V² control with capacitor current ramp compensation using lossless capacitor current sensing

Abstract

V 2 control is a popular control architecture in point-of-load Buck converters due to its simplicity and fast transient response. V 2 control with ceramic capacitor has instability issue. This paper reviews the existing solutions and their limitations are studied. A solution using output capacitor current ramp to stabilize the control loop is proposed. The analog capacitor current sensing method based on impedance match is used in the implementation of lossless current sensing. A small signal equivalent circuit is proposed for the analysis of the proposed control method. The proposed solution is superior to existing compensation methods for the excellent transient response for a wide range of duty cycle. The effectiveness of the concept and implementation are verified by experimental results.

Overview, equivalences and design guidelines of v¹ concept: A voltage mode control that behaves as a current-mode with near time-optimal response

“…Consequently, from (6), zi = K v /(KiC) « l/t r . This is the same design guideline obtained in [11] from the Describing Function modeling. Additionally, z 2 needs to be placed so that the phase margin is at least larger than 60° to avoid a peaking in the output impedance [17].…”

“…Some ripple-based controls such as v 2 [1,2] and v 2 i c [6,11] only sense the output voltage but they are in nature current-mode controls. This is because the output voltage has inherently information of the capacitor current (from the ESR of the capacitor or by using a trans-impedance amplifier).…”

“…Using the capacitor current to improve the dynamic response is not a new concept since it dates back at least to 1986 [10], but [6] improves the idea by using a simple lossless sensor of the capacitor that only measures the output voltage and that takes into account the ESL of the output capacitor. In 2013, [11] proposed the same concept with a different implementation of the sensor of the current. Later in 2014, Google™filed a patent including the same concept as in [9] but with a different sensor of the capacitor current [12].…”

Improved transient response of controllers by synchronizing the modulator with the load step: application to v2ic

“…This way, the control presents a very fast response under load perturbations. The error of the output voltage is also added in the fast loop and, consequently, a step in the reference voltage In the literature, a simplified version of V 2 I C control, where the reference voltage is not included in the fast loop, is presented in [4] with constant on-time modulation and no artificial compensating ramp (see Fig. 2).…”

Comparison of the behavior of voltage mode, V² and V²I<inf>c</inf> control of a buck converter for a very fast and robust dynamic response

“…The current through the output capacitor is sensed indirectly by means of a trans-impedance amplifier [9] in order not to introduce losses in the converter. The reference [10] • subsection III-A explains the proposed optimization algorithm to design controls of power converters. This method is used later to optimize the controls of the comparison for a very fast and robust dynamic response.…”