Flexible Fault Management

November 24, 2015
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The Factorized Power Architecture (using the PRM and VTM) provides an efficient means of providing power under rapidly varying load conditions. In the event of a fault, the VTM module shuts down automatically and remains shut down until restarted by the PRM module. The addition of a Fault Management Circuit allows for much more flexibility in the response to a fault.

The Fault Management Circuit described here can be used for a PRM driving one or two VTMs in series or parallel. The circuit is compatible with the PRM / VTM families that support the ENABLE, VAUX and TM pins. The circuit causes the power supply to attempt periodic restarts during the fault condition, so that power is restored automatically once the fault condition has been removed.

Concept and Design Considerations

A fault condition could be

  • A short circuit or overcurrent on the VTM output;
  • An overvoltage on the input power; or
  • A thermal fault in a VTM.

If one of those conditions occurs, the VTM responds by shutting down its output. In some cases, the PRM will continue operation: for instance, if the PRM and VTM are located far apart and the VTM shuts down due to a thermal fault, the PRM would continue to regulate. To restart the power supply, it would be necessary to shut down the PRM and restart it with a pulse applied to the ENABLE pin.

In a circuit with two VTMs, there is a possibility that one VTM would shut down due to a fault and the other VTM continue to provide load current, all of which would flow through the body diodes of the faulted VTM’s rectifiers. This may result in permanent damage to the faulted VTM, as well as potential overheating issues. The Fault Management Circuit must quickly acknowledge a faulted VTM and shut down the entire system to prevent this scenario.

The Fault Management Circuit senses the TM pin from the VTM; when TM goes low, indicating that the VTM has shut down, the Fault Management Circuit shuts down the PRM by bringing the ENABLE pin of the PRM ground. The diagram below shows a conceptual block diagram for one PRM with one VTM.

Block Diagram of Fault Management Circuit Connections with One VTM

Figure 1 – Block Diagram of Fault Management Circuit Connections with One VTM

There are many possible implementations of the Fault Management Circuit. One implementation is shown in below. This circuit is low cost, uses little board space and satisfies the performance requirements of a broad range of systems. It gets its power and ground from the PRM’s VAUX and SGND pin, respectively. The components are off the shelf resistors, capacitors and N-channel MOSFETs.

Fault Management Circuit Diagram

Figure 2 – Fault Management Circuit Diagram

Download the Application Note

This post is an extract from Vicor Application Note 33. To learn more complete the form below to request the full application note.

 

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