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Buck converter PFM/PSM in light load questions

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S.Quoc.S

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Hello,

I'm making a current mode control buck converter for wide input range. For high load, I designed the PWM scheme, and it worked already. I'm working now for the light load optimized operation mode. I want something with lower switching frequency corresponding to the load current, so I looked in the literature and papers, and found the PFM and the skip mode thing (PSM).
I'm confused between PFM and PSM, since there is no clear definition as far as I found. Are they the same?

From what I've seen for PSM (skip mode) designs then the fixed frequency switching clock pulse is still used as normal PWM. It is just that the PSM logic will determine to gate the clock at next cycles or not based on comparing the output voltage and the reference voltage. For PFM designs, there is no such clock synchronization in the scheme. If the output is below threshold then controller will issue constant pulses to pump power to load. If output is fine then just shut all switches.

By the way, is the compensation ramp to prevent sub-harmonic oscillation necessary for current mode control Buck in DCM and PFM/PSM mode? I read that there is still oscillation for >2/3 duty cycle and a compensation ramp with a slope of at least 0.086 of the down cycle slope should be used to ensure full duty cycle range. I simulated without one, and saw no oscillation at all. Could anyone comment on this?
 
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Easy peasy

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slope comp only needed in CCM, min slope to add is 1/2 of down slope inductor current signal, V/uS, only needed above 45% pwm duty cycle.

note; highest Vout gives highest downslope, requiring highest slope comp.
 

S.Quoc.S

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slope comp only needed in CCM, min slope to add is 1/2 of down slope inductor current signal, V/uS, only needed above 45% pwm duty cycle.

note; highest Vout gives highest downslope, requiring highest slope comp.
Thank you for the confirmation, then I will disable the slope generator during DCM skip mode.
 

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