Using a digital oscilloscope to test the switching power supply method steps
For those accustomed to making high-bandwidth measurements with oscilloscopes, power supply measurements may seem 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.
