Intrinsically safe switching supply?

[Raytel Matt]

Hi,
I've been asked to design a univesal input, Dual 12volt output intrinsically safe power supply for use within one of our products which is for use within explosive atmosperes.
Have you any ideas of how we go about this and what we should consider (i.e. creepage and clearance within the transformer), isolation ets?

Have you any tips or sample circuits?
What is the best topology?

The transient response must be fairly lively as one of the secondarys will be for energising a relay coil.

Thanks

Matt

 

[FvM]

You have to consider the design practices, that apply to intrinsically safe circuits in general.

Isolation, clerance etc. of the transformer plays a role, if the isolation barrier is involved with the intrinsic saftey, which can't be clearly seen from your post.

 

[Raytel Matt]

Thanks for the reply. This is my concern, if i start adding clearance within the transformer i am surely going to severly effect the efficency of the transformer/supply?
The circuit will have two circuits; intrinsically safe circuit (for logic etc) and a circuit for the relay coil (the coil to contact clearance within the relay is too small, i will be switching the relay from the safe circuit via opto).
The IS circuit will contain a zener barrier, current limiting resistor and fuses.

Any ideas of which switching topology to use and have you any suggections regarding transformer selection?

Thanks

matt

 

[FvM]

I guess, you intend to create an IS circuit without protective earth connection of the zener barriers? Then the requirements for the transformer will be rather high, because it must keep a safe isolation despite of overvoltages from the non-IS circuit. Zener barriers with protective earth connection would be easy, but most likely unwanted.