Why does an SMPS use LCR on INPUT?

Question

I have this 5 V output SMPS I bought a while ago. I was wondering why there are LCRs connected to the input of my SMPS. I don't understand why it's there.

BTW, I'm an electrical engineering student, and I know the basics of LCR, but I am not sure what it's doing here.

Do you mean the resistor near the yellow thing, the yellow thing and the transformer-like thing ? — Long Pham, Jun 28 '18 at 14:41
It's a filter which prevents (reduces) the SMPS switching noise from going back into the mains. — Nick Alexeev, Jun 28 '18 at 14:55

score 11 · Accepted Answer · edited Jun 29 '18 at 06:28

11

The yellow thing is an X-capacitor, C11 in the above schematic. The resistor is to discharge said X-capacitor within legal requirements (usually 34 V within 1 second after unplugging), R14 in the schematic. L is an common mode choke, X3 in the schematic. They all form the EMI supression from the power supply to the mains, again within legal requirements.

Please see this old question for more information: X capacitor selection for SMPS power supply

edited Jun 29 '18 at 06:28

Peter Mortensen

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answered Jun 28 '18 at 14:46

winny

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And block spikes from mains too. – Long Pham Jun 28 '18 at 14:48
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I was trying to figure this for few months and decided to ask here today. I also searched in google, but didn't know that I will be able to find results until I use X-Capacitor word. Thanks for your explanation – Aimkiller Jun 28 '18 at 15:07
@Aimkiller You are welcome! – winny Jun 28 '18 at 16:06

score 11 · Answer 2 · answered Jun 28 '18 at 14:58

This is a filter that sits between the mains power line and the SMPS. Its goal is to suppress electromagnetic interference (EMI) and (to some extent) protect the circuit from spikes on the line. The basic structure of such a filter looks like this:

These filters use specially-rated capacitors to handle the high AC voltage. The capacitors used between the hot and neutral lines (inputs 1 and 2, outputs 3 and 4) are called "X-class capacitors", or "X capacitors" for short. The capacitors used between a hot or neutral line and ground are called Y-class capacitors. The X and Y stuff is just a safety rating; it means the capacitors can survive the line conditions.

Also in the filter is a pair of coupled inductors which form a common-mode filter. This allows the differential line voltage through, but blocks any voltage spikes that are on both lines at once.

Thanks for you explanation. both answers explained it well for me. — Aimkiller, Jun 28 '18 at 15:10
The safety rating doesn't just mean the caps will survive the line conditions. It also means that if the cap is subjected to conditions it cannot survive, it will fail open rather than shorted. — supercat, Jun 28 '18 at 23:46

Why does an SMPS use LCR on INPUT?

2 Answers2