Here is a question.
The absolutely majority flyback converter schematics I have seen have the Y capacitor value of 1nF or 2.2nF. This value is chosen probably for meeting the legal requirement of maximal leakage allowance and for prevention from unwanted shocks.
If for a moment we take out the legal requirement from consideration, and look at it purely from a circuitry function point of view, how large the Y capacitor can be before it starts to reduce efficiency or to compromise the function of the flyback converter?
I ask this because every time we double the size of the Y capacitor value, the CM noise would be theoretically reduced by 6dB, which is a lot!
The absolutely majority flyback converter schematics I have seen have the Y capacitor value of 1nF or 2.2nF. This value is chosen probably for meeting the legal requirement of maximal leakage allowance and for prevention from unwanted shocks.
If for a moment we take out the legal requirement from consideration, and look at it purely from a circuitry function point of view, how large the Y capacitor can be before it starts to reduce efficiency or to compromise the function of the flyback converter?
I ask this because every time we double the size of the Y capacitor value, the CM noise would be theoretically reduced by 6dB, which is a lot!
I will have to dig into the IEC relevant standards. If what you say is true, then every powered USB hub is only Class III compliant when not connected to a desktop PC.
This is the 'generic' cap in question on a unit I have been working on.
It can not be made bigger in value because to do so causes excessive current to be available to flow from chassis to ground when such a path exists. In other words via an unsuspecting user.
If the unit is first coupled up to other equipment that actually does complete a legitimate and direct ground path then the problem becomes one of excess ground current flowing. If all that equipment is connected to a 3 pin outlet that includes earth leakage detection then, depending on the size of the cap, the leakage current may approach or exceed the trip value.
It can not be made bigger in value because to do so causes excessive current to be available to flow from chassis to ground when such a path exists. In other words via an unsuspecting user.
If the unit is first coupled up to other equipment that actually does complete a legitimate and direct ground path then the problem becomes one of excess ground current flowing. If all that equipment is connected to a 3 pin outlet that includes earth leakage detection then, depending on the size of the cap, the leakage current may approach or exceed the trip value.
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Yes. The 680pF in your schematic is low enough. On my DVD player it has 470pF. But I have seen many schematics having 1nF and some 2.2nF. The higher the value of the capacitor is the higher the leakage current is and the lower the EMI/CM noise is.
My thought is this. Commercial EMI filters (Corcon, Schaffner, etc) can use 2 x 2.8nF Y capacitors that generate about 0.4mA leakage current. So if I increase this capacitor to 2.2nF or even 3.3nF, it is not going to be worse than an EMI filter. In combination with an EMI filter, it may have 0.6mA leakage current. This is still very far from the 30mA needed to trigger the house safety switch (RCD). Provided that the chassis is earthed this should be OK. That is exactly the reason I want to earth my DVD player chassis.
My thought is this. Commercial EMI filters (Corcon, Schaffner, etc) can use 2 x 2.8nF Y capacitors that generate about 0.4mA leakage current. So if I increase this capacitor to 2.2nF or even 3.3nF, it is not going to be worse than an EMI filter. In combination with an EMI filter, it may have 0.6mA leakage current. This is still very far from the 30mA needed to trigger the house safety switch (RCD). Provided that the chassis is earthed this should be OK. That is exactly the reason I want to earth my DVD player chassis.
Hi Mooly and others,
I hope you can help me with one more question. I want to install an EMI filter into the DVD player. The best found so far is the Schaffner FN2090-1-60. But this is rated for 1A current. The 3A version is too large to fit into the DVD player.
I found the DVD player has a 2A fuse. Would a 1A rated EMI filter be good enough? I guess so but am not sure. For a linear supply, 1A at 240V is good for 10A at 24V, but for SMPS, I can't be 100% sure because it operates completely differently. The SMPS supply has +5V 2.5A output, +12V 1.6A output and -12V 0.1A output.
I hope you can help me with one more question. I want to install an EMI filter into the DVD player. The best found so far is the Schaffner FN2090-1-60. But this is rated for 1A current. The 3A version is too large to fit into the DVD player.
I found the DVD player has a 2A fuse. Would a 1A rated EMI filter be good enough? I guess so but am not sure. For a linear supply, 1A at 240V is good for 10A at 24V, but for SMPS, I can't be 100% sure because it operates completely differently. The SMPS supply has +5V 2.5A output, +12V 1.6A output and -12V 0.1A output.
I would think a 1A filter would be fine. The high fuse value is to prevent nuisance blowing either at start up or during any 'brownouts' of the mains supply.
The SMPS will draw its minimum current on 240v mains, if you used it on 110v then the current draw would more than double in order to maintain the same power delivery to the load. Probably still under 1A average though.
The SMPS will draw its minimum current on 240v mains, if you used it on 110v then the current draw would more than double in order to maintain the same power delivery to the load. Probably still under 1A average though.
good luck with your project.- Status
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