[88] PSRR and Low-Performance Control Loops

PSRR becomes a major problem with poorly-designed control loops.

Introduction

Modern power converters are moving towards high efficiency, high density, and high switching frequencies. Vendors of power supplies are doing an outstanding job of meeting these goals with a range of new technologies. At the same time, despite the use of advanced control chip technology, many control loops have suffered a quantum drop in performance, and this is showing up as a problem with the power supply rejection ratio (PSRR). 

 

Power Supply PSRR Measurements

When I first started working in power supplies, design and test procedures were based on rigorous military and aerospace testing standards. For switching power supplies, the important measurements to collect were loop gains, output impedance, and audiosusceptibility (often referred to as power supply rejection ration, or PSRR, these days.) Loop gains were easy to make, just involving cutting a trace on a board and injecting small test signals. Output impedance can be tougher, depending on the power level of the supply being tested, as significant current must be driven into the output terminals for low impedance systems. 

PSRR is also hard to measure since the input line must be broken, and signal added to the input dc bus. Details of how to make these measurements are given in [1]. We make PSRR measurements in order to predict how much noise gets through to the output of the power supply from the input bus. Shortly after the start of the 1980s, current-mode control became widespread in the power supply industry, and measurements of PSRR became less common in the following years. Why? Because the noise rejection using current-mode control was so outstanding that there was really no need to measure it any more. 

Recently, however, PSRR has again become an important measurement. The only reason for this to suddenly re-emerge as a concern is because many power supplies are being designed with poor performance in this regard. Noise on the input voltage rail disturbs the output sufficiently to create regulation issues and system failures. 

Four events have led to deteriorated PSRR performance: 

Demand for high density has raised the switching frequencies.

MLC capacitors with very low ESRs are now available with high value capacitance.

Inductor values have reduced to minimize size.

Digital controllers are becoming more and more widespread. 

Paradoxically, all of these events should be producing much better control performance, but things have become worse in many cases. If the technology is implemented properly, they can do much better but this is usually not the case. 

 

Voltage-Mode PSRR Performance

Figure 1 shows a buck converter operating at 200 kHz with conventional components. The output capacitor is a large electrolytic type, sized to produce low ripple on the output. The inductor has been selected to provide reasonable ripple. The controller can be any kind – analog, digital, voltage-mode or current-mode. 

Fig 1

Figure 1: Buck Converter with Feedback Controller.

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