Help...problem with test circuit of SG3524..with duty ratio.

[AbhimanyuSingh]

This is one of the test circuits given in datasheet of SG3524, with a little modification, mostly insignificant, like changing the value of inductor and using just one BJT instead of 2, as given in datasheet. I am not getting any output there. The expected % volts is not there. The signal out of pin 13 has a duty ratio of almost 0. What could the reason be. What possible mistake have I made. Even if I change the voltage at pin 2 (off course remove this voltage divider and put a rheostat), I dont find any change in duty ratio.
Also once it happened, in another occasion that I was getting 2 volts at that point once I switched on the circuit and that voltage remained there even after i switched off the power supply. What could be a reason for this.
The circuit is built on a bread-board. So its a little lousy, but there were no loose-wires.
I am posting the circuit diagrams. The first file is the circuit i have built and the second file is the test circuit given in datasheet of Texas Instruments.

 

[kabiru]

Re: Help...problem with test circuit of SG3524..with duty ra

Hi,as I can see in the diagram pin 13 and 12 are link together and they are out me phase with the other,try and use only one output either pin 12 or 13 and see the response.

 

[FvM]

Re: Help...problem with test circuit of SG3524..with duty ra

use only one output either pin 12 or 13

To achieve up to 100% duty cycle, both outputs are usually connected in parallel, although not required for 28 to 5 V buck. But no problem.

I doubt, that the transistor can achieve 1A with a few mA base current. In addition, without a base-emitter resistor, the switching behaviour will be very poor. It's a bad idea to use a single transistor instead of a darlington without applying respective changes to the circuit.

Anyway, the circuit can be expected to give regulated output with a light load (possibly a minimal load is required). If it doesn't work at all, it's rather a problem of wrong breadboard wiring.

 

[AbhimanyuSingh]

Re: Help..problem with test circuit of SG3524 duty ratio..

After I put the base-emittor resistor, there appeared a pulse on pin 12 and 13,when they are not shorted together, both of duty ratio 0.5. But the duty ratio do not changes if the voltage at pin 2 is changed. The voltage at pin 1 is 0, since the output voltage is 0.
I can see pulse of 0.5 duty ratio on the base of PNP BJT, which oscillates between 2 V to 20 V. The voltage at emittor of the PNP BJT is continously 20 Volts. Still the voltage at the collector never increases. What keeps it always low.?

 

[FvM]

Re: Help...problem with test circuit of SG3524..with duty ra

I can see pulse of 0.5 duty ratio on the base of PNP BJT, which oscillates between 2 V to 20 V. The voltage at emittor of the PNP BJT is continously 20 Volts.

Look sharp. Either the transistor isn't a PNP, it's wrongly connected or - burned.

 

[AbhimanyuSingh]

Re: Help...problem with test circuit of SG3524..with duty ra

Thank you again..
The transistor is a PNP. Its connected right. I checked it 1000 times. I will assume its burned.