Block Diagram Of A Typical Switch Mode Power Supply
The power supply repair is one of the most challenging tasks for an electronic repairer and once you have mastered the circuit and the repair technique, to troubleshoot other type of circuits such as the colour, vertical, audio, high voltage and so forth would be much easier. If you are aware of how power supply or switch mode power supplies work, then you are ready to repair any kind of power supply problems in any type of equipment which include the smaller power supplies used in the notebook or the laptop (Both equipment use smaller size of electronic components but the working principle is the same). Basically, all of the power supply functions are almost the same which is to produce output voltages for various secondary circuits.
The working principle of switch mode power supply is different from the linear type. First the AC voltage enters the RF filter circuit where its function is to prevent the Power Supply unit from causing interference on the main wiring and to a full wave rectifier (bridge rectifier) which converts the AC to produce an uneven DC output and then filter by a large filter capacitor (usually 220 Micro Farad and up to 450 volts).
The clean DC voltage will then be given to start up resistors and to the input of switch mode power transformer. Once the voltage passed through the high ohms resistor (start up resistors) the voltage would drop to a value where it then goes to the VCC supply pin of Pulse width modulation (PWM) IC. The Run DC circuit that consists of a resistor and a diode will maintain the power IC stable operation.
Once the PWM IC received the voltage, it will produce a signal to drive the transistor (normally FET) and produces a change in the magnetic field in the transformer primary winding. The changing magnetic field induces voltage in the secondary windings.
Each of these AC voltage produced by the secondary windings is then rectified, filtered, and regulated to produce a clean DC voltage. One of the main DC output voltages is the B+ voltage. The output from the B+ voltage supply is then connected, through a sampling error detection circuit and “feedback” loop back to the PWM IC. When the voltage from the B+ supply rises or drop a bit, the PWM IC will act to correct the output.