While the output voltage was still remaining within manufacturers +/−5% specification, the waveform associated with the burst-mode operation triggered the equipment to enter an under-voltage-lockout condition and remain latched off. To resolve the issue, the manufacturer was able to adjust the PSUs burst-mode activation setpoint to be just below the equipment’s worst-case quiescent load. As a result, the customer did not have to make any changes to their equipment, while still managing to meet the no-load power consumption requirements associated with DoE Level VI.
Case 2: RF emissions
In another instance, a customer was upgrading to a new Level VI-compliant power supply during a product redesign. Of course, the new system needed to be re-evaluated against the current safety and electromagnetic compatibility (EMC) standards for information technology equipment (ITE). When the EMC test lab was evaluating the system for electromagnetic compatibility, they found that the system exceeded emissions limits while idling, but passed during normal system operation. This spawned a bit of a conundrum as power supplies will typically exhibit the greatest emissions at full load. Further evaluation, however, revealed that the customer’s idle load was so low that the power supply was still operating in burst-mode. As a result of the converters resonant topology, the PSU emissions were extremely low during normal operation, and actually higher at light-load.
Although the power supply did pass emissions testing at light-load on its own, the pass margin was within a few dB. The combination of the power supply emissions and equipment emissions under these conditions was enough to cause a system-level failure. A review of the control ICs burst-mode operation revealed the root of the issue.
For this particular control chip operating in burst-mode, the switching frequency was actually modulated over a large range and was not fixed. Fixed frequency operation was restored under heavier-loads. The light-load switching frequencies were not as effectively attenuated by the converters EMC filters, which had been designed for the normal operating frequency. Additionally, though not the case in this particular situation, burst-mode operation of higher-power converters is also frequently accompanied by a shutdown of any active power factor correction (PFC) circuitry, as 61000 series harmonic current regulations are not applicable to light-load operating conditions. This shut down contributes to the release of harmonic content onto the mains and also possibly onto external radiators (cables) that would otherwise be mitigated by the active PFC. In a similar fashion to first example scenario, the manufacturer was able to adjust the burst-mode setpoint to a suitable level for the application and still keep the no-load power consumption within limits.
With the ever-increasing use of power conversion products in households and offices worldwide, efficiency mandates and the technological advances that make them possible are absolutely essential. However, it is important to consider the ramifications of those efficiency-boosting measures. In both of the cases outlined in this article, the solution was relatively simple and required very little redesign. Going forward, as efficiency requirements grow ever-more stringent, and power supply technologies become more complex, additional measures will need to be considered to mitigate the side effects brought on by these new technologies.