Switching power supply Y capacitor between primary and secondary ground - Page 1
Forum section: electronics / beginners

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 18, 2015, 02:04:57 pm
content: Hi there,first of all - I would like to say that I am having very basic knowledge on switching power supplies. I am just a newbie on this topic.I just found an old DELL laptop charger and I decided to try to understand how it works.I am proud that I managed by myself to understand what is doing what in this power supply (of course not in details, but I wasn't completely lost.)I started reverse engineering it, but then I found somebody have already done this.Here is the schematic:http://320volt.com/wp-content/uploads/2012/01/schema-circuit-dell-pa12-19v-notebook-adapter-smps-1d07012-3-34a.pngI would like to ask if someone can explain in details the role of the capacitor and the 40M resistor between primary and secondary ground.I know it has to do with filtering of hi frequency noise - but I'd like to know more details about it.Thanks in advance!

author: katzohki
author_rank: Frequent Contributor
timestamp: September 18, 2015, 05:32:28 pm
content: Here's a couple resources that might help explain:http://powerblog.vicorpower.com/2013/06/what-are-y-capacitors/http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdfX and Y capacitors have to be safety rated.

author: jitter
author_rank: Frequent Contributor
timestamp: September 19, 2015, 05:42:20 am
content: Quote from: pplaninsky on September 18, 2015, 02:04:57 pmThe Y-class cap is commonly found in switch mode power supplies between primary and secondary for EMC reasons. It will give HF stray currents a short path back to the primary to prevent radiated emissions from the secondary side. Without it, the leads on the secondary side could radiate this HF leading to interference.Google "capacitor between primary and secondary" and you will find technical articles on this.

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 19, 2015, 05:51:44 am
content: Hey guys,I did Google it and read a lot about the Y caps.Thanks!However, I couldn't find any information about the 40M resistor.All examples I looked into just have the Y cap and not other connection between primary and secondary.I suppose this 40M resistor is some kind of safety feature - but still the idea eludes me.Thanks!

author: C
author_rank: Super Contributor
timestamp: September 19, 2015, 06:35:43 am
content: 40M resistorIf you have a signal that is not balanced you can build a DC offset.A square wave = 0 DC offset, but any change from this and you have a offset.Motor drives and PWM in place of DAC use this.Can be a big problem on communication lines. For 8b/10b one of the bits is used to maintain balance across the 10 bits.As you lower that resistance you could be causing ground loops with other things connected to laptop.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2015, 03:52:27 pm
content: The 40 M rsistor can disipate static electricity. So the resistor prevents the Y-capacitor to accumulate to much DC voltage, which could be a shocking experience to the user or damage the computer (ESD) or the y-cap.

author: Phoenix
author_rank: Frequent Contributor
timestamp: September 20, 2015, 04:00:44 am
content: The resistor chain stops the secondary from floating away from the primary. Because it is a chain of high value resistors it can be considered a high impedance isolation barrier from a safety standard perspective. Ie a hazardous voltage on the primary side will not present a hazard to users or equipment on the secondary side with a single failure (considering creepage clearance and impedance).

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 20, 2015, 01:13:02 pm
content: Thanks for all the replies!I think I got the idea with the 40M resistor chain.