TNY256 - what is that "Y1 Safety" capacitor for ? - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st June 2012, 05:25 PM   #1
nowhere is offline nowhere  Israel
diyAudio Member
 
Join Date: Jun 2012
Default TNY256 - what is that "Y1 Safety" capacitor for ?

Hello,

I stumbled upon TNY256 chip: http://www.powerint.com/sites/defaul...ocs/tny256.pdf .

The chip is apparently used in many of those small switching power supplies plugged directly into electrical outlet.

On page 7 of the datasheet I see "Figure 12. 5.5 W AC Adapter with Universal Input (85-265 VAC)." - typical application circuit. And on page 8 "Figure 13. Recommended PC Layout for TinySwitch without Under-Voltage Lock Out Resistor.".

I don't understand why/what for "C5 2200 pF Y1 Safety" on page 7 is needed. I.e. I don't understand why capacitive connection between primary and secondary network is needed, especially taking into account that there is already parasitic connection through windings of T1 transformer.

Or that's the purpose - to protect from T1 parasitic capacitance of from breakdown, i.e. from possible electrical discharge going through insulation between primary and secondary winding of T1 ?
  Reply With Quote
Old 21st June 2012, 05:30 PM   #2
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Its to reduce common mode conducted/radiated EMI.
  Reply With Quote
Old 21st June 2012, 05:35 PM   #3
nowhere is offline nowhere  Israel
diyAudio Member
 
Join Date: Jun 2012
Quote:
Originally Posted by wrenchone View Post
Its to reduce common mode conducted/radiated EMI.
Your reply doesn't explain the word "safety". The word is typically used in conjunction with implied danger, and what is that implied danger is the thing I want to find out.
  Reply With Quote
Old 21st June 2012, 06:07 PM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
The text says it is part of an EMI filter. Not sure how this works, but as the cap connects the output to live mains it has to be a safety cap.
  Reply With Quote
Old 21st June 2012, 06:14 PM   #5
nowhere is offline nowhere  Israel
diyAudio Member
 
Join Date: Jun 2012
Quote:
Originally Posted by DF96 View Post
The text says it is part of an EMI filter. Not sure how this works, but as the cap connects the output to live mains it has to be a safety cap.
Yes and no. The "no" part is that EMI most of the times is mentioned in conjunction with intentionally introduced frequency jitter and just one time in conjunction with the capacitor in question.
  Reply With Quote
Old 21st June 2012, 06:26 PM   #6
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
The cap has to be rated for safety - an ordinary disc cap is not OK, even if it's rated for high voltage. In this case, a Y1 rating is needed, meaning that it is certified by the various safety agencies for to safely bridge the primary-secondary barrier and is 100% tested for a high withstand voltage. You can find out more about safety and EMI issues by checking out AN-15 at the PI web site. And yes, iit's an EMI filter cap and is instrumental in reducing common mode EMI. The transfomer parasitic capacitance from primary to secondary actually is the cause of a lot of the common mode EMI, the transformer primary being a prinncipal source of noise. The cap forms a short return path from secondary return to primary. Proper transformer construction is also instrumental in reducing common mode EMI.

Last edited by wrenchone; 21st June 2012 at 06:31 PM.
  Reply With Quote
Old 21st June 2012, 07:25 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
The jitter spreads out the interference, thus getting it under the required level set by the relevant EMC standard. This seems to be the modern way of 'reducing' interference - just spread it out so more spectrum users get a bit less interference each. I think this could be regarded as an example of malicious compliance. Spark transmitters are banned, yet jittered SMPS have a similar effect and are accepted.
  Reply With Quote
Old 21st June 2012, 07:49 PM   #8
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Jitter only gives you about a 5-6dB reduction in EMI at best. It can help to reduce the size of some EMI components (assuming layout and everything else is good, but it does not render an EMI filter unnecessary.
  Reply With Quote
Old 21st June 2012, 07:53 PM   #9
diyAudio Member
 
Join Date: Jan 2012
Location: Pennsylvania
Blog Entries: 1
The engineer(s) who designed this circuit will be the best judges on the value of said capacitor. Is it for health safety [connected between RTN and power line] and/or EMI reduction?.
  Reply With Quote
Old 21st June 2012, 09:16 PM   #10
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Excuse me, I am one of the engineers...The capacitor is in the circuit for the purpose of EMI reduction, and needs to be proply safety rated (Y1) because it bridges the primary-to-secondary barrier. It has no safety benefit. It one of the first things to add for common mode noise reduction.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Y1 and Y2 Capacitor in SMSP circuit Sara001 Power Supplies 1 4th June 2012 01:42 PM
Seen the New "SawStop" Table Saw Safety Device? Andersonix Equipment & Tools 3 25th April 2005 08:02 PM
Oscilloscopes and safety measures. (Split from "I want to buy an Oscilloscope") Jennice Parts 36 1st March 2005 06:41 AM
SMPS -- "safety and the 5 cent zener" jackinnj Solid State 2 28th October 2004 02:44 AM


New To Site? Need Help?

All times are GMT. The time now is 08:18 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2