IGBTs (Insulated Gate Bipolar Transistors) are ideal for applications with bus voltages in the hundreds to thousands of volts. As minority carrier devices, IGBTs have better turn-on characteristics than MOSFETs in this voltage range, while having a gate structure very similar to MOSFETs for easy control. In addition, since there is no need for an integrated reverse diode, this gives manufacturers the flexibility to choose a fast "composite package" diode optimized for the application, as opposed to intrinsic MOSFET diodes, which have reverse recovery charge Qrr and reverse recovery The time trr increases as the rated voltage increases.
Of course, the improved turn-on efficiency comes at a price: IGBTs typically have relatively high switching losses, which can reduce the application switching frequency. The trade-offs between these two and other application and production considerations have created the conditions for the creation of several generations of IGBTs and different sub-classes of devices. The large number of products makes it important to use a rigorous process when sizing, as this will have a significant impact on electrical performance and cost.
The method for IGBT selection is mainly through the following aspects:
1. Selection of IGBT rated voltage
After the three-phase 380V input voltage is rectified and filtered, the maximum value of the DC bus voltage: Under the condition of switching operation, the rated voltage of the IGBT is generally required to be higher than twice the DC bus voltage, and the 1200V voltage is selected according to the voltage level of the IGBT specification. grade IGBT.
2. Selection of IGBT rated current
Taking a 30kW inverter as an example, the load current is about 79A. Due to the current overload when the load is electrically started or accelerated, it is generally required to withstand 1.5 times the overcurrent within 1 minute. The maximum load current is about 119A, and 150A is recommended. current rating of IGBTs.
3. Selection of IGBT switch parameters
The switching frequency of the inverter is generally less than 10kHZ, and in the actual working process, the on-state loss of the IGBT accounts for a relatively large proportion. It is recommended to choose a low-on-state IGBT.
4. Factors Affecting IGBT Reliability
(1) Gate voltage
When the IGBT is working, there must be a forward gate voltage. The commonly used gate driving voltage is 15~187, and the maximum is 20V. The gate voltage has a great relationship with the gate resistance Rg. When designing the IGBT driving circuit, refer to the IGBT Datasheet. The rated Rg value in , design appropriate driving parameters to ensure a reasonable forward gate voltage. Because the working state of the IGBT has a great relationship with the forward gate voltage, the higher the forward gate voltage, the smaller the turn-on loss and the smaller the forward voltage drop.
In the case of bridge circuits and high-power applications, in order to avoid interference, when the IGBT is turned off, a negative voltage is applied to the gate, generally -5-15V, to ensure the turn-off of the IGBT and avoid the influence of Miller effect.
(2) Miller effect
In order to reduce the influence of Miller effect, improvement measures are adopted in the IGBT gate drive circuit:
(1) different gate resistances Rg,ON and Rg,off are used for turn-on and turn-off to ensure the effective turn-on and turn-off of IGBT;
(2) gate-source A capacitor c is added in between to discharge the energy generated by the Miller effect;
(3) a negative gate voltage is added when it is turned off. In practical design, a reasonable combination of the three is better for improving the Miller effect.
IGBT is the main power switching device mainly used in the inverter, and it is also the main working device in the inverter. Reasonable selection of IGBT is the premise to ensure the reliable operation of IGBT. type IGBT is preferred. According to the shed characteristics of the IGBT, the gate drive structure is reasonably designed to ensure the effective turn-on and turn-off of the IGBT and reduce the influence of the Miller effect.
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