Power Integrity: Measuring, Optimizing, and Troubleshooting Power Related Parameters in Electronics Systems by Sandler Steven

Author:Sandler, Steven
Language: eng
Format: epub
Publisher: McGraw-Hill Education
Published: 2014-11-17T16:00:00+00:00

FIGURE 8.25 TPS40200 output impedance and BK8540 Eload impedance along with the minor loop gain, Zsource/Zload.

In this case, the gain does not reach one and, therefore, does not have a PM since it is defined as the phase at a gain of one. The highest gain occurs at approximately 62 kHz, which coincides with the maximum impact seen in Fig. 8.23.

The magnitudes of the source impedance, load impedance, and the source and load combined are tabulated for three frequencies in Table 8.4.

TABLE 8.4 Source and Load Impedance, Minor Loop Gain, and the Source and Load Combined

The peak gain is 0.267, occurring at 62.2 kHz, and at 30.1 kHz and 201.4 kHz the gain is much lower. Though we are trying to measure the impedance of the DC/DC converter, we are actually measuring the impedance of the DC/DC converter in parallel with the impedance of the electronic load. At 30.1 kHz and 201.4 kHz, the measured impedance is reasonably close to the impedance of the DC/DC converter, while at the peak gain point the measured impedance of 126 mΩ is 25% lower due to the interaction of the DC/DC converter with the impedance of the electronic load.

Though we are generally concerned with the stability of a filter interacting with the negative resistance of the switching converter, this instability is also the basis of many quartz crystal and resonator oscillators.

Consider the circuit in Fig. 8.26, which includes a resonator (source) and a negative resistance (load). The series resistance of the resonator and the negative resistance are set to the same value. At the resonator series resonant frequency the source impedance consists of only the real resistance Rs and no imaginary terms. The negative resistance load impedance is -Rs.

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