Hello,

I am in need of some clarification on slope compensation. After reading some articles on Rohm, and the often suggested paper from Rob Sheehan on how to apply current mode control theory, I am just utterly confused.

For example:
1. "The Reason Why Compensation Ramp Slope must be at least 1/2 down slope" by Rohm
2. Robert Sheehan's paper " Understanding and Applying Current-Mode Control Theory "

https://techweb.rohm.com/knowledge/dentatsu/s-dentatsu/s-dentatsu03/3790
https://www.ti.com/lit/an/snva555/snva555.pdf?ts=1674662544454&ref_url=https%3A%2F%2Fwww.google.com%2F

I see three different suggestions on the slope of the comp ramp, and I am sure I have read so much that I am lost.
1. Rohm paper's math is 1/2 the inductor down slope (presume independent of topology - buck, boost, buck-boost)
2. Robert's paper I swear says 2 different things between page 5 and 6: a) " By adding a compensating ramp equal to the down-slope of the inductor current ", and b) " the optimal slope of the ramp presented to the modulating comparator input is equal to the sum of the absolute values of the inductor upslope and down-slope

Is the correct comp ramp 1) 1/2 down slope, 2) down slope, 3) sum of the up and down slopes?

What in the world am I missing? Even if we presume a peak current mode control buck.

 

One of the purposes of slope compensation is to prevent sub-harmonic oscillation, (Basso, 2014, pp. 173-175). in conjunction with Eq. (2.163), takes 2+ pages to develop, Basso posits:

\[ S_a\;>\;\cfrac{S_2}{2}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2.163)\]​

\( \text{where } S_a\text{ is the slope of the compensating signal and }S_2\text{ is the slope of the inductor discharge} \)​

​

The accompanying text says:

This is the minimum value that guarantees the stability for all operating duty ratios. In the literature other choices are often proposed, such as compensation levels up to 75%. One should keep in mind that overcompensating a converter seriously hampers its dynamic behavior but also reduces it maximum peak current and hence available power...​

​

I consider this a pretty definitive statement on the matter, but your mileage may vary.

 

Which book from Basso, so I can pick it up from Amazon? I know he has several.

I find it a little disappointing that this particular subject is not clear, IMO.
If I presume a fixed slope compensation ramp as some devices do, and then apply 1) 1/2 down slope, 2) downslope, finally 3) sum of up and down slopes to make sure I am choosing the optimal inductor, that leads to an increasing inductor value with the largest inductor value being calculated with the sum of the up and down slope option. If 1/2 downslope is the optimal slope balancing sub-harmonics and transient behavior then the calculated inductor value using #3 would hinder my converter's transient performance.

 

Basso, C., Switch-Mode Power Supplies, 2014
https://www.amazon.com/Switch-Mode-Power-Supplies-Second-Simulations/dp/0071823468/ref=sr_1_4?crid=19LT8KT0RKJ62&keywords=Basso,+Christophe&qid=1674696736&sprefix=basso,+christophe,aps,258&sr=8-4

Get the chaplain to punch your TS card. Everything is a compromise. IMHO you should avoid obsessing over the inductor value there are much bigger fish to fry.

 

I had a copy in my personal library - First Edition. I should consult my own bookshelf more often to remember what I have.

I'll disagree about the inductor. Choosing the inductor value for ripple is important, but also stability. Many low power converters have internal slope compensation and knowing how to determine the boundaries with limited knobs to turn will help the system designer.