Need a snubber or DC circuit breaker

[bwilson4web]

Hi,

I'm planning to replace the perfectly good, NHW11 traction battery with one from ReInvolt that uses NHW20 modules:


The perfectly performing NHW11 battery will be retired to "plug-in" duty to provide additional charge for the ReInvolt battery. But I am running into a safety issue, how to 'fuse' the power cables.

With AC power, there is a zero crossing point so any arc as the circuit breaker contacts open will be extinguished. But with DC, there is no zero crossing. So far, I'm looking at:

• Digikey $75, 30A, 300 VDC - a purpose built circuit breaker, I would need four.
• snubber - a resistor capacitor that sinks the current as the contacts open to extinguish the arc . . . a design and testing challenge (how to test arc suppression of the arc suppression device.)
• two-step, relay - the first relay opens to shunt the power through a second relay with a resistor to reduce current flow. Then the second relay opens and with a snubber, the arc is suppressed.
• solid-state - instead of using air-contacts, a power MOSFET is used to isolate the external wires. I suspect these are somewhat expensive too.

It is a safety issue because in an accident, the power cables might short. A battery power, fire is no laughing matter.

Target goal:

• 30 A, 300 VDC - the maximum power drain for the NHW11 battery.

Bob Wilson

[dcb]

How about a 30 amp fuse?

[JasonG]

I have fallen in love with IGBTs.
Built in zener diode for protection.

IGBT 30A 600V D2PAK - STGB20NC60V

Just make sure to drive it fully on and off with pull up/down resistors.

For motors, they have a low gate voltage and consumption. A 555 will drive them continuously !

[dcb]

but you have to tell a transistor solution to stop conducting somehow in an accident, possibly without means to test that it actually works. Fuses were meant for just this sort of thing, simple and efficient and reliable and cheap. The case here is if there is an accident and somehow the cables get connected, which can be minimized with appropriate insulation/dividers.

[JasonG]

My mistake, I thought he was looking to switch them.

I know they make 30A circuit breakers for RVs but i doubt they are rated for 300VDC.

[gone-ot]

...snubber = suitably sized series RC combination (typically 0.1µF/47-51 ohm), also called a "quench" circuit. Capacitor "size" varies with "rate-of-change" of the current.

...typically used on multi-phase, power grid, SCR-commutating and switching circuits.

[bwilson4web]

Originally Posted by JasonG

I have fallen in love with IGBTs.
Built in zener diode for protection.

IGBT 30A 600V D2PAK - STGB20NC60V

Just make sure to drive it fully on and off with pull up/down resistors.

For motors, they have a low gate voltage and consumption. A 555 will drive them continuously !

Thank you! We may have a winner.

They are cheap enough I can put a current limiter on each of the four wires. A simple latch-up if the current exceeds 30 A will solve the problem. Add recessed, push button reset and LED indicator and we're good to go. In fact, the same reset switch can also work as an ON/OFF switch. Best of all, I don't have the replacement safety issue and potential costs of blown fuses. My only concern is whether they are unidirectional and I may have to diode protect and double them.

The other possibility are some of the high voltage, solar array fuses. I found one rated at 30 A., 600 V. for $7.00 each. If I add a 'fuse change' switch, this could also work. The fuse is naturally bi-directional. But I can't use the fuse change switch because interrupting 29 A will still draw an arc and eat the switch contacts. I still need a power, DC switch.

Bob Wilson

[dcb]

I would separate your safety concerns from the control concerns. $7 is cheap compared to some of the current sensors our there and the time required to turn it into a dynamic fuse.

Also, derate the IGBTs, and heat sink them, I wouldn't expect a 30 amp igbt to be happy at a constant 30 amps.

Look at the igbt datasheet to see if it has built in diode protection and gate voltage requirements.

And consider using two spdt switch to control them, that ties the gate to ground or 15v (check datasheet) source, just to keep it simple, safe and reliable, where off means off, with a low gate resistor so they turn on/off quickly. You can also tie it into the high side of the pack easily with spdt.

You will want a tiny bit of delay between turning one off and turning the other on (to ensure the igbt has actually turned off). A simple mechanical interlock (sliding bar) on the switches that only allows one switch on at a time would provide plenty of delay for the igbt to turn off and also allow you to turn them both off. So it goes, turn off pack 1, slide bar, turn on pack 2.

I don't understand why you need 4 switches? You have two packs and only need to switch one leg of the pack in or out. Switching both legs is inefficient and not common practice.

[bwilson4web]

The NHW11 packs have two banks of 9 and 10 module pairs. The interconnect is open when the car is off. I want the ability to transfer energy as needed between the two banks of the two packs even if (especially when) the NHW11 is powered off.

I am not willing to disable the pack 'interconnect' as the "orange safety plug" is part of the emergency responder training.

I've been reading the IR datasheets and I believe there is some pad over the 30 A. at 100 C part ratings.

Bob Wilson

[JasonG]

IGBTs were designed as flyback coil drivers and have a built in diode protection circuit, so no external one is needed.
They are rated at continuous current levels at 75 deg C. They can handle 10% over for brief periods, but i dont reccommend it. They heat up much less than regular transistors.
Some have a gate voltage of 5 volts, others need full rail voltage to fully turn on.

As for the "You will want a tiny bit of delay between turning one off and turning the other on (to ensure the igbt has actually turned off." They swith in milliseconds. I have driven them at 20KHZ and found them to handle it. Some are rated at 1MHZ but you dont need to waste money on those.

While i havent abused them real bad, i have over loaded and reversed polaritied them and have not blown one up (yet).
If you are switching both neg and pos you will need one N type and one P type, guess why