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Old 22nd Jan 2019, 9:11 pm   #1
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
Default High side high voltage MOSFET switching with 3V logic drive?

Hi All,

Im working on a project to give modern microprocessor control to vintage 1980s military radio equipment. Due to the majority of the control system being interfaced with 3V logic, I find myself with a puzzle,

I need to use 3V logic outputs, from MCP23017 I2C port expanders, to switch between 18 different high voltage lines generated from a potential divider stack. This is 18 levels between 0 and +80V, currently selected by a 18way wafer switch.

Im thinking, I need p-channel MOSFETs to do the high side switch, but because these need biasing from the HT line to the gate, clearly then need another transistor, either an n-channel MOSFET or NPN, to control it from the 3V logic?

Would I be correct in thinking then that the drive transistor would also need to be rated for the high voltage on the line its driving the switch for?

This is a moderate HT but very little current, and also very very limited space in the radio, so I need small devices as well.

Can anyone suggest suitable devices, that are also low cost? If im looking at 36 MOSFETs to switch 18 lines, I need compact and cheap!

I could of course use different voltage ratings depending on the line being switches, which should allow me to use cheaper parts where the high rating isnt required.

The circuit im working on can be seen in this post on my blog http://g7mrv.blogspot.com/2019/01/ht-chain-issues.html

I'd be pleased to hear any suggestions as to circuits, devices etc, especially if anyone knows an IC that could do the task for me!

Regards
Martin G7MRV
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