (dv/dt)max is using Laplace Transform.
Why (and how) would you use a Laplace transform to determine a specification that is already given? Can you explain what you mean in more detail?



Can the thyristor be modeled as having a threshold voltage and on resistance? Does is have parasitic capacitance (I'm assuming so according to your previous description) and inline inductance? How complicated of a model do you want to use? These issues seem like they would be important. For a first order approximation, I would just replace the thyristor by some typical on resistance value (and perhaps the parasitic cap) and then ensure the ratings are not breached for this resistor.
 
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  • #5
snubber

I thought I'd look at the dv/dt ratio when the thyristor is off (immediately after). In this case I can follow the loop from Vcc to ground and use Laplace to determine V(output) -> dV(output)/dt. From this I was thinking that I would pick a value for either L or C and calculate the other one that I didn't choose. Then I'd do the same thing for the current (di/dt), when the thyristor is turning on and make sure that these values of L and C satisfy this parameter.

Another thing that I was thinking was that when the thyristor turns off...the inductor and the capacitor might go into resonance ?
 
  • #6
OK, that sounds pretty good. But it seems like you still need some sort of a model for your thyristor, because you're going to need to know what the state of the circuit is immediately before you "open" the path through the thyristor.

Consider the on-steady-state and a strictly on-resistance model for the thyristor:
There will be an initial current through the inductor given by IL,0 = 120V/(Rthyristor + 1Ω).
There will be an initial voltage across the capacitor given by VC,0 = IL,0Rthyristor.
When the thyristor is switched off, this current IL,0 must go into the cap.

I would start my analysis there, and then increase the order of approximation by including a parasitic cap on the thyristor if you get bored. I imagine that should be good enough, since these values don't really seem to be critical operational values. In fact, if your #1 concern is that the max specs must absolutely not be breached, then just get the largest L and largest C that you can.

About the resonance, I imagine that's why there's a res in parallel with the cap.
 

1. What are dv/dt and di/dt values?

Dv/dt and di/dt values refer to the rates of change of voltage and current, respectively. They are important factors to consider in circuit protection as they can cause high voltage spikes and rapid changes in current flow, which can damage electronic components.

2. Why is it important to find L & C values for circuit protection?

Finding the appropriate values for inductance (L) and capacitance (C) is crucial for circuit protection as they help to limit the effects of high dv/dt and di/dt values. L and C values act as filters and can absorb or redirect these sudden changes in voltage and current, preventing damage to the circuit.

3. How do I calculate L & C values for circuit protection?

L and C values can be calculated using the formula L = V/(di/dt) and C = I/(dv/dt), where V is the voltage rating, I is the current rating, and di/dt and dv/dt are the maximum rates of change for current and voltage, respectively. These values can also be determined experimentally or by consulting a circuit protection specialist.

4. Can L & C values be adjusted for different circuits?

Yes, L and C values can be adjusted to suit different circuits. The values can be chosen based on the specific voltage and current requirements of the circuit, as well as the maximum dv/dt and di/dt values that need to be limited. It is important to carefully select L and C values to ensure effective circuit protection.

5. Are there any other factors to consider besides L & C values for circuit protection?

Yes, there are other factors to consider for circuit protection, such as the type of circuit, the environment in which it will be used, and the type of components being protected. It is important to carefully assess all these factors to choose the most suitable protection methods and components for a circuit.

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