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Texas Instruments Third quadrant operation of GaN Application notes
Application Report
SNOAA36 – February 2019
Does GaN Have a Body Diode? - Understanding the Third
Quadrant Operation of GaN
Bingyao Sun
ABSTRACT
Gallium Nitride (GaN) FETs are providing designers with a viable alternative to Si MOSFETs in high power
density applications. This is due to their significant advantages over Si, including small junction
capacitance, lack of body diode, and no reverse recovery loss. These advantages enable more efficient
and compact power converter designs, driven by a higher switching frequency. Nowadays, the major
commercial GaN FETs are lateral high-electron-mobility transistors (HEMT). Without the p-n-doping drift
region in the structure, GaN illustrates unique characteristics in the third quadrant operation. This
application report provides a detailed explanation of the diode-like behavior of GaN in reverse current
conduction, based on the study on its lateral structure. The third quadrant operation naturally happens
when GaN is applied as rectifiers. This application report also shows how to minimize the third quadrant
loss of GaN FETs used as rectifiers by leveraging a short dead time or adaptive dead time control.
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Contents
Introduction ...................................................................................................................
Mechanism of Reverse Conduction in GaN FETs.......................................................................
Operation and Loss Estimation in the Third Quadrant ..................................................................
How to Minimize the Dead Time Losses .................................................................................
Summary ......................................................................................................................
References ...................................................................................................................
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6
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List of Figures
1
Bridgeless Totem-Pole PFC Using GaN as the High-Frequency Switching and Si MOSFET as the Rectifier.. 2
2
Cross Section of the Lateral Structure of GaN FETs ................................................................... 2
3
Conditions to Turn on the Channel in the Forward and Reverse Conduction ....................................... 3
4
Simplified Behavior of GaN in the First and Third Quadrant ........................................................... 3
5
The Third Quadrant Operation in the Boost Converter ................................................................. 4
6
Illustration of Adaptive Dead-Time Control ............................................................................... 5
7
Implementation of Adaptive Dead Time Control in TIDM-1007 ........................................................ 5
8
Power Saving with Adaptive Dead Time in TIDM-1007
................................................................
5
List of Tables
Trademarks
1
Introduction
GaN is enabling higher power density and higher efficiency power converters. In contrast to Si MOSFETs,
GaN offers faster switching speed, smaller output charge, and lower switching losses. Unlike a MOSFET,
there is no p-n junction within the lateral structure of a GaN FET and no body-diode and no reverse
recovery charge (Qrr). By eliminating the reverse recovery losses, GaN enables efficient high switching
frequency operation in the hard-switched topologies. For example, in the bridgeless totem-pole power
factor correction (PFC) under continuous conduction mode (CCM), using superjunction MOSFETs are not
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Mechanism of Reverse Conduction in GaN FETs
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practical due to their high reverse recovery losses. As shown in Figure 1, by using GaN FETs as the highfrequency switches, the reverse recovery loss is fully eliminated and the switching-related loss is
significantly reduced. The inverter for AC drive is another beneficial topology, where the low switching loss
and lack of reverse recovery loss downsize the heat sink in the compact servo drives and integrated motor
drives.
High- LowFreq. Freq.
S1
S3
Rload
VAC
S2
S4
Figure 1. Bridgeless Totem-Pole PFC Using GaN as the High-Frequency Switching and Si MOSFET as the
Rectifier
Third quadrant operation occurs for a power MOSFET when the current flows from the source to the drain
terminal through the body diode or the channel. Although GaN FETs have no body diode, the symmetry of
the device helps conduct in the third quadrant with diode-like behavior. Similar to the situation with the Si
MOSFET, it is recommended not to add an antiparallel diode with the GaN FET to conduct the reverse
current. Adding an antiparallel diode adds output capacitance to the switch node and increases switching
losses. Instead, the third quadrant losses can be minimized by optimizing the dead time.
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Mechanism of Reverse Conduction in GaN FETs
The lateral GaN structure is comprised of a Source and Drain connected by a two-dimensional electron
gas (2DEG) channel. The gate voltage controls the conductivity of the channel. Figure 2 shows the
simplified cross-section of the lateral GaN structure, illustrating the symmetry of the channel region
between the source and the drain. In third quadrant operation, the drain and the source switch positions.
The drain potential is lower than the gate turning on the GaN device and allows the reverse conduction
without a body diode.
Figure 2. Cross Section of the Lateral Structure of GaN FETs
The condition to turn on the channel in the forward conduction (the drain to the source) is that the gate-tosource voltage, Vgs, is higher than the threshold voltage Vth, as Figure 3 illustrates. With the channel on,
Vds can be calculated using Equation 1.
2
Does GaN have a body diode? - Understanding the third quadrant operation
of GaN
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Operation and Loss Estimation in the Third Quadrant
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D
Vgd
D
G
G
Vgs
S
S
Vgs>Vth
Vgd=Vgs+Vsd>Vth
Figure 3. Conditions to Turn on the Channel in the Forward and Reverse Conduction
Vds= IdsRon
where
•
Ron is the on-resistance of the channel in the first quadrant
(1)
When Vgs is smaller than Vth, the current cannot conduct in the first quadrant, but can flow in the third
quadrant. When the current flows in the reverse direction (third quadrant), the drain and source terminal
can swap termination. The condition to turn on the channel for reverse conduction is the gate-to-drain
voltage Vgd is higher than the threshold voltage, Vth. Like a diode, the channel in GaN FET selfcommutates and Vsd is self-biased so that Vgd can reach Vth and the channel conducts the reverse current.
Under this operation, Vsd is biased by Vth- Vgs and increases with the reverse current, Isd as Equation 2
illustrates. In Equation 2, Ron_reverse, the equivalent on-resistance in the third quadrant with the off gate is
dependent on Isd as the device operates in the saturation region. It is recommended to refer to IV-curve in
the datasheet to estimate Vsd in the third quadrant. When the gate is on during reverse conduction, Vgd,
equals to (Vgs + Vsd), which is higher than Vth, enabling the channel fully on with the same on-resistance as
Ron in the first quadrant.
Vsd ≈ (Vth - Vgs) + IsdRon_reverse
where
•
Ron_reverse is the equivalent on-resistance in third quadrant with the off gate
(2)
Compared with a Si MOSFET body diode, GaN FETs have higher Vsd drop as Vth - Vgs is usually higher
than 0.7 V when the gate is off. This may induce a higher third quadrant loss of GaN over Si MOSFET.
Figure 4 shows the simplified behavior of the GaN FET in the first and the third quadrant. The analysis
above is true for both enhancement and depletion mode GaN FETs.
Figure 4. Simplified Behavior of GaN in the First and Third Quadrant
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Operation and Loss Estimation in the Third Quadrant
To further explain the operation, a synchronous boost converter using GaN is shown inFigure 5 as an
example. When S1 turns off, S2 goes into the third quadrant operation. Before S2 turns on, S2 operates like
a diode, following the behavior in the red curve in Figure 4 in the third quadrant. After the dead time, td2,
the S2 channel is on and S2 behaves following the blue curve in Figure 4 in the third quadrant. After S2
turns off, S2 returns to the diode behavior in the duration of td1.
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3
How to Minimize the Dead Time Losses
iL
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Vds_S2
+
Vout
Vds_s2
Vin +
S1
S2
Vout
iL
Vrev1
Vrev2
S1 on
td1
S2 on
td2
Figure 5. The Third Quadrant Operation in the Boost Converter
The difference of the behavior above from Si MOSFET is a high Vds drop, Vrev1, and Vrev2 during the dead
time td1, td2. Use Equation 3 to calculate the dead time loss.
Pdt = fswtd1Vrev1iL_vl + fsw(td2 - tr)Vrev2iL_pk
where
•
•
•
•
fsw is the switching frequency
tr is the rising time for S1 Vds
iL_pk, iL_vl are the peak and valley inductor current respectively
Vrev1 and Vrev2 are the corresponding third quadrant voltage drop under iL_vl, iL_pk
(3)
For the hard-switched edge, the dead time is determined by the input charge, the gate driver propagation
delay and mismatch. For the soft-switched edge, the dv/dt slew rate is another main constraint.
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How to Minimize the Dead Time Losses
Designers have a number of options to minimize the dead-time losses of GaN.
• Select a Proper Turn-Off Gate Voltage for GaN.
Section 2 explains that Vsd during dead time is a function of the turn-off gate voltage. The turn-off gate
voltage can be optimized between the dead-time loss and the turn-off speed. In the hard-switched half
bridge, the turn-off gate voltage must be low enough to avoid a false turn-on when the opposite switch
turns on. This is internally optimized for the designers for integrated GaN solutions such as the
LMG3410R070.
• Minimize the Dead Time.
Minimizing the dead time is the most straightforward solution. The dead time can always be well-tuned
based on the maximum dead time needed. For example, in the reference design of 1 MHz 1 kW High
Efficiency and High Power Density Resonant Converter using TI GaN LMG3410R070, the dead time is
100 ns and the dead time loss on GaN under full power is only 0.2 W, taking only 0.7% of total loss.
• Adaptive Dead-Time Control
For the topologies like PFC, adaptive dead-time control is more beneficial. In PFC, the inductor peak
current varies and the turn-off dv/dt slew rate is not constant in each switching cycle. The fixed dead
time is determined by the worst case with the lowest dv/dt slew rate, resulting in higher dead-time
losses in the operation with a higher dv/dt slew rate.
The principle of the adaptive dead-time control is demonstrated in Figure 6 where the optimum dead
time is almost equal to the switch-node capacitor charging time. The optimum dead time is dependent
on the inductor current and switch node capacitance and requires real-time correction in each
switching cycle. By doing so, the ‘diode’ operation period, shown as tdiode in Figure 6, can almost be
eliminated.
4
Does GaN have a body diode? - Understanding the third quadrant operation
of GaN
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SNOAA36 – February 2019
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How to Minimize the Dead Time Losses
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tdiode is too large
Current sampling point
Sync FET turns on too late
Vds_active
iL
Active FET on
Sync FET on
Optimal td
Figure 6. Illustration of Adaptive Dead-Time Control
Generally, there are two types of implementation. With digital control, the dead time for the next switch
event can be calculated based on the operating conditions. For example, in the 3.3 kW GaN-based totempole CCM PFC TIDM-1007 with C2000TM MCU, the dead time is calculated using Equation 4 before the
synchronous rectifier GaN FET turns on. Figure 7 shows the diagram of its implementation. In TIDM-1007,
at high line 230 Vrms, the power loss savings using the adaptive control compared with a fixed dead time
is shown in Figure 8.
td = CswVo / iL,AVG
where
•
•
•
•
td is the minimal dead time for the next switching event
Csw is the total parasitic capacitor from the switching node to ground
Vo is the output voltage
iL,AVG is the sampled inductor current in the current cycle
Senseing
iL, AVG
Absolute
value function
iL, AVG
GLL, AVGG
Limitter
A.D.T.
Calculation
A.D.T
(4)
Limitter
2COSS VOUT
iL,AVG
Update
ePWM DBRED
DBRED = A.D.T
Figure 7. Implementation of Adaptive Dead Time Control in TIDM-1007
1.4
Power Savings (Watts)
1.2
1
0.8
0.6
0.4
0.2
0
500
1000
1500
2000
2500
Power (Watts)
3000
3500
Figure 8. Power Saving with Adaptive Dead Time in TIDM-1007
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Summary
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With analog control, the adaptive dead-time control is implemented in some controllers for synchronized
rectifiers. The switching mode voltage Vsw is monitored and compared with a predetermined threshold
voltage to provide a detection signal to turn on the synchronized rectifier. A short period, like 12 ns to 15
ns, may be needed between the detection signal and the turn-on signal to leave enough dead time. See
the UCC24612 High Frequency Synchronous Datasheet for a design example.
5
Summary
The lack of p-n junction of the GaN HEMT in the lateral structure eliminates the body diode and reverse
recovery loss, which significantly reduces the switching loss in the hard switched topologies, such as the
bridgeless totem-pole CCM PFC. To conduct current in the reverse direction, GaN FET self-commutates
and behaves like a diode due to the symmetry of the lateral structure, but tends to have a large Vsd. With a
short dead time, the dead-time losses can be minimized. Adaptive dead-time control is an effective way to
further reduce the loss by correcting the dead time based on the operating condition.
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References
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•
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E. A. Jones, F. Wang, and B. Ozpineci. (2014) Application-based review of GaN HFETs. IEEE
Workshop on Wide Bandgap Power Devices and Applications, pp. 24-29.
https://ieeexplore.ieee.org/document/6964617
Edward Andrew Jones. (2016) Review and Characterization of Gallium Nitride Power Devices.
University of Tennessee. https://trace.tennessee.edu/utk_gradthes/4048/
Does GaN have a body diode? - Understanding the third quadrant operation
of GaN
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