Transformer with Unipolar Flux
Transformer with Unipolar Flux:
Fig: Winding voltage and core-flux waveforms for a forward type SMPS supply
- Many switched mode power supply circuits use only one controlled switch. The winding current and core-flux for most of these transformers are unidirectional.
- The figure shows the typical winding voltage along with the corresponding core-flux waveform for a forward converter.
- When the forward converter switch is turned on the primary winding is subjected to input dc voltage.
- As soon as the primary winding is turned-off, the tertiary winding starts conducting and the voltage across primary goes negative with a magnitude that equals the product of input voltage and the turns ratio between the primary and tertiary windings.
- The maximum duty ratio (Dmax) of the switch is also limited by the turns ratio between the primary and tertiary winding to allow resetting of the transformer flux.
- The maximum input voltage (Vmax), switching frequency ‘f’(=1/T) and the maximum duty ratio (Dmax) are related with the peak magnitude of core-flux is calculated as:
Vmax Dmax = f φm NP = f Bm Ac NP
- This equation may be compared with the equation for the peak magnitude of flux in the transformer core for a typical value of Dmax = 0.5 (which corresponds to the case when primary and tertiary windings have identical number of turns). Because of unipolar nature of flux the utilization of core (in terms of emf generation) is poorer here.
- The primary to secondary turns ratio (NP/ NS ) for the forward converter can be estimated as the H-bridge converter. Accordingly, NP/ NS = Vmin Dmax /(Vo VR), where Vo is the required output voltage and VR denotes the voltage drop in output rectifier and filter circuit.
- The maximum rms current through the secondary winding can be equated to and the window area (Aw) requirement is given by
- We get, the VA rating of the transformer is given as: , which may be rewritten as
- The symbols used also denote the same. Knowing the window area, the transformer core selection and other designs are done as described in connection with the H-bridge topology.
- The extra tertiary winding of a forward converter transformer carries only magnetization current, which is a quite small and even a thin gauge wire will serve the purpose.
- However, with the addition of tertiary winding the insulation requirement of the transformer increases significantly and hence the window utilization factor (kw) becomes low.