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Producing a very small boost converter

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acharnley
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2019/04/10 12:08:25 (permalink)
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Producing a very small boost converter

I've been grappling with making the following circuit work but inductor saturation is pretty much constant. I need about 5mA, no more than 10mA at about 16V and while I should have used a driver but I'm out of board space and time is of the essence.
 
I've had this working before but that was by taking the gate resistor down to 10 ohm or 0 ohm. TBH I'm not sure why it no longer works, I changed the PIC (physically), frequency from 62KHz-500KHz, FET and tried numerous value inductors from 4.7uH to 22uH (they're rated from 250-550mA ripple so should be plenty). 

What I believe is happening is the PIC pin impedance is too high, which I thought was odd as the FDV301N has so little capacitance. I tried slowing the PWM frequency and increasing the inductor to one rated 1.5A ripple and the problem was much worse. Would a darlington be any faster?

Duty can go down to 0% (so DCM). This is expected as there's so little load.



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    acharnley
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    Re: Producing a very small boost converter 2019/04/10 13:12:50 (permalink)
    +1 (1)
    I'm sorted. For anyone that has this problem it may seem a good idea to go for the lowest capacitance FET (aka easier to drive) but the higher Rgs prevents the inductor from charging, which quickly leads to duty increase and saturation.
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