Abstract: This paper addresses the overcurrent protection problem of IGBT drivers by proposing separate component drive overcurrent protection circuits and modular drive overcurrent protection circuits. The paper presents both circuits, analyzes their working principles in detail, and points out their advantages and disadvantages. Keywords: IGBT; separate component drive overcurrent protection circuit; modular drive overcurrent protection circuit. IGBTs are the preferred power devices for high-power switching power supplies and other power electronic devices due to their low saturation voltage drop and high operating frequency. However, like thyristors, IGBTs have low overload resistance [1-2]. Therefore, how to design an IGBT drive overcurrent protection circuit with complete overcurrent protection function is a problem that designers must consider. This paper summarizes the design methods of IGBT drive overcurrent protection circuits from an application perspective. 1. Overcurrent protection principle of drive overcurrent protection circuit. According to the technical data of IGBT, IGBT can withstand a maximum of 2 times the rated current within 10μS. However, frequent overcurrent will cause the device to age prematurely [3]. Therefore, the design principle of IGBT drive overcurrent protection circuit is as follows: First, when the overcurrent value is less than 2 times the rated current value, the protection can be achieved by instantaneously blocking the gate voltage; Second, when the overcurrent value is greater than 2 times the rated current value, since instantaneously blocking the gate voltage will make di/dt very large, a high peak voltage will be induced in the main circuit. Therefore, a soft turn-off method should be used to reduce the gate voltage to zero voltage within 2μS-5μS [4], until it finally becomes a reverse voltage of -5V; Third, an appropriate gate drive voltage should be used. Based on the above ideas, drive overcurrent protection circuit is now divided into discrete element drive overcurrent protection circuit and module drive overcurrent protection circuit. 2 Design of overcurrent protection circuit for drive 2.1 Overcurrent protection circuit for separate element drive Taking the overcurrent protection circuit for multi-power supply drive as an example, the overcurrent protection circuit for separate element drive [5] is shown in Figure 1. In Figure 1, T1, T4 and T5 constitute the drive circuit of IGBT, and DZ1, T3, D2 and C4 constitute the delay step-down circuit. T6, 555 integrated circuit and optocoupler LP2 constitute the delay circuit. When normally turned on, T1 and T4 are turned on. Due to the effect of D1 and R6, the circuit at point B will not exceed the breakdown voltage of DZ1. At this time, T3 is cut off, the potential at point D will not drop, the delay circuit does not delay, and T2 is cut off. When the IGBT flows through the short circuit current, the collector-emitter voltage drop of the IGBT rises. At this time, the potential at point C rises, and the rise time t1 is obtained by equation (1) [6]. In equation (1), VCC is the power supply voltage in volts; V1 is the breakdown voltage of DZ1 in volts; τ2 = R2 × C2 is the time constant in seconds; VC2 is the initial voltage of capacitor C2 in volts. When the potential at point C rises to the breakdown voltage of DZ1, T3 turns on, C4 discharges, the potential at point D drops, that is, the potentials at points F and G drop, and the gate drive voltage of IGBT drops. At the same time, optocoupler LP2 turns on, and the delay circuit starts timing. This timing time t2 is obtained from equation (2) [6]. In equation (2), VCC is the power supply voltage in volts; V2 is the 555 switching level in volts; τ2 = (R14 + R15) × C5 is the time constant in seconds; VC5 is the initial voltage of capacitor C5 in volts. If the overcurrent fault is eliminated within the 555 timer t2, the potential at point C drops back to its original value, DZ1 and T3 immediately turn off, C4 starts charging, the potentials at points F and G rise, the gate voltage of the IGBT returns to its original normal value, and the IGBT continues to work normally; if the overcurrent fault is not eliminated within the 555 timer t2, the 555 outputs a high level, which drives the optocoupler LP1 through T7, CD4043 and CD4081, causing the potential at point A to drop and remain, T1 to turn off, T5 to turn on, and the gate-emitter voltage of the IGBT finally becomes -5 volts, causing the IGBT to turn off, thus achieving delayed voltage reduction overcurrent protection. The total time t required from the occurrence of the overcurrent fault to the complete shutdown of the IGBT is t = t1 + t2 (3) In equation (3), the units of t, t1 and t2 are all seconds. In addition, the principle of the single-supply driven overcurrent protection circuit is similar to that of the multi-supply driven overcurrent protection circuit mentioned above, and can be found in reference [7]. It should also be noted [8]: (1) Select a suitable gate drive voltage value; the positive voltage value is generally 12V-15V, with 12V being the best, and the reverse voltage is generally 5V-10V; (2) Select a suitable gate series resistance value, generally a few ohms to a dozen ohms; (3) Select a suitable gate emitter parallel resistance value or Zener diode. From the above analysis, it can be seen that the overcurrent protection circuit driven by the discrete element is complex, but the design is flexible. 2.2 Module Drive Overcurrent Protection Circuit Taking the EXB841 series as an example, the module drive overcurrent protection circuit [9-10] is shown in Figure 2. In Figure 2, pin 9 is the reference ground, pin 2 has a potential of 20V, and pin 1 has a potential of 5V. When a high-level drive signal is applied between pins 14 and 15, the upper transistor in the complementary output stage of EXB841 is turned on, and the IGBT is turned on; conversely, when the input is low, the IGBT is turned off. The internal overcurrent protection circuit of EXB841 determines whether the IGBT is overcurrent by detecting the collector-emitter voltage Vce of the IGBT. Its judgment formula is: Vce + V1 + VD ≥ V2 (4) In equation (4), V1 is the potential of pin 1; VD is the forward voltage drop of diode D connected to pin 6; V2 is the breakdown voltage of the internal diode of EXB841. If V1=5V, VD=1V, V2=13V, that is, when Vce=7V, it is the overcurrent protection voltage threshold. When Vce<7V, the protection circuit does not work. Its protection function is: when there is an overcurrent, the gate-emitter drive voltage is reduced and combined with the slow turn-off technology [11]. After detecting a short circuit for 2μS, the gate drive voltage is reduced and drops to 0V within 10μS. During this period If the short circuit is eliminated within the specified time, the gate drive voltage returns to its normal value. If the fault persists, a fault signal is output from pin 5. After a certain time delay, the gate-emitter voltage of the IGBT eventually reaches -5 volts, simultaneously blocking the input signal. This avoids a hard shutdown caused by immediately stopping the input signal, which could lead to overvoltage breakdown of the IGBT. Its drawbacks are: 1. The negative gate voltage is too low, reducing the reliability of the IGBT; 2. There is no overcurrent signal lockout function, so if an overcurrent fault occurs, delayed protection shutdown cannot be achieved within the current operating cycle. In addition, the principles of the IR series, M579 series and VC37 series modular fast drivers are similar to those of EXB841, and will not be repeated here. Please refer to references [12], [13], [14] and [15]. 3 Conclusion The above introduces several IGBT drive overcurrent protection circuits. The overcurrent protection circuit of the discrete component drive is complex, but the design is flexible and the protection function is comprehensive. The overcurrent protection circuit of the modular drive simplifies the circuit design and has certain protection functions, but these protection functions are limited. When using it, we should also consider expanding its functions. As for which method to use in actual application, it depends on the actual situation. References: [1] Zhang Li. Fundamentals of Modern Power Electronics Technology [M]. Beijing: Higher Education Press, 2000. [2] Lin Weixun. Fundamentals of Power Electronics Technology [M]. Beijing: Machinery Industry Press, 1990. 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