Thermal Runaway Limiting

A project log for 45W RF SSB CW AMP (EBAY) DIY Build = 55W Upgrade

This is a guide to build & improve this $14 DIY Ebay kit as the OEM docs (also incl. here) are borderline Chinglish.

mosaicmercmosaicmerc 08/30/2017 at 23:240 Comments

For continuous use temperature compensation on the output transistors is required to prevent thermal runaway.

This was achieved with a pair of SMT 47K, 3960 Beta NTC thermistors direct soldered to the source pins for ground and thermal reference. The thermistors were jumpered per the image to the RV1 and RV2 joints with L6 and L7.

The Results with no cooling fan @ 27C ambient.

After about 30 mins to hit steady state:

  1. DC bias to the gates reduced by 110mV (thermistors at work)
  2. Overall current draw dropped from 4.69A @ 14.5V to 4.59A
  3. Overall RF out dropped from 45W 'cold' to 43.5W steady state
  4. Output transistor TO-220 TAB temperature held at 92C peak.

Note the images below for assembly and thermal performance.