Working principle of power frequency transformer and switching power supply
The working principle of the power frequency transformer is relatively simple. The power frequency AC voltage input by the primary coil is converted into a magnetic field, which is transmitted to the secondary coil through a magnetically conductive material (usually a silicon steel sheet) to induce a voltage. The output frequency is the same as the input frequency, and the voltage is reduced according to the ratio of primary and secondary coil turns (if there are more secondary turns, it is a boost). Since the output of the transformer is alternating current, and 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 transformation element of the switching power supply is still a transformer, and it also follows the rule that the voltage ratio is equal to the turns ratio. Different from the power frequency transformer, the switching power supply needs to increase the operating frequency, that is, it needs to change the low-frequency AC voltage into a high-frequency AC voltage, which requires the realization of an additional control circuit. Because the operation of the circuit requires direct current, the input AC voltage must first be rectified to become a direct current voltage before it can be controlled by the subsequent circuit. Let's take a commonly used mobile phone charger circuit as an example to briefly understand the working principle of the switching power supply.
After the input 220V AC voltage is rectified and filtered, it will become a DC voltage of about 310V (that is, the peak value of the 220V AC voltage). Next, this DC voltage needs to be converted into a high-frequency AC voltage. To turn this voltage into high-frequency alternating current, the easiest way is to use a switch to quickly open and close the switch, so that the direct current can be turned into a high-speed pulsed direct current voltage. The component that realizes this switch is a transistor. Transistors, including commonly used triodes and field effect transistors, etc., these two components can be used as electronic switches, that is, controlled by the voltage of a pin (the base of the triode and the gate of the field effect transistor), just The other two pins can be controlled on and off.
With the switch, the next step is to have a circuit to control the switch. The function of this circuit is to output a high-speed switching signal to control the on and off of the switch tube. This circuit is called an oscillation circuit. There are many kinds of oscillating circuits in switching power supplies, no matter which one, the function is to provide control signals to the switching tube.
After the control of the control circuit, the input voltage changes from low-frequency alternating current to high-frequency pulsed direct current voltage, which is input to the transformer for step-down, and the voltage output by the transformer will also be rectified and filtered to become direct current output, which is provided to the load Work. Different from the power frequency transformer, the switching power supply also has a part of the voltage detection circuit, which will feedback the output voltage signal to the primary control circuit of the transformer for voltage regulation after detection, so that the output voltage of the switching power supply is stable. performance has been improved, and can have a wide input voltage range. Therefore, the working process of the switching power supply is actually realized by several processes of AC-DC, DC-AC, and then AC-DC.
There may be a question here, isn’t the transformer only able to pass AC power, why can the DC power of the switching power supply also be transformed through the transformer? It is true that the transformer can only pass through alternating current. Specifically, it needs a change in magnetic flux. Since the power frequency alternating current is a sine wave and has positive and negative half cycles, it will produce a change in magnetic flux. The switching power supply uses the switching tube to convert the direct current into pulsed direct current. The switching tube changes from cut-off to conduction, and then from conduction to cut-off, which will also produce changes in magnetic flux.
