Re: uBITX adjustable power supply?

Jerry Gaffke

Look at fig 2 of that wikipedia page for the boost converter:

If the inductor is large enough, the inductor current will be constant.
When the switch is on, all of that inductor current gets shunted to ground, 
and energy gets stored in the inductor's magnetic field as the current rises slightly.
When the switch is off, that inductor current must go somewhere, so the voltage rises
and it pushes it's way through the diode.

The switch inside the chip can handle a max of 4 Amps,
that sets the maximum inductor current we can allow.
So we have up to 4 amps through the switch when the switch is on,
and those same 4 amps going out to the load when the switch is off.

As the wikipedia page explains, the duty cycle of the switch is equal to the output voltage over the input voltage,
As the output voltage rises, the amount of time that the 4 Amps is going through the diode is reduced.

So max current out is 4 Amps if Vout = Vin, and max current is reduced as Vout rises.
Specifically, max current out is 4Amps*Vin/Vout.
So if boosting from 12v to 24v, max current is 2 Amps.
If boosting from 5v to 25v, max current is 4*5/25 = 0.8 Amps.

That assumes all parts are perfect, which they are not, actual max output current will be somewhat less.
And the output will get plenty noisy as Vout rises, since the duty cycle of inductor current through
the output diode is reduced, and the inductor and output cap will be less than ideal.


On Fri, Jul 20, 2018 at 07:16 AM, Jerry Gaffke wrote:
Here's a tutorial on boost converters:

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