A Practical Fly-Back Converter
A practical fly-back converter:
Fig: A practical Fly back converter
- The loss part of the circuit is to be kept in mind while designing for rated power. The designed input power (Pin) should be equal to Po/η, where Po is the required output power and η is the efficiency of the circuit.
- A typical figure for η may be taken close to 0.6 for first design iteration. Similarly one needs to counter the effects of the non-ideal coupling between the windings. Due to the non-ideal coupling between the primary and secondary windings when the primary side switch is turned-off some energy is trapped in the leakage inductance of the winding.
- The flux associated with the primary winding leakage inductance will not link the secondary winding and hence the energy associated with the leakage flux needs to be dissipated in an external circuit (known as snubber). Unless this energy finds a path, there will be a large voltage spike across the windings which may destroy the circuit.
- The snubber circuit consists of a fast recovery diode in series with a parallel combination of a snubber capacitor and a resistor. The leakage-inductance current of the primary winding finds a low impedance path through the snubber diode to the snubber capacitor. The diode end of the snubber capacitor will be at higher potential. To check the excessive voltage build up across the snubber capacitor a resistor is put across it. Under steady state this resistor is meant to dissipate the leakage flux energy. The power lost in the snubber circuit reduces the overall efficiency of the fly-back type SMPS circuit.
- A typical figure for efficiency of a fly-back circuit is around 65% to 75%.
- In order that snubber capacitor does not take away any portion of energy stored in the mutual flux of the windings, the minimum steady state snubber capacitor voltage should be greater than the reflected secondary voltage on the primary side.
- This can be achieved by proper choice of the snubber-resistor and by keeping the RC time constant of the snubber circuit significantly higher than the switching time period. Since the snubber capacitor voltage is kept higher than the reflected secondary voltage, the worst-case switch voltage stress will be the sum of input voltage and the peak magnitude of the snubber capacitor voltage.
- A practical fly back converter contains taken close to 0.6 for first design iteration. Due to the non-ideal coupling between the primary and secondary windings when the primary side switch is turned-off some energy is trapped in the leakage inductance of the winding.
- Similar to the rectification done for the secondary winding. The rectified tertiary voltage will be nearly proportional to the secondary voltage multiplied by the turns-ratio between the windings. The rectified tertiary winding voltage also doubles up as control power supply for the PWM controller.
- For initial powering up of the circuit the control power is drawn directly from the input supply through a resistor connected between the input supply and the capacitor of the tertiary circuit rectifier. The resistor ‘RS’ is of high magnitude and causes only small continuous power loss.
- In case, multiple isolated output voltages are required, the fly-back transformer will need to have multiple secondary windings. Each of these secondary winding voltages are rectified and filtered separately. Each rectifier and filter circuit uses the simple diode and capacitor as shown earlier for a single secondary winding.
- In the practical fly-back converter there is a tertiary winding is used for voltage feedback, it may not be possible to compensate exactly for the secondary winding resistance drop as the tertiary winding is unaware of the actual load supplied by the secondary winding. However for most applications the small voltage drop in the winding resistance may be tolerable. Else, one needs to improve the voltage regulation by adding a linear regulator stage in tandem or by giving a direct output voltage feedback to the control circuit.