Automotive 48Vdc needs isolation? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: November 16, 2021, 02:41:09 pm
content: Hi,For an automotive bus, if you have say a 120Vdc (or higher) bus from the  on the car...and you want to convert it to 48Vdc, 100A, then would you need isolation? I assume its a regulatory thing?

author: BradC
author_rank: Super Contributor
timestamp: November 16, 2021, 02:52:27 pm
content: Quote from: Faringdon on November 16, 2021, 02:41:09 pmI "assume" you know Benny Hill?

author: Faringdon
author_rank: Super Contributor
timestamp: November 16, 2021, 03:21:17 pm
content: Thanks, yes, i see that Benny Hill wouldnt bother isolating it, but it seems a waste of time, because the chance of someone getting shocked with >48VDC is very very low.With a non isolated converter, you can put in protection, to disconnect the >120vdc if a fault occurs.And lets face it, the >120vdc source is still on the car anyway, even if you isolate.So if theirs a crash.....the isolation transformer possibly isnt going to protect you from the >120VDC anyway.I would have thought a reasonable , non-isolated solution could be found.You might say theres a danger to service engineers..but ive worked on non isolated high voltages above 400VDC and no one said that i was in danger...no regs against it.I was regarded to be a "trained person".A customer isnt likely to come into contact with >48VDC with suitable protection, even if non isolated.its interesing, that you can get isolated 48VDC smps's, if eg from a 2 transistor forward, the secondary side, depending on transformer turns ratio, can pulse up to above 120Vdc.So it just makes you wonder, when thats on the secondary side, and officially, its "isolated".

author: Slh
author_rank: Regular Contributor
timestamp: November 16, 2021, 05:55:24 pm
content: I'd suggest that you have a read though UN ECE regulation 100. If it's a motorbike you're interested in then I think it's regulation 136 (spoiler alert, they're basically the same except 100 says "cars" and the other "motorbikes"). If you're designing something for use in the state's then you'll want to find the appropriate SAE one that I think says something very similar.The quick answer is that yes you need to isolate it.

author: SeanB
author_rank: Super Contributor
timestamp: November 16, 2021, 07:04:34 pm
content: Yes, to comply with the SELV regulations it needs to be isolated, otherwise the insulation and touch requirements are the same as the 120V it is derived from, which means you need to have the same safety requirements, and it gets difficult to provide proper insulation. Making it isolated also allows you to have it be either SELV or PELV, and thus you can connect it to the chassis of the vehicle if needed. Insulation for 100V is a lot easier to arrange than insulation that can survive 2500VAC for 30 seconds, which is the standard if it is not isolated, and this also includes every part of the 48V system, including controls.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 16, 2021, 11:34:34 pm
content: It depends on the kind of source and how things are grounded. There are lots of nonisolated solar charge controllers that accept up to 150V and output 48V.

author: David Hess
author_rank: Super Contributor
timestamp: November 17, 2021, 05:29:33 pm
content: Quote from: Faringdon on November 16, 2021, 02:41:09 pmIf it is not isolated, then a ground fault on the source could result in up to 168 volts at a high current being exposed, so the 48 volt output needs to be treated the same as a 168 volt 100 amp source as far as safety.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 17, 2021, 05:57:27 pm
content: If the 48V is contained within your product and not accessible in any way (communication etc. is isolated), then don't bother with isolation.Otherwise, do isolate it.

author: JohnG
author_rank: Frequent Contributor
timestamp: November 18, 2021, 02:26:46 am
content: If it's a non-isolated buck converter, and the top switch fails short (the usual failure mode), you have 120VDC on the output, which is lot worse than pulses > 48V at the switching frequency, but whose average is 48VDC, so don't equate the two. 120VDC can do some serious damage, including starting fires in addition to being a shock hazard. It might not even blow a fuse or breaker.A safety isolation switch is not necessarily cheap or easy, either, especially if you are interrupting a high-ish voltage DC current. An isolated design can prevent a lot of other issues like low frequency ground currents, etc, and if one of the primary switches fails, the output fails safe. It can also keep failures on the 48V side from propagating upstream.A lot depends on your source and load, i.e. the system you are fitting your 4.8 kW converter in. What happens if the high side switch fails in a non-isolated converter? That's a rhetorical question, since you know your system better than I do.John

author: IanB
author_rank: Super Contributor
timestamp: November 18, 2021, 02:34:56 am
content: Quote from: Faringdon on November 16, 2021, 02:41:09 pmCan a car electrical system provide 4.8 kW? (Just asking, out of curiosity.)

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 18, 2021, 03:29:24 am
content: Quote from: IanB on November 18, 2021, 02:34:56 amAt that high voltage, yes.http://priups.com/others/bill/index.html

author: IanB
author_rank: Super Contributor
timestamp: November 18, 2021, 06:22:58 am
content: Quote from: NiHaoMike on November 18, 2021, 03:29:24 amAh...anelectriccar can!

author: Siwastaja
author_rank: Super Contributor
timestamp: November 18, 2021, 03:30:01 pm
content: Do not use transformer to "increase safety" against switch short, this is a ridiculous idea that jumps up on this forum every now and then. This kind of primitive thinking screams of false sense of security. Namely: if one of your feedback loop resistors fail, or any other part of the feedback, a transformer isolated topology is happy to supply overvoltage! High side transistor fail-as-short is literally theonlyscenario where transformer isolation helps protect the secondary. Everything else remains.Instead: use fuses properly for fire protection. When not applicable (i.e., current-limited situation, or possibility of high-resistance "shorts"), add more accurate e-fuses, or other supervisor circuits. Protecting lower voltage side of the circuit can be done using a completely redundant, independent overvoltage protector, which can be of series pass type, or crowbar+fuse. These strategies are all equally recommendable regardless of whether the supply itself is isolated or not.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 18, 2021, 03:40:26 pm
content: The answer is a big "It depends", primarily on exactly what you are using the 48V for.In a car situation, the HV system is isolated from ground, and there is usually active isolation monitoring, so as well as worrying about whether the 48V needs to be isolated for downsrtream reasons, you also need to consider what can happen to a nonisolated 48V supply that might make the HV isolation detection unhappy.

author: bdunham7
author_rank: Super Contributor
timestamp: November 18, 2021, 04:41:42 pm
content: Assuming you mean you want 48V from the DC bus of an existing electric or hybrid car, yes the conversion will need to be isolated--no ifs ands or buts. Doing it otherwise could be insanely dangerous. Whether or not the output is isolated or referenced to the vehicle ground is a separate question entirely. The DC bus/traction battery of an EV is very carefully isolated and monitored, the only place you see any use of that power in a non-isolated manner is within the core modules of the car--the traction motor inverters, inside the battery modules and things like that.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 18, 2021, 07:47:14 pm
content: Quote from: bdunham7 on November 18, 2021, 04:41:42 pmAs you say, the non-isolated battery HV is already in various modules, including the motor, where the HVDC in windings is separated from the metal casing with fairly thin sheets of insulation materials (like glassfiber reinforced polyester sheet). Being a mechanical part in demanding conditions, insulation breakdown is a real risk. This is exactly why the status of insulationismonitored, to create an active extra layer of protection above the passive protection which isn't good enough alone.There is no fundamental reason why you couldn't extend the same thinking into whatever retrofit module. You just need to perform equally good, or better than the vehicle designers did with the inverter and motor.There is going to be an non-isolated part in any case - that is, wiring to the isolating converter, and the input side of said converter. If the 48V load is of similar simplicity, then it carries no more risk than the non-isolated side of the isolated converter.For the same reason, when designing such retrofit module, you can't avoid complying with all relevant regulations and manufacturer guidelines regarding accessing the battery HVDC terminals. In that big picture, whether the last piece of the device is behind a layer of isolation is finally quite insignificant. So depending on case, it's either needed, or it isn't. It's going to be a demanding project anyway.

author: JohnG
author_rank: Frequent Contributor
timestamp: November 19, 2021, 12:06:26 am
content: Quote from: Siwastaja on November 18, 2021, 03:30:01 pmAll additional failure mechanisms you propose can occur in isolated and non-isolated topologies. A transformer mitigates different failure mode, which is a high side FET short of a non-isolated converter. In a non-isolated converter, you are forced to have yet another series switch, further complicating the circuit, adding cost, and having a negative impact on overall reliability.Your suggestions for protection are good, but can add significant cost, and an isolated topology can save overall cost in some situations. I have had customers find this out the hard way and move back to using isolated converters with measurable cost savings and reliability improvements.I do not know if this is relevant to the OPs situation. That is why I suggested that the OP needs to consider the rest of the system. A proper failure analysis will provide insight for the proper choice.I have designed non-isolated stepdown converters of much higher power, so I am not against them. I am well aware of the fact that faults can also affect transformer-isolated systems, and that often they cost more. However, they have real benefits in a lot of systems, including system reliability and safety benefits.John

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2021, 09:11:58 am
content: If the designer fails the basic design of a buck in such a way that the high side FET has tendency of failing short, then mitigates the problem by adding the complexity of turning the converter into isolated version, I would not trust that designer, unless the application is non-critical. Of course it can work by luck, assuming the FET dimensioning/protection was the only mistake they made, and by adding complexity of isolated feedback they did not make any further mistake there (could happen more easily, because now the feedback path is more complex).Even a simple solder joint issue can turn a feedback network into something else. This is why high-end supplies have crowbars even if they are isolated.If the pass element in a regulator/converter designed by me fails short and causes problems, I accept responsibility and fix the problem. Not by adding isolation at the expense of customer, but instead finding the root cause, rectifying it, and if the supply is powering some really expensive special gear, add the $2 crowbar functionality as an independent safety layer.Independent safety layers are standard practice today, for example almost all li-ion based products use another supervisor circuit with their own FET power switches. Yet these do not add massive cost, a few $. It's a good idea even if you are confident your product does not fail providing overvoltage.

author: rs20
author_rank: Super Contributor
timestamp: November 19, 2021, 09:17:54 am
content: Quote from: Faringdon on November 16, 2021, 03:21:17 pmGuy: I have a 240V AC to 5V DC converter to use as a phone charger. Should it be isolated?Faringdon: It seems a waste of time, because the chance of someone getting shocked with >5VDC is very very low.Me: Um, no, that's not how this works...Guy: This is a 240V AC to 1kV DC converter. It's isolated -- is it safe to touch the outputs?Faringdon: Oh yeah the risk of getting shocked by 1kV is high, so it would be dangerous: except you have isolation, so you're safe!Me: Um, no, that's not how this works...

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2021, 09:20:49 am
content: Quote from: rs20 on November 19, 2021, 09:17:54 amThis is ridiculous because we know nothing about Faringdon's use case. Yes, we can guess, and I agree, Faringdon likely needs isolation but such guesses are cargo cult engineering.Your LED bulbs definitely do not have isolated supplies. Your microwave oven with plastic buttons likely does not have isolated supply.The 5V phone charger is isolated because the output is accessible to the user, and it would be difficult and expensive to use isolated connectors, isolated charging cables, and completely isolate each and every phone and all their interfaces.

author: rs20
author_rank: Super Contributor
timestamp: November 19, 2021, 09:28:07 am
content: I apologise, perhaps I leaned too hard on overinterpreting a single sentence. But at the same time, the higher an output voltage, the less likely it is that isolation is useful... so I stand by the point that "it seems a waste of time, because the chance of someone getting shocked with >48VDC is very very low" seems a universally completely backwards statement.

author: Nusa
author_rank: Super Contributor
timestamp: November 19, 2021, 09:47:54 am
content: Don't apologize...that response was on point, given the history.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2021, 12:37:18 pm
content: Really no need to apologize. Treez is of course classic and I know some can't stand him, but I think he opens very interesting topics, that is, whenever we don't try to directly solve his exact problems, but instead use the opening as a starting point for discussion of relevant aspects. And while doing so, we should raise the bar.IMHO, the OP likely shouldn't be doing this project at all, so from that perspective, the isolation question is an irrelevant detail. Yet it's an interesting starting point for discussion.