Flyback converter

The main element of the flyback converter is a multi-winding storage choke ^[1] (which is sometimes called a transformer , although the processes occurring here and in the transformer have significant differences).

There are two main stages of the circuit: transferring energy from the primary power source to the inductor and transferring the energy of the inductor to the secondary circuit (secondary circuit).

When the key is closed, the voltage of the power supply is applied to the primary winding of the inductor. In the inductor, the current in the primary winding and the magnetic flux in the magnetic circuit begin to increase almost linearly, and therefore, energy is accumulated. The key is usually a transistor . The EMF induced on the secondary winding locks the diode, and there is no current in the secondary winding. When the key is opened, the current in the primary winding disappears, but the magnetic flux in the inductor cannot change instantly, so an almost linearly decreasing current begins to flow in the secondary winding, unlocking the diode. This current charges the capacitor and enters the load. In the first stage, the load is powered only by the energy received by the capacitor during the second stage. The repetition rate of the steps is usually in the range from 20 kHz to 1 MHz ^[2] .

The regulation of the output voltage is carried out by changing the duration of the current pulses in the primary winding. In the feedback circuit (not shown in the diagrams), in the key management device, pulse width modulation (PWM) or pulse frequency modulation (PFM) can be used ^[2] .

Some microcircuits for such converters do not have a full-fledged pulse-width modulator (PWM - when the pulse duration changes from 50 ... 70% to 0 to change the output voltage) and operate in the "start-stop" mode. That is, the microcircuit constantly works with maximum power, if the voltage rises above the switching threshold - the microcircuit turns off and stops “pumping” pulses into the transformer until it drops, and then starts working again with maximum power. This mode of operation, in comparison with PWM, creates a lot of interference , the output voltage ripples, the load on the smoothing capacitor, power transistor, rectifier diodes increases, but this is not essential for battery charging , power supply of digital circuits.

Flyback converters are widely used as power sources for various equipment with power up to 200 W: televisions, audio and video equipment, computer peripherals and computers themselves.

They are used in energy-saving lamps and LED lamps, in addition to the cheapest schemes with linear drivers .

Also used in the chargers of mobile phones and laptops .

On the basis of flyback converters, amateurs also produce inverter sources of welding current , since the load characteristic of the flyback converter is sharply falling, which is optimal from the point of view of arc stabilization. But such converters are characterized by large dimensions in comparison with linear and push-pull, therefore they are not produced commercially.

The widespread use of flyback converters has led to the appearance on the world market of electronic components of special microcircuits that provide the construction of flyback converters with a minimum number of external elements (for example, the TOPSwitch series microcircuits).

Advantages of flyback converters:

• significantly smaller dimensions and weight compared to power supplies containing a transformer at a frequency of 50 Hz;
• significantly less use of expensive materials ( copper )
• insensitivity of the flyback converter to the load short circuit ;
• the ability to control the output voltage over a wide range, as well as maintaining the required output voltage in the face of changing voltage of the supply network ;

Due to the fact that in the flyback converter, the accumulator choke is connected to the primary network and to the load at various points in time, the transmission of interference from the network to the load and back is excluded, which is also an advantage of the flyback converter.

Disadvantages flyback converters:

• power is limited by the energy stored by the inductor (in practice, no more than 200 W);
• increased level of electromagnetic interference generated both in the mains and in the load;
• larger dimensions compared to other pulse converters at the same power.
• such an IIP does not work at idle (that is, without a connected load); therefore, modern controller microcircuits have a function to automatically turn off the converter when the load is reduced.

• Switching voltage regulator
• Push-pull converter
• Voltage inverters
• Rumkorf Coil

• Kostenko S. Evolution of flyback pulsed IP // Radio No. 8 (2002) and No. 9 (2002)