Flyback converter

Fig. 1: Schematic of a flyback converter

The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and any outputs. More precisely, the flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. When driving for example a plasma lamp or a voltage multiplier the rectifying diode of the buck-boost converter is left out and the device is called a flyback transformer.

Structure and principle

Fig. 2: The two configurations of a flyback converter in operation: In the on-state, the energy is transferred from the input voltage source to the transformer (the output capacitor supplies energy to the output load). In the off-state, the energy is transferred from the transformer to the output load (and the output capacitor).

Fig. 3: Waveform - using primary side sensing techniques - showing the 'knee point'.

The schematic of a flyback converter can be seen in Fig. 1. It is equivalent to that of a buck-boost converter, with the inductor split to form a transformer . Therefore the operating principle of both converters is very close:

• When the switch is closed (top of Fig. 2), the primary of the transformer is directly connected to the input voltage source. The primary current and magnetic flux in the transformer increases, storing energy in the transformer. The voltage induced in the secondary winding is negative, so the diode is reverse-biased (i.e., blocked). The output capacitor supplies energy to the output load.
• When the switch is opened (bottom of Fig. 2), the primary current and magnetic flux drops. The secondary voltage is positive, forward-biasing the diode, allowing current to flow from the transformer. The energy from the transformer core recharges the capacitor and supplies the load.

The operation of storing energy in the transformer before transferring to the output of the converter allows the topology to easily generate multiple outputs with little additional circuitry, although the output voltages have to be able to match each other through the turns ratio. Also there is a need for a controlling rail which has to be loaded before load is applied to the uncontrolled rails, this is to allow the PWM to open up and supply enough energy to the transformer.

Operation

The flyback converter is an isolated power converter, therefore the isolation of the control circuit is also needed. The two prevailing control schemes are voltage mode control and current mode control (in the majority of cases current mode control needs to be dominant for stability during operation). Both require a signal related to the output voltage. There are two common ways to generate this voltage. The first is to use an optocoupler on the secondary circuitry to send a signal to the controller. The second is to wind a separate winding on the coil and rely on the cross regulation of the design.

The first technique involving an optocoupler has been used to obtain tight voltage and current regulation, whereas the alternative approach was developed for cost-sensitive applications where the output did not need to be as tightly controlled but up to 11 components including the optocoupler could be eliminated from the overall design. Also, in applications where reliability is critical, optocouplers can be detrimental to the MTBF (Mean Time Between Failure) calculations.

Recent developments in primary-side sensing technology, where the output voltage and current are regulated by monitoring the waveforms in the auxiliary winding used to power the control IC itself, have improved the accuracy of both voltage and current regulation.

Previously, a measurement was taken across the whole of the flyback waveform which led to error, but it was realized that measurements at the so-called knee point (when the secondary current is zero, see Fig. 3) allow for a much more accurate measurement of what is happening on the secondary side. This topology is now replacing ringing choke converter (RCC) in applications such as mobile phone chargers.

Limitations

Continuous mode has the following disadvantages, which complicate the control of the converter:

• The voltage feedback loop requires a lower bandwidth due to a zero in the response of the converter.
• The current feedback loop used in current mode control needs slope compensation in cases where the duty cycle is above 50%.
• The power switches are now turning on with positive current flow - this means that in addition to turn-off speed, the switch turn-on speed is also important for efficiency and reducing waste heat in the switching element.

Discontinuous mode has the following disadvantages, which limit the efficiency of the converter:

• High RMS and peak currents in the design
• High flux excursions in the inductor

Applications

• Low-power switch-mode power supplies (cell phone charger, standby power supply in PCs)
• Low-cost multiple-output power supplies (e.g., main PC supplies <250 W)
• High voltage supply for the CRT in TVs and monitors (the flyback converter is often combined with the horizontal deflection drive)
• High voltage generation (e.g., for xenon flash lamps, lasers, copiers, etc.)
• Isolated gate driver

See also

• Forward converter
• Joule thief - Minimalist switchmode converter example

References

• Billings, Keith (1999), Switchmode Power Supply Handbook, McGraw-Hill, ISBN 0-07-006719-8 {{citation}}: Unknown parameter |ed= ignored (help)