Real-Time Oscilloscope Accurate Duty Cycle Tracking for Power Converter Analysis

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Rohde & Schwarz GmbH & Co. KG

MEV Elektronik Service GmbH

Switching converter stability is critical to power supply design. Converter stability is often ensured by frequency loop and load transient responses. While frequency loop response is becoming more important, load transient response is still commonly used.

Power supply designs must be validated for loop stability to ensure proper and stable operation. Today, frequency loop response is the first choice for measuring converter loop stability. Frequency loop response uses small-signal AC analysis, where a small sinusoidal signal is injected into the loop to measure gain and phase over a wide frequency range in an open loop. Measured gain and phase values are plotted against the frequency in a Bode plot diagram to directly obtain gain margin, phase margin and crossover frequency. In load step response testing, a large current step is applied and then the voltage response needs to be measured and analyzed.

Large signal measurements are performed in a closed loop, which are very different from open loop systems. Output voltage needs to be analyzed in the time domain to estimate and determine converter stability. Figure 1 shows an exemplary step-down converter to test load transient response.

Having the load step generator connected to a converter output terminal is vital when rapidly changing the load current. Since PWM signals control the power plant in control loops, measuring the positive duty cycle during the load step can enhance load transient response when visualizing unknown effects. This measurement requires an instrument where the positive duty cycle can be measured with high sample rates over the complete recording period. The cycle-by-cycle measurement must be displayed as a waveform over time.

Accurate Duty Cycle Tracking for Power Converter Analysis

This challenging task can be tackled with a modern oscilloscope, which allows engineers to measure the positive duty cycle over a long recording period even at higher PWM switching frequencies. The MXO 5 oscilloscope from Rohde & Schwarz, for example, offers sufficient bandwidth, a high sampling rate and a large amount of memory needed for this task. All positive duty cycles in an acquisition can be used to visualize variations over an entire acquisition in a track. The tracks for each measurement in a single cycle can be displayed over time.

A typical load transient waveform included in the track waveform is illustrated in Figure 2. Figure 2 shows the standard output voltage and current waveforms for three consecutive load steps. The positive duty cycles for the controller output are also displayed and used to create a track. In theory, the track waveform mirrors the output voltage waveform, since the duty cycle regulates the power plant to maintain constant output voltage. A good sample application to showcase the benefits of the track function is a DC/DC switching converter in a full bridge topology with synchronous rectification. The isolated converter operates at a switching frequency of 100 kHz and converts 48 V input voltage to 12 V output voltage. The output current is set to 8 A maximum and the output load step is generated with an electronic load.

Before applying the load steps at the converter output, users need to complete several steps to visualize the positive duty cycle as a track waveform: First, a channel setup with a probe selection must be defined, then a trigger to catch load step events at the controller output. The positive duty cycle measurement function needs to be activated reference voltage percentage levels are to be defined (e.g. 20 %, 50 %, 80 %). To accurately measure a PWM signal with sharp edges, the user must define a sufficient sampling rate ≥100 Msample/s. To catch a whole sequence (at least one current step from low-to-high and another form high-to-low) a sufficient recording length has to be selected. Last but not least, the user needs to activate the track function within the measurement sub menu and optimize the vertical scaling.

Uncovering Hidden Transients in Power Converters

Once the setup has been completed, the user is then able to configure the electronic load in order to apply a load step between a low current value (20 % of the maximum load) and a high current value (80 % of the maximum load). Upon detection of a valid trigger condition, the waveforms will be displayed on the screen in accordance with the illustration depicted in Figure 3. The upper window displays the acquisition of two load steps in either direction.

The output voltage is measured on channel 1, while the output current is measured on channel 2. Additionally, the PWM control signal (channel 3) and the track waveform for the positive duty cycle are displayed. The zoom window shows that output voltage drops only for approximately 300 μs before re-entering steady state operations. The deviation between the 20 % and 80 % loads in a steady state is only 2.4 mV as measured by the cursor function. Upon entering a steady state, the converter exhibits a distinct level of 26% as opposed to the previously observed value of 24 %. The deviation reveals an effect, which does not meet the expectations described in Figure 2.

According to the definition and theory, the duty cycle should be independent of the load current. Reviewing control theory shows the 2 % deviation comes from higher conduction losses caused by higher output current. The higher losses are mainly generated in the transformer and output rectifier. The additional losses need to be equalized by increasing the positive duty cycle and the track function allows this complex measurement task to be performed.

With four channels and a bandwidth of 350 MHz to 2 GHz, the MXO 5 is the oscilloscope of choice for engineers looking to verify load transients on any PWM-controlled power converter where in-depth analysis is required to uncover details of system behaviour. Additional features such as large memory and trace capabilities help users to identify and understand the intricacies of converter operation. (heh)

* Marcus Sonst is Senior Application Engineer Oscilloscopes at Rohde & Schwarz.

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