Detailed explanation of the principle and application circuit diagram of UC3842

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1 Introduction to UC3842 internal working principle

Figure 1 shows the internal block diagram and pin diagram of the UC3842. The UC3842 uses a fixed-frequency pulse width controllable modulation method. There are 8 pins, and the functions of each pin are as follows:

Pin 1 is the output of the error amplifier, and the external resistor-capacitor is used to improve the gain and frequency characteristics of the error amplifier;
The 2 pin is the feedback voltage input terminal, and the voltage of the pin is compared with the 2.5V reference voltage of the non-inverting terminal of the error amplifier to generate an error voltage, thereby controlling the pulse width;
Pin 3 is the current detection input terminal. When the detection voltage exceeds 1V, the pulse width is reduced to make the power supply in intermittent operation.
The 4th pin is the timing terminal, and the operating frequency of the internal oscillator is determined by the external resistance time constant, f=1.8/(RT×CT);
5 feet are public ground;
The 6-pin is a push-pull output terminal, and the inside is a totem pole type. The rising and falling time is only 50 ns and the driving capability is ±1 A;
7 feet is the DC power supply terminal, with under- and over-voltage lockout function, the chip power consumption is 15mW;
Pin 8 is a 5V reference output with 50mA load capability.

Figure 1 UC3842 internal block diagram

2 UC3842 switching power supply circuit

Figure 2 is a switching power supply circuit composed of UC3842. The 220V mains is filtered by C1 and L1 to remove electromagnetic interference, and the negative temperature coefficient thermistor Rt1 is current limited. After VC rectification and C2 filtering, the resistor R1 and the potentiometer RP1 are stepped down. It is added to the power supply terminal (7-pin) of UC3842 to provide the startup voltage for UC3842. After the circuit is started, the rectification and filtering voltage of the winding 34 of the transformer provides the normal working voltage for UC3842, and the voltage divider for R3 and R4. The inverting input terminal of the error amplifier provides a negative feedback voltage for the UC3842. The rule is that the higher the voltage of the pin, the smaller the duty cycle of the driving pulse, thereby stabilizing the output voltage. The R6 and C8 externally connected to the 4th and 8th pins determine the oscillation frequency, and the maximum oscillation frequency can reach 500KHz. R5 and C6 are used to improve gain and frequency characteristics. The square wave signal outputted by the 6-pin is driven by the R7 and R8 voltages to drive the MOSFEF power tube. The energy of the primary winding 12 of the transformer is transmitted to the windings of the input side. After rectification and filtering, the DC voltages of different values ​​are output for the load. Resistor R10 is used for current detection. It is filtered by R9 and C9 and sent to the 3rd pin of UC3842 to form a current feedback loop. Therefore, the power supply composed of UC3842 is a double closed-loop control system, and the voltage stability is very high. When the voltage of the UC3842 is high, the 3-pin voltage is high. At 1V, the oscillator stops vibrating, protecting the power tube from overcurrent and damage.

When the circuit is powered up, the external startup circuit provides the required startup voltage of the chip through pin 7. Under the action of starting the power supply, the chip starts to work, and the pulse signal generated by the pulse width modulation circuit is driven by the 6-pin output to drive the external switching power tube. The signal generated by the power tube operation is converted into a low-voltage DC signal by the sampling circuit and fed back to the 3-pin to maintain the normal operation of the system. After the circuit works normally, the low-voltage DC signal fed back by the sampling circuit is sent to the internal error comparison amplifier via pin 2, compared with the internal reference voltage, and the generated error signal is sent to the pulse width modulation circuit to complete the pulse width modulation. The purpose of stabilizing the output voltage. If the output voltage becomes high for some reason, the sampling voltage of the 2 pin also becomes high, and the pulse width modulation circuit narrows the width of the output pulse, so that the on-time of the switching power tube becomes shorter, and the output voltage becomes lower, thereby Make the output voltage stable and vice versa. The sawtooth oscillator circuit produces periodic sawtooth waves whose period depends on the external RC network at pin 4. The generated sawtooth wave is sent to the pulse width modulator as its duty cycle, the pulse period of the pulse width modulator output is constant, and the pulse width varies with the magnitude of the feedback voltage.

3 circuit debugging

The debugging of this circuit needs to pay attention to: First, adjust the potentiometer RP1 to make the circuit start, the starting current is about 1mA; Second, the DC voltage provided by the winding of the transformer 34 after starting the vibration should make the circuit work normally, the range of this voltage is about Between 11 and 17V; the third is to change the R4 according to the value of the output voltage to determine the amount of feedback; the fourth is to determine the size of the sense resistor R10 according to the protection requirements, usually R10 is 2W, 1Ω or less.

For more information on UC3842, please refer to the PDF document!