The origin of the BUCK circuit, with three evolutionary circuits

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The origin of the BUCK circuit, with three evolutionary circuits [Copy link]

The origin of Buck

I don't want to talk about the history of power electronics. After decades of development, it has evolved from the low efficiency and large size of the initial linear power supply to the current high frequency, small size and high efficiency. The following will introduce how the Buck converter, one of the three most basic topologies, has evolved. Anyone
who has studied electronics should know how to get the desired voltage value (low) from a voltage (high). The simplest way may be to divide the voltage through resistors, as shown below.

This method is the most convenient and quickest. Now, general voltage sampling basically adopts this method, but what if the power is slightly higher? Since R1 and R2 are connected in series, the loss on R1 cannot be ignored. If the required voltage value is much lower than the input voltage, the efficiency of the circuit will be extremely low. Try to transform the circuit and replace R1 with a triode, which is the current LDO model, as follows:

Through the modification, the loss on R1 is transferred to the transistor Q1. Since Q1 bears the voltage difference between input and output, the efficiency of the circuit is also relatively low. In order to improve the efficiency, the transistor was previously working in a linear state. Can it be changed to a switching state? In this way, the transistor only has switching loss and conduction loss, and the loss will be greatly reduced. It can be changed to the following circuit:

The circuit has a working cycle time of Ts and a conduction time of Ton, so the duty cycle is D=Ton/Ts. However, the output voltage is highly correlated with the switch state. When S1 is turned on, there is an output voltage, and when S1 is turned off, there is no output voltage. However, the output load always requires continuous energy supply, which is unacceptable for the output load. This requires decoupling, and the introduction of energy storage components and capacitors at a certain position of the converter , so that even when the input S1 is disconnected, the output capacitor can continue to output energy to ensure the stability of the output voltage.

If you do this, do you see what kind of effect it will bring? Since the voltage across the capacitor cannot change suddenly, when S1 is closed, a very large surge current will be generated in the line , which will not only cause noise and EMI problems, but S1 may also be damaged. Therefore, it is necessary to limit the current, as follows:

After adding the current limiting resistor R2, there will not be such a large impact current at the moment S1 is closed. However, since R2 is connected in series in the main power circuit, the resistor will consume power. In this way, the power consumption reduced on the switch may eventually be consumed by the added resistor. Therefore, in order to maximize efficiency, R2 can be transformed into an inductive element. In principle, an inductive element only stores energy but does not consume energy. As we all know, the current at both ends of an inductor cannot change suddenly, so when the switch S1 is closed, the inductor can well suppress the impact current without consuming energy. As shown below:

This solves the surge current caused by C1 when S1 is closed, but what happens when S1 is disconnected? As mentioned earlier, the current at both ends of the inductor cannot change suddenly. When S1 is suddenly disconnected, it is equivalent to a sudden change in the current of the inductor. Since there is no freewheeling circuit, the energy stored in the inductor will be consumed in the form of "arcing", which will produce a very large voltage spike. Therefore, in order to provide a freewheeling path for the inductor L1, a freewheeling diode needs to be added, as follows:

In this way, when S1 is suddenly disconnected, the energy of L1 will be freewheeling through the diode, so we also call it a freewheeling diode. Of course, in order to improve efficiency, the freewheeling diode can be replaced with a MOSFET, as follows:

Thus, a synchronous Buck converter is created. The inductor can be placed in different positions to transform into different topologies. If it is placed at the input end, it is a Boost converter, and if it is placed at the bottom, it is a Buck-boost converter. Therefore, there are actually only three basic converters, and many other topologies are the evolution of these three basic converters. For example, the forward converter is the isolated version of the Buck converter, and the flyback converter is the isolated version of the Buck-boost converter.

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bigbat

Thank you very much, it would be great if there were calculations for all the circuits