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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!!