Circuit Equations Under Discontinuous-Flux Mode & comparison
Circuit Equations under Discontinuous –Flux Mode:
Fig: Fly-back converters waveform under discontinuous flux
- When the Fly-back converter is operating in the discontinuous flux mode, it is operating in the Mode 3 type of operation.
- During mode-3 of the circuit operation, primary and secondary winding currents as well as voltages are zero.
- The load, however, continues to get a reasonably steady voltage due to the relatively large output filter capacitor.
- With the turning ON of the switch, the primary winding current starts building up linearly from zero and at the end of mode-1 the magnetic field energy due to primary winding current rises to (Lpri I2p )/ 2.
- This entire energy is transferred to the output at the end of mode-2 of circuit operation. Under the assumption of loss-less operation the output power (Po) can be expressed as
, where fswitch (= 1/T) is the switching frequency of the converter.
Continuous Versus Discontinuous Flux Mode of Operation:
- A practical fly-back type SMPS circuit will have a closed loop control circuit for output voltage regulation. The controller modulates the duty ratio of the switch to maintain the output voltage within a small tolerable ripple voltage band around the desired output value.
- If the load is very light, very small amount of energy needs to be input to the circuit in each switching cycle. This is achieved by keeping the ON duration of the switch low, resulting in low duty ratio (δ).
- Within this small ON time only a small amount of current builds up in the primary winding. The off duration of the switch, which is (1-δ) fraction of the switching time period, is relatively large. Mode-2 duration of the circuit operation is also small as the magnetic field energy is quickly discharged into the output capacitor.
- Thus, at light load, the circuit is in mode-3 for significant duration. As the load increases the mode-3 duration, during which there is zero winding currents and zero flux through the core, reduces and the circuit is driven towards continuous flux mode.
- The circuit operation changes from discontinuous to continuous flux mode if the output power from the circuit increases beyond certain value. Similarly if the applied input voltage decreases, keeping the load power and switching frequency constant, the circuit tends to go in continuous flux mode of operation.
- For better control over output voltage, discontinuous flux mode of operation is preferred. However, for the given transformer and switch ratings etc., more output power can be transferred during continuous flux mode.
- A common design thumb rule is to design the circuit for operation at just-continuous flux mode at the minimum expected input voltage and at the maximum (rated) output power.