Modularity is the general trend of switching power supply development. It can be composed of modular power supply to form a distributed power system. It can be designed as an N+1 redundant power supply system, and realizes the capacity expansion in parallel mode, so that the bulk weight of the entire power supply is reduced. The current stress of the device is small, which improves the reliability of the system. The switching power supply of this study is a single power supply with a small load. When the load is large (current exceeds 1.7A), the automatic switching is used to supply parallel power to the dual power supply. The external characteristic drooping method is used to realize the current sharing of each power supply, and the switching power supply is enhanced. Load capacity and improve power supply efficiency.

1 system design

1.1 DC-DC converter circuit topology

This design selects the boost chopper circuit, and its circuit schematic is shown in Figure 1. The boost rail wave circuit is selected as the main topology of the DC-DC transform.

Step-up chopper circuit principle

Figure 1 Principle of boost chopper circuit

1.2 System performance indicators

This design uses the double-ended driver integrated chip TL494 to output the PWM pulse to control the conduction of the main switch to control the voltage output. With the ATmega128 microcontroller as the core, it realizes automatic switching from single power supply to dual power supply and parallel current supply when high current is realized. Switching power supply with load capacity and improve power supply efficiency. The system hardware is mainly composed of the minimum system of the single chip microcomputer, the PWM control chip TL494, the switching power supply boosting main circuit, the current detecting circuit and the D/A conversion circuit. The system output DC voltage is adjustable from 18~45V, which can be adjusted by keyboard. The maximum output current reaches 4A. It can measure and display the output voltage and output current. It has fast adjustment speed, low voltage regulation rate and low load regulation rate. High efficiency, small output ripple and so on.

1.3 System implementation block diagram

In the comparison of the integrated schemes, the ATmega128 is selected as the main control chip. After the D/A conversion, the reference voltage is supplied and compared with the output feedback voltage, so that the TL494 generates the corresponding PWM square wave, and the totem pole driver is used to control the Boost boost circuit. The output voltage is adjustable. The INA169 is used for current sampling, optocoupler and IRF9540 to form an automatic switching circuit. The overall block diagram of the system design is shown in Figure 2.

System design block diagram

Figure 2 System design block diagram

2 Theoretical analysis and calculation

2.1 Selection of Inductance of Energy Storage Components

Calculating the correct inductor value is important for choosing the right inductor and output capacitor for the smallest output voltage ripple. The inductor used in this design is a ferrosilicon-aluminum two-wire wound inductor. Its core loss is much lower than that of iron powder core and high magnetic flux. It has low magnetostriction (low noise) and is a low-cost energy storage material. Stable performance at high temperatures.

2.2 Switch tube selection

The design of the MOSFET is IRF540, and the N-channel enhanced field effect power transistor packaged in the trench process is commonly used in DC to DC converters, switching power supplies, TV and computer monitor power supplies, and has low conduction. Resistive, fast switching, low thermistor and other significant advantages, its drain-source voltage V_DSS up to 100V, on-current I_D up to 23A, its on-resistance R_DS(on) "77mΩ, allowing maximum tube consumption PCM Up to 50W, meeting the circuit requirements.

2.3 Selection of freewheeling diode

Switching power supply output rectifier diodes typically use Schottky diodes or fast recovery diodes. Because of its reduced forward voltage and almost no reverse recovery time, the rectifier diode used in this design is SS35, which is a low-power Schottky diode with a reverse voltage of 50V and a forward voltage drop of only 0.6V. Left and right, with high surge current capability.

2.4 PWM pulse width modulation circuit

The core of the PWM controller circuit uses a dedicated integrated chip TL494. Through appropriate external circuits, not only PWM signal output can be generated, but also various protection functions. The TL494 contains an oscillator, an error amplifier, a PWM comparator, and an output stage circuit. The external circuit of this design is shown in Figure 3.

TL494 external circuit

Figure 3 TL494 external circuit

Pins 1 and 2 of pin TL494 are the in-phase and inverting input of error amplifier 1, 1 is connected to output voltage feedback terminal IN1, pin 2 is connected to D/A port, feedback signal and preset signal are compared and amplified by error amplifier. The pulse width is output from pin 8 and then through the totem pole circuit to control the switch IRF540 to turn on. In order to protect the output transistor of the TL494, the voltage is divided by R30 and R31, and the intermittent adjustment voltage of 0.3V is applied to the 4th pin, and the whole power supply takes a single 16V power supply.

2.5 MOSFET drive circuit

In the system, the N-channel MOSFET model is IRF540N, and its turn-on voltage is 2~4V. However, in order to ensure its full conduction, it is generally required to provide a gate voltage of about 10V. To this end, the design uses a simple, reliable, low-cost totem pole circuit as the drive circuit for the MOSFET (as shown in Figure 4). The PWM signal is amplified by the first transistor 9014, and then maintained by a complementary circuit composed of the rear-stage NPN-type transistor 9014 and the PNP-type transistor 9015 to supply a voltage of about 11 V to the gate G of the MOS transistor. The circuit has better performance during the turn-on and turn-off of the switch: fast and reliable turn-on, and there is no high-frequency oscillation of the rising edge; at the moment of turn-off, the driver circuit can provide a low-impedance path for the MOSFET gate Rapid bleed of the source-to-source capacitance voltage. The circuit inputs and outputs are just reversed. That is, when PWM is low, the gate gets a high voltage, and the MOS transistor is fully turned on; when PWM is high, the gate voltage is almost 0, and the MOS transistor is turned off.

Totem pole drive circuit

Figure 4 totem pole drive circuit

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