Y capacitor value for SMPS

Thread Starter

artmaster547

Joined Jan 6, 2016
410
Hi all,
I wondered if anyone could assist me, I am designing to different converters:
1) Flyback converter
2) Dual Active Bridge

1)With the first one I am trying to select the Y/X capacitor that will go across the GNDs of the primary and secondary side of the flyback transformer, however I cannot find any resources on how to calculate or select the capacitance of this capacitor can anyone provide me with information about this please?
2) With the second topology the same again I am trying to select the X/Y capacitors for the high frequency current path across the primary and secondary sides does anyone have any information on this please?

kind regards

Art
 

Bernard

Joined Aug 7, 2008
5,262
A few things that might be helpful:
Primary & secondary voltage
Power
Frequency
Allowable ripple.
 

Hymie

Joined Mar 30, 2018
708
Points to note:-

The schematic you attached has a rated output of 36W (12V * 3A) and not 2.5kW.

There are no X/Y capacitors shown on the schematic.
 

Thread Starter

artmaster547

Joined Jan 6, 2016
410
Points to note:-

The schematic you attached has a rated output of 36W (12V * 3A) and not 2.5kW.

There are no X/Y capacitors shown on the schematic.
hi yes sorry the flyback is 36W and the DAB is 2.5kW apologies for not making that clear the X/Y cap in theory should go across the grounds on primary and secondary side it is something I want to add but I want to know what value this capacitor should be for the flyback firstly
 

Hymie

Joined Mar 30, 2018
708
I see many schematics similar to this that incorrectly assign the 0V of the rectified primary circuit as Earth. There is no earth connection shown in the circuit; if the 0V of the rectified mains were to be connected to earth, a large fault current would flow as a result.

Only Y class capacitors should be connected between a primary circuit and earth (Y2), the limiting factor on the capacitor value is the resultant leakage current the capacitor is passing.
Based on mains voltage/frequency, at 50nF the leakage current would be somewhere around 3.5mA.
Typical Y2 capacitors (bridging basic insulation) on commercial units range from 1 – 10nF.

The capacitor bridging the reinforced barrier (primary to secondary) must be a Y1 class. The limiting factor on this capacitor value is the resultant leakage current (from primary to secondary) permitted by the applicable safety standard. Note that this leakage current will very much depend on the frequency of the voltage across the capacitor.
Typically Y1 capacitors (bridging reinforced insulation) on commercial units are limited to 2n2F.

The purpose of the above Y capacitors is to limit the rf interference/immunity in the PSU – ideally the minimum capacitance values achieving the required circuit performance should be used to avoid excessive leakage currents.
 

Bordodynov

Joined May 20, 2015
2,394
Here is a good article in Russian:
http://bsvi.ru/setevye-filtry-i-pomexopodavlyayushhie-kondensatory/
If there are difficulties with translation, then I will help.
It is possible to reduce the capacitor value Y by installing a shield in the transformer between the windings. The main thing is that this screen is not a short-circuited loop! There are other ways to reduce parasitic capacitance between the primary and secondary windings. For example, use two ferrite cores. On one core, the input winding, and on the second secondary. The connection between the windings through a short-circuited coil that passes through both cores. Structurally, it can be a screen that covers both cores. By the way, in medical power supplies Y-capacitors are not used.
and
http://bsvi.ru/uploads/9ef84c26f387_AC96/rfi_fact.pdf
 

Thread Starter

artmaster547

Joined Jan 6, 2016
410
G
Here is a good article in Russian:
http://bsvi.ru/setevye-filtry-i-pomexopodavlyayushhie-kondensatory/
If there are difficulties with translation, then I will help.
It is possible to reduce the capacitor value Y by installing a shield in the transformer between the windings. The main thing is that this screen is not a short-circuited loop! There are other ways to reduce parasitic capacitance between the primary and secondary windings. For example, use two ferrite cores. On one core, the input winding, and on the second secondary. The connection between the windings through a short-circuited coil that passes through both cores. Structurally, it can be a screen that covers both cores. By the way, in medical power supplies Y-capacitors are not used.
and
http://bsvi.ru/uploads/9ef84c26f387_AC96/rfi_fact.pdf
Great those resources are perfect thank so much :)
 

Thread Starter

artmaster547

Joined Jan 6, 2016
410
I see many schematics similar to this that incorrectly assign the 0V of the rectified primary circuit as Earth. There is no earth connection shown in the circuit; if the 0V of the rectified mains were to be connected to earth, a large fault current would flow as a result.

Only Y class capacitors should be connected between a primary circuit and earth (Y2), the limiting factor on the capacitor value is the resultant leakage current the capacitor is passing.
Based on mains voltage/frequency, at 50nF the leakage current would be somewhere around 3.5mA.
Typical Y2 capacitors (bridging basic insulation) on commercial units range from 1 – 10nF.

The capacitor bridging the reinforced barrier (primary to secondary) must be a Y1 class. The limiting factor on this capacitor value is the resultant leakage current (from primary to secondary) permitted by the applicable safety standard. Note that this leakage current will very much depend on the frequency of the voltage across the capacitor.
Typically Y1 capacitors (bridging reinforced insulation) on commercial units are limited to 2n2F.

The purpose of the above Y capacitors is to limit the rf interference/immunity in the PSU – ideally the minimum capacitance values achieving the required circuit performance should be used to avoid excessive leakage currents.
great thank you :)
 
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