SMPS auto start in half bridge topology

[BennVenn]

I've just scored myself a 12v 60A multi mode lead acid charger with blown BJT's in the power stage. Its of Chinese origin but I'm very happy with the build quality and design. Very neat layout, logical, going to be a pleasure to repair.

The power stage is pretty much plain as they come, standard half bridge, BJT drive transformer etc... This got me wondering though, how do these things start themselves up? The switchmode IC (pinout identical to TL494/KA7400 etc) is powered from the high current secondary winding on the transformer. There is an aux winding going to what would be two diodes but they were not fitted.

The spec sheet says regulated output from 0-60A output which implies a battery is not required to start the SMPS up.

So! with no resonant primary startup circuit, no biasing on the primary BJT's and no way for the TL494 to get a pulse through the drive transformer, how does it start up?

Maybe it does require a battery to kick it off? There is no way for current to flow from the battery back past protection diodes. Or is it something about the half bridge topology (voltage divider caps?) that gets an initial pulse through to start the whole lot up?

[amyk]

If it's anything like a half-bridge ATX PSU you'll find a high-value resistor between the primary DC rail + and one of the transistor bases.

[wraper]

If it's anything like a half-bridge ATX PSU you'll find a high-value resistor between the primary DC rail + and one of the transistor bases.

Sure that you won't find anything like that in ATX PSU because they don't need it. They have additional standby PSU.

[amyk]

Oops. I meant AT, not ATX.

[BennVenn]

Biasing is the same for both halves of the bridge yet it starts up with no battery attached. Looks like more reading will be required.

The secondary side rectifier (one of three) has gone short which explains the destruction in the primary. Dave's trick of a bit of spit on the finger revealed (part number sanded off) a Visahy part rated at 40amps, couldnt get the following two digits. I'll go with 40v? (It is a 6cell Lead acid charger)

All the base drive resistors are open but the biasing resistors are fine. Just replaced the resistors and 'borrowed' the BJT's from a smps downlight converter. Powers up fine and I can see the 3 states of the charging circuit switching as it powers a 12v halogen lamp (CC, CV then float)

Order placed with RS for the 6 original BJT's + 2 new schottky rectifiers. Cheap fix for a very expensive charger!