Operating Principle of the Quasi-Resonant Converter
For the operation of QR converter there are two main
phases of operation (see Figure 20): a charging phase where
the system behaves as a LR 1st order system and a resonant
phase where the system acts as LRC 2nd order system. QR
converters operate according to a two phase sequence where
during the first phase the coil (Lr) is charged keeping
the switch T1 in an on state and delivering power to the load
due to the current in the inductor. During the second phase
the energy stored in the inductor is transferred to
the resonant capacitor (Cr) and partially dissipated into
the load, which is represented to the bottom layer of the Pot.
The energy dissipated into the resistor is the actual energy
delivered to the load. To evaluate the circuit operation over
both the steady-state and switching portions, we will break
to waveform into four intervals: (0−t0), (t1−t2), (t2−t3), and
(t3−t4). From the previous interval (before the time 0)
the resonant thank was oscillating. At the 0 time on the graph
in Figure 20 the diode D1 is conducting and the gate of T1
is switched off. This continues until time t0. At t0 the current
goes from negative to positive and starts to flow through T1.
Therefore, in the QR converter the turn-on switching losses
are theoretically eliminated, the Miller effect is absent and
the reverse recovery current of the diode flows through T1
without any further losses into the resonant circuit. In the QR
the turn off losses are relevant due to the transition between
the high current and the high voltage. In fact at t1 switch T1
is turned off while the current is still high and this leads to
an overlap with the voltage causing turn off losses into
the device. Also the Miller effect leads to an increase of
the losses. After the turn off of the device the resonant thank
starts to oscillate. This resonant phase can be split in to
intervals t1−t2 voltage across the device positive and current
into the coil positive and t2−t3 voltage across the device still
positive but current into the coil negative.