Switch Mode PS input section: resistor across input to common mode choke? - Page 1
Forum section: electronics / beginners

author: ItsASecret
author_rank: Newbie
timestamp: December 01, 2023, 10:19:36 pm
content: I'm tearing down some spare SMPS's I have lying around to get a better understanding of how they work. I've traced in the input section, everything up to the bridge rectifier, and I'm puzzled at the purpose of the resistors labeled R1A and R1B in the circuit:Probably something simple I'm missing, but I haven't really seen it on many example circuits out there.Otherwise, I think I have it largely figured out? C1A and C1b are 'line-to-line' protection against differential-mode noise, U1 (should say T1 but you get the idea) is a common-mode choke to protect against common mode noise, and the thermistor RT4 is to protect against inrush current.Any help is appreciated!

author: ItsASecret
author_rank: Newbie
timestamp: December 01, 2023, 10:28:50 pm
content: As usual, minutes after posting a question online, I just found a description that sounds right:They could be there to provide a discharge path for the input capacitor when mains are disconnected. Seems to make sense, and the high value would mean actual draw while the supply is in use would be minimal.

author: Konkedout
author_rank: Frequent Contributor
timestamp: December 02, 2023, 04:40:34 am
content: Yes when you pull the plug out of the power outlet, those X capacitors can have dangerous voltage on them. The discharge of those capacitors might be relatively slow. You would then have dangerous voltage present briefly across the prongs of the AC plug. Those resistors discharge the X capacitors more quickly so there is less chance of getting a shock from the plug prongs after pulling the plug from the outlet.

author: MathWizard
author_rank: Super Contributor
timestamp: December 04, 2023, 09:53:53 pm
content: A lot of times they are left out, since it's assumed there are circuits after the rectifier that will run down all the caps in the path. But yeah sometimes on some circuits, that could take a while.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 05, 2023, 03:26:32 am
content: Quote from: MathWizard on December 04, 2023, 09:53:53 pmNo, you shouldn't assume that. The bulk cap on most SMPS design are often substantial enough that it may prevent X caps from discharging within reasonable time from downstream quiescent current alone. Combined with vampire power legislation, it is a large enough problem that warrant a dedicated X cap discharger IC.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2023, 08:10:21 am
content: Quote from: ArdWar on December 05, 2023, 03:26:32 amIt can be drawn down via PFC front-end as well. But this could still be disabled (intentionally, on models with such a function, or unintentionally say in fault conditions), and then the filter and rectifier section will be left floating when unplugged.Specifically, there are safety standards relating to energy available on plugs, within some duration from being unplugged and some available energy storage amount or residual voltage present. Obviously capacitors store energy, so this is a wise precaution. Example, IEC 62368-1 sec. 5.5.2.2.Resistors are a good enough solution when the capacitors are small; in larger supplies, the total can be many uF, and an active solution becomes attractive -- SMPS controllers with integrated X-cap discharge function is one example, stand-alone components are another (usually having a sort of charge-pump supply to a bidirectional switch; when AC stops, the charge pump discharges, the switch turns back on, and the device goes to a modest resistance (~10k?) until terminal voltage goes away).Tim

author: Milmat1
author_rank: Contributor
timestamp: December 07, 2023, 06:26:26 pm
content: So you're saying the resistors are to discharge the BFC ? Which is not shown on the diagram ...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2023, 07:43:48 pm
content: No, just in front of the rectifier.Tim