The output of this then goes to a low voltage boost converter: MCP16251. That's an awesome device that can generate a stable 5V supply from an input voltage as low as a diode drop. Actually, that's what the input practically is. But that input can also rise to 25V, which is way outside the MCP16251's max input voltage of 5.5V. That's why I need some form of pre-regulator (or a totally different approach to turn 0.7V into 5.0V).
The boost converter understandably doesn't allow a large voltage drop between that 0.7V supply and its own input. I checked, the circuit still but barely works with a 3 ohm resistor in series. That drastically reduces its output current, but the final 5V load only consumes a few hundred uA. The input current of the MCP16251 is around 15mA in this situation, resulting in a 75mV maximum drop. In the actual solution, I want to allow 50mV max.
So I am in need for a voltage regulator or limiter that doesn't exist. I thought of using a JFET but there are none with sufficiently low voltage drop at zero gate. My current way to go is to use a PTC fuse, followed by two strong diodes in series to GND. At small voltages, the PTC fuse is conducting and has little resistance, and as the voltage rises the diodes start conducting and eventually cause the fuse to heat up and increase its resistance. That works well but has two drawbacks: a) current consumption peaks at more than 0.5A at a certain input voltage, and b) PTC fuses are not really designed for repeated action.
Do you guys have any other and maybe better idea?
The application circuit will be a high current (50A) ideal diode that works with a minimum input and output voltage of 0V under full load. That allows it to use it to charge large ultracapacitors. Sounds impossible, but isn't
