Advanced power loss analysis using oscilloscope

The higher data speeds and GHz-class processors used in new Switch Mode Power Supply (SMPS) architectures are creating new pressures for power supply designers. To address these demands, designers are adopting new techniques like synchronous rectifiers, active power filter correction and higher switching frequencies.

However, these techniques bring unique challenges such as high power dissipation at the switching device, thermal runaway and excessive EMI/EMC. During the transition from an “off” to an “on” state, the power supply experiences higher power loss. The inductors and transformers isolate the output voltage, smooth the load current and are subjected to switching frequencies. This results in power dissipation and occasional malfunctioning because of saturation.

The measurement of power loss at the switching device and inductor/transformer assumes great importance because the power dissipated in an SMPS determines the thermal effect on the power supply and its overall efficiency.

Accurate power loss measurement
The MOSFET power transistor, driven by a 40kHz clock, controls the current. The MOSFET in Figure 1 is not connected to the AC main ground or to the circuit output ground. Hence, taking a simple ground referenced voltage measurement with the oscilloscope would be impossible because connecting the probe’s ground lead to any of the MOSFET’s terminals would short-circuit that point to ground through the oscilloscope.

Making a differential measurement is the best way to measure the MOSFET’s voltage waveforms. With a differential measurement, you can measure VDS—the voltage across the MOSFET’s drain and source terminals. VDS can ride on top of a voltage ranging from tens to hundreds of volts, depending on the range of the power supply.

There are several methods to measure VDS:

• Float the oscilloscope’s chassis ground. This is not recommended because it is unsafe for the user, the device under test (DUT) and oscilloscope.
• Use quasi-differential measurement, employ two conventional passive probes with their ground leads connected to each other and use the oscilloscope’s channel math capability. However, the passive probes, in combination with the oscilloscope’s amplifier, lack the CMRR to block any common mode voltages adequately.
• Use a commercially available probe isolator to isolate the oscilloscope’s chassis ground. The probe’s ground lead will no longer be at ground potential, and you can connect the probe directly to a test point. Probe isolators are effective but are more expensive than differential probes.
• Use a true differential probe on a wideband oscilloscope which allows the accurate measurement of VDS.

For current measurements through the MOSFET, clamp on the current probe then finetune the measurement system. Many differential probes have built-in DC offset trimmers. With the DUT turned off and the oscilloscope and probes fully warmed, set the oscilloscope to measure the mean of voltage and current waveforms. Use sensitivity settings that will be used in the actual measurement. With no signal present, adjust the trimmer to null mean value for each waveform to zero. This step minimizes the chance of a measurement error, which results from quiescent voltages and current in the measurement system.

Correcting errors

Before making any power loss measurement in an SMPS, synchronize the voltage and current signals to eliminate propagation delay. This process is called “deskewing.” The traditional method calls for calculating the skew between the voltage and current signal and then manually adjusting the skew using the oscilloscope’s deskew range. However, this is a tedious process. It is simpler to use a deskew fixture that can be purchased with an oscilloscope. To deskew, connect the differential voltage probe and the current probe to the deskew fixture’s test point. The deskew fixture is driven by either the Auxiliary output or cal-out signal of the oscilloscope.

Measuring the dynamic switching parameter is simple if the emitter or the drain is grounded. On a floating voltage, however, there is a need to measure a differential voltage. A differential probe can be used to accurately characterize and measure a differential switching signal. Hall Effect current probe allows the user to view the current through the switching device without breaking the circuit. By deskewing, the propagation delay caused by the probes is eliminated.

The next step is to compute the power waveform and measure minimum, maximum and average power loss at the switching device for the acquired data. Knowing power loss at turn on and turn off enables you to work on voltages and current transitions to reduce the power loss. During the load change, the control loop of SMPS changes the switching frequency to drive the output load.

In a real-world environment, the power supply is continuously subjected to a dynamic load. It is important to capture the entire load-changing event and characterize the switching loss to make sure it does not stress the device. Today, most designers use an oscilloscope with deep memory (2MB) and a high sampling rate to capture events in the required resolution. However, this presents the challenge of analyzing a huge amount of data for the switching loss points, which stresses the switching device. The key to solving this challenge is choosing the application-focused software that automates the process.

Another way to reduce power dissipation comes in the core area. From the typical AC/ DC and DC/DC circuit diagram, the inductor and transformer are the other components that will dissipate power, thereby affecting power efficiency and causing thermal runaway. Typically, inductors are tested using an LCR meter which utilizes a test signal—the sine wave. In a switched power supply, the inductors will be subjected to high voltage, high current switching signals, which are not sinusoidal.

As a result, power supply designers need to monitor the inductor or transformer behavior in a live power supply. However, testing with LCRs may not reflect a real-life scenario. The most effective method of monitoring the behavior of the core is through the B-H curve because it quickly reveals inductor behavior in a power supply.

The inductor and transformer will have different behavior during the turn on time and steady state of the power supply. In the past, to view and analyze B-H characteristics, designers had to acquire the signals and conduct further analysis on a PC. With modern oscilloscopes and power measurement software, users can do the B-H analysis directly on the instrument giving them an instantaneous view of inductor behavior.