Principle of Operation of Forward converters
Principle of operation of Forward Converters:
Fig: Basic topology of a Forward converter
- The basic circuit of a forward converter is basically a dc-to-dc buck converter with the addition of a transformer for output voltage isolation and scaling.
- When switch ‘S’ is turned on, input dc gets applied to the primary winding and simultaneously a scaled voltage appears across the transformer secondary. Dotted sides of both the windings are now having positive polarity. Diode ‘D1’, connected in series with the secondary winding gets forward biased and the scaled input voltage is applied to the low pass filter circuit preceding the load.
- The primary winding current enters through its dotted end while the secondary current comes out of the dotted side and their magnitudes are inversely proportional to their turns-ratio. Thus, as per the assumption of an ideal transformer, the net magnetizing ampere-turns of the transformer is zero and there is no energy stored in the transformer core.
- When switch ‘S’ is turned off, the primary as well as the secondary winding currents are suddenly brought down to zero. Current through the filter inductor and the load continues without any abrupt change. Diode ‘D2’ provides the freewheeling path for this current.
- The required emf to maintain continuity in filter-inductor current and to maintain the forward bias voltage across D2 comes from the filter inductor ‘L’ itself.
- During freewheeling the filter inductor current will be decaying as it flows against the output voltage (Vop), but the presence of relatively large filter capacitor ‘C’ still maintains the output voltage nearly constant. The ripple in the output voltage must be within the acceptable limits.
- The supply switching frequency is generally kept sufficiently high such that the next turn-on of the switch takes place before the filter inductor current decays significantly. Needless to say, that the magnitudes of filter inductor and capacitor are to be chosen appropriately.
- The idea behind keeping filter inductor current nearly constant is to relieve the output capacitor from supplying large ripple current.
- As per the basic circuit topology of a forward converter, the inductor and the capacitor together share the load-current drawn from the output. Under steady state condition, mean dc current supplied by the capacitor is zero but capacitor still supplies ripple current. For maintaining constant load current, the inductor and capacitor current-ripples must be equal in magnitude but opposite in sense.
- Capacitors with higher ripple current rating are required to have much less equivalent series resistor (ESR) and equivalent series inductor (ESL) and as such they are bulkier and costlier. Also, the ESR and ESL of a practical capacitor causes ripple in its dc output voltage due to flow of ripple current through these series impedances. Since the output voltage is drawn from capacitor terminal the ripple in output voltage will be less if the capacitor is made to carry less ripple current.
- For better understanding of the steady-state behavior of the converter, the circuit’s operation is divided in two different modes: mode-1 and mode-2. Mode-1 corresponds to the ‘on’ duration of the switch and mode-2 corresponds to its ‘off’ duration.
The following simplifying assumptions are made before proceeding to the detailed mode-wise analysis of the circuit:
- ON state voltage drops of switches and diodes are neglected. Similarly, leakage currents through the off state devices is assumed zero. The switching-on and switching-off times of the switch and diodes are neglected.
- The transformer used in the circuit is assumed to be ideal requiring no magnetizing current, having no leakage inductance and no losses.
- The filter circuit elements like, inductors and capacitors are assumed loss-less.
- For the simplified steady-state analysis of the circuit the switch duty ratio (δ), is assumed constant.
- The input and output dc voltages are assumed to be constant and ripple-free. Current through the filter inductor (L) is assumed to be continuous.