Boost converter debugging

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Fish-in-a-barrel

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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?
 
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Neat!

So you only needed a 100uH inductor?

I only understand about half of that assembly, but it looks like your PWM period was 36us (~28kHz) at 80% duty cycle?

I hadn't really looked at the Capture/Compare/PWM module on the PIC. Frankly, I didn't know what it was used for. Guess I should take a look at that.

I really appreciate seeing such a clear, working example of what I'm after. Thanks!
 
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The PWM was software, if the voltage dropped below the set voltage it output a 30uS pulse to kick a pulse into the storage capacitor then waited untill the voltage dropped again, so only P - proportional only. Very simple but worked well. I think the most important part is the current carrying capacity of the inductor.

Mike.
 
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With 100 μH, a 36 μs pulse and a 12 V supply, the peak current in the inductor is 4.32 A. That gives plenty of charge to overcome the capacitance of the MOSFET, but you need the input capacitor, the output capacitor, the diode and all the wiring between those components to withstand that much current.

I suggest a much larger inductance so that the peak current will be much smaller.

A circuit like that will have a duty cycle of less than 1% and it is impossible to control the voltage without measuring it. Pommie's circuit operated with zero current most of the time, as I described in post #16.

You need to look at the saturation current of the inductor. Inductors saturate when the current is too high, and they loose inductance. That means you get loads more current flowing than you expect.

The rapid change of current can cause problems and it's a good idea to have the negative side of the output capacitor as close as possible to the MOSFET source.

In the original circuit, the input was shown with the negative of the input capacitor connected to the earth symbol, and the negative of the output capacitor connected to a pad labelled GND. Those have to be connected together for the circuit to work.
 
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Merry Xmas from Canada.

You can get away with just about any feedback method with small hysteresis for small % ripple. and some RC anticipation to prevent overshoot (Kd) But open loop boost is like a car with full throttle and no shocks on the springs with resonant pulses getting damped by load changes from leakage with boost duty cycle. This low power could be run from 12 or even 9V.

So always use Vgs > 2*Vgs(th)max (aka >2*Vt max) or Vgs > 2.5*Vt for Vt=2 to 4V
and always use negative feedback for boost regulators and simulate R for load.
 
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Merry Christmas everyone!

So let's see if I've been following along correctly.

Let's say I wanted to stick with the 5V gate level even though I have 20V right there because I'm stubborn or something. An onsemi FDD7N25LZTM (datasheet) has these specs:
  • Drain to Source Voltage (Vdss): 250 V
  • Current - Continuous Drain (Id): 6.2A (Tc)
  • Drive Voltage (Max Rds On, Min Rds On): 5V, 10V
  • Vgs(th) (Max) @ Id: 2V @ 250µA
It looks like this would comfortably meet all my requirements if I drive it with 5V and 250µA. And that 250µA is only used when the gate is switching (so it's not like I'm burning 1.25W continuously) but the larger the current available, the faster the MOSFET can switch, which keeps it cool.

If I use a large enough inductor, could I get away with a high-voltage transistor instead of a MOSFET? Something like a MPSA42 has the voltage range, and a 5mH inductor should keep the peak current under 500mA if my on duration is under 125µs. That feels theoretically possible, but awfully easy to overload.
 
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Bipolar transistors are used just as often as MOSFET's, in fact probably more often - certainly in domestic electronics SMPSU's bipolar were far more popular than MOSFET's as they are cheaper and more reliable.

However, in either case, you still need to arrange a driver - the I/O won't sink or source enough current to charge/discharge the gate capacitance fast enough, and the device will get hot.
 
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Don't be fooled by Vgs(th) max. This is the threshold (th) voltage that the MOSFET will start to turn on - not the voltage it's turned on. It's safer to think of it as the limiting voltage the MOSFET is guaranteed to be off.

Mike.
 
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Tony Stewart said:
DT looks good
Click to expand...
It's alright. The free version has very few limitations that I've run into (you can only do 2-layer boards is the only one I can think of). At the end of the day, I'm able to build schematics and layout PCBs that get printed without a problem.

The downsides are that it's clunky, and it's not as well supported as other tools. For example, no one ever seems to have component libraries for it so I'm always converting KiCAD downloads.

I've been meaning to try out Fusion360 and/or KiCAD, but that's a lot of time to invest when I'm already proficient with DipTrace.
 
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Tony Stewart said:
170 5W 92% efficient 1V error ripple <100 mV typ http://tinyurl.com/yp7zadrt
View attachment 143806

Is this good enough?

Is this TMI ? (too much info on simulation)
Click to expand...
That simulation shows pulses every 5μs or so, and it's using a 1N400x, which has a reverse recovery time of around 2μs. I don't think that the reverse recovery time is simulated properly, if at all.

I think that 4.7μH is far too small for a practical converter that needs 2 mA. All the currents are changing far too fast and it would be far easier to have a much larger value of inductor, which would mean that the switch is turned on for a longer time. That would mean that there would not be as much need to turn the MOSFET on and off quickly.
 
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Multiplying 5V to 180 = 36 and power is < 0.5W per tube, Although over designed, <4W in & 3.5W out peak current was >2Apk for speed with 0.1V ripple on 170Vdc

This Burp charger could be reduced in capacity from 2.9A for 100 us by raising L . But this has superior efficiency from 5 to 30V in and 0.5 to 5W out . Some might want more than 1 tube.

Reducing C load uF to nF increases ripple from 0.1V to several V, but reducing 5W to 0.5W by raising L can accomplish same.

It was just a quick and dirty design and may be easily tweaked to different requirements. ( It was far from perfect for everyone) One would normally use an IC for this.

TY for comments.
 
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If I wrote my design spec as say 1
2us recovery time I simulated with cap across diode by simulator was too slow (run time) , but it worked fine with 5 us off time 100kHz .

If I reconsider start time as 2 seconds and only 0.5W out @ 170V or 1 J, then the 5V input needs to provide 1J/5V/2s = 100 mA average and with 50% d.f. a peak current for trapezoid current of 3 to 4 x average or 400 mA peak in theory or 0.5A with losses in higher L DCR using 100:1 ratio for R/L which is more gentle and well with USB limits.

I didn't have time to create a better diode model, but you are right, always choose a fast recovery diode or run it at 50 kHz and deal with peak currents in DCM.
 
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In case anyone was wondering how it's going...



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.

I've gotten just shy of my target of 180V. At 89% DC I can get 175V with just slightly less than target amperage!

I have two obvious problems left to resolve:
  1. 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.
  2. The efficiency is garbage. I'm drawing north of 1W and consuming a little less than 400mW across my dummy load.
My goal now is to get that stable voltage all the way to target, plus a little bit of wiggle room. After that I'll work on the efficiency. I'm a little surprised that I have 1W going somewhere, yet nothing seems to be heating up.

Big thanks to everyone that chipped in their knowledge! It's been a lot of help getting this far, and I've spent a lot of time educating myself based on all your responses.
 
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