Main circuit—The path through which the power current flows in the switching power supply. The main circuit generally includes switching devices, energy storage devices, pulse transformers, filters, output rectifiers, and other power devices in the switching power supply, as well as power supply input terminals and load terminals.
There are many types of switching power supplies (DC converters). When researching and developing power systems, it is extremely important to fully understand the basic types of switching power supply main circuits and their working principles.
The main circuit of the switching power supply can be divided into two types: isolated and non-isolated.
1. Types of non-isolated circuits:
Non-isolated – the input and the output are electrically connected and are not isolated.
1.1. Tandem structure
Series connection - In the main circuit, the switching device (the switching transistor T shown in the figure below) is connected in series with the input terminal, the output terminal, the inductor L, and the load RL.
The switch tube T alternately operates in both on/off states. When the switch tube T is turned on, the input terminal power supplies power to the load through the switch tube T and the inductor L, and simultaneously charges the inductor L, when the switch tube T is turned off. When the back electromotive force in the inductor L causes the freewheeling diode D to be automatically turned on, the energy stored in the inductor L passes through the loop formed by the freewheeling diode D, and continues to supply power to the load R, thereby ensuring continuous current at the load end. .
The series structure can only obtain an output voltage lower than the input voltage, so it is a buck conversion.
1.2. Parallel structure
Parallel - In the main loop, the switching device (switching transistor T shown in the figure below) is connected in parallel with the output load in relation to the input.
The switch tube T alternately operates in both on/off states. When the switch tube T is turned on, the input terminal power source charges the inductor L through the switch tube T, and the freewheeling diode D is turned off, and the load R is powered by the electric energy stored in the capacitor. When the switch tube T is turned off, the freewheeling diode D is turned on, and the input terminal power supply voltage is superimposed with the self-induced electromotive force in the inductor L, and then the load R is supplied through the freewheeling diode D, and the capacitor C is simultaneously charged. .
It can be seen that in the parallel structure, an output voltage higher than the input voltage can be obtained, and thus it is a boost type conversion. And in order to obtain a continuous load current, the parallel structure has higher requirements on the output filter capacitor C than the series result.
1.3. Polarity reverse converter structure
Polarity reversal - The output voltage is opposite to the polarity of the input voltage. The basic structural feature of the circuit is that in the main loop, the inductor L is connected in parallel with the load with respect to the input.
The switch tube T alternately operates in both on/off states, and the working process is similar to the parallel structure. When the switch tube T is turned on, the input end power source charges the inductor L through the switch tube T, and the freewheeling diode D is turned off. The load RL is powered by the electrical energy stored by the capacitor; when the switch T is turned off, the freewheeling diode D is turned on, and the self-induced electromotive force in the inductor L supplies power to the load RL through the freewheeling diode D, and simultaneously charges the capacitor C; The reverse polarity of the freewheeling diode D causes the output to obtain a voltage output of opposite polarity.
2. Type of isolated circuit:
Isolation—The input and output are not electrically connected. The energy is transmitted by the magnetic coupling of the pulse transformer, and the input and output are completely electrically isolated.
2.1. Single-ended forward
Single-ended - one-way drive of a pulse transformer through a switching device;
Forward--the phase relationship of the original/sub-side of the pulse transformer ensures that when the switch is turned on and the primary side of the pulse transformer is driven, the transformer supplies power to the load at the same time.
The biggest problem of this circuit is that the switch tube T operates alternately in the on/off state. When the switch tube is turned off, the pulse transformer is in the "idle" state, in which the stored magnetic energy will be accumulated to the next cycle until the inductor The device is saturated, causing the switching device to burn out. The flux reset circuit formed by D3 and N3 in the figure provides a channel for venting excess magnetic energy.
2.2. Single-ended flyback
The flyback circuit is opposite to the forward circuit. The original/paying phase relationship of the pulse transformer ensures that when the switch is turned on and the primary side of the pulse transformer is driven, the transformer does not supply power to the load, that is, the original/paid side is switched on and off. . The problem that the magnetic energy of the pulse transformer is accumulated is easy to solve. However, due to the leakage inductance of the transformer, a voltage spike will be formed on the primary side, which may break through the switching device. A voltage clamping circuit is needed to protect the circuit formed by D3 and N3. From the circuit schematic diagram, the flyback type is very similar to the forward type. On the surface, it is only the difference between the same name of the transformer, but the circuit works differently, and the functions of D3 and N3 are different.
2.3. Push-pull (transformer center tap)
The characteristic of this circuit structure is: symmetrical structure, the primary side of the pulse transformer is two symmetrical coils, the two switch tubes are connected in a symmetrical relationship, and the wheel is broken, and the working process is similar to the class B push-pull power amplifier in the linear amplifying circuit.
Main advantages: high frequency transformer core utilization (compared to single-ended circuit), high power supply voltage utilization (compared to the half-bridge circuit to be described later), large output power, low voltage of both bases Flat, the drive circuit is simple.
The main disadvantages are: low utilization of the transformer windings and high requirements on the withstand voltage of the switching tubes (at least twice the supply voltage).
2.4. Full bridge
The characteristic of the circuit structure is that four identical switching tubes are connected into a bridge structure to drive the primary side of the pulse transformer.
In the figure, T1 and T4 are a pair, driven by the same group of signals, and simultaneously turned on/off; T2 and T3 are another pair, driven by another group of signals, and simultaneously turned on/off. Two pairs of switch tube wheels are turned on/off, and a positive/negative alternating pulse current is formed in the primary coil of the transformer.
Main advantages: Compared with the push-pull structure, the primary winding is reduced by half, and the switching tube withstand voltage is reduced by half.
The main disadvantages are: the number of switching tubes used is large, and the parameters are required to be consistent, the driving circuit is complicated, and it is difficult to achieve synchronization. This circuit structure is usually used in ultra-high power switching power supply circuits above 1KW.
2.5. Half bridge
The structure of the circuit is similar to the full bridge type, except that two of the switch tubes (T3, T4) are replaced by two equal-value capacitors C1 and C2.
Main advantages: It has a certain anti-unbalance ability, and the circuit symmetry requirements are not very strict; the adaptable power range is large, from tens of watts to kilowatts; the switch tube withstand voltage requirements are lower; the circuit cost is higher than the full bridge circuit Inferior. Such circuits are often used in various unregulated output DC converters, such as electronic fluorescent lamp drive circuits.
(Edit: Bright)
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