Trying to improve my amp's thermal management by moving a MOSFET  - Page 1
Forum section: electronics / beginners

author: gjpmho
author_rank: Contributor
timestamp: August 30, 2023, 09:26:00 am
content: Hi All,I'm trying to improve the thermal management in the Power Amplifier Section of my integrated amp. There is a particular hot spot I'm worried about. I am measuring 80C+ on the case of a SOT-223 transistor (Q12/112 - ZVP2120G). Is this too hot? This puts the junction near 100C, according to the SOT-223 junction-to-case thermal resistance of around 12C/Watt. I've calculated about 3/4 Watt (Vds * I) of heat being dissipated. The copper padding area under the MOSFET drain and PCB thickness are not dissipating the heat quick enough and heating up nearby components, like q19/119 and VR1/101 (trimmer). Another motivator is a noticeable sound degradation when ambient temperatures are above 25C hotter, where the amp sounds strained. I’ve added a small surface mount heatsink on top of the case via thermal tape. This helps a bit (~5C?) but is not a permanent solution. Thermal camera shows the heatsink is around 70C. However, the thermal tape will wear out and I don't want to worry about a loose heatsink rolling around. I've also considered adding a heatsink under the PCB which again means glue that will break down over time. No fans, since they will add dust and I don't want to turn them on and off.I’m thinking of moving the Q12/112 to a large DPAK breakout board with a fitted heatsink, and secure that to output device heatsinks above the PCB (these heatsinks run around 40-50C).I don't know if the location of this transistor is important for thermal coupling. Should it be near the same temperature as Q14/114 and Q19/119?1. An inch of wiring from the DPAK breakout board to the PCB pads.Will this create considerable noise? Will weaving the wiring reduce that noise?2. I plan to connect the heatsink to the drain which as I understand introduces EMI via the heatsink and can cause other issues? Again I don't know if these would be noticeable. I figure at best this setup will emulate the same MOSFET just in a THT package. Or do I electrically insulate with a SIL pad heatsink and connect it to ground? As it turns, per design, the heatsink for Q14/114 are connected to the Drain.In the end will I trade less heat (in the PCB) for noticeable noise and or other issues? This is just one component...Notes:I've converted several surface mount power resistors to THT above board to cool down the PCB. I am using a thermal camera to record temperatures. My goal is no more than 70-80C junction temperature with ambient temperature near 25C.ZVP2120G datasheet:https://www.diodes.com/assets/Datasheets/ZVP2120G.pdfBreakout board:https://oshpark.com/projects/SmLNbAES/view_designBreadboard:https://www.digikey.com/en/products/detail/sparkfun-electronics/PRT-08808/7387401Heatsink:https://www.mouser.com/ProductDetail/532-574502B33GStock photo of the amp:https://www.hifi-inside.com/images/phocagallery/hifi-ht/creek/integrated-amplifier/thumbs/phoca_thumb_l_destiny-1-creek.jpgPhotos of Q112 covered by the current heatsink, silk screen, and schematics attached.Any help would be appreciated. I'm trying to get a perspective and relevant details of the scale of noise or any other issues I might be introducing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2023, 11:33:32 am
content: Does it need to operate in 75C ambient or something?100C is 100% fine to leave as-is. Semiconductors handle temperatures much higher than your finger can, don't worry about it.Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: August 30, 2023, 12:30:12 pm
content: The objective is to decouple the heatsink from the transistor chip and leadframe.Use a bit of grounded shim as a screen. Place it between the transistor and the heatsink.That keeps the hf current from flowing in the heatsink body.Take care how you route shim ground back to PSU common.Now you can bolt the heatsink safely to the chassis.

author: gjpmho
author_rank: Contributor
timestamp: August 30, 2023, 03:24:52 pm
content: Thanks, I've read up to 75-80C is a safer more desirable junction temperature than 100C. It also is heating up nearby components. And during hot weather (25-30C+) the amp sounds strained as if the loud button is on, if that makes sense. Others have reported similar issues of sound degradation during hot weather. I suspect the sound degradation is caused by overheating... But yes the datasheet shows the maximum junction temperature is 150C.

author: gjpmho
author_rank: Contributor
timestamp: August 30, 2023, 03:26:37 pm
content: Would a thermal SIL pad work? And connect the heatsink to main heatsink, which is grounded?