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Power supply black box enhances customer diagnostics

08 Dec 2014  | Subodh Madiwale, Vuong Tran

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Customer returns can cause a major distress to power supply and power system manufacturers. Returns are often provided with limited and ambiguous information regarding the failure mechanism and with vague details about the environmental conditions and the 'scene of the crime'. The time-pressure to diagnose and report comprehensive failure analysis based on sketchy information continues to contract. The cost associated with debugging and fixing issues naturally increase as the design progresses to different stages from the prototyping to production and to the end customer – not considering relationship damage and credibility.

In addition, a significant percentage of customer returns appear to be 'no-fault-found' returns. Such returns can often be dismissed and never correctly diagnosed but risk masking real failure mechanisms in the end customer system. For example, any failure causing an over-temperature trip will shut-down in the customer equipment but when returned to the manufacturer may appear to function normally.

The problem can be solved using built-in 'Black Box', similar in concept to an airplane flight recorder and capable of capturing critical information prior to shut-down. This article describes how the ADP1055 PMBus digital controller for isolated power supply systems uses its internal Black Box to record and store critical information. The data can be used by power system manufacturers and end-system users to improve diagnosis of field customer returns and improve the robustness of the qualification process.

Black box operation
The Black Box feature of the ADP1055 can record to the EEPROM vital data about the faults that cause the system to shut down. The Black Box diagnostics tool can be considered in two parts: Firstly the 'First Flag ID' feature records the first instance of failure such as over current/voltage/temperature etc.; and secondly, as the controller encounters such a fault a snapshot of the telemetry is captured (figure 3). This information is saved to the embedded non-volatile EEPROM, where it can be later retrieved for debugging purposes. In the presence of multiple faults, the First Flag ID that caused the system to shut-down is captured into the Black Box, along with all the telemetry information.

Since there are several parameters being measured in a digitally controlled power supply, the ADP1055 utilises dedicated (not multiplexed) sigma-delta ADCs that are averaged over time for each measurement such as voltage, current, and temperature, and to ensure that the accurate data is captured, the measured quantity is recorded into the Black Box at the moment of shut-down.

This Black Box feature is extremely helpful in troubleshooting a failed system during testing and evaluation. If a system is recalled for failure analysis, it is possible to read this information from the EEPROM to help investigate the root cause of the failure.

Several options are available for recording to the Black Box, which includes:
 • No recording, Black Box disabled
 • Only record telemetry just before the final shutdown
 • Record telemetry of final shutdown and all intermittent retry attempts (if device is set to shut down and retry)
 • Record telemetry of final shutdown, all retry attempts, and normal power-down operations using the CTRL pin or the OPERATION command (as described by PMBusTM)

Black box contents
Two pages (PageA, PageB) of the EEPROM are dedicated for storing the Black Box contents. This allows for a total of 16 records (each page comprises of eight records with 64B each). The two pages form a circular buffer for recording Black Box information with data that gets overwritten on every 16th record.

The EEPROM is a page-erase memory, meaning an entire page must be erased before the page can be written to. Due to the page erase requirement of the EEPROM, after writing the eighth record of any page, the next page is automatically erased to allow for continuous Black Box recording.

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