The Method of Measuring Switching Power Supply with Digital Oscilloscope
Power supplies come in a wide variety of types and sizes, from traditional analog-type power supplies to high-efficiency switching power supplies. They all have to face a complex and dynamic working environment. Equipment loads and demands can change dramatically in an instant. Even an "everyday" switching power supply can withstand momentary peaks well in excess of its average operating level. Engineers who design a power supply or a power supply to be used in a system need to understand how the power supply operates under static conditions as well as 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 the oscilloscope 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 Needs
Ideally, every power supply should behave like the mathematical model it was designed for. But in the real world, components are defective, loads can vary, power supplies can be distorted, and environmental changes can alter performance. Also, changing performance and cost requirements complicate power supply design. Consider these questions:
How many watts can the power supply sustain beyond its rated power? How long can it last? How much heat does the power supply dissipate? What happens when it overheats? How much cooling airflow does it need? What happens when the load current increases significantly? Can the device maintain the rated output voltage? How does the power supply deal with a dead short on 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 can be straightforward because of their relatively low frequencies. In fact, there are many challenges in power measurement that designers of high-speed circuits never have to face.
The entire switchgear may be high voltage and "floating", that is, not connected to ground. The pulse width, period, frequency and duty cycle of the signal can vary. Waveforms must be captured and analyzed faithfully to detect anomalies in the waveform. This is demanding on the oscilloscope. Multiple Probes—Single-ended, differential, and current probes are required at the same time. 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 different amplitudes in one acquisition.
Switching power supply basics
The dominant DC power architecture in most modern systems is the switching power supply (switching power supply), which is known for its ability to handle varying loads efficiently. 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 mostly (ideally) lossless components: switching transistors, capacitors, and magnetics.
The switching power supply device 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, showing the power conversion section, 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 erratic voltage spikes. Equally important, they consume very little power in both the on and off states, are highly efficient and generate low heat. Switching devices largely determine the overall performance of a switching power supply. Key measurements on switching devices include: switching loss, average power loss, safe operating area, and others.
