Power Supply switched mode

Circuit Description of Variable Switching Power Supply The input AC is stepped down using a step-down transformer T1 to 18V AC which is further rectified by a bridge rectifier. The rectified output is filtered by 1000 capacitor C1. The Zener diode ZD1 is used to form the reference voltage and the output voltage is selected by potentiometer VR1. Transistor T1 forms an emitter follower, and the voltage at the emitter of T1 is equal to the voltage at the varying point of VR1. Transistor T2 is used to sense the output voltage. Timer IC (IC1) is here configured as a monostable multivibrator (timer circuit) for the period of 0.2ms. The collector of PNP transistor T3 is connected to pin 2 of IC1 and is used to trigger. When the circuit is switched on, the voltage at T1 is equal to the voltage at the center point of VR1. If the voltage at the center point of VR1 is 8 volt, the emitter of T2 is also at 8 Volt and the base is at zero. When transistor T2 is in an off state no voltage is developed across R4 and transistor T3 continues in the off state. When transistor T2 conducts the voltage at the collector is developed and passes through resistor R4. This turns transistor T3 on and voltage at pin 2 of IC1 becomes more than Vcc/2. This led output at pin 3 of IC1 goes high and transistor T5 starts conducting and goes into saturation, while capacitor C3 gets charged when the voltage across it reaches 5.6V. After 0.2 ms the output of IC1 goes low and the capacitor C3 discharges through the load. When the voltage at pin 3 of IC1 decrease from 5.6V, transistor T2 goes off which further turn off transistor T3. This produces a negative trigger at pin 2 of IC1. Its output goes high and further turns on the transistor T4. This process is repeated continuously. The power loss is very low because transistor T4 only operates in saturation and cut-off mode. When output load is increased, the capacitor C3 gets charged due to leakage current. To avoid this problem resistor R10 is used. To increase the current capacity of the circuit use a high power transistor instead of SK100 (T4) or add one SK100 to its parallel. Here IC1 is used as a comparator and also provides some delay to output change. If the output ripple is high, decrease the value of C2 or omit it altogether.