power supply Why is my simple boost converter giving me such a high peak output voltage

Posted on Feb 6, 2014

The supply voltage is 5V and I am seeking to increase it to 12V with a load current of 1A, or an output power of 12W. I selected a switching frequency of 20kHz. By my math, I need a duty cycle of 0. 583 to do this, so the on time should be 29. 15 µs. Assuming an efficiency of 0. 90, the input power will be 13. 34W and the input current 2. 67A. What I expected to see is a voltage of 5V, perhaps with a slight ripple,

at point 2 (between the inductor and the NMOS) and a voltage of 12V with a ripple at point 3 (between the diode and the capacitor). Instead, what comes out is what looks like total chaos - I get a peak voltage of 23V that oscillates around 11. 5V at point 2 and a slightly lower peak voltage of just over 22. 5V that oscillates around 17V at point 3: On the hunch that my switching frequency might be too low, I tried increasing it to 200kHz (T=5 µs, Ton=2. 915 µs) and now I get something more like what I was looking for, which is a peak voltage of 12. 8V at point 2 (oscillating between that and 0V) and a peak of 12V at point 3 (oscillating about 11. 8V): There was significant ripple in the voltage. I tried increasing the size of the inductor to 100 µH but all it seemed to affect was the startup oscillation. So I increased the capacitance to 10 µF, and that seemed to work, the voltage oscillation at point 3 is much smaller. The image above is the result with a 10 µF capacitor.

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