IEEE Power Electronics Magazine - March 2015 - 37

Output Voltage (Normalized)

input voltage range. The primary resonant frequency is associated with C r and L r , where L r incorporates the leakage
+VIn
inductance of the transformer. At resonance, the on time for
Lr ir
each switch is chosen to complete a resonant half-cycle,
iS
delivering charge to the output. Between each resonant half+
Lm
VOut
cycle, a dead time allows the magnetizing current, which
+
built up during the on time, to charge and discharge the
Vsw
Vx
capacitance of the switch node. As long as the magnetizing
Cr
-VIn
inductance contains sufficient energy and sufficient dead
GND
time is allowed, the circuit will achieve ZVS.
fig 1 The simplified schematic of an LLC converter.
Regulation is achieved by varying the on time above
and below the resonant half-cycle period. This operation
switching frequency, the ac losses, such as skin effect and
is shown in Figure 2. Point A represents the nonregulated
proximity effect, provide a significant increase in transoperation point. At this point, during each on time, a halfformer power dissipation. Because of these effects, the
sinusoid of current flows to the output. At point B, the on
transformer often limits the switching frequency. Howtime is reduced to decrease the output voltage or compenever, using GaN can reduce these transformer losses.
sate for light load or high line voltage. In this case, the magnetizing current decreases, but ZVS may be enhanced by
residual current in the resonant inductance. At point C, the
Performance Enhancements Using GaN Transistors
on time is extended and the conversion ratio is increased,
The switching-related loss of an LLC converter can be detercompensating for a low line voltage or
mined by examining operation at the
high load condition.
dead time. During the converter's
There is a limit on how much regudead time, the magnetizing current
The switching-related
lation range exists around the resocharges or discharges the switching
nant point. Point D represents a connode to achieve ZVS. The relevant
loss of an LLC
dition where the output voltage is no
capacitance at the switch node is the
converter can be
longer rising with respect to a reducsum of the transistors' time-related
tion in switching frequency. In the case
output capacitance and the value of
determined by
shown here, there is hardly any regulathe additional parasitic capacitance at
examining operation
tion range above the nominal point at
this node. The time-related output
full load or during overload conditions.
capacitance, which is the total output
at the dead time.
At this point, full ZVS is no longer
charge Q OSS divided by the voltage
possible, resulting in high switching
VDS , is relevant here as we are looklosses. Loss of ZVS is a known issue
ing for the time taken to charge this
with the LLC converter and has device-level ramifications,
capacitance by a current source. This equivalent capaciwhich are discussed in the next section.
tance, denoted by C SW, is given by
The magnetizing current, which is necessary to achieve ZVS, contributes to the curC SW = 2 ) C OSS (tr) + C par .
rent flowing in the primary side. At low output power, this current can be a significant
portion of the input current and contribute
LLC Regulation Characteristic
significantly to the converter's loss. Reduc1.6
Below
Above
ing the magnetizing will necessitate longer
1.5
Resonance
Resonance
dead times to achieve ZVS and will thus
1.4
increase the currents during the on time for
12% Load
a given load current. The magnetizing induc1.3
25% Load
tance can be optimized for a given imple1.2
C
mentation, switching frequency, and design
50% Load
1.1
power level.
100% Load
A
1
The most difficult design aspect of the
D
LLC power stage is the transformer. When
0.9
B
150% Load
switching at hundreds of kilohertz, numer0.8
0.5
0
1
1.5
2
ous high-frequency effects come into play
while considering the design of this transSwitching Frequency (Normalized)
former. As nearly all the current in the
transformer windings is sinusoidal at the
fig 2 The regulation characteristic of an LLC converter.
March 2015

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