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#qmx QMX and waste heat from the finals
#qmx
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Hi all, So, I wonder if there is any construction possible to dissipate the heat of the finals to bottom cover of the housing. My idea is: a piece of copperplate soldere to a bolt which is connected with the washer which is on top of the finals.? The copperplate must be in tight touch with the bottom of the housing. |
开云体育Frank,No problem here with heat even in a chat mode with key down time of 10 minutes or more. I think the QMX doesn't get as hot as the QDX. 73, Cliff, AE5ZA
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The flat surface of the BS170 contacts the surface of the PC board.? Being a six layer board this should dissipate heat better than the QDX.? (I do put a little thermal paste on mine to help conduct.)? Hans has explained that the washer on top of the BS170 is to press the BS170s down to make good contact with the board and not primarily to dissipate heat.
I have lost one BS170 on my QMX most likely due to high SWR possibly during? auto antenna tuning or even transmitting without any load - though I don't recall? doing this.? Even with better heat sinking it may not be possible to protect the BS170s from the power reflected into the radio from high SWR.? I believe Hans may build in firmware protection based on the SWR hardware sensing in the QMX. |
开云体育I've been following this thread and others regarding what takes out the Finals. I've built 2 QDXs and one QMX. Both the QDX rigs blew the finals within a short time of building. I don't know for sure why in either case. After that I put some serious heat sinking in the QDXs and haven't lost them in the 9+ months since. The other day I really, unintentionally for sure, tested the theory of an open antenna blowing the finals with high SWR. I was in a hurry and hit TX and saw no power out. Switched the meter to SWR and saw a SWR of 10. By that time it had been in TX at least 10 seconds. Quickly killed the TX and connected the antenna. Tried again and all was good. I've had several QSOs in Olivia chat mode with key down times of up to 10 minutes since. All is good. No damage evident yet.?Why the finals endured my attempt at their destruction I can't say, but I'm sure heat sinking helped. 73, Cliff, AE5ZA
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On Tue, Aug 22, 2023 at 06:45 PM, Chris KB1NLW wrote:
The flat surface of the BS170 contacts the surface of the PC board.? Being a six layer board this should dissipate heat better than the QDX.? (I do put a little thermal paste on mine to help conduct.)? Hans has explained that the washer on top of the BS170 is to press the BS170s down to make good contact with the board and not primarily to dissipate heat.Ah yes - makes sense. Mainly I was worried about IC503, which is over the discharge surface and gets very warm. |
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I strongly recommend using a VERY thin layer of thermal paste between the BS170’s and the PCB. Mechanical surfaces are not perfectly flat - neither the flat of the FET package nor the board. A thin layer of paste fills in these microscopic gaps. Don’t glob the paste down as this will make things worse. Use something like a credit or debit card to gently scrape away excess paste for optimum heat transfer. On Tue, Aug 22, 2023 at 10:29 AM Frank <dc8fg@...> wrote:
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I also lost BS 170 in a QDX unfortunately when it failed it put 12V on the driver (74ACT08) and killed it, so I had to replace it.
Hopefully Hans will eventually get protection against high SWR into a not too distant firmware upgrade.? I would also like to see a low power mode for antenna tuning.? I do have an external 50 ohm attenuator that allows safe tuning. |
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Won't work well because you are requiring heat flux through steel (the bolt) to get to the Cu . What you want is a thick Penny? (buy one from a coin dealer for example)? , put it on grinding paper say P80 grit? backed? by a wet? glass plate or kit bench and flatten it and then use P200 to get? smooth surface then Drill precisely in ctr using? a ctr finder if you have one ( or scribe against two chords ) or mound in a lathe chuck to find the ctr .? Cu has amazing density cw with what most people think - It's about 70% of steel. this makes it particularly good at absorbing the pulses of heat it would typically get in voice SSB operation.? (but not so good for? continuous digital modes). Brass is an unacceptable substitute .? The big problem is the rounded back of the BS? 170? cases - potentially you can lap them to a small rectangle say 1.6mm across x the TO92 case height? by hand and a jig? for improved HT but if you have the soldered on the PCB then you are sod out of luck unless you want to do a pull and resolder job. |
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The BS170 (finals) are low power devices with a thermal impedance of 150C/W.? Trying to reduce the internal chip temperature by lowering the already low external thermal impedance is pretty futile.?
(thermal impedances add together just like electrical impedance). So imagine a bad SWR causing the dissipation in the four finals to dissipate 2W, 0.5W each, then the rise within the transistor will be 75C.? If you find the mounting surface feels painfully hot to touch (cannot keep a finger on it) then it is 44C only a rise of 15C to 20C over the case temperature - a fraction of the transistors internal temperature rise.? Someone can take the time to do the thermal modeling, its quite simple. |
开云体育Wayne,There is just a hole through the plate to hold it in place and a foot that you can't see almost at the left hand edge under the plate the same as the one that is visible on the right hand side. It was made from a piece of aluminum molding that happened to have those two feet in the right place. I may try to put a piece of it under the finals on the QMX, but without the feet, as I'm sure there isn't enough room for the extra height of the feet. I also added the diode across L14 so hopefully it will be as bullet proof as I can make it and baring operator "destructive" testing should last a long time. Here's the reference from John Z to the diode "fix" in case you are interested. The diode simply is in parallel with the 47uH inductor L14, with the orientation shown in the photo provided by Paul W9AC. Cathode towards the power supply connection, anode to the transformer center tap. Any of the diodes he mentioned ?(1N4448, 1N4148, 1N914) will do a fine job of absorbing the end-of-transmission voltage spike it produces. I have used a 1N914. /g/QRPLabs/message/105256 73, Cliff, AE5ZA
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Thanks, Cliff. You've been a tremendous help on this. Where did you find the heat sink you have on top of the finals? Wayne KB4DSF On Wed, Aug 30, 2023 at 11:41?AM Cliff <ae5zaham@...> wrote:
-- Wayne Greene |
开云体育Wayne,I had it lying around from a Raspberry Pi 3B that I had put in a different enclosure with different cooling so the heat sink wasn't needed anymore. Wish I had more of them. 73, Cliff, AE5ZA
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Thanks, Cliff. I don't have any left over from my RPIs, but I found something on Amazon. I'm going to give that a try. Wayne KB4DSF On Wed, Aug 30, 2023 at 11:59?AM Cliff <ae5zaham@...> wrote:
-- Wayne Greene |
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Before making a radical change to the PA transistor (BS170s) thermal design it is necessary to fully understand the the thermal parameters.? As Hans ha pointed out:
? ?BS170 Thermal Resistance
?220C/W??(package only) 175
°
C
/
W
?(plated through-holes)
145
°
C
/
W
?(plated through-holes and?
0.25
i
n
c
h
2
?pad per lead) |
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I believe Chris is correct.? Me personally, I have never added a heatsink to any of my QCX, QCX+, QCX-mini, QDX and QMX, and I have yet to burn up any BS170s other than that night earlier this year when I lay in bed half-asleep at 4:30am in the morning listening to a massive thunderstorm crashing around, failing to calculate which flash belonged to which boom and therefore was it coming or going, and failing to quite motivate myself to get out of my nice warm bed and haul myself up to the lab and unplug the antenna coax.? In my opinion you're looking for solutions to a non-existent problem. On the other hand, it's harmless, and if it makes you happy, well, live and let live ;-)? 73 Hans G0UPL On Thu, Aug 31, 2023 at 5:14?PM Chris KB1NLW via <chrisrey1=[email protected]> wrote: Before making a radical change to the PA transistor (BS170s) thermal design it is necessary to fully understand the the thermal parameters.? As Hans ha pointed out: |
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Before making a radical change to the PA transistor (BS170s) thermal design it is necessary to fully understand the the thermal parameters.? As Hans ha pointed out:
? ?BS170 Thermal Resistance
? ? ? ? 220C/W? ?(package only) ? ? ? ? 175C/W???(plated through-holes)
? ? ? ? 145C/W???(plated through-holes and?0.25?inch sqr ?per lead)? With the multi layer board this is probably equivalent. It appears that the thermal path from a TO-92 (BS170) package is lowest through the pins. This is demonstrated by the significant improvement to 145C/W with good pin heat sinking. (From my investigations it appears that more than 50% of the conduction is through the pins). FYI conductivity of copper is ~400x that of epoxy. With a fast electrical transient thermal capacities may attenuate any observable rise outside the transistor chip before it fails. Adding an additional structure under the transistors is questionable: ? ? ? ? Added interfaces to between the transistors and the PC board/case primary heat sink ? ? ? ? In the cramped area of the heat sink there will be little air flow. ? ? ? ? I would strongly recommend against adding anything between the pins and the board. Test Good engineering practice mandates measuring the temperature of the current heat sink under the BS170s and comparing the temperature of the proposed heat sink.? Must be done in a full assembled QMX.? |
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Here I am obsessing about putting thermal compound on the outside of package, when duh, of course the leads are going to be the lowest resistance path for heat out of there.
That leaves the question of where the heat goes once it's in the ground plane. Maybe most of it gets conducted to the enclosure, and out the shielding of your antenna cable, via the BNC? I've got some silicone thermal pad, thick enough to fill the gap between the bottom of the PCB and the enclosure, I wonder how effective that would be at pulling heat out to the enclosure too. |
Timothy Fidler
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