|
I have observed that any linear amplifier when operated on keydown for more than a few seconds starts to reduce the output power gradually. My sbitx v3 amplifier is using IRF510s. The output power starts at 25 watts on CW keydown, registering 100 v peak to peak. However, in about 30 seconds, the output is reduced to 20 watts.? This is probably due to the drift of gate bias with temperature. - f?
|
Quick question about the transistor choice. Could you also use IRF520 transistors as well as the 510s? I am wondering as I have some 520s left over from another project.?
WK4DS?
On Sat, Nov 18, 2023 at 10:45 Ashhar Farhan < farhanbox@...> wrote: I have observed that any linear amplifier when operated on keydown for more than a few seconds starts to reduce the output power gradually. My sbitx v3 amplifier is using IRF510s. The output power starts at 25 watts on CW keydown, registering 100 v peak to peak. However, in about 30 seconds, the output is reduced to 20 watts.? This is probably due to the drift of gate bias with temperature. - f?
|
Farhan,
I know you have always preferred the IRF510s, but have you considered the IRF530s? I have noticed that most of the cheap Chinese Linear amps use them to get a good 45-50 watts. I just bought a Micro 50 Plus amp that uses a set of 530s. From all the research I have done on the amp, it seems that it is pretty stable and reliable as long as we don't exceed 50 watts. Might work great for a steady 40 watts on the V3.
Joel
N6ALT
|
Joel, The all the IRF series exhibit this behavior. Just monitor the RF Output over 1 minute of key down.?
On Sat, Nov 18, 2023, 10:15 PM Joel Caulkins/N6ALT < caulktel@...> wrote: Farhan,
I know you have always preferred the IRF510s, but have you considered the IRF530s? I have noticed that most of the cheap Chinese Linear amps use them to get a good 45-50 watts. I just bought a Micro 50 Plus amp that uses a set of 530s. From all the research I have done on the amp, it seems that it is pretty stable and reliable as long as we don't exceed 50 watts. Might work great for a steady 40 watts on the V3.
Joel
N6ALT
|
Several things...
One IRF510 spec is 100V breakdown.? More than that and poof.
Also sustained current should be 4A or less (the 5.6A is for
saturated switch mode).
So the question is why it it 100V P-P?? It suggests an open relay,
wrong band LPF selected or seriously bad SWR.? ?Last case is that
the Drain load (transformer) is too high.
100V P-P across 50 ohms is well over 25W,? if its across a 50 ohm
load thats about 35W.? At 13V that's pushing the device really hard
to saturation most likely.? Load current at that power is likely near
a indicator and heating will be significant (for small heatsinks) if linear.
I have several linears using IRF510 mosfet, and I do not see that.
Be wary as when the die heats the gate threshold decreases at
roughly 10mv/degree-C and at that power its likely 100C++ if
not higher.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
Added is the bias circuit that I use and created by W1GHZ and N1JEZ
for use with high power amps? using RF rated LDmos parts.? Its posted
to show an example, not the only way.
I could only post an excerpt as groups.Io seemed to have a fit.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
Allison, This is for two IRF510s in push pull. Each draws about 2A. Input is 4Ax 13.5v = 54watts. The 100v peak to peak is at the secondary of the PA transformer. The drain swing is about 35v peak to peak. - f
On Sat, Nov 18, 2023, 11:25 PM ajparent1/kb1gmx < kb1gmx@...> wrote: Several things...
One IRF510 spec is 100V breakdown.? More than that and poof.
Also sustained current should be 4A or less (the 5.6A is for
saturated switch mode).
So the question is why it it 100V P-P?? It suggests an open relay,
wrong band LPF selected or seriously bad SWR.? ?Last case is that
the Drain load (transformer) is too high.
100V P-P across 50 ohms is well over 25W,? if its across a 50 ohm
load thats about 35W.? At 13V that's pushing the device really hard
to saturation most likely.? Load current at that power is likely near
a indicator and heating will be significant (for small heatsinks) if linear.
I have several linears using IRF510 mosfet, and I do not see that.
Be wary as when the die heats the gate threshold decreases at
roughly 10mv/degree-C and at that power its likely 100C++ if
not higher.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
why is there an npn transistor used as temp sensor?? would not a
simple 1n4148 also do the same thing??
dg9bfc sigi
Am 18.11.2023 um 19:13 schrieb
ajparent1/kb1gmx:
toggle quoted message
Show quoted text
Added is the bias circuit that I use and created by W1GHZ and
N1JEZ
for use with high power amps? using RF rated LDmos parts.? Its
posted
to show an example, not the only way.
I could only post an excerpt as groups.Io seemed to have a fit.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
DJ9BFC,
Yes it could and no not in that circuit.? However to get the needed thermal
coefficient it needs to be two diodes in series.
The npn is a 2n3904 that is mounted on the heatsink near the device.
It was picked as well characterized, doesn't hurt that its cheap.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
Farhan,
You didn't specify where or details.? This is the revised sBitx or some proto?
What the bias when cold and after immediately after a keydown session?
While my amps do not exhibit this, I've seen it in others especially when??
heating is an issue.? ?
Examples are the usual Ebay "70" watt amps?and similar copies.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
I put a copy of the full article in the files area under my call.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
It would be interesting to see if the drop in power is reduced with aluminum nitride insulators.? Previous work has suggested it will decrease the rise in temperature of the die
For a very simplified transmitter, it does reasonably well! ?Different goals and different constraints on different designs. ?Each has its place. ??
toggle quoted message
Show quoted text
On Nov 18, 2023, at 13:40, ajparent1/kb1gmx <kb1gmx@...> wrote:
?I put a copy of the full article in the files area under my call.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
Gordon, I failed to mention but I have tried it with Aluminium Nitride, Silpads (Grey and Blue) and Mica insulators, there was no difference at all. I should get a contact sensor to measure the rise of the heatsink temperature to know how these two are related. I will further document this over the next few days.
It would be interesting to see if the drop in power is reduced with aluminum nitride insulators.? Previous work has suggested it will decrease the rise in temperature of the die
For a very simplified transmitter, it does reasonably well!? Different goals and different constraints on different designs.? Each has its place. ??
Gordon Kx4z? On Nov 18, 2023, at 13:40, ajparent1/kb1gmx <kb1gmx@...> wrote:
?I put a copy of the full article in the files area under my call.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
Farhan;
I switched out the silpads in my DE for Alumina Ceramic insulators. The attached thermal image photos show the stock silpad under the right FET and an Alumina Ceramic insulator under the left FET. The transmitter was key-down at 30 Watts for maybe 5 seconds. I saw a pretty dramatic difference between the two. I had planned to take some better images, as well as make a short movie showing the temperature difference resulting from an insulator change-out, but the FET on the right was having no more of it and shorted out.
No doubt that Aluminum Nitride insulators would work even better. I was able to get a lifetime supply of Alumina Ceramic insulators from Amazon in two days for not a lot of $$, so I'm happy with this solution.
My conclusion is that there is room for improvement from the stock silpads.
I posted a video in the files section a couple months ago - it shows that the FETs stabilize in temperature relatively quickly.
73; Steve, N3SB
|
Ashhar, can I first of all congratulate you on all the work on the sbitx V2 and V3. This new V3 I have upgraded to is superb and has gone well above all my expectations. It must surely has made the project more appealing to the new and old enthusiasts.
Farhan;
I switched out the silpads in my DE for Alumina Ceramic insulators. The attached thermal image photos show the stock silpad under the right FET and an Alumina Ceramic insulator under the left FET. The transmitter was key-down at 30 Watts for maybe 5 seconds. I saw a pretty dramatic difference between the two. I had planned to take some better images, as well as make a short movie showing the temperature difference resulting from an insulator change-out, but the FET on the right was having no more of it and shorted out.
No doubt that Aluminum Nitride insulators would work even better. I was able to get a lifetime supply of Alumina Ceramic insulators from Amazon in two days for not a lot of $$, so I'm happy with this solution.
My conclusion is that there is room for improvement from the stock silpads.
I posted a video in the files section a couple months ago - it shows that the FETs stabilize in temperature relatively quickly.
73; Steve, N3SB
|
Ashhar thanks for all that experimentation!!!!
toggle quoted message
Show quoted text
On Nov 18, 2023, at 21:43, Owen Baldwin <owenbaldwin@...> wrote:
? Ashhar, can I first of all congratulate you on all the work on the sbitx V2 and V3. This new V3 I have upgraded to is superb and has gone well above all my expectations. It must surely has made the project more appealing to the new and old enthusiasts.
Farhan;
I switched out the silpads in my DE for Alumina Ceramic insulators. The attached thermal image photos show the stock silpad under the right FET and an Alumina Ceramic insulator under the left FET. The transmitter was key-down at 30 Watts for maybe 5 seconds. I saw a pretty dramatic difference between the two. I had planned to take some better images, as well as make a short movie showing the temperature difference resulting from an insulator change-out, but the FET on the right was having no more of it and shorted out.
No doubt that Aluminum Nitride insulators would work even better. I was able to get a lifetime supply of Alumina Ceramic insulators from Amazon in two days for not a lot of $$, so I'm happy with this solution.
My conclusion is that there is room for improvement from the stock silpads.
I posted a video in the files section a couple months ago - it shows that the FETs stabilize in temperature relatively quickly.
73; Steve, N3SB
|
After I modified the output stage circuit at SBitx v2, I only now had the opportunity to test it in reality. There were several reasons for this, I didn't have a usable antenna, which is important. After switching to v3, I tried the device on the air for the first time. I replaced the AlN insulators when it was first discussed here on the forum. I also used a very small amount of Arctic Sliver 5 additive on both sides of the ceramic insulator. This is the weakest link in the heat conduction used here. I find that in long-term FT8 use, the heatsink on the back is approx. 40 degrees, then the current consumption decreases compared to the start. So the regulation takes place so that the output power decreases slightly (according to the instrument, about 1W). I use the device from 12.5V with a current limit of 6.00 Amps. I replaced the IRFZ24Ns once. Since this is from early production, it ran for some reason during setup. The point is that so far I am satisfied with the heat conduction, I have made about 50 connections with the freshly made top-fed delta loop antenna. I scaled it to 7MHz and it has good parameters at 7-14-21 MHz. I would like to thank Ashar for his persistent work. The design is well developed for visual and usability reasons. I didn't take a picture for this topic, but you can find them in another topic. -- Gyula HA3HZ
|
Most prominently exhibited by those poorly designed amplifiers that do not account for thermal effects to bias related voltages. ?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ?
toggle quoted message
Show quoted text
On Nov 18, 2023, at 12:36 PM, ajparent1/kb1gmx <kb1gmx@...> wrote:
?Farhan,
You didn't specify where or details.? This is the revised sBitx or some proto?
What the bias when cold and after immediately after a keydown session?
While my amps do not exhibit this, I've seen it in others especially when??
heating is an issue.? ?
Examples are the usual Ebay "70" watt amps?and similar copies.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
|
Whow about real RF transistors?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ?
toggle quoted message
Show quoted text
On Nov 18, 2023, at 10:45 AM, Joel Caulkins/N6ALT <caulktel@...> wrote:
?Farhan,
I know you have always preferred the IRF510s, but have you considered the IRF530s? I have noticed that most of the cheap Chinese Linear amps use them to get a good 45-50 watts. I just bought a Micro 50 Plus amp that uses a set of 530s. From all the research I have done on the amp, it seems that it is pretty stable and reliable as long as we don't exceed 50 watts. Might work great for a steady 40 watts on the V3.
Joel
N6ALT
|
I have noticed this even on LDMOS. i dropped AFT05MP075, a 75 watts LDMOS transistor in place of the IRFs. In this setup, there are no thermal pads. The LDMOS is directly soldered onto a small copper block that was bolted to the heatsink. That too exhibited this behaviour.? I also (dangerously) tried tweaking the bias while the output reduced, during transmission to see if the output level can be re established with bias change. That didn't work either. - f
On Sun, Nov 19, 2023, 1:51 PM K9HZ < bill@...> wrote: Whow about real RF transistors?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it:
email:??bill@... ? On Nov 18, 2023, at 10:45 AM, Joel Caulkins/N6ALT <caulktel@...> wrote:
?Farhan,
I know you have always preferred the IRF510s, but have you considered the IRF530s? I have noticed that most of the cheap Chinese Linear amps use them to get a good 45-50 watts. I just bought a Micro 50 Plus amp that uses a set of 530s. From all the research I have done on the amp, it seems that it is pretty stable and reliable as long as we don't exceed 50 watts. Might work great for a steady 40 watts on the V3.
Joel
N6ALT
|
WK4DS
Joel Caulkins/N6ALT
HA3HZ
K9HZ