Principle of driving high-power IGBT using 2SD315AI design

Common IGBT driver modules include TLP250 and EXB841/840 series. However, in the development of DC/DC converters for fuel cell city buses, it was found that the on-board DC/DC converters often work in high-power or ultra-high-power states, and the IGBT instantaneous drive current in this state is large, requiring high reliability, so the traditional drive circuit can no longer meet its use requirements. After research and analysis, the Swiss CONCEPT company's dedicated integrated drive module 2SD315A for driving and protecting IGBT or power MOSFET was selected as the driver of high-power IGBT (800A/1200V). The driver integrates intelligent drive, self-test, state feedback, DC/DC power supply, and complete isolation of the control part and the power part. The actual road condition operation experiment of the on-board 90kW DC/DC converter shows that the effect is good.

IGBT drive requirements

The IGBT drive requirements are closely related to its static and dynamic characteristics, that is, the gate positive bias, negative bias and gate resistance have different degrees of influence on the IGBT's on-state voltage, switching time, switching loss, short-circuit withstand capability, and the conversion rate of the switch tube C and E pole voltage. Its switching function is to form a channel by adding a positive gate voltage, provide base current to the PNP transistor, and turn on the IGBT. Conversely, adding a reverse gate voltage eliminates the channel, and the reverse base current flows, turning off the IGBT. According to such characteristics, the drive circuit for it should meet the following requirements:

IGBT is a voltage-type drive with a threshold voltage of 2.5 to 5V and a capacitive input impedance. Therefore, the IGBT is very sensitive to the gate charge and requires a low-impedance discharge circuit, that is, the connection between the drive circuit and the IGBT should be as short as possible.

·Use a driving source with low internal resistance to charge and discharge the gate capacitance to ensure that the gate control voltage Vge has sufficiently steep leading and trailing edges, so that the switching loss of the IGBT is as small as possible. In addition, after the IGBT is turned on, the gate driving source should be able to provide enough power to put the IGBT
in a saturated state, otherwise the IGBT is easily damaged.

·When the driving level +Vge increases, the on-state voltage drop and switching loss of the IGBT decrease, but the Ic increases when the load is short-circuited
, and the time the IGBT can withstand the short-circuit current decreases, which is not good for its safety.

·During the shutdown process, in order to extract the stored charge of the PNP tube as quickly as possible, a negative bias Vge must be applied, but it
is limited by the maximum reverse withstand voltage between G and E of the IGBT.

·The gate drive circuit of the IGBT should be simple and practical, and it is best to have its own protection function for the IGBT and have strong anti-interference
ability. ·Since IGBT is mostly used in high-voltage occasions in power electronic equipment, the drive circuit and the control circuit should be strictly isolated

in terms of potential .

2SD315A driver module

The 2SD315A module can drive 1200A/1200V IGBTs, with a DC switching frequency higher than 100kHz, and can achieve 0-100% duty cycle adjustment through ±15A gate current. In addition, it has perfect status monitoring of power supply and current to protect the module and power switch tube; the driver module is divided into several parts, including interface unit, electrical isolation, and driver unit. Each driver channel has electrically isolated the control circuit and the main power circuit, as shown in Figure 1

The driving ability and protection performance of the driver module are issues that people are more concerned about. The following are some experiences in the application:

· Since the gate of the IGBT has a high input impedance, its gate is easy to accumulate charge in the absence of a gate discharge circuit, and the gate oxide layer is very fragile and can only withstand a withstand voltage of ±20V, which is easy to cause gate-source breakdown and damage the IGBT. In the actual circuit, a gate-source bias of ±15V is used to improve the short-circuit tolerance of the IGBT.

· In order to improve the steepness of the leading and trailing edges of the control pulse and prevent oscillation, and reduce the voltage spike pulse of the IGBT collector, the series resistance of the gate is reasonably selected according to the user manual of the module, which can not only obtain a good driving pulse, but also control the transition time of the IGBT on-off state change.

· Use the external resistor Rth to define the tube voltage drop when the power tube is turned on. When it is greater than the defined maximum tube voltage drop, the monitoring circuit will output a fault alarm signal and turn off the power tube, thereby protecting the IGBT.

When the power supply voltage to the drive module is too low, it will affect the reliability of the drive circuit, and the monitoring circuit will
send a fault signal to the module, which will put the entire module in a blocked state and protect the safety of the system. Here, the working state of the module is represented by the level of the two SO output pins on the module, and is connected to the protection circuit through a logical relationship to realize state detection.

Application Examples

This driver module has two working modes: direct mode and half-bridge mode. When the driver works in direct mode, there is no connection between the driver's drive channels, and the two channels are always driven at the same time. In the half-bridge mode, the MOD input terminal is connected to GND, InA inputs the PWM signal, and InB inputs the enable signal (high level is valid, low level blocks all channels). Since the two state output terminals SO1 and SO2 are connected together, the two drive channels output the same fault signal. The dead time is determined by the external circuits of RC1 and RC2 on the module, so that the two output signals of the driver will not be high at the same time.

The following points should be noted when using 2SD315A to drive high power density IGBTs:

· The setting of the working mode MOD and the selection of the reference resistor Rth are the prerequisites for the correct use of the driver module. It is necessary to pay attention
to the setting of the dead time in the half-bridge working mode and the matching relationship between the Rth size and the power switch tube model.

· The appropriate gate resistor Rg is very important for driving IGBTs. If Rg is too large, the transition time of the IGBT on-off state change will be prolonged and the energy consumption will increase; but if Rg is too small, di/dt will increase, which may cause gate voltage oscillation and trigger
mis-conduction, which may damage the IGBT in severe cases. The minimum selectable value of Rg is determined by the following formula:

Where △U is the difference between the positive and negative gate bias voltages; Ig(max) is the maximum current that the drive circuit can provide.

· Pay attention to the wiring between the drive module and the main power switch tube. The gate drive wiring has a great influence on preventing potential oscillations, slowing down the rise of gate voltage, reducing noise loss, reducing gate power supply voltage or reducing the number of gate protection circuit actions. Therefore, the distance between the output stage of the driver and the IGBT should be minimized, and the drive signal should be transmitted with twisted wire.

The 2SD315A drive module is set in direct mode, and the pin MOD is directly connected to the +15V power supply; the reference resistor is 47kΩ; the gate drive resistor is 2Ω; the application circuit of the drive module is shown in Figure 2.

In the figure, S1 and S2 represent two 800A/1200V IGBTs of EUPEC, and Figure 3 shows the drive waveform of the main power switch tube obtained in the experiment. It can be judged from the shape and amplitude of the waveform that the IGBT is working normally.

As can be seen from Figure 3, the rising and falling edges of the waveform are steeper, and it takes less than 1 microsecond from IGBT shutdown to turn-on, which greatly reduces the switching loss; the module can provide the appropriate positive gate-source voltage to the IGBT and shut it down reliably; all of these provide important guarantees for the normal operation of the IGBT. In addition, in the experiment, it was found that in order to obtain a flatter positive and negative waveform, appropriate capacitors can be connected in parallel at both ends of the module's COMx and Visox pins for adjustment.

When a fuel cell city bus is running under various road conditions such as starting, shifting, and braking, the IGBT (800A/1200V) in its high-power DC/DC is often at a high voltage of 300-400V, or is subjected to a high current of nearly 300A in a very short time (≤200ms). The power output ranges from tens of kilowatts to hundreds of kilowatts. The 2SD315A isolated drive module used with it has fully met the driving requirements for power demand under different road conditions in the actual urban road conditions. It has traveled safely and trouble-free for a total mileage of more than 40,000 kilometers and is a high-power drive module with excellent performance.