Buck converter with current-mode control

Figure 4 shows a plot of the control-to-output loop gain with different values of external ramp. The additional resonant poles will give up to o additional phase delay. Besides, Figure 4 clearly shows the transition from current-mode to voltage-mode control as the slope compensation is increased.

This article presents a new hybrid feedback structure, as Figure 5 a shows. The idea of hybrid feedback is to stabilize the control loop by using an additional capacitor feedback from the primary LC filter. The outer voltage feedback from the output through resistor divider is defined as the remote voltage feedback and the inner voltage feedback though capacitor C F will be referred to as the local voltage feedback hereafter.

The remote feedback and local feedback carry different information on the frequency domain. Specifically, the remote feedback senses the low frequency signal to provide good dc regulation of the output, while the local feedback senses the high frequency signal to provide good ac stability for the system.

Figure 5 b shows the simplified small signal block diagram for Figure 5 a. The resulting equivalent transfer function see Equation 31 and Equation 32 in Appendix II of a hybrid feedback structure differs significantly from the transfer function of conventional resistor divider feedback. The new hybrid feedback transfer function has more zeros than poles, and the additional zeros will lead to o phase ahead at the resonant frequency determined by L 2 and C 2.

Therefore, with the hybrid feedback method, the additional phase delay in control-to-output transfer function will be compensated for by the additional zeros in the feedback transfer function, which will facilitate the compensation design based on the complete control-to-feedback transfer function. Apart from those parameters in the power stage, there are two more parameters in the feedback transfer function.

The feedback transfer function has been simplified to a new form see Equation 33 in Appendix II. As long as the condition is satisfied, the control system will be easily stable. The control-to-feedback transfer function G P s can be derived by the product of the control-to-output transfer function G vc s and the feedback transfer function G FB s.

The compensation transfer function G C s is designed to have one zero and one pole. The asymptotic Bode plots of the control-to-feedback and compensation transfer function, as well as closed-loop transfer function T V s , are shown in Figure 8. The following procedures show how to design the compensation transfer function.

Determine the cross frequency f c. Since the bandwidth is limit by f z1 , choosing an f c smaller than f z1 is recommended. Calculate the gain of G P s at f c , then the middle frequency band gain of G C s should be the opposite number of G P s.

Place the compensation pole at the zero f z2 caused by the ESR of output capacitor C 1. According to Figure 8, the closed-loop transfer function T V s has crossed 0 dB three times. The Nyquist plot is used to analyze the stability of closed-loop transfer function, as Figure 9 shows. Since the plot is far away from —1, j0 , the closed loop is stable and has adequate phase margin. Free downloadable software shows the characteristics of the current-mode buck converter.

In this article, Dr. Ridley presents a summary of current-mode control for the buck converter. A free piece of analysis software, the second in a series of six, is provided to readers of this column to aid with the analysis of their current-mode buck converters. In the last article, the complications of modeling power circuits were discussed in some detail for a buck converter with voltage-mode control.

Even for that simple configuration, the analysis can have different levels of complexity. This will depend on how many parasitic components are included in the analysis, and any assumptions made about their relative values. Current-mode control is the preferred approach, implemented as shown in Figure 1. Figure 1: Buck converter with current-mode control. The green components show the current feedback; without these, the control is voltage-mode.

A whole new world of mathematical complexity arises when current-mode control is used for a power supply. The dynamic analysis of current mode involves advanced techniques, including discrete-time and sampled-data modeling.

A free piece of analysis software, the final one in a series of six, is provided to readers of this column to aid with the analysis of their current-mode buck-boost converters. The buck-boost converter or flyback converter in its isolated version is the most popular converter for generating low power with multiple output voltage levels. The converter can be run in many different modes — discontinuous conduction mode DCM , continuous conduction mode CCM , quasi-resonant mode DCM with switching at the bottom of the DCM ring wave , and various options of fixed or variable frequency.

The choice of operation depends on the power level, the application, and the control chip used. Regardless of the mode of operation of the power stage, current-mode control is almost always used, as shown in Figure 1.

Many designers strive to always keep their converter in DCM in order to avoid some of the complexities of control. This is not necessary if current mode control is used, and keeping the converter DCM under every condition of line and load can create large peak stresses in the semiconductors. Figure 1: Buck-boost converter with current-mode control.