Solutions to issues with the design of a DC regulated power supply
Design of DC Stabilized Power Supply
The design of the three-phase rectifier transformer includes: the connection mode 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. As shown in Figure 1, +7V low-voltage drive is mainly used for phase-locking. Its current is small and voltage is low. Type 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 to limit current; +12V is used for power supplies of computers and integrated circuits, with small current and low voltage. However, stable voltage and small ripple coefficient are required, so capacitors and three-terminal regulators 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.
1. Secondary example current calculation and capacity determination
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.
1. Determination of the connection mode of the secondary winding
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.
Rectifier circuit design
The three-phase rectifier circuit usually has a three-phase half-wave rectifier circuit and a three-phase bridge rectifier circuit. Since the output average voltage of the three-phase bridge rectifier circuit is high, the voltage ripple is small, and the quality factor is high, the bridge rectifier circuit is often used. The choice of diode type on the bridge arm is mainly determined by its rated voltage and rated current, and the rated current and voltage are determined by the average load current and voltage. The calculation formula is: ID=(1/3)½·Id, ID( AV) = ID / 1.57, UDn = (1 ~ 2) 2½·U2, the model of the rectifier can be determined by checking the diode manual with ID (AV) and UDn.
Design of filtering and voltage stabilizing circuit
1), filter circuit and device selection
The rectifier filter circuit usually has filter circuits such as capacitors, inductors, and RC. Inductive filtering is realized by using the inductance to generate counter electromotive force to the pulsating current and hinder the current change. The larger the inductance, the better the filtering effect. It is generally used in the field where the load current is large and the filtering requirements are not high. The RC filter circuit is a filter circuit used by connecting resistors and capacitors. Since the resistor will reduce a part of the DC voltage, the DC output voltage will decrease, so it is only suitable for small current circuits. Capacitor filtering is to use the charging and discharging effect of the capacitor to make the rectified output voltage stable, and the voltage amplitude increases, the filtering effect is good, and it is suitable for various rectifying circuits. The selection of the filter capacitor is mainly the determination of the type, capacity and withstand voltage value. Commonly used rectifier filter capacitors include aluminum electrolytic, tantalum electrolytic, polyester, and monolithic capacitors. Aluminum electrolytic capacitors have large leakage current, low withstand voltage and operating temperature (up to +70°C), but large capacity; tantalum electrolytic capacitors have small leakage current, higher withstand voltage and operating temperature than aluminum electrolytic capacitors, and are generally used for higher requirements places; polyester capacitors have large insulation resistance, low loss, low operating temperature (up to +55°C), small capacity, but high withstand voltage; monolithic capacitors can be made small in size and high in withstand voltage. The performance and thermal performance are relatively stable, but the capacity is small. Generally, when the rectified output current is large, electrolytic capacitors must be used to filter and stabilize the voltage; if the output current is small, ordinary capacitors or electrolytic capacitors can be used for filtering. If the DC output voltage has ripple coefficient requirements or in order to prevent high-frequency noise, use electrolytic capacitors It is better to be used in parallel with small-capacity non-polar capacitors: small-capacity capacitors can filter out high-order harmonics in pulsating DC, and electrolytic capacitors can filter out large-value low-frequency components, and the voltage stabilization range is wide and the effect is good. The rectification and filtering circuit does not require too much capacity and withstand voltage of the capacitor. Generally, the capacity of the capacitor is estimated according to the output current. If the output current is large, the capacity will be large; if the current is small, the capacity will be small. However, if the capacity is too large, the output voltage value will be reduced, and if it is too small, the voltage ripple will be large and unstable. Refer to Table 1 to determine the capacity. The withstand voltage value is generally 1.5 to 2 times the working voltage of the connected circuit.
2), voltage regulator circuit and device selection
There are two kinds of voltage stabilizing circuits: discrete component voltage stabilizing circuit and integrated voltage stabilizing circuit, among which integrated voltage stabilizing circuit is mainly used for rectifying circuit with low voltage and small current. . When choosing, you must first determine the series, whether it is a positive power supply or a negative power supply, whether it is adjustable or fixed, and then select a specific model according to its rated voltage and rated current; at the same time, when the voltage stabilizer is connected to the rectifier circuit, some Protective components, such as connecting a diode at the I/O terminal to prevent a short circuit at the input terminal, connecting a small capacitor between the input terminal and the ground, can limit the input voltage amplitude, etc.
The design of DC power supply is relatively simple in theory, but further analysis, research, practice and summary are needed in specific engineering design.
