Measuring Phase Margin of a DC-DC Converter – KBA222529

Version: **



How do you measure phase margin?



Phase margin can be easily measured using a Frequency Analyzer (FRA) made by NF Corporation, as shown in Figure 1. Figure 2 shows the FRA 5097 adapter for loop gain measurement. Figure 3 shows the DC-DC converter phase margin measurement setup with FRA.

Figure 1. FRA 5097

               (Image by NF Corporation)


Figure 2. FRA 5097 Adapter for Loop Gain Measurement


          (Image by NF Corporation)


Figure 3. DC-DC Converter Phase Margin Measurement Setup

The steps for measurement are as follows:

Step 1. Injection point selection. Generally, the injection point is between the output and the high-side feedback resistor in the feedback path, as shown in Figure 4. To inject the signal, connect the signal source output to two sides of the injection resistor at injection point. A small resistance should be used so that the output voltage is not influenced. However, if the resistance is too small, it will cause the signal to be heavily affected by noise. The recommended value for an injection resistor is 51W. Solder the 51-W resistor directly on to the target PCB and attach an enameled wire to both sides of resistor for signal pickup.

Step 2. FRA connection. Connect the FRA adapter (Figure 2.) hook probes to the enameled wires soldered on both sides of the injection resistor. Specifically, connect the red hook probe to the side that is close to the output end of the feedback path. Connect the yellow hook probe to the side that is close to the feedback resistor. Connect the black hook probe to ground of the feedback path, as shown in Figure 4.


Figure 4. Adapter Probe Setup

Step 3. FRA configuration. The sweep frequency should range from 1 kHz to 1 MHz. FRA’s signal amplitude should be set between 1/100 to 1/50 of the DC output voltage. For example, 50~100 mVpeak is suitable for a DC output of 5 V.

Step 4. Measurement. Press the oscillator output AC/DC ON button, as shown in Figure 5. Then press either the UP or DOWN key to start the measurement, as shown in Figure 6. The UP and DOWN keys are used to measure in the order of increasing and decreasing frequency. Either way, the resulting waveforms should be the same.

Figure 5. OSC ON Button

Figure 6. MEASURE SWEEP UP and DOWN Buttons

Step 5. Result analysis. To get the phase margin value, turn the marker knob and move the marker to the position where the red line intersects the 0dB line. As shown in Figure 7, the phase margin value is displayed on the top right of the screen as *θ.

Figure 7. Example Result