Tertiary Winding
Figure 1(a).shows the circuit diagram for the forward converter with tertiary winding added to the basic configuration. Such converters are used in computers, word processors, televisions, etc.
Operation of Tertiary Winding
When the transistor Q is turned on, due to the winding polarities, diode Dm is reverse-biased and does not conduct. However, when Q is turned off, Dm is forward-biased and the current flows through the tertiary winding as shown in Fig.1(b).
The residual energy in the transformer core is returned back to the dc source via diode Dm and the tertiary winding. This is how the tertiary winding helps to demagnetize the core and avoids the core saturation.
Due to the dot convention as shown in Fig. 1(a), the primary winding and tertiary winding will never carry current simultaneously. Associated waveforms are shown in Fig. 2. The dark areas on the waveforms of Fig.2 show the magnetizing-demagnetizing current, given as,
Tertiary Winding Advantages
(i) It needs only one power transistor as the switching element:
(ii) Simple driver circuits are required as compared to forwarding converters.
Tertiary Winding Disadvantages
(i) Voltage rating of the power switch is twice the supply voltage Edc.
(ii) Size of the transformer is large.
(iii) If multiple outputs are used, then it gives regulation problems at light loads.
(iv) Costlier compared to Ryback converters.