Why Do I Need a Vector Network Analyser?
A switched-mode power supply (SMPS) is a type of power supply that uses switching technology to convert electrical power from one form to another. Various applications use these power supplies, ranging from small electronic devices to large industrial systems. One of the critical challenges in designing and testing SMPSs is characterising their control-loop stability, output impedance and input filtering. An engineer can use a vector network analyser (VNA), such as the Omicron-Lab Bode 100 (1 Hz to 50 MHz), to characterise these accurately and efficiently.
One of the main benefits of using a VNA to characterise an SMPS is its ability to measure the power supply's small-signal stability. The VNA injects a small signal into the control loop and measures the loop gain and phase against frequency. This reveals the phase margin and gain margin of the loop. By measuring the stability of an SMPS, designers can ensure that the power supply will operate reliably and avoid problems such as oscillation or poor transient response.
Another benefit of using a VNA to characterise an SMPS is its ability to measure the power supply's output impedance and power supply rejection ratio (PSRR). Output impedance against frequency shows how well the supply holds its voltage when the load changes quickly, and PSRR shows how much input ripple and noise reach the output. Both help designers choose the output capacitors and tune the control loop.
A VNA also helps with the EMC design of an SMPS. EMC management is essential for switched-mode power supplies because they can generate electromagnetic interference (EMI) that can interfere with other electronic devices. The VNA does not measure the emissions themselves (that needs an EMI receiver or spectrum analyser), but it can measure the attenuation of the input EMI filter, the impedance and parasitics of the filter's capacitors and inductors, and check that the input filter does not destabilise the converter. This helps designers get the filter right before compliance testing.
In conclusion, engineers can use a vector network analyser (VNA) as a powerful tool to accurately and efficiently characterise switched-mode power supplies (SMPS). By using a VNA to measure the SMPS's loop stability, output impedance, PSRR and input filter performance, designers can ensure that the power supply will operate reliably and give it the best chance of meeting EMC standards first time. Using a VNA can save time and money for the designer and the manufacturer.
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