Measuring Power Dissipation in Switching Power Supplies Using a DPO Oscilloscope

Publisher:CreativeDreamerLatest update time:2012-08-13 Source: EDNCHINAKeywords:DPO Reading articles on mobile phones Scan QR code
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Changing power requirements are driving architectural changes in switching power systems, and being able to measure and analyze power consumption in next-generation switch-mode power supplies (SMPS) is critical. New power supplies that support much higher data speeds and gigahertz processors require higher currents and lower voltages, placing new pressures on power designers in terms of efficiency, power density, reliability, and cost. To meet these demands, designers are adopting new architectures that include synchronous rectifiers, active power factor correction, and higher switching frequencies. These technologies also bring new challenges, such as high power dissipation on the switching devices, temperature rise, and excessive EMI/EMC effects.

A key parameter to understand these effects is the power loss that occurs during the switching process. During the transition from the "off" state to the "on" state, the power supply incurs higher power losses. On the other hand, the power loss when the switching device is in the "on" or "off" state is lower because the current flowing through the device or the voltage applied to the device is quite small.

The inductors and transformers associated with the switching devices smooth the load current and isolate the output voltage. These inductors and transformers are also affected by the switching frequency, incur certain power losses, and occasionally fail due to saturation.

Since the power dissipated in a switching power supply determines the overall efficiency and thermal effects of the power supply, measuring the power losses in the switching devices as well as inductors and transformers is very important, especially in terms of indicating power efficiency and temperature rise. Therefore, engineers need measurement and analysis equipment that can quickly and accurately measure and analyze instantaneous power losses under changing load conditions.

Designers who need to accurately measure and analyze the instantaneous power losses of different devices face the following challenges:
● How to set up test equipment to accurately measure power losses
● Correct errors caused by transmission delays in voltage probes and current probes
● Calculate power losses in non-periodic switching cycles
● Analyze power losses when the load changes dynamically
● Calculate core losses of inductors or transformers

Fortunately, comprehensive power analysis software has emerged on the market that runs on the latest generation of digital phosphor oscilloscopes and has a common "look and feel" with the oscilloscope user interface, providing intuitive navigation capabilities and ease of use. This power measurement and analysis application software can help switch-mode power supply designers accurately perform power loss analysis on switching devices and magnetic devices, and perform detailed input/output analysis. Key features of these software include the "Hi-Power Finder" assistance tool (described in more detail below), comprehensive report generation capabilities, a ripple viewer, the ability to perform magnetic measurements and a fast and efficient deskew function.

Test Setup for Accurate Power Loss Measurements

Figure 1 is a simplified circuit diagram of a switch-mode power supply. A MOSFET driven by a 40 kHz clock controls the current. The MOSFETs in Figure 1 are not connected to the AC mains ground or the circuit output ground, so simple ground-referenced voltage measurements using an oscilloscope are not possible because connecting the probe ground lead to any MOSFET terminal would short that point through the oscilloscope ground.


The best way to measure the voltage waveform of a MOSFET is to make a differential measurement. With a differential measurement, you can measure the drain-to-source voltage (V DS ), which can be on top of tens to hundreds of volts, depending on the supply range.

There are several ways to measure V DS : 1. Float the chassis ground of the oscilloscope. This method is not recommended because it is very unsafe and can easily cause personal injury to the user, damage to the device under test, and damage to the oscilloscope. 2. Use two traditional passive probes with their ground leads connected to each other while using the oscilloscope's channel math function. This measurement is called the "quasi-differential" method. However, passive probes used in conjunction with the oscilloscope's amplifier lack sufficient common-mode rejection ratio (CMRR) to adequately block any common-mode voltage. This setup will not adequately measure voltage, but existing probes can be used. 3. Use a commercial probe isolator to isolate the oscilloscope's chassis ground. The probe's ground lead is no longer at ground potential, and the probe can be connected directly to the test point. Probe isolators are an effective solution, but they are expensive, costing 2 to 5 times more than differential probes. 4. Use a true differential probe on a wideband oscilloscope to accurately measure V DS . To measure the current through the MOSFET, the user first clamps on the current probe and then fine-tunes the measurement system. Many differential probes have built-in DC offset trimmers. With the device under test turned off, fully "warm up" the oscilloscope and probes, and set the oscilloscope to measure the average of the voltage and current waveforms at the sensitivity settings that will be used in the actual measurements. When no signal is present, adjust the trimpot to zero the average of each waveform to 0 V. This minimizes measurement errors caused by quiescent voltages and currents in the measurement system.





Correcting Errors Caused by Propagation Delays in Voltage and Current Probes

Before making any power loss measurements in a switching power supply, it is very important to synchronize the voltage and current signals to eliminate propagation delays, a process called “deskewing.” Traditional methods require calculating the offset between the voltage and current signals and then manually adjusting the offset using the oscilloscope’s deskew range. However, this method is very time-consuming.


However, this process can be greatly simplified by using a high-bandwidth digital phosphor oscilloscope equipped with a deskew fixture and power measurement software. To correct for de-skew, differential voltage and current probes can be connected to the test points of the deskew fixture. The deskew fixture is driven by the oscilloscope's auxiliary output or calibration output signal. The deskew fixture can also be driven by an external source when needed. The deskew function of the power analysis software will automatically set up the oscilloscope to calculate the propagation delay caused by probing. The deskew function then automatically offsets the offset using the oscilloscope's deskew range. The test setup is now ready for accurate measurements. Figures 2 and 3 illustrate the current and voltage signals before and after deskew.


Calculating Power Loss on Non-Periodic Switching Signals

If the emitter or drain is grounded, then measuring dynamic switching parameters is straightforward. However, with floating voltages, differential voltages must be measured. To accurately characterize and measure differential switching signals, a differential probe is required. Hall-effect current probes allow viewing of the current flowing through the switching device without interrupting the circuit. The automatic offset correction feature of the power analysis software can be used to eliminate the propagation delay caused by the probe. The minimum power loss,

maximum power loss, and average power loss of the switching device are measured on the acquired data, and the "Switching Loss" function in the software will automatically calculate the power waveform. The data is then presented as turn-on loss, turn-off loss, and power loss, as shown in Figure 4. This provides useful data for analyzing the power consumption on the device. Knowing the power loss at turn-on and turn-off, the user can adjust the voltage and current conversion to reduce power loss.


During load changes, the control loop of a switching power supply changes the switching frequency to drive the output load. Figure 5 shows the power waveform when switching the load. Note that the power loss in the switching device also changes when the load changes. The resulting power waveform has a non-periodic nature. Analyzing non-periodic power waveforms can be a tedious task. However, the advanced measurement capabilities of the power analysis software automatically calculate the minimum power loss, maximum power loss, and average power loss, providing more information about the switching device.

Analyzing Power Losses During Dynamic Load Changes

In real-world environments, power supplies are constantly exposed to dynamic loads. Figure 5 shows that the power losses that occur during switching also vary during load changes. It is critical to capture the entire load change event and characterize the switching losses to ensure that they do not reach the limits of the device.

Today, most designers use oscilloscopes with deep memory (2 Mbyte) and high sampling rates to capture the event at the required resolution. However, this approach presents a challenge in terms of the large amount of data that needs to be analyzed from the switching loss point. The power analysis software “HiPower Finder” eliminates the challenge of analyzing deep memory data, and Figure 6 shows a typical result. Figure 7 goes a step further and shows the number of switching events in the acquired data as well as the maximum and minimum switching losses. The desired switching loss point can then be viewed by entering a range of interest. The user simply selects the point of interest within the range and lets “HiPower Finder” locate that point in the deep memory data. The cursor will connect to the requested area. While locating the point, the software can be used to zoom in on the area around the cursor position to view the activity in more detail. This, combined with the switching loss feature mentioned earlier, allows for quick and efficient analysis of power consumption in switching devices.

Calculating Power Losses in Magnetic Devices

Another way to reduce power loss comes from the magnetic core area. From the typical AC/DC and DC/DC circuit diagrams, inductors and transformers also consume power, which affects power efficiency and causes temperature rise.

Generally, inductors are tested using LCR meters, which generate sine wave test signals. In switch-mode power supplies, inductors transmit high voltage, high current switching signals, which are not sinusoidal. As a result, power supply designers must monitor the behavior of inductors or transformers in real power supplies. Testing with LCR meters may not reflect the actual environment.


The most effective way to monitor the behavior of the magnetic core is to use the BH curve, which will quickly reveal the behavior of the inductor in the power supply. Power analysis software can quickly perform BH analysis on an oscilloscope without the need for expensive dedicated tools.

Inductors and transformers have different behaviors during the power supply startup process and in steady state. In the past, in order to view and analyze the BH characteristics, designers had to acquire the signal and further analyze it on a PC. With oscilloscope software, it is now possible to perform BH analysis directly on the oscilloscope software and instantly view the inductor behavior, as shown in Figure 8.


This magnetic analysis capability also automatically measures power losses and inductance values ​​in a real power supply environment. To derive the magnetic losses on an inductor or transformer, the user simply measures the power losses on the primary and secondary sides. The difference in these results is the power loss on the core. In addition, under no-load conditions, the power loss on the primary device is the total power loss on the secondary side, including core losses. These measurements can reveal information related to the area of ​​power dissipation.

Summary

This article describes the main features of the power measurement and analysis software, including the ability to measure power losses on switching devices, the "HiPower Finder" function and BH analysis, providing tools for making quick measurements on switch-mode power supplies. When using a digital phosphor oscilloscope, the software allows the user to quickly locate areas of power dissipation of interest and view the behavior of power dissipation in dynamic situations.

Keywords:DPO Reference address:Measuring Power Dissipation in Switching Power Supplies Using a DPO Oscilloscope

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