SMPS Mains Filter

[codex653]

ok so i've gotten 2 designs completely worked out for some SMPS's, one being a buck converter, and the other being a buck boost...i'm gonna be using them for an amplifier i made, so i needed both the +/- voltages, in case you were wondering about the buck boost design.

anyways on to the real question....i was looking around at different stuff online and apparently it is a good idea to put a mains filter before the diode bridge so that you don't get some high frequency currents going back into the mains wiring...apparently that can screw some things up. one of the circuit diagrams i came across was this right here:

http://www.lcdmonitorrepairebook.com/wp-content/uploads/2010/04/smps-schematic.png

now, this schematic is not for the power level i will be using (200 watt or so per polarity on my SMPS) , nor is it the actual circuitry for the SMPS...i was just looking at the filter on the mains wiring and components.

Is the filter used on this circuit a pretty typical design? I've taken apart some computer monitors before and have seen something very similar to this inside of them, i just never realized what purpose they served. Is there any specific math that goes along with designing these types of filters? I'm sure there is some sort of math, as there is for anything in electronics, but does a filter require particular values of components, or is it just something like "use 'such and such' range of capacitance, and 'such and such' range of inductance and you will be just fine"?

i've never really delved into power supply filters before as i have not needed them until now, so any help would be appreciated!

 

[tcmtech]

I use them on some of my homemade power supplies and grid tie inverter units. Basically just find one from something that used a similar amount of power and call it good.

Technically you can get real detailed and nit picky about the over all filter design though but most of the time its not necessary unless some odd problem pops up.

So why do you need a buck and a boost SMPS? I would just stack the outputs of two of what you posted here and call it good. It looks like a simple and easy to replicate design that may be possible to be built out of some reworked computer power supply's.

I have seen a few of them that have basically the same primary power circuits and use the same control IC. Its just a matter of rewinding the transformers and redesigning their secondary circuits to be a single voltage output that way you get the benefit of a pre made primary side and a good power to size ratio with minimal poke and guess parts matching behind it that comes with its own shielded case and built in cooling system.

Its just a thought.

 

[codex653]

like i said, the circuit in the schematic i posted is not the actual switching circuitry i'm using....as for the reasons for a separate buck and buck boost is because of the power requirements of my amplifier...it requires a +/- 30v input....so i have the buck converter for the positive polarity, and the buck boost converter (aka, inverting buck) to get the negative polarity. and yes, the supply will have it's own cooling and shielding system..haha don't want any of that EMI getting into the amplifier circuitry! especially with it being in such close proximity!

 

[tcmtech]

I follow the need for the positive and negative power requirements, I have rebuilt and modified many an amplifier in my day, but for the most part that can be done with two identical power supplies by connecting the negative of one to the positive of the other and using that as the mid point common for the power supply. That way you have the equivalent of a center tapped 60 volt power source but still maintain the independent regulation for each section.

The other option is to use two secondary windings set up to make the +- 30 volts and only use the voltage sensing off one for the feed back to the primary side for regulation. Thats how car audio amplifiers typically do it being the power output for music typically has a naturally balanced load for each half of the power supply.

 

[codex653]

oh! huh i never thought of that! oh well, it's too late now haha i'll keep that in mind for future projects! thanks!