"Control Loop Design linear and switching power supply" is the latest book Power Electronics former columnist Christophe Basso's. This works explore the engineers really need to know focus on compensation and stability of a given knowledge of the control system. This book contains excerpts of the relevant chapters of the stability criteria.
In electronics, an oscillator capable of generating a sinusoidal excitation signal from the circuit. In a wide variety of configurations, the oscillator involves the acceleration noise inherent in an electronic circuit using an oscillator. Noise level on the power rises, and then began to self-excited oscillation. Constituting the module shown in FIG. 1 can be composed of such circuitry. As you view, this configuration looks very close to what we control the configuration of the system.
Figure 1: an oscillator is essentially an error signal, the control system does not interfere with the output signal changes.
In our example, the excitation noise is not input, but the voltage level Vin, which is injected to initiate the oscillator as an input variable. Direct channel transfer function is constituted by H (s), the return passage comprising a G (s) block. To analyze this system, we first write the equation by changes in the relationship between output voltage and input variable transfer function:
In this equation, the product of G (s) H (s) referred to as the loop gain, which is labeled T (s). To Our system is converted into a self-excited oscillator, the output signal must be present, even if the input signal has disappeared. To meet this goal, we must meet the following conditions:
Figure 2: an oscillation condition can be expressed in a Nyquist or Bode plot of FIG.
Under conditions of these two equations, we obtain the steady-state oscillation condition. This is called the Palestinian Cauthen (Barkhausen) standard proposed by the German physicist Barkhause in 1921. Practically speaking, in a system control loop, which indicates the correction signal output is no longer resist, but the form of the phase return, is exactly the same as the amplitude of the excitation signal. Equation (6) and (7) represents the loop gain, the curve passes through the 0 dB axis, and just at this point by 180 ° in phase lag influence Bode diagram (Bode plot) in. Nyquist analysis, the change in the relationship between the loop gain of the imaginary and real part of the relative frequency is plotted, this point corresponds to -1, j0. Figure 2 shows two curves of the oscillation condition is satisfied. If the system is slightly deviate from these values (e.g., temperature drift, gain variation), either the output of the oscillation will decrease exponentially to 0, amplitude or divergence, until a higher or lower supply rail. In the oscillator, the designers tried as much as possible to reduce the gain margin, the oscillation conditions under a variety of operating conditions can be met.
Stable condition
As you know, the goal is not to build an oscillator control system. We hope that the control system provides high-speed, accurate and non-oscillatory response. Therefore, we must avoid oscillation or configured to meet the divergent conditions. One way is to limit the system reacts frequency range. By definition, a frequency range or bandwidth, corresponding to the decrease of 3 dB from input to output of the closed loop transmission channel frequency. The bandwidth of the closed loop system can be considered as a frequency range, within this range the system is considered to be excellent in response to the input (i.e. set-point follow effectively suppressed or disturbance). We will see later in the design stage, we do not directly control the closed loop bandwidth, but will control the crossover frequency (crossover frequency) fc-- This is an open-loop analysis with the relevant parameters. These two variables are usually outline considered equal, but we will see that this is only set up on one condition. However, they do not differ too far, both interchangeable in the discussion.
We have seen that the open loop gain is an important parameter in our system. When the gain is present (i.e., | T (s) | "1), the system operates in closed loop dynamics, can compensate for the disturbance input or responding to setpoint changes. However, there is a limit system response: The system must provide a gain frequency disturbance signals involved. If the set point change fast disturbance frequency component of the excitation signal to less than the system bandwidth, which indicates the lack of the frequency gain: the system becomes slow and not respond, the operating state of the loop waveform change as not responding. So, whether it requires infinite bandwidth it? No, because increasing the bandwidth widening like a funnel diameter: Of course you can collect more information, and the input vibration to react more quickly, but the system will also receive a dummy signal (spurious signal), as converter in some cases they produce noise and parasitic parameters (e.g., the switching power supply output ripple). Therefore, mandatory limit the bandwidth in the range of your real application requirements. Bandwidths are too wide to impair the anti-noise performance of the system (such as its strong suppression of spurious signals outside). , Reading the full text, the technology area
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