Here the following Transformers will be introduce
- converter transformer
- power and Distribution Transformer
COVERTER TRANSFORMER:
FORWARD CONVERTER
Input voltage get reflected into secondary side( Vinr=Vin/n where N=NP/Ns) get impressed on the secondary node. From there on we have DC-DC converter with an input Voltage Vinr and an output voltage Vo. secondary winding conducts at the same time as the primary. This lead to almost complete flux cancellation inside the core.
Only magnetization current component remain, irrespective of load. At zero load it is entire current through primary and switch. As soon as we try to draw some load current the secondary winding current increases and so do the primary. Each current increases proportionally to the load current. core never see any change in the net ampere turns.
All the conditions inside the core i.e the flux, the magnetic fields, the energy stored and even the core loss are dependent only on the magnetizing current of course the windings themselves have a different story to relate. windings bear entire brunt , not only of the actual load current but also of the sharp edges and consequent high frequency content of the pulsed current waveform.
Magnetizing current component like a “parallel leakage inductance” we need to subtract this from total switch current , and then only find that the primary and secondary current scale according to the turn ratio.
To achieve a steady state , even a transformer need to ” reset” every cycle (along with the output choke) but magnetization energy is uncouple to secondary because of out put diode direction so we use tertiary winding or other means to neutralize when switch is off. During off period when leakage energy is zero or in circulation (depending on topology) no voltage is present at primary or secondary. energy freewheel in secondary through flywheel diode. Duty cycle of forward converter under no circumstance be remain less than 50%… Transformer always remain in DCM. where as choke is normally in CCM
FORWARD TRANSFORMER
During the non-power delivery cycle of the primary, proper transformer reset time is achieved when the ON time is less than its OFF time (duty cycle is less than 50 percent). In other words, the primary winding itself acts as the reset winding. Having the OFF time longer than the ON time will always reset the transformer.
720w two switch synchronous rectifier
1). the rectifier diode has been replaced by a MOSFET to increase efficiency and to compensate for signal delays that are caused by the leakage inductance of the secondary transformer winding.
The change in inductor current is load independent. The output voltage depends only on the duty cycle and the input voltage, it is load independent
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