Hi,

I'm powering a high power LED (lumled) via a MOSFET from a PIC. The MOSFET is switching the LED load at 125KHz (fixed). The LED load when on is 2.56Amps (measured) and the entire ciruit is powered from a mini ATX 100W supply +3.3V. I've noticed if the ripple is greater than 0.5 volts, it seems to effect the LED and so I'd like to remove this on my board.

When I put a scope on the supply it's generating ripple during the LED switched on period and is affecting other logic circuits (i.e. the control conputer).

I have searched the web about L/C power filters and PIE filters but trouble is most talk about transformers (half/full rectified) and the ones that talk about switchers often have access to parameters I don't know about my PC supply.

So can someone suggest a way I can *easily* calculate the inductor (H) and capactor (C) in an effort to attenuate as much of this ripple as possible? I love worked examples if you have time!

Thanks

Dan

 

Can you provide a sketch of the circuit for us to examine? We can and will guess at a fix but I can tell you from experience that a schematic will clinch the deal.

hgmjr

 

Hi,

I'm powering a high power LED [...] switching the LED load at 125KHz (fixed).

If you drop the frequency to something closer to 125 Hz instead, the ripple will probably not cause (too much) trouble, since several cycles will average out (I assume that your problems is some kind of aliasing effect).

If you insist on using an LC (or CLC) filter, just go for the largest inductance that will fit and is able to handle the current. For the cap(s), it's better to go with several smaller than one large. to lower the total ESR.

In short - Just grab what you have available, the larger the L and C, the lower the filter frequency.