Hi guys,

Could anyone suggest a suitable switch for the following operation?

Objective:

SW1 is normally closed (N.C.) since buck #1 is the main operator. SW2 normally open (N.O.) because it is used as a backup.

Both SW1 & 2 to be controlled with 8051 logic level, that is, 0V for LOW and 3.3V for HIGH.

Average current through each switch is about 1A, therefore I'm selecting a 2X1A = 2A rated current SW for tolerance.

Absolute voltage = +100V DC with respect to GND, but potential difference (P.D.) across the switch terminals really depends on the R(on).

Operation:

When a fault occurs at Buck #1 (short or open circuit), the 8051 MCU detects and turn off SW1 and on SW2 simultaneously. The step-down operation is transferred from Buck system #1 to #2.

I've searched through the internet and has narrowed down to 2 options.

1. Mechanical switch (relay)
   • Slower response but totally cut off operation with no leakage current
   • Not sure if a relay could be controlled by 3.3V logic levels output? Even if it could, would it be safe as 1 terminal = +100V, and the other to 3.3V?
2. Electronic switch (MOSFET)
   • Any off-the-shelves unit where I could conveniently connect this SW with my existing system? Perhaps with 3 pins; PIN 1 -> +100V DC input; PIN 2 -> Buck convertor; PIN 3 -> logic level?
   • Or do I need to design from scratch an appropriate switching circuit with the use of MOSFET? I did see a few schematics online but those are like for lower voltage system in the range of 12V to 20+ V?

I am certain someone must have done this before and I hope I could get some good suggestion overhere!

Attached is my operation layout to facilliate better understanding.

Thanks in advance!

 

1. Mechanical switch (relay)
   • Slower response but totally cut off operation with no leakage current
   • Not sure if a relay could be controlled by 3.3V logic levels output? Even if it could, would it be safe as 1 terminal = +100V, and the other to 3.3V?
2. Electronic switch (MOSFET)
   • Any off-the-shelves unit where I could conveniently connect this SW with my existing system? Perhaps with 3 pins; PIN 1 -> +100V DC input; PIN 2 -> Buck convertor; PIN 3 -> logic level?
   • Or do I need to design from scratch an appropriate switching circuit with the use of MOSFET? I did see a few schematics online but those are like for lower voltage system in the range of 12V to 20+ V?

1. This exists, example: http://search.digikey.com/us/en/products/IM01GR/PB1060CT-ND/1209988
It is safe.

2. That's an optorelay or solid state relay. example: http://search.digikey.com/us/en/products/CPC1916Y/CLA209-ND/654909
NC version should also be available. However I think your system can also work with NO only.

You can of course build the whole thing from scratch. A few transistors , maybe an optocoupler etc.

The advantage of the solid state relay: it is faster.
If you want to detect a failure of the converter, e.g. a short circuit, a mecanical relay maybe not fast enough to open before your fuse burns.

Your major challenge will actually be to detect a short circuit fast enough in order not to damage anything, there could also be a short circuit at the output (load) you have to consider.