CHARACTERISTIC ANALYSIS OF SIC MOSFET AND RESEARCH ON THE DRIVE CIRCUIT FOR SIC MOSFET
Peng Mao*, 1 Ru-Ru Mei1 Mao Zhang2 Wei-Ping Zhang1
1Department of Information Engineering North China University of Technology
Beijing, China 100144, C. N.
2Department of of Automation
Beijing Information Science and Technology University Beijing, China 100192, C. N.
Key Words: SiC MOSFET, drive circuit, crosstalk, LTSpice.
ABSTRACT
The differences on the material properties of silicon carbide (SiC) and silicon (Si) are compared in this paper. The electrical characteristics of MOSFET based on SiC and Si are also analyzed, and the switch trajectory is obtained by the simulation. A model is proposed in this paper to analyze the influence of drive resistance during the switching process. Then, the drive circuits suitable for typical circuits such as single-switch power circuit, bridge power circuit and dual-switch forward circuit adopting SiC MOSFET are obtained. In addition, the crosstalk problem in the bridge circuit is also analyzed and four improved auxiliary circuits are provided. Their advantages and disadvantages are compared by simulation. Some key tips about the layout design of the SiC MOSFET drive circuit are also proposed.
I. INTRODUCTION
The power semiconductor devices have a critical im- pact on switching power converters. Conventional power semiconductor devices based on silicon materials are sub- ject to many limitations, such as low withstand voltage and low temperature resistance. The emergence of silicon car- bide (SiC) material allows the solution of this problem to no longer be limited to changing the device structure. At pre- sent, the SiC power devices with high withstand voltage and high temperature resistance are mostly used in the aerospace power systems. The volume and weight of solid-state transformers made of SiC devices in the new smart grid sys- tem have been reduced and SiC power devices have also been added to the power control unit of new energy vehicles.
SiC material can improve the power density of switching converters and also miniaturize electronic devices.
In this paper, first, the material properties of SiC and Si are compared. Second, the electrical properties of MOSFETs made from two materials are analyzed, and the switching trajectories are obtained by simulation. Then, the suitable drive circuits of SiC MOSFET for different power converters are studied and LTSpice simulation is used for the bridge circuit. By comparing the advantages and disadvantages, some key tips for the drive circuit layout are provided. The structure of the paper is shown in Table 1.
II. CHARACTERISTICS COMPARISON OF SIC MOSFET AND SI MOSFET
1. Physical characteristics comparison between the SiC material and Si material
The comparison between the physical characteristics
*Corresponding author: Peng Mao, e-mail: maopeng@ncut.edu.cn
Table 1 Structure of the paper
Characteristics comparison between the SiC MOSFET
and Si MOSFET
Comparison between the physical characteristics of the SiC material and Si material Characteristics analysis of SiC MOSFET
Effect of drive resistance on the switching characteristics of SiC MOSFET
SiC MOSFET drive circuit
For single-switch power con- version circuit
Direct drive circuit Isolated drive circuit
Drive circuit consisting of the integrated chip
For bridge power conversion circuit
Analysis of the crosstalk problem
Solutions of the crosstalk problem
Auxiliary circuit 1 Auxiliary circuit 2 Auxiliary circuit 3 Auxiliary circuit 4 For dual-switch forward power conversion circuit
Layout design of the drive circuit
Fig. 1 Comparison between the physical characteristics of the SiC material and Si material
of the SiC and Si materials is shown in Fig. 1. The band ap of the SiC material is about three times larger than that of the Si material, such is the case for the SiC material’s thermal conductivity as well. The insulation breakdown field strength of the SiC material is about ten times higher than the Si material’s. The saturated electron drift velocity of the SiC material is twice that of the Si material. The physical expression equation for the MOSFET's on-re- sistance [1] is given by Equation (1)
2
, 3
0
4 B
on min
r T
R u
ε ε μE
= (1)
where ݑ, ߤ and ܧ் represent the breakdown voltage,
Fig. 2 Test circuit
carrier mobility and breakdown electric field strength of the material, respectively. It’s shown in Equation (1) that the on-resistance of the SiC material is much smaller than that of the Si material in the same voltage and current levels.
This unique physical property does not only help reduce switching losses but also enables the SiC material to operate in temperatures of up to 1000K. Thus, it’s the first choice for high-power, high-voltage device manufacturing applica- tions.
2. Characteristics analysis of the SiC MOSFET In this paper, Cree’s SiC MOSFET C2M0280120D and STMicroelectronics’ Si MOSFET STW11NM80 are taken as examples. The test circuit shown in Fig. 2 is used [2-3], and the switching trajectories of the SiC MOSFET and Si MOSFET simulated with LTSpice are obtained.
The transfer characteristic curves of MOSFETs with dif- ferent materials are shown in Fig. 3. At normal temperature,
g
Fig. 3 Comparison of transfer characteristic curves
Fig. 4 Switching trajectories of SiC MOSFET and Si MOSFET
the threshold voltage of silicon-based MOSFET is 4 ~ 6V, while that of SiC MOSFET is only about 2V. However, because of the small negative temperature coefficient of the SiC MOSFET threshold voltage, the threshold voltage would further decrease as the temperature increases. Thus, the gate-source would be highly susceptible to voltage ring,
Fig. 5 Driving equivalent circuit of the gate-source
so a false turn-on phenomenon may occur with the increase of temperature. Therefore, the negative gate voltage is usually used in practice when turning off MOSFET. The switching trajectories of SiC MOSFET and Si MOSFET are illustrated in Fig. 4. As it can be clearly seen in Fig. 4 that the SiC MOSFET has a smaller transconductance and the body diode has a smaller reverse recovery current, thus the switching loss of the entire device is greatly reduced.
When the gate voltage ugs is 12V, C2M0280120D can be turned on. In order to get smaller on-resistance, the gate voltage ugs should be further increased to reduce the con- duction loss. The gate drive voltage recommended in the manual is generally -5V / 20V.
3. Effect of drive resistance on the switching character- istics of SiC MOSFET
In addition to the material used in the device affecting the switching characteristics, the effects of parasitic param- eters are also significant. The researchers have analyzed the effect of parasitic inductance on the switching process of SiC MOSFET through experimental methods. Based on the typical ambipolar diffusion equation, the analytical model is established to extract the parasitic capacitance pa- rameters of SiC MOSFET and analyze the effect of parasitic capacitance on the switching process of SiC MOSFET.
Moreover, the selection of the drive resistance Rg has a cer- tain effect on the switching speed of SiC MOSFET.
An analysis model using a typical RLC series second- order circuit to study the effect of the drive resistance Rg is proposed in this paper as shown in Fig. 5.
According to the circuit theory, when the drive re- sistance satisfies the following condition, the damping co- efficient of the circuit is greater than 0.707. When it is equal to 0.707, the voltage across the equivalent capacitance Ciss may appear to slightly overshoot, but the rise slew is faster.
1.4 g
g
iss
R L
≥ C (2)
In fact, Ciss is a variable. A different Rg value is selected
d
gs
(a)SiC MOSFET switching trajectory
(b) Si MOSFET switching trajectory
gs
gs
gs gs gs gs gs gs gs gs
d d
ds
ds
g g
gs iss
Fig. 6 Effects of Rg on ugs, uds and id Turn on / Turn off
to simulate the effects of ugs, uds and id when SiC MOSFET is turned on and off in this paper. The simulation waveforms are shown in Fig. 6. The smaller the Rg, the more seriously the voltage across the gate-source rings, but the turn-on and turn-off times are shorter. So the switching loss is smaller.
The larger the Rg, the longer the turn-on and turn-off times.
III. SIC MOSFET DRIVE CIRCUIT
By comparing the electrical characteristics the of sili- con-based IGBT and SiC MOSFET, the design require- ments for the SiC MOSFET drive circuits are proposed in reference [4-6]. Considering the differences in electrical characteristics between the SiC MOSFET and Si MOSFET, the distinguished difference in the drive circuit is the drive level. The traditional Si MOSFET ugs is usually 0 / 15V, while the SiC MOSFET ugs is generally -5 / 20V. In addi- tion, SiC MOSFET is mostly used in high-frequency switching circuits, and the influence of parasitic parameters is more significant. Therefore, it is also necessary to con- sider the PCB layout of the drive circuit. The following three drive circuits using SiC MOSFET and their basic lay- out tips are mainly discussed in this paper.
1. For single-switch power conversion circuit i. Direct drive circuit
When designing a drive circuit for a single-switch power converter, reducing the switching losses and switching noise is the primary purpose. At present,
Fig. 7 Direct drive circuit and gate drive waveform
the most widely used is the totem-pole structure output drive circuit. This structure output greatly reduces the switching losses while enhancing the drive capa- bility. The direct drive circuit and drive waveform
g=
g=
g=
g=
g=
g=
ugs/ V ugs/V
uds/ V uds/ V
i /A i /A
-1A 11A 8A 5A 2A
304.88μs
t / (120ns / grid) t / (120ns / grid)
305.00μs 305.12μs 305.24μs 305.36μs -1A 11A 8A 5A 2A
+Vcc
-Vcc R1
R2 C1 D
C2
1
2
g drive
(a) Direct drive type circuit
(b) Gate drive waaveform -6V
0V 6V 12V 18V
t / (1600ns / grid) -2V
351.2μs 352.8μs 354.4μs 356.0μs 357.6μs 359.2μs 360.8μs 9V
28V
Fig. 8 Transformer isolated drive circuit and gate drive waveform
commonly used in single-switch power conversion cir- cuits are shown in Fig. 7. The drive resistance is in parallel with a diode, where R1 should satisfy Equation (2) in order to optimize the turn-on process. When turned off, the diode D is turned on and the drive re- sistance becomes smaller. Thus, the MOSFET turn- off process is accelerated and the turn-off loss is re- duced. An optimized drive circuit is proposed in ref- erence [7-9], which accelerates the switching process of the SiC MOSFET to reduce the switching losses by adding a boost module to the gate loop.
ii. Isolated drive circuit
Direct drive circuits are suitable for Boost type converters, single-ended forward and flyback convert- ers, and so on. For Buck type converters, the isolated drive circuit is required. As shown in Fig. 8, the transformer winding can realize electrical isolation.
The turns ratio of the original side and secondary side determines the voltage level. Furthermore, the totem- pole structure output is used in the secondary side to increase the drive capability while making energy and transit the signal separately. When the drive level is high, the drive signal is amplified by the transformer.
Fig. 9 Integrated driver module of SiC MOSFET
Fig. 10 Bridge leg in the bridge circuit
Hence, T2 is turned on, and the voltage across the gate and source is high level. In addition, the ratio of C1
and C2 can be adjusted to obtain different positive and negative gate voltages, which satisfy the drive level re- quired by SiC MOSFET. Compared with the direct drive circuit of Fig. 8, although a negative gate voltage can be generated, delays still exist when turning off the drive circuit, and the delay time is about 200 ns.
iii. Drive circuit consisting of the integrated chip For the great capability SiC MOSFET, the drive circuit consisting of the integrated chip is usually used.
In addition to the integrated module driver’s strong SPW M D1
R1
T1
N:1 T VCC
D3
D2
R2
T2
T3
C1 C2
g
s
(a) Transformer isolated drive circuit
(b) Gate drive waveform
0V 2V 4V 6V 8V 10V
t / (1200ns / grid)
-6V 6V 0V 12V 18V
DC/DC DC/DC
+12V +18V
-5V
DC+
DC+
IXDD614
IXDD614 ISO7842
ISO7842 PWM Enable
HS-pwm Overcurrent
Detection
Fault detection
PWM Enable
Overcurrent Detection LS-pwm
Voltage Detection
Voltage Detection +18V
-5V
gd1
gs2 gd2
Cgs1
VCC
Turn on
Turn off Q2 Q1
L
Fig. 11 Improved auxiliary circuit
anti-interference ability, its rapidity is also a reason for its popularity. For the half-bridge module consisting of SiC MOSFET, a high-speed integrated driver mod- ule CGD15HB62LP is introduced by CREE, as shown in Fig. 9. It consists of the isolated DC / DC converter, digital isolator ISO7842 and fast MOSFET driver chip.
It has two independent output channels, which can be used as the drive bridge circuit or be treated as an ordi- nary drive circuit. The module is powered by 12V and can output a +18V / -5V drive square wave with up to 115 kHz switching frequency. The input terminal includes the power supply pin, enable pin and PWM signal pin. Compared with the general drive circuit, fault detection, short circuit protection and under-volt- age protection is also integrated in this module.
Moreover, plug and play technique is also adopted in the design to reduce the influence of the parasitic in- ductance in the gate loop. Consequently, it is widely used in the industry.
2. For bridge power conversion circuit i. Analysis of crosstalk problem
For the bridge circuit, there is a crosstalk problem
between the upper and lower switches of the same bridge arm [10-16]. Due to the low threshold voltage of the SiC MOSFET, the forward voltage spike caused by crosstalk may cause the switch to be turned on mistakenly. At the same time, the negative gate source voltage maximum rat- ing is small, and an excessive negative gate voltage spike may breakdown the switch. Therefore, when designing the drive circuit suitable for the bridge converter, how to suppress crosstalk voltage spikes needs to be considered.
In response to this problem, many common suppression methods have been proposed at home and abroad, such as increasing drive resistance, gate-source in parallel with capacitance, negative gate voltage turn off, and ac- tive Miller clamping. Although these methods have a certain effect on suppressing negative gate voltage spikes, the switching speed is slowed down and the switching loss is increased by them. A bridge arm of the dual active bridge circuit is taken as an example in this paper as shown in Fig. 10. The four optimized drive circuits shown in Fig. 11 are compared. For the simplifi- cation of analysis, only the influence of the voltage spike generated on the lower switch Q2 when the upper switch Q1 is turned on and off is considered in this paper.
g
C1 C2
(a) Auxiliary circuit 3
(c) Auxiliary circuit 3 (d) Auxiliary circuit 4
(b) Auxiliary circuit 4 g
R1
R1
R2
R3
R3
D
g
s D
g
s R2
C1
C2
C1
R2
R1
R3
R2
R1
Table 2 The maximum of positive and negative spikes in different drive circuits
Drive circuit type Positive voltage spike at turn-on Negative voltage spike at turn-off
Traditional drive circuit 1.72V -17.3V
Auxiliary circuit 1 -3.4V -5.9V
Auxiliary circuit 1 -3.19V -8.23V
Auxiliary circuit 1 0.67V -6.98V
Auxiliary circuit 1 2.69V -6.90V
Table 3 The advantage and disadvantage of different drive circuits
Drive circuit type Advantage Disadvantage
Traditional drive circuit Crosstalk spike is too large
Auxiliary circuit 1 Crosstalk spike is too large Negative voltage source is used Auxiliary circuit 1 Simple circuit structure Slower switching speed
Auxiliary circuit 1
Passive devices generate negative gate voltage instead of negative
voltage source
Forward voltage spike suppression effect is general
Auxiliary circuit 1 Positive voltage spike suppression is better
Reduce the switch turn-off speed to suppress the negative voltage spike
ii. Solutions of crosstalk problem (1) Auxiliary circuit 1
The influence of crosstalk is reduced by the auxiliary circuit 1, which connects two BJTs in parallel with the drive resistance R1 to realize the clamping effect. When the upper switch Q1 is turned on, the voltage at the center of the bridge arm rises sharply, forming a charge circuit with Cgd2 and R1 of the lower switch Q2. Thus, the T2 is turned on by the voltage drop across R1, and the voltage across the Q2 gate- source is maintained the turn-off gate voltage by the clamping effect of the negative gate voltage source.
Consequently, the influence of the positive crosstalk voltage spike is reduced. When the upper switch Q1
is turned off, a discharge circuit consists of Cgd2 and R1
of the lower switch Q2. Therefore, the T1 is turned on by the voltage drop across R1, and the voltage across the gate-source would be clamped at the turn-off gate voltage. As a result, the negative crosstalk voltage spike is reduced.
(2) Auxiliary circuit 2
In the auxiliary circuit 2, when the upper switch Q1 is turned on, the T1 in the lower switch drive circuit is in off state, and the voltage across the gate-source is
clamped at the turn-off gate voltage. Moreover, the spike current caused by the voltage rise at the center of the bridge arm flows through R2 and C2. Hence, the value of equivalent capacitance Cgs2 is lager, the switching speed is slowed down, and the amplitude of the positive interference voltage across the gate-source of the lower switch is reduced. When the upper switch Q1 is turned off, T1 in the lower switch drive cir- cuit operates in the saturation state, and the spike cur- rent caused by the voltage drop at the center of the bridge arm flows through R1 and C1. What's more, the value of equivalent capacitance Cgs2 becomes larger, the switching speed is slowed down, and the amplitude of the negative interference voltage at the gate-source of the lower switch is reduced.
(3) Auxiliary circuit 3
The negative gate voltage is generated by R2, R3
and C2 in the auxiliary circuit 3. When the upper switch Q1 is turned on, the positive voltage spike caused by the crosstalk would be suppressed by the negative gate voltage generated by C1 in the lower switch drive circuit. Nevertheless, the effect is not significant. When the upper switch Q1 is turned off, a part of the current in the lower switch drive circuit
Fig. 12 Simulation waveform of improved drive circuit
flows through R3, and T1 is turned on. In addition, C2
is connected to the circuit, and the value of C2 is much larger than Cgs2. Thus, the low impedance discharge circuit is provided for the load current, and the negative crosstalk voltage spike is suppressed.
(4) Auxiliary circuit 4
The auxiliary circuit 4 is proposed based on the im- provement of active Miller clamp circuit. When the upper switch Q1 is turned on, the spike interference cur- rent flows through the Cgs2 and R1 of the lower switch Q2, and the voltage drop generated causes T1 to be turned on.
Moreover, C1 is connected to the circuit, and the value of C1 is much larger than Cgs2. In consequence, the low impedance discharge circuit is provided for the
current of Cgs2, and the positive crosstalk voltage spike is suppressed. When the upper switch Q1 is turned off, T1 in the lower switch drive circuit is in off state and T1
in the upper switch drive circuit is turned on. Addi- tionally, C1 is connected to the circuit, and the value of equivalent capacitance Cgs2 becomes larger. As a re- sult, the upper switch turn-off speed is slowed down and the negative crosstalk voltage spike is suppressed.
The data in Table 2 and Table 3 can be obtained from the simulated waveform shown in Fig. 12. Compared with the traditional drive circuit, these four optimized drive cir- cuits have a certain suppression effect on the crosstalk spike. The auxiliary circuit 1 has the best suppression effect, but a negative voltage source should be used in the circuit.
-9V 15V 21V
9V 3V -3V -9V
15V 21V
9V 3V -3V
-9V 15V 21V
9V 3V -3V
-9V 15V 21V
9V 3V -3V
-9V 15V 21V
9V 3V -3V
-9V 15V 21V
9V 3V -3V 282.0μs
t / (400ns / grid)
(b) Auxiliary circuit 2 simulation waveform (a) Auxiliary circuit 1 simulation waveform
(d) Auxiliary circuit 4 simulation waveform (c) Auxiliary circuit 3 simulation waveform
-9V 15V 21V
9V 3V -3V
g1,s1 g1,s1
V g1,s1 V(g1,s1)
282.4μs 282.8μs 283.2μs 283.6μs 284.0μs
t / (400ns / grid) t / (400ns / grid)
282.8μs 282.0μs 282.4μs 282.8μs 283.2μs 283.6μs 284.0μs 282.0μs
t / (400ns / grid)
282.4μs 282.8μs 283.2μs 283.6μs 284.0μs
V(g2,s2) V(g2,s2)
g2,s2 g2,s2
Fig. 13 Drive circuit of dual-switch forward converter
Switching speed is slowed down in the auxiliary circuit 2.
The positive suppression effect of the auxiliary circuit 3 is not significant. The suppression effect of the auxiliary circuit 4 is significant, but the switching speed of the power switch is reduced and the switching loss increased.
3. For dual-switch forward power conversion circuit In the dual-switch forward converter, the bridge arm consists of the switch and the diode. Furthermore, there is no short circuit problem of the upper and lower switches compared to the bridge circuit, and its reliability is high.
The common drive circuit is shown in Fig. 13. The front stage of the transformer is added to the totem-pole amplifier circuit to increase the current drive capability. The sec- ondary coil of transformer with 20V high level and -20V low level is produced by PWM signal. When the auxiliary circuit input terminal is 20V, the BJT is off and the voltage across the gate-source is 20V. When the auxiliary circuit input terminal -20V, the BJT is on, and the voltage across the gate-source is clamped to -5V by the zener, satisfying the SiC MOSFET drive level requirement.
4. Layout design of drive circuit
The Layout design has a critical impact on perfor- mance the switching power supply. As mentioned above, the voltage ring would be caused due to improper selection of Rg value. The voltage or current ring would also be caused by the inductance generated by the lead. In order to avoid this problem, the following layout tips are given to reduce influence of the distributed inductance.
The point of the source is used mostly as the reference in the drive circuit. The large induced voltage would be generated by the fast changing di / dt in the gate-source loop.
This value may reach the threshold voltage of the SiC MOSFET, which may cause the device to be turned on mis- takenly or damaged. Therefore, when designing the lay- out of SiC MOSFET drive circuit, it is recommended to use
Fig. 14 Practical tips of PCB layout
the Kelvin connection shown in Fig. 14(a). For the bridge circuit, the same location where the source of the lower switch of different bridge arm is connected is usually treated as the reference potential of the entire drive circuit. Con- sequently, the inductance of the source lead is reduced and the switching speed of the SiC MOSFET is slowed down.
Therefore, the distance between two sources during the lay- out should be minimized, and the two lower switches should be placed symmetrically as shown in Fig. 14(b). In addi- tion, the drive circuit should be connected to the gate-source as closely as possible to reduce the gate-source loop area.
Moreover, some decoupling capacitors can be placed close to auxiliary power
IV. CONCLUSIONS
In this paper, the switching characteristics of SiC MOSFET and Si MOSFET are compared and analyzed. A method of selecting the drive resistance is proposed. The drive circuits suitable for different power converters are dis- cussed respectively. The LTSpice software is used to sim- ulate the drive circuit for suppressing the crosstalk spikes in the bridge circuit. The advantages and disadvantages of each circuit are also summarized. The suitable drive cir- cuit can be selected to meet different requirements.
Finally, three tips are also given for the drive circuit lay- out, which can further reduce the switching loss. Based on the research results of this paper, the theory to select SiC MOSFET drive circuits is provided in the practical application.
g
C1 +Vcc
-Vcc T1
T2
s R1
R2 D1
R3 R4 D2 T
g
s
g d
s Control Q
Signal
Drive Circuit
(a) Kelvin connection
g d
s Q1
s g d Q2
Drive Drive
g1 g2
(b) Bridge circuit layout tip
ACKNOWLEDGEMENTS
Great supports were given by Beijing Key Lab. of Green Lighting Power Supply for Integration Manufacture and Science Innovation Project as well as Innovation and entrepreneurship Project for college students of NCUT.
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Manuscript Received: Feb. 28, 2019 First Revision Received: Apr. 26, 2019 and Accepted: Jan. 14, 2020
