The job of switching power supply, who will, point about UC3842

UC3842 adopts a fixed operating frequency pulse width controllable modulation mode, with a total of 8 pins, each pin has the following functions: The pin is the output terminal of the error amplifier, and the external resistor-capacitance element is used to improve the gain and frequency characteristics of the error amplifier; The pin is the input terminal of the feedback voltage, and the voltage of this pin is compared with the reference voltage of the inverting terminal of the error amplifier to produce the error voltage, thereby controlling the pulse width; The pin is the current detection input terminal, when the detection voltage exceeds 1V, the pulse width is reduced to make the power supply in an intermittent working state; The pin is the timing terminal, and the operating frequency of the internal oscillator is determined by the external resistance time constant, f=; The feet are public ends; The foot is the push-pull output terminal, and the inside is a totem pole type, the rise and fall time is only 50ns, and the driving capacity is 1A; The pin is the power supply end of the DC power supply, with undervoltage and overvoltage locking functions, and the power consumption of the chip is 15MW; The pin is a 5V reference output with a load capacity of 50mA.

C1 is a high-voltage filter capacitor, but the withstand voltage should be greater than 300V, and the capacity should be larger. 7. Angular voltage divided by r3r4There is a current value, and then r2

UC3842 is a PWM control chip that is commonly used in switching power supply control circuits. Here's how it works:

The controller receives feedback signals: In the switching power supply, a feedback loop is usually formed through components such as inductors and capacitors, and the output voltage or current information is fed back to the controller. The UC3842 receives a feedback signal through an internal comparator and converts it into a digital signal.

Generate PWM signal: The UC3842 compares the feedback signal to an internal reference voltage to produce a PWM (Pulse Width Modulation) signal. The duty cycle of the PWM signal indicates the magnitude of the output voltage, and the controller adjusts the duty cycle of the PWM signal according to the difference between the feedback signal and the reference voltage, thereby adjusting the output voltage.

Drive switch tube: The PWM signal is reversed through the inverter and output to the drive circuit of the switch. The on-off and cut-off of the switch control the on/off of the output circuit, so as to adjust the output voltage and current.

The UC3842 also includes some protection functions, such as short-circuit protection, overload protection, etc., which can effectively protect the safety of switching power supplies and loads.

Stable output voltage: The on-time of the switch is adjusted through the PWM signal, the output voltage is controlled, and the output DC attitude first voltage becomes more stable after being processed by the output filter circuit.

In general, UC3842 realizes stable control of the output voltage and open rot current of the switching power supply through feedback control and PWM regulation, which makes the switching power supply have the characteristics of high efficiency, low cost, small size and high reliability.

The UC3842 is a commonly used switching power controller IC that is commonly used in applications such as DC-DC converters and AC DC converters.

The working principle of UC3842 switching power supply is as follows:

1.The input voltage is obtained by the rectifier and filter hand source circuit, and is used as the input voltage for the PWM control circuit.

2.The PWM signal generated by the UC3842 chip controls the switching of the MOSFET switch on and off, enabling output voltage regulation of the DC-DC or AC-DC converter.

3.The controller adjusts the duty cycle of the PWM signal according to the difference between the output voltage and the set voltage, so that the output voltage is stable near the set value.

4.When the controller detects a fault or overload, it will control the PWM signal through the feedback circuit to reduce the output voltage and protect the electric rock leather circuit.

The working principle of UC3842 switching power supply can be simply summarized as follows: adjust the on-off cycle of the MOSFET switch through PWM to realize the output voltage regulation of DC-DC or AC DC converter, and achieve the purpose of stable output from coarse difference.

1. The 5th foot of UC3842 is the ground.

2. First test the 5th pin and the 8th pin, the 8th pin outputs a reference voltage of 5V, and the test of this pin can quickly judge the quality of UC3842.

3. If the voltage of the 8th pin is normal, there is no problem with UC3842 under normal circumstances, and there is no need to test the others.

4. If the voltage of the 8th pin is not there, measure the 7th pin to see if the power supply voltage is correct. However, the voltage value of the 7 pins depends on the specific circuit design.

The pin is the output of the error amplifier. The pin is the input terminal of the feedback voltage, and the voltage of this pin is compared with the reference voltage of the inverting terminal of the error amplifier to produce the error voltage, thereby controlling the pulse width; The pin is the current detection input terminal, when the detection voltage exceeds 1V, the pulse width is reduced to make the power supply in an intermittent working state; The pin is the timing terminal, and the operating frequency of the internal oscillator is determined by the external resistance time constant, f=; The feet are public ends; The foot is the push-pull output terminal, and the inside is a totem pole type, the rise and fall time is only 50ns, and the driving capacity is 1A; The pin is the power supply end of the DC power supply, with undervoltage and overvoltage locking functions, and the power consumption of the chip is 15MW; The pin is a 5V reference output with a load capacity of 50mA.

1. When repairing the switching power supply, first use a multimeter to detect whether the power components are broken down and short-circuited, such as power supply rectifier bridge stack, switch tube, high-frequency and high-power rectifier tube; Whether the high-power resistor that suppresses the inrush current is blown out. Then check whether the resistance of each output voltage port is abnormal, and if the above parts are damaged, they need to be replaced.

2. After the first step is completed, it cannot work normally after the power is turned on, and then the power factor module (PFC) and pulse width modulation component (PWM) should be detected, and the relevant information should be consulted, and the functions of each foot of the PFC and PWM modules and the necessary conditions for the normal operation of the modules should be familiarized.

3. Then, for the power supply with PFC circuit, it is necessary to measure whether the voltage at both ends of the filter capacitor is about 380VDC, if there is a voltage of about 380VDC, it means that the PFC module is working normally, and then detect the working state of the PWM component, measure the power input terminal VC, the reference voltage output terminal VR, start control VSTART VTont VThe terminal voltage is normal, and use the 220VAC 220VAC isolation transformer to supply power to the switching power supply. Use an oscilloscope to observe whether the waveform of the CT end of the PWM module to the ground is a sawtooth wave or triangle with good linearity.

4. In the maintenance practice of switching power supply, there are many switching power supplies using UC38 series 8-pin PWM components, and most of the power supplies cannot work because the power supply starting resistance is damaged, or the chip performance is reduced.

5. When there is no VC after the R circuit is broken, the PWM module cannot work, and the resistor with the same power resistance value needs to be replaced. When the startup current of the PWM module increases, the R value can be reduced until the PWM module can work normally. Sometimes due to the failure of the peripheral circuit, the 5V voltage of the VR terminal is 0V, and the PWM component does not work, when repairing the power supply of the Kodak 8900 camera, when encountering this situation, the external circuit connected to the VR terminal is disconnected, and the VR changes from 0V to 5V, the PWM component works normally, and the output voltage is normal.

6. When there is no voltage of about 380VDC on the filter capacitor, it means that the PFC circuit is not working normally, and the key detection pins of the PFC module are the power input pins VC, the start pins Vstart Control, CT and RT pins and V0 pins. The V0 terminal is welded to the solder spot on the board, and the filter capacitor is measured with a multimeter with a voltage of 380VDC. When the VSTART control terminal is low and the PFC does not work, it is necessary to check the relevant circuit connected to the periphery at its endpoint.

You need to point to UC3844 switching power supply with a voltage of 7 pins and 13V and eight pins, but there is no voltage, and the chips have been replaced several times.

Since the chip has been replaced and it is still broken, it is definitely not a problem with the spike chip. First of all: 1. See if the 8 pins have a short circuit to the ground on the PCB board The pin voltage is 13V, whether it has not reached the rated working point, increase the high voltage of the pin to 15V, try 3, check whether there is any device damage in the loop.

4. Check whether the winding that supplies power to the 7 pins in the switching power supply is damaged. The absence of the vref means that the loop is open. We also have to see if there is a problem with the reckless circuit.

Whether there is an output from other channels of the switching power supply.

384x series, there are many mature circuit diagrams on the Internet, go to the power supply related forums to find them, there are so many.

Detailed explanation of the circuit UC3842 circuit that electronics beginners must understand.