POL Measurement Functions Aid Power Systems Design

Jun 1, 2004 12:00 PM
By David Morrison, Editor, Power Electronics Technology

Typically, point-of-load converters (POLs) aren't viewed as diagnostic tools. However, that may change as power supply vendors begin to incorporate new measurement and reporting functions into their modules. The Z-series POLs, recently introduced by Power-One as part of the company's digital IBA product line, illustrate the benefits of having diagnostic capabilities in a POL. Although Power-One's implementation of these functions may be unique, it's reasonable to expect other vendors may follow suit in offering similar features.

In Power-One's Z-series POL, each converter can measure its own output voltage, output current and temperature. An on-board ADC digitizes this data, which is collected by the company's digital power manager (DPM). The DPM stores the voltage, current, temperature and status data for each of up to 32 POLs in ring buffers that are cyclically updated and then can be accessed by the system through an I²C bus. If desired, the DPM can continuously write the collected data into an external I²C memory. In the case of a system failure, the last 10 sets of data will be copied into nonvolatile memory for subsequent troubleshooting.

Power-One refers to these data measurement, reporting and storage capabilities as “telemetry” to distinguish these operations from the conventional voltage monitoring that most POLs perform. Typically, the POL will simply monitor its output voltage and generate an alarm if the voltage falls outside its prescribed limits.

POL telemetry offers several benefits. The most basic of these is the elimination of those external components previously used for voltage monitoring. A more interesting payoff is the ability to characterize the system's power requirements without removing the POLs and installing test equipment. This capability, in turn, allows for optimization of the power system. By taking current and temperature readings on each POL, the system designer can measure onboard load currents, and determine whether the POLs and environmental conditions, such as airflow, are sufficient to meet the load requirements.

Where multiple POLs are being paralleled for current sharing, the collected data may be used to confirm the correct number of converters is in place. Where the initial design is found to be overly conservative, it may be possible to eliminate one or more POLs. The telemetry functions also may allow system designers to assess the long-term reliability of their power components by measuring and collecting data on their performance in system before any systems are shipped. In the event of a system failure — even one in the field — these same functions may be used to troubleshoot the equipment without taking more invasive or destructive measures.

The Z-series POLs measure their output voltage and temperature with 2% accuracy. The output current is measured by reading the voltage across the ESR of the output inductor. Unlike other methods that may rely on additional series components, this technique doesn't interfere with POL performance.

In measuring current through the inductor, the POL compensates for temperature variations of the ESR. However, this measurement technique yields accuracy in the 10% to 20% range, reflecting the unit-to-unit variations in inductor ESR. The company plans to increase its monitoring resolution in a future product for those customers who want greater precision on monitored data.

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