Ⅰ. Preface

Power semiconductor devices, as we all know, are power electronic switches. Thousands of times (kHz) can be switched in a second, and high-speed power devices can reach tens of kHz or even hundreds of kHz. The device's voltage change rate dv/dt and current change rate di/dt increase as the switching speed increases. The device material has the greatest impact on dv/dt and di/dt, followed by the device's voltage, current, temperature, and drive parameters. To help you better understand high-speed power semiconductor devices, today we're going to look at  SiC and GaN as examples, and see how big the dv/dt and di/dt of high-speed power devices are?

Figure. 1

Ⅱ. Definition of switch transient parameters

Let's start with a look at some of the major parameters of the SiC MOSFET switching transient. The image is taken from Cree's official website's datasheet for the  SiC MOS power module. Turn-on time ton, turn-on delay time td(on), turn-on current rise rate di/dt on, turn-on voltage drop rate dv/dton, and current rise time tr are all important parameters of the turn-on transient, as shown in the following diagram:

Figure. 2

Turn-off time toff, turn-off delay time td(off), turn-off current drop rate di/dt off, turn-off voltage rise rate dv/dtoff, and current drop time tf are some of the essential parameters of the turn-off transient. As depicted in the diagram:

Figure. 3

Let's concentrate on the switching transient's voltage and current rate of change and ignore the other parameters. We may deduce two important points from the aforementioned device switching curves: ① The  SiC MOS  switch's transient rise and fall times tr and tf mostly refer to the current. It's possible that it's because they're on the same scale. After all, this option isn't particularly important. It is mostly used to characterize the device's speed. Friends with differing viewpoints are welcome to share them with me. ② Transient voltage and current change rate in  SiC MOS switches The current and voltage amplitudes are selected at a rate of 40% to 60% of the current and voltage amplitudes. Because the device's switching transient state is nonlinear, this is quite simple to comprehend. The easiest way to convey the problem is to choose the area with the highest change rate.

Ⅲ. DV/DT, DI/DT quantitative analysis

Let us see how big the dv/dt and di/dt of SiC MOS are after learning the definition of switching transient characteristics of SiC MOS. For this presentation, we'll use Cree's 1200V 300A module as an example. CAS300M12BM2 is the device's model number. The module's physical map and internal circuit are as follows:

Figure. 4

The relationship between dv/dt, di/dt.  and gate resistance of SiC MOSs at 600V bus voltage and 300A current at 25°C is shown in the diagram below. It can be seen that when the device's gate resistance rises, the device's switching becomes more temporary. The state's di/dt  and dv/dt will drop, making it easier to comprehend. The device's switching speed will slow down as the gate resistance rises.

Figure. 5

There is an important piece of information to remember here. The MOS device's gate can directly regulate the current change rate, however the IGBT  gate resistance has only a minor impact on the turn-off current change rate.

Assuming the device's gate resistance is 2 ohms, the SiC MOS turn-on transient's dv/dton and di/dt on are 17.5V/ns and 9A/ns, respectively, while the turn-off transient's dv/dtoff and di/dtoff are 12V/ns and 12A/ns, respectively. Some of your pals may have a difficult time grasping this concept. To make it clearer, we'll compare it to other weak current indications later.

Let's compare the quicker GaN  -HEMT (Gallium Nitride, High Electron Mobility Transistor,  Gallium Nitride -High Electron Mobility Transistor) straight above:

Figure. 6

The turn-on speed of GaN  MOS is 4 times quicker than SiC MOS under the same on-resistance, while the turn-off speed is 2 times faster. It has achieved 100V/ns, and there is no di/dt data here (GaN current measurement is a difficulty), indicating that the speed will be significantly faster than that.

Figure. 7

Friends who are interested can read the IGBT  handbook. The IGBT's dv/dt and di/dt are calculated in us, and if SiC and GaN are calculated as well, the GaN's turn-off dv/dt is calculated in us. It can reach 100,000V/us, and the SiC MOS module's turn-off di/dt can reach 12,000A/us. This is a vast amount of information. Is it inconceivable to think about tens of thousands of amperes and tens of thousands of volts?

But, thinking back, the sentence "1us can move tens of thousands of amps of current and tens of thousands of volts" is a false proposition; in fact, GaN and SiC are impossible to change the voltage of tens of thousands of V within 1us, and tens of thousands of A current, but rely on IGBT.   IGCT, IEGT,  SCR, but I have seen many academic papers researching SiC power devices above 10kV (the current is very small), I only want to highlight GaN and the fact that the dv/dt and di/dt of SiC devices are really enormous, so don't be fooled! If compared to a sharp sword, such a fast edge will cut iron like mud, blow hair, and break hair.

Let's compare high-speed switching transients in power devices to weak current signals to better comprehend them. Everyone is familiar with the most commonly used  DSP  in power electronics, so let's look at the  DSP GPIO number's output edge transition time. The rising edge and falling edge times of the  TM320F28335 GPIO  are shown in the diagram below:

Figure. 8

The maximum rising and falling edge of DSP common IO is just 8ns, as can be shown. This is a relatively short time, but the signal level is also quite low, at only 3.3V. As a result, the GPIO's dv/dt is around 0.41V/ns. If there is no comparison, there is no harm. The original power device's dv/dt speed is faster than the DSP GPIO output level's transition speed

Some of your buddies may have reservations. This is merely the speed of regular IO when DSP was first introduced, and it is not indicative. Then Lao Geng double-checked certain data. The edge jump of digital logic levels frequently used in  TTL, CMOS,  LVDS, and  ECL  is shown below. When the time is changed, the fastest can reach 100 frames per second, and the bandwidth has reached 3.5 gigabits per second. The amplitude of the level decreases as the signal speed increases, hence most of them use differential signals. The  ECL level swing, for example, is only 0.8V, hence the level's dv/dt is merely 8V/ns. There is still a small difference when compared to the most powerful gadgets.

Figure. 9

Naturally, there are high-speed signals that I am unaware of. Friends who are interested can find them on their own.

Ⅳ. DV/DT, DI/DT negative effects?

The parasitic characteristics in the circuit might finally reflect their "value" due to the rapid switching speed of dv/dt and di/dt,  The di/dt of 12A/ns will generate a voltage drop of 12V at 1nH, and the capacitance of 12V/ns at 1pF will generate a current of 12mA, and the parasitic parameters in the power main loop may be much bigger than 1nH and 1pF. Because it is difficult for most isolation transformers to produce a parasitic capacitance of many pF, current will flow freely in the circuit even if the isolation circuit is impotent, therefore high-speed power devices rely on these parasitic characteristics. The application of the power circuit presents additional issues for the power circuit's design.

Figure. 10

Excessive dv/dt and di/dt will also broaden the EMI radiation spectrum, in addition to the previous impacts.

Figure. 11