[Introduction]There are two sides to everything: SiC MOSFETs have faster switching speeds, which can significantly reduce device switching losses compared to IGBTs and improve system efficiency and power density; however, high-speed switching also produces greater dv/dt and di/dt, poses additional challenges for both motor insulation and EMI design in some motor control fields.
Application pain points
The power of the high-speed air compressor controller in the hydrogen fuel system is around 35kW, the speed is as high as 100,000 rpm, the output frequency can reach 2000Hz, and the modulation frequency is more than 50kHz. It is a common design, and SiC MOSFET is a good solution.
However, the high dv/dt and harmonics of SiC can cause heating of the air compressor coil and motor shaft current.
There are two general countermeasures:
1. Use a large gate resistance to drive the SiC MOSFET to suppress dv/dt, but it will significantly increase the switching loss and affect the efficiency.
2. The output filter is used to suppress the harmonic current and reduce the dv/dt on the motor side, but the volume will account for more than one third of the controller, which will increase the cost, and the introduction of the filter will also cause a certain loss.
The above two typical designs, at the expense of loss and efficiency, seem to “have both fish and bear’s paw”…
In response to the above design pain points, Infineon innovatively launched the 2L-SRC series of driver ICs, which combined the SiC switching characteristics to optimize the configuration of Rg, in order to achieve the best of both worlds. Please refer to the following decomposition for the specific process:
SiC switching characteristics
Figure 1 is a schematic diagram of the switching characteristics of SiC, describing the relationship between dv/dt and Ic, dv/dt and Rg, and Esw and Rg
From the curve in Figure 1 above, it can be seen that the maximum dv/dt value of SiC will appear when small current is turned on and large current is turned off. By increasing Rgon and Rgoff, the maximum dv/dt value of turn-on and turn-off can be reduced respectively, but SiC The switching loss Esw will increase accordingly.
Using 2L-SRC’s Problem Solving Ideas
In fact, it is not difficult to solve the problem. Last year, Infineon launched the 2L-SRC driver IC. Combined with the IGBT switching characteristics in the field of motor control, a method of using large Rg for small current and small Rg for large current is proposed (for details, please refer to the AN document at the end of the article).
Here we can still learn from its ideas and develop corresponding Rg optimization strategies for the switching characteristics of SiC MOSFETs.
In order to facilitate the understanding of the whole process, this article assumes the relevant curves in Figure 2 and Figure 3 based on the SiC trend curve in Figure 1, as the basis for the subsequent 2L-SRC driver IC optimization Rg and circuit simulation analysis.
(The curves in Figure 2 and Figure 3 are based on rational assumptions and are for reference only. The actual curve should be based on the actual measurement of SiC.)
Turn-on resistance Rgon strategy for optimal configuration of dv/dt(on)
Figure 2. 1200V SiC MOSFET dv/dt(on) vs. Ic curve
The dv/dt characteristic is turned on by SiC, assuming two curves of Rgon=5Ω and Rgon=10Ω, and the preset dv/dt limit value as shown in Figure 2; Rgon can be divided into two parts according to the output current Ic, when the current Id=[0,50A]The interval is turned on with Rgon=10Ω, when the current =[50A,200A]The interval is changed to Rgon=5Ω to turn on. Compared with the traditional driving scheme (Rgon=10Ω for the whole current range), the Eon loss advantage of small resistance turn-on can be obtained in the medium and large current range (50A, 200A).
Turn-off resistance Rgoff strategy for optimal configuration of dv/dt(off)
Figure 3. 1200V SiC MOSFET dv/dt(off) vs. Ic
Also by the dv/dt characteristic of SiC turn-off, we can also divide the current interval into two, as shown in the assumed conditional curve in the figure below, when the current =[133A,200A]Use a large resistor Rgoff=12Ω to turn off, and when the current =[0A,133A]Use a small resistor Rgoff=6Ω to turn off.Compared with the traditional driving scheme (Rgoff=12Ω for the full current range), it can be used in the small and medium current range.[0A,133A]Gain the Eoff loss advantage of small resistive turn-off.
Summary of the driving strategy for optimizing the configuration of dv/dt
According to the above case analysis, the driving resistance Rg control strategy is optimized, and the current is divided into three parts: small current, medium current, and large current, corresponding to different gate resistance settings, and then at the predetermined current threshold value Rgon and Rgoff are calculated. Switch to achieve the purpose of optimizing the drive, as shown in the following figure:
Figure 4. Control strategy of driving resistor Rg based on Figure 2 and Figure 3
Realization of Drive Circuit Based on 2L-SRC
According to the above ideas and processes, it is not difficult to obtain the relevant driving resistance Rg configuration strategy.
The ancients said, “If you want to do good work, you must first sharpen your tools.” How to use 2L-SRC driver IC to achieve it?
2L-SRC driver IC product and function introduction
Figure 5.2L-SRC (1ED3240MC12H) functional block diagram
The typical functional block diagram of 2L-SRC, as shown in Figure 5, is a simple 8pin design. Just in addition to the regular IN input and OUT output, another set of OUTF output is added. According to the logical relationship between the /INF signal and the IN signal level, the OUTF state can be flexibly configured to act at the turn-on and turn-off moments of the conventional OUT output.
Combined with the figure below, you can more intuitively understand the four main states when the 2L-SRC driver IC is connected to a gate resistor:
Figure 6. 2L-SRC (1ED3240MC12H) drive resistor Rg configuration diagram
● When OUTF is only enabled when it is turned on, the turn-on resistance Rgon=R1//R3, and the turn-off resistance Rgoff=R2;
● When OUTF is only enabled when it is turned off, the turn-off resistor Rgoff=R2//R4, and the turn-on resistor Rgon=R1;
● When OUTF is turned on and off at the same time, the turn-on resistance Rgon=R1//R3, and the turn-off resistance Rgoff=R2//R4;
● When both the turn-on and turn-off of OUTF are disabled, the turn-on resistance Rgon=R1, and the turn-off resistance Rgoff=R2;
Regarding the state relationship between OUTF, control signal /INF and input signal IN, as shown in Figure 7, the specification has a detailed interpretation, so I won’t go into details here:
Figure 7. State diagram between OUTF and input IN and control/INF ((1ED3240MC12H))
The core logic of OUTF is:
● At turn-on time, when the input signal IN level jumps high, /INF signal is low level (0), then OUTF is enabled at turn-on time;
● At turn-off time, when the input signal IN level jumps low, /INF signal is high level (1), then OUTF is enabled at turn-off time.
2L-SRC-driven Rg configuration (based on the control strategy in Figure 4)
Based on the control logic of the 2L-SRC driver IC and the optimized control strategy of the driving resistor Rg in the above SiC case, we can further refine the configuration of the 2L-SRC driver IC as follows:
Figure 8. Schematic diagram of 2L-SRC (1ED3240MC12H) driving resistor Rg based on Figure 4
Based on the drive resistor configuration in Figure 8, it can be obtained: R1=10Ω, .R2=12Ω, R3=10Ω, R4=12Ω
The final drive and control strategy is as follows:
● When current =[0,50A]At this time, /INF is waveform sequence A, OUTF is only enabled at turn-off time, at this time Rgon=R1=10Ω, Roff=R2//R4=12//12=6Ω;
● When current=(50A, 133A]/INF is waveform sequence B, OUTF is enabled at turn-on and turn-off time, at this time Rgon=R1//R3=10//10=5Ω, Rgoff=R2// R4=12//12=6Ω;
● When the current=(133,200A]/INF is the waveform sequence C, and OUTF is only enabled at the turn-on time, at this time Rgon=R1//R3=10//10=5Ω, Rgoff=R2=12Ω;
PS: In order to achieve different current intervals, give different /INF waveform sequences, or add a simple control loop. For example, the output current value is sampled or estimated in real time, the current range of the instantaneous current is determined, and then the corresponding signal is given by the host computer (such as CPLD, DSP, etc.) to the /INF pin of the driver IC.
Application Case Simulation Reference
Based on the above 2L-SRC variable Rg configuration strategy, we built a PLECS circuit, referring to the application conditions of high-speed air compressors, selected SiC half-bridge modules, and carried out simulation verification and comparison of two-level three-phase Inverter circuits.
The relevant simulation conditions are as follows:
SiC Easy Half-Bridge Module: FF6MR12W2M1_B70 (1200V/200A AlN)
Heat sink temperature Th=50C, Fsw=50kHz, fo=1kHz, Io=142Arms, Vdc=600V, Modi=1.0, SVPWM, PF=0.95
Traditional drive scheme: Rgon=10Ω, Rgoff=12Ω, Rg full range fixed resistance
2L-SRC drive scheme: Rgon=5, 10Ω, Rgoff=6, 12Ω, Rg switches with the current interval (Figure 8)
Figure 9. Comparison of simulation results between traditional Rg control and 2L-SRC Rg control
Full text summary
The article combines the SiC switching characteristics and the Rg optimization configuration of the 2L-SRC driver IC, plus the SiC case analysis based on certain reasonable assumptions, and the final simulation comparison, the effect has already appeared.
Review the opening question: 2L-SRC+SiC, can you get both fish and bear’s paw? You must have the answer in your mind.