Switching DC step-down from 320V to 160V

[NiHaoMike]

This sounds very interesting. Could you give me more specific information?

https://www.microchip.com/en-us/product/SR087
It's limited to 50V but might be possible to use for higher by using a zener diode to lift the low side. Or just replicate the design using a common comparator chip. (I have done something similar although at low voltage, deriving about 6V from a 12V transformer which then gets regulated to 5V with a LDO, the 17V or so from the main rectified supply is used for some of the analog circuits on the same board.)

Without isolation from mains power you're builidng a death trap.

Isolation would only make things a little safer at 160V.

[DavidAlfa]

You microwave oven uses nearly 4KV, and guess, it's not isolated.
Calling it a death trap, but you don't know where this is being applied.

[Wolfram]

Without active feedback, your buck converter will enter discontinous conduction mode (DCM) under light load conditions, and the output voltage will no longer be proportional to the duty cycle. So practically you need either active feedback, or to replace the diode with a controlled switch in order to operate in forced CCM, both are valid solutions. Active current limiting is also strongly recommended. This project comes to mind as a potential source of inspiration: https://ludens.cl/Electron/latsup/latsup.html

[soldar]

Without active feedback, your buck converter will enter discontinous conduction mode (DCM) under light load conditions, and the output voltage will no longer be proportional to the duty cycle. So practically you need either active feedback, or to replace the diode with a controlled switch in order to operate in forced CCM, both are valid solutions. Active current limiting is also strongly recommended. This project comes to mind as a potential source of inspiration: https://ludens.cl/Electron/latsup/latsup.html

I believe that is the purpose of R4, to provide a minimum load but, you are correct that it needs to be taken into account.

I had no visited ludens.cl in quite some time. Good to see it is still around. That circuit would work for me although it is way more powerful than I need.

A plus for that circuit is that it uses UC3842 of which I must have many in my junk pile. Against it is that I would need to wind the inductance with a secondary winding to supply the IC. I will consider it even though the power I need is 40W or less.  Thanks.