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Simplify switched-mode power-supply measurements using dedicated oscilloscope software

Thu, 04/04/2013 - 11:25am
Art Pini, Technical Marketing Engineer, Teledyne LeCroy

The analysis of a switched-mode power supply requires the ability to measure device, control loop, and line-related functions.  To this end, most oscilloscope manufacturers have created power analysis software, which combines all these elements into a simple-to-use toolbox.  Such software simplifies power analysis by automating the setup of even these relatively complex functions.  The software facilitates device and control-loop analysis to characterize power supply stability under load changes, line changes, soft-starts, dropouts, hot swaps, and short circuits.  It allows observation, on a cycle-by-cycle basis, of the behavior of the supply. In the absence of a power analysis package, debug of power supply issues becomes a much more time-consuming, manual task.
   
Figure 1 shows an example of such software: Teledyne LeCroy’s Power Analysis software option.


Device and safe operating area measurements
Every switching device has a maximum voltage, current, and power specified by the device manufacturer and displayed on its technical application note. Reliability of the power supply is dependent on not exceeding these limits. Safe Operating Area plots help confirm operating margins.

The dialog box at the bottom of the figure shows the options for Analysis Type including Device, Control Loop, and Line Power.  This example shows measurements of FET losses and Safe Operating Area (SOA).  The upper trace shows the FET's drain-source voltage, and the center trace shows the FET's drain current waveform. The lower trace is the power dissipated by the FET.  Much of the power dissipated by the FET occurs during the switching transitions.  The parameter readouts, located under the display, show the turn-on, conduction, turn-off, off-state and total losses along with the switching frequency.  The highlighted area clearly delineates each portion of the power supply’s operating cycle also showing where each of the loss measurements is made. 

In Figure 1, the amplitude of current vs. amplitude of voltage is displayed as an XY trace known as a Safe Operating Area plot.  The horizontal Axis in the X-Y plot is voltage while the vertical axis is current.  The upper-right corner of the XY trace represents maximum power. The primary interest in the SOA plot is to determine if the device exceeds its maximum voltage, current, or power ratings.  Thanks to the long acquisition memory in modern oscilloscopes, it is possible to find SOA violations that occur for only a few cycles after an event, such as a short circuit or during startup.

Control loop measurements
Every power supply has a feedback loop that monitors the output voltage or current and keeps the device’s output level constant despite changes in the load.  This means that the power device conducts longer if the output voltage is too low.  Most switched-mode power supplies use pulse width (PWM) or frequency modulation (FM) in their control loops.  Analysis of the loop dynamics requires the ability to demodulate these signals.  Power analysis software should include easy-to-use modulation analysis capabilities for this purpose. 

Modulation analysis functions produce a time-domain display that represents the modulated parameter in a time vs. time graphical plot. They are convenient tools for intuitively viewing the time-domain response of the entire control loop, including any time constants added by the pulse width modulator.  One may perform modulation analysis or measure duty cycle, period, frequency, or pulse width using parameters. 

Figure 2 illustrates the response to a step load change of a PWM-based control loop.


The upper trace, C1, is the gate-to-source drive signal to a MOSFET.  This PWM signal is demodulated using a track function of duty cycle shown in the lower Control Loop trace. The Control Loop function displays the duty of the gate drive signal as a function of time, which is time-synchronous with the source waveform.  One may use the zoom features, as in trace Z1, to see the duty cycle of each individual cycle and the corresponding value of the track plot. This relates each point in the track function to the source waveform.  It is easy to see that the control loop initially overshoots and then recovers in about 800µs.  The time scale of this acquisition is 200µs per division and the vertical scaling of the track of duty cycle is 2% per division.  Using the parameter table measurement results, the pulse duty cycle before the load change is approximately 4.8%.  After the change, it increase to 15.5% and then quickly recover to 9.7%.  The measurement parameters Frequency, Period, Width, and Duty Cycle are read out simultaneously showing the current, mean, minimum, maximum, standard deviation, and number of measured values in the statistics.

Figure 3 shows a similar study.  In this example, power analysis software runs on an oscilloscope that acquires a 20 ms record (including every gate-drive pulse from power supply startup) until it reaches steady state.


The modulation analysis display shows the pulse width of every cycle of the gate-drive signal as it occurs.  It is easy to observe the soft-start circuit's performance.  The minimum and maximum parameters read the range of pulse-width variations as 222 ns to 5.09 µs.

Note that the use of a relatively long acquisition memory of 40 Mega samples (MS) allows digitizing of the measured waveforms at 2 GS/s for a time resolution of 500 ps.  This particular oscilloscope offers a maximum memory length of 256 MS.  Because the switching frequency of this supply is only 64 kHz, the 2 GS/s sample rate provides more than ample time resolution for the measurement. 

Conclusion
Measurements of device and control-loop parameters as well as line-related functions are integral parts of the analysis of switched-mode power supplies. Oscilloscopes can do this job out of the box, but a much easier solution can be found in power analysis software that instrument manufacturers make available as options.

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