Author Topic: Why is the voltage mode control Buck converter isotope triangle wave better for  (Read 392 times)

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Offline PCBindex88

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The control circuit of the voltage mode control Buck converter adopts a PWM control mode, and the output voltage and the reference voltage Vref generate a control signal vcon via an error amplifier. Since the horizontal width and height of any point on the sawtooth wave are linear, when compared with the control voltage vcon, a pulse u having a width proportional to the amplitude of the control voltage can be obtained. When the input voltage fluctuates or the load changes, the closed-loop negative feedback maintains the stability of the output voltage by changing the duty ratio of the control signal u.
Like the sawtooth wave, the isosceles triangle wave can also be used as the PWM control drive signal. Through simulation and experiment, I found that the converter system is more stable when the isosceles triangle wave is used for PWM control than the sawtooth signal.
What is the reason for this?
 

Offline David Hess

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In practice I have never noticed a difference but for the same frequency, the triangle wave has a slope twice as great so noise at the comparator input produces half of the timing variation per edge.  However since there are two comparator edges instead of one and the noise is uncorrelated, the timing variation will be 1/Sqrt(2) instead of 0.5 (one half).

I doubt the above difference caused what you found though.
 

Offline adamcord

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That's an interesting observation, but what exactly do you mean when you say the system is "more stable"?
 

Offline David Hess

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That's an interesting observation, but what exactly do you mean when you say the system is "more stable"?

I mean with the greater slope of the triangle wave, the PWM signal has less jitter even though two edges are contributing noise instead of just one with a saw-tooth.  But like I said, this should be insignificant compared to other sources of noise.

Switching power supply controllers typically use a saw-tooth for ease of implementation reasons and because the reset time of the saw-tooth is an easy way to generate fixed dead-time.
 

Offline adamcord

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Thanks for the details, but my question was in response to the original post. I'm fairly new here, did I reply incorrectly?
 

Offline David Hess

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Thanks for the details, but my question was in response to the original post. I'm fairly new here, did I reply incorrectly?

You did not quote what you were responding to so it was not clear.
 


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