Download With the output diode rectifier configuration at the point when the left

With the output diode rectifier configuration at
the point when the left side starts to transition
all the output current is going through Lo and a
corresponding current is flowing through the
primary of T1 ( In reality there will also be
some magnetizing currents but these are
negligible as long as the output current is
relatively large).
As soon as Qa turns off and the voltage at the
Qa source starts to fall, a corresponding
voltage will start to develop across Lr as
indicated by the figure to the left. This will
lead to a di/dt in the primary.
As the voltage across Lr increases as the voltage on QB falls, the di/dt in the primary will
eventually exceed the di/dt that is occurring as a result of the output voltage across Lo and the
transformer turns ratio. When this happens, the voltage across the secondary stays at zero and
current is drawn from both diodes in such a manner as to keep the primary and total secondary
currents balanced.
Once the voltage across Qb reaches zero volts, Qb can turn on and the di/dt in the primary will
now continue to change at a fixed rate as a function of Lr and the input voltage. This will be
accompanied by a similar transition in current from the D1 diode to both diodes and then as the
current in the primary reverses direction. When the primary current has reversed and is now
equal to the instantaneous current in the Lo inductor (taking into account the turns ratio) all the
current will be flowing in the D2 diode and D1 will not be conducting. At this point there will be
a voltage start to build across the secondary of T1 driving the voltage at the anode of D2 high.
This rate of rise will be limited by the capacitive loading on the output (interwinding
capacitance of Lo, diode D1 capacitance and reverse recovery and other parasitics).
The output of the transformer at the anode of D2 should quickly achieve the reflected primary
voltage at which point the rate of rise of current will be controlled by the output inductor and the
output voltage as tne converter is in the power stroke and the leakage inductance is negligible.