"A time-varying piece-wise linear waveform is necessary for PWM operation. In
the case of voltage mode control, a ramp signal is generated inside the controller and used for the time-varying waveform. However, for the purpose of PWM, other linear waveform can be used. In particular, the triangular waveform can be employed for the PWM process." - Quoted from Prof. Choi's book.
Review through many papers, they mainly used ramps instead of other "time-varying piece-wise linear signal generators" such as triangular waveform.
I am wondering why?
P/S: In order to design ramp, high speed comparator should be used. However, the high speed comparator is power-hungry block.
You mean that the ramp generator will consume more power than the triangular generator? Both waveform carriers will generate the same current ripple at the same Ts (for buck example). Then, the problem just is that the design and making of ramp and triangular generator. Which one is better ? I think as an IC designer you should know that answer.
Thanks Mr. Nguyen,
Related to IC, ramp is more power-hungry. That's why I am still wondering. I suspect the reason linking to control of PWM, not power.
Regarding the power-hungry issue, I think it depends on the comparator architecture.
Since, many papers talking about ultra-low power DC-DC converters (power delivered to the load in micro range ) with high-efficiency and they use comparators as well, so these comparators do not consume too much.
Thanks Ragheb,
I suspect it relates to control, not power. However, the ramp needs a higher-speed comparator as compared to triangle if all the terms are setup similarly.
I believe the main reasons are that (a) pulse width can be better controlled with a triangular waveform, and (b) harmonics produced will be less, due to lower dV/dt. In terms of overall power, it shouldn't make much difference because this part of the circuit is not operating with high V or current. I suppose the triangle may take a little more power to produce, but negligible difference in terms of overall circuit efficiency.
Thank you for your comments Ms. Lois Gray. Recently, I found that the triangle bring out better charge balance in relation to the output capacitor of the DC-DC converter; thus eliminating the ripple. Also, I fully agree with you about the harmonic produced will be lower (that may help lower EMI, e.g. ). Under the perspective of power consumption, the triangle consumes less power than ramp generator since the slope is lower as compare to the ramp counterparts. This is mostly because the ramp need high speed comparators.
Anyway, thanks all for nice discussion.
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