28-12-2012, 03:11 PM
Adaptive Digital Drive of IGBTs for High Power Applications
1Adaptive Digital Drive.pdf (Size: 320.71 KB / Downloads: 36)
Literature Survey
The IGBT is attractive because of the high input impedance at the gate and the control that can be exercised over the switching by the gate drive. In its most basic form, the gate drive applies a step voltage to the gate input capacitance via a gate resistor. The gate resistor should be chosen to limit the dv/dt to an acceptable range, particularly at turn on, without introducing unacceptable delays. High current IGBTs have a specified gate resistor value, which is optimized for low switching losses and acceptable device stresses ( dv/dt, di/dt, current overshoot, and voltage overshoot). Another feature of modern IGBTs is the permissible use of their active region to the extremes of the safe operating area, typically for 10 s.
As the characteristics of insulated gate bipolar transistors have been constantly improving. Its utilization in power converters operating at higher voltage and higher frequencies has become more common. A central issue in reducing the size and cost of IGBT power converters is to control or limit dv/dt and di/dt during the switching process. Load side snubbers and clamp circuits are bulky and expensive. Increasing the gate resistors values is cheap and simple but switching times as well as power losses are increased.
ACTIVE GATE CONTROL
AVC refers to the direct control of the IGBT collector emitter voltage within a feedback loop. It is a classic feedback control method which reduces the dependence of the performance on the main plant (in this case the IGBT). The collector emitter voltage is the feedback term to the control loop, and follows the reference. Thus the IGBT voltage is controlled. The method has been successfully applied to a wide range of IGBT devices.
SERIES CONNECTION OF IGBTs
Series connection of IGBT or any other power devices is attractive. If the operating voltage can be increased for a given power rating, the operating current can be decreased. This leads to the advantage of that the power connections can be made smaller and the effects of stray inductance can be reduced greatly. Also transformers are no longer needed when operating at high voltages. Therefore, the equipments can be made smaller and lighter. Devices with lower voltage ratings usually have higher operating frequencies. Therefore, with IGBTs series connected, the operating frequency can be increased, which will also improve the performance of the equipments. When the operating frequency has been increased, at the same rating, the series connection of IGBTs can have lower power losses than one single IGBT. Therefore, series connection of IGBTs are of great advantage and desirable in industry. However, due to individual parameter differences of the series-connected IGBTs, it is difficult to ensure a proper voltage balance between them, and transient or steady-state voltage unbalances could cause the failure of these devices.
REASONS OF VOLTAGE UNBALANCING
Among all the factors that can cause voltage unbalancing during IGBT series connection, the most important factor is the device characteristic difference. As we know, the manufacture process of IGBT is very complex, including etching, oxide growth, deposition, diffusion and many other procedures. Many of these procedures can’t be very precise and therefore, even the
IGBTs have the same design and are manufactured at the same time, there will be some small differences among them. The gate-to-emitter capacitor, gate-to-collector capacitor, output capacitor and many other components have small differences and these factors would cause voltage unbalancing in switching transients and static voltage balancing which could cause the failure of these devices. For example, as shown in Fig , the two IGBTs have different I-V curves although they are the same model. When IGBT1 and IGBT2 are series connected, the current flowing through them will be the same but the voltage on each device is quite different which means the static voltage sharing is very bad. From the figure it can be seen that IGBT2 will go beyond its SOA faster than IGBT1.
VOLTAGE BALANCING
Voltage balancing can be achieved by using AVC technique. The voltage imbalance during switching is prevented from destroying the device by actively clamping the voltage across the device. If the voltage of the device with the highest voltage is controlled to be clamped to a reference voltage which is less than the voltage rating of IGBT, the overvoltage naturally distributes across the other devices in the series connection. This ensures a safe operation of power devices without reducing the speed of the IGBTs since the device voltage is limited when it exceeds the voltage reference. It also acts only on those devices which have overvoltages. Thus the efficiency will be comparable to ideal balanced case efficiency. In addition, the control does not act when the voltages are balanced, which is also important because devices that are already balanced should not be slowed down.