New Technique for Non-Invasive Testing of Regulator Stability

Focus:

In devices such as fixed-output voltage regulators where the control loop cannot be accessed by the user, designers cannot take gain-phase or Bode plot measurements to determine the control-loop stability. Load-step testing only provides a rough indication of stability and not an accurate measure of phase margin. Moreover, it’s not just the stability of the device that matters, but rather the stability of the device in the application, where fast-switching loads and interconnects, can affect device stability. This article describes a technique for measuring the stability of voltage regulators (linear or switching) in system. The technique is described as noninvasive because it does not require that any circuit connections be broken and it works where the control loop is inaccessible. This technique uses a vector network analyzer (VNA) and a voltage-controlled current source to measure the output impedance of the device under test in circuit with output filter and load connected. The technique is demonstrated on a fixed output linear regulator as well as an adjustable linear regulator. As the authors explain, once output impedance of the regulator is measured, its Q can be determined, and from that the phase margin of the device’s control loop can be calculated. Alternatively, if the VNA measures group delay, that value together with the resonant frequency can be used to calculate phase margin.

What you’ll learn:

• How to determine the stability of a voltage regulator in circuit without cutting traces
• How to determine the stability of a voltage regulator in cases where the control loop in not accessible

Notes:

A Bode 100 VNA and a Picotest J2111A current injector were used In these experiments. Although the Bode 100 has some software features that automate the phase margin calculation, other models of VNAs could be used for this output impedance measurement. However, according to the authors, their Picotest current injector is a unique device.

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Author & Publication:

Steven Sandler and Charles Hymowitz, AEi Systems, Power Electronics Technology, Sep 01 2011

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