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Diode-resistor : Choosing the resistance value

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OwlFowl

Thu Apr 24 2008, 03:17AM

Registered Member #1441 Joined: Sun Apr 13 2008, 03:40PM
Location:
Posts: 10

Hi again!

After a long time of figuring out what type of snubber circuit I should use, I've finally decided to use the parallel diode-resistor variation to cut down decay time instead of the v switch or the half bridge for pure simplicity.
According to here, the value of the resistor is determined by the supply voltage, current at turn-off, and the maximum switching device (in my case it's an SCR) voltage. I've really tried to find an equation to find the safe resistance value for my system, but I find them very vague.

I would like to know the relation between these variables so I can calculate the resistance value by myself

*I'm guessing I'll need a very, very high watt but low resistance resistor.

Thx in advance

...

Thu Apr 24 2008, 07:11AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Basically, you need to make the resistor large enough so that your reverse diode does not explode when it tries to stop the reverse pulse it is designed to.

So, that in mind, you need to figure out what the maximum current pulse your diode is rated for (this is usually about 10x the average current), and then the maximum reverse voltage you expect to see (this depends on the ESR and ESL of your caps/work coil/buss/etc).

The latter value can be tricky to calculate, but if you provide more details on the system we should be able to point your in the correct direction...

OwlFowl

Fri Apr 25 2008, 01:59AM

Registered Member #1441 Joined: Sun Apr 13 2008, 03:40PM
Location:
Posts: 10

Hmm I'm a bit confused on this part. In HvWiki, it says that the resistor value is limited by the switch voltage since a high resistance value creates a higher voltage spike of the switch. So am I also looking at how much voltage my switch can handle also?

I think my whole concept of the diode-resistor setup is wrong. From what I've gathered my concept is that a diode can be placed anti-parallel with the coil to circulate the current after turn-off instead of negatively charging the caps. Or that a resistor can be placed in series with the diode which is anti-parallel to the coil in order to decay the current every time it loops around. Is that right? Or is my whole concept completely wrong..

Well...
diode: 600V, 1500A pulse
SCR: 600V, 650A pulse
caps: 400V, 470uf
coil: supposed to be 12 layers 18awg 9mm inner diameter so that's 0.5 ohms, 1.13 mH, 293 Amps which hits zero at 4ms and goes maximum down to -140V, -100A

I still want to calculate my own resistance value even if it's tricky, though

...

Fri Apr 25 2008, 02:15AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The wiki is somewhat misleading, the voltage that you use for your switch only indirectly affects the resistance of the snubber resistor in the sense that the higher the voltage you use the higher the reverse voltage will be.

The main factor to use to calculate the resistance of the resistor is the maximum current the diode can handle. If you have an inductor, the current that it will put out is the current you used to charge it (300A) divided by the resistance. If you can handle 1500A you would need .2ohms of resistance, which is less than the resistance of your coil so you should have no worries.

The other idea to consider is the maximum voltage reversal that your caps can handle. Luckly, since you are not adding any resistance this value is only that voltage drop across the diode, which is about 2v, which shouldn't give you any problems.

TheMerovingian

Fri Apr 25 2008, 01:24PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

You need to make sure that the switch voltage isn't exceeded even in worst case:

Switch voltage is:

Before firing: Vcap (initial cap voltage)
During firing: Vforward (or I* Rmosfet), it is in the 5-10V range
After firing (during turn-off): Vswitch = Vcap-residual+ R*I+Vdiode (with I decaying exponentially depending on L/(R+Rcoil) ratio)
Final voltage= Vcap-residual.

To know worst case Vswitch voltage you need to RLC-simulate the circuit to know the peak current, then use this formula to determine the correct resistor (wirewound) value:

Vwcswitch = Vcap + R*Ipeak , so choose R accordingly. It is a worst case value, because at Ipeak, Vcap will be smaller, but this leaves a bit of room for safety.
Alternatively use my RLC simulator, that includes peak switch voltage during turn-off

Coilgun simulator

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