Abstract: Currently, most high-power IGBT drivers used in engineering employ single-gate resistor switching devices, which cannot maximize the efficiency of the devices. To address the shortcomings of these driver products, this paper proposes a digital driver with a CPLD as the core controller, utilizing digital technology to improve the performance of the devices during operation. The paper discusses the structure of this digital driver and the working principle and function of each component. This driver utilizes comprehensive IGBT status detection circuitry, multi-channel fault detection and protection circuitry to improve the safety and reliability of the driver and the entire system. Finally, experimental waveforms of the digital driver's power supply and drive stage operation are presented, demonstrating the correctness and feasibility of this proposed solution.
1. Introduction
With the development of power electronic device technology, the market for high-power devices in rail transit, DC power transmission, wind power generation and other fields has developed rapidly. Among them, IGBT devices have performed particularly well. In specific application conditions, each IGBT module requires a dedicated driver. The IGBT driver has a significant impact on the operating performance of the IGBT [1-3].
Traditional IGBT driver technology using analog circuits is relatively mature and stable. However, since most of the driver parameters are set in hardware, parameter adjustment is relatively complex. Different drivers must be designed for different IGBT models. This paper uses a digital controller to control the gate, which allows for flexible modification of the driver's software parameter settings to adjust the IGBT's performance. For different IGBT models, only different drivers need to be loaded, which can solve the model matching problem of traditional driver products.
2. Overall driver solution
As shown in Figure 1, the driver mainly consists of five parts: a high-level isolated power supply, an optical fiber communication interface, a power amplifier circuit, a detection and protection circuit, and a digital controller.
2.1 High-level isolated power supply
In the design of high-voltage IGBT drivers, power supply design is one of the key parts. The output power of the power supply determines the operating frequency that the IGBT can operate in actual work. If the power supply output power is insufficient, undervoltage may occur when the IGBT device is operating at high frequency, which will lead to increased IGBT losses or even damage to the IGBT.
In this design, the LM5025 chip from TI is used to design a flyback DC-DC circuit (circuit diagram shown in Figure 2). The primary side of the circuit has a current sensing soft-start function. When the voltage drop across the current sensing resistor reaches 0.25V, it provides good overcurrent protection for the power supply.
2.2 Fiber Optic Communication Interface
In the interface design of the user's main control system communication, optical fiber communication with strong anti-interference capability is selected to prevent the control signal from being interfered with and triggering falsely. HFBR-1522 and HFBR-2522 optical fibers are selected, and the optical fiber circuit is shown in Figure 3.
2.3 Power Amplifier Circuit
A MOSFET with very low on-resistance is selected as the switching device to construct the IGBT gate power output circuit, as shown in Figure 4. Simultaneously, multiple gate resistors are used for switching to adjust the IGBT performance under different conditions. During normal IGBT switching, the switching speed can be controlled by adjusting the gate resistors to optimize device efficiency. When the IGBT malfunctions (e.g., a short circuit), the operating state can be controlled by adjusting the gate resistors to prevent damage and protect the device.
2.4 Detection and Protection Circuit
To prevent any abnormal malfunctions during IGBT operation, the driver needs to monitor the IGBT's status parameters. If an abnormality is detected, the driver will automatically take protective measures and notify the master controller.
Undervoltage detection: Currently, IGBT manufacturers recommend a gate voltage of ±15V for IGBT device operation (maximum gate withstand voltage is ±20V). If the IGBT device operates below 15V, according to the relationship between the IGBT device's saturation voltage drop VCE and the gate voltage (as shown in Figure 5), the IGBT saturation voltage drop will increase as the gate voltage decreases, leading to increased IGBT device losses and potentially damaging the device. Therefore, it is essential to monitor the voltage at the driver's output gate. If undervoltage turn-on occurs, the driver must immediately implement protection. It is also important to note that the IGBT's short-circuit current is proportional to the gate voltage. Therefore, if the gate voltage exceeds +15V when the device is turned on, a larger short-circuit current will occur if the device malfunctions. Thus, the driver must ensure that the gate turn-on voltage is within a reasonable range.
Vce voltage detection: Vce voltage detection can provide IGBT device parameters to the digital controller. The controller can determine the operating status of the IGBT through the Vce voltage, and thus take corresponding strategies to control the IGBT in different ways.
Overvoltage protection: During the IGBT device turn-off process, due to the parasitic inductance of the bus circuit, the turn-off voltage will generate a voltage overshoot spike with an amplitude of ΔV=Ls*di/dt. If the spike voltage exceeds the rated voltage of the IGBT device, the IGBT device will be broken down, causing device damage.
The overvoltage protection circuit uses a TVS diode connected in series with a current-limiting resistor, with the collector connected to the gate (as shown in Figure 6). When a voltage spike exceeding a set value (less than the IGBT's rated value) occurs at the collector during turn-off, the TVS diode conducts in reverse, injecting current into the gate through the current-limiting resistor, slowing down the IGBT turn-off speed (reducing di/dt), thereby limiting the voltage spike. Users select the appropriate number of TVS diodes and the breakdown voltage parameters of a single TVS diode according to specific application conditions.
di/dt detection: (As shown in Figure 7), the equivalent diagram inside the IGBT module shows that due to the parasitic inductance between the main electrode circuit and the auxiliary electrode circuit connected within the IGBT module, the main current IC flows in from the main electrode and out through the parasitic inductance L during IGBT module operation. According to the formula V = -L * di/dt, the induced voltage V is directly proportional to the di/dt value. The di/dt value of the main current Ic can be obtained by detecting the induced voltage generated on L. During IGBT module operation, if the induced voltage is higher or lower than the set value, it is considered that the device's di/dt is abnormal, requiring protection and reporting a di/dt fault to the main control system.
Digital controller function
The digital controller primarily controls the power amplifier circuit to drive the IGBT devices based on the input signals. Simultaneously, it determines the operating status of the IGBT devices based on feedback signals from the detection circuit, and immediately protects the IGBT devices according to the set strategy if an abnormality occurs.
3. Driver-level testing
To preliminarily determine the correctness and feasibility of the basic functions of the driver stage, the sample board in Figure 8 was used for testing. The test sample was a 3300V-1500AIGBT module, and a general-purpose double-pulse test method was adopted. The test waveform is shown in Figure 9.
As can be seen from the test results above, the driver board can safely turn the IGBT module on and off under the test conditions.
4. Areas for improvement
The TVS diode series connection method used in the digital IGBT driver overvoltage protection detection proposed in this paper is only effective against parasitic overvoltages generated by di/dt during IGBT turn-off. However, this method also has drawbacks. If the user sets the overvoltage protection threshold improperly, or if the system experiences frequent or prolonged overvoltages during operation, the IGBT gate may be damaged by high voltage, or an IGBT that should be turned off may be forcibly turned on, resulting in a short circuit between the upper and lower IGBTs and damaging them. Therefore, a Vce detection circuit can be added to monitor the collector voltage in real time and formulate a protection strategy.
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