Testing a power supply – Stability (Part three)
This is the third and final part in a three-part series which discusses how to properly test a power supply to ensure it will work reliably over various operating conditions. The series is intended to provide the design engineer with a sufficient understanding about some, but not necessarily all, of the testing needed to verify a reliable power supply design.
Part 1 addressed how to accurately measure power supply efficiency. In Part 2 we covered various noise sources and how to properly measure them with an oscilloscope. We also discussed output errors created by line and load transients.
This Part 3 discusses power supply control loops and how to measure stability. We discuss Bode plots and what to look for when testing stability.
See Video 4 below
Why measure stability?
A power supply is a closed loop amplifier; it takes in electrical energy and converts it to electrical energy in another form, at a specific regulated voltage and/or current. Power supplies regulate by sensing the output and comparing a portion of it to a reference voltage. The difference between the sense signal and the reference is amplified and then used to control the power stage of the regulator to keep the voltage (or current) constant (Figure 1).
Figure 1. A typical power supply control loop.
Power supplies employ negative feedback from the output back to an error amplifier to ensure proper regulation over various operating conditions (load changes, temp changes, input voltage changes, etc.). As with any stable closed-loop system, one must ensure the closed-loop gain is less than one at frequencies of operation or risk oscillation and/or other non-desirable characteristics.
Negative (or degenerative) feedback works in opposition to external influences such as changes in output voltage caused by load changes or drift of component values. The negative feedback term of a power supply must be sufficiently out of phase with the input or establish a gain of less than one to ensure proper operation.
The traditional method of ensuring stability in a closed feedback circuit is to measure and plot the gain and phase for the complete path around the loop. From this measurement a safety margin can be calculated and determined if it is acceptable for the system under test. Phase and gain margin will be expanded upon below.
While loop characteristics can be simulated, real world system level characteristics such as PCB and connector impedances are difficult to accurately model, especially with lower cost simulation tools. So an actual stability measurement is necessary to understand actual loop stability.When can control loop stability be ignored?
Some power supply ICs were designed to help minimize the concerns related to stability. Using these ICs assumes a stable design, if one follows strict design guidelines and maintains specific operating conditions. For example, the Simple Switchers™ or Swift™ product families from TI are designed for ease of use and operate properly over clearly defined ranges of input and output conditions with a specific implementation. If a design is held within the defined application limits and the designer follows printed circuit board (PCB) examples, the circuit should be stable.
Some regulator architectures do not employ traditional amplifier feedback loops, but use level comparators with hysteresis to maintain the output voltage. These hysteretic control loop regulators, often called DCAP or constant-on-time (COT), employ an inherently stable control loop. Though not always necessary, it’s always a good practice to measure stability in any closed-loop power system.