Boost converter not working

[abbyrox]

I am using a converter to boost 10V to 17.5V load current load resistance 47ohms. An SG3525 for the PWM signal at 138khz. The problem is that the boost converter shows short circuit(constant current on the power supply) when I increase the SG3525 voltage above 7.4V. In other words, when my IC is providing PWM, the boost circuit is being short circuited, the mosfet doesnt seem to be turning off. Inductance value is 39uH, capacitor is 29uF. Please help me out here guys! :-|

 

[BradtheRad]

Your values should work (according to my simulation). Duty cycle should be around 45 percent.

A boost converter requires more than one feedback.
(1) It needs to measure volt level on the load (by volt reading or current reading).
(2) It needs to measure current through the coil to turn off the mosfet, and to measure current in order to turn it on.

If the mosfet stays on, then your IC may not be getting a large enough swing on a sense resistor somewhere. Its ohm value must be high enough to create a sufficient voltage drop across it.

 

[ajayamrita]

BradtheRad, thanks a lot for your input and I get what you are saying. But can you be a little more specific on what is to be done?

 

[BradtheRad]

I'm sorry I'm not familiar with the SG3525.

Someone at a message board (link below) says it is tricky to use properly. It talks about having to close a feedback loop.

https://www.diyaudio.com/forums/power-supplies/98944-sg3525-loop.html

 

[picgak]

abbyrox
I can help you if you can post your circuit

 

[abbyrox]

My apologies for replying late. I have attached the circuit diagram. The mosfet is switching for a low duty cycle but for a higher duty cycle, the mosfet does not switch off. Both cases i get the output same as the input.
**broken link removed**

- - - Updated - - -

My apologies for replying late. I have attached the circuit diagram. The mosfet is switching for a low duty cycle but for a higher duty cycle, the mosfet does not switch off. Both cases i get the output same as the input.
**broken link removed**

 

[mtwieg]

Setting the comp pin voltage with a potentiometer like that won't work. The comp pin is the output of an op amp, so you should set its output by configuring it as a unity gain amp, and control it using its inverting input (with a pot, if you want). That should allow you for easier open loop control. Of course you should eventually use closed loop control with a proper feedback loop.

 

[picgak]

Hi abbyrox
For testing
1. remove the pot
2. connect pin2 to pin 16
3. ground pin 1 (actually feedback is to be given to this pin)
4. connect a capacitor 100n to pin9
5. insert a resistor 4.7R between pin11 and gate of FET
6. connect a resistor 1.5k from gate to ground of FET
Now test the circuit