How UC3843 and uc3842 work

The probe for domestic switch needle KG-300K has a needle head diameter of 3.0mm and is designed as a normally open switch needle.

Photocoupler

UC3842A UC3843A Information

The UC3842A and UC3843A are high-performance fixed-frequency current-mode controllers designed for offline and DC-to-DC converter applications, offering designers a cost-effective solution with minimal external components. These integrated circuits feature a trimmable oscillator, precise duty cycle control, temperature-compensated reference, and a high-gain error amplifier. The current-sampling comparator and the high-current totem-pole output make them ideal for driving power MOSFETs. Other protection features include input and reference undervoltage lockout with hysteresis, cycle-by-cycle current limit, programmable output dead time, and single-pulse measurement latch. These devices are available in 8-pin dual-in-line plastic packages and 14-pin plastic surface-mount packages (SO-14). The SO-14 package's totem-pole output stage includes separate power and ground pins. The UC3842A has low-voltage lockout thresholds of 16V (on) and 10V (off), making it suitable for offline converters. The UC3843A is designed for low-voltage applications with lockout thresholds of 8.5V (on) and 7.6V (off).

Features:

Precise duty cycle control via adjustable oscillator discharge current.
Current mode operates up to 500kHz.
Automatic feedforward compensation.
Latched pulse-width modulation.
Internally trimmed reference voltage with undervoltage lockout.
High-current totem-pole output.
Undervoltage lockout with hysteresis.
Low startup and operating current.
Direct interface with ON Semiconductor’s SENSEFET products.

Figure 1

Figure 2 - Pin Diagram

The following figure shows the UC3842 application circuit diagram for a display.

Image 3

UC3842 Good and Bad Judgment and Identification Method Among the domestic electronic equipment, the most commonly used integrated circuit model for the power supply PWM control circuit is UC3842 (or KA3842). It is frequently encountered, so here’s a brief introduction to the judgment method of UC3842: After replacing the damaged components, do not install the switch (MOSFET), power up to measure the voltage of pin 7 of UC3842. If the voltage fluctuates between 10-17V, the other pins also have fluctuating voltages, indicating that the circuit has started, and UC3842 is basically normal; if the voltage of pin 7 is low, and the other pins have no voltage or do not fluctuate, then UC3842 is damaged. Adding a DC voltage of about +17V between pins 7 and 5 of UC3842. If pin 8 has a voltage of +5V and pins 1, 2, 4, and 6 also have different voltages, the UC3842 is basically normal, the working current is small, and it is not easy to damage itself. The most common cause of damage is when the power switch (MOSFET) shorts out, causing a high voltage to be applied from the G pole to its 6 pin, burning it. In some models, the G-grounded protection diode is omitted. When the power switch (MOSFET) is damaged, the current-limiting resistors of the UC3842 and G poles must be damaged. Simply replace them at this time. It should be noted that the source (S pole) of the power switch is usually connected to a small resistance, high-power resistor as an overcurrent protection detection resistor. The resistance of this resistor is generally between 0.2 and 0.6. Above this value, there is a phenomenon that the load cannot be loaded (that is, the voltage of the secondary pole is low). Since the working voltage and output power of UC3842 (KA3842) are far from UC3843 (KA3843), the 3842 series and 3843 series also have a big difference in starting voltage and closing voltage. The former has a starting voltage of 16V, and the voltage is turned off at 10V; the latter has a starting voltage of 8.5V and a shutdown voltage of 7.6V. These two series of ICs cannot be replaced directly. If it is necessary to replace the former with the latter, the circuit must be modified. Therefore, this must be paid attention to in the maintenance work.