Measurement and analysis with MSO/DPO oscilloscopes - Part 2

The prevailing DC power supply architecture in most modern systems is the SMPS because of its ability to efficiently handle changing input voltages and loads. It minimises the use of lossy components such as resistors and linear-mode transistors, and emphasises components that are (ideally) lossless.

SMPS devices also include a control section containing elements such as pulse-width-modulated regulators, pulse-rate-modulated regulators and feedback loops.

The technology rests on power semiconductor switching devices such as metal oxide semiconductor field effect transistors (MOSFET) and insulated gate bipolar transistors (IGBT). These devices offer fast switching times and are able to withstand erratic voltage spikes. Additionally, transistors dissipate very little power in either the on or off states, achieving high efficiency with low heat dissipation.

For the most part, the switching device determines the overall performance of an SMPS. Key measurements for switching devices include:

• Switching loss;

• Safe operating area;

• Slew rate.

Figure 1: SMPS components that are characterised with DPOxPWR power analysis software.

Transistor switch circuits typically dissipate the most energy during transitions because circuit parasitics prevent the devices from switching instantly. The energy lost in a switching device, such as MOSFET or IGBT, as it transitions from an off to on state is defined as turn-on loss. Similarly, turn-off loss is the energy lost when the switching device transitions from an on to an off state.

Transistor circuits lose energy during switching due to dissipative elements in the parasitic capacitance and inductance and charge stored in the diode.

Switching loss measurements are made on complete cycles within the selected region of the acquisition (by default, the entire waveform) and the statistics of those measurements are accumulated across the acquisition, but not between acquisitions.

A major challenge in measuring turn-on and turn-off losses is that the losses occur over very short time periods, while the losses during the remainder of the switching cycle are minimal. This requires that the timing between the voltage and current waveforms is very precise, that measurement system offsets are minimised and that the measurement’s dynamic range is adequate to accurately measure the on and off voltages and currents.

The probe offsets must be nulled out, the current probe must be degaussed to remove any residual DC flux in the probe and the skew between channels must be minimised.

The other major challenge is the high dynamic range required for accurate switching loss measurements. The voltage across the switching device changes dramatically between the on and off states, making it difficult to accurately measure both states in a single acquisition.

There are three ways to determine the correct values with the MSO/DPO series:

• Measure the voltage drop across the switching device during conduction. Because this voltage is typically very small compared with the voltage across the switching device when it is not conducting, it is generally not possible to accurately measure both voltages at the same vertical setting on the oscilloscope;

• Provide the RDS(on) value (best model for MOSFETs) based on the device data sheet. This value is the expected on-resistance between the drain and source of the device when it is conducting;

• Provide the VCE(sat) value (best model for BJTs and IGBTs) based on the device data sheet. This is the expected saturation voltage from the collector to the emitter of the device when it is saturated.

The safe operating area of a transistor defines the conditions over which the device can operate without damage; specifically how much current can run through the transistor at a given voltage. Exceeding these limits may cause the transistor to fail.

Figure 2: Sample mode.

Figure 3: Average mode.

The SOA is a graphical test that accounts for limitations of the switching device such as maximum voltage, maximum current and maximum power and ensures that the switching device is operating within specified limits.

The device manufacturer’s data sheet summarises certain constraints on the switching device. The object is to ensure that the device will tolerate the operational boundaries that the power supply must deal with in its end-user environment. SOA test variables may include various load scenarios, operating temperature variations, high and low line input voltages and more.

A user-definable mask is created to ensure that the device adheres to defined tolerances in regard to voltage, current and power. Mask violations are reported as failures in the power application.

To verify that the switching device is operating at maximum efficiency, the slew rate of the voltage and current signals is measured to verify that the circuit is operating within specifications.

The oscilloscope is used to determine the slew rate of the switching signals by using measurement cursors, simplifying gate drive characterisation and switch dv/dt or di/dt calculations.

Ideally, the output of a DC power supply should not have any switching harmonics or other non-ideal noise components. Realistically, that is not possible. Output analysis measurements are essential to determine the effects of variations in input voltage or load on the output voltage.

These measurements include:

• Modulation analysis;

• Ripple.

The digital phosphor acquisition technology of the MSO/DPO4000 and MSO/DPO3000 series offers advantages when troubleshooting designs, especially when identifying excessive modulation effects in a switching power supply. These oscilloscopes have a 50,000 wfm/s waveform capture rate, which is many times higher than that of a typical digital storage oscilloscope.

This provides two advantages when investigating modulation effects. First, the scope is active more of the time and less time is spent processing waveforms for display. Thus the oscilloscope has more chances to capture the modulation.

Second, the digital phosphor display makes it easier to see the modulated waveforms in real time. The display intensifies the areas where the signal trace crosses most frequently, much like an analog scope. The modulation is dimmer than the main waveform that repeats continuously, making it easier to see.

Measuring modulation effects with this company’s oscilloscope is claimed to be easy. Modulation is important in a feedback system to control the loop. However, too much modulation can cause the loop to become unstable. The waveform is dimmer in regions where the modulation is less frequent. The red waveform is a maths waveform, showing the trend in cycle-to-cycle pulse width measurements made on an IGBT gate drive signal as the power supply’s oscillator starts up.

Figure 4: Hi-res mode.

Since the maths waveform represents pulse width measurement values (with units of time), variations in pulse widths may be measured using cursors.

The maths values represent the trends in the selected modulation measurement across the acquired waveform. In this case, it represents the response of the oscillator’s control loop during start-up.

This modulation analysis could also be used to measure the response of the power supply’s control loop to a change in input voltage (‘line regulation’) or a change in load (‘load regulation’).

Ripple is the AC voltage that is superimposed onto the DC output of a power supply. It is expressed as a percentage of the normal output voltage or as peak-to-peak volts.

Linear power supplies usually see a ripple that is close to twice the line frequency (~100 Hz), whereas switching power supplies may see a switching ripple in the hundreds of kHz.

The power supply is integral to virtually every type of line-powered and battery-operated electronic product, and the switchmode power supply has become the dominant architecture in many applications. A single switchmode power supply’s performance - or its failure - can affect the fate of a large, costly system.

To ensure the reliability, stability, performance and compliance of an emerging SMPS design, the design engineer must perform many complex power measurements.

Automated power measurements like harmonics, power quality, switching loss, safe operating area, slew rate, modulation and ripple ensure fast analysis while simplified set-up and deskew of probes provides maximum accuracy.