Is a $1 Power Supply Any Good? + Wave Winners
3 quick & easy power tests you can run with an oscilloscope: ripple, turn-on, and turn-off
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I recently came across some cheap boost converters on Amazon, these claim to take 5V in and boost it up from Vin to 25V. I’m a little skeptical, but I was curious how well they actually worked, so join me for a couple quick tests that I like to use to do a quick check of a supply, and we’ll put this thing through its paces.
First off, let’s measure the output ripple. You can do this manually, but I prefer to use the power app on the scope. To set it up, we probe the output and setup our measurement. We can get a better measurement with a better probe, but this standard passive probe will work fine. To set this up manually put your channel into AC coupling mode, zoom in, and make your measurements.
We have a micro USB input, so naturally we’ll use 5V as our input voltage, and our output voltage is clocking in around 20 V.
So, when we make the ripple measurement we see our peak-to-peak and RMS values on our 20V output.
I’m pretty happy with that, it’s just a couple percent. Ripple on a supply rail translates to noise, though, so for audio work or jitter sensitive designs this is probably not a good supply to use.
With my cursors I can see this is about an 350 kHz signal, but to double check it won’t hurt to turn on an FFT. I’ll do FFT, set my start and stop frequencies, and I like to use “max hold” to make things easier to see. Only use this for consistent signals – it’s not a good option for 1-off measurements.
I can see my peak, and a couple harmonics. I also see some 1/f action here, but my gut says this is also likely an artifact of this sawtooth shape. Notice how much this varies from cycle to cycle? That’s going to smear out the power over a range of frequencies, which is what we’re seeing here.
Let’s also take a closer look at the signal – do you see these little spikes here? If I measure the frequency of them with my cursors, I would bet this scope that it’s caused by the boost converter’s switching frequency. It looks like 1 MHz, but don’t be fooled. Let’s grab a few single captures and see what we get. It looks like these spikes happen every 350 kHz. Let’s see what we find if we pull the part number. It looks like there’s a 350 kHz internal PWM signal driven by an oscillator. There’s also a mention of a 1.2 MHz fixed operation mode – which is pretty easy to see on the scope as well.
Data sheets are great, but being able to verify it with the scope gives me a lot more confidence in this converter.
The last note on this measurement is that we’ve done it all with a simply resistive load, this performance will almost certainly change under a dynamic load, and you could test this with an electronic load or SMU.
So, we learned a lot from the ripple. The other two things I like to check are the turn-on and turn-off characteristics.
I’m going to use the scope app to measure these, but you can also do this manually by setting up a single-shot trigger
Let’s walk through the wizard and check our turn-on. The thing to watch out for when a power supply turns on is inrush current. When applying power right, the inductors and capacitors aren’t automatically at steady state, they have to charge up. This can pull a lot of current and do some damage – so it’s worth checking.
In this case, we see some interesting behavior. It boots up to one voltage level, sits there, and then moves up to the final output level. If we check the data sheet for the switching chip, we see that it actually has a built-in soft-start function. And it looks like it works! You’d still want to test this on various loads before moving on.
And last for today, the turn-off test. What you don’t want to happen when you turn a supply off is have weird power spikes or discharges into your system. You can set this up the same way as the turn-on test, but again I’m going to use the app.
And, a simple power off test shows things perform as expected.
Overall, this is a killer little supply and well worth the $1 just to have it on hand if I need something quick.
The power app also does a LOT more than just this, you can do a full suite of switching tests and all kinds of other measurements that are really tricky to make manually.
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