DIY boost converter calculator

For many small projects, its cheaper and easier to DIY a boost converter than to buy a specialty chip. DIY converters are usually not as efficient but they’re quick & cheap!

The above schematic section shows how I designed a 30-60V vacuum fluorescent tube display driven from a microcontroller pin.
Tubes such as VFDs, Nixies, Decatrons, etc require high voltage to light the gas in the tube. In order to reduce cost, we use a microconrtoller to make a boost converter and avoid paying \$5 for a seperate chip. We can do this because we don’t need a precision output and the current draw is mostly constant. The boost regulator is run open-loop there is no feedback resistor divider as it isn’t necessary as long as the input voltage is within a reasonable range…

The microcontroller runs at 8MHz so the 8-bit PWM output is 31250 Hz. The inductor and output capacitor is calculated below. The diode is a standard Schottkey type. The switch just has to be able to handle the max voltage plus some for safety. Note that this design is meant for ‘static’ output currents, not for variable current draw designs. There is no feedback and its very approximate! This is not for precision electronics!
The boost circuit works by connecting the power inductor L1 to ground that current can flow through it by turning on Q2. After a little bit of time, we disconnect the from ground (by turning off Q2) this means that there is no longer a path for the current in L1 to flow to ground. When this happens, the voltage across the inductor increases (this is the electric property of inductors) and charges up C6 . When the voltage increases to the level we want it to be (30V+) we turn on Q2 again which allows the current in L1 to flow back to ground. If we do this fast enough, and C6 large enough, the voltage on C6 is smoothed out and we get a nice steady high voltage.

The timing of turning off/on Q2 allows us to modify the output voltage. Normally there is a feedback resistor to the microcontroller but it is not here because we are running it open-loop. To drive Q2 we use the PWM output from the microcontroller and adjust the duty cycle to vary brightness.

These sorts of designs can be easily made with a 555, once you have the PWM output, connect it up to Q2! For this simple calculator, enter in the freqency, voltage ranges and current ranges and the duty cycle, inductor and current requirements will be displayed!

More on the calc page!

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1. Does this let you vary the brightness of the fluorescent tube by varying the PW?

Comment by . — August 20, 2009 @ 2:42 pm

2. Durr.

“To drive Q2 we use the PWM output from the microcontroller and adjust the duty cycle to vary brightness.”

Comment by . — August 20, 2009 @ 2:43 pm

3. Hi,

I follow your blog having recently got into Arduino’s and that. A brief comment about the circuit:

1N5817 diode only has a reverse breakdown of 20V (see http://www.fairchildsemi.com/ds/1N%2F1N5818.pdf). So whenever the MOSFET is turned on, the anode of the diode is at ground and the cathode is at the +UB output voltage.

This would mean that any voltages above 20V on the output would cause the diode to breakdown and be damaged. The voltage rating of the diode must be at least rated for the output voltage of the boost converter circuit.

So for higher voltages, you could use a 1N5819 which goes up to 40V reversed. You could also use a conventional fast rectifier like a UF4001 or UF4002 (http://www.fairchildsemi.com/ds/UF%2FUF4004.pdf) to get 50V and 100V reverse voltage respectively or just select a higher voltage Schottky.

Also, a resistor divder from the output and fed back to the input (with a little decoupling) could be used for feedback and control (albeit with a slow response) of the PWM driving the circuit.

Perhaps some of this could be included in the calculations page?

B. Regards,
James

Comment by James — August 21, 2009 @ 2:17 am

4. James, good point, thank you for reminding me! I will update that. MBR160 are nice and stocked at our fav. suppliers

5. which microcontroller is used for delivering pwm signal?if instead of schottky diode, if we use rectifier diode,will circuit will work?

Comment by vijaya jadkar — August 31, 2009 @ 4:33 am