Electronics > Projects, Designs, and Technical Stuff

To Double or not to double?

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MagicSmoker:

--- Quote from: NivagSwerdna on July 17, 2019, 05:26:10 pm ---Sadly I opted for a chip TPS61175 with an inbuilt switch rated at 38V. (I said it was alike not the same  :) )
--- End quote ---

Yeah, not really what I would describe as, "very much like..."

The switch has to withstand the output voltage, at a minimum, so you have no choice but to use a doubler or tripler. Ripple goes up dramatically, of course, but that's rarely a problem at high frequency and low load current/power. One advantage is that the coupling capacitor in the multiplier automatically provides short circuit protection, which is otherwise impossible to do in the conventional boost (you can protect the switch from overcurrent, but not the output diode).



NivagSwerdna:

--- Quote from: MagicSmoker on July 17, 2019, 08:01:00 pm ---Yeah, not really what I would describe as, "very much like..."
--- End quote ---
The principle is the same as the MC34063... and the question of the doubler?

Buriedcode:
You can certainly add a charge pump voltage doubler, but its voltage will sag under the 50mA so you won't get 70V.  But its worth trying it, and seeing what boost voltage you'd need to get 70V out the charge pump (it'll be over 35V), then if its low enough, you can just connect the feedback to the charge pump output. It only costs a couple of diodes and a couple of caps.
I would add a low value resistor in series with the fly capacitor, as sometimes hooking a charge pump to a boost converter can stop it switching.

Alternatively, coupled inductors (1:1) are easily obtained, and will give you double the voltage output (or rather, half the peak switch node voltage) if you use it in an auto transformer config.  You could also try other ratios, but these are harder to find. A 1:2 for example would mean the peak switch voltage is ~23V.

David Hess:
Charge pumps work great when driven by a boost switching regulator.  Move the feedback network to the output of the charge pump to compensate for diode and capacitor losses.

Beware however when operating close to the switch's maximum voltage.  Making up for losses in the charge pump will raise the voltage seen by the switch.

NivagSwerdna:
Total success and a nicely glowing IV-18.  I am getting 69.7V out of the doubler.  Perfect!

Thanks

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