PSUB modification for 14V use

OH2NLT 16.01.2012




Old GSM/Tetra base station power supplies are already available in the surplus market. This is high quality RF-quiet telecom spec psu but with one handicap. Available outputs are

Not very useful selection for 13,8V ham equipment feed. So I decided to find out if the 24/26V psu can be changed to 13.8V. PSUB is available under many different brands but I have found all being the same construction. I have following brands

IL-Power, Efore, TH-elektroniikka There are also models build with SMT components around. OH2GWE is studying his treasure.

I managed to find also nice stand-alone box for the PSUB from flea market.

I also had two broken units. I decided to start the project trying to repair at least one of them and then try to modify the voltage.

Both units had the same fault. Large PFC switching fet 20A IRFP460 had failed and 27R 11W soft start resistor had also failed. I did not have such fets in my junk box but luckily in the same psu 24V switcher is done with 14A IRFIP450 fets. So I got neded parts from second broken unit. Input rail voltage after PFC is 400VDC in these power supplies. Repair attempt was a success.

Next step was to try modify the24/26V PCU down to 13.814V. Research how the 26V-control loop behaves when the reference is changed for lower output. Trim pot was used to get quick empirical results. It showed up that no modifications for the control loop filtering are necessary. With light load regulator is in the burst mode with about 600ns on time. Load increaced to about 20W regulator goes in to the pwm mode. Cycle time is about 10us. Required modification is just changing the TL431 regulator output sampling resistor value. 3k97 was calculated for the new value. Standard 3k9 resistor was used to replace the original 8k25. Bottom view. There are extensive Fault / Alarm logic in the PSUB. All voltages do have over and under voltage alarm. So 26V under voltage alarm comparator operation had to be modified. Solder 22k resistor as done in this picture and you have about 12.8V under voltage alarm for the new 13.8V setup. Again success.


Modification if nice enclosure for the PSUB is not available. Connector posts can be mounted directly to the PSUB chassis. Possibly useful +/-12V outputs was also mounted in to the PSUB rear plate. Final test done with real equipment.

Main board modification can also be done without removing the board. Connect 7k65 resistor over the original one. Near enough value can be build from 6k8+820R resistors. Yet another successful modification done.

Bonus. PSUB connector spec