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

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.

• What voltage is the 9V battery actually supplying under such a heavy load?
– user16324
Dec 29, 2012 at 13:52
• 8.2+ The converter is pulling 800mA from it. I got it working by lowering the switching frequency to 25kHz at a 97% duty cycle. It can now power two Nixie lamps at 5mA each while maintaining 145 vDC :) The battery lasted for about 40 minutes.
– user3657
Dec 29, 2012 at 17:51

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.

• I lowered the frequency to 25kHz (from 550kHz) at a 97% Duty cycle. It can now power the lamps while maintaining 145 vDC. The battery lasts a little over half an hour before the voltage drops too much :)
– user3657
Dec 29, 2012 at 17:53
• If you haven't also increased the value of inductance you're using, then you're wasting a lot of power in the inductor, by essentially saturating it with the maximum current that the battery can supply. At 25 kHz, you should have at least 3.6 mH in order to keep the peak-to-peak ripple current under 100 mA. A regular alkaline battery (about 500 mAh capacity) should be able to run this for a couple of hours. Dec 29, 2012 at 18:09
• I'll try that, I'll need to order the inductors. For now I'll also try using 6 AA in series. This isn't some major project. One of the engineers at my internship gave me two nixie lamps as a gift, and he told me to bring in a circuit if I get it powered with a 9 volt.
– user3657
Dec 29, 2012 at 18:18
• I figured you were trying to make some sort of steampunk gadget, such as a Nixie Tube Wristwatch... ;-) Dec 30, 2012 at 2:00