Abstract : IGBT drive protection circuits are fundamental to the reliable, stable, and efficient operation of IGBTs, and are indispensable for the reliable operation of IGBT-based systems. This paper presents an IGBT drive protection circuit applied to a diode-clamped three-level frequency converter. After introducing the main characteristics of IGBTs, it describes the design of an IGBT drive protection circuit with high reliability, high integration, and high efficiency using CONCEPT's 2SD315 module. Finally, the actual drive circuit using the 2SD315AI-33 drive module is presented.
Keywords : IGBT, drive circuit, three-level inverter, 2SD315
1. Introduction
An IGBT (Insulated Gate Bipolar Transistor) is a composite, fully controllable, voltage-driven power electronic device composed of a BJT (Bipolar Junction Transistor) and a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). It combines the advantages of a MOSFET's high input impedance and a GTR's low on-state voltage drop. While GTRs have a low saturation voltage drop and high current density, they also require a large drive current. MOSFETs, on the other hand, have very low drive power and fast switching speed, but also a large on-state voltage drop and low current density. The IGBT combines the advantages of both devices, offering low drive power and a low saturation voltage drop, making it the preferred device in today's power electronics industry.
The development level of power electronic devices largely determines the development level of power electronic products. Currently, the limitations imposed by voltage and current levels on power electronic devices have become a bottleneck for the development of the power electronics and electrical drive industries, with IGBTs being a prime example. How to maximize the functionality of existing IGBT technology is a primary and crucial issue. Besides reasonable software control methods, the IGBT drive and protection circuit is undoubtedly another important aspect. Currently, many IGBT control and protection circuits exist, but their integration and reliability are still not perfect. This article, based on an introduction to the basic performance of IGBTs, presents an IGBT drive and protection circuit based on the CONCEPT 2SD315 module.
2. IGBT Characteristics and Driver Design
This article takes the FF450R17ME3 IGBT from Eutec as an example. According to its technical manual, when the junction temperature is 125℃, the voltage drop between the collector and emitter is UCE=900V, the gate drive voltage is UGE=±15V, IC=450A, and the current limiting resistor is RG=3.3Ω, Tdon=100ns and Tdoff=1000ns. However, the switching speed is different when the gate resistor RG is different. When RG is small, the time constant between it and the gate capacitor is short, which makes the deep saturation conduction time of the IGBT short, and vice versa. The switching speed of the IGBT directly affects the system efficiency, but considering the side effects of di/dt and du/dt on the IGBT itself, the current limiting resistor cannot be too small. The selection of the current limiting resistor can generally refer to the following formulas: [1]~[2]
In the formula, U <sub>CN</sub> and I<sub> CN </sub> represent the rated voltage and rated current of the IGBT, respectively. Figure 1 shows the voltage drop versus current curve of the FF450R17ME3 transistor. As can be seen from the figure, when the gate drive voltage is less than 12V, the voltage drop increases sharply when the turn-on current reaches a certain value. Although the IGBT can be turned on at 12V, the turn-on loss is relatively large. At 8V, 9V, and 10V, the voltage drop increases linearly when the current reaches a certain value, indicating that the current carrying capacity has reached its limit. Therefore, considering all factors, the gate drive voltage should be greater than 12V. In practical engineering, 15V is generally chosen. Considering faster turn-off speed and improved anti-interference capability, a reverse bias voltage of -10~-15V should be applied. In addition to the above considerations, the drive circuit also needs to consider the isolation of the drive signal, the isolation of the drive power supply, the isolation between the control section and the main circuit, and whether there is interlocking and dead-time control between the various switching signals.Figure 1. Voltage drop versus current curve of FF450R17ME3 transistor
3. IGBT Protection Circuit
IGBT failures are generally caused by three factors: overcurrent, overvoltage, and overheating. Overvoltage is further divided into collector-emitter overvoltage and gate-emitter overvoltage. Regarding overcurrent protection, many manufacturers' technical data indicate that IGBTs can withstand up to twice their rated current for a short period. However, frequent overcurrent exposure will cause premature aging of the device. As shown in Figure 3, if the gate drive signal amplitude is 15V, when the IGBT carries a rated current of 450A, the voltage drop across the transistor is approximately 2.4V. Based on this characteristic, an IGBT overcurrent protection circuit can be designed. Figure 2 shows a typical protection circuit for the 2SD315. Based on the circuit structure, the parameter configuration is calculated. As shown in the figure, when the IGBT is off, VT1 is on, and the non-inverting input of the comparator is 0. At this time, a 150μA current flows through Rth to form a reference potential at the inverting input of the comparator, and the comparator output is low. When the IGBT is on, VT1 is off. The reference potential still exists. A 1.4mA current, after a delay through capacitor Ca, flows through resistors Rm, VDM1, VDM2, and the IGBT before entering the reference ground. Therefore, a potential is formed at the non-inverting input of the comparator. Its amplitude is determined by the voltage drop URm across resistor Rm, the voltage drops UD across diodes VDM1 and VDM2, and the voltage drop UCE across the IGBT. Since URm and UD are fixed values, the potential at the non-inverting input of the comparator is determined by UCE. As shown in Figure 1, for a specific IGBT model, the voltage across its terminals and the current flowing through it have a certain curvilinear relationship under on-state conditions. As the current flowing through the IGBT increases, the voltage UCE across the IGBT also increases. When UCE increases to the point that the potential at the non-inverting input of the comparator is higher than that at the inverting input, the comparator output flips, thus blocking the IGBT drive pulse. The circuit parameters can be calculated as follows: Since the IGBT can withstand twice its rated current for a short time, it can be appropriately selected in practical applications. At the instant the IGBT turns on, the voltage UCE across the IGBT does not immediately enter a steady state, but rather undergoes a short transition process. During this stage, the IGBT protection circuit will issue an incorrect detection signal. By adding a capacitor Ca for delay, the IGBT protection circuit can avoid this transition process and achieve the correct protection function. Moreover, this capacitor can also filter out external interference signals on UCE to a certain extent, reducing the occurrence rate of protection malfunction. The parameters of Ca are calculated as follows: 4. Introduction and Application of 2SD315AI-33 Figure 3: Outline of 2SD315AI-33
4.1 The 2SD315AI-33 is a drive module designed by CONCEPT GmbH, Switzerland, specifically for the reliable operation and safe running of 3300V high-voltage IGBTs. It is based on a dedicated chipset and consists of other necessary components. This module uses pulse transformer isolation and can drive two IGBT modules simultaneously. It provides a drive voltage of ±15V and a peak current of ±15A, featuring accurate and reliable drive functions, flexible and adjustable overcurrent protection, undervoltage detection, and an operating frequency exceeding megahertz. Electrical isolation reaches 6000VAC.
Figure 4 Functional block diagram of 2SD315AI-33
Figure 3 shows the external dimensions of the 2SD315AI-33, and Figure 4 shows its functional block diagram. It mainly consists of a DC/DC converter circuit, an input processing circuit, a drive output circuit, and a logic protection circuit. The DC/DC converter circuit isolates the input section from the operating section. Its input processing circuit consists of the LDIO01 and its peripheral circuitry. Since the PWM signal generated by the control circuit cannot directly pass through a pulse transformer, especially when the frequency and duty cycle of the pulse signal vary significantly, the LDIO001 is specifically designed for this purpose. This dedicated integrated chip encodes the input PWM signal so that it can be transmitted through the pulse transformer. Because the device has an internal Schmitt trigger, it has no special edge steepness requirements for the input signal and can provide quasi-static state signal feedback. Designed as an open-collector chip, it can adapt to any logic level and directly generate dead time. These advantages make the interface both easy to use and flexible, thus eliminating the need for many peripheral components required by other dedicated circuits. The core chip for the drive output and logic protection circuit is the IGDOOI. It integrates functions such as transformer interface, overcurrent and short-circuit protection, blocking logic generation, feedback status recording, power supply monitoring, and output stage identification. Each IGD is used for one channel, and its specific functions are to decode the PWM signal from the pulse transformer, amplify the PWM signal, detect and protect against short circuits, overcurrents, and undervoltage of the IGBT, and feed back the status to the LDI to generate the response time and blocking time of the short-circuit protection. 4.2 The actual application circuit is shown in Figure 5:
Figure 5. Practical application circuit of 2SD315AI-33
4.3 Experimental Waveforms
Figure 6 shows the gate drive signal waveform of the drive circuit designed using the method described in this paper when applied to a three-level frequency converter.
5. Summary
For the correct use of IGBTs, in addition to adding necessary snubber circuits, designing a good drive protection circuit is crucial. Before designing the drive protection circuit, it is essential to carefully study the key external characteristics of the switching device, which is vital for the correct design of the drive protection circuit. This article presents a drive protection circuit designed based on the analysis of the IGBT transient process and other characteristics. Leveraging the high integration and high reliability of the 2SD315AI-33 module, the overall circuit achieves high performance.
