Working principle of power frequency transformer and power supply
The working principle of a power frequency transformer is relatively simple. The power frequency AC voltage input from the primary coil is converted into a magnetic field, which is transmitted through a magnetic material (usually silicon steel sheet) to the secondary coil to induce voltage. The output frequency is the same as the input frequency, and the voltage is reduced according to the ratio of the first stage coil turns (if there are more secondary turns, it is a boost). Due to the fact that the transformer outputs alternating current, while most electrical circuits use direct current, the voltage output by the transformer needs to be rectified, filtered, stabilized, and other circuits to become a relatively smooth and stable voltage for the load circuit to work.
The core transformer component of the switching power supply is still a transformer, and it also follows the rule that the voltage ratio is equal to the turn ratio. Unlike power frequency transformers, switching power supplies need to increase their operating frequency, which means they need to convert low-frequency AC voltage into high-frequency AC voltage, which requires the implementation of additional control circuits. Because the operation of the circuit requires direct current, the input AC voltage needs to be rectified to become direct current voltage before it can be controlled through the subsequent circuit. Taking a commonly used mobile phone charger circuit as an example, let's briefly understand the working principle of a switching power supply.
After rectification and filtering, the input 220V AC voltage will become a DC voltage of around 310V (i.e. the peak of 220V AC voltage). Next, it is necessary to convert this DC voltage into high-frequency AC voltage. The simplest way to turn this voltage into high-frequency alternating current is to use a switch that quickly opens and closes, thus converting direct current into high-speed pulse direct current voltage. The component that implements this switch is a transistor. Transistors, including commonly used transistors and field-effect transistors, can be used as electronic switches, that is, by controlling the voltage of one pin (the base of the transistor and the gate of the field-effect transistor), the other two pins can be controlled to turn on or off.
With a switch, the next step is to have a circuit that controls the switch. The function of this circuit is to output high-speed switch signals to control the conduction and cutoff of the switch tube. This circuit is called an oscillation circuit. There are many types of oscillation circuits in switching power supplies, regardless of which one is used to provide control signals to the switching transistor.
After being controlled by the control circuit, the input voltage changes from low-frequency AC to high-frequency pulse DC voltage. It is input to a transformer for voltage reduction, and the voltage output by the transformer is also rectified and filtered to become DC output, providing it to the load for operation. Unlike power frequency transformers, the switching power supply also has an additional voltage detection circuit, which detects the output voltage signal and feeds it back to the primary control circuit of the transformer for voltage regulation. This improves the stability of the switching power supply's output voltage and allows for a wide input voltage range. So the working process of a switching power supply is actually achieved through several processes of AC DC, DC AC, and then AC DC.
There may be a question here, isn't a transformer only capable of AC power, why can DC power from a switching power supply also be transformed through a transformer? Transformers can only pass through AC power, specifically requiring changes in magnetic flux. Power frequency AC power is a sine wave with a positive and negative half cycle, which will result in changes in magnetic flux. Switching power supplies convert direct current into pulsed direct current through a switch tube. The switch tube changes magnetic flux from cut-off to conduction, and then from conduction to cut-off.
