Flyback converter
The flyback converter , also high-downconverter , English flyback converter , is an operation mode for DC-DC converter . It is used to transfer electrical energy between an input and an output side of galvanically separated direct voltages. The inverse converter , which works in a similar way, has no galvanic separation between the two sides.
Flyback converters are found in primary switched mode power supplies low-power (less than 250 W, as a separate standby power supply in larger and PC power supplies ) in voltage transformers in electronic equipment, etc. The high-voltage generation in flash units and for picture tubes from television sets to continue the production spark in automobiles are examples of this.
The flyback converter, which is used for power conversion of direct voltage to another voltage, from the oscillator circuit of the blocking oscillator to distinguish.
functionality
The principle of the flyback converter is that a small amount of energy is stored in the magnetic field of a transformer, consisting of the ideal transformer L 1 and L 2 and the main inductance L h . The 1st phase is the "charging" of the main inductance, the 2nd phase is the "discharging" via the secondary side. This cycle is run through with a switching frequency of several thousand times per second, so that a quasi-continuous flow of energy is created from the generator to the consumer side.
The 1st phase is the leading phase with the switch closed, the 2nd phase the blocking phase with the switch S open.
During the conducting phase (0… t 1 ) the diode D blocks (plus at the cathode) and only one current flows through the main inductance L h , which is caused by the input voltage U e . The winding L 2 is de-energized. A magnetic tension builds up in the air gap of the coil . In this phase there is no energy transfer, the output voltage is only held by the capacitor C.
When switch S opens, the blocking phase begins (t 1 ... T). The current I 1 suddenly becomes zero due to the open switch. However, since the current cannot jump through the main inductance L h , it flows via the ideal transformer, i.e. L 1 and L 2 , and via the diode D to the output. There it charges the capacitor C to the output voltage U a . This current decreases linearly and finally becomes zero in intermittent operation when all the energy has flown out of the coil, ie the coil is "discharged" (t 2 ). Then the switch closes again, the lead phase begins again and the cycle starts again. The actual energy transport to the secondary side takes place during the blocking phase , which is why this circuit is called a flyback converter .
A non-ideal coil has winding capacities that were also charged at the beginning of the blocking phase. The energy stored there, together with the coil, leads to a damped natural resonance oscillation ( resonant circuit ) after the coil has delivered its entire current (t 2 ... T).
In practice, a transistor is used as switch S , with a switching frequency of mostly over 20 kHz (just above the audible range to avoid interference) up to approx. 500 kHz - higher frequencies allow the use of smaller coils, but result in higher losses in the Switching element and in the diode.
The "storage transformer"
Magnetically coupled coils, as used in the flyback converter, are similar to transformers. However, they differ significantly from transformers, as the entire energy transferred between the individual states is temporarily stored in the magnetic field. In ordinary transformers, only a small amount of magnetic energy is stored in the core because of the simultaneous power consumption and output. In conventional transformers, the magnetic core does not have an air gap, whereas the cores in flyback converters always have an air gap as in coils, in which a substantial part of the magnetic field energy is stored by the high magnetic voltage that occurs there. Depending on the design, the air gap is attached in the area of the center leg in the case of E-cores, for example, and is no longer visible from the outside.
With switched-mode power supplies based on the flyback converter principle, the storage transformer is much smaller and lighter than a 50 Hz transformer thanks to the high operating frequency. However, it is larger than the transformer in other switched-mode power supply topologies; no additional storage choke is required for the flyback converter.
particularities
The output voltage of flyback converters depends on the load, it is in principle unlimited, i. In other words, in the unloaded, unregulated flyback converter, it increases until the rectifier diode, the switching transistor or the load are destroyed.
In most cases it is therefore necessary to regulate the flyback converter. An unregulated flyback converter always transmits the same power at constant voltage, namely the stored energy of the coil multiplied by the operating frequency (number of conducting / blocking phases per second). If the flyback converter transfers more energy than the consumer needs, the voltage on the consumer increases. In simple cases, a Zener diode can be connected in parallel to the consumer , which converts the excess power into heat.
For the regulation, a measuring winding is often attached to the choke or the storage transformer, which at the same time takes over the auxiliary voltage supply; the voltage from this winding is compared with a reference value. The result is then fed to control electronics that readjust the duty cycle of the switching frequency . Because of the relatively large spread between the windings, this form of control is not particularly good. But it is simple and has the advantage that you can influence several output windings at the same time. In the case of particularly good controls, on the other hand, the output voltage on the secondary side must be compared with a reference voltage and the deviation must be transferred to the primary side via an optocoupler (in order to achieve galvanic isolation if necessary) or fine control must be provided on each secondary side.
In quasi-resonance mode, the measuring winding is used to detect zero voltage (ZVD) when the storage transformer has given up all of its energy. This minimizes switching losses, since the switching process starts with the falling edge, supported by the natural resonance oscillation. Thus, the converter never switches against the natural resonance, which reduces the EMC radiation . Valley switching is a further refinement, in which the switching process takes place at the apex of the natural oscillation.
Another advantage is the principle short-circuit strength of the flyback converter.
With regard to the design of the control and the storage inductance, a distinction is made between intermittent and non- intermittent operation. When the power and duty cycle of the circuit breaker is low, intermittent operation occurs: The current in the switching transistor is triangular. In non-discontinuous operation, the inductance is still live when the circuit breaker is switched on, the current in the switch is trapezoidal (sloping leg at the top).
Example of a flyback switched-mode power supply
To operate a flyback converter on the power grid , the AC line voltage is rectified via a rectifier bridge and smoothed with an electrolytic capacitor. At 230 V ~ there are approx. 325 V- (= U e ).
The adjacent figure shows a complete circuit diagram of a flyback converter switch-mode power supply. The basic circuit diagram is shown in the partial diagram with only one output voltage. The component names have been taken over into the principle circuit diagram.
The middle winding of the storage transformer is the primary winding, L1 is used for the auxiliary voltage supply for the control, L4 is used to regulate the voltage and determine the point in time at which the magnetic field in the core has become zero. The winding on the right is the secondary winding and has multiple taps to produce different output voltages. This winding and thus the output voltages are galvanically separated from the mains voltage.
As with other switched-mode power supplies, the winding direction of the windings is important: in the example circuit shown here, all windings except for L4 are wound in the same direction, i.e. as shown. The winding direction is shown as in the example circuit diagram with asterisks or dots at the beginning of the winding.
Security advice:
When making electrical measurements on a primary switched-mode power supply during operation, it makes sense to use an isolating transformer for the purpose of galvanic separation from the mains.
Advantages and disadvantages
The advantages and disadvantages compared to other switching converter topologies are shown below:
Advantages:
- Simple design (no additional storage choke is required for flyback switched-mode power supplies)
- All output windings supply an output voltage that can be regulated via an auxiliary winding according to the number of turns
- Very high output voltage possible even with a moderate gear ratio, whereby the secondary rectifier does not have to block significantly more voltage than the value of the output voltage for a very short blocking phase (advantageous for small high-voltage generators)
- Very large input voltage range; advantageous when realizing power supply units for different mains voltages in different countries (power supply units with wide range input)
- The flyback converter only transfers its energy to the secondary side when the circuit breaker on the primary side opens. Diodes on the secondary side block when the circuit breaker closes. Therefore, flyback converters are basically short-circuit proof.
Disadvantage:
- Major problems with electromagnetic compatibility
- Larger transformer due to higher effective current load and unipolar use of the magnetic flux.
- High switching losses in the primary-side circuit breaker, as it switches off when the current is at its maximum and the voltage rises very steeply.
