Maximum efficiency verification and characterization of switching power supplies

To ensure the best efficiency with an acceptable compromise, it is important to verify and characterize the design of the switch mode power supply (SMPS). This can be done by measuring the switching power loss and magnetic power loss to determine the power supply efficiency; measuring the power quality and harmonics to understand the role of the switch mode power supply on the power line.

The greatest energy loss in power systems typically occurs during power conversion in AC/DC and DC/DC power supplies. Energy conservation is a top priority in almost every design, so switching power supplies that convert between 80% and 90% of power are the mainstream. Ideally, all power supplies work according to mathematical models. However, in the real world, there are various problems, such as defective components, variable loads, line power distortion, and frequent environmental changes. To ensure optimal efficiency with an acceptable compromise, it is critical to verify and characterize the design of the switching power supply. To accomplish these tasks, it is usually necessary to measure the switching power loss and magnetic power loss to determine the efficiency of the switching power supply, as well as measure the power quality and harmonics to understand the role of the switching power supply on the power line.

Measuring switching losses

The switching transistors in a switching power supply switch quickly, minimizing energy losses. For a switching power supply, the switching transistor loses the most energy when it dissipates a small amount of heat in the on or off state. Energy losses occur during switching because the energy stored in the diode and the energy stored in the parasitic inductance and capacitance are released. "Turn-off losses" refer to the losses in the device from on to off. "Turn-off losses" also refer to the energy lost when the switching device is turned from off to on. Here is the formula to calculate the energy losses incurred during switching:

Where: ETRANSITION refers to the energy loss generated by the switch during the switching process; vA(t) refers to the instantaneous voltage of the switch; iA(t) refers to the instantaneous current of the switch; t1 refers to the time when the switching is completed; t0 refers to the time when the switching starts.

The total energy loss in the entire switching cycle is composed of turn-on switching loss, turn-off switching loss and conduction loss. The following is the calculation formula for the total loss: ELOSS = ETURN-ON + EON + ETURN-OFF. In the formula: ELOSS refers to the energy loss of the transistor during the switching cycle; ETURN-ON and ETURN-OFF are both switching losses; EON refers to conduction loss.

Analyzing the above losses is necessary for characterizing the power supply and estimating its efficiency. An oscilloscope can be used to measure the switching losses (Figure 1). Using an oscilloscope with professional power analysis software, the switching losses and conduction losses of multiple switching cycles can be measured to determine the characteristics of the device at different times. From the measurement statistics, the changes in the measurement results can be observed. It is a challenge to accurately measure the turn-on loss and turn-off loss because the loss only occurs for a short period of time and rarely occurs during the rest of the switching cycle. Determining the above losses requires precise timing of the voltage and current waveforms, and the deviation of the measurement system must be minimized.

Figure 1 Oscilloscope with professional power analysis software

Figure 1 An oscilloscope with professional power analysis software can display switching losses and conduction losses over multiple switching cycles to determine the characteristics of the device at different times Figure 2 An oscilloscope with power analysis software can be used to measure the power loss of a single-winding inductor. Channel 1 ($ trace) is the voltage across the inductor, and Channel 2 (blue trace) is the current through the inductor measured with a non-intrusive current probe. The power measurement software automatically calculates the power loss and displays it in a graph (278.1 mW)

Measuring magnetic power loss

Inductors and transformers usually have relatively low power losses and are often used by switching power supplies to filter and change voltage levels. The impedance of an inductor increases with increasing frequency, blocking more high frequencies than low frequencies. This characteristic is beneficial for filtering power inputs and outputs.

The transformer couples the AC voltage and AC current of the primary winding to the secondary winding, increasing or decreasing the signal level of the voltage or current (one of them). The primary of the transformer can accept a voltage of 120V, and by proportionally increasing the current of the secondary, the voltage of the secondary is reduced to 12V. The primary and secondary of the transformer are not electrically connected, and isolation is still required between circuit elements.

Magnetic power losses affect the efficiency, reliability, and thermal performance of the power supply. There are two types of power losses associated with magnetic components: iron loss in the core and copper loss in the copper windings. Magnetic losses are equal to the sum of iron loss and copper loss. Iron loss is composed of hysteresis loss and eddy current loss, while copper loss is caused by the resistance of the copper winding wire.

From the datasheet provided by the core vendor and the results from an oscilloscope with power measurement software, the total power loss and core loss can be derived. The copper loss can then be calculated from these two values. Once the power loss components are known, the cause of the power loss in the magnetic components can be determined.

The method for calculating the total power loss of a magnetic component depends in part on the type of component being measured. The device being measured can be a single-winding inductor, a multi-winding inductor, or a transformer. Figure 2 shows the measurement results for a single-winding inductor. Channel 1 ($ trace) is the voltage across the inductor, and Channel 2 (blue trace) is the current through the inductor measured with a non-intrusive current probe. The power measurement software automatically calculates the power loss and displays it in a graphical form (278.1 mW).

Figure 2 Measurement results of a single-winding inductor

To achieve the best performance, designers generally specify magnetic components using hysteresis curves obtained from manufacturers. The performance range of the magnetic component core material is specified in the characteristic curve. In order to ensure that the operating voltage and operating current remain in the linear region of the hysteresis curve during operation, it is necessary to verify the magnetic components in the switching power supply. The use of dedicated power measurement software can greatly simplify the steps of measuring magnetism with an oscilloscope. In many cases, only the voltage and excitation current need to be measured, and then the software will complete the calculation of the magnetic measurement. Magnetic measurements can be performed on single-winding inductors or on transformers equipped with primary and secondary current sources.

Figure 3 This figure shows the hysteresis curve of the transformer. Channel 1 ($ trace) is the transformer voltage, Channel 2 (blue trace) is the primary current, and Channel 3 (gold trace) is the secondary current. The software determines the excitation current based on the data from Channel 2 and Channel 3.

Figure 3 Hysteresis curve of transformer

Figure 4 Measured results without two probe deflection correction (5.141W)

In Figure 3, Channel 1 ($ trace) is the transformer voltage, Channel 2 (blue trace) is the primary current, and Channel 3 (gold trace) is the secondary current. The software determines the excitation current based on the data from Channel 2 and Channel 3. Some power measurement software can also accurately plot the hysteresis curve of magnetic components and characterize their characteristics. Before the software can plot the hysteresis curve, the number of turns, magnetic length, and cross-sectional area of ​​the core must be entered.

Detection Considerations

When measuring power with an oscilloscope, the voltage and current from the drain to source node of a MOSFET (metal oxide semiconductor field effect transistor) switching device or the collector to emitter node of an IGBT (insulated gate bipolar transistor) must be measured. This requires high voltage differential probes and current probes. Each probe has its own characteristic propagation delay. The difference or offset between the two delays can cause inaccurate power measurements and distorted timing measurements.

Understanding the effect of probe propagation delay on maximum peak power and area measurements is important. After all, power is the product of voltage and current. If the two variables being multiplied are not perfectly timed, the result will be wrong. When the probes are not properly deskewed, the accuracy of measurements such as switching loss will be compromised. For example, Figure 4 shows the result (5.141W) that the engineer measured without first deskewing the two probes. The results shown in Figure 5 demonstrate the importance of probe deskew. This example shows that the offset can cause a 5.3% measurement error. Performing accurate deskew reduces the error in the peak-to-peak power loss measurement.

Some power measurement software can automatically deskew the selected probe combination. It adjusts the delay between the voltage channel and the current channel through an active signal to eliminate the propagation delay between the probes. Static deskew (if available) is based on the fact that the specific voltage and current probes have constant and repeating propagation delays. Static deskew automatically adjusts the delay between the selected voltage channel and the current channel based on an embedded table of the propagation times of the selected probes.

Additionally, differential and current probes can have a small offset. Since offset affects accuracy, it should be offset before measurements are made. Some probes have an automatic offset method built into them. Used in conjunction with an oscilloscope, such probes can remove any DC offset errors from the signal path. Current probes A large input current is generated in the core of the transformer, and degaussing removes any residual DC flux.

Power lines

For AC/DC power supplies, power line measurements such as power quality and line harmonics are important for characterizing the interaction of switching power supplies and the environment in which they are used. In reality, power lines always have distortion and impurities, so they never supply an ideal sine wave. In addition, switching power supplies present the characteristics of a nonlinear load to the source, so the voltage and current waveforms are not ideal. Switching power supplies will draw current during a certain part of the input cycle, causing harmonics in the input current waveform. Determining the impact of these distortion factors is an important part of power engineering design.

Figure 5 After deskewing the signal in Figure 4, the peak amplitude increases to 5.415W or 5.3% more.

Figure 6 shows the results of harmonic analysis of the power supply load current.

The software automatically calculates current harmonics and determines the total harmonic distortion (THD) relative to the fundamental and RMS values

To determine the power consumption and power distortion of the power line, the power quality at the input stage is measured. Engineers traditionally use power meters and harmonic analyzers to measure power quality, but digital oscilloscopes with power measurement software have become a better choice.

Oscilloscopes have many advantages. The test instrument must be able to capture harmonic components up to the 50th harmonic of the fundamental. Power line frequency is usually 50 Hz or 60 Hz, depending on applicable local standards. Sometimes in military and avionics applications, line frequency can be 400 Hz. Signal distortion can contain finer frequencies, and modern oscilloscopes with high sampling rates can capture fast-changing events with great resolution. In contrast, conventional power meters are slower to react and miss signal details. The record length of an oscilloscope is generally sufficient to capture a complete cycle, even at high sampling resolution.

Current harmonics

Switching power supplies tend to generate harmonics, mainly out-of-order, which can be fed back to the grid. As more switching power supplies are connected to the grid, this effect begins to accumulate. For example, when more desktop computers are added to an office, the total proportion of harmonic distortion fed back to the grid increases. Distortion causes heat to accumulate in the cables and transformers of the grid, so minimizing harmonics becomes particularly important. Normative standards such as EN/IEC61000-3-2 specify the power quality for special non-linear loads.

With an oscilloscope with power measurement software, harmonic analysis is as simple as ordinary waveform measurements. Figure 6 shows the results of a harmonic analysis of the power supply load current. In this case, the software automatically calculates the current harmonics and determines important values, such as the total harmonic distortion (THD) relative to the fundamental value and the root mean square (RMS) value. The above measurements help analyze whether it complies with standards such as EN/IEC61000-3-2 and Military Standard 1399. Some software automatically compares the measurement results with the standard to quickly check whether the device is qualified.

Almost every electronic product has a power supply. Switching power supplies have become the mainstream of the market due to their energy-saving efficiency of up to 90%. In order to verify and test the design of switching power supplies to ensure that they can function well in actual environments, it is often necessary to measure switching power loss, magnetic power loss, power quality and power harmonics. Although power supply measurements are complex, oscilloscopes with appropriate detection tools and automated measurement software can make measurements simple.