Full Bridge SMPS primary DC blocking capacitor

Full Bridge SMPS primary DC blocking capacitor...(capacitor in series with primary)

Does any reader know of a book that explains ......

1. Whather or not this capacitor is needed in Voltage mode control?
2. Whether or not this capacitor is needed in Current mode control?
3. Whether or not this capacitor is needed in Current mode control with added slope compensation?
4. What voltage rating this capacitor should be?
5. Does this capacitor charge up to the full rail voltage in overload situations where cycle-by-cycle currnet limiting occurs.

I have just bought the £64 book ("SWITCHING POWER SUPPLY DESIGN")by Pressman Billings and Morey, but it does not answer the above questions...other than just saying that a series capacitor is needed, and explaining how to derive its Faradic value.

Hi gentleman!
You can use marty brown power supply cookbook . it is incredible.
Best wishes
Goldsmith

Hi goldsmith,

thanks , i have seen that book, and it does not tell of the information of the top thread.

The capacitor's function is pretty much the same as it is in a half bridge converter. It's necessary when using voltage mode control in order to prevent staircase saturation. In current mode control the capacitor must not be used. In peak current mode operation the primary needs a DC current path.

This is discussed briefly in Switchmode Power Supply Handbook, 3rd ed (Billings and Morey). It doesn't seem to go into the ratings of the capacitor. But I imagine the most critical spec is RMS current handling. The DC voltage it has to block is just determined by the volt-time product imbalance applied by the bridge. The actual capacitance is probably not important, so long as it does not form any severe resonances near the switching or crossover frequencies.

In a modern design you'll probably be using current mode control so you don't need to worry about the capacitor.

tahnks mtwieg,

I presume that the capacitor is also not needed in current-mode-with-added-slope-compensation?

...because thats actually a combination of both curent and voltage mode, so you would think that maybe the capacitor was needed there?

...i appreciate the most critical spec is its current handling, but its also important to make this capacitor as small and cheap as possible, so we really need to get it as small as possible, with the lowest possible voltage rating.....the problem is , what is that voltage rating, and why?....no book on the planet seems to explain this.

If you want to implement both a voltage mode and peak current mode control, then you will have to take some special measures to protect against staircase saturation. Like what national semiconductor does with using average current mode as well.

Like I said, the voltage rating is determined by the volt-time product imbalance, which is a tricky thing to predict. Basically the largest difference in duty cycle between the two half cycles, multiplied by you DC bus voltage. It might be 2 volts or 20 volts. Can't really say without doing empirical measurements on a variety of devices.

We've concluded that a DC blocking capacitor is needed in series with the primary for the Voltage Mode Half Bridge, so in that case, do you know why the Voltage mode half bridge schematic on page 25 of the LM5039 datasheet does not include a DC blocking capacitor?

LM5039 datasheet
https://www.ti.com/lit/ds/symlink/lm5039.pdf

We've concluded that a DC blocking capacitor is needed in series with the primary for the Voltage Mode Half Bridge, so in that case, do you know why the Voltage mode half bridge schematic on page 25 of the LM5039 datasheet does not include a DC blocking capacitor?

Click to expand...

Hi Treez
Half bridge converter deals with two capacitors in series together that somehow will make the circuit ( and core ) balanced so DC blocking capacitor is not needed ! of course i've saw many of the circuits using that , but personally i've never used that , in a half bridge converter .
Best Wishes
Goldsmith

goldsmith said:

Hi Treez
Half bridge converter deals with two capacitors in series together that somehow will make the circuit ( and core ) balanced so DC blocking capacitor is not needed ! of course i've saw many of the circuits using that , but personally i've never used that , in a half bridge converter .
Best Wishes
Goldsmith

Click to expand...

Or to be more precise, the two bus splitting capacitors function as a DC blocking capacitor.