As promised, I will post details of the progress of an isolated switching regulator to use with the General Purpose Power Supply design.
https://www.eevblog.com/forum/projects-designs-and-technical-stuff/general-purpose-power-supply-design-7488/It seems to be becoming a project in its own right, and there is no reason it couldn't be used with other low dropout regulator circuits, so I am making it a new thread.
Here is the test circuit I am using that is based around a standard IC - the UC3842. Not perfect for the final application, but it is fine to test out the other components in the circuit.
The problem is not getting it to work - I had it working fine over a week ago.
The problem is choosing the specifications and parameters that best match the available parts in the market. The biggest problem is that there are cheap MOSFET and Schottky diodes with amazing specs as long as I keep the maximum voltage rating under 40V. If I go above 40V, the parts are more expensive, and the specifications are not as good.
The problem is if I go after a big range, then choosing a transformer that keeps the MOSFET voltage well under 40V means that the schottky diode has to handle way over 40V. If I keep the schottky diode voltage under 40V, then the MOSFET has to handle way over 40V.
Also I have to make decision about the input range. Ideally is would go from 3V to 20V, but this is unlikely to happen. For one thing, to output 25V at 1A with a 3V input, I would need over 10A of input current. If I reduce maximum output current to 250mA at maximum voltage, then the current in at 3V is reduced to 2.5A which is a better value.
I will get into the test later, but first some notes on the test circuit. Th UC3842 only starts up at 15V and shuts down at 10V. I could go for the UC3843, but I prefer to find a modern device that does not need a current sense resistor. (They use the voltage drop across the MOSFET instead).
I would love to find a device with integrated switching FETS, but I think it is impossible unless I narrow the range specs a lot.
The secondary is fully isolated. The feedback circuit keeps the switching regulator output at one IR LED drop + one V
be drop above the linear supply input reference. If the linear supply output drops below this (in current limit mode), the switching regulator will follow the output down. I do not want to drive the switching regulator from the output all the time, as I do not want it to increase the voltage if the load is supplying an overvoltage.
A second rectifier on the secondary captures the negative voltage swing of the transformer, as this is exactly proportional to the input voltage. So the micro running the supply (if you are using a micro) knows what the switching supply input voltage is, and can adjust the maximum currents to suit. More soon ...
Richard.