Circuit stability characterizes the behavior of dynamic circuits under disturbances, encompassing two typical scenarios: the system's ability to maintain its original steady state when encountering transient disturbances during operation; and the system's ability to autonomously return to its original steady state after the disturbance is eliminated. This property is a core indicator for evaluating the safe operation of power systems and the anti-interference capability of electronic equipment.
Judgment method
Stability is determined based on the location distribution characteristics of the roots of the system's differential equation:
When all the system poles are located in the left half of the complex plane, it exhibits asymptotic stability.
If poles exist located on the imaginary axis or in the right half-plane, the system may experience sustained oscillations or divergence.
Influencing factors
System parameter settings directly affect stability performance:
Phase margin and gain margin of the feedback loop
Energy storage characteristics of energy storage elements (inductors, capacitors)
Harmonic distortion effect introduced by nonlinear elements
The limiting effect of distributed parameters on high-frequency response
Application areas
Circuit design involves many techniques, but also many pitfalls. This article will introduce common pitfalls in circuit stability design.
Myth 1: Product failure = product unreliability. Sometimes, product problems are not due to R&D issues. There have been cases where equipment intended for medium to high-development regions in China worked well domestically, so it was exported to Colombia, but it frequently failed there.
The cause of the malfunction is that the altitude of moderately and more developed areas in mainland China is relatively low. Therefore, the airtightness of the equipment is challenged in high-altitude areas, and the increased pressure difference between the inside and outside of the equipment leads to an increased leakage rate.
The project was initiated with only low altitude in mind, so their design is fine. That's what your boss required. Whoever decided to export this model to Colombia is the real culprit.
If the CEO in charge of R&D participates in decision-making without raising any objections, he is practically the biggest sinner. After all, it's forgivable for a sales executive to make decisions without understanding the technology, but the VP of Technology's mistake is simply incompetence.
Product reliability is the ability to perform its intended function within a specified time and under specified conditions.
Readers must carefully savor this definition, investigate things to gain knowledge, and see who can achieve more knowledge when investigating this definition. The conditions in the application often exceed the prescribed conditions, and this exceedance is very likely implicit.
Myth 2: Credit limit reduction is easy and there are no problems. Anyone can reduce their credit limit, just like drawing. Everyone can draw, but not everyone can make a living by drawing. Here's a simple summary:
Devices with the same function but different manufacturing processes have different derating factors;
Adjustable devices and fixed devices have different derating factors;
Different loads require different derating factors;
The derating factor differs for conductors of the same specification when used in multi-turn and single-turn applications;
Some parameters cannot be reduced in rate;
Temperature reduction and dehumidification should not be overlooked.
Myth 3: Components are safe to use. Why is component failure often referred to as "burning"? The reason is that most component failures are thermal failures. The component's ambient temperature is not the same as the overall system ambient temperature. The component's ambient temperature is affected by the heat dissipation of other components in the chassis, and generally the component's ambient temperature is higher than the overall system ambient temperature.
Myth 4: Reliability is unrelated to mechanics and software. Installation, wiring, layout, and painting all affect electrical performance.
Electromagnetic compatibility, poor soldering, heat dissipation, vibration and noise, corrosion, and grounding are all related to the structure.
Software error prevention, error detection, error correction, and fault tolerance measures can avoid mechanical and electronic defects.
Myth 5: Simple components do not require datasheets. When designing, you must obtain the datasheets for all components, read all the graphs, charts and parameters, and finally establish a connection between the design and these curves.
Myth 6: Maintainability is irrelevant to me. What is the purpose of reliability work for electronic products? It's to make money!!! How do you make money? By increasing revenue and reducing costs. Increasing revenue is difficult, reducing costs is easy.
Don't always try to save on material costs. Saving on material costs will only increase repair costs. You're just changing the time between dying sooner and dying later. You're going to die sooner or later anyway, so why bother? It's better to die sooner and be reborn sooner.
The best approach is to prioritize maintainability and eliminate that cost, which translates into genuine profit.
Myth 7: Poor process control means a lack of technical personnel. Poor process control is not just a problem of process personnel; it is a process of building a value chain.
Requirements of design engineers for components;
Manufacturer selection by the purchasing engineer;
The control measures in the inspection process should include sections on key performance indicators of the components.
The testing methods should not introduce failure mechanisms or damage to components;
The assembly process should also avoid introducing damage, such as controlling the temperature of the wave soldering oven and applying anti-static treatment to the manual soldering table;
The factory inspection process should check for any issues that may lead to product failure due to drift in component parameters, and the repair process should not introduce failures.
As can be seen from the above, a few process engineers cannot guarantee that problems will be resolved.
Therefore, the specific approach can be summarized as establishing consistency. The premise of consistency is that the designers provide sufficient and prioritized technical information, and the process is only based on the design drawings and design documents to ensure that the manufacturing reliability is infinitely close to the design reliability.
Myth 8: Strengthening testing can solve reliability problems. Since this is listed as one of the top ten myths, its definition is naturally wrong. There are three main reasons: some problems simply cannot be detected through simulation testing;
Testing methods = engineering calculations + specification review + simulation experiments + electronic simulation;
The corresponding low-temperature operating time cannot be calculated from the results of the temperature enhancement test.
Circuit stability analysis is a crucial step in ensuring that a circuit operates stably under various operating conditions. Here are some commonly used methods and steps to help you perform circuit stability analysis:
1. DC Operating Point Analysis
Determine the operating point of the circuit under DC conditions to ensure that each component operates within its normal operating range.
2. AC Small Signal Analysis
Analyze the circuit's response under small-signal AC conditions to determine its gain and phase characteristics. Specific steps include:
Construct a small-signal equivalent circuit.
Calculate the transfer function (gain and phase).
Use a Bode plot to analyze the relationship between gain and phase as a function of frequency.
3. Stability Analysis of Feedback Loop
For circuits with feedback, especially amplifiers and control systems, stability analysis is crucial.
Open-loop gain and phase analysis: Analyze the open-loop gain and phase characteristics by disconnecting the feedback loop.
Nyquist Plot: This plot is used to draw and analyze Nyquist plots to determine the stability of a system. The Nyquist criterion is an important tool for assessing the stability of feedback systems.
Bode plot: Uses Bode plots to analyze gain crossover frequency and phase margin. Gain margin and phase margin are important parameters for measuring system stability.
4. Transient Analysis
The response of an analog circuit to transient inputs (such as step jumps or pulses) is observed to determine whether the output is stable or exhibits oscillations. Transient analysis using simulation tools (such as SPICE) can provide information about the circuit's dynamic performance.
5. Root Locus Analysis
Plot the root locus of the system to observe the trajectory of the system poles as the gain changes. The root locus plot can help understand the impact of the location of the system poles on the system's stability.
6. Pole-Zero Analysis
Analyze the location of the poles and zeros of the system's transfer function. The poles of the system determine its dynamic behavior and stability. Ensure that all poles are in the left half-plane (S-plane) to guarantee system stability.
7. Frequency Response Analysis
Frequency response testing: Test the circuit's response at different frequencies to ensure that the circuit has the expected response characteristics within the design frequency band.
8. Simulation tools
Stability analysis is performed using circuit simulation software (such as SPICE, MATLAB/Simulink), specifically including:
SPICE simulation: Perform AC analysis, transient analysis, and frequency response analysis.
MATLAB/Simulink: Perform open-loop and closed-loop stability analysis using the Control Systems Toolbox.
Example: Circuit stability analysis using MATLAB/Simulink
Suppose we have a simple feedback amplifier circuit, we can use MATLAB/Simulink to perform stability analysis. Here are the basic steps:
Establish the circuit model: Build a model of the feedback amplifier in Simulink.
Open-loop analysis:
Insert the "open-loop gain" module and disconnect the feedback loop.
Use the "Frequency Response" tool to plot the open-loop gain and phase Bode plot.
Closed-loop analysis:
Restore the feedback loop.
Use the "Root Locus" tool to draw the root locus diagram of the system.
Use the "Transient Response" tool to simulate a step response and observe whether the system is stable.
