Research on high frequency reliability of silicon carbide MOSFETs resonant circuit

Silicon carbide MOSFETs could process high power at high switching frequency with little loss. The driving circuit produce loss, which is proportional to the switching frequency. Resonant gate driving circuit, which has a inductor to produce resonant with a parasitic capacitor to recycle energy is used to reduce energy loss in the high frequency situation. Firstly, this paper introduces the simplest resonant gate driver. After that, three different types of resonant gate driver with their similar versions are introduced. In these optimized circuits, the third resonant gate driver (RGD3) is the simplest to control and has the lower energy loss, but the second resonant gate driver (RGD2) has faster switching speed and the first resonant gate driver (RGD1) provides more loops to energy feedback.


Introduction
In 1952, the Si has been made in laboratory.After this time, with the development of the technology, more and more material has been invented.At twenty first century, with the development of technology material like SiC gradually takes place of Si in some area.
SiC has a series of advantages.Firstly, SiC has high rate of saturated electron drift, so it could have higher switching rate to improve system efficiency [1][2].Secondly, SiC has high blocking voltage; therefore, it could be put into high-voltage or the high-power applications [3].Thirdly, SiC has low onstate resistance , which means it could reduce conduction loss and improve the instrument efficiency [4][5].Lastly, the SiC has higher bandgap width; thus, it could be used in higher operation temperature [6].
As a part of SiC electrons, SiC MOSFET is increasingly used in electrons due to its advantage on low loss, high withstand voltage and fast switching speed.Major manufacturers have launched their own products like Cree has made C3M0065090D, C3M0120090D and E3M0065090D and Rohm has made SCT3080KL, SCT3105KR, and SCT3120AL.
This article pays attention to the SiC MOSFET because it is relatively mature, and has been used many area.
However, SiC electronic devices still have some problems.Firstly, most SiC switches do not have a short circuit protection, but it will cost a lot to buy a SiC driven circuit, which have a short circuit protection; thus, compared with traditional circuit, SiC electronic devices cost a lot.Secondly, because of the parasitic inductance and SiC MOSFET anti-parallel diode, the switch of the SiC electronic devices generates a large rate of voltage and current change between its drain and source.At the switching instant, SiC switch produces large voltage and current overshoot, which increase the risk of the SiC breakdown.Thirdly, it is made sure that the SiC is useful in high-frequency circuit, but the corresponding driven circuit has not developed.In the 90s of the last centuries, the concept of driven circuit had been invented, which could reduce the loss and keep high frequency at the same time.In 2017, some researchers had applied resonant gate driver to SiC MOSFETs, achieving low loss and high frequency [7][8][9].Resonant driven circuit could be designed to have lower loss and more perfect, and these will be discussed in the article.The article researches basic resonant driving circuit and analyzes its problems.After that, this article analyzes some complex driven circuit bases on the fundamental one and gives some advice about choosing the driven circuit.
Firstly, the first part presents the context and the significance of the SiC MOSFETs and points out the resonant driving circuit is important to reduce loss for the driven circuit.
Secondly, the second part introduces the intern structure of SiC MOSFETs.
After that, this part introduces the basic low frequency square wave driving circuit and points out some problems the circuit have.
Thirdly, the fourth part gives a resonant gate driver to solve the problems in high frequency.This part lists the advantage and disadvantage of resonant gate driver; after that, three improved resonant gate driving circuits are provided to show different design and uses of resonant driver.Some similar circuits are also provided to indicate the advantage and disadvantage of these three circuits clearly.
Lastly, the last part concludes the use of resonant gate driver for the silicon carbide MOSFETs.

Principle and problems of the gate driving circuit
Figure 2. driven circuit.This driven circuit is totem pole structure, which is shown in figure2.Traditional driving circuit often uses low frequency, so square wave is used to control the switch tube to open and close.When the SiC MOSFET Q want to be turned on, firstly, switch S1 is turned on but S2 is turned off to make the energy source VCC connect to the circuit, and current starts flow to charge the parasitic capacitor Ciss.When the voltage across Ciss rises to VCC, switch Q closes and makes the circuit run.In other mode, when switch S2 is closed but S1 is open, negative voltage VEE connects to the switch Q and parasitic capacitor Ciss discharges until the voltage across capacitor Ciss is VEE.When the voltage across capacitor Ciss is lower than the threshold of SiC MOSFET Q, switch Q turns off.At low frequency situation, duty cycle is much greater than the time constant RgCiss, so gate driven voltage approximates as a square wave.
In this circuit, at each switching period, energy on the parasitic capacitor Ciss is completely depleted.The formula of the circuit power loss Ploss is below where VCC and VEE are the gate voltage of SiC MOSFET at the time when it is turned on and turned off, Qg is the gate charge of SiC MOSFET, fs is the switching frequency.As shown in formula (1) The formula of the gate charge Qg is shown in formula (2), where Ig is the current and t is the time.
In high frequency situation, circuit depletion of parasitic capacitor Ciss will be higher due to higher energy loss on the resistor Rg.If circuit uses lower current to get lower loss, SiC MOSFETs will need more time to be charged, which decreases the switching frequency.Furthermore, if switching frequency increases, circuit will need higher current to achieve faster speed about turning on and turning off switch transistor.However, the current Ig is constant, so it is hard to increase the current fast.These are problems about balance energy loss, switching frequency and current size.To reduce the circuit depletion and make SiC MOSFETs could be used in high-frequency situation, the concept of resonant gate driving circuit is put up [10][11][12].The simplest resonant gate driving circuit could be seen in Figure3, because of the switch is SiC MOSFETs, negative voltage source VEE is added.
In this circuit, an extra inductor Lr is added, and switching driven voltage is changed to sinusoidal or approximately sinusoidal voltage.There will be a resonant between the inductor Lr and the parasitic capacitor Ciss, which means the capacitive energy and the resonant inductive energy could be converted to each other in the form of electric and magnetic fields to store energy and reduce energy loss.
In the traditional square wave driving circuit, energy on capacitor Ciss dissipates at each switching cycle [13][14].In resonant driven circuit, the loss is only on parasitic resistor Rg.The first topology circuit of the simplest resonant circuit is shown in figure4 [15][16][17].Because the switch Q is SiC MOSFETs, negative voltage VEE is added to the circuit.Compared with the simplest resonant gate driving circuit, two diodes D1 and D2 are added to provide extra current loop to help the energy flow back to the energy source.This circuit includes 4 models.

First improvement method of resonant circuit
For the model1, closing switch S1 and opening switch S2, voltage source VCC charges the inductor Lr and the capacitor Ciss.when VGS is equal to VCC + VD1, charge is finished, so the switch Q is closed and the voltage VGS is fixed.
At the model2, opening switch S1 and S2, energy at the inductor Lr flows back to the source VCC passing by the diode D1.
At the mode3, opening switch S1 and closing switch S2, capacitor Ciss discharges.Energy stored in capacitor Ciss transmits to inductor Lr and voltage on the capacitor Ciss becomes VEE + VD2.
At the mode4, opening switch S1 and S2, inductor Lr discharges current to the source VCC.
In the condition of low duty cycle, there is no resistor and energy transmit between the capacitor Ci and the inductor Lr, which reduces circuit loss.Nevertheless, in low duty cycle, because the circuit has 4 model, the circuit is more difficult to control.
In the condition of high duty cycle, the circuit has many disadvantages.Firstly, the circuit has less time to discharge, so the loss becomes bigger.Secondly, in model1 and model3, if the duty cycle is big, current from the inductor Lr will flow along the diode VD1 and VD2, which also increases conduction loss.
In this circuit, four S1, S2, S3 and S4 are used to control the switch Q.Because of four switches, this circuit could control the drive model with higher flexibility, which means more energy feedback from inductor Lr and faster switching speed.On the other hand, four switches mean circuit becomes more difficult to control.The increase of the switch S3 and S4 also produces more energy loss.This circuit is shown in figure6 [21].Because the switch Q is SiC MOSFETs, negative voltage VEE is added to the circuit.Compared with the circuit in figure3, capacitor C0 is added should be big enough to stroe the energy.This circuit could be divided into 4 models.
In the model 1, when the gate voltage Vgs reaches VCC+VD1, switch S1 closes but S2 opens.At this time, swithc Q closes.After this time, current from inductor Lr flows to the source VCC, which means erergy feedback.After energy feedback is finished, VCC starts to provide energy to the inductor Lr and capacitor C0, and current flows in opposite direction.
In the model 2, opening both switchs S1 and S2, capacitor Ci and inductor Lr have resonance.Because of the current in inductor Lr is high, capacitor Ci discharges fastly to the inductor Lr.
In the model 3, switch S1 is opened but S2 is closed, current from inductor Lr flows to capacitor C0, voltage at capacitor Ciss become VEE, swithch Q turns off .
In the model 4, both swithchs S1 and S2 are opened, and the capacitor Ciss and inductor Lr have resonance.After this situation, current has opposite direction and inductor Lr starts to charges capacitor Ciss.Current in inductor Lr is high, so the capacitor Ciss could be charged fastly.When the switch S1 is turned on again, circuit will restart at model 1.
The first advantage of the circuit is when the swithch Q need to open or close, the circuit current ILr is high, so the switch Q could be opened and closed fastly.
The second advantage is the switch S1 and S2 could achieve ZVS.
The circuit also has some disadvantages.Firstly, if the time cost of switch S1 or S2 opened is a long time, circuit produces circulation.The second disadvantage is if the original current of the switch opening and closing is different, model 2 and model 4 will cost different time, which makes the circuit hard to control. .single tube resonant gate driving circuit2.This is another type of resonant gate driving gate circuit 2 that is shown in figure 7 [22].In this circuit, two switches S3 and S4 are added.This circuit overcomes circulating current because the switch S3 and S4 could cut off the path of circulating current.This circuit could also control switch S3 and S4 precisely to control the value of current, which increase circuit efficiency.
The disadvantage is that the circuit uses more switches S3 and S4, which makes the circuit become more complex to control.Switches S3 and S4 also cause more energy loss.The resonant gate driving circuit 3 could see in figure 8 above [23][24] This circuit is so smple , because it just adds two diodes to avoid the current flowing back.The first advantage is two switches are easy to control.The second advantage is the diode D1 and D2 avoids the current flow back, so there will be less energy loss.The third advantage is that because of the resonance between capacitor Ci and inductor Lr, gate voltage could be big that a small source voltage VCC could make the circuit work.

Third improvement method of resonant circui
The disadvantage is when the switch Q opens or closes, the switch float to the ground, so the circuit could be easy to be interfered.

Conclusion
In this paper, some resonant gate driver for the silicon carbide MOSFETs is researched.Compared the traditional circuit in high frequency situation, resonant gate driver is a better choice for SiC MOSFETs in high frequency situation.First resonant gate driver (RGD1) has low energy loss in low duty ratio because two freewheeling diodes provides extra energy recycle path.If use more switches to control the circuit, more depletion will be produced.Second resonant gate driver (RGD2) is a current mode driving circuit.During the time of gate voltage rise and decline, big circuit current makes the silicon carbide MOSFETs charge and discharge fast.Added switches also reduce circuit loss but increase circuit complexity and conduction loss.The third resonant gate driver (RGD3) is just added two diodes, which has no influence of the number of models of the circuit, so the circuit is easy to control.Diodes blocks current flow back to the voltage source to reduce circuit loss.In researching resonant gate driver, the ability of designing and analyzing circuit is improved, and these results provide technical support for researching high frequency reliability of silicon carbide MOSFETs resonant circuit.

2 .Figure 1 .
Figure 1.model of power SiC MOSFET.Figure1 shows the main parasitic parameters of a SiC MOSFET device, and this figure ignores parasitic inductance that arisen due to some factors like device packing.In this figure, Cgs represents the gatesource parasitic capacitance, Cgd represents the drain-gate parasitic capacitance, and Cds represents the output parasitic capacitance.Rg indicates the gate input resistance.

4 .Figure 3 .
Figure 3. basic resonant gate driving method.To reduce the circuit depletion and make SiC MOSFETs could be used in high-frequency situation, the concept of resonant gate driving circuit is put up[10][11][12].The simplest resonant gate driving circuit could be seen in Figure3, because of the switch is SiC MOSFETs, negative voltage source VEE is added.In this circuit, an extra inductor Lr is added, and switching driven voltage is changed to sinusoidal or approximately sinusoidal voltage.There will be a resonant between the inductor Lr and the parasitic capacitor Ciss, which means the capacitive energy and the resonant inductive energy could be converted to each other in the form of electric and magnetic fields to store energy and reduce energy loss.In the traditional square wave driving circuit, energy on capacitor Ciss dissipates at each switching cycle[13][14].In resonant driven circuit, the loss is only on parasitic resistor Rg.

6 4. 2 .Figure 6 .
Figure 6.resonant gate driving circuit 2 RGD2.This circuit is shown in figure6[21].Because the switch Q is SiC MOSFETs, negative voltage VEE is added to the circuit.Compared with the circuit in figure3, capacitor C0 is added should be big enough to stroe the energy.This circuit could be divided into 4 models.In the model 1, when the gate voltage Vgs reaches VCC+VD1, switch S1 closes but S2 opens.At this time, swithc Q closes.After this time, current from inductor Lr flows to the source VCC, which means erergy feedback.After energy feedback is finished, VCC starts to provide energy to the inductor Lr and capacitor C0, and current flows in opposite direction.In the model 2, opening both switchs S1 and S2, capacitor Ci and inductor Lr have resonance.Because of the current in inductor Lr is high, capacitor Ci discharges fastly to the inductor Lr.In the model 3, switch S1 is opened but S2 is closed, current from inductor Lr flows to capacitor C0, voltage at capacitor Ciss become VEE, swithch Q turns off .In the model 4, both swithchs S1 and S2 are opened, and the capacitor Ciss and inductor Lr have resonance.After this situation, current has opposite direction and inductor Lr starts to charges capacitor Ciss.Current in inductor Lr is high, so the capacitor Ciss could be charged fastly.When the switch S1 is turned on again, circuit will restart at model 1.The first advantage of the circuit is when the swithch Q need to open or close, the circuit current ILr is high, so the switch Q could be opened and closed fastly.The second advantage is the switch S1 and S2 could achieve ZVS.The circuit also has some disadvantages.Firstly, if the time cost of switch S1 or S2 opened is a long time, circuit produces circulation.The second disadvantage is if the original current of the switch opening and closing is different, model 2 and model 4 will cost different time, which makes the circuit hard to control.

Figure 7
Figure7.single tube resonant gate driving circuit2.This is another type of resonant gate driving gate circuit 2 that is shown in figure 7[22].In this circuit, two switches S3 and S4 are added.This circuit overcomes circulating current because the switch S3 and S4 could cut off the path of circulating current.This circuit could also control switch S3 and S4 precisely to control the value of current, which increase circuit efficiency.The disadvantage is that the circuit uses more switches S3 and S4, which makes the circuit become more complex to control.Switches S3 and S4 also cause more energy loss.
. Because the switch Q is SiC MOSFETs, negative voltage VEE is added to the circuit.Compared with the simplest resonant circuit, two diodes D1 and D2 are added.The circuit is a simple circuit, and there are four models in this circuit.Model 1, switch S1 closes but S2 opens, and the capacitor Ciss is charged from the source voltage Vcc.When the inductor current wants to be negative, diode D1 prevents the current and makes energy store in inductor Lr and capacitor Ciss.Voltage at the capacitor Ciss is VCC and the switch Q is closed.Model2, both switch S1 and S2 are opened.Because of the diode D1, the circuit is an open circuit, so voltage in inductor Lr and capacitor Ci does not change.Model3, opening switch S1 but closing S2, capacitor Ciss and inductor Lr discharge.Voltage at the capacitor Ciss become VEE and switch Q is opened.Model4, both switch S1 and S2 are opened, and the circuit is open circuit.