SUPER-DUPER PIN DIODE Needed - RF Cafe Forums
Post subject: SUPER-DUPER PIN DIODE needed Posted: Mon
Oct 17, 2005 5:58 am
I never used pin diodes
in my projects, hence looking for your, guys, advice.
project is a magnetic loop autotune antenna. Currently it is tuned
by vacuum cap driven by stepper and works fine with average tuning
speed of 3-4 seconds. On lower bands It needs additional caps and
these added by jennings vaccum relay that also controlled by the
same CPU. These caps and relays are 6kV/20Amps rated, lower rated
arching, checked. The loop itself is a very high Q circuit and has
reactive impedance. Although 100 watt PEP is the power I have to
multiply it by Q=900 in order to rate these components sufficiently.
Now I want to use my FH (freq. hooping) radio with my antenna and
it requires 4-5 millisec tuning time for the antenna. This means
that I have to switch from motor tuning to SPST relay switcheable
capacitor bank. However, all suitable relays have 2.5-3ms switching
time (it is measured, all of them rated 10ms) which means there
is almost no room left for calculations. These relays have 10-12
mOhms in closed contacts which is next to bad as larger resistance
causes antenna radiating resistance less than sum of other resistances
and lowers Q and efficiency dramatically.
The question is:
Is the nowdays pin diode technology advanced enough to offer
following parameters: (reverse voltage 10-15kV, open diode resistance
0.01 Ohms, Current 20-30 Amps (at 30 MHz)). Are they available commercially?
In what packages? (SMD is preffered).
I think switching speed
is out of qustion, all of them are fast enough.
Browsing on pin
diode webs I noted that most powerful an low resistance (best I
found is 0.1 Ohm) are biased with quite high current. I have only
3 Amps for 16 diodes.
Thanks and regards,
Post subject: Super-duper PIN diodePosted:
Mon Oct 17, 2005 10:52 pm
I don't think you'll
find what you're looking for.
To the best of my knowledge,
there are no SMT diodes of any variety rated at 10-15 kV. One reason
is that we're talking electric field densities near or over the
breakdown of air at STP for most SMT packages.
look at the values you name for current and resistance. Even though
you can't run a PIN diode at zero forward current, taking your low
end estimate of 20 Amps and 0.01 Ohms on resistance, P = I squared
R gives 4 Watts of power dissipation: rather a lot for an SMT.
Finally, you have to have sufficient carriers in the intrinsic
region to sustain the currents you need. As a rule of thumb, that
means a forward ON-state current of more than 20 Amps - and you
only have 3/16 A!
On the bright side, 1 millisecond for a
calculation, even on a cheap uP, seems like a long time!
Post subject: Posted:
Tue Oct 18, 2005 3:00 am
Thanks for that.
look at http://www.microsemi.com/datasheets/hum3002-04
These ones, if three or four in parallel are would be
a subject for experiment (20-30 mOhms) if were not packaged in these
railways styled bolts. Also, it makes me optimistic that since these
already come quite close to my specs I can hope on soon availability
of better ones.
However reverse volatge is still too low.
Currently I'am switching 7kV/12A rated multilayer ceramic caps by
6kV/12A/0.012Ohms relays. These caps in most cases added to vaccum
cap at nearly full capacitance (typical situation) in parallel,
thus current is divided between them and they getting warm very
The nature of entire circuit, as you can see, does
not allow for conducting of precise measurements. The q-factor is
derived as calculation from measurement of bandwidth at resonance
curve. Thus, though powered by 100 Watt PEP the reactive power has
to be considered as much as 90 kW. Lots of guessing.
fact that other guys managed to do similar job with 1 kW (!) powered
transmitter makes me very intrigued. By what means??? All known
relays are too slow, reed relays too weak and have huge capacitance
to it's coil... Magic.
Thanks and regards,
Post subject: Super-duper PI diodesPosted:
Wed Oct 19, 2005 1:52 am
Regarding those Microsemi PIN diodes:
Thanks for bringing those to my attention. Maybe your
task isn't so impossible after all. But what's the problem with
the package? They obviously need to get rid of about 50W of heat.
subject: Posted: Wed Oct 19, 2005 5:29 am
In my case I do not
face trouble of heat dissipation this much. First of all I do not
have CW all the time, SSB heats the diode by carrier that appears
during modulation only, pauses are for cooling. The other thing
is typical fashion of communications is 10:1, e.g. you listen more
than speak. For that reason conventional air cooling would be sufficient.
Also construction, having SMD enveloped multilayer caps and chip
like pin diodes placed over copper clad flan (I think FR-4 also
will do, however) makes whole thing much more organized, compact
and technologically easier for assembling. With these bolts and
strip lided caps entire construction will look like 1930s SX-28
Post subject: Posted: Wed Oct 19, 2005 5:43 am
they need their bolts exactly because 0.1Ohms resistance, should
they have made three same crystals on common piece of waffer and
packaged into ceramic or plastic cube shaped SMD, they would get
3 times less lossy unit with no heatsink requirements for 50W for
sure. Just rerate by frequency, not all use them for microwave...I
think this package is bit overengineered.