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Monday, July 2, 2012

Update1: DC boost converter: MOSFET switching method?

In my previous post i showed a modest 12v to ≈70V boost converter. I used the signal generator of my DSO Quad to switch the MOSFET and was about going the popular way of using a 555 timer chip in astable configuration to do the switching when I had the idea of using the famous astable multivibrator/oscillator circuit (pair of NPN transistors, 4 resistors, 2 capacitors).
I was concerned about the calculations of resistors and capacitors values to set the frequency and duty cycle, so i decided for 50% duty cycle which simplifies the calculation process. 

Schematic using iCircuit App. for iPhone

Normally you would determine the frequency using:  F = (1 / 0.693*(R2C1 + R3C2))
But in case of 50% duty cycle: F ≈ 0.721 / RC  (while R2 = R3 and C1 = C2)

I did choose 10K for R and 1nF for C so I have approximately 72 KHz 

The multivibrator circuit on breadboard

Important notice: everything here is experimental .. 
and such a DC converter and switching method are not a suitable for application!!


  1. how do you attach this to the mosfet in your boost converter as there is only 1 lead on it and on this there are 2 leads

  2. The GND of the multivibrator circuit connects to GND of the MOSFET circuit! and the other lead drivers the MOSFET gate. More important , if you power this pulse circuit with 5V you need to use a logic level MOSFET or use a MOSFET driver ...

    1. Could you perhaps design a boost converter which gives out 500v from 18v which has no ic's and is switched using the same oscillator

    2. please note Im not a professional electronics engineer, You can try this circuit:
      You need few things .. 500V rated capacitor at the output .. A MOSFET or a switching Power transistor to handle high current and the same with the inductor .. and on the switching side a transistor to handle the 18V Input which means a very high current too .. I would suggest you to use 12V instead of 18v ...

    3. Please put out a schematic with the switching circuit connected to the boost converter when you have the time

    4. The link i posted in the previous comment has the switching component in it! You can control the freq. using the POT shown in the schematic.
      By the way, you dont have to use the same components as the schematic, try with components you have and im sure you will get some good results.

    5. at what voltage will i need to power the oscillator to use a regular mosfet instead of a logic level mosfet or a driver?

    6. Depends on what MOSFET you will be using and what kind of transistor for oscillator! .. Normally I would use 5V for the oscillator but it wont be driving the MOSFET hard enough ... so you gotta start from 5 and go up while watching the MOSFET

  3. MOSFETs usually require 10 to 12 volts to fully turn on. Look at the datasheet to find out more. But what is very important is to turn the MOSFET on and off quickly. If it is turn on and off slowly it spends more time in the linear region and the dissipation increases (it will get hotter and waste power).

  4. Unless you use a logic level MOSFET ;-) (Not that I used one in this circuit or u did know about it at that time)