Using a Digital Oscilloscope to Measure a Switching Power Supply
Power supplies come in a wide variety of types and sizes, from traditional analog power supplies to high-efficiency switching power supplies. They all face complex and dynamic work environments. Equipment loads and demands can change significantly in an instant. Even "everyday" switching power supplies must be able to withstand instantaneous peaks that far exceed their average operating levels. Engineers designing power supplies or systems that will use power supplies need to understand how the power supply operates under static conditions as well as under worst-case conditions.
In the past, characterizing the behavior of a power supply meant measuring quiescent current and voltage with a digital multimeter and performing painstaking calculations with a calculator or PC. Today, most engineers turn to oscilloscopes as their preferred power measurement platform. Modern oscilloscopes can be equipped with integrated power measurement and analysis software, simplifying setup and making dynamic measurements easier. Users can customize key parameters, automate calculations, and see results in seconds, not just raw data.
Power supply design issues and their measurement requirements
Ideally, every power supply should behave like the mathematical model for which it was designed. But in the real world, components are defective, loads can change, power supplies can distort, and environmental changes can alter performance. Moreover, changing performance and cost requirements also make power supply design more complex. Consider these questions:
How many watts of power can the power supply sustain beyond its rated power? How long can it last? How much heat does a power supply dissipate? What happens when it overheats? How much cooling airflow does it require? What happens when the load current increases significantly? Can the device maintain rated output voltage? How does the power supply respond to a complete short circuit at the output? What happens when the input voltage of the power supply changes?
Designers need to develop power supplies that take up less space, reduce heat, reduce manufacturing costs, and meet stricter EMI/EMC standards. Only a rigorous measurement system can enable engineers to achieve these goals.
Oscilloscope and power measurements
For those accustomed to making high-bandwidth measurements with an oscilloscope, power supply measurements may be simple because of their relatively low frequency. In fact, there are many challenges in power measurement that high-speed circuit designers never have to face.
The entire switchgear may be at high voltage and "floating", that is, not connected to ground. The pulse width, period, frequency and duty cycle of the signal will all change. The waveform must be captured and analyzed faithfully to find any anomalies in the waveform. This is demanding on the oscilloscope. Multiple probes – Single-ended probes, differential probes, and current probes are all required. The instrument must have a large memory to provide recording space for long-term low-frequency acquisition results. And it may be required to capture different signals with widely varying amplitudes in a single acquisition.
Basics of switching power supply
The dominant DC power architecture in most modern systems is the switching power supply (switching power supply), which is well known for its ability to effectively handle changing loads. The power signal path of a typical switching power supply includes passive components, active components, and magnetic components. Switching power supplies use as few lossy components as possible (such as resistors and linear transistors) and mainly use (ideally) lossless components: switching transistors, capacitors and magnetic components.
The switching power supply equipment also has a control part, which includes a pulse width modulation regulator, a pulse frequency modulation regulator, and a feedback loop 1 and other components. The control section may have its own power supply. Figure 1 is a simplified schematic diagram of a switching power supply, which shows the power conversion part, including active devices, passive devices and magnetic components.
Switching power supply technology uses power semiconductor switching devices such as metal oxide field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs). These devices have short switching times and can withstand unstable voltage spikes. Equally important, they consume very little energy, are highly efficient and generate low heat, whether in the on or off state. Switching devices determine to a large extent the overall performance of a switching power supply. The main measurements of switching devices include: switching loss, average power loss, safe operating area and others.
