Electronic – smps transformer capacitance effects

switch-mode-power-supplytransformer

What are effects of interwinding capacitance in smps power transformer? What is the influence? For example, i have 83khz smps transformer with windings made of litz wire. The capacitance between windings is 250pF (i measured it by short-circuiting both windings and using u1732c agilent RLC meter). Is it good or bad?
Is electrostatic shield between primary and secondary helpful to reduce value of inter-winding capacitance?

Best Answer

What are effects of interwinding capacitance in smps power transformer?

The easiest way to think about the main noticable effect is to lump that distributed capacitance into one component: -

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Then consider what mid-point voltage you have on the primary and what the possible impedance to earth is: -

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At the switching frequency, the peak-to-peak voltage on the primary winding might be 230V x 1.414 = 325 volts. This is the rectified DC bus voltage converted into a square wave at the switching frequency. The "effective" mid point voltage is about 160 Vp-p (an 83 kHz square wave approximately).

For an SMPS, the incoming lines are either coupled via capacitors to earth or live/neutral is regarded as "earth". We can now paint a scenario where we have 160 Vp-p at 83 kHz coupling via 250 pF to the output secondary winding. The scenario is likely to have very little impedance where I've shown a resistor and capacitor in parallel because of the switching frequency: -

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This of course creates a lot of common-mode noise on the output of the SMPS and what manufacturer's do is add capacitors from the output to ground (if ground is available) to form a high frequency potential divider with the 250 pF interwinding capacitance. Thus, the high frequency 160 Vp-p seen on the secondary is reduced considerably with respect to earth and the device passes EMC testing.

When the SMPS uses a two-pronged earthless AC connection, a capacitor is usually added from secondary back to the "steady side" of the primary in order to try and flush the output noise down to earth via AC wiring. This produces a knock on effect of now seeing AC mains frequencies on both output wires of the SMPS - the subject of many questions on this site.

Typical example used by Power Integrations: -

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Note the 2.2 nF 250V AC capacitor just above the transformer - this is that capacitor used to reduce output noise but also leaches 50/60 Hz to the output.

Is it good or bad?

It is what it is. Bigger transformers will have more capacitance and this cannot be avoided sometimes.

Is electrostatic shield between primary and secondary helpful to reduce value of inter-winding capacitance?

Only if you can connect it to earth - if you can't it probably makes the primary-secondary cross-coupling even greater.

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