Y-rated capacitor

[CheapRussianBeer]

Hi guys. When you look at offline flyback/bridge/pushpull etc. converters you often see a "Y-rated" capacitor between primary and secondary coils like one shown on the second attached picture. Existence of that cap does not spoil galvanic isolation, so the circuit is decoupled even though technically there is a cap between input and output. I've got a philosophical question: What if you use same capacitors as supressing/current limiting(not sure what these are properly called), by putting them in series with load, will that circuit be considered as galvanically isolated or not? If so why has no one used that design in very low power offline supplies?

[SeanB]

You can do that, but it will still only be possible to do the isolation with very small value capacitors, so the current into the load is under the threshold of causing damage. thus if your circuit can operate on under 500uA maximum, and is fine with much less, then you can make an isolated supply using class Y capacitors as isolation.

Not sure on the exact value for current, but it is pretty low to allow you to have the double isolated rating and still have class Y capacitors in there, and typical capacitor values are under 1nF in most cases for that between line and earth and neutral and earth.

[Benta]

The Y capacitors are not connected to the output, they are connected to ground.

When that's said, you'll find capacitor-drop power supplies in a lot of applications, but only when the load and all other connections are fully isolated.

[Zero999]

The Y capacitors are not connected to the output, they are connected to ground.

No, Y capacitors can be connected to the output or ground. In a double insulated power supply, there's no earth connection and the Y capacitors are connected between the primary and secondary side. The Y capacitors divert and RF capacitively coupled across the transformer back to the primary side.

Only Y1 capacitors fulfil the requirements for reinforced insulation and is the only type of capacitor allowed for connecting the primary side to the secondary in a power supply. Theoretically two Y2 capacitors could be used in series but in practise no one bothers.

http://www.kemet.com/Lists/FileStore/900%20Series%20Product%20Training%20Module.pdf

[DBecker]

The Y capacitors are not connected to the output, they are connected to ground.

It is common to see Y capacitors coupling the line to the output.  This couples any RF picked up on the low voltage side to the earth-referenced AC supply.

The capacitor is commonly placed on the side of the line most likely to be connected to neutral.

[Benta]

I was commenting on the schematic the OP posted.

It's still not a good idea to build a power supply based on Y-capacitors, and it will NOT provide isolation.

[CheapRussianBeer]

SeanB
There are capacitors up to 10nF available with Y1 rating(almost 2$/piece, yet still) And theoretically you can connect them in parallel.

on 50Hz 10nF will give you 1/2*Pi*f*C = 1/6.28*50*0.00000001 = 318kOhm, with 220V rms in my power socket it can provide almost 0.154W of pure power! That is a lot.

Benta
Y capacitors are connected not to ground, but to the device housing which is in direct access to users hand. And of course it is a good idea to build a supply like that, who are you taking me for? It's a brilliant idea!

[Benta]

It's a brilliant idea!

Yes, and one that's been built thousands of times. Capacitor-drop PSUs are used all over the place, but are always 100% galvanic isolated from the users.
Normally X2 capacitors are used (they have higher value), for 12 V, 20 mA @ 230 V input, 470 nF is normal.

There is NO galvanic isolation in such a circuit.

[CheapRussianBeer]

Benta
So, placing a cap between primary and secondary in flyback converter is normal, a cap between casing and AC phase is ok, but you can't use same cap as current limiter? Sounds a bit unreasonable to me. Pic in OP-post is not very related, since I've found it in google right before starting the topic.

[Benta]

Your original question was:

What if you use same capacitors as supressing/current limiting(not sure what these are properly called), by putting them in series with load, will that circuit be considered as galvanically isolated or not? If so why has no one used that design in very low power offline supplies?

I think I've answered all of that.

[T3sl4co1l]

Galvanic isolation depends on the standard defining it, but it's usually: a touch or ground-leakage or isolation-leakage current of a few mA, maximum, at the highest possible line or isolation-barrier voltage.

While using Y-caps as coupling capacitors might count as galvanic isolation, you can only transmit as much power across that barrier as there is leakage current.  Not particularly handy.

Tim

[Zero999]

So, placing a cap between primary and secondary in flyback converter is normal, a cap between casing and AC phase is ok, but you can't use same cap as current limiter? Sounds a bit unreasonable to me. Pic in OP-post is not very related, since I've found it in google right before starting the topic.

Of course you can use Y capacitors for current limiting but the current is tiny so it's not worth it.

Y capacitors can also be used to couple much higher frequencies. Here's an example of a solid state relay circuit which does that. The Y1 capacitors can be small 100pF because the frequency is high.

There are capacitors up to 10nF available with Y1 rating(almost 2$/piece, yet still) And theoretically you can connect them in parallel.

on 50Hz 10nF will give you 1/2*Pi*f*C = 1/6.28*50*0.00000001 = 318kOhm, with 220V rms in my power socket it can provide almost 0.154W of pure power! That is a lot.

That would be a bad idea because the leakage current would most likely exceed the legal limits.

Also, the leakage current would be given by the values of the two capacitors in parallel but the usable current is calculated from both capacitors in series.

The maximum allowable leakage current for a double insulated appliance is 0.25mA, which means the maximum current your device can use is half of that: 0.125mA.
http://www.pat-testing-expert.com/support/leakage-tests.php
http://slpower.com/reference/An113%20Leakage%20Current.pdf