RFC: Switching mode power supply schematics

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nitrousnrg
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Joined: Tue Jun 24, 2008 5:31 pm

Re: RFC: Switching mode power supply schematics

Post by nitrousnrg »

first things first: thanks Jim :-)

Ok, this is the topology of my SMPS:

Image

At the left you can see the inductor, the controller, diode and capacitor. After the smps LC, I added an RC (0.1ohm+4.7uF ceramic), before the smps, I added a pi filter (4.7uF-1uHy-100uF), not shown in that pic. On the right, two 5v regulators.


This kind of SMPS has a lot of noise in the input, and low noise on the output.

This is the ripple at the input, without the pi filter, around 1 ampere of load.
Image

And here is the same test, with the pi filter
Image

Scope settings: 10mV/div, 1usec/div
Image

On the output side (6v) at 1.5 amperes you can see something *like* this:
Image
I'm not 100% sure if it is the exact picture, but the waveform at the output was exacly like that. I think it was 20mV per div, damn, could be 50mV, anyway, it doesn't made it through the regulator. Those 8MHz spikes are always present, and could be caused by a bad quality probe. The next time I import something, I'll buy some beads to try to supress that one.

After the regulators (5v) I can't measure any ripple. In the 5v line I can only see those 8mhz spikes, but I measure the same waveform in the ground; and my puma spin1 (withouth a smps) has a very similar signal in the ground, so I'd say it is noise being picked by my probe (which is not a great one, btw).

Given my equipment (and my university's) I can't go further without getting some better quality stuff. With time I'll pick some USB scope, and a bunch or probes.

For your reference, this is what I measure at the battery when the engine is running. There is nothing mine in there, its just a stock carbureted car. 10mV per division, and you're not seeing how big the ignition spikes are
Image
Marcos
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