Re: How to create a ramp generator for boost converter contr
I think this frequency is fine, I build 100kHz boosts on breadboard and they work. There will be some efficiency loss due to charging and discharging the stray capacitances, but I don't think you are going for an efficiency record..
Anyway, here is a common ramp gen I have used. It charges a cap to Vcc/2, then discharges it to 0 and starts again.
Here's how it works - On startup, R1/R2 create a threshold of Vcc/2, and the comparator gives a low output so the cap can charge up. When the cap charges up to Vcc/2, the comparator goes high. This shorts out R2, changing the reference voltage to Vcc/10 and discharges the cap quickly. Once the cap falls below Vcc/10, the comparator goes low and the cap can begin charging again.
Q3/Q4 is a current mirror used to charge the cap with a constant current. The current is (Vcc-0.65)/R7. You could simply use a 10k to Vcc to charge the cap, but the ramp would have a slight curve to it - not a big deal for experimentation, but that curve means that the gain of the PWM modulator would change slightly as the duty cycle changed - you can tell your lecturer that so they think you are a PWM master!
Usually, when I use this circuit to build a boost converter, I use the output of the comparator to force a reset on the PWM flip-flop, giving a nice max duty cycle gate and preventing the loop from locking up in the ON state.
Do you want me to draw the rest of the boost?