Design of DC regulated power supply and analysis of its series connection application

1. Introduction

With the rapid development of power electronics technology, DC power supplies are widely used. Their quality directly affects the working performance of electrical equipment or control systems. At present, the basic links of various DC power supplies on the market are roughly the same, including AC power supply, AC transformer (sometimes not necessary), rectifier circuit, filter voltage stabilization circuit, etc. This article takes the design of DC power supply powered by three-phase AC power supply as an example to introduce the solutions to some problems in DC power supply design. It also explains the problem of using multiple DC regulated power supplies in series in practical applications.

2. Design of DC regulated power supply

2.1 Design of rectifier transformer

The design of three-phase rectifier transformer includes: the connection mode of primary and secondary windings, the calculation of secondary voltage, the calculation of primary and secondary current, the calculation and determination of capacity, the selection of structural form, etc. Among them, the connection mode of primary and secondary windings and the determination of secondary voltage are the key points of our analysis. This article takes the design of three DC power supplies of a stepper motor driver as an example for detailed introduction, and the schematic diagram is shown in Figure 1.

Figure 1 Schematic diagram of DC power supply design for stepper motor driver

1. Determination of secondary side voltage

The secondary voltage is not only related to the load voltage (i.e. the voltage of the DC regulated power supply to be designed) and the rectifier circuit, but also to the voltage stabilizing device. For those with high requirements, a bridge rectifier circuit is selected, and capacitor filtering and voltage stabilization are used. For those with low requirements, no voltage stabilization or capacitor stabilization is required. As shown in Figure 1, +7V low-voltage drive is mainly used for phase locking. Its current is small and voltage is low. Voltage fluctuation has little effect on the working state of the drive power supply, so no voltage stabilization is required; +110V is used for high-voltage drive, intermittent power supply and high frequency. Large current and current change rate will produce very high overvoltage, so electrolytic capacitors are used for voltage stabilization and resistors are used for current limiting; +12V is used for power supply of computers and integrated circuits. It has small current and low voltage, but requires stable voltage and small ripple coefficient, so capacitors and three-terminal regulators are used for two-stage voltage stabilization. For different voltage stabilization methods, the secondary voltage has different determination methods. Theoretically, the calculation formulas for these three voltages are the same, namely U2=Ud/2.34 or UL=Ud/1.35. The calculated three secondary voltages are 5.2V, 81.5V and 8.9V respectively. However, 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 rectifier system is generally estimated by the formula UL=(0.9~1.0)·Ud. If the DC side is filtered by electrolytic capacitors, the output average value will increase, and it is generally estimated by the formula UL=Ud/2½. If the DC side is stabilized by capacitors and three-terminal regulators, in order to expand the voltage regulation range, Ud should generally be increased by 3~6V, and then estimated by the formula UL=(0.9~1.0)·Ud. The three secondary voltages determined in this way are: UL7=0.9×7=6.3V, UL110=110/2½=78V, UL12=16×0.9=14.4V.

2. Calculation of primary and secondary current and determination of capacity

The secondary current is determined according to the load current and the rectifier circuit. In Figure 1, a three-phase bridge rectifier circuit is used. The formula I2 = (2/3) ½ Id is used to calculate the three effective values ​​of the secondary currents: 3.26A, 6.5A, and 1.63A, and then the three secondary voltages and currents are obtained. According to the principle that the primary and secondary powers of the transformer are approximately equal, the primary current I1 = 1.45A can be obtained, and the capacity of the transformer is S = 953VA. The transformer model is selected according to 1.5kVA.

3. Determination of primary and secondary winding connection method

The winding of the three-phase transformer can be connected in star or Δ shape as needed. The three-phase rectifier circuit is generally used for high-power (i.e., load power above 4kW) rectification, and the transformer is usually connected in Y/Δ and Δ/Y. The Δ/Y connection method can make the power line current have two steps, which is closer to a sine wave, with less harmonic influence, and is more commonly used in controlled rectifier circuits; the Y/Δ connection method can provide a single-phase AC power supply and reduce the secondary winding current, and is generally used in high-power diode rectifier circuits; for small-power three-phase transformers, it is sometimes connected in Y/Y type, although this connection method will introduce harmonics into the power grid. But after all, its power is small and the impact is small. In short, when choosing, you should consider the impact on the power grid, and try to reduce the winding current and reduce the winding insulation level. In Figure 1, the current of 7V and 12V is relatively small and the voltage is low, so the star connection is selected; the current of 110V is large and the voltage is not too high, so the Δ connection is selected, which can greatly reduce the current in the winding, reduce the winding wire diameter, and extend the 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 is selected to reduce the winding voltage and the insulation of the winding.

2.2 Rectification circuit design

Three-phase rectifier circuits usually include three-phase half-wave rectifier circuits and three-phase bridge rectifier circuits. Since the three-phase bridge rectifier circuit has a high average output voltage, small voltage ripple, and high quality factor, the bridge rectifier circuit is often used. The selection of the diode model 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 current and voltage of the load. The calculation formula is: ID = (1/3) ½ · Id, ID (AV) = ID / 1.57, UDn = (1 ~ 2) 2 ½ · U2, and the rectifier tube model can be determined by checking the diode manual based on ID (AV) and UDn.

2.3 Filter and voltage stabilization circuit design

1. Filter circuit and device selection

Rectification and filtering circuits usually include capacitor, inductor and RC filtering circuits. Inductor filtering is achieved by using the inductor to generate back electromotive force for the pulsating current and hinder the current change. The larger the inductance, the better the filtering effect. It is generally used in fields with large load current and low filtering requirements. The RC filter circuit is a filter circuit connected by 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 uses the charging and discharging effect of the capacitor to make the rectifier output voltage stable, and the voltage amplitude is increased, the filtering effect is good, and it is suitable for various rectifier circuits. The selection of filter capacitors is mainly determined by the type, capacity, and withstand voltage value. Commonly used rectifier filter capacitors include aluminum electrolytic, tantalum electrolytic, polyester, monolithic capacitors, etc. Aluminum electrolytic capacitors have large leakage current, low withstand voltage and working temperature (maximum +70℃), but large capacity; Tantalum electrolytic capacitors have small leakage current, higher withstand voltage and working temperature than aluminum electrolytic capacitors, and are generally used in places with higher requirements; Polyester capacitors have large insulation resistance, small loss, low working temperature (maximum +55℃), small capacity, but high withstand voltage; Monolithic capacitors can be made very small, withstand voltage can also be made very high, chemical properties and thermal properties are relatively stable, but capacity is small. Generally, when the rectifier output current is large, electrolytic capacitors must be used for filtering and voltage stabilization; when the output current is small, general capacitors or electrolytic capacitors can be used for filtering. If there is a ripple coefficient requirement for the DC output voltage or to prevent high-frequency noise, it is better to use electrolytic capacitors and small-capacity non-polar capacitors in parallel: small-capacity capacitors can filter out high-order harmonics in pulsating DC, and electrolytic capacitors can filter out large-value low-frequency components, with a wide voltage stabilization range and good results. The rectifier filter circuit does not have high requirements on the capacity and withstand voltage of the capacitor. Generally, the capacity of the capacitor is estimated according to the output current. The larger the output current, the larger the capacity; the smaller the current, the smaller the capacity. However, too large a capacity will reduce the output voltage value, while too small a capacity will cause large voltage pulsation and instability. The capacity can be determined by referring to Table 1. The withstand voltage value is generally 1.5 to 2 times the working voltage of the connected circuit.

2. Voltage stabilization circuit and device selection

There are two types of voltage regulator circuits: discrete component voltage regulator circuits and integrated voltage regulator circuits. Integrated voltage regulator circuits are mainly used for low-voltage and low-current rectifier circuits. They are small in size, simple in circuit, high in voltage regulator accuracy, and easy to use and debug. When choosing, you must first determine the series, whether it is a positive power supply or a negative power supply, and whether it is adjustable or fixed. Secondly, select the specific model based on its rated voltage and rated current. At the same time, when the voltage regulator is connected to the rectifier circuit, some protective components should be added appropriately, such as connecting a diode at the I/O terminal to prevent the input terminal from short-circuiting, and connecting a small capacitor between the input terminal and the ground to limit the input voltage amplitude.

The design of DC power supply is relatively simple in theory, but further analysis, research, practice and summary are needed in specific engineering design.

3 Series application of DC regulated power supply

DC regulated power supplies are widely used. Sometimes two or more power supplies are used in series. Here is a brief introduction to this application.

Figure 2 Series voltage regulator circuit

3.1 Circuit composition and working principle

Figure 2 is a series voltage regulator circuit, which consists of a sampling circuit, a reference circuit, a comparison amplifier and an adjustment circuit. R1, R2 and RP form a sampling circuit, and R1, R2 and RP are called sampling resistors; R3 and V2 form a reference circuit, R3 is the current limiting resistor of VZ, and VZ provides a reference voltage to the emitter of V2; V2 is a comparison amplifier tube, which amplifies the change in the output voltage of the voltage regulator circuit and then sends it to the base of the adjustment tube; V1 is an adjustment tube, which plays an adjustment role.

The voltage stabilization process is as follows: When the output voltage U0 changes, the change in U0 is sampled and added to the base of the amplifier tube V2 through the sampling circuit. The reference circuit composed of R3 and Vz provides the reference voltage Uz for the emitter of V2. The amplifier circuit composed of V2 and R4 compares and amplifies the sampled voltage and the reference voltage, and then outputs an adjustment signal to the base of the adjustment V1, controlling V1 to adjust so as to keep U0 basically unchanged.

3.2 Series use of DC regulated power supplies

1. Use in sequence

The so-called series connection means that the polarity of the power supply is connected end to end, as shown in Figure 3.

Figure 3 Schematic diagram of power supply series connection (sequential series connection)

When the a and b terminals are open, the two power supplies are independent power supplies and can work normally. The output voltages are E1 and E2 respectively. The voltages at the a and b terminals should be the algebraic sum of the two electromotive forces, Uab=E1+E2; after connecting the loads R1 and R2, since the direction of the load current is consistent with the direction of the external output current when the two power supplies work normally, the two power supplies can also work normally, Uab=E1+E2, and the entire circuit conforms to Ohm's law for the entire circuit I=(E1+E2)/(R1+R2). Each power supply has a load current passing through it, and the voltage on the power supply remains unchanged (ignoring the internal resistance).

2. Reverse use

The so-called reverse series connection means that the power supplies are connected head to head or tail to tail, as shown in Figure 4. Figure 4 can be equivalent to Figure 5.

When the a and b terminals are open, the two power supplies are independent power supplies and can work normally. The output voltages are E1 and E2 respectively. The voltages at the a and b terminals should be the algebraic sum of the two electromotive forces, Uab = E2-E1; add the loads R1 and R2. Since E2>E1, the current direction on the load is as shown in the figure (i.e., I2). It is opposite to the current direction output by power supply 1, forcing power supply 1 to stop working, and the output voltage is 0. At this time, the load current will form a loop through the sampling circuit. Therefore, the "output voltage" of the left power supply is the voltage on the sampling circuit R1, R2 and RP. Assuming that the current reference direction of power supplies 1, 2 and the sampling circuit is as shown in the figure, I3 = I1 + I2, that is, the voltage on the sampling circuit U = I3 (R1 + R2 + RP). If you look to the right from the sampling circuit, it can be equivalent to a power supply with an electromotive force of E3 and an internal resistance of R0, where E3 = U. If E3＞E1, Iv1 is forced to be cut off, IIVC1 is approximately equal to 0, and the power supply really stops working. At this time, the "voltage output by the power supply" U is taken as = I3(R1+R2+RP) (it is not the actual voltage output by the power supply 1, but the voltage of E2 shared by the sampling resistor), where I3=E2/(R1+R2+(1R1+2R2+1Rp)). The size of the voltage is mainly determined by the size of E2, R1, R2, lRl, lR2, and 1RP. The entire circuit does not conform to Ohm's law of the entire circuit. I2≠(E2-E1)/(R1+R2). The voltage at this time should be greater than E1; E3 取样电阻的功率问题。如E3=E2，负载电流为0；E3＜E1情况与上述相反，E2电源没有真正工作。

From the above analysis, we can know that for the case of DC regulated power supply in series, if the power supply is connected in series, each power supply can work normally, and the whole circuit complies with Ohm's law of the whole circuit; if the power supply is reversed, the power supply with a relatively small stable voltage output cannot really play the role of a power supply, and the load current does not pass through the power supply under any circumstances. The specific situation is as follows: when the voltage on the sampling circuit is greater than the output voltage of the regulated power supply, the power supply is forced to stop working. At this time, only the sampling circuit of the power supply is connected to the circuit, and the voltage at both ends of the power supply is also the voltage at both ends of the sampling circuit (should be greater than the stable output voltage), and the whole circuit does not satisfy Ohm's law of the whole circuit; when the voltage on the sampling circuit is less than the stable output voltage, the power supply works normally, and the whole circuit satisfies Ohm's law of the whole circuit, but the load current only passes through the sampling circuit. The current on the sampling circuit should be the sum of the load current and the power supply adjustment current with a relatively small voltage. The power of the sampling resistor should be considered when using it. Therefore, when we use two or more regulated power supplies, we should use the reverse series of power supplies reasonably according to the specific situation. In order to avoid the phenomenon that the actual value is very different from the theoretical value.