Y capacitor Query

[Techman_7]

Hello All,
For a AC input supply, there is limit of current flow through Y cap.

But, for DC input supply for DC converter or BLDC motor controller, I think there should not be limit of current through Y cap, so how to select Y cap for DC applications?

Is it only depend on common mode inductance w.r.t. filter cut off frequency?

 

[KlausST]

Y-capacitor for DC?? can you geve more details?

Y-Capacitors are conneted to EARTH-GND. And the AC current according V_AC / X_c should be below dangerous value.
But on DC ... the capacitor current is negligible.
And DC connected to EARTH-GND .. I´m not sure if this is a thing.

Klaus

 

[mtwieg]

It's not clear how what role the Y cap has in this "DC application". A schematic would be helpful.

The limit on current through Y caps is based on leakage current limits in safety standards. These leakage currents are typically due to the Y cap being connected to the mains in some way. Though in theory leakage currents can be present even without mains connections. Again, need a schematic to say more.

 

[D.A.(Tony)Stewart]

Can you describe your issue or uncertainty? I will try.

Ground, by definition, is 0V—a reference point we all agree on. Protective Earth (PE) ground, though, is a different beast. It’s not just any ground; it’s critical for safety and RF shielding. Y-capacitors, on the other hand, are specific to AC grid applications. They need to handle high grid voltages while keeping leakage current low—typically under 100µA, though specs vary by country (often <2.5mA max). Both AC-DC and DC-DC converters require attention to output common-mode (CM) noise, not just input CM noise.

Y-caps are designed for high-voltage (HV) input CM filtering, but since DC systems usually aren’t high-voltage, they don’t need the same HV-rated caps.
Take laptop chargers as an example of DC-DC caps preventing interference with PE ground. These chargers often have a 3-pin AC input with PE grounding but deliver an isolated, floating DC output, like ~19V. I ran into an issue at the University of Toronto while setting up a custom stepper gantry. We had a 12V AC-DC supply feeding an Arduino, connected via USB to an ASUS laptop with a 19.5V charger. The USB wouldn’t connect. Unplugging the charger fixed it. The culprit? High CM leakage impulse noise from the 19V charger, coupled with poor CM rejection in the USB interface. Switching to a PE-grounded PC tower worked because the PE ground’s low impedance suppressed the CM noise on the charger’s 0V line, thanks to the transformer’s small leakage capacitance (in pF). Other fixes could’ve included using a laptop with an external VGA monitor (also PE-grounded), adding a low-ESR capacitor (like a Y-cap or larger) between the laptop’s floating 0V case and PE ground, or grounding the laptop case itself. Another CM interference issue can pop up with external microphones picking up noise from the laptop charger at twice the grid frequency, causing a hum or buzz. PE grounding on laptops was once considered risky due to potential ground faults in appliances with high voltage, but this is less of a concern for stationary PC towers.

Common Mode
The transformer in a charger has coupling capacitance that generates significant CM noise on the laptop, especially with high-frequency (HF) leakage. If you touch a grounded appliance (like a stove or washer) while holding a plugged-in laptop, you might feel a “safe” 100µA leakage current. I’ve noticed this myself—not with my hand, but with my wrist or kneecap brushing the edge of a laptop while barefoot on grass. It’s a slight tingle or mild burn if the contact area is small. This is generally deemed safe by UL, CSA, and EU standards. If I had a Y cap to PE from the laptop to PE, the leakage might have been attenuated.

For DC-DC converters, the input needs a low source impedance to keep the converter happy, so low-ESR capacitors are a must for line regulation stability. But if you overdo the suppression, you might kill the noise feedback the converter needs to stay stable, leading to a loss of phase margin. Series ferrite beads are often added before the secondary input cap to balance things out.

Differential Mode
Here’s an anecdotal gem: I once design a “chaos noise generator” in the R&D lab with cascaded DC-DC converters on a PCB (step-down followed by step-up). It worked except I could hear running water in the lab from everywhere bouncing off the walls. It turned out to come a 1 cm round through-hole coil in a low-power 5V-to-12V boost converter acting as "voice coil" and like filtered white noise (brown noise)! The fix? A low-ESR differential bulk capacitor between the two DC supplies. Problem solved.

 

[FvM]

If you deal with isolated DC sources or loads, Y-capacitors are often used for common mode noise supression. High voltage DC applications (solar inverters, electric vehicles, storage batteries) involve a protective earth or chassis ground where Y:-capacitors can connect to.

 

[24vtingle]

I think there should not be limit of current through Y cap, so how to select Y cap for DC applications?

There's no actual way of exactly calculating it...but you know that common mode is high frequency so use a cap with low stray series inductance if possible.....and if you can do that with a high Faradic value cap , then thats all the better....
Say for example you have a DCDC in a metal enclosure...you need to connect the ground of the DCDC to the enclosure at one place....this can be a direct wire connection (ie an "infinite value capacitor"), or a capacitive connection.
Often you dont want a direct wire connections between GND and the metal enclosure because you dont want the enclosure to be electrically DC connected to the enclosure.

You might actually want the cap to fail open like a y capacitor so in that case you have to pick an actual y capacitor and then you are restricted to the low values that y capacitors come in.

Page 77 of this shows the Y caps in a DCDC...

https://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf

..But doesnt give the value....generally you shouldnt need that much since y cap noise is high frequency....and lower value caps tend to have less stray inductance.
You see people using 1 or 2nF's.....but it might be ok to use up to 1uF. But watch the voltage rating in relation to any transients that you may get between chassis and circuit gnd.

 

[Techman_7]

Y-capacitor for DC?? can you geve more details?

Y-Capacitors are conneted to EARTH-GND. And the AC current according V_AC / X_c should be below dangerous value.
But on DC ... the capacitor current is negligible.
And DC connected to EARTH-GND .. I´m not sure if this is a thing.

Klaus

Yes KlausST, the Y cap is connected to earth but input supply is DC. I think in this case cap impedance is not criteria for selection of Y cap value and we can connect any value of Y cap (limited to EMI filter cut off frequency).

Can you pls confirm this?

--- Updated Apr 20, 2025 ---

There's no actual way of exactly calculating it...but you know that common mode is high frequency so use a cap with low stray series inductance if possible.....and if you can do that with a high Faradic value cap , then thats all the better....
Say for example you have a DCDC in a metal enclosure...you need to connect the ground of the DCDC to the enclosure at one place....this can be a direct wire connection (ie an "infinite value capacitor"), or a capacitive connection.
Often you dont want a direct wire connections between GND and the metal enclosure because you dont want the enclosure to be electrically DC connected to the enclosure.

You might actually want the cap to fail open like a y capacitor so in that case you have to pick an actual y capacitor and then you are restricted to the low values that y capacitors come in.

Page 77 of this shows the Y caps in a DCDC...

https://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf

..But doesnt give the value....generally you shouldnt need that much since y cap noise is high frequency....and lower value caps tend to have less stray inductance.
You see people using 1 or 2nF's.....but it might be ok to use up to 1uF. But watch the voltage rating in relation to any transients that you may get between chassis and circuit gnd.

Thanks CupofTea.
But I think, Y cap value is limited to EMI filter cut off frequency.
Can you confirm this ?

--- Updated Apr 20, 2025 ---

It's not clear how what role the Y cap has in this "DC application". A schematic would be helpful.

The limit on current through Y caps is based on leakage current limits in safety standards. These leakage currents are typically due to the Y cap being connected to the mains in some way. Though in theory leakage currents can be present even without mains connections. Again, need a schematic to say more.

Its DC application. Y caps are connected between DC supply and earth. In this case I think any value of Y cap can be selected but limited by EMI filter cut off frequency.

--- Updated Apr 20, 2025 ---

Hello All,
For a AC input supply, there is limit of current flow through Y cap.

But, for DC input supply for DC converter or BLDC motor controller, I think there should not be limit of current through Y cap, so how to select Y cap for DC applications?

Is it only depend on common mode inductance w.r.t. filter cut off frequency?

I make one parameter clear here.
DC voltage range is 9V - 20V

--- Updated Apr 20, 2025 ---

Can you describe your issue or uncertainty? I will try.

Ground, by definition, is 0V—a reference point we all agree on. Protective Earth (PE) ground, though, is a different beast. It’s not just any ground; it’s critical for safety and RF shielding. Y-capacitors, on the other hand, are specific to AC grid applications. They need to handle high grid voltages while keeping leakage current low—typically under 100µA, though specs vary by country (often <2.5mA max). Both AC-DC and DC-DC converters require attention to output common-mode (CM) noise, not just input CM noise.

Y-caps are designed for high-voltage (HV) input CM filtering, but since DC systems usually aren’t high-voltage, they don’t need the same HV-rated caps.
Take laptop chargers as an example of DC-DC caps preventing interference with PE ground. These chargers often have a 3-pin AC input with PE grounding but deliver an isolated, floating DC output, like ~19V. I ran into an issue at the University of Toronto while setting up a custom stepper gantry. We had a 12V AC-DC supply feeding an Arduino, connected via USB to an ASUS laptop with a 19.5V charger. The USB wouldn’t connect. Unplugging the charger fixed it. The culprit? High CM leakage impulse noise from the 19V charger, coupled with poor CM rejection in the USB interface. Switching to a PE-grounded PC tower worked because the PE ground’s low impedance suppressed the CM noise on the charger’s 0V line, thanks to the transformer’s small leakage capacitance (in pF). Other fixes could’ve included using a laptop with an external VGA monitor (also PE-grounded), adding a low-ESR capacitor (like a Y-cap or larger) between the laptop’s floating 0V case and PE ground, or grounding the laptop case itself. Another CM interference issue can pop up with external microphones picking up noise from the laptop charger at twice the grid frequency, causing a hum or buzz. PE grounding on laptops was once considered risky due to potential ground faults in appliances with high voltage, but this is less of a concern for stationary PC towers.

Common Mode
The transformer in a charger has coupling capacitance that generates significant CM noise on the laptop, especially with high-frequency (HF) leakage. If you touch a grounded appliance (like a stove or washer) while holding a plugged-in laptop, you might feel a “safe” 100µA leakage current. I’ve noticed this myself—not with my hand, but with my wrist or kneecap brushing the edge of a laptop while barefoot on grass. It’s a slight tingle or mild burn if the contact area is small. This is generally deemed safe by UL, CSA, and EU standards. If I had a Y cap to PE from the laptop to PE, the leakage might have been attenuated.

For DC-DC converters, the input needs a low source impedance to keep the converter happy, so low-ESR capacitors are a must for line regulation stability. But if you overdo the suppression, you might kill the noise feedback the converter needs to stay stable, leading to a loss of phase margin. Series ferrite beads are often added before the secondary input cap to balance things out.

Differential Mode
Here’s an anecdotal gem: I once design a “chaos noise generator” in the R&D lab with cascaded DC-DC converters on a PCB (step-down followed by step-up). It worked except I could hear running water in the lab from everywhere bouncing off the walls. It turned out to come a 1 cm round through-hole coil in a low-power 5V-to-12V boost converter acting as "voice coil" and like filtered white noise (brown noise)! The fix? A low-ESR differential bulk capacitor between the two DC supplies. Problem solved.

Hello Tony,
The Y cap is between DC supply and earth and DC voltage range is 9V-20V.
I think, Y cap value is limited to EMI filter cut off frequency and not on leakage current.
Can you confirm this?

 

[D.A.(Tony)Stewart]

Hello Tony,
The Y cap is between DC supply and earth and DC voltage range is 9V-20V.
I think, Y cap value is limited to EMI filter cut off frequency and not on leakage current.
Can you confirm this?

Yes, there is no safety issue since it is low voltage DC, so the value is not limited by the safety ratings for leakage current which applies to Y caps with I = V*2pi f C e.g. 1nF @ 50 Hz 230V is 72 uA . If you can measure the impedance to PE ground and the CM noise level then you can compute the attenuation from the impedance ratio by choosing C with the pulse noise that comes from the switching rates observed.

 

[KlausST]

I make one parameter clear here.
DC voltage range is 9V - 20V

Y capacitors are safety capacitors .. to pretect human life.
but 20V is SELV .. it does not need safety capacitors.

So any capacitor is good, for example a ceramics capacitor. No need for safety cerfication.

Klaus

 

[Techman_7]

Y capacitors are safety capacitors .. to pretect human life.
but 20V is SELV .. it does not need safety capacitors.

So any capacitor is good, for example a ceramics capacitor. No need for safety cerfication.

Klaus

Thanks. But what about value ?

 

[KlausST]

Hi,

your headline says "Y capacitor query" ... "Y" means safety.
But there is no safety issue - no matter what value you use.

--> Use whatever value you want/need.

Klaus

 

[24vtingle]

Its pretty much a guess but take into account the stuff i explained you previously.
There is no picking it to accord with any "EMI cut off frequency" like you mentioned.

I am stupid because i still call them Y caps even when they are not y caps like in your LV case....otherwise , what do we call them?...i suppose "common mode filter caps", or "chassis caps".....i dunno.
Anyway, i used to work with the Engineer that designed many of the PSUs for the UK military....they told me that adding such "Y caps" and ferrite beads etc , in order to solve common mode noise, is like "throwing currants into a cake mix".
So basically, its dealing with all the strays, which are time consuming to calculate, so pretty well guess or use previous experience or some intuition.......eg, use the highest cap value possible but ensure that you dont have too much stray L in the cap.....so you may in fact want to use a lower value cap......or one low , one high, its up to you.