# Isolating Pack voltage from 12V battery/chassis



## CaptConan (Jul 12, 2010)

I understand the importance and necessity of keeping the high voltage pack isolated from the 12V aux battery which itself is grounded to the frame of the car, but for the controller logic to work, The pack voltage and 12V aux system need to share a common ground reference. How is everyone achieving this? Does a DC-DC converter accomplish the job or are optoisolators used?
If it is the latter, then exactly how does a device with 4 or more pins link the commons?


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## major (Apr 4, 2008)

CaptConan said:


> I understand the importance and necessity of keeping the high voltage pack isolated from the 12V aux battery which itself is grounded to the frame of the car, but for the controller logic to work, The pack voltage and 12V aux system need to share a common ground reference. How is everyone achieving this? Does a DC-DC converter accomplish the job or are optoisolators used?
> If it is the latter, then exactly how does a device with 4 or more pins link the commons?


What controller do you have?


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## CaptConan (Jul 12, 2010)

I'm designing my own: basically the 12V aux battery will power the uC-->Optoisolator-->gate driver. Then the gate driver connects to the gate of the IGBTs which of course are on the high side(eventually 144V). My fuji IGBTs are rated safe for +-20 Vge, 600 Vce, 300A. The Miller knee occurs at 8V on the gate. Hoping to punch that to about 10 or 11V.


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## major (Apr 4, 2008)

CaptConan said:


> I'm designing my own: basically the 12V aux battery will power the uC-->Optoisolator-->gate driver. Then the gate driver connects to the gate of the IGBTs which of course are on the high side(eventually 144V). My fuji IGBTs are rated safe for +-20 Vge, 600 Vce, 300A. The Miller knee occurs at 8V on the gate. Hoping to punch that to about 10 or 11V.


The drives I've worked with, both DC and AC, have internal isolated power supplies for everything. I would not use the 12V aux for anything except maybe the main contactor coil and fan.


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## CaptConan (Jul 12, 2010)

How do you tie your grounds together?


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## major (Apr 4, 2008)

CaptConan said:


> How do you tie your grounds together?


Hi Cap,

If you have isolated supplies, you can tie them together at any potential  Ground is a funny term. Is it earth? Or is it common? Or is it the frame?

I assume you are doing a DC controller, a chopper. You have the high voltage (battery) junction, Vce. And you need a control (gate driver), Vge. The emitter (e) is common to both Vce and Vge. 

When you have your main switch on the high side, a potential difference develops between the emitter and battery negative. So the gate drive supply must be isolated from the main power source (high voltage battery).

Some choppers put the switch on the low side. Then the emitter is at battery negative potential all the time and the gate supply can be common at that point.

Regards,

major


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## Tesseract (Sep 27, 2008)

CaptConan said:


> ...The pack voltage and 12V aux system need to share a common ground reference.


No they don't need - or even should! - share a common ground.

The power side of a converter/inverter should always float w/r/t to the control side. There are many ways to skin this cat, but given that a dc motor controller needs to span the full 0-100% duty cycle range the usual choice is to use isolated power supplies and optocouplers to transmit the gate drive signal to the switch (whether it be a MOSFET, IGBT, etc...). It does not matter whether the switch is on the high side or low side - once you need complete isolation (ie - your pack is above 48V) you automatically have the ability to control the switch in either position.


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## CaptConan (Jul 12, 2010)

Thanks guys, I've changed my design. Next step is finding a cheap DC/DC converter. I see there was a thread where the guys were trying to hack an ATX power supply but it seems to have died out.


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## DJBecker (Nov 3, 2010)

major said:


> If you have isolated supplies, you can tie them together at any potential  Ground is a funny term. Is it earth? Or is it common? Or is it the frame?


Not only should the traction circuit be isolated from frame ground, it's best to have a leakage monitor/alarm to insure that they remain that way.

If you are designing one of the circuits (controller, etc), you can implement this with a simple voltage divider. Supply battery voltage through a pair of high value (100K-1000K ohm) resistors safely connected inside the high voltage section. On your low voltage board, monitor each side with 1K-10K resistors as the other half of the voltage dividers. By putting the high value resistor close the source you limit external exposure to the uA range.

Of course if you are designing a smart battery monitor, it's easier. Just put the leakage check on the master controller. It already has to deal with full battery voltage and communication isolation.

Don't expect that your monitor will report absolutely zero leakage. Everything has a little leakage current, including the monitor you just built. It's common for controllers to connect the isolated circuit through to ground through a very high value resistor or a leaky high voltage decoupling cap.

As far as picking the isolation point for the motor controller, that's up to your design. If you have a separate low voltage control and high voltage switching boxes, you might choose to isolate the output of the low voltage box (which protects the control electronics from wiring faults and induced current), at the input of the power box (which prevents high voltage from escaping when power semiconductors drop their "semi") or both.

One simple solution is to use an optoisolated gate driver. The drawback here is that they are a step down in current and speed from the best gate drivers.

For the gate driver power supply, you might consider a "bootstrap" circuit instead of a DC-DC converter module. The average power used by a gate driver is pretty low, and a 20V-30V supply isn't useful for much else, so a sleazy clamped charge pump might be an acceptable low-cost choice.


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## frodus (Apr 12, 2008)

How many watts do you need? what pack voltage?

I have an isolated DC-DC converter. Its a 200W 100-200VDC input 12VDC output (trimmable to 13.2V) that I'm not using if you want to buy it off me. Its a Vicor VI-251.


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