Switching regulators

hi there
not sure if they exist im looking for a switching regulator where the output starts at 0v the ones ive found have a min of 1.2 ish i cent seem to find one that starts at 0v
thanks

Odd request. Why do you need output at 0V?

Needing an adjustable voltage regulated DC power source that can reach down to absolutely 0 volts DC is usually only possible without being an expensive lab type power supplies, it's not a trivial specification to meet.

retrolefty:
Needing an adjustable voltage regulated DC power source that can reach down to absolutely 0 volts DC is usually only possible without being an expensive lab type power supplies, it's not a trivial specification to meet.

The ratio between input and output voltage is an important parameter for a switching
regulator - having this ratio variable over a large range makes everything harder,
so allowing the output to go down to 0 isn't normally seen (in fact most regulator
chips have a Vref of 1.1V from a band-gap circuit, so 1.1V is the minimum possible
output.

A way to make this work is to have a variable-output linear voltage regulator after
a switching regulator. Another way perhaps is having a variable-output switching regulator followed by a fixed-drop linear regulator circuit (which is the same thing as a
shunt regulator, the simplest example being a zener diode).

I have a bench supply with fully variable output voltage - the way it works is there
is a controllable linear regulator, and the input to this comes from a multiple-tap
transformer via a bridge rectifier. A sense circuit detects the output voltage and
switches some relays to choose the best match from the transformer-taps. This
is intermediate in efficiency between a switched supply and a fully linear one.

Yes, it's possible.

The problem with a simple buck converter delivering 0V output is that the output voltage (crudely) is given by switching duty cycle multiplied by input voltage. And because it takes a finite time to turn mosfets on and off, if the mosfet goes on for any time at all you have an output >0V. There is no control action to reduce the output back to the demanded value.

You could fix that by providing a negative-acting stage to work in opposition to the buck converter, perhaps one which sinks a fixed current, IN. Then to provide 0V output, the buck converter just needs to supply IN. To provide 200mA, the buck converter supplies 200mA+IN. That way, you always have a minimum load on your buck converter which helps you define the switching frequency range and the operating locus.

There are plenty of simple switching regulator ICs which can operate as an inverting regulator, or you could create one with a comparator and a mosfet driver circuit.