Future PSU designs

[exxos]

Prototype PCB arrived today :) I have just ordered the parts from Farnell. Not sure when I will get chance to build it, but I hope soon :)

I have been trying to work out how to fit a voltage selector switch to the PCB, but I think it is just not going to be possible :( I do not really like the idea of routing mains on the PCB when it is all low-voltage stuff there. Then the other way would be to simply solder the transformer wires direct to the voltage selector switch, but then not everyone would want the switch, and they would still be the problem of where to mount it and how. So I am thinking the voltage selector switch just not going to happen.

What I could simply do is not solder the primary transformer wires, and let the customer decide which voltage to wire it at. This way I would only have to keep stock of one power supply as it could be configured for either voltage, and this would also save me a lot of work also..

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[exxos]

A quick test on the 5V rail. Farnell didn't have the right resistor in stock, so a trimmer pot was used.

I can't see any ripple or noise under 2 amp load.. In fact my bench PSU is generating noise which the scope is picking up, which is being swamped out by my ceiling LED lightings anyway :roll:

Even taking into account the noise part, I cannot see any ripple at all. So the output ripple is "undetectable" in this case..

The fact that my LED lighting is swamping out the PSU noise.. must mean its going to be low. How low, no idea, but its very good ;)

Will try and solder the 12V rail tomorrow to test.

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[rpineau]

:bravo: :cheers:
Rodolphe

[exxos]

Managed to get the 12V working today.

It seems when a datasheet says leave the enable pin floating to enable the device, it does not actually work :roll:

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[rpineau]

Since when do you trust datasheet !!!! :lolbig: :sarcasm:

[exxos]

Since when do you trust datasheet !!!! :lolbig: :sarcasm:

:lolbig: :blonde:

[exxos]

I have been doing a little bit more testing and trying to figure out the noise and ripple results better..

Main problem is they always seem to have about 20mV noise on the oscilloscope probes even before I start actually taking measurements. So these results are likely a lot worse than they really are..

These are the 5 V noise figures..

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So actually have about 28mV P-P.


This is the ripple figures..

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Which are to 20mV P-P. Though the average figures are more around 10mV.


The 12 V rail is to be a little worse for some reason..

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Here we have 80mV P-P noise. Ripple is actually about 10mV. But it is difficult to work out if it is just interference on the scope probe is all actually noise on the power rails. But I am still looking into this currently.

Considering this power supply does not have the output ripple filter, it's still doing really well. Plus it has much less output capacitance than my previous power supply design. Thing to bear in mind here is that they will be capacitors on the motherboard as well which will lessen the noise and ripple a lot. So of course again it is important to use quality capacitors on the motherboard.

EDIT:

Oddly the spikes I'm seeing are at 175Hz :shrug: can only assume the scope is picking this is interference somewhere.

I put the digital filter setting to filter out anything above 5 MHz on my scope.. And this helped a great deal.. The regulators do not run anywhere near that speed.. In any case, the results seen are a lot better now..

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Realistically the worst spikes are 40 mV P-P. Majority the ripple is at around 20 mV P-P. This is of course under a 1.2 amp load.. The 5 V line is also around 20 mV P – P and this is under a 2.3Amp load. Where 20mV is 0.02 Volts, just to put it in perspective :)

Aside from the fact that my measuring techniques suck, the actual results of the power supply should realistically be better than what I am actually measuring.

EDIT2:

I found this in the IC datasheet which I had forgot about..

The device minimizes excessive output overvoltage transients by taking advantage of the overvoltage powergood
comparator. When the regulated output voltage is greater than 106% of the nominal voltage, the
overvoltage comparator is activated, and the high-side MOSFET is turned off and masked from turning on until
the output voltage is lower than 104%.

So based on 12V @ 6% is 0.72V. So the voltage spikes are never going to be higher than 12.72V. Which is part of its protection system :)

[exxos]

Okay so I figured this out yesterday.. Turns out the 12 V fan I was using to cool my dummy load resistors was the cause of this noise :roll: I put my finger on the fan to stop it, all these odd noises issues disappear..

... So back to the plot... To be continued...

[exxos]

I have finished tweaking the new layout.. Really the next step is to construct a prototype .. I don't know when I will have time to do that..

I will likely get 3-10 PCBs made, if anybody is interested in some of them then let me know... PCB price is likely to be expensive such a small run though.

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[exxos]

PCB's arrived.. need to find some time to build one up next...

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