Pin not driven ERC error with common mode choke

Considering this schematic:

I get an ERC error at each of the converter’s inputs (notice the arrows). The errors are:

Pin connected to other pins, but not driven by any pin: Pin ? (Power input) of component U1 is not driven (Net ??).

But if I place the power flags past the choke (EMI_Filter_LL_1423 in the symbol library) then it works. I can’t see what the problem might be though. Any ideas?

You get the error message because every power input pin must be connected to a power output (for ERC to pass). The pwr_flag simply is a symbol with a single power output for telling ERC: “yes this net is connected to a power source”.

Important to note is that ERC works on a net basis. It has no knowledge about what symbols represent. So if you place a power flag before a passive then this info is lost to ERC after the passive.

Thank you @Rene_Poschl for your reply. I wasn’t aware that the #PWR_FLAG would not get past the passive. For the rest, I understand your explanation but due to my lack of experience I cannot think of a nice solution – would you have any suggestions off the top of your head?

Sorry, perhaps I gave up too soon! :slight_smile: How about this?

Note the extra ±VIN labels.

Now you short circuited your choke! (Use the highlight net label to see what you did)
Just add the PWR_FLAG after the choke or ignore the ERC warning.

Now you short circuited your choke! (Use the highlight net label to see what you did)

It was bound to happen. :see_no_evil: The sooner the better I suppose. :grin:

Just add the PWR_FLAG after the choke or ignore the ERC warning.

#PWR_FLAG after the choke sounds good to me.

Thank you so much for your help!

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Note, on both of your schematic screen-shots you aren’t using a power flag for the -VIN signal. You are using a GND power symbol. The power symbols are for making net connections and are pin type PowerInput. Putting the GND symbol between the choke and the -VIN pin of U1 will short the -VIN input with the -VOUT pin. Probably not what you want.

As it is, you have the -VIN input to your filter network connected to the -VOUT of U1. Not sure if that is intentional common GND reference, or accidental isolation breaking.

Thanks for your input @SembazuruCDE.

The power symbols are for making net connections and are pin type PowerInput.

Ok, so that clarifies what the manual says. Would it be correct to use two #PWR_FLAG symbols, one each on +VIN and -VIN then?

As it is, you have the -VIN input to your filter network connected to the -VOUT of U1.

Are you referring to my misuse of the GND symbol or to the wire connecting the two via C5? The former is a mistake, the latter is intentional; following thn15_emi_consideraton.pdf (sic) in the manufacturer’s application notes (ZIP file).

Yes. The #PWR_FLAG symbol does not make any net connections. It just provides a PowerOutput pin to drive all the connected PowerInputs for ERC. That’s all it does.

The misuse of the GND symbol. :wink: I was figuring that C5 is for shunting any AC noise on the output to ground.

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Also double check that the two GND symbol and GND label do not create a short. As far as i remember different label classes in the same sheet can create a connection if they result in the same net name.

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Your recollection seems to be correct: it was creating a short.

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