Boost Converter

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Boost Converter Capacitor Charger

The boost converter is a tricky charger to get going, but is essential for charging large cap banks in a short time and while on the go! Basically a 555 timer drives a DC-DC converter, the boost convert in this case, which boosts the voltage to the desired value.


The converter consumes around 12A peak, and is designed for 12V use. The charge time for a 4700µF, 430V bank is 30 seconds. When the desired voltage is reached the 555 is turned off automatically, and the LED lights. NEVER run the boost converter without a load, the voltage spikes will kill it quickly.

Inductance and Frequency

If you feel a need to modify the circuit, I’ve made a frequency/ inductance/ power calculator. It’s an Excel spreadsheet, so you’ll need Excel unfortunately. Download it here. A lower frequency and higher duty cycle increase the inductor current and output power. However the switching device must be able to handle this current. For faster charge times a lower frequency + paralleled IRFP450s could be used.


Having trouble? Check the points below.

  • If you hear a high pitched squeal everything should be working, check the connection to the capacitor bank.
  • If the timer is working but nothing is happening make sure the inductor diode and IGBT are all connected correctly. Also make sure you are using a proper inductor.
  • Capacitor charges to 9 - 12 volts? This means the mosfet isn’t switching the inductor off. Check the gate drive and 555.
  • Will only charge to XXX volts, then slows down or stops? This is almost always caused by using too little input power. I’ve tested this design to 445V, so it will work up to there from 12V.
  • Always check that the 555 timer is still alive.


As you can see they can be built quite small, using surface mount components will further decrease size. Use a heatsink on the IRFP450, it heats above 400V.

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