PIN Diode Switch - RF Cafe Forums
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Post subject: PIN Diode Switch
Unread postPosted: Tue Dec 14, 2004 5:06 am
Joined: Fri Aug 27, 2004 11:25 am
Location: Cape Town, R.S.A.
I’ve been searching for PIN diodes for use in a High power RF switch. The approach in mind was the standard use of shunt diodes with quarter wave lines. The substrate to be used is aluminium clad microstip.
It seems that manufacturers typically connect the cathode to the ground plane (effectively making it the heat-sink of the device).
With this arrangement a positive voltage switches the diode on, while a relatively large negative voltage (in order of -50V) is used to reverse bias the device.
My question then is, what stops one from physically reversing the diode, thereby reverse biasing with a high positive voltage? The goal is to limit system power supply requirements.
Is there some physical reason (heat dissipation, grounding etc) limiting one to the conventional grounded cathode configuration, as I have been unsuccessful in finding devices configured otherwise.
Unread postPosted: Tue Dec 14, 2004 12:51 pm
When you are talking about "The goal is to limit system power supply requirements." you must mean your particular budget on each supply. It will take the same power to turn on the diode regardless of where it is referenced to.
The diode can be used reversed, but you assumption about the heat dissapation is correct. The n region is much larger than the p region and has more suface area in contact with the external contacts. Therefore you want this to be the surface that dumps the heat. Some switches are designed with series diodes so obviously the RF track can absorb the heat for some designs.
Every case is different and you may be able to get away with reversing the diode if your power calculations show sufficient margin.
P.S. I assume you are not planning to hot switch the diode, hot switching is a much more dificult design, I would avoid this if possible.
Unread postPosted: Wed Dec 15, 2004 2:52 pm
Thanks for the response.
Actually, the idea was to limit the number of power supply modules required. In some previous designs we have used a dedicated -50V supply for switching the devices off.
Hence the need for such PSU module may be eliminated by sharing existing supplies for the PIN diode switching. (The standard practice of the use of COTS switch-mode PSU modules is convenient, but it can also be quite costly)
Regarding your comments on heat dissipation, I think you have answered my question as to why it seems that grounded anode PIN diodes are not freely available. (Note: SMT diodes would allow one to connect as desired, but generally these do not handle high level RF power very well)
This is strictly a ‘cold’ switching application and no complications are expected here.