Fish-in-a-barrel
New Member
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?
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?