I already had a square wave with duty cycle 50%. I use this signal to control ON/OFF of switches in Boost converter. Now I want to modulate the square wave signal by PWM. The control should keep frequency unchanged with D=50% and only change pulse width. In other words, I want to change the pulse width of that signal based on feedback voltage from output. Is there a simple way to do that in CMOS technology? I want a simple solution and don't want to use ramp wave because it consumes too much power. Also I don't want to use microcontroller or commercial ICs.
A simple control IC may fit your requirement. Check this out and find a suitable parts based on your converter parameters.
http://www.ti.com/lsds/ti/power-management/step-up-boost-converter-products.page
The datasheet will provide the common connection which you can just follow and not need to fully understand it.
Well, thank you but I want to design it myself. I don't want to use microcontrollers or ICs.
It is the RC combination of the external circuit that decides the duty cycle.A potentiometer of appropriate value with a chosen capacitor will suffice your requirement.
best regards
vasanth
A simple way to change the pulse width without a ramp is to:
1. create a delayed copy of the original input signal (you can use current starved inverter chains to do this).
2. Use a logical operation such as AND to combine the two signals. The offset in delay between the two cells results in a narrowing of the combined pulse at the output of the gate.
Think carefully about how quickly you want the switching element to change between off and on. Do it too slowly and it will dissipate power during the transition reducing efficiency and generating heat.
What is meant by "keeping the duty ratio constant at 50%" or equivalent of that and "PWM"?? In PWM, the duty ratio is varied. So one cannot keep the duty ratio as 50% and vary it too. Please clarify. Are you needing to vary PWM so that average duty ratio is 50%?? One method could be to modulate the frequency and use a divider which always gives 50% duty ratio.
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