27
September 2016
P
ower
E
lectronics
of ZVS is to use energy stored in the parasitic
capacitance of the switching device along with
inductor current to losslessly commutate the
switch capacitance, instead of hard-switch-
ing where the transistor forces commutation
and dissipates the energy stored in the device
capacitance. Hard switching is very com-
mon in the PFC stage of a power supply. Fig-
ure 1 shows the typical boost converter stage
switching waveform at the moment the tran-
sistor turns on. At t0 the inductor current is
flowing through the diode into the bus capac-
itor. The switch node Vsw voltage is therefore
at the bus voltage. At t0, Q1 is turned on, and
it begins conducting current, which ramps up
to the inductor current IL at t1. Note that the
voltage at the switch node Vsw has not moved
yet. If the diode was perfect and there was no
reverse-recovery charge, then the switch node
voltage would begin to move towards zero at
t1. But if D1 is a PN junction diode (or the
body diode of a synchronous rectifier), then
the diode cannot immediately stop conduct-
ing, so the current in Q1 continues to ramp
up, as does the corresponding reverse current
in the diode. This continues until t2, when the
diode recovers its ability to block voltage, and
stops conducting. At this point, there is a sig-
nificant reverse recovery current on top of the
steady state inductor current, and the transis-
tor Q1 is supporting this peak current while
simultaneously supporting the full bus volt-
age across its drain-source. This leads to the
high peak power dissipated in Q1 during the
turn-on interval, as shown by the P(t) curve
(which is the product of device current times
voltage) peaking at t2.
Finally, between t2 and t3, the current
through Q1 discharges the capacitance of the
switch node and drives the voltage to zero,
thus dissipating the energy stored in the diode
capacitance and Q1’s own self-capacitance.
To summarize, in a hard-switched turn-on,
there are 3 main energy loss mechanisms
each cycle. 1) Commutation or crossover
loss: proportional to current risetime. Faster
turn-on means lower loss. 2) Reverse recov-
ery loss (does not apply for Schottky diode),
depends mostly on the diode characteristic.
Diodes with large Qrr, like the body diode of
a superjunction FET, can have extremely large
Qrr and completely dominate the turn-on loss.
3) Eoss loss: this is the energy stored in the
capacitance of the switch node (including the
switch itself, the diode, and parasitic capaci-
tance in the inductor) that gets dissipatively
discharged each time the switch is turned on.
As a simple example of zero voltage switching
(ZVS) , consider the same boost PFC circuit as
in the previous example, except this time the
control strategy is different. Instead of contin-
uous conduction mode (CCM), the current is
allowed to reach zero each cycle. Of course this
means that the ripple current of the PFC stage
Figure 2. ZVS soft-switching achieved through Critical Conduction Mode CrCM
Figure 3. Charge and energy versus voltage for
both superjunction (blue) and GaN (red). The
dotted lines represent charge (left axis) and the
solid lines represent energy (right axis).
