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Why is half-bridge capacitor rated to 400V?

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grizedale

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Hello,

Why is the 1uF/400V capacitor used in the follwing schematic?

https://i56.tinypic.com/124kbvp.jpg


...i am speaking of the capacitor in series with the primary winding of the half-bridge smps.

This capacitor only needs to be rated to 250V.

The DC Bus (primary side ) is 310V.

Why have they overreated it...these such film caps are very big and expensive, and theres no need to use a 400V version.
 

A lot of people will derate 2X on capacitor BV. Do you live
in a nominal world where manufacturers ship only defect
free product?
 

There may be many reasons for that... for example:
- The line protection is specified at 275 V (nominal operation voltage), however, during transient surges or other events this voltage is exceeded, provably clamping at 300 - 350 V, this is why most of the elements are rated at 400 V. This circuit may be designed for real life applications where you can expect line overvoltages, surges, etc, not a perfect 220 V RMS Sinewave input! In addition, if you want to meet some EMC requirements you have to test the device (and survive) with voltage surges, burst, etc.
- Capacitor aging depends on working voltage, using higher voltage capacitor would reduce derating compared to a lower voltage one...
- There may be other reasons.
 
Isn't the 1uF capacitor a blocking cap put there to avoid flux walking? It is required to be of a non polarized type as it has to carry bidirectional current. Its rating should be specified as an example 1uF/250Vac.

For example checkout cap C102 in this application note

Regards :)
 

technoqwerty123 said:
Isn't the 1uF capacitor a blocking cap put there to avoid flux walking? It is required to be of a non polarized type as it has to carry bidirectional current. Its rating should be specified as an example 1uF/250Vac.

For example checkout cap C102 in this application note

Regards :)
Click to expand...
This is what I was thinking. The cap's AC rating is determined by the rectified line input voltage, but its DC rating is often much higher than the AC rating. So if that 400V is the DC rating then the AC rating is probably around 250 or 275Vrms, which fits fine.
 
technoqwerty123

The cap to which you refer does stop flux-walking.

In your FSFA2100 device, the cap looks to be performing some type of resonat activity aswell.

That PWM controller looks very interesting indeed.

Its a fixed frequency ZVS controller for half bridge and flyback., as you know.

So i woudl say your cap is mostly for resonance reasons.....but yes cuts out dc component too.

Also, i wonder why Resonant converters like FSFA2100 never ever have multiple outputs.?
 
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In your's schematic, power supply section is not shown. here is a similar circuit

When there is asymmetrical load in a half bridge supply that is when one side is drawing more current , float poin is pulled to that side. Now filter capacitor are also not operating at 1/2 Vcc. In addition to other factors mentioned already, this is also a situation.
 
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