Digital Oscilloscope Measurement of Switching Power Supplies
Power supplies come in a wide variety of types and sizes, from traditional analogue-type power supplies to highly efficient switching power supplies. They all face complex, dynamic operating environments. Equipment loads and requirements can change dramatically in an instant. Even "day-to-day" switching power supplies need to be able to withstand instantaneous peaks well above their average operating level. Engineers designing power supplies or systems that will use power supplies need to understand how the power supply will behave under static conditions as well as under worst-case conditions.
In the past, characterising the behaviour of a power supply meant measuring quiescent currents and voltages 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 customise 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 work like the mathematical model for which it was designed. But in the real world, components are flawed, loads change, power supplies can be distorted, and environmental changes can alter performance. Also, changing performance and cost requirements complicate power supply design. Consider these issues:
How many watts can the power supply sustain beyond its rated power? How long does it last? How much heat does the power supply emit? What happens when it overheats? How much cooling airflow does it require? What happens when the load current increases dramatically? Can the unit maintain its rated output voltage? How will the power supply cope with a complete short circuit at the output? What happens when the input voltage to the power supply changes?
Designers need to develop power supplies that take up less space, reduce heat, cut manufacturing costs, and meet more stringent EMI/EMC standards. Only a rigorous measurement system will allow engineers to achieve these goals.
Oscilloscopes and Power Measurement
For those accustomed to high-bandwidth measurements with oscilloscopes, power supply measurements may be simple because of their relatively low frequency. In reality, there are many challenges in power measurements that high-speed circuit designers never have to face.
The voltage across the switching device can be high and "floating", i.e., not grounded. The pulse width, period, frequency and duty cycle of the signal can vary. Waveforms must be captured and analysed to detect anomalies. This is a demanding requirement for oscilloscopes. Multiple probes - Single-ended probes, differential probes, and current probes are also required. The instrument must have a large memory to provide room to record results from long, low-frequency acquisitions. And it may be required to capture different signals with widely varying amplitudes in a single acquisition.
Switching Power Supply Fundamentals
The dominant DC power architecture in most modern systems is the switching power supply (switched-mode power supply), which is well known for its ability to cope efficiently with varying loads. The electrical energy signal path of a typical switching power supply includes passive devices, active devices, and magnetic components. Switching power supplies use as few lossy components as possible (e.g., resistors and linear transistors) and primarily use (ideally) lossless components: switching transistors, capacitors, and magnetic components.
Switching power supply equipment also has a control section, which includes components such as a pulse-width modulation regulator pulse-frequency modulation regulator, and a feedback loop1. The control section may have its own power supply. FIG. 1 is a simplified schematic of a switching power supply showing the electrical energy conversion section, which includes active and passive components as well as magnetic components.
Switching power supply technology uses power semiconductor switching devices such as metal oxide field effect transistors (MOSFETs) with insulated gate bipolar transistors (IGBTs). These devices have short switching times and can withstand unstable voltage spikes. Equally important, they consume very little energy in either the on or off state, resulting in high efficiency and low heat generation. Switching devices largely determine the overall performance of a switching power supply. Key measurements of switching devices include: switching losses, average power loss, safe operating area and others.
