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Evaluating DC-DC Converters and Passive PDN Components

Application Notes

Introduction

Switch-mode DC-DC converters/voltage regulators are widely used in electronic equipment in a variety of industries. One industry that’s driving the most recent technical evolutions of DC-DC converters is computer equipment. Here, DC-DC converters play an important role for power integrity (providing a steady Vdd voltage regardless of the load variations) in the power supply circuits called PDNs (power distribution networks); which consist of bare PC board power planes, DC-DC converters, and passive PDN components such as bypass capacitors mounted on the power planes. The most significant trend in PDN of the computer equipment is that the load devices such as MPU’s, FPGA’s, and ASIC’s are continuously evolving toward faster operating speeds and lower operating voltages. Also, various voltage levels are required by these LSI’s (3.3 V, 2.5V, 1.5 V, 1.2 V, etc). In accordance with these trends, distributed power architecture is becoming common, where the low-voltage DC-DC converters are mounted at the point-of-load (POL) to improve the power integrity in high performance computer systems (i.e. servers and network infrastructure equipment). To make the DC-DC converters quickly respond to load variations of high-speed LSI’s, it is more important than ever to optimize the balance of the response speed and the stability of the feedback loop circuits. To minimize the transient voltage fluctuations due to large load current variations and ensure the voltage is maintained within narrower margins of the low voltage systems, it is necessary to verify that the output impedance of the DC-DC converter is suppressed to an extremely small value of milliohm order. In addition, PDN designers need to evaluate the impedance of passive PDN components such as bypass capacitors up to high frequencies beyond the DC-DC converter’s loop bandwidth, where the passive PDN components suppress the impedance between the power and ground planes. Accurately knowing the characteristics of each passive PDN component helps improve the quality of PDN design using simulation tools. Also, by measuring the entire PDN impedance of the PC board after mounting the passive components, we can verify if the desired target impedance is achieved as simulated. In general, the frequency range of interest is up in the hundreds of MHz range, which is an upper limit frequency for suppressing the PDN impedance with on-board passive components.

This application note describes the measurement methods for evaluating the frequency domain characteristics of DC-DC converters and associated passive PDN components by using the E5061B-3L5 LF-RF network analyzer with Option 005 impedance analysis function (5 Hz to 3 GHz). The first part of this document discusses how to measure feedback loop characteristics of DC-DC converters. The second part discusses impedance measurements of DC-DC converters and the passive PDN components.

Table of Content

Evaluating Feedback Loop Characteristics of DC-DC converters

  • DC-DC Converter Basic Theory of Operation
  • Evaluating Feedback Loop Characteristics of DC-DC Converters

Evaluating Impedance of DC-DC Converters and Passive PDN Components

  • Evaluating Output Impedance of DC-DC converters
  • Evaluating impedance of passive components

Evaluating Feedback Loop Characteristics of DC-DC Converters

DC-DC Converter Basic Theory of Operation

First of all, let’s quickly summarize the basic theory of operation of the DC-DC converter. The example shown here is a simple non-isolated single-phase buck converter with voltage-mode control.

The block diagrams and the timing charts shown in Figure 2 explain the basic operation of a buck DC-DC converter. The input DC voltage Vin is converted to pulsed voltage with a switch (MOSFET). It is on/off condition is controlled by the feedback loop circuit, the pulsed voltage is converted to the output DC voltage Vout with the charging and discharging operations of the output LC filter.

When the switch is turned on, the current Ion flows through the inductor L, and the power is delivered to the output capacitor Cout and the load, then Vout is increased.

If Vout reaches a certain voltage level, the switch is turned off and the energy that was charged to L by the current Ion generates the current Ioff and delivers the power to the load together with the energy that was charged to Cout, then Vout is decreased. If Vout reaches a certain level, the switch is turned on and Vout is increased again. The output voltage level is determined by the pulse duty ratio.

The longer the period Ton, the higher the output voltage. The shorter the period Ton, the lower the output voltage. When a current higher than a certain level continuously flows through the inductor L, the averaged output voltage is calculated as Vout = Ton/(Ton + Toff) x Vin. By repeating this on/off operation while monitoring the output voltage and adjusting the pulse duty ratio, the regulated output DC voltage is obtained regardless of the load variations.

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