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
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
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.