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PA 50W on RD16HHF1
#20m
#50w
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On Sun, Apr 25, 2021 at 01:06 PM, Andy V. Borisenko wrote:
The first step is to become a magician. Because you're gonna need some pretty slick magic to get 50w out of a 16w device ;-) |
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Am 25.04.2021 um 14:24 schrieb Jesus Christ on Toast via groups.io:
On Sun, Apr 25, 2021 at 01:06 PM, Andy V. Borisenko wrote:anopther problem ist the maximum drain voltage of tejh RD16HHF1, not good to use with 20 V of supply voltage |
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There is some magical thinking with regard to RD15hhf parts.
RTFM, in this case, the data sheet. First its a 16V max part. Its dissipation is to low for a pair to sustain more than 20W. Its layout is different than IRF510. Will it work... once your past the mechanical issues, and limit power out to maybe 20W and deal with the fact that for all that effort you gain little. When it comes to that if you limit the IRF510 to 16V or less you get the same power reduction and its also about as SWR tolerant as the RD16HHF. Allison ------------------------------- Please reply on list so we can share. No private email, it goes to a bit bucket due to address harvesting |
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At this point I think it's best to say that it's not worth the effort to try and "improve" the 50w PA.
It works, it's paramaters are known and replacement FET's are cheap. As for the input SWR, I wouldn't really worry unless it's some ridiculous value. The input and output impedance of any PA device will vary across frequency by at least some amount and so will the *nominal* impedance of the drive source. An input SWR of 2.1 (but no more) is quite typical of even most commercial equipment, but should never really be any more. |
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On Sun, Apr 25, 2021 at 10:53 AM, ajparent1/kb1gmx wrote:
There is some magical thinking with regard to RD15hhf parts. Can someone explain this? I'm trying to understand maximum values of RF transistors...and the RD16HHF1 datasheets I see don't say 16v anywhere. I see 20v and 50v ratings...and I see things about rules of thumb elsewhere...but how do I determine from the datasheet of an RF transistor what the maximum DC supply voltage is?
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Thanks for any help with my learning!?
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-Nate
N8BTR |
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Nate,
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When Allison says a max of 16 volts, she means the power supply into the rig.
Transmitting on 80m with the QMX from a 12v supply into a 50 ohm dummy load can cause
peak voltages of more than 60 volts from drain to ground at those BS170's.?
And 60v is the absolute max listed in the BS170 datasheet.
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In addition to reading the datasheet for the FET's, you also need to know how the circuit works.
That is not a trivial task, unless like Allison you have spent a career doing this stuff.
Here's a tutorial on a Voltage Mode Class D push-pull amp such as used on the QMX:
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Why such high voltages?
As described in that tutorial, the action of transformer T501 causes twice the power supply voltage to appear
across each BS170 when it is not conducting.? In addition, the LPF in the QMX blocks
the undesired harmonic currents from going out to the load, which raises voltages further.
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For those wishing to see what's going on, I'd recommend using
the LTSpice simulator on a model of the QMX final.? Such a model can be found
in the left-most of the four simulations found in this file:
The discussion is in this thread:??
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Jerry, KE7ER
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On Mon, Mar 24, 2025 at 05:20 AM, Nate Reik wrote:
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>>in a 50 watt amplifier, I want to replace the IRF510 with the RD16HHF1.<<<
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NO!? The pinout is not the same making for odd crossover mods.
? ? ? ? ? The max operating votlage for RD16HHF1 is 16V, no exceptions.
? ? ? ? ? ?The max power is 16W per device and a pair cannot do 50W
? ? ? ? ? ?without emitting smoke.
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Allison ------------------ Post online only,? direct email will go to a bit bucket. |
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>>>>I see 20v and 50v ratings...and I<<<
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The 20V is a hard limit gt gate brakdown votlage.? ?The 50V is the peak voltage
the drain can see in operation before the internal silicone goes back to sand.
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The device was specifically designedd for 12V nominal? use.? To do that you
need margins as peak RF van easily exceed 3X 12V and 4X unders poor swr
is not unlikely,
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The IRF510 is a 100V device...
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Allison ------------------ Post online only,? direct email will go to a bit bucket. |
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Applies to LINEAR push pull amps.?
The 50W amp is not linear and the rules are not quite the same.
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Generally for high power amps LTspice reaults are bluesky
in practice.? It cannot apply all of the strays and parasitics
of the end circuit.? I've used Genesys at work but the
license will take your breath away.
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Allison ------------------ Post online only,? direct email will go to a bit bucket. |
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Nate,
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Some additional details:
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In that old thread I pointed to, I suggest L502 is causing larger voltages across the BS170's than there should be:
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?That file with four different LTSpice simulations shows why I think this is the case:?
The first of the four simulations is a model of the QMX as it arrives from QRP Labs, the maximum voltage across the BS170's is 40v.
When looking at the QMX with a scope, we can see even higher voltages across the BS170's, sometimes more than 60v.
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In the second simulation, L502 has been removed.? The maximum voltage across the BS170's is now the 24v we would expect.
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In the third simulation, a simple series LC circuit is used instead of the multi-band LPF of the QMX.
Now the voltage across the BS170's is a nice square wave, not the sharp peaks of the second simulation.
The current through the BS170's is a sinusoid.
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I feel that L502 is not appropriate here, making the QMX amplifier a weird cross between a Voltage Mode Class D amp
and a Current Mode Class D amp.? In the fourth simulation is the equivalent Current Mode Class D amp.
Here it is the current through the BS170's that is a square wave, and the voltage that is a sinusoid.
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For those that don't have the free (and highly recommended) program LTSpice installed
to simulate these things, I have added a pdf of the LTSpice schematic to the files section:
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Jerry, KE7ER
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On Mon, Mar 24, 2025 at 06:46 AM, Jerry Gaffke wrote:
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>>>>Ahhh, so thus 50v divided by 3 or 4.... Giving 12.5-16.6V. Got it.?
So it would be OK for 20v IF you had a perfect output load matched to the transistor? Makes sense. Thanks, Gary!?
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No. Don't even go there.
At 20V to get 50W out you would be very close to max Idd.? ?Assume 60% efficient that means
for 50W out you need 83W DC in and your are very close to the 5A peak Idd at 20V.? A pair could
but that tends to push the max Vdd.? To get that power you would also have to exceed max
input power or push very close to it.
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What that means is every margin needed for reliability would be at the edge.
Generally any feedline or antenna fault would kill any try.? Quickly!
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Magical thinking will not make the RD16hhf1 into a RD70HHF (a 70W device)!
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Allison ------------------ Post online only,? direct email will go to a bit bucket. |
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Indeed, LTSpice does not always agree with real hardware.
Especially with something like the QMX final and LPF's.
Some of the details don't get properly modeled.
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But it's way better than just staring at a schematic and confidently thinking?
you know how it works.? In the case of the QMX final, you are almost certainly wrong.
(Well, true for most of us anyway.)? Very educational.
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Jerry, KE7ER
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On Mon, Mar 24, 2025 at 08:41 AM, ajparent1/kb1gmx wrote:
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Tronald Dump
Gary W9TD
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