Q: What is the charging process of the gate charge Qg during the turn-on of the IGBT device?
A: The relationship between the gate charge QG and the gate voltage vge is shown in Figure 1 in the data sheet for IGBT devices such as the IHZA50N65EH7. Figure 2 is a simplified schematic diagram of an IGBT device. The charging process of gate charge can be divided into the following three regions. The capacitor CGE is charged between the time period AB, and when the gate voltage VGE reaches the gate threshold voltage VGETH, the IGBT device starts working. In the time period BC, the charging process of the gate is determined by the feedback capacitance CGC (also called Miller capacitor), at this time, the VCE voltage is continuously reduced, and the current IGC discharges the gate through CGC, and the gate voltage remains constant at this time, this phenomenon is called Miller platform. During the CD period, the IGBT device enters saturation, and the DVCE DT will drop to zero, at this time, the gate current IG continues to charge the gate-emitter capacitor CGE, and the gate voltage continues to rise.
Figure 1
Figure 2Q: In practical application, how to determine the value of the external gate resistance of IGBT devices?
A: The selection of gate drive resistance has a great influence on the switching characteristics and switching losses of IGBTs. In the IGBT data sheet, the gate resistance RGON RGOFF is an important condition in the switching loss test conditions shown in Figure 3, and the principle selected is the value under the specified device test conditions, for example, the value of RGON should be guaranteed to be not ** at room temperature and 1/10th current.
In addition to limiting the charge and discharge current of the gate and affecting the switching speed of the IGBT, the gate resistor also has other effects, such as: increasing the power loss of the drive loop; Reduction of electromagnetic interference; Prevents gate oscillation; Avoid parasitic turn-on of the device, etc. In order to take full advantage of the switching performance of IGBT devices, gate drive circuits often use independent gate resistors for turn-on and off, as shown in Figure 4. The shutdown loop series fast recovery diode can make the gate shutdown resistance less than the turn-on resistance, which is mainly due to the fact that for some power devices, the turn-off delay time is often longer than the turn-on delay time.
Figure 3
Figure 4 On the other hand, to avoid parasitic turn-on of the device, as shown in Figure 5, if the shutdown gate resistance is large, the IGBT is turned off during the shutdown process under the action of high DV DT and Miller capacitance CGC, according to the formula.
The gate voltage of the IGBT will be raised, once the VGE voltage is higher than the threshold voltage VGETH, it will cause the parasitic opening of the IGBT device, if the half-bridge circuit occurs the upper and lower tube pass-through phenomenon, it will affect the reliability of the system. If the turn-off resistor is too small, it may cause a large VCE voltage spike due to too high a DI DT during shutdown, resulting in damage to the device. Therefore, there is a trade-off between switching speed and reliability when selecting a gate resistor. In the IGBT data sheet, Figure 6 shows the curve of IGBT switching loss and gate resistance under the specified test conditions, which can also be used as a reference for designers. However, in order to ensure that the selection of gate resistance is truly applicable to the actual system application, the experimental verification should be carried out in the actual system. For more information about gate resistor selection and precautions, please refer to the application note: AN2015-06
Figure 5
Figure 6Q: What is the cause of gate voltage waveform oscillation of IGBT devices?
A: In power electronics applications, gate voltage waveform oscillations are often seen. As shown in Figure 7, the driving current IG flows through the gate resistor RG of the drive loop, the parasitic capacitance CG of the parasitic inductance LP and IGBT devices, forming a resonant circuit, and oscillation occurs under the action of excitation. In order to avoid or suppress oscillation, the most important point is to optimize the layout of the PCB drive circuit and reduce the parasitic inductance LP of the drive loop. On the one hand, trace lengths can be shortened to keep the gate loop as short as possible, typically reducing parasitic inductance by 1 nh mm. When an IGBT device has a Kelvin Emitter pin, as shown in Figure 8, the return end of the drive loop can optionally be connected to the Kelvin Emitter pin.
Figure 7
Figure 8 In the design, when the above optimization method cannot be achieved due to geometrical reasons, a large gate resistance can be used to increase the damping effect of the drive loop and to suppress gate oscillation. Simply increasing the gate resistance will reduce efficiency and may result in a longer delay time in the switching process, so the resistor value should be chosen carefully. In addition, if PCB space allows, the gate resistor should be as close to the gate pins of the IGBT device as possible.
Q: What is the minimum turn-on time for an IGBT device?
A: When the IGBT or diode chip is first turned on, it will not be filled with carriers immediately, and when the IGBT or diode chip is turned off when the carriers are spreading, the current change rate DIC DT or DIF DT may increase compared to the shutdown after the carrier is fully filled. Due to the increase in DI DT and the effect of the commutation stray inductance, the IGBT can generate a higher voltage overshoot when it is turned off, which may also cause an increase in the reverse recovery current of the diode, resulting in a "snap off" phenomenon. Whether or not this phenomenon occurs is mainly related to the chip technology, voltage and load current. It should be noted that different devices exhibit different voltage overshoot phenomena during the turn-on process, which means that there is no uniform conclusion on the effect of the minimum turn-on time. In the application, the minimum turn-on time of the IGBT device needs to be adjusted according to the actual situation.
Q: Can IGBT devices withstand backpressure? Is the reverse blocking voltage capability related to the VCE breakdown voltage?
A: As shown in Figure 9, compared to MOSFETs, IGBT devices do not contain body diodes, so IGBT devices can only flow current in one direction, that is, from the collector direction to the emitter direction, so IGBT devices should avoid being subjected to reverse voltage. Since most industrial applications are inductive loads, it is necessary to de-parallel a diode chip next to the IGBT chip, or simply select an IGBT device with an integrated diode chip. For example, IGBT devices with the new 600 V Trenchstop Performance technology, which are mainly used in industrial drives, solar inverters, and large household appliances, are available with and without parallel diodes, as shown in Figure 10.
Figure 9, Figure 10Q: How to calculate the operating junction temperature (TVJ) of an IGBT device?
A: Calculated according to the following formula:
where thermal resistance.
For the values of the IGBT device at the time of transient pulse, you can refer to the transient thermal resistance curve in the data sheet, as shown in Figure 11.
is the total loss of the IGBT device, including switching loss and conduction loss. The switching loss of a single cycle can be obtained by testing the switching waveform of the IGBT device, which is integrated by multiplying the voltage and current. The calculation of the system loss of IGBT in the application can be found in the following article: Dry goods|How to evaluate the loss of IGBT modules in the system.
is the shell temperature of the IGBT device. The final calculated operating junction temperature of the IGBT device cannot exceed the maximum operating junction temperature in the data sheet.
This is shown in Figure 12.
Figure 11, Figure 12Q: When selecting the gate driver of the IGBT device, how to calculate the maximum peak current provided by the driver chip for the IGBT device?
A: When choosing a gate driver, an important parameter is the maximum peak current of the driver. This parameter can be estimated using the following formula:
where the peak current (a) that must be supplied to the driver;
is the positive gate voltage (V) used to turn on the IGBT;
is the negative gate voltage (V) or 0 used to shut down the IGBT;
is the gate resistance inside the IGBT ( ).
is the gate resistance ( ) outside the IGBT
0.7. The correction factor for calculating the peak current in practical application. If a gate-emitter capacitor CGE ext is added to the outside of the IGBT driver, the approximate approach is to equate this capacitor to an internal gate resistance short, i.e.
This parameter can be set to 0. If different gate resistors RGON and RGOFF are used, then the peak current required is determined by the smaller resistor, and then the gate driver is selected.
Q: How to understand the meaning of SC data for short circuit circuit in the data sheet of IGBT module?
A: As shown in Figure 13, the short-circuit time TP indicates how long the short-circuit will affect the IGBT device, and the short-circuit characteristics of the IGBT are usually related to several parameters, the driving voltage VGE, the DC bus voltage VCC and the set-emitter voltage VCE, the short-circuit time TP, the junction temperature TVJ, and the IGBT technology. Figure 14 is a schematic diagram of the voltage and current waveform of the short circuit test. It is important to note that the short-circuit time TSC only applies to the conditions specified in the data sheet. If the driving voltage of the IGBT is higher vge, the short-circuit current will be larger; If the DC bus voltage is higher, the energy accumulated during the short circuit is higher; If the initial junction temperature TVJ is higher, the junction temperature will also be higher when the IGBT is short-circuited. All of these factors result in a shorter short-circuit time that IGBTs can withstand. If the actual short circuit time is too long, the IGBT device may be damaged. The data sheet only gives a maximum short-circuit time under the specified conditions as a reference.
Figure 13, Figure 14Q: Is it possible to solder the IGBT modules of the Easy Econo series for crimp assembly?
A: According to the international standard IEC 60068 series standard Section 2 (260°C < = 10s), the standard specifies that the welding temperature of the module is 260 °C and the welding time is up to 10 seconds. During the welding process, the maximum permissible case temperature of 223 must not be exceeded. For more detailed welding requirements, please refer to AN2005 06.
Q: What is the CTI value of a single tube for IGBT in To247 package?
A: When designing the PCB layout of IGBT single tube applications, the CTI value is often referred to to determine the creepage distance. In general, the mold compounds of IGBT single-tube packages belong to Class II materials (CTI is 400-600V), and the specific CTI value cannot be guaranteed, but it can be determined that the CTI value range for IGBT device TO247 package is 400-600. The CTI value of the IGBT module can be viewed in the data sheet of the corresponding product, as shown in Figure 15.
Figure 15 If you need to purchase IGBT single tube, apply for sample testing, BOM matching and other needs, **Customer service WeChat: 13310830171.