Switching power supplies have become increasingly popular in electronic devices due to their advantages of stability, high efficiency, and high power density. They use various switching technologies, with pulse width modulation (PWM) being one of the key techniques used.
In a voltage-type PWM controller like the TL494, the principle is to compare the output voltage (Uo) of the power supply with a reference value to obtain the error voltage (Ue). The error voltage is then processed by a regulator, which generates a control voltage (U). This control voltage is compared with a sawtooth wave signal generated by a sawtooth wave generator. By changing the output duty cycle of the PWM, the switching of the power supply’s switch transistor can be controlled to adjust the output voltage.
To design a boost type regulated output switching power supply, the TL494 is used as the core PWM controller, along with a single-chip microcomputer MSP430F449. This system utilizes the TL494’s capability to compare and control the output voltage, adjusting the duty cycle of the PWM signal to achieve stable output.
1. The overall scheme of the system
The main circuit is responsible for stepping down the voltage using an isolation transformer, rectifying the AC voltage using a rectification bridge, and then stepping up the DC voltage using a DC-DC Boost converter.
The feedback loop plays a crucial role in regulating the output voltage and current. It includes components such as voltage and current sensors for measuring the output values, small signal amplifiers to amplify the measured signals, an analog-to-digital converter (A/D converter) to convert the signals into digital form, and a control voltage output section for providing the necessary control signals to adjust the PWM duty cycle.
The protection circuit ensures the safety and reliability of the system by incorporating voltage and current detection mechanisms. These circuits monitor the input and output voltages as well as the current levels and trigger appropriate protective measures when certain thresholds are exceeded.
The human-computer interaction part involves the integration of a keyboard and display control. This allows users to interact with the system, inputting commands or parameters via the keyboard, and receiving feedback or relevant information through the display.
Overall, the system layout you described seems to have a well-defined structure with distinct functional blocks interacting with each other.
2. Main circuit design
2.1 DC_DC main loop topology
This system uses Boost circuit for DC-DC conversion. The output voltage is controlled by adjusting the duty cycle of the switch tube. The switching tube is turned off and turned on alternately, the inductor L will store and release energy alternately, and the voltage will rise after the inductor L stores energy. The capacitor C keeps the output voltage balanced, and the relationship between the output and input voltages is: Uo=Uin(ton+toff)/toff. The desired output voltage can be obtained only by changing the on-off duty cycle of the switch tube. Its schematic diagram is shown in Figure 2.
2.2 TL494 circuit design
TL494 has a built-in 5V reference voltage reference source, 5-pin and 6-pin are externally connected with capacitors and resistors, which can generate a corresponding sawtooth wave and then send it to the comparator for comparison to generate an oscillation signal of a certain period. The oscillator frequency is fosc=1/RTCT. Pin 4 is the dead-time control terminal, and pin 13 is the output mode control terminal. The chip contains two identical error amplifiers. The output terminal is isolated by a diode and then sent to the non-inverting terminal of the comparator. It is compared with the sawtooth wave voltage at the negative terminal to determine the width of the output voltage. The width adjustment process is controlled by the voltage of pin 3, and can also be controlled by the error amplifier. Two amplifiers can be used independently for feedback voltage and overcurrent protection.
In this application circuit, pin 3 and pin 13 of TL494 are grounded, a 51kn resistor is connected between pin 2 and pin 3, and pin 1 is used as an input terminal. The 5-pin and 6-pin are respectively connected with a 1000pF capacitor and a 2.7kQ resistor to generate an oscillation frequency of 50kHz. The over-current protection measures use 15-pin to connect to the resistor divider and 16-pin to connect to the action voltage value. Its implementation circuit is shown in Figure 3.
2.3 Switch tube drive circuit
The switch tube drive uses the MOSFET dedicated drive chip IR2110 from IR Company. As shown in Figure 4, the input signal is the PWM signal output by TL494, and its output signal can directly drive the switch tube.
2.4 Voltage and current sampling and small signal amplification
Since the voltage value after A/V conversion is very small when the current is very small, it needs to be amplified before it can be accurately collected. Here, the PGA program-controlled amplification method is selected to amplify the acquisition signal by 0 to 1000 times, ensuring that both small and large currents can be acquired through program control.
2.5 Selection of D/A converter and A/D converter
The selection of the D/A converter is directly related to the output accuracy of the system and the stability of the closed-loop adjustment. Here, Maxim’s serial port 16-bit high-precision D/AMAX5441 is selected as the D/A converter’s output control voltage for TL494. And adopt MSP430F449 to have 12 A/D converters, can meet the data acquisition precision requirement of this system and can reduce cost, simplify the circuit.
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