Boost converter debugging

[Fish-in-a-barrel]

Howdy, first time poster. I'm a software engineer that's always enjoyed tinkering with electronics, and I'm working on a project to expand my skill set a little. An important part of this project is a boost converter to create 180V power for some nixie tubes. That's what I'm having trouble with right now.

I pieced together a design from various conflicting sources and calculators across the Internet.

I bread-boarded a 5V to 12V version and that worked fine so I built a schematic, had a PCB printed and put it together.

Here's the relevant portion of the schematic:

I've got a +20V power source, and I'm providing a 200kHz PWM signal from a PIC (that's the RA2 port at the bottom).

This is all kinda/sorta working. It's definitely boosting the voltage, but it's doing so in weird steps that I can't work with. At 25% duty cycle, it puts out about 80V and that climbs slowly as I increase the DC%. Around 30% it suddenly jumps from 90V to 145-165V and then levels off. At around 53% it jumps again to 230-240V! That's more than D1 is rated for, but it's hanging in there so far.

I hooked my oscilloscope up to MOSFET drain, and the waveform is weird. In the 25-35% duty cycle regime, there are three distinct peaks in the voltage every period. When it suddenly jumps to 145V, one of the peaks disappears. When it jumps again to 230V another peak disappears and I'm left with just one peak.

90V:

145V:

230V:

The PWM signal isn't the cleanest, but I don't think it's the problem:

Any ideas on what's going on here?

 

[Fish-in-a-barrel]

Raise your input voltage just a skosh. With the conversion ratio you are running it should put you over the top.

I would, but the final goal is to use USB-PD to power the device, so 20V is as high as I can go.

 

[Nigel Goodwin]

I've been doing a lot of learning, reading, experimenting, and waiting for parts from DigiKey. Without intending it, my current design looks an awful lot like what Nigel posted on Dec. 24. Apparently he knows a thing or two.

Or google does

I googled and found various driver examples, including IC ones (which I also tried), however the simple push/pull I settled on worked just as well as the IC did, and is considerably cheaper (plus used parts we have in stock).

As there's an example on my desk which has finished charging over night, here's a quick picture. The top RH corner of the lower board is the buck converter (12V down to Li-Ion charging).

 

[Diver300]

89% DC is the ragged edge. If I push to 90%, the input current takes off and everything goes to hell. I figure I'm either over-taxing some component, or the drive signal gets muddy and the MOSFET isn't switching off entirely.

You have to make sure that you aren't saturating the inductor. If the current doesn't reduce enough during the off-time, the current will get to a larger value at the end of the on-time. Once the inductor is saturated, you will get a massive increase in current in the inductor, with no more energy stored.

Can you used the oscilloscope to measure the current in the inductor?

Just measuring the gate and drain voltages on the MOSFET would be a good start. When the MOSFET is on, there will be some drain voltage due to the current and the Rds(on) of the MOSFET. If the drain voltage goes up linearly while the MOSFET is on, that's fine. If the drain voltage goes up faster before the gate voltage comes down at the end of the on-time, that shows that the inductor is saturating.

When making those measurements you will be looking at quite small voltages, less than 1 V, on the drain of the MOSFET. When the MOSFET turns off, the voltage will jump up to the output voltage. That will drive the oscilloscope well over the input range that you are looking at, but oscilloscopes are not damaged by turning the sensitivity up.

 

[Fish-in-a-barrel]

Final update: V2 board is working great! I'm hitting 180V without a problem.

Naturally, I'm already working on V3, which includes the USB PD integration. That's the last bit I want to prototype before designing the final project.

Thanks again for all the help!