Subject : Power Electronics
Unit : DC to DC Converters
Selection of Transformer Turns Ratio, Filter Circuit Inductor and Capacitor
Selection of Transformer Turns Ratio:
- The primary to secondary turns ratio of the transformer is decided in accordance with
.
- For the required output voltage (V_{O}), the turns ratio N_{S}/N_{P} is found after considering the minimum magnitude of input supply voltage () and the maximum allowable duty ratio (δ).
- The maximum duty ratio of the converter, as discussed above, is constrained by the primary to tertiary winding turns ratio but the choice of primary to tertiary winding turns ratio is often governed by the voltage stress that the switch must withstand.
- Higher voltage stress will mean higher cost of switch. If the tertiary winding turns is kept very high, the switch voltage stress reduces but allowable duty ratio of switch and the power output of the converter becomes low and diode ‘D_{3} ’ voltage rating increases.
- Thus an optimum design needs to be arrived at to maximize the performance of the converter.
Selection of Filter Circuit Inductor and Capacitor:
Fig: Typical waveform of a practical Forward converter circuit
- The transformer’s secondary voltage is rectified and filtered suitably to get the desired quality of output voltage waveform. The filter inductor and capacitor values need to be chosen optimally to arrive at a cost-effective, less bulky power supply.
- Mean (dc) value of inductor current equals the load current. The filter capacitor merely supplies the ripple (ac) current of switching frequency. Linear relation between the output voltage and the switch duty ratio, the inductor current is desired to be continuous.
- In case the inductor current becomes discontinuous the linearity between switch duty ratio and output voltage is lost and the output-voltage controller circuit, which is often designed using linear control theory, is not able to maintain the desired quality of output voltage.
- Hence filter inductor should be chosen to be sufficiently large such that under expected range of load current variation, the inductor current remains continuous.
- In many cases the minimum value of load current may not be specified or may be too low. If the load connected to the output is very light or if there is no load, the inductor current will not remain continuous.
- Hence, as a thumb rule, the filter inductor size may be chosen such that the inductor current remains continuous for more than 10% of the rated load current. At 10% of the load, the inductor current may be assumed to be just continuous.