Post subject: PIN Diode Switch
Tue Dec 14, 2004 5:06 am
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.
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.
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
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.
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
Unread postPosted: Wed Dec 15, 2004 2:52 pm
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.
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.