# Controlling DC/DC Converter's Output and Its Galvanic Isolation



## dougingraham (Jul 26, 2011)

iruraz said:


> But converter does not have galvanic isolation. It means that high voltage is not separated from low voltage equipments with transformer. Does this situation contain risk?


Yes, it is risky. Your car frame is probably at traction pack negative. This means you will get shocked if you are touching the car and happen to touch the traction pack. If you add to this a charger that is not isolated then you potentially have input mains voltage on the frame of your car. There are lots of scenarios where accidents happen when the DC-DC isn't isolated.


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## iruraz (Sep 4, 2012)

dougingraham said:


> Yes, it is risky. Your car frame is probably at traction pack negative. This means you will get shocked if you are touching the car and happen to touch the traction pack. If you add to this a charger that is not isolated then you potentially have input mains voltage on the frame of your car. There are lots of scenarios where accidents happen when the DC-DC isn't isolated.


@dougingraham, thanks for reply. Is the risk about usage of common ground high voltage with low voltage? If it is, how can I prevent this risk? As far as I understand I have to use isolation transformer in the system.

There is only a statement below in its manual. Is it enough to ensure its isolation?



Kind regards.


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## dougingraham (Jul 26, 2011)

iruraz said:


> Is the risk about usage of common ground high voltage with low voltage? If it is, how can I prevent this risk? As far as I understand I have to use isolation transformer in the system.


Yes, common ground is typically the problem here. (Note: It could also be the high side of the traction battery connected to the +12 volts, it depends on the topology of the DC-DC converter.)

The DC-DC converters with isolation will have a transformer as part of the switching supply circuit. The simplest switching supply will not have an isolation transformer in the middle which makes it cheaper but also gives a common path for ground. You can't just add a transformer to a DC-DC supply because transformers require an AC waveform. If the DC-DC converter you have is not isolated then I recommend a hunt for one that is. An isolated system will not show voltage between your car's 12v system and the traction pack when it is in operation. There are other devices that can leak between the systems so don't just assume it is the DC-DC converter. Instrumentation is a known source of issues. Anything that connects to both the traction pack and to the car's 12V systems can cause a leak. People have even seen brush dust in a DC motor cause frame leaks although from your diagram this one wont be an issue for you.

DC-DC converters are often repurposed AC input power supplies. The supplies that have PFC (Power Factor Correction) front ends can accept a large range of input voltages (usually 90-240 VAC and 110-340 VDC) and are isolated.


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## iruraz (Sep 4, 2012)

dougingraham said:


> If the DC-DC converter you have is not isolated then I recommend a hunt for one that is. An isolated system will not show voltage between your car's 12v system and the traction pack when it is in operation. There are other devices that can leak between the systems so don't just assume it is the DC-DC converter. Instrumentation is a known source of issues. Anything that connects to both the traction pack and to the car's 12V systems can cause a leak. People have even seen brush dust in a DC motor cause frame leaks although from your diagram this one wont be an issue for you.
> 
> DC-DC converters are often repurposed AC input power supplies. The supplies that have PFC (Power Factor Correction) front ends can accept a large range of input voltages (usually 90-240 VAC and 110-340 VDC) and are isolated.


I have purchased already Delphi's DC/DC converter. As I mentioned befor there is only one statement and it is :



> From Manual:
> Please note the Universal 2.2kw Converter high voltage is isolated from low voltage and case ground. Use extreme
> caution when working around high voltage. Only experienced technicians should attempt to troubleshoot high
> voltage issues. In all cases HV insulated probes should be used when measuring high voltage.


Is it enough to ensure its isolation?

Best regards.


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## Tesseract (Sep 27, 2008)

iruraz said:


> I have purchased already Delphi's DC/DC converter. As I mentioned befor there is only one statement and it is :
> 
> 
> 
> ...


Yes, this means the high voltage side is isolated from the low voltage side. Either English is not your first language... or you shouldn't be messing with this stuff.


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## dougingraham (Jul 26, 2011)

iruraz said:


> I have purchased already Delphi's DC/DC converter. As I mentioned befor there is only one statement and it is :
> 
> 
> 
> Is it enough to ensure its isolation?


Based on that statement it is isolated. As I mentioned an isolation transformer can only work if you have AC. In a switching power supply there is always AC present (even if the input is DC) so it is a fairly straightforward matter to include the isolation as part of the voltage buck or boost stage. It is a huge advantage to do this isolation in the switching stage because the switching frequency is a much higher frequency than the line frequency of 50 or 60 hz. Typically 30khz but it can go into the multi-megahertz range. The higher frequencies mean smaller capacitors and inductors/transformers are needed. At 50/60hz the transformer needed for a couple thousand watts is heavy, bulky and not all that efficient.

If you want to test for a frame leak take a 100k ohm resistor and connect it between your car frame and your traction pack positive. Measure the voltage between the frame and the traction pack positive. It will be essentially zero. Now connect the resistor between the frame and the traction pack negative. Measure the voltage between the frame and the traction pack negative terminal. Again you should see essentially zero volts. If you see a voltage on either then you have a frame leak. The 100k ohm resistor is not completely necessary, but because of capacitance effects your meter might not settle down so using a resistor will ensure you get a good result. If you have a frame leak then the resistor could be used all by itself as it will get warm. With a 300 volt traction pack there would be 1 watt of heat dissipated. So if you hook up the resistor and it gets hot you don't even need to put a meter on it. You have an issue.


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## iruraz (Sep 4, 2012)

Tesseract said:


> Yes, this means the high voltage side is isolated from the low voltage side. Either English is not your first language... or you shouldn't be messing with this stuff.


@Tesseract, you are right English is not my first language. But there is no clear expression about isolation. From your point of view "
Please note the Universal 2.2kw Converter high voltage is isolated from low voltage and case ground." is enough to ensure isolation. They should have written details about isolation (for instance they could write : it has galvanic isolation or it has isolation transformer etc.). I wanted to ask you for peace of mind.



dougingraham said:


> If you want to test for a frame leak take a 100k ohm resistor and connect it between your car frame and your traction pack positive. Measure the voltage between the frame and the traction pack positive. It will be essentially zero. Now connect the resistor between the frame and the traction pack negative. Measure the voltage between the frame and the traction pack negative terminal. Again you should see essentially zero volts. If you see a voltage on either then you have a frame leak. The 100k ohm resistor is not completely necessary, but because of capacitance effects your meter might not settle down so using a resistor will ensure you get a good result. If you have a frame leak then the resistor could be used all by itself as it will get warm. With a 300 volt traction pack there would be 1 watt of heat dissipated. So if you hook up the resistor and it gets hot you don't even need to put a meter on it. You have an issue.


@dougingraham, thanks for detailed explanation and test technique. When the chassis is ready I will test it. 



dougingraham said:


> Based on that statement it is isolated. As I mentioned an isolation transformer can only work if you have AC. In a switching power supply there is always AC present (even if the input is DC) so it is a fairly straightforward matter to include the isolation as part of the voltage buck or boost stage. It is a huge advantage to do this isolation in the switching stage because the switching frequency is a much higher frequency than the line frequency of 50 or 60 hz. Typically 30khz but it can go into the multi-megahertz range. The higher frequencies mean smaller capacitors and inductors/transformers are needed. At 50/60hz the transformer needed for a couple thousand watts is heavy, bulky and not all that efficient.


I have no more knowledge about isolation technique. But as far as I understand it is an obligation to use tranformer for isolation. Please correct me if I am wrong.

By the way, I sent mail about converter's isolation. And according to their answer yes it has isolation. And it is clear now. I think it is not hard to write "it has galvanic isolation" or "it has isolation transformer" etc. 

Thanks for your interest.

Best regards.


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## Tesseract (Sep 27, 2008)

iruraz said:


> @Tesseract, you are right English is not my first language. But there is no clear expression about isolation. From your point of view "
> Please note the Universal 2.2kw Converter high voltage is isolated from low voltage and case ground." is enough to ensure isolation. They should have written details about isolation (for instance they could write : it has galvanic isolation or it has isolation transformer etc.). I wanted to ask you for peace of mind.


No problem, though I suggest that you go to your "UserCP" here and fill in some details like your location. Your written English is quite good and so I wasn't sure why you seemed to be having such a tough time with the Delphi description.

As for needing more details about the isolation, there is only one practical way to provide isolation in a power transfer circuit - a transformer. In other words, there is no other way of providing the isolation so there is no need to be all that specific about it.


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## iruraz (Sep 4, 2012)

Tesseract said:


> No problem, though I suggest that you go to your "UserCP" here and fill in some details like your location. Your written English is quite good and so I wasn't sure why you seemed to be having such a tough time with the Delphi description.


@Tesseract, in fact I haven't realized empty details  I will consider your warning.



Tesseract said:


> As for needing more details about the isolation, there is only one practical way to provide isolation in a power transfer circuit - a transformer. In other words, there is no other way of providing the isolation so there is no need to be all that specific about it.


Thanks for technical details.


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