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HV Resistor layout

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jcconverters:
Hello!

I was wondering why in a Isolation meter (like a High voltage multimeter that checks isolation between HV battery lines a chassis in EV) this strange layout is used in the voltage divider for HV+-, is because of optmizing the space, clearance, creepage... would not be another layout be more appropiated.

Thanks!

exmadscientist:
Creepage and clearance are the concerns. Also fitting it into the available space as the requirements can call for a lot of distance. Clever designers will make use of the voltage divider's actual division to relax the spacing closer to the end of the division.

This one... does not look clever. Or even sane. I guess they needed that conformal coat to get it working?

T3sl4co1l:
Looks fine to me.  Seems to be adequate clearance between components depending on position in the string, and it more or less fills the available area.  A more orderly arrangement might take up less space, but if they weren't limited on space, that doesn't matter.  The stray capacitances are surely not going to be very straightforward, but I'm guessing this is low frequency where a flat frequency response is more or less trivial (no compensation needed).  That leaves a large number of available states, that is to say, positions and orientations of the available components, and without any other constraints, the layout shown is about as likely as any other.

Personally, I always apply the constraint that, sure it doesn't matter, but if I can, why not?  Or, if it doesn't affect anything else (dev time included!), make those as optional constraints.  So, a more orderly and symmetrical layout would give a somewhat flatter frequency response, or have better CMRR near cutoff, or other metrics like that; or the constraint to make it look pleasing, symmetrical, that sort of thing (a generally low-entropy goal, except when I do shit like this, https://www.seventransistorlabs.com/Images/AltiumStepRoute.png ); or even to abuse the available entropy and, say, spell out something cheeky with the components/traces.

Tim

floobydust:
The concern is creepage and clearance between/across resistor strings, the connectors, most of the odd looking layout is to keep generous spacings to the corner mounting screws.
I'm not sure what the operating voltage and surge voltage specs are for an EV here.

I would say it's not a really great design, looks more like something from a worried, inexperienced designer and PCB layout person. The exposed metal (for mounting) on the TE/AMP connectors is a concern, what is that about. There's no conformal coating there, you can see the mask. Oops shoulda had a slot and them further apart. Common noob mistake is thinking there's conformal coating somewhere it isn't, and then using the incorrect spacings.

Vishay MELF MMA 0204 operating voltage is 200V each, 1.5mm pad spacing and 19 of 10k resistors to the divider node... is a bit crazy to make it good for 3.8kV operating? Yes they need to be AEC-Q200 and high stability.

This kind of circuit (voltage monitoring) was a problem with smart energy meters. Without really impulse testing it, if the spacings are adequate is just a theory especially with moisture present.
Smart meters being in OVC Cat. IV would breakdown and arc inside, and because there are no fuses, it started bad house fires. Then UL/CSA realized they should probably have some standard (UL 2375) and get smart meter manufacturers to comply and actually test their products.

It's a good question - to what standard are the EV makers testing to for HV spacings? Safety standards always lag behind so it's likely this is loosy-goosy.

T3sl4co1l:
Those are standard connectors for CCFL application, I *think*.  Unless they're just bog standard Micro-Fit or whatever.  Not sure.

More to the point, the pads for those mounting nails are much closer than the thru-air distance indicated, and the creepage even closer still.  That's likely the biggest compromise in this design.  Possible there's board routs, invisible under the connectors, but clearly they don't cover the width of the connector so will be of limited value if present.

Tim

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