Flyback converter

The flyback converter, and high - step-down converter, flyback converter english, is a design of DC-DC converters. It is used to transfer electrical energy between an input and an output side galvanically isolated DC voltages. From the principle similarly functioning inverse converter has no galvanic isolation between the two sides.

Flyback converters can be found in primary switched-mode power supplies, low power ( less than 250 W, as a separate standby supply in larger and PC power supplies ), voltage transformers in electronic equipment, the high-voltage generation in flash units and for picture tubes of televisions, but also the generation of the spark in automobiles are examples.

The flyback converter, which is used for power conversion of direct voltage to another voltage is to be distinguished from the oscillator circuit of the blocking oscillator.

Operation

The principle of the switching regulator is that a small amount of energy in the magnetic field of a transformer consisting of the ideal transformer L1 and L2 and the main inductance Lh is stored. The first phase is the " loading " of the main inductance and the second phase, the "Unload" on the secondary side. This cycle is run through with switching frequency of several thousand times per second, so that a quasi-continuous energy flow arises from the producer to the consumer side.

The first phase is the conducting phase with closed, the second phase, the blocking phase with open switch S.

During the conducting phase (0 ... t1), the diode D (positive to cathode), and it will only be a current through the main inductance Lh is what caused by the input voltage Vin. The winding L2 is de-energized. It builds up in the air gap of the coil a magnetic voltage. At this stage there is no energy transfer, the output voltage is maintained only by the capacitor C.

Opens the switch S, the blocking phase (t1 ... t ) begins. The current I1 is abruptly to zero, but since the current through the main inductance Lh can not jump through the open switch, it flows through the ideal transformer, ie L1 and L2, and through diode D to the output. He charges the capacitor C to the output voltage at Vout. This current decreases linearly and is in continuous mode eventually becomes zero when all energy is drained from the coil, the coil thus " discharged " is ( t2). Thereafter, the switch closes, the conducting period begins again, and the cycle begins anew. The actual power transmission to the secondary side taking place during the blocking phase, therefore, this circuit is referred to as a flyback converter.

A non-ideal coil winding has capacity that was at the beginning of the blocking phase also charged. The energy stored there leads to the coil together to form a damped natural resonance oscillation ( oscillation circuit ) after the coil has discharged its entire electricity (t2 ... T).

In practice a transistor as the switch S are used, the switching frequency is typically about 16 kHz is selected ( just above the audible range in order to avoid noise ) to about 500 kHz - the higher frequencies allow the use of smaller coils, but require higher losses in the switching element and the diode.

The " memory transformer "

Magnetically coupled coils, as used in the flyback converter, similar to transformers. However, they differ significantly from transformers, since the total transmitted energy is temporarily stored between the individual states in the magnetic field. In ordinary transformers is only slightly magnetic energy stored due to the simultaneous power and release in the nucleus. The magnetic core has in conventional transformers, no air gap, whereas the seeds in flyback converters always have an air gap as with the coil, in which a substantial part of the magnetic field energy is stored by the high magnetomotive force occurring there. Depending on the design of the air gap, for example, in E- cores is arranged in the area of the center leg and is no longer visible from the outside.

For switching power supplies according to the flyback converter principle, the storage transformer is due to the high operating frequency but much smaller and lighter than a 50 -Hz transformer. However, it is larger than the transformer at other switching power supply topologies; for no more choke is needed for flyback converter.

Special

The output voltage from flyback converters is determined by the load, it is in principle unlimited, i.e., it rises at the unloaded unregulated flyback converter to such an extent that the rectifier diode, the switching transistor or the load to be destroyed.

In most cases, therefore, a control of the flyback converter is necessary. An unregulated flyback converter always transmit at a constant voltage, the same performance, namely, the stored energy of the coil times the operating frequency (number Leit-/Sperrphasen per second). If the flyback energy transfer as the consumer just needed, the voltage at the load. In simple cases, parallel with the load are connected, a Zener diode, which converts the excess power into heat.

For controlling a measuring coil is often mounted on the reactor or the storage transformer, which at the same time takes over the auxiliary power supply; comparing the voltage from said coil with a reference value. The result of the control electronics are then fed to the re-adjusts the duty cycle of the switching frequency. Because of the relatively large scatter between the windings, this form of control is not very good. But it is simple and has the advantage that it can affect multiple output windings simultaneously. In particularly good arrangements you have on the other hand, secondary side compare the output voltage with a reference voltage and transmit the deviation via an optocoupler to the primary side, ( about need to achieve if the electrical isolation ) or provide a fine control on each secondary side.

In the quasi- resonant operation, the measuring winding is used for zero-voltage detection ( CVP), when the memory transformer has delivered its total energy. This switching losses can be minimized, in which the switching operation at the falling edge, supported by the natural resonant oscillation starts. Thus, the converter also never turned against the self-resonant what the EMC emissions reduced. A further refinement is the valley in which switching of the switching operation takes place at the apex of the natural vibration.

Another advantage is the principal short-circuit rating of the flyback converter.

With regard to the interpretation of the legislation and the storage inductance distinction is intermittent and continuous mode. In low-power and low duty cycle ( duty cycle ) of the circuit breaker occurs continuous mode: The current in the switching transistor is triangular. In continuous mode the inductor when the circuit breaker is still energized; the current in the switch is trapezoidal ( diagonally rising limb above).

Example of a flyback switching power supply

To operate a flyback converter to AC power, the AC line voltage is rectified by a bridge rectifier and smoothed with a electrolytic capacitor. At that lie at 230 V ~ 325 V approx ( = Ue).

In the adjacent figure, a complete circuit diagram of a flyback switching power supply is shown. In the field the block diagram is shown with only one output voltage. The component names have been adopted in principle diagram.

The mean winding of the storage transformer, the primary winding L1 of the auxiliary power supply is used for the control, voltage regulation, and L4 is the determination of the timing at which the magnetic field has become essentially zero. The right-hand winding and the secondary winding has a plurality of taps, to produce different output voltages. This winding and thus the output voltages are galvanically isolated from the mains voltage.

As with other switching power supplies also is the winding sense of windings of importance: in the adjacent example circuit windings are all in the same direction down to L4, that is, as shown, wrapped. It represents the winding sense as in the example circuit diagram with asterisks or dots dar. the winding start

For electrical measurements on a primary switch mode power supply during operation, the use of an isolation transformer for the purpose of galvanic isolation from the mains is useful.

Pros and Cons

The following are the advantages and disadvantages compared to other switching converter topologies are shown:

• Easy to set up (with flyback switching power supplies no additional storage inductor is needed)
• All output windings provide a controllable via an auxiliary winding output voltage according to their number of turns
• Very high output voltage even with a moderate ratio possible with the secondary rectifier with a very short off phase does not have to block a lot more voltage than the value of the output voltage ( advantageous for small high voltage generators)
• The flyback converter transmits its energy to the secondary side only when the circuit breaker opens to the primary side. Diodes in the secondary lock during the closing of the circuit breaker. Therefore flyback converter are basically short-circuit proof.

• Larger problems with electromagnetic compatibility
• Increased transformer due to higher RMS current stress and unipolar Terms of magnetic flux.
• High switching losses in the primary side power switch since it switches off the current maximum and the voltage rises very steeply.