1. Home
  2. General Electronics
  3. How to cool a collector of a transistor that is capacitance sensitive?

How to cool a collector of a transistor that is capacitance sensitive?

Hi, I have a circuit with many transistors in TO126 package that have to be cooled. They should be mounted onto a grounded Heatsink but the collector has RF on it and this node is very capacitance sensitive. I took transistors with low capacitance, but when I mount the collector to the heat sink, even a thick alumina insulation washer makes me over

2 pF per transistor, which is almost double the transistors collector to base capacitance :-( (0.64cm*0.77cm)/0.15cm * 0.0885pF/cm*Er with Er around 8 :-(

What can I do instead?

Greg

Reply to
Greg
Loading thread data ...

Is it a tuning issue?

Any chance of neutralizing the capacitance?

Is the frequency high enough to make a strip line design practical - then your heat sink might be part of the circuit board or mounted to it.

Minimize it by using a small part mounted heat sink? And fan . . .

In P.A. stage in a VHF application you might use part of the output tank or filter inductance. We used to press TO18 transistors into the end of 1/4" copper tubing tank circuits to cool them . . .

--

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Reply to
default

Off top of head,

formatting link
may be suitable.

Reply to
bhauth

Hi,

it is for broadband use so the capacitance can not be tuned away by a tank circuit. Also the output/load impedance is too high (500 ohms) to include the capacitance into a microstripline. I thought about using bare chips, but the transistors are not available as chip (at least not at the quantities we need). If they were, I could maybe use thick AlN as an isolator as it has a good ratio of thermal conductance to dielectric constant (180w/mk to 8). I know only BeO (260w/mk to 6.5, toxic) and diamond (2000w/mk to 5.7, a bit expensive) with better ratios. (Do you know better/cheaper ones?) I also thought about using AlN cubes with 3x3x3mm placed directly under the position of die mount under the TO126 package, but don't know how to fix the cubes and the transistors on them. Thermal conductivity of the 3x3x3 cube would be enough and I could keep the dielectric constant of the remaining area of the package to heat sink at 1.

Any other ideas?

Greg P.S.: Would like to not use fans because of noise and maintainance issues

Reply to
Greg

What's wrong with the obvious answer of sticking them in oil, anyway?

Reply to
bhauth

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.