Modes of Circuit Operation - Forward Type Switched Mode Power Supply
Mode-1 of circuit operation:
Fig: Current path & Equivalent circuit in Mode- 1
- Mode-1 of circuit starts after switch ‘S’ is turned ON. This connects the input voltage, Edc, to the primary winding. Both primary and secondary windings start conducting simultaneously with the turning on of the switch. The primary and secondary winding currents and voltages are related to their turns-ratio (NP / NS), as in an ideal transformer.
- As switch ‘S’ closes, diode D1 in the secondary circuit gets forward biased and the input voltage, scaled by the transformer turns ratio, gets applied to the secondary circuit. Diode D2 does not conduct during mode-1, as it remains reverse biased.
- The output circuit consisting of L-C filter and the load gets a voltage equal to
during mode-1. This voltage is the maximum achievable dc voltage across the load, corresponding to δ = 1.
- Mode-1 can be called as powering mode during which input power is transferred to the load
Mode-2 of Circuit Operation:
- Mode-2, to be called as freewheeling mode, starts with turning off of the switch ‘S’.
- Points ‘P’ and ‘N’ of the equivalent circuit are effectively shorted due to conduction of diode ‘D2’. The inductor current continues to flow through the parallel combination of the load and the output capacitor.
- During mode-2, there is no power flow from source to load but still the load voltage is maintained nearly constant by the large output capacitor ‘C’. The charged capacitor and the inductor provide continuity in load voltage.
- Since there is no input power during mode-2, the stored energy of the filter inductor and capacitor will be slowly dissipating in the load and hence during this mode the magnitudes of inductor current and the capacitor voltage will be falling slightly.
- To keep the load voltage magnitude within required tolerance band, the converter-switch ‘S’ is turned on again to end the freewheeling mode and start the next powering mode (mode-1).
- Under steady state, loss in inductor current and capacitor voltage in mode-2 is exactly made up in mode-1.
- It may not be difficult to see that to maintain load voltage within the desired tolerance band the filter inductor and capacitor magnitudes should be sufficiently large.
- To keep the filter cost and its physical size small these elements should not be unnecessarily too large. Also, for faster dynamic control over the output voltage the filter elements should not be too large.
- The filter inductor, capacitor, transformer and the heat sinks for the switching devices together account for nearly 90% of the power supply weight and volume.
- One important factor that directly influences the size of the filter circuit elements and the transformer is the converter’s switching frequency. High frequency operation of switch ‘S’ will help in keeping the filter and transformer size small. The switching frequency of a typical forward converter may thus be in the range of 100 kHz or more. The higher end limit on the switching frequency comes mainly due to the finite switching time and finite switching losses of a practical switch.
- The switch and the diodes have been assumed to be ideal, with no losses and zero switching time. Control over switch duty ratio, which is the ratio of ON time to (ON OFF) time, provides the control over the output voltage ‘VO’.