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Buck converter with SG3524 and IR2117, problem with duty cycle.


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I have to build a buck converter, with variable input from 10 V - !5 V. Use a PI controller to maintain the output at 5 V. I have used SG3524. Now, it is clear that I should be able to attain a pulse width of duty ratio 33% to 50%, exactly speaking. And wider range of duty ratio will be even better. Also I am getting duty ratio of 20% to 70% from SG3524. But the problem comes after that. The output of gate driver IR2117 which is supposed to follow its input, does not follow it for a pulse of duty ratio more than 30%. Why is it so? If i decrease the duty ratio below 30 % the output of IR2117 follows input, but not for more than 30%. There is nothing mentioned so on the datasheet. The frequency i m using is 100 khz. is there any other option for me, like change the gate driver. Can anyone suggest me a gate driver which wont be having this problem. Or else can anyone tell why is this happening.
Ask if have missed some information to provide.

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Yes you've missed some information: the schematic.

There should be no need to manually adjust the duty cycle, the SG3524 should do it automatically. The duty cycle should also depend on the load current and will generally be a bit higher than the calculations suggest because of the losses.

The IR2117 uses bootstrapping to provide the high side drive required for an N-channel MOSFET so it's normal to have a maximum operating duty cycle but it should be higher than 30%. My guess is that the bootstrapping capacitor is too small so the undervoltage protection is kicking in: the maximum on time is a factor of the current drawn from the capacitor, the off time and the minimum operating voltage.

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  • 3 weeks later...

Thanks Hero999..
It was this bootstrap problem and I dealt with it.
Yeah I know I dont have to manually adjust the duty cycle, but at initial stage I used to adjust the positive input of comparator of SG3524 manually. I get good duty cycle from both SG3524 and IR2117. But there is a new prob now.
I have to connect a PI controller in my circuit to keep the output voltage equal to 5 volts. I have passed the output (of the buck converter, which is supposed to be 5 Volts, after scaling it down) to the positive output of the comparator of SG3524. The negative input of comparator is output of PI controller and PI controller's input is the output of comparator. I know this sounds confusing. If you ppl need the schematic, I will upload it next time. I have used LM324N for PI controller. The prob that I see is, "P" controller works alone fine. "I" controller works alone fine. I.e. if they are broken in to parts and connect them , they individually work fine. But when I short the input and output of "P" and "I" controller, the output no more remains as expected. Why dont the "P" and "I" work together properly?
Also when I use "P" and "I", I use the same IC for both.

Thanks in advance.


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  • 3 months later...

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