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?
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. 
The sooner the better I suppose. 
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!
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. 
I was figuring that C5 is for shunting any AC noise on the output to ground.
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.
Your recollection seems to be correct: it was creating a short.
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How about this?