Application of Integrated PWM controller MB3759 in Switching Power Supply

Application of Integrated PWM controller MB3759 in Switching Power Supply

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Source: Power Technology Author: Ma Xinxia song mingzhong Qian cunjian Cai fangwei

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O Introduction
As an energy-efficient power supply, switching power supply represents the development direction of regulated power supply. Switched Mode Power Supply (SMPS) is an electric conversion device consisting of a switch circuit controlled by the duty cycle. The core of the switching power supply is the power electronic switch circuit. According to the requirements of the load on the output voltage or steady flow characteristics of the power supply, the feedback control circuit is used to control the duty cycle, control the switch circuit. With the continuous development of control technology and component technology, the performance of all aspects of the switching power supply is constantly improving, and the capacity is also expanding.
The control and protection circuit mainly processes signals and is a "weak current" circuit, but it controls the switching devices in the main circuit. If a mistake occurs, it will cause serious consequences and cause the power supply to stop working or damage. Many indicators of the power supply, such as voltage regulation and Steady Flow precision, ripple, output characteristics, and so on, are also related to the control circuit. Therefore, the design quality of the control circuit is crucial to the power supply performance. This paper introduces the composition and principle of the Integrated pulse width modulation (PWM) controller (MB3759), and analyzes in detail the control circuit, driving circuit and Protection Circuit of the switching power supply consisting of MB3759 controller, it has some value for the design of the switch power supply.

1. Composition and principle of integrated PWM controller (mb3759)
Composition of 1.1 PWM controller (mb3759)
The function of the PWM control circuit is to convert a continuously changing analog signal to a PWM signal with a fixed switching frequency and a duty cycle following the continuous change of the input signal. Switching Power Supply PWM control integrated chip uses the model MB3759 (FUJITSU), uses a fixed frequency PWM control mode, its MB3759 chip internal structure diagram l, the internal circuit consists of a high-frequency oscillator, a PWM comparator, a reference voltage source, an error voltage amplifier, a driving circuit, and a blocking circuit. The control chip has two voltage comparator. pins 1, 2, 15, and 16 are positive and negative input terminals of the voltage comparator, and Pin 3 is the unified output end of the voltage comparator. At the same time, the output of the error amplifier can also be opened to users. Users can design PI controllers as needed. The pins 5 and 6 can be connected to the oscillating capacitor and resistor. The vibration frequency of the oscillator is determined by the external resistor and capacitor, And the capacitance and resistance value are adjusted according to the circuit frequency. Pin 8 is the trigger pulse output port and uses the current totem output so that the chip can directly drive the switch with low power. T trigger is used to divide the output and obtain a square wave of l/2 with a duty cycle of 50% and a frequency of oscillator frequency, after the two complementary square waves output by the T trigger are used as the comparator to output the PWM signal and perform the "or non" operation, the duty cycle of the two complementary methods is O ~ 50% PWM signal, considering the existence of dead time, the maximum duty cycle is usually 45% ~ 47.5%. PIN 13 is the block control, pin 14 is the reference voltage, Pin 12 is the operating voltage, and pin 4 is the dead zone control end. Once a high level input, the chip output pulse is blocked, and the DC voltage output is zero.

Working principle of 1.2 PWM controller (mb3759)
The feedback channel of the PWM controller (MB3759) consists of the voltage error amplifier EA, the PWM comparator, the latch and the driving circuit. Pin 1 serves as the feedback signal of the DC output voltage. Pin 2 is connected with the reference voltage of the chip output, serves as the reference input of the error amplifier, and Pin 3 inputs the voltage feedback of the main circuit.
Triggered by a clock pulse, when the power tube is activated and the inductance (power tube) current rises to the threshold value determined by the EA output, the PWM comparator is flipped, the latches are reset, the drive pulse is switched off, and the inductance current drops, when the next clock pulse arrives, the latch is set and the switch is re-activated. When the input voltage changes, the increase slope of the inductance current changes, and the output duty cycle changes to suppress the changes in the input voltage. This is a feed-forward adjustment process with extremely fast response; the load disturbance is adjusted by changing the current threshold value through the EA. The peripheral circuit 2 of the MB3759 chip is shown in.

2 Circuit Analysis
2.1 Drive Circuit
The driving circuit is the interface between the control circuit and the main circuit, which is closely related to the reliability and efficiency of the switching power supply. The driving circuit requires high speed, can provide certain driving power, and has high anti-interference and noise isolation capabilities.
The driving signal is applied between the gate-to-source (MOs) of the switch device. In the full-bridge circuit, the source-to-source potential difference of different switch devices is very large and changes at high speed. Therefore, the drive circuit must be isolated. The switch power supply adopts a transformer isolation driving circuit. 3 shows that the circuit has simple structure, low cost, high isolation voltage, low transmission delay, and no additional power supply.

2.2 Protection Circuit
In order to make the single-chip switching power supply work stably, safely and reliably for a long time, its control circuit should contain a protection circuit to avoid damage to the switching power supply due to circuit faults, improper use or changes in environmental conditions. The switching power supply protection circuit mainly includes output overvoltage, output undervoltage and overcurrent protection.
The internal control voltage of the switch power supply is the auxiliary switch power supply composed of S2 and T3 components. The T3 secondary side has two sets of coils, A group outputs control voltage (VCC) Through rectification and filtering of components such as D16 and C30, and a group uses rectification and filtering of components such as D17 and C32 as the under voltage detection signal.
The output Overvoltage of the switch power supply and the composition of the output undervoltage protection circuit are shown in Figure 4. Working Principle of the switch power supply protection circuit: The undervoltage protection circuit consists of components such as 17902/2 and 17902/4, the overvoltage protection circuit consists of S103, D14, and other components. It consists of 17902/l and 1790l/1 components to form an overcurrent protection circuit. Undervoltage, overvoltage, and overcurrent fault signals use components such as D103 to control the dead-time pin 4 of the PWM chip for protection. As a result, the chip stops working and cannot output the pulse normally. The voltage output is zero. The function of the Protection Circuit of the switch power supply is to immediately stop the switch circuit in the case of abnormal use of the output overvoltage or the output undervoltage.

3 power supply performance test
3.1 Voltage Adjustment Rate
When the input grid voltage changes from the rated value AC 110V to ± 10%, the 24 V fluctuation of the output voltage of the regulated power supply is 0.8%, which meets the requirements of Class C.
3.2 Load Adjustment Rate
When the output voltage is 24 V, the load current is at O ~ 3.9A fluctuation power output value, 24 V fluctuation is 3.3%.
3.3 Ripple Voltage
The peak value of the ripple (including noise) of the output voltage is 1.4% at the rated output voltage and load current,

4 Conclusion
At present, various new technologies in the field of switching power supply are attracting widespread attention, and various technologies are emerging. This switching power supply uses common integrated circuits and power modules with high reliability in industrial environments, and uses the feedback control circuit and the duty cycle control method to control the switching circuit. The technical features of the switch power supply make it small, light weight, strong anti-interference performance, stable output voltage, fast voltage dynamic response, high cost performance, easy to use, and other advantages.