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List: elecraft
Subject: Re: [Elecraft] KPA-500 270V ERR
From: David Gilbert <ab7echo () gmail ! com>
Date: 2021-08-27 2:08:40
Message-ID: cea73ccb-ee51-7cec-4f1a-8a6e9d618631 () gmail ! com
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Well, at the risk of offending Fred ...
A PIN diode has an intrinsic (I) region between the P and N junctions.
The I region is almost completely undoped and therefore the carrier
lifetimes are long relative to the frequency of the signal being
switched. A PIN diode is turned on with CURRENT bias, and as long as
the signal current is less than the bias current the carriers live long
enough in the I region that the signal just swishes the carriers back
and forth without the diode ever becoming reverse biased ... and that
holds pretty much no matter what the signal peak to peak voltage is.
You turn off a PIN by removing the bias current ... or better yet,
reversing it to pull out the carriers. The hard part of making a normal
PIN diode is that you have to start with an intrinsic wafer and then
diffuse first from one side and then the other side ... and while you're
diffusing the second side the dopant from the first side wants to
contaminate the I region. (There are other techniques but they get even
weirder).
A PN diode, on the other hand, is forward biased (i.e., "on") as long as
the control VOLTAGE exceeds the signal voltage. As soon as the voltage
across the diode reverses, mobile carriers are soon swept out of the
junction area and the diode becomes reverse biased and shuts off. As
you say, that reverse bias could from either the control voltage OR a
higher than expected signal voltage if the reverse peaks of the signal
voltage exceed the control voltage. How fast all of that that happens
depends upon the carrier lifetimes in the junction region and the
frequency of the signal being switched. Fast switching diodes are built
with high doping levels to give a narrow junction and short carrier
lifetimes, but some high voltage diodes have low doping levels, wide
junctions, and therefore moderately slow carrier recombination times.
If it just happens that the carrier lifetimes are long enough compared
to the signal frequency, it can work. However, I think it's generally
bad practice to use a standard diode as a PIN because unexpectedly high
signal voltages can reverse bias the diode and create signal distortion
... plus the fact that diode manufacturers are NOT designing standard
diodes for use as a PIN diode, and they are NOT specifically trying to
control carrier lifetimes during fabrication. A pair of high voltage
switching diodes can both fully meet the data sheet specs, while one
works quite a bit better than the other when used as a PIN. That which
works with today's batch might not work with tomorrow's.
73,
Dave AB7E
On 8/26/2021 3:58 PM, jerry wrote:
> On 2021-08-26 15:19, David Gilbert wrote:
>> Pedantic? If those were truly PIN diodes like you called them the
>> 270v bias wouldn't have been used there.
>>
>
> Actually, I'm finding this interesting. So with real PIN diodes, you
> don't have
> to strongly bias them off?
>
> The HV is obtained in various ways. W6JL does diode switching with a
> tube amp, so he has ample HV available. My K2 has a crystal
> oscillator feeding a toroid transformer and a voltage multiplier.
> Hans Summers uses a voltage doubler running off the RF output of his
> amplifier. I have heard discussions of people
> using fluorescent ballast transformers.
>
> A complication is that the voltage at a transmitter output - even
> though it's nominal 50 ohms - can rise quite high with high SWR,
> especially in the "looks like an open circuit" direction. So even
> though modern LDMOS PA transistors are rated to tolerate extremely
> high SWRs, your equipment is limited to the SWR that produces a
> voltage lower than the diode bias. Hans Summers' design is elegant -
> since he's rectifying and multiplying the RF itself, the DC is
> guaranteed to be higher than the RF itself. Only thing is - there is
> surely a delay in creating that bias.
>
> I will say that the QRP-labs approach somehow results in the
> cleanest, most transparent-sounding QSK I've ever heard. Better than
> my K2. But it's only one band, the selectivity is just ok, and it has
> no AGC...
>
> - Jerry KF6VB
>
>
>
>
>> Dave AB7E
>>
>>
>> On 8/26/2021 2:56 PM, Fred Jensen wrote:
>>> This morphed into a pedantic discussion of solid state diodes.
>>> Chuck's original question was about the 270 V error. I'm fairly
>>> sure the only purpose for the 270 V in a KPA500 is bias for the T-R
>>> switch. He asked for suggestions. The T-R switch and the source
>>> for the 270 V might be a good place to start troubleshooting.
>>>
>>> 73,
>>>
>>> Fred ["Skip"] K6DGW
>>> Sparks NV DM09dn
>>> Washoe County
>>>
>>> On 8/26/2021 2:23 PM, David Gilbert wrote:
>>>>
>>>> High voltage rectifier diodes would have very low doped N-regions,
>>>> which means fairly long carrier lifetimes so no doubt they work
>>>> somewhat like a PIN diode if properly biased. But they aren't
>>>> really the same thing, and I still say that a PIN diode has
>>>> performance advantages in many situations. They are more difficult
>>>> and costly to build, though.
>>>>
>>>> I used to manage a large semiconductor entity that manufactured
>>>> these things.
>>>>
>>>> 73,
>>>> Dave AB7E
>>>>
>>>> On 8/26/2021 2:05 PM, jerry wrote:
>>>>> Using 1N4007's and 1N5208's ( I could have the last # wrong ) as
>>>>> "Poor Man PIN
>>>>> diodes" is pretty established practice in the amateur community.
>>>>> Look up
>>>>> W6JL on QRZ.com. He has an extensive article about it on his page
>>>>> there.
>>>>>
>>>>> Also Hans Summers of qrp-labs uses them in his 50W QSK amplifier.
>>>>>
>>>>> Apparently, these kilovolt rectifiers behave very much like PIN
>>>>> diodes,
>>>>> at speed.
>>>>>
>>>>> - Jerry KF6VB
>>>>>
>>>>> On 2021-08-26 11:46, David Gilbert wrote:
>>>>>> That sounds odd to me. True PIN diodes are current actuated
>>>>>> devices.
>>>>>> The carrier lifetimes are long enough that the devices are "on" even
>>>>>> if the signal peaks exceed the voltage of the control current ... as
>>>>>> long, of course, as the control current exceeds the signal current.
>>>>>>
>>>>>> On the other hand, I've heard that Elecraft has sometimes used
>>>>>> regular
>>>>>> switching diodes instead of PINs, and for those you need a voltage
>>>>>> that exceeds the peak voltage of whatever you're switching.
>>>>>>
>>>>>> Both types of diodes have their place ... using one where the other
>>>>>> works better is not good practice.
>>>>>>
>>>>>> 73,
>>>>>> Dave AB7E
>>>>>>
>>>>>> On 8/26/2021 11:33 AM, Fred Jensen wrote:
>>>>>>> I think the 270 V circuit is DC bias for the PIN diodes in the
>>>>>>> TR-switch.
>>>>>>>
>>>>>>> 73,
>>>>>>>
>>>>>>> Fred ["Skip"] K6DGW
>>>>>>> Sparks NV DM09dn
>>>>>>> Washoe County
>>>>>>>
>>>>>>> On 8/26/2021 10:32 AM, Chuck Chandler wrote:
>>>>>>>> My KPA-500 has been running great for several years. Recently
>>>>>>>> I noticed it
>>>>>>>> was making the occasional extra "click" or "thunk" upon
>>>>>>>> powering up or
>>>>>>>> down. Usually it would "click" when powered up and again when
>>>>>>>> powered
>>>>>>>> down, but recently a few seconds after the first click there
>>>>>>>> would be
>>>>>>>> another one, maybe more of a thunk sound.
>>>>>>>>
>>>>>>>> This afternoon I turned it on and when I pushed the
>>>>>>>> Operate/Standby button
>>>>>>>> I got a fault light and the 270V ERR message on the display.
>>>>>>>> The menu HV
>>>>>>>> option shows 68.4V, varying a bit, on my 240V line. I put it on
>>>>>>>> the bench
>>>>>>>> and opened up the top cover. There is a smell of overheated
>>>>>>>> components,
>>>>>>>> maybe burnt but more like too hot.
>>>>>>>>
>>>>>>>> I've submitted the contact form on the Elecraft support page. Any
>>>>>>>> suggestions? For now, I'm running barefoot...
>>>>>>>>
>>>>>>>> 73 de Chuck, WS1L
>>>
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>>
>> ______________________________________________________________
>> Elecraft mailing list
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