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EEYORE

Mon Mar 30 2009, 04:36PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello all,
So I was looking around at the vicor app notes and found this:

It is about making an output filter to greatly reduce the ripple. I am seeing 1vp-p ripple on my output. I could greatly reduce this with an inductor and two tantalum capacitors. Problem is, is that they call for two 270uF capacitors! Thats $$$ methinks for tantalums of high quality.
They dont give any sort of equation to relate results to parts used. Does anyone here have any experience with Pi filters? (Thats what this is). Could I get away with cheaper parts? Also, they dont mention whether or not the inductor HAS to be an air core or not. If an air core is fine, I could easily wrap one with some 10awg wire.

My laser doesnt seem to mind the ripple, but I just hate knowing I could make something better

Thanks!
Matt

...

Mon Mar 30 2009, 06:54PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

68ufd tantalum isn't actually that expensive, especially if you only need one good for 5v you can get one for less than $.5 from mouser. A 30v 100mR one woudl set you back about $3, but that doesn't seem like it would be much compared to what your supply costs. As to the inductor, they mention using one on a micrometals core (lower right hand side of the data sheet) so I would go with that (plus micrometals has free samples).

EEYORE

Mon Mar 30 2009, 09:41PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

... wrote ...

68ufd tantalum isn't actually that expensive, especially if you only need one good for 5v you can get one for less than $.5 from mouser. A 30v 100mR one woudl set you back about $3, but that doesn't seem like it would be much compared to what your supply costs. As to the inductor, they mention using one on a micrometals core (lower right hand side of the data sheet) so I would go with that (plus micrometals has free samples).

Hello,
Thanks for the tip on the samples! As for the capacitance, the data sheet calls for 270uF for the 5 volt output. This will need to be higher no doubt as I am trimming down around 2.8volts.
Matt

...

Tue Mar 31 2009, 03:44AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Sorry, i was just looking in the bottom corner, it looks like a 6.3v 220uf cap will run about $2 for a nice low esr one (12mR), or you can get 470ufd ones for about $3, still not much when you look at the total cost of the system

In any case, do you see the ripple out the output to the diode? It seems like a 1v ripple (out of a 2.8v supply) would kill anything i have ever built

Also, a pi filter isn't really as complicated as one might think it is (it got its name from looking like a pi symbol, not some nasty math involved with it) Wikipedia has some info on it, and there is a forum post

that seems to have a pretty simple way to figure out how well a given set of values will work. Basically, if the ripple is to high get a bigger inductor (which is usually cheaper than more capacitors) and if that doesn't solve it tack on more capacitors. Also, you may find that it helps to stick a few reasonably large tantalum (maybe 50-200ufd) around on your power board to help stabilize things a bit (it seems like seeing a 1v ripple is completely wrong, and that there is something weird going on with your load in that case)

EEYORE

Tue Mar 31 2009, 04:12AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

... wrote ...

Sorry, i was just looking in the bottom corner, it looks like a 6.3v 220uf cap will run about $2 for a nice low esr one (12mR), or you can get 470ufd ones for about $3, still not much when you look at the total cost of the system

In any case, do you see the ripple out the output to the diode? It seems like a 1v ripple (out of a 2.8v supply) would kill anything i have ever built

Hello,
The ripple, or at least what looks to be ripple, is deffinately visible on the scope (At first youll see a nice solid line at 2.8volts or so, and then zooming in and slowing down the scope as well as lowering the volts/div scale will reveal a deffinate ripple). Its about what the specs for the flatpacs describe. The engineer at vicor said to expect up to a 250mV ripple for the 5 volt supply. The app notes state that this will worsen as voltage is trimmed down. It shouldnt be a problem at all considering the 56A rating of the laser, but I want this to be an all purpose laser diode driver. This no doubt would create trouble for a low power, bare laser diode. Where did you find the tantalums? I checked mouser and they didnt seem to have much.

I will look again. (Want to order from them to save shipping as some other parts I need are from them only).

EDIT: For some reason, the ringing went down when I adjusted the dang o-scope. I think the scope is got some issues or something. Now the ripple looks like its 300mV p-p.
Arg! Maybe I was mistaken about the ripple and was misreading on the wrong scale...
Matt

...

Tue Mar 31 2009, 11:07PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

That 300mv ripple seems more reasonable, but a filter is still a good idea. see

for mousers stock of capacitors, just select the capacitance you want and then sort the result by price and keep looking till you find one that is cheep but has sufficiently low ESR and sufficiently high voltage

EEYORE

Wed Apr 01 2009, 04:57AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

... wrote ...

That 300mv ripple seems more reasonable, but a filter is still a good idea. see

Mouser sucks! Anything I look at is obsolete or out of stock...Arg! Thats the case with them usually...At least they have some things I need. Anyways, I found some on ebay for 2$ each.

I got 5 of them and will put them all in parallel.

Id LOVE to get the ripple down to 10mV p-p! I would feel so much better then. 300mV still sucks.
Everything is on its way for this Pi filter. Hope it works!
I will keep you guys updated. Also, the supply runs well now all trimmed down and whatnot. Even at 30A, the heatsink on the mosfets/resistors barely warms (with fan running).

EDIT: So I decided to hook up a laser diode and measure the voltage across the laser diode with the scope. Now it seems that there is such little ringing that I cant accurately measure it. It starts as 100mV max and then seems to go to "zero" as current goes up. I had been measuring voltage with the supply just shorted. I guess I should have put a diode in before and looked. So it would seem that the ringing isnt all that bad, but a Pi filter will still be in order so there is NO (or little ) ringing at low current.
Matt

EEYORE

Sat May 02 2009, 06:57PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello all,
Some work has been done on this driver. I got a sample of the iron core for the filter but, it is way too small to use! Haha! There must have been some error on vicor's part with the specifications. Anyways, I found a ferrite core that I had used long ago in my DRSSTC for driving the full bridge. It is one of those electronic goldmine type that has no specifications. I wound two turns onto the core using 12awg wire and hooked it up with the capacitors. It instantly killed all ringing, but seems to be acting as a bandpass filter now?

I am measuring the waveform across the laser diode. From 0-8amps, the trace is just a thin solid line. From 8-about 15amps, the ringing comes back at around 50mV. Above 15amps the ringing goes away again, back to a thin solid line.

I dont like that. It bothers me, but I figure that any laser diode that requires at least 8amps ought to be fine with a 50mV ripple. Seems low power diodes would get pissy, but there is no ripple under 8amps.

I have basic knowledge of filters, so I really dont know what its going on here. Any ideas? More turns on the inductor, fewer? I was surprised to see the ferrite core, work so instantly well.
Matt

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