[Theme Month] Power Knowledge You Will Understand Once You Know It - From Buck-Boost to Flyback
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There are many kinds of power supply topologies, but if we can understand one topology, we can understand other topologies. This is because the basic elements that make up various topologies are the same.354902For isolated power supplies, the circuit topology that most people are exposed to is probably flyback. But when we first started making power supplies, we didn't know how to design them, and we didn't even understand analysis. The only thing we could do was to imitate (well, to put it bluntly, it was plagiarism). This state lasted for a while before we began to slowly understand it. But for novices, if we can evolve from the basic topology structures BUCK and BOOST to more complex topologies, then it will become very easy for us to understand various topologies in a comprehensive way. In fact, to understand isolated power supplies, compared to non-isolated DCDC, we need to understand one more basic element - the transformer. Then many basic principles can also be evolved through basic topologies. This article analyzes the evolution process. In order to analyze the flyback circuit, let's start from the origin of flyback. Flyback evolved from buck-boost, one of the three most basic circuits. So the analysis of buck-boost will definitely help the analysis of flyback, and buck-boost seems to be simpler than flyback, at least it does not have a transformer. The following will begin to evolve buck-boost, and eventually will evolve into flyback. Buck-Boost circuit - a step-down or step-up chopper, whose output average voltage U0 is greater or less than the input voltage Ui, with opposite polarity. Figure 1 is the prototype circuit of buck-boost. Wrap the inductor L around a parallel coil, as shown in Figure 2: Disconnect the two parallel coils of L and change the turn ratio to 1:n, as shown in Figure 3: 354905[/attach] [color=rgb(25, 25, 25) 25)]Move the diode in Figure 3 along the loop so that the cathode faces outward, and change the position of the output voltage V and the ground. (The function of the diode is unidirectional conduction. There is no other branch current in the circuit. The two positions in a loop can have equivalent effects.) (Buck Boost achieves reverse voltage, but our isolated power supply does not require reverse voltage, so we need to swap the polarity of the power supply.) As shown in Figure 4: Move the Q in Figure 4 Follow the loop and move to the bottom of the transformer, as shown in Figure 5: (The switch can be located anywhere, but we don't want the Vgs voltage value of the MOS to be too high.) 354907 Changing the winding direction of the transformer will form a Flyback. 354908 The above shows that our study of the behavioral characteristics of buck-boost is of great help to the study of the behavioral characteristics of flyback. Because the working processes of the two circuits are extremely similar. It’s just that in the buck boost topology, there is only one inductor for energy storage, while in the flyback circuit, it is a transformer. The electromagnetic energy on the primary side can transfer energy to the secondary side when the primary circuit is suddenly disconnected. For the Buck-Boost topology: The first working state: mosfet Q is turned on, and diode D is turned off. As shown in Figure 8: At this time, the input power supply charges the inductor. The energy originally charged in the capacitor supplies power to the load, maintaining its original voltage. The second working state: Mosfet Q is turned off, diode D Turn on. As shown in Figure 9:354911[/attach] At this time, the inductor will maintain the original current. Let's take a look at the working process of flyback: Assume that this flyback circuit still works in stable CCM State. In state 1, mosfet Q is turned on, diode D is turned off, and the circuit is shown in the figure. It is similar to the state 1 of BuckBoost we just mentioned. At this time, the inductor is charged and the capacitor maintains the current of the load. In the flyback state 2, Mosfet Q is turned off, diode D is turned on, and the secondary side of the transformer charges the load and capacitor. What we just discussed is the CCM situation. Flyback has another situation DCM. The flyback working in DCM has one more working state 3 than in CCM. Working state 1 and working state 2 are the same as working states 1 and 2 of CCM. In working state 3, Mosfet Q and diode D All are in the off state. The three working state experience time are d1Ts, d2Ts, d3Ts.25)]What we just discussed is the CCM situation. Flyback has another situation DCM. The flyback working in DCM has one more working state 3 than in CCM. Working state 1 and working state 2 are the same as working state 1 and 2 of CCM. In working state 3, Mosfet Q and diode D are both in the off state. The three working state experience times are d1Ts, d2Ts, and d3Ts respectively. 25)]What we just discussed is the CCM situation. Flyback has another situation DCM. The flyback working in DCM has one more working state 3 than in CCM. Working state 1 and working state 2 are the same as working state 1 and 2 of CCM. In working state 3, Mosfet Q and diode D are both in the off state. The three working state experience times are d1Ts, d2Ts, and d3Ts respectively.
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