Introduction to Flyback Converters: Parameters, Topologies, and Controllers

qwqwqw2088

Introduction to Flyback Converters: Parameters, Topologies, and Controllers [Copy link]

The flyback converter is a versatile power electronic device used in applications such as medical equipment and laptop computers. Also known as an isolated buck-boost converter, this converter has a simple circuit that can regulate the output voltage (VOUT) of the system while minimizing electromagnetic interference (EMI).

Parameters and topology of flyback converter In a flyback converter, the inductor is split to form a coupled inductor, which is also called a flyback transformer. The coupled inductor isolates the input of the converter from its output. Figure 1 is a schematic diagram of a flyback converter, which consists of the following:

Figure 1: Flyback converter topology

Flyback Converter Considerations There are some important factors to consider when selecting a flyback converter, including determining some basic parameters such as VIN, VOUT, LP, and LS. Here are some additional considerations:

The transformer turns ratio NP:NS (NP is the number of turns of the primary winding, NS is the number of turns of the secondary winding) directly affects VOUT. If NS increases, VOUT increases proportionally; if NS decreases, VOUT also decreases proportionally. The relationship between NP and VOUT is inversely proportional, NP increases, VOUT decreases proportionally; and vice versa.

The duty cycle is the ratio of the on-time to the total switching period (tON / τSW). The duty cycle determines VIN based on VOUT and the transformer turns ratio; the higher the duty cycle, the higher the VOUT.

Protection mechanisms and isolation capabilities are critical for flyback converters to meet safety standards such as UL 1577 and IEC 62368. Protection features can be optimized for EMI performance to ensure that the device does not operate under suboptimal conditions.

Flyback Converter Operation The essence of the flyback converter is to store and transfer energy. Its working cycle includes the on-time (tON) and the off-time (tOFF), which are controlled by the switching state of the MOSFET (see Figure 2). During tON, the MOSFET is in the on-state, and the current flows from the input through LP to charge the coupled inductor; during tOFF, the MOSFET is in the off-state, the coupled inductor is demagnetized through the diode, and then the current charges COUT and supplies power to the load. This process can be simplified into the following steps:

tON begins. When the MOSFET turns on, current flows through LP and energy is stored in the transformer's magnetic field.
tON ends.

tOFF begins. When the MOSFET turns off, the stored energy is transferred to the output through the secondary diode/MOSFET, charging COUT and increasing VOUT.
tOFF ends.

Figure 2: tON and tOFF

This cycle repeats continuously to achieve VOUT regulation. Although the flyback converter follows the above overall process, some other processes and modes can still be selected to improve efficiency.

qwqwqw2088

qwqwqw2088