Many electronics enthusiasts who are new to maintenance often find that these notebook adapters with switching power supplies seem more complex and harder to repair. In reality, the internal circuit designs of power adapters for various brands and models of notebook computers and LCD monitors are quite similar. For beginners, mastering a typical principle of such switching power supplies can make repairs much easier. In the following section, this article will use a typical adapter that employs the KA3842 control chip in a notebook and LCD power adapter as an example to explain its working principle and maintenance methods. Figure 1 shows the circuit schematic of the adapter's switching power supply. Figure 2 displays the pin functions of the Samsung power supply control chip KA3842 used in the power supply. The basic working principle of the circuit is to convert a 220V AC voltage into a 19V DC voltage output, with an output current of approximately 3A. The fundamental working process of the circuit is as follows: The 220V AC input voltage passes through the bridge rectifier circuit D2 (KBP206G), and is then filtered by C1 to produce a DC voltage of around 300V. This voltage is applied to the field-effect switch tube via the 1~2 winding of the switching transformer T1. The Ql (K2543) D pole receives the voltage, while the other path is stepped down by R4 to generate approximately 17V of starting voltage to power ICl (KA3842) pin 7, and a 5V reference voltage is produced from the pin 8 output of ICl’s internal reference generator. At this point, the internal oscillator begins to oscillate. A widening pulse (PWM) is output from the 6th pin, driving the switching transistor Q1 to operate in the switching state. The D-pole output current of Q1 generates an induced voltage on the Tl primary winding, which is coupled to the magnetic core. The induced voltage generated on the secondary 5-6 winding is rectified and filtered by Schottky diode Q2 and capacitor C4 to produce a 19V DC voltage output. To ensure the stability of the output voltage, the 19V output voltage is subjected to error sampling at the output terminal using R13 and R14. The sampled voltage is compared and error-amplified by the three-terminal adjustable shunt reference IC3 (TL431), which drives the optocoupler IC2 (pC817). The error voltage is amplified and sent to the first pin of ICl (KA3842), changing the width of the output pulse from pin 6 via the internal PWM circuit to adjust the switching time of Q1, thereby regulating the output voltage. Through such a feedback control process, the output voltage is ultimately stabilized at 19V. There are also several overvoltage and overcurrent protections in the circuit: 1. The induced voltage of the primary 3-4 winding of the switching transformer is rectified and filtered by D4 and C2, sending a voltage of about 17V to the 7th pin of Icl to maintain the normal operation of IC1 (the voltage of 300V is supplied to the 7 pin of R4 step-down supply because the current is small as the starting voltage). When the output voltage rises due to some reason, the circuit voltage will also rise. When the voltage exceeds 22V, the Zener diode D1 will reverse breakdown, causing the voltage at the overcurrent protection terminal of IC1 pin 3 to rise above 1V. At this point, the IC will be internally shut down, stopping the pulse output of pin 6, achieving the purpose of overvoltage protection. 2. If the current of the switch Q1 becomes too large for any reason, the voltage across the S1 pole of Q1 will increase. When the voltage rises significantly, the voltage at IC1 pin 3 will also rise. This will also cut off the output of pin 6 of IC1 to protect against overcurrent. 3. The R10, C8, and D6 connected to the D pole of the switch Q1 form a spike absorption loop, absorbing the spike-induced pulse generated on the 1-2 winding of T1 during the Q1 cutoff period to prevent Q1 from being broken down. Zgar International (M) SDN BHD , https://www.zgarvape.com
June 23, 2025