may i know to adjust the rise time and fall time using pwm controller chip??? any pwm controller to recommend???

 

may i know to adjust the rise time and fall time using pwm controller chip??? any pwm controller to recommend???

Get rid of parasitic capacitance and inductance.

 

thanks for the reply.. what do you mean by get rid of the capacitance and inductance??

 

perhaps you are confusing the PWM duty cycle and the terms rise and fall times. the speed at which a circuit can move an output from ground to Vcc is rise time. the opposite of that is fall time. they are not related to the timing of the PWM waveform that is programmed by the chip. rise and fall times are sometimes referred to as slew rates.

 

may i know to adjust the rise time and fall time using pwm controller chip??? any pwm controller to recommend???

 

In PWM we add saw-tooth wave to msg signal & it is applied to a slicer. depending on slicing level we get the pwm o/p. o/p is '0' when ever i/p below slicing level & is constant whenever i/p exceeds this level so we can't change the risetime & fall time

 

thanks for the reply. but can it be done with without programming???

 

perhaps you are confusing the PWM duty cycle and the terms rise and fall times. the speed at which a circuit can move an output from ground to Vcc is rise time. the opposite of that is fall time. they are not related to the timing of the PWM waveform that is programmed by the chip. rise and fall times are sometimes referred to as slew rates.

Thank you. That is correct. A PWM controller has no explicit control of rise and fall times. Reread carefully what I said. I did not say capacitance and inductance, those nouns are modified by the adjective PARASITIC.

If you meant high time and low time then you you should be able to control both the frequency, and the duty cycle. The duty cycle is the ratio of high time to low time and is usually expressed as a percentage. A 555 can be used to implement a PWM waveform as numerous other threads on this board have demonstrated. Tell us more about the particular requirements you have for this PWM signal.

 

thanks for the reply.. the requirements for the pwm signal is to control the turn on and turn off for IGBT switching performance. how to control the amplitude of the square wave but the voltage output must be 15v..

 

thanks for the reply.. the requirements for the pwm signal is to control the turn on and turn off for IGBT switching performance. how to control the amplitude of the square wave but the voltage output must be 15v..

Think that you getting you terms mixed-up. Amplitude is the voltage in a wave form. What you are needing to change in a PWM signal is the 'duty cycle'. Duty cycle is the time the signal is high verses the time it is low, this is what PWM does. Read this, maybe it will help you - http://en.wikipedia.org/wiki/Pulse-width_modulation

 

i roughly get what you mean, so am i alright to say that, say i can't control the amplitude??? the duty cycle for my project is 50%.

 

You can control the output voltage if you like. If your duty cycle is fixed at 50%, then it is just a square wave. It is not PWM without any modulation of the duty cycle.

 

may i know to adjust the rise time and fall time using pwm controller chip??? any pwm controller to recommend???

The MC34063 is a popular part - there are a couple of calculator utilities online for working out the external components.

 

thanks for the reply.. what do you mean by get rid of the capacitance and inductance??

If you aim for a high switching frequency; parasitic reactances in wiring, component leads etc become significant, you end up not getting the frequency you expected from the calculated component values.

Pushing the envelope on switching frequency is when you have to use synchronous rectifiers because ordinary ones have too much Trr, you also have to use ultra low ESR electrolytics.