Bode plot, a tool for characterizing PSU dynamic control behavior
Control loop speed and regulation stability are identifiable.
Representative tools of ADI include 'LTpowerCAD' and 'LTspice'.


One tool that can be used to quickly assess whether a designed power supply unit (PSU) meets the requirements for dynamic control behavior is the 'Bode plot'.

Power supplies typically maintain a fixed output voltage through a control loop. This control loop can be stable or unstable. Furthermore, the control loop can be adjusted quickly or slowly. Control loops can often be described using Bode plots.

Bode plots give you an immediate indication of how fast or slow a control loop is, and especially how stable its regulation is.

The regulator above converts a high input voltage to a low output voltage. The goal is to regulate the output voltage VOUT as precisely as possible. For this purpose, a control loop is integrated into the circuit via a feedback (FB) pin, which detects voltage changes at VOUT.

The control loop must respond quickly to ensure that VOUT is always readjusted as accurately as possible. The output voltage must be readjusted whenever the input voltage or load current changes.

The above is a control loop gain curve in a Bode plot. This curve provides two important pieces of information: It can be read at the frequency where the gain is 1, or 0 dB.

In the control loop shown above, the so-called crossover frequency occurs at approximately 80 kHz. This frequency must not exceed 1/10th of the switching frequency set in the switch-mode power supply. Failure to do so can result in unstable operation.

The second piece of information is the area under the gain curve, i.e. the integral of the function. The higher the DC gain and crossover frequency, the better the control loop maintains the output voltage at a constant level.

The above is a phase curve from a Bode plot. The most important value read from this plot is the phase margin, which indicates the stability of the control loop. The phase margin can be read from the crossover frequency obtained from the gain plot. See [Figure 2].

In this example, the crossover point is at 80 kHz. Therefore, the phase margin in [Figure 3] is approximately 60°. A phase margin less than approximately 40° is considered unstable.

The control loop is optimally configured when the phase margin is between 40° and 70°. Within this range, a reasonable compromise exists between fast regulation and excellent stability. When the phase margin is greater than 70°, the system is very stable, but regulation tends to be extremely slow.

Typically, Bode plots are not provided in switching regulator data sheets because they are too dependent on the circuit design. Each operating condition, such as input voltage, output voltage, and load current, as well as the switching frequency used and the selection of external components such as inductors and output capacitors, can have a significant impact.

For this reason, Bode plots are often generated using computational tools such as LTpowerCAD® or simulation tools such as LTspice® . These tools can quickly verify that the planned circuit meets the requirements for dynamic control behavior.



This article is a summary of the article titled "Power Supply Design: How Bode Plots Can Help You Meet the Requirements for Dynamic Control Behavior" by Frederik Dostal, FAE, Analog Devices (ADI), Munich, Germany.