AC output single capacitor half-bridge transformer switching power supply (part 1) Switching power supply principle and design (serial 36)

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AC output single capacitor half-bridge transformer switching power supply (part 1) Switching power supply principle and design (serial 36) [Copy link]

1-8-2-2. AC output single capacitor half-bridge transformer switching power supply Figure 1-39 is the working principle diagram of the single capacitor half-bridge transformer switching power supply. The single capacitor here means that the upper voltage divider capacitor C1 or the lower voltage divider capacitor in Figure 1-36 is omitted. Therefore, the single capacitor half-bridge transformer switching power supply in Figure 1-39 is relative to the dual capacitor half-bridge transformer switching power supply in Figure 1-36. The half-bridge transformer switching power supply in Figure 1-36 uses two capacitors to divide the voltage to power the switching transformer, so we call it a dual capacitor half-bridge transformer switching power supply; the half-bridge transformer switching power supply in Figure 1-39 uses one capacitor to power the switching transformer, so we call it a single capacitor half-bridge transformer switching power supply. Unless otherwise specified, we call both of them half-bridge transformer switching power supplies. By the way, in Figure 1-39, the upper voltage-dividing capacitor C1 in Figure 1-36 is omitted. However, if the upper voltage-dividing capacitor C1 is retained and the lower voltage-dividing capacitor C2 is removed, this pull-up single capacitor half-bridge transformer switching power supply can also work normally, and has the same electrical performance as the pull-down single capacitor half-bridge transformer switching power supply in Figure 1-39, except that the voltage output polarity is just the opposite. When the single capacitor half-bridge transformer switching power supply starts to work, since the capacitor C1 is not fully charged in advance, the positive and negative half-cycles of the voltage waveform output by the switching power supply are asymmetric. The output voltage is always higher in the positive half-cycle than in the negative half-cycle. It takes a period of time for the output voltage to stabilize. When the switching power supply starts to work, the control switches K1 and K2 are turned on and off, and the capacitor C1 starts to charge and discharge repeatedly. When the capacitor C1 starts to charge and discharge, the average value of the voltage across the capacitor C1 will continue to rise, that is, the amount of charge stored in the capacitor C1 when charging is greater than the amount of charge released when discharging; it takes a period of time, when the amount of charge charged and discharged by the capacitor C1 is completely equal, that is, when the voltage across the capacitor C1 is exactly equal to half of the input voltage Ui, the output voltage of the single capacitor half-bridge transformer switching power supply begins to stabilize. Let's further analyze the working principle of the single capacitor half-bridge transformer switching power supply in detail. When the control switch K1 is just turned on, the input power Ui is added to the primary coil a and b of the switching transformer through the capacitor C1 to power the switching transformer. At the same time, the capacitor C1 also starts to charge, and the current flowing through the capacitor C1 can be regarded as consisting of two parts. Part of the current i1 is the excitation current flowing through the primary coil N1 winding of the switching transformer. We can regard the primary coil N1 winding of the switching transformer as an inductor, which is equivalent to the power supply voltage Ui charging the capacitor C1 through the control switch K1 and the inductor L. The other part of the current i2 is the current flowing through the secondary coil N2 winding of the switching transformer and refracted to the primary coil. This part of the current is equivalent to n times the output current of the secondary coil of the power transformer, where n is the turns ratio of the secondary coil of the switching transformer to the primary coil. This is equivalent to the power supply voltage Ui charging the capacitor C1 through the control switch K1 and the equivalent load resistance R, please refer to Figure 1-40. In Figure 1-40, Figure 1-40-a is a schematic diagram of the power supply voltage Ui charging the capacitor C1 through the control switch K1 and the primary coil N1 winding of the switch transformer when the control switch K1 is turned on, and Figure 1-40-b is a diagram of the current flowing through the primary coil N1 winding of the switch transformer equivalent to the sum of the excitation current i1 and the load current i2. If you want to accurately calculate the circuit of Figure 1-40-a or 1-40-b, you need to solve a set of differential equations, and the calculation is very complicated. However, we know that in a circuit composed of an inductor and a capacitor, the voltage across the capacitor rises according to a sine curve when charging, and the voltage across the capacitor drops according to a cosine curve when discharging; in a circuit composed of a resistor and a capacitor, the voltage across the capacitor rises according to an exponential curve when charging, and the voltage across the capacitor drops according to an exponential curve when discharging. In a circuit composed of an inductor and a capacitor in series, the voltage across the capacitor rises according to a sinusoidal curve when it is charged, and its working principle is also easy to understand. In a circuit composed of an inductor and a resistor, or a capacitor and a resistor in series, the voltage across the inductor and the capacitor changes according to an exponential curve when they are charged; however, the voltage across the inductor decreases according to an exponential curve, while the voltage across the capacitor increases according to an exponential curve; if the inductor and the capacitor are charged in series at the same time, then the voltage across the inductor and the capacitor will change together according to a pair of conjugate exponential curves. According to Euler's formula, the algebraic sum of the two conjugate exponents is exactly a sine function or a cosine function. For the detailed process and analysis of capacitor charging and discharging, please refer to the relevant content of equations (1-114) and (1-115) in the previous section "1-7-2. Transition process of switching power supply circuit", and we will not repeat it here.