Boost converter can't power load. What are the limitations?

Question

I'm trying to power a nixie lamp with a 9 volt battery. The boost converter charges up to 160vdc, the nixie lights, but quickly goes out (or half lights) as it pulls more current ~= 5mA. My boost circuit can't provide more than 3mA without dropping voltage to 125vdc? 125vdc isn't enough to keep a nixie lamp lit.

What limits the current output of a boost converter, or, what are the main limitations?

My boost circuit has a
470uF input cap
2.2 uF output cap
100uH inductor
irf740 mosfet
and a 400v diode without any markings.

The switch signal is from a 555 astable with R1=240 ohm, R2=12 Ohm, and a 10nF Cap. This makes a 95% Duty at 546.6kHz. I'm using a higher frequency to try and lower the current ripple on the indutor since it's only a small axial inductor. I have three 300uH inductors in parallel to share the current.

Should I use two 9 volts in series?

Any ideas or input? Two 9 volts in series works, but I wanted to be able to fit one in a small package.

Answer 1

Do you have a decent oscilloscope? Looking at the waveforms in your circuit should be instructive.

At 550 kHz, a 95% duty cycle means that the off time of the MOSFET is just under 91 ns. This is the only time during which current can be delivered through the diode to the output capacitor. You'll need to be driving the MOSFET very hard to get it to switch that quickly in the first place (the 200 mA available from the 555 is probably not enough), and you'll need a very fast diode as well. Your "unmarked" diode is probably not up to the job.

Also, driving an SMPS with a fixed duty cycle in order to get a particular voltage ratio only works in "continuous conduction mode", which means that the current must be flowing in the inductor at all times. In discontinuous mode, you need to use feedback to get any sort of voltage regulation.

For what you're trying to do, you would be better off using a transformer driven with a square wave, and rectifying the output of that.

Finally, I hope you're not expecting much in terms of battery life. Even if you ultimately achieve 80% efficiency in your converter, you're asking it to provide 1W of power, which is a lot for a 9V battery.