Problems in the design of a DC regulated power supply solutions
Design of DC Stabilized Power Supply
The design of the three-phase rectifier transformer includes: the connection method of the primary and secondary windings, the calculation of the secondary side voltage, the calculation of the primary and secondary side current, the calculation and determination of the capacity, and the selection of the structural form. Among them, the connection mode of the primary and secondary windings and the determination of the secondary side voltage are the contents of our key analysis. This article takes the design of three DC power supplies of a stepper motor driver as an example to introduce in detail.
Determination of the secondary side voltage
The secondary voltage is not only related to the load voltage (that is, the DC regulated power supply voltage to be designed) and the rectifier circuit, but also related to the voltage stabilizing device. For the bridge rectifier circuit with high requirements, use capacitor filter to stabilize the voltage and stabilize the voltage with a voltage stabilizer. For those with low requirements, you can not stabilize the voltage or use capacitors to stabilize the voltage. +7V low-voltage drive is mainly used for phase-locking, its current is small, voltage is low, voltage fluctuation has little effect on the working state of the drive power supply, no need for voltage regulation; +110V is used for high-voltage drive, intermittent power supply and high frequency , large current and current change rate will produce high overvoltage, so electrolytic capacitors should be used to stabilize the voltage, and resistors should be used to limit current; +12V is used for power supplies of computers and integrated circuits. The current is small and the voltage is low, but the voltage is required to be stable. The wave coefficient is small, so a capacitor and a three-terminal regulator are used to stabilize the voltage in two stages. For different voltage stabilization methods, the secondary voltage has different determination methods. In theory, the calculation formulas of the three voltages are the same, that is, U2=Ud/2.34 or UL=Ud/1.35, and the calculated three secondary voltages The voltages are: 5.2V, 81.5V and 8.9V, but the results of such calculations are not suitable in practice. Therefore, some quantities must be determined by engineering estimation formulas. For example, the three-phase irreversible rectification system generally uses the formula UL=(0.9 ~1.0)·Ud estimate, if the DC side is filtered by an electrolytic capacitor, the average value of the output will increase, which is generally estimated by the formula UL=Ud/2½; if the DC side is stabilized by a capacitor and a three-terminal voltage regulator, in order to expand the stability Voltage range, Ud generally should be increased by 3 ~ 6V, and then estimated by the formula UL = (0.9 ~ 1.0) · Ud. The three secondary voltages thus determined are: UL7=0.9×7=6.3V, UL110=110/2½=78V, UL12=16×0.9=14.4V.
2. Current calculation and capacity determination of primary and secondary cases
The secondary current should be determined according to the size of the load current and the rectifier circuit. In Figure 1, a three-phase bridge rectifier circuit is used, and the effective values of the three secondary currents are obtained by using the formula I2=(2/3)½Id: 3.26 A, 6.5A, 1.63A, you get 3 secondary voltages and currents. According to the principle that the primary and secondary power of the transformer are approximately equal, the primary current I1=1.45A can be obtained, the capacity of the transformer is S=953VA, and the transformer model is selected according to 1.5kVA.
3. Determination of the connection mode of the primary and secondary windings
Three-phase transformer windings can be connected in a star or delta shape as required. Three-phase rectification circuits are generally used for high-power rectification (that is, the load power is above 4kW), and the transformers are usually connected into two types: Y/Δ and Δ/Y. The Δ/Y connection can make the power line current have two steps, which is closer to the sine wave, and the harmonic influence is small, and the controllable rectification circuit is used more; the Y/Δ connection can provide single-phase AC power, reducing the secondary Winding current is generally used in high-power diode rectifier circuits; for small-power three-phase transformers, it is sometimes connected into Y/Y type, although this connection method will introduce harmonics to the power grid. But after all, its power is small and its impact is small. In short, when choosing, we should not only consider the impact on the power grid, but also minimize the winding current and reduce the winding insulation level. In Figure 1, the 7V and 12V currents are relatively small, the voltage is low, and the star connection method is selected; the 110V current is large, and the voltage is not too high, and the Δ-shaped connection method is selected, which can greatly reduce the current in the winding, reduce the diameter of the winding wire, and extend the length of the winding. Service life; although the line voltage of the primary winding is high (380V), the transformer capacity is only 2kW, and the primary current is 1.45A, so the star connection method can reduce the voltage of the winding and the insulation of the winding.
