Comparing Power Supplies

RE: Comparing Power Supplies

DAB — Mon, 13 Feb 2017 19:07:36 GMT

Great post John.

Your observations about the difference between switching and linear power supplies takes me back.

There were a number of discussions on the differences when the switchers started appearing.

The biggest reason was power transfer efficiency. Your linear supplies waste a lot of energy, which is why the switching supply now dominates the market.

DAB

RE: Comparing Power Supplies

D_Hersey — Mon, 13 Feb 2017 12:46:15 GMT

It seems that the light-bulb loads are a little hinky. Maybe you should make one consisting of an R and a MOSFET that shunts another R with the first one, thus increasing the load for a potential supply. You could drive the Q with a 555, or even better, a uP. It shouldn't be too hard to do this

RE: Comparing Power Supplies

jc2048 — Sun, 12 Feb 2017 23:20:55 GMT

With a 10V supply and a cold load of 0.8 ohms you are asking the supplies to give you 12.5A. The commercial supply current limits and reduces the voltage to 4V (4V/0.8ohms = 5A). It does that nice and precisely. As the bulb warms, the resistance increases and the PSU can increase the voltage, still on the limiting. That's the initial ramp. When the current is down to 5A, it goes over to voltage control (the handover is a little uncertain) and you have the transient part that you would have measured if you had stuck to the rated current.

Your linear supply does the same thing, but the current limit isn't precise at all and it's very uncertain at the start, though the handover is better.

The switcher looks like it limits at about 8.7A, which isn't precise at all if it's meant to be 5A and that might just the limiting it does to protect itself. This time the bulb warms much quicker (obviously). The transient part is quicker than the linears, but not too much so. The switcher has better control of the output - the output smoothing caps are smaller and it can throw them around much quicker.

It would be worth looking at the noise of your linear supply without the switcher on the other trace (physically disconnect it, don't just turn the trace off; even better turn the switcher itself off). Even with the best scopes, there's a certain amount of contamination between the channels, particularly with spikey, high-frequency noise. If you still see the spikey noise, try turning off other equipment that might be injecting noise through the mains lead. I would not expect 700mV of noise like that from a linear design (where is it going to come from?) - the true noise figure should be a few mV at most. My bench supplies - cheapish ones - quote <1.0mV rms for currents over 3A and <0.5mV rms for currents less than 3A in the spec, though I haven't tried to measure the figures, but I don't remember much in the way of mains filtering (it's a while since I had a look inside one) so they may be prone to feeding noise through.

RE: Comparing Power Supplies

mcb1 — Sun, 12 Feb 2017 22:14:17 GMT

Nice test.

I suspect that once you add a non linear type load (harmonics, induction), you'll start seeing other differences.

The linear supplies we used to make way back suffered from instability, and they therefore needed 'slugging' which then made their reaction times slower.

So it becomes a game of compromises and trade-offs.

It's nice to see that your 'low cost' models perform very well and make an excellent addition to any workshop.

Mark