Transformer Noise Issues

Transformer audible noise usually results from pushing the core too hard, loose windings, etc.  Varnishing and vacuum impregnation can help in some circumstances.

I've attached a PI Expert design file with an optimized EF20 core transformer.  Hopefully this gives you a better starting point.

Regards,

The Traveler

HI,

there is no attachment

Sorry about that.

-The Traveler

I located an early Power Integrations Application Note that address audio noise suppression.  I've attached it to assist you with your efforts in reducing transformer audbile noise in your design.

Regards,

The Traveler

okay, thank you

Dear sir, i need urgent help for my lnk626 based psu. i have dessigned dual 5 volt output circuit using LNK626 ic and according to piexpert suite i have constructed trasforner. i suppose to get 5 volt at output but im getting 16 volt and 20 volt at v1 & v2. i have used exact values what i got from piexpert but i have repalced with sMD pkg. please go though with my design. 

Hallo, can anybody tell me what the role is from the capacitor placed beween the primary and the secondary winding (see example.pdf) ?

Where can I find more information about that topic?

Best regards

Switched mode power supplies use what is known as a "flyback converter" to provide voltage conversion and galvanic isolation. A core component of this converter is a high frequency transformer.

Practical transformers have some stray capacitance between primary and secondary windings. This capacitance interacts with the switching operation of the converter. If there is no other connection between input and output this will result in a high frequency voltage between the output and input.

This is really bad from an EMC perspective. The cables from the power brick are now essentially acting as an antenna transmitting the high frequency generated by the switching process.

To suppress the high frequency common mode is is necessary to put capacitors between the input and output side of the power supply with a capacitance substantially higher than the capacitance in the flyback transformer. This effectively shorts out the high frequency and prevents it escaping from the device.

When desinging a class 2 (unearthed) PSU we have no choice but to connect these capacitors to the input "live" and/or "neutral". Since most of the world doesn't enforce polarity on unearthed sockets we have to assume that either or both of the "live" and "neutral" terminals may be at a sinificant voltage relative to earth and we usually end up with a symmetrical design as a "least bad option". That is why if you measure the output of a class 2 PSU relative to mains earth with a high impedance meter you will usually see around half the mains voltage.

That means on a class 2 PSU we have a difficult tradeoff between safety and EMC. Making the capacitors bigger improves EMC but also results in higher "touch current" (the current that will flow through someone or something who touches the output of the PSU and mains earth). This tradeoff becomes more problematic as the PSU gets bigger (and hence the stray capacitance in the transformer gets bigger).

On a class 1 (earthed) PSU we can use the mains earth as a barrier between input and output either by connecting the output to mains earth (as is common in desktop PC PSUs) or by using two capacitors, one from the output to mains earth and one from mains earth to the input (this is what most laptop power bricks do). This avoids the touch current problem while still providing a high frequency path to control EMC.

Short circuit failure of these capacitors would be very bad. In a class 1 PSU failure of the capacitor between the mains supply and mains earth would mean a short to earth, (equivalent to a failure of "basic" insulation). This is bad but if the earthing system is functional it shouldn't be a major direct hazard to users. In a class 2 PSU a failure of the capacitor is much worse, it would mean a direct and serious safety hazard to the user (equivilent to a failure or "double" or "reinforced" insulation). To prevent hazards to the user the capacitors must be designed so that short circuit failure is very unlikely.

So special capacitors are used for this purpose. These capacitors are known as "Y capacitors" (X capacitors on the other hand are used between mains live and mains neutral). There are two main subtypes of "Y capacitor", "Y1" and "Y2" (with Y1 being the higher rated type). In general Y1 capacitors are used in class 2 equipment while Y2 capacitors are used in class 1 equipment.

So does that capacitor between the primary and secondary sides of the SMPS mean that the output is not isolated? I've seen lab supplies that can be connected in series to make double the voltage. How do they do that if it isn't isolated?

Some power supplies have their outputs hard-connected to earth. Obviously you can't take a pair of power supplies that have the same output terminal hard-connected to earth and put them in series.

Other power supplies only have capactive coupling from the output to either the input or to mains earth. These can be connected in series since capacitors block DC.

answared by Peter Green , 

http://electronics.stackexchange.com/questions/216959/what-does-the-y-capacitor-in-a-smps-do

Hi Santhosh,

Thank you for choosing Power Integrations.

Was the output voltage taken with no load connected to the output? You would need at least a 2kohm preload resistor at the output. Here is an excerpt from the linkswitch-CV application note which discusses pre-load resistor and output voltage.

"For designs where output voltage regulation must be maintained at zero load, start with a resistor value that represents a load of approximately 25 mA at the nomimal output voltage. For example, for a 5 V output use a pre-load resistor value of 2 kohms. For designs where the output voltage can rise under no-load conditions, select the pre-load resistor value such that the output voltage is within the maximum output voltage specification. Limit the maximum voltage rise at no-load to <50% of the normal output voltage to minimize increases in input power due to increases in the primary clamp and bias winding dissipation. Since a pre-load resistor also increases the no-load losses, where the specification allows, adjust the no-load voltage to trade-off lower no-load input power with high no-load output voltage as needed."

You may refer to the link below for the application note.

https://ac-dc.power.com/sites/default/files/product-docs/an45.pdf

hello,
now that voltage regulation problem fixed by adding that load resistor. now the problem with output current as soon as load connect output voltage drop about 0.5vp-p and noise come from transformer (because of core loose ) is this core loose will effect on output current ? and now how to make a stable power supply using this design ?

Hi Jom,

For more information on the Y-capacitor (the capacitor placed between the primary and secondary windings) including safety implications, you can refer to the application notes (AN-15) below:

https://ac-dc.power.com/sites/default/files/product-docs/an15.pdf

Hope this helps.