Low voltage and high current switching power supply based on UC3825

1.  Design of switching power supply

The basic structure of the switching power supply mainly consists of 7 parts: input rectification and filtering circuit, high-frequency switching converter circuit, rectification output circuit, control circuit, protection circuit, auxiliary power supply and display circuit.

1.1 Main Circuit

The main circuit topology of this design is shown in Figure 1. The input voltage is 220 V from the city grid. The AC/DC/AC/DC conversion process is achieved through RC filtering, bridge rectification, full-bridge inverter, high-frequency transformer, and output rectification, and the required 15 V DC regulated power supply is finally obtained.

1.1.1 Input filter and rectification (AC/DC)

The low-voltage, high-current switching power supply is very sensitive to high-frequency interference signals and surge current at the moment of power-on. In order to ensure the stable operation of the circuit and eliminate various interferences from the power grid, the input 220 V mains power first passes through the RC filter circuit to suppress the peak voltage. The voltage after high-frequency filtering is rectified by the rectifier circuit to obtain a DC voltage. The filter capacitor behind the bridge rectifier circuit has a charging and discharging function to filter out the AC component after rectification.

1.1.2 High-frequency switching converter (DC/AC)

It is an important part of the switching power supply. The inverter circuit adopts full-bridge conversion. Four IGBT switch tubes form the four arms of the bridge. Each IGBT is connected in parallel with a high-speed power diode. Its clamping effect is to reduce the voltage spike generated by the transformer when the switch tube is converted from on to off, so as to protect the switch tube from breakdown. IGBT, insulated gate bipolar transistor, has the advantages of high input impedance, fast speed, good thermal stability and simple drive circuit, as well as low on-state voltage and high withstand voltage. The gate of the switch tube IGBT receives the PWM signal. When a positive voltage is applied to the gate, a channel is formed in the MOSFET and the base current is provided to the PNP transistor, thereby turning on the IGBT. At this time, the conductivity is modulated by injecting from the P region to the N region, reducing the resistance value of N-, so that the high-voltage IGBT also has a low on-state voltage drop. When a negative voltage is applied to the gate, the channel in the MOSFET disappears. The base current of the PNP transistor is cut off and the IGBT is turned off. T1, T4 and T2, T3 are switched on and off at high frequency in turn, converting the DC voltage into AC voltage, and then into the required isolated output AC voltage through the high-frequency isolation transformer. The high-frequency transformer here adopts an iron-based niobium-copper nanocrystalline toroidal core high-frequency transformer, which has the characteristics of low loss, low leakage inductance and small size.

1.1.3 Output Rectification and Filtering (AC/DC)

The inverter voltage output by the high-frequency isolation transformer is rectified by the rectifier circuit composed of a high-power high-frequency rectifier diode SBD (i.e. Schottky diode), and then filtered by the LC filter circuit to output a DC voltage. The shunt at the output end samples the output and transmits it to the control circuit for control and regulation to output a stable voltage.

1.2 Control Circuit

The control circuit is an important guarantee for the stable operation of the switching power supply. UC3825 is selected as the control chip, which consists of an oscillator, PWM comparator, PWM latch, output driver, current limiting comparator, overcurrent comparator, reference voltage source, fault latch, soft start circuit, undervoltage lockout, etc.

The maximum switching frequency of UC3825 can reach 1MHz, and the maximum transmission delay time of the output pulse is 50ns. It has functions such as soft start control and undervoltage lockout. The control circuit of the switching power supply is designed by applying the functions of UC3825 . The two pulse output terminals of UC3825 provide PWM drive signals for the switch tube IGBT and output pulses alternately. Therefore, the frequency of the output pulse of each output terminal is 1/2 of the oscillator frequency. The frequency of the oscillator is 200 kHz, so the frequency of the output PWM pulse is 100 kHz. The duty cycle of the output pulse is adjusted within 0% ~ 50%. In order to avoid the bridge arm short circuit, the dead time is often set, so it is generally not 50% in practical applications. The waveforms of 4, 6, 11, and 14 pins of UC3825 are shown in Figure 2. The waveforms of 11 and 14 pins are coupled to the gate of the IGBT through the output of the drive circuit to control the opening and closing of the IGBT. The drive circuit of the switch tube is shown in Figure 3.

In practical applications, the IGBT drive circuit has a considerable impact on the operation of the IGBT. Therefore, the drive circuit must have several requirements: it can provide appropriate forward and reverse voltages to ensure reliable opening and closing of the tube; it has strong dynamic drive capability; the signal transmission is basically delay-free, which improves the working speed; when an abnormal situation occurs, the IGBT can achieve soft shutdown and protect the tube from overvoltage and overcurrent. The drive circuit used can provide + 15 V and - 15 V forward and reverse voltages, the opening and closing delays are controlled within 1us, and it has overvoltage and short-circuit protection. In addition, it has a fault soft shutdown function.

In order to obtain a stable output, real-time sampling is performed at the output end. As shown in Figure 4, the sampling current flowing through the sampling resistor will generate a voltage drop, which is used as a feedback signal and input into the voltage comparator to compare with the given reference voltage to generate a difference. After comparison and amplification by the error amplifier, the output difference signal is compared with the sawtooth wave (or triangle wave) to change the width of the output pulse. When the output is greater than the reference voltage, the pulse width duty cycle is reduced, otherwise, the duty cycle is increased to achieve a stable output.

1.3 Protection Circuit

Considering the characteristics of the switching power supply and the actual electrical characteristics, in order to make it work safely and reliably in harsh environments and sudden faults, the characteristics of UC3825 are fully utilized in the actual production, and protection circuits such as voltage and current protection circuits and soft start protection are designed. Soft start is achieved through the external capacitor of the soft start (SOFT, START) pin. After the power is turned on, the external capacitor of the soft start pin is discharged, the pin is at a low level, the error amplifier outputs a low level, and the switching power supply has no output voltage.

When the internal current source charges the capacitor connected to the soft start pin, the error amplifier output voltage gradually increases until the closed-loop regulation function starts to work, and the output voltage of the switching power supply gradually increases to the rated value. Once the level of the current limit (ILIM) pin exceeds 1.2 V, the fault latch is set and the output pin becomes low; at the same time, the capacitor connected to the soft start pin is discharged with a current of 250uA. After the soft start capacitor is discharged, when the current limit pin level drops below 1.2 V, the fault latch will not output pulses. At this time, the fault latch is reset and the chip starts the soft start process.

Overcurrent protection and overvoltage protection are sampled in the main circuit and input into the protection circuit to protect the power supply. The protection circuit is shown in Figure 5 and Figure 6.

A current detection transformer is added between the switch conversion circuit and the high-frequency transformer, and the detection value is input to the 9-pin current limiting terminal of UC3825. When it is detected that the switch current reaches the upper limit current, the upper limit current comparator outputs a high level. The output of the comparator is the S terminal of the upper limit current trigger. When S is a high level, the trigger output is high, that is, the base of the NPN tube connected to the trigger output becomes high, the NPN tube is turned on, and FB is pulled low, thereby protecting the circuit from overcurrent problems.

The voltage is sampled directly and in real time at the output end of the main circuit. After comparison with the given voltage, the signal is amplified and input to pin 2 of UC3825 after isolation to control the duty cycle of the PWM signal, thereby controlling the change of the output voltage of the main circuit.

Overheat protection is implemented by checking the on and off of the thermal relay in the system. When the switching power supply is working normally, the thermal relay is in the normally on state. Once the temperature overheats and exceeds the rated value, the thermal relay switch will be disconnected, and the on and off signal will be fed back to the control circuit to achieve the effect of overheat protection.

1.4 Auxiliary power supply

The auxiliary power supply is used to supply power to the control circuit, and is divided into two parts: one is the power supply for UC3825 and other control parts, and the auxiliary power supply for the control part. The other is the power supply for the voltage feedback link. This design connects the 220 V voltage of the power grid to the power frequency transformer, and provides 5 V, 12 V, 18 V and other working voltages for the chip UC3825, phase loss protection, undervoltage, overvoltage protection and other circuits.

1.5 Display Circuit

On the control panel of the switching power supply, there are ammeters and voltmeters to display the output current and voltage values. The auxiliary power supply is provided to the ammeter, and the voltmeter provides 5 V voltage to drive the digital tube. The display panel is also equipped with overheating, phase loss, abnormality indicator lights, and constant current and constant voltage conversion switches. As the external display part, the real-time working status of the system can be clearly observed to ensure the safe and stable operation of the switching power supply.

2 Experimental Tests and Results

In the test experiment, a variable load car is used as the debugging load. The number of parallel resistors is changed as needed. For the control of output voltage and current, the variable sampling resistor on the control board can be changed, and then the UC3825 adjusts the duty cycle of the PWM signal to achieve the expected output value. After debugging, the machine is also subjected to an aging test. The test results of the prototype show that the pulsating voltage peak of the power supply output is small, the output current is smooth and stable, the fluctuation amplitude is small, and it has the advantages of high efficiency, energy saving, and small volume and weight.

3 Conclusion

A 15 V / 1 200 A low voltage, high current, high power switching power supply was developed, and the pulse width modulation chip UC3825 was used as the control core to control the entire system. At the same time, the soft start circuit, undervoltage, overvoltage and overcurrent protection circuits were designed to increase the reliability and stability of the switching power supply. At present, the system has completed experimental debugging, and the main circuit can work safely and stably with load. This design provides a strong theoretical and practical guarantee for the application of high-frequency switching voltage in the electroplating industry.