15v Halogen switched mode power supply

[cmgill]

Hi,

This is my first post so excuse if I go on or am unclear.

Basically I have a power supply board inside a unit that drives a 150w 15v halogen lamp. Now the transistors have had to be replaced as they overheated. What please could cause this, could it be the toroidal coil thats pulling too much current, and if so what replacement would anyone recommend.

This smps employs the BUL59 transistors apparently they are arranged in a "push-pull" configuration, but it did get very hot.

Look forward to your replies.

Regards

Colin.

PS. This happens to be a disco lighting fixture, I know this is diyaudio but I am a bit of a loss. Help!!!

[Mooly]

Well I can only offer at best general comments... I have never worked on anything like you describe, or know much about it, although I am very familiar with working on SMPS in commercial equipment etc... but never lamp dimmers However it's always good to expand your knowledge.

First a link to something that may be like you describe,
http://www.datasheetcatalog.org/data...cs/mXyzxvz.pdf

Transistors used as switches (The BUL59) are used such that they are either fully on, so there is little voltage across them but large current flowing, or fully off, high voltage across them but no current flowing. Used like this they run "cold".

Problems... if they are hot the usual cause is that the switching isn't correct, if they are ina a push/pull arrangement there may be "overlap" in the conduction where both devices are on for a short time resulting in the overheating, or the drive waveform may be wrong.

If the unit has a control circuit using an IC etc, any electroylitic capacitors are always favourite failure items, they are highly stressed in these types of application. Cap failure is the cause of 95% of PSU problems in SMPS.

Other than that it's hard to say without seeing a full circuit and testing and measuring... they are so many possibilites.

[Eva]

This is probably a self oscillating circuit. Transistor selection is critical, the wrong type of transistors won't switch fast enough (or won't have a low enough saturation voltage) and will get really hot and fail. Be aware of fake transistors.

Open capacitors or resistors, or shorted diodes (or wrong type), may give the same result.

Were the old transistors really damaged? Replacing stuff that is not damaged is not a good idea in switching power supplies, unless you know that the new parts are exactly the same as the old ones.

Note that transformers are very unlikely to fail.