Power Loop Stability Evaluation Method

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Power Loop Stability Evaluation Method [Copy link]

1. Loop Stability Evaluation Index The indicators for measuring the stability of the switching power supply are phase margin and gain margin. At the same time, the crossover frequency should also be used as a reference indicator. (1) Phase margin refers to the phase corresponding to the gain dropping to 0dB. (2) Gain margin refers to the gain size corresponding to the phase of 0deg (actually attenuation). (3) Crossover frequency refers to the frequency value corresponding to the gain of 0dB. Phase margin, gain margin, and crossover frequency are shown in the figure (Bode plot).

Phase tolerance is the phase margin, and gain tolerance is the gain margin.

2. Verification of the stability of the switching power supply control loop The frequency response characteristics of the switching power supply loop, if the load characteristics of the power supply at a certain frequency when the gain is equal to 1 (0dB) and the phase shift is 180°, then the power supply control loop will have positive feedback in phase (this phase shift plus the original set 180° phase shift, the total phase shift is 360°), so there is enough energy to return to the system and maintain oscillation at this frequency. In order to avoid similar destructive instability in the power supply system, usually, the loop control circuit will use feedback compensation components to reduce the gain of the high-frequency end, so that the switching power supply remains stable within the preset frequency range. The wiring diagram of the loop stability test is shown in Figure 2. The signal resistor (Inserted Resistor) is connected to the loop control circuit of the power supply, and a swept frequency interference signal (OUT) with a specific amplitude and frequency range is injected. When the interference signal reaches a certain amplitude, the loop system responds to the interference signal. Then, the frequency response analyzer is used to simultaneously perform ratio tracking measurements on the power loop output terminal (CH2) and the power loop interference signal injection terminal (CH1). The relationship curve between frequency, gain and phase can be obtained - the Bode Plot Graph, as shown in Figure 1. The links that should be paid attention to during the actual test process are the position of the injected resistor and the resistance value. In order to reduce the measurement error, the actual measurement generally selects a resistor of 10 to 100Ω; the size of the interference signal generally requires that its amplitude cannot exceed 5% of the output voltage, otherwise the measured result is inaccurate.

Loop Stability Test Wiring Diagram

III. Loop Stability Evaluation Index

In engineering, it is generally believed that under room temperature, standard input and normal load conditions, the phase margin of the loop is required to be greater than 45° to ensure the stability of the system under various errors and parameter changes. When the load characteristics and input voltage change greatly, it is necessary to consider that the loop phase margin should be greater than 30° under all load conditions and input voltage range. The crossover frequency, also known as bandwidth, can reflect the speed of the control loop response. It is generally believed that the wider the bandwidth, the better the ability to suppress the dynamic response of the load, the smaller the overshoot and undershoot, the faster the recovery time, and the more stable the system. However, due to the influence of the right half plane zero point, as well as the limitations of raw materials, the bandwidth of the operational amplifier cannot be infinite, and the bandwidth of the power supply cannot be increased indefinitely. It is generally taken as 1/20 to 1/6 of the switching frequency. In summary, the stability of the power supply loop can generally be determined from the following three principles: (1) At room temperature, standard input, and normal load conditions, when the closed loop gain is 0dB (no gain), the phase margin should be greater than 45 degrees; if the input voltage, load, and temperature change range is very large, the phase margin should not be less than 30 degrees. (2) Synchronously check that when the phase is close to 0deg, the closed loop gain margin should be greater than 7dB. In order not to approach the unstable point, it is generally believed that a gain margin of more than 12dB is necessary. (3) At the same time, analyze the power supply characteristics based on the tested Bode diagram. The crossover frequency is closed at 20dB/Dec, and the bandwidth is generally 1/20 to 1/6 of the switching frequency. If there is a loop analyzer, it will be easy. Under low temperature conditions, typical input and typical output, measure the open loop Bode diagram, and judge according to the above criteria. The problem is: our small and medium-sized enterprises often do not have sufficient testing conditions. In the absence of conditions for testing the Bode diagram, how can we analyze and judge the loop stability? If there is no loop analyzer, you can judge by the output dynamic load response. The test conditions are: operate at the lowest temperature specified in the specification (such as -25℃), rated input voltage, no additional output capacitance, and switch from half load to full load to half load (the current change rate can be set according to the standard change rate of 0.1A/us of industrial power supply). If the output dynamic response can be achieved like this, it can be basically determined that the loop is very stable: Jitter at Phase Point

For a typical PWM switching power supply, if the phase point jitter is too large, the system will usually be unstable (that is, the insufficient phase margin mentioned earlier, the performance in the time domain under dynamic load conditions). For a 200~500K PWM switching power supply, the typical jitter value should be less than 1ns.