Switching power supply can be adjusted if it is adjusted

The control theory of switching power supplies is a very abstract and sometimes daunting thing. System instability is a common problem, how to adjust? Why is there a problem with the system being mass-produced? There are many materials that talk about theory, and there are many mathematical theories that need to be understood. So how do you understand the loop stability of a switching power supply?

Cedar told me a metaphor, I think it is very good, first share it with everyone:

The feedback loop is like a wife giving her husband a back. At the beginning, my wife said, "I didn't eat, so I didn't have the strength?" The wife began to work harder, and she always added, and her husband said: "So much effort, you want to kill people?" Repeatedly, it is a loop instability. the process of.

Today I saw another metaphor of a man and a woman for your reference, as follows:

Let's talk about the pole first. A simple example is an RC filter. It is open circuit to DC C and short circuit to infinite high frequency C, so the amplitude of Bode plot is flat before the pole, and starts to decrease by -20dB/dec after the pole. The feeling of the pole is a man. Men usually start to be enthusiastic, but most of them can't stand the test of time. Whether it is for love or for the gradual loose hair, men are probably the same. This zero is of course a woman. A simple example is the ESR zero of a capacitor. In DC, the resistance of the capacitor is infinite. As the frequency increases, the resistance decreases continuously. After the pole, the resistance of the remaining ESR resistor is no longer reduced. A man is a fire, a woman is a water, and a woman is not as strong as a man, but most of them have more endurance than a man. Women are also very impressed with love. It is not difficult to understand the difference between men and women by seeing Anna Karenina and his lover.

After talking about men and women, it is the turn of two men. Unfortunately, I have been staying near San Francisco for a long time, but I don’t want to talk about comrades (good words, bad things). An LC filter forms the double pole. The two men inevitably have conflicts, which is like the high Q value. An LC with no parasitic resistance has an infinitely high Q value, which amplifies the signal at that resonant frequency. This is why we can receive different radios on the radio when we adjust a small capacitor. The two men are in conflict with each other. Power is not a good thing. The degree of conflict depends on the parasitic resistance value, or on the strength of the persuasion (scientific name is damping). A system with a high Q value, the phase is very fast from 0 ° to -180 °, very easy to be unstable, and difficult to compensate. Therefore, a system with high efficiency (small resistance component) is not easy to stabilize. Make compensation for unstable systems. Compensation usually adds a zero, but at the same time it will produce a high frequency pole. For example, adding a capacitor to the feedback terminal produces a zero and pole pair. The understanding of the zero pole pair is to talk about a love. Zero first intervenes, just as a woman is mostly at the beginning of a romantic relationship. For most men, that is the only time he is interested in accompanying a woman to a shop. The man and the woman came together after the truth, the truth, or at least the true oath of the mountain. Most of the love of a man is going to happen. If it happens before the pole of his life, it will be a tragedy. On the contrary, it will be called immortal love. For the feedback system, a pole reduces the amplitude (for stability) and also reduces the phase margin (not conducive to stability); a zero increases the amplitude (not conducive to stability), but increases the phase Margin (for stability); so they all did a good thing, a bad thing. Only the zero point of the right half plane, she increased the amplitude and reduced the phase margin, which is to do two bad things. Such a woman can only be described by a witch. A simple example is the boost circuit: when the active tube is turned on, the inductor stores energy; when the diode is turned on, the inductor delivers the stored energy to the load. The current drawn by the load is approximately IL(1-D). Deriving two variables, the inductance hinders the current rise at low frequencies, and only -ILd at high frequencies. As I already know, the amplitude is from zero to constant, just like the ESR of the capacitor is a zero, the difference is that there is a negative sign. As the load increases, D will increase to provide more current. However, since the output current is directly proportional to (1-D), the increase in D instantaneously causes less output current. It is this negative sign that turns a woman into a witch. As we all know, we use negative feedback systems. If there is more output, just reduce it in the place of control, and the change will not be too big. However, most of the loop itself has phase lag. If the signal of a certain frequency is 180° when the phase of the loop is delayed by 180°, the 180° with negative feedback is 360°. Negative feedback becomes positive feedback. Moreover, each time the amplitude of the loop turns larger, it is naturally unstable. Therefore, the stability condition of the system is that when the gain of one revolution is 1 (0 dB), the phase lag is less than 180 ° (considering the margin, generally less than 135 °). The idiot in "Dust Settled" can also understand that it is like a man (phase lag 90 °, quite a pole) or a half man (phase lag 135 °, quite a half pole) die (reach 0 dB) ). Let's look at an example. For a current-type buck, the current on the inductor is limited, so the poor inductance loses the floor (strictly speaking, the front row of the floor). The main circuit is left with only one pole of Rload and Cout (man 2) and the ESR zero of output capacitor (woman 1). Of course, the control part must have a very low frequency pole (Men 1). In other words, we have two men, the danger of instability, the key to see where the ESR zero (Women 1) is. The zero frequency of the electrolytic capacitor is very low, so it is very likely that a man is partially neutralized, so no compensation may be required. The zero frequency of the ceramic capacitor is very high, so we are likely to compensate by adding a woman (usually a zero-point pair, that is, a love can be stabilized). For the voltage type buck, L and Cout form the double pole (Men 2 and Man 3), plus the pole of the control part (Men 1). We may face three men. There is no doubt that in order to die like a man, we have to add one or two pole-zero pairs. Obviously the voltage type buck is not easy to stabilize. Like a swaying teenager, it is inevitable that you can mature a few times in love. Finally, we will talk about how the zero pole of the switching circuit is derived. I really want to squat in this step and still decide where the card is, or talk about history is more interesting. When the switching power supply appears, the general control theory is very mature, but it is all about a fixed circuit. Switching power supplies are not only in a state, but sometimes there are even more than three states. These states correspond to different state equations. How do you describe the overall circuit? In fact, very understandable. For example, if you take a step east, then go north and describe your trajectory as 50%*east+50%*nor=north. If you go three steps to the east, then go one step north, describing your trajectory is 75% * East + 25% * North = East North. The state equations of different switching states are weighted and added, and small signal interference is added. After finishing, the zero poles of different circuits are obtained.