Tl783c figure 22 (Adjustable regulator with current boost and current limit)

Victor,

What is your dropout voltage over the BJTs? At these power levels thermal problems can easily arise. For example, if you Vin is 85V, you are disappating (85V-80V)*8A = 40 Watts across the pass device, which will need heatsinking and possibly forced air cooling to keep the pass device in a safe operating area.

Regards,

David

Hi David:

Vin=92-95 volts, Yes the pass device is mounted on a black heat sink with the other two pnp transistors To-3 style.

and the fan is blowing down on the devices. 

Victor,

Did you say both the TL783 and NPN are getting damaged? At the same time of one or the other.

The current limit in Figure 20 is not very precise. Do you know if the failure occurs during power up, power down , or during a load fault?  

Hi Ron:

Sometimes the tl-783  and sometimes the npn is shorted both base to collector and base to emitter. We have recently installed a fuse (6)amp slow blow) between the source of the dc and the pc board. Both the input and output capacitors take a long time to discharge. The failure occurs mostly during power down, the supply draws idling around 1.5 amps.

Regards,

Victor Morita

Victor,

If VIN falls faster the VOUT at power down, then the NPN transistor can breakdown and may be damaged.

A high voltage diode from 125V (anode) to VOUT (cathode) will fix that.

The red line shows current flow for VOUT>VIN and NPN base emitter breakdown.

Hi Ron:

You are suggesting  a 200v, high current diode.(10 amps). Is this correct.

Regards,

Victor Morita 

Victor,

At least 200V. The current is based on how fast the VIN falls. 10 amp should be more than enough.

Hi Ron:

When we put a eight amp diode across the npn it works but the side effect is we cannot adjust the tl783 down to 80volts dc it stays around 90-100vdc(100vdc is the input voltage).

Victor,

Is Cathode connected to NPN collector and Anode connected to NPN emitter?
On many diodes the band marks cathode side.

If it is backward then VOUT will be close to VIN.

Hi Ron:

I have it working now. I have had to substitute the TIPL762 with a buv21G because the Tipl762 only can carry 6 amps

continuous. Turning on and turning off now doesn't blow the buv21g.  A direct short on the output finally shorts the

buv21g both base to collector and base to emitter. which I don't understand.

Victor,

With a shorted output, there will be the full input voltage across the transistor at the same time as maximum current. Even with a heat sink, the transistor won't survive long. A slow blow fuse should protect the transistor. 

Hi Ron:

    Since my application is 8 amp continuous max. Are you saying use a 9 amp slow blow fuse.

Regards,

Victor Morita

Victor,

I don't have a lot of experience sizing fuses. I do think 9 AMP SB will be good.