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| Zbig:
I've just got an ARRAY 3721A. I'm quite impressed with it so far but I'm a bit surprised by one design decision. Peeking under the heatsink assembly, I could see eight pass transistors in what looks like a TO220 packages to me. Granted, the heatsink is beefy, there are two fans and the airflow is tunneled within a metal sheet shield but isn't asking the poor TO220s to dissipate 50W each maximum (the unit is rated 400W) stretching it a bit? I'm pretty sure they knew well what they were doing but still, why not TO247? I could imagine the thermal compound going bad on one of them and the heatsink-mounted temperature sensor not even noticing as the localized hot spot from a single TO220 wouldn't affect the average heatsink temperature much. Or am I overthinking it? I realized the last sentence didn't make much sense but anyway, seems quite strange to use TO220 in such device. |
| forrestc:
--- Quote from: Zbig on September 07, 2016, 09:49:08 am ---I've just got an ARRAY 3721A. I'm quite impressed with it so far but I'm a bit surprised by one design decision. Peeking under the heatsink assembly, I could see eight pass transistors in what looks like a TO220 packages to me. Granted, the heatsink is beefy, there are two fans and the airflow is tunneled within a metal sheet shield but isn't asking the poor TO220s to dissipate 50W each maximum (the unit is rated 400W) stretching it a bit? I'm pretty sure they knew well what they were doing but still, why not TO247? I could imagine the thermal compound going bad on one of them and the heatsink-mounted temperature sensor not even noticing as the localized hot spot from a single TO220 wouldn't affect the average heatsink temperature much. Or am I overthinking it? I realized the last sentence didn't make much sense but anyway, seems quite strange to use TO220 in such device. --- End quote --- I just picked a random FET datasheet with TO-220 and TO-247 options. This one has 0.45*C/W junction-case thermal resistance listed for TO-220, TO-247 and D2PAK. So, for heatsinked applications, it seems like it should be good (50W *0.45C/W = 22.5*C temperature rise, with a 175*C junction temperature max means the case needs to stay under 152.5*C)... So, this doesn't seem like a lot of a stretch. One would have to look at the thermals for the exact transistors and also the thermal resistance of the heatsink (case to ambient). |
| Zbig:
Thanks for checking that. Actually, it's case to heatsink resistance I'm more worried about. Do they specify that as well for the one you've checked? |
| nctnico:
IIRC you have to count on 0.5C/W for direct heatsink mounting (non isolated) when it comes to a TO220. After that you have to add the thermal resistance of the heatsink which can be another 0.5C/W even with air cooling. All in all that 50W easely adds up to a 75 degree C difference between the air and the die of the transistor. |
| Jester:
--- Quote from: MikeO on September 06, 2012, 06:02:16 am ---Hello all, I came across this thread while googling for a manual for a Kikusui PLZ300W electronic load. I need an electronic load for a project I took on, and am on a tight budget. I looked at the Array units and they seem nice. I ended up buying the PLZ300W, on ebay, for $360US, including expedited shipping. My main reason for making this choice was budget, and, in a previous life, I shared an identical unit with a couple of engineers and a couple of techs. I was in that lab for ten years, and that load was no spring chicken when I got there. It performed flawlessly for all the time I was there, and it was in high demand (lab was engineering department for a battery and charger manufacturer). I didn't over-think it, and went with what I know. I doubt it has the features many of you seek, but, as far as dependability and accuracy per dollar, it's a worthy consideration (if buying used). I think we all love new gear, but I've been so burned by Chinese made goods, I generally do what I can to avoid them. It may seem racist but, really, I've thrown away A LOT of Chinese made tools; frankly, I think there is some cultural differences, and they do not embrace the meaning of the word "quality" in the same way. Regardless, I did just buy that 1052E. I don't do much engineering these days, but have this one job, and need to set up a mini lab at home. I'm trying to do it for under $1000US, so as not to suck all the profit out of the job. I bought a Rigol 1052E direct from the manufacturer, two HP bench supplies (6289A, 6291A), and the PLZ300W, and, so far, with shipping, I've spent $833US. I still need a simple, low-speed function generator, and an array of test leads, but I think I'll make my budget. Considering that I've not been watching ebay for good prices, let alone hit up the HAM radio folks, but rather selected and purchased these items today, at the "buy it now" premium, I think one could do even better. Anyway, my point is, 30 year old equipment doesn't tend to inspire confidence, and usually lacks great, modern features, but the old thru-hole gear is very dependable and very repairable. Best of all, you can choose quality levels you couldn't otherwise get into the budget. The one caveat is the dark years of electronics: the previous decade. When Europe introduced the RoHS and WEEE legislation, it all really went bad. The lead free solders, tin whiskering, delaminated PCBs (from the higher heat for new solders) made for some of the worst lived electronics in the history of, well, electronics! I avoid gear from that era, whether it be an electronic load, or a car stereo. You can keep it So, there are some opinions from some random guy on the internet. Best of luck in your endeavors. Regards, Mike O' --- End quote --- FWIW, I have a couple of old school Kikusui loads (PLZ72W and PLZ152WA), still working fine. The one thing I really like about this old school equipment is the ease of use. Much like my go to Agilent E3615A power supply, you turn it on and adjust to the desired value. I had and sold an Array power supply nearly identical to the Array load being discussed, it worked very well functionally however the user interface sucked. The loud beep on every key touch was also annoying until I put a piece of tape over the transducer. The one thing I did not like about the PL72W was the single line display, so I replaced it with a cheapo but accurate ebay dual V/I display that fits in the original cutout. |
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