Measuring impedance of DC-DC converters and passive PDN components

If you use the shunt-thru method and use 1 MΩ inputs instead of 50 Ω, you can ignore the DC block on the T port.

Other methods for measuring high-voltage converters include the current-voltage sensing method, or a similar method using the Picotest J2111A current injector (www.picotest.com). These methods are less accurate than the shunt-through method but are better suited for high-voltage converters (up to 40 Vdc).

Figure 15. Example of configuration for shunt-through measurement method

In order to accurately measure extremely small resistances in the milliohm range, the probe contact resistance should be very small during the measurement process. The measurement terminal should be in contact with the device under test through a 2-port probe [1] [2]. In practice, we recommend that you solder the measurement terminal to the device under test. If the two measurement terminals are combined and the device under test is contacted by single-ended probing, the lead of the measurement port should be as short as possible, because its residual impedance will directly affect the measurement accuracy of the milliohm impedance.

Figure 16 is an example of using the 2-port probing method. In the figure, two homemade probes are connected to the ends of the test cable, and the probes are in contact with the output terminals of the device under test. Homemade probes can be made using SMA connectors (cut off its three ground pins, and then use the remaining ground pins and the center pin for probing) or SMA semi-rigid cables (cut the cable short, strip the middle conductor, and then solder a short pin to the outer conductor).

When performing a THRU response calibration, make sure the electrical length of the THRU component is approximately equal to the electrical length of the two probes.

Figure 16. Probing example of the shunt-through method

DC-DC Converter Output Impedance Measurement Example

Figures 17 and 18 show measurement examples of the output impedance of a 5 V to 3.3 V DC-DC converter using the shunt-through method and the E5061B gain-phase test port. The device under test is the same converter used in the loop gain measurement example in the previous chapter, and the measurement frequency range is 10 Hz to 10 MHz. The IFBW is set to Auto/Maximum 10 Hz, and the port T attenuator is set to 0 dB. During the measurement, the power of the excitation source is set to 10 dBm; when doing the through response calibration, the power of the excitation source is set to -5 dBm.

Note: When powering on or off the converter, we recommend temporarily changing the attenuation value of the T port attenuator from 0 dB to 20 dB to prevent the converter's transient output voltage from overloading the measurement receiver. If the instrument enters overload protection mode due to a transient high voltage, the method to recover is: Press the [System] button on the instrument panel, select the "Overload Recovery" and "Clear Overload Protection" buttons.

The | Z | traces are plotted using the impedance analysis function (gain-phase shunt-through method) of the E5061B-005. The left trace in Figure 17 shows the measurement results of | Z | with the converter and electronic load turned off.

As shown in the figure, the output impedance of the converter in the off state indicates the self-resonant impedance response of the converter's output capacitor. The trace on the right is the trace of | Z | measured under 0.3 A load conditions. As shown in the figure, | Z | is limited to less than 2 mΩ in the low frequency range by the feedback loop of the converter. Due to the unique receiver architecture of the gain-phase test port, the E5061B can correctly measure small impedances in the milliohm range, even at measurement frequencies below 10 Hz, and the measurement results are not affected by the ground loop of the test cable between the stimulus source and the receiver.

Figure 18 shows the traces of | Z | measured under 1 A and 2 A load conditions. As shown in the figure, in the low frequency range, the impedance of the device under test is higher than that under 0.3 A load condition. It is usually necessary to measure the output impedance under various load conditions, which allows us to know whether the output impedance of the device under test can be maintained within our desired target and whether the impedance change is small enough when the load condition changes.

Another important thing is to make sure that the trace of the output impedance does not have large forward peaks, as that will cause transient noise under all load conditions.

Figure 17. DC-DC converter output impedance measurement

At power-off state and 0.3 A load,
start frequency = 10 Hz, stop frequency = 10 MHz,
source power = 10 dBm (-5 dBm for THRU calibration)
T port: ATT = 0 dB, Zin = 50 Ω, R port: ATT = 20 dB, Zin = 50 Ω

Figure 18. DC-DC converter output impedance measurement

1 A and 2 A load conditions,
start frequency = 10 Hz, stop frequency = 10 MHz
Source power = 10 dBm (-5 dBm for thru calibration)
T port: ATT = 0 dB, Zin = 50 Ω, R port: ATT = 20 dB, Zin = 50 Ω