UC3842 model chip detailed explanation, UC3842 schematic, information

Pin 1 COMP is the output of the internal error amplifier. Normally, a feedback network is formed between this pin and pin 2 FB, with an external resistive-capacitive element to determine the gain and frequency response of the error amplifier.

Pin 2 FEED BACK is the feedback voltage input. This pin is compared with the reference voltage (usually +2.5V) at the same input of the internal error amplifier to generate a control voltage to control the width of the pulse.

Pin 3 ISENSE is the current sampling terminal. When the detection voltage exceeds 1V narrow the pulse width to make the power supply in intermittent operation.

As shown in the figure below, in the peripheral circuit, a small resistance sampling resistor is connected in series with the source of MOS tube Q1 to convert the pulse current into a voltage, and this voltage is fed into pin 3 to control the pulse width. When the load current exceeds the rated value or the output is short-circuited, causing an increase in switching tube Q1 current, the voltage on R2 is fed back to the 3-pin, and when the voltage on R2 is greater than 1V, the conduction width is narrowed by the internal current amplifier, and the output voltage decreases, and at the same time, the UC3842 operating voltage decreases. When falling below a certain voltage, UC3842 stop output, overcurrent protection circuit work to protect the power tube. After the short-circuit phenomenon disappears, the power supply automatically returns to normal operation.

Pin 4 RT/CT is the timing terminal. The operating frequency of the internal oscillator is determined by the external resistor-capacitor time constant, f=1.8/(RT×CT).

Pin 5 GND is the ground.

Pin 6 OUT is the output, this pin is a totem pole type output with a rise and fall time of only 50ns and a drive capability of ±1A.

Pin 7 Vcc is the DC power supply. With under- and over-voltage lockout function, the chip power consumption of 15mw. When the supply voltage is lower than +16V, UC3842 does not work, at this time, the power consumption is below 1mA. The input voltage can be obtained from the high-voltage step-down through a large resistance resistor. After the chip works, the input voltage can fluctuate between +10 and +30V, and stop working below +10V. The power consumption during operation is about 15mA, and this current can be provided by a feedback resistor.

Pin 8 VREF is the reference voltage output, can output accurate +5V reference voltage, with 50mA load capacity.

UC3842 voltage regulation rate of up to 0.01%, the maximum operating frequency of 500kHz, start-up current is less than 1mA, the input voltage of 10 ~ 30V, the reference voltage of 4.9 ~ 5.1V, the operating temperature of 0 ~ 70 ℃, the output current is 1A.
 

The reference application circuits are as follows
Circuit 1, UC3842 working process

The reference application circuits are as follows

Circuit 1, UC3842 working process

1. The circuit consists of UC3842 switching power supply circuit, 220V utility by C1, L1 filter electromagnetic interference, rectified by D1, C2 filter, negative temperature coefficient of the thermistor RT1 current limitation;



2. rectified by the bridge stack, through the resistor R1, R2 step-down added to the UC3842 7-pin power supply, for the UC3842 to provide the start-up voltage;



3. After the start of the circuit transformer secondary winding ② rectifier filter voltage on the one hand, to provide the chip with normal operating voltage, on the other hand, by R4, R5 voltage divider to the error amplifier 2 feet inverted input, to provide negative feedback voltage for the chip, the rule is that the higher the voltage of the foot drive pulse duty cycle is smaller, in order to stabilize the output voltage;



4.4 feet and 8 feet external R7, C8 determines the oscillation frequency, the maximum value of its oscillation frequency up to 500KHz;



5.1 and 2 feet of the external R6, C6 is used to improve the gain and frequency characteristics. R5 is changed according to the numerical magnitude of the output voltage to determine the amount of its feedback;



6.6 pin output square wave signal through R8 to drive the MOS tube;



7. The energy of the transformer primary winding ① is transferred to the secondary winding, and after rectification and filtering, different values of DC voltage are output for the load;



8.3 pin is the current sampling pin. Resistor R10 is used for current detection, according to the protection requirements to determine the size of the detection resistor R10, usually R10 is 2W, 1Ω or less. When the power supply overload, 3842 protection action, so that the duty cycle is reduced, the output voltage is reduced, the 3842 supply voltage is also reduced, when low to 3842 can not work, the entire circuit is closed, and then rely on R1, R2 to start the next start-up process. R9, C9 filtered into the UC3842 3 feet to form a current feedback loop, so the power supply composed of UC3842 is a double closed-loop control system, the voltage stability is very high.

Attention.

In this protection state, the power supply only works for a few switching cycles, and then enters into a very long time (several hundred ms to several s) startup process, the average power is very low, and even if the output is short-circuited for a long period of time, it will not lead to the damage of the power supply. Due to leakage inductance and other reasons, some switching power supply in each switching cycle has a large switching spike, even in the duty cycle is very small, the auxiliary voltage can not be reduced to a low enough, so generally in the auxiliary power supply rectifier diode strung on a resistor and capacitor, used to absorb the turn-on moment of the spike voltage. Using this circuit, care must be taken to select a relatively low auxiliary voltage, for 3842 is generally 13 to 15V, so that the circuit is easy to protect.
 

1. The circuit realizes the isolation of output and input, weak and strong power because of the use of photocoupler, which reduces electromagnetic interference and has a strong anti-interference capability, and it is sampling the output voltage, which has a very good voltage stabilization performance.



2. The circuit changes the output of the error amplifier by adjusting the gain of the error amplifier instead of adjusting the input error of the error amplifier, thus changing the duty cycle of the switching signal. This topology not only has fewer external components, but also employs a three-terminal adjustable voltage regulator in the voltage sampling circuit so that the output voltage remains essentially unchanged when the load undergoes a large change.
 

JoinWin is a professional electronic components distributor and partners with original manufacturers, provides genuine integrated circuits and connectors, supply hard-to-find and discontinued parts. To get accurate data for millions of electronic parts.Enjoy affordable prices for quality products.