The car charger is based MC34063 chip which can function in both buck and boost configurations.
Thankfully, it's an almost exact replica of the suggested circuit inside the MC3063A datasheet.
So I decided to modify the circuit from the buck (step-down) configuration to the boost (step-up) one.
A random paper with musings:
A trace had to be cut, the inductor was replaced with the diode and was soldered to the bottom side. Everything else stayed the same.
Did it work?
Absolutely yes but there are a few caveats:
- without designing the circuit yourself and just using existing components you are throwing away efficiency
- the inductor has to pass two times more current now so it gets really hot
- efficiency is down in the 20-50% range
I was trying to use this circuit as an emergency travel USB charger but the efficiency is very poor. The idea was as well to couple it with one of those wind-up flashlights but I could never get the chip to start reliably and it kept waking the phone up which draws more power than it delivers.
I have a design on the bench and I'm waiting for a few parts but the emergency USB charger will be completed.