Laurent Beaurenaut, Senior Staff Application Engineer, Electric Drive Train Systems, Infineon Technologies AG, Germany
In order to fulfil increasingly stringent requirements to improve fuel economy and to reduce emissions, the automotive industry is looking for innovative solutions. In the future, e-Mobility is expected to play a major role in the overall reduction of CO2 emissions. The mass production of this technology is highly depending on the introduction of cost effective and reliable power electronics. Infineon's product portfolio, scaling from bare dies and discrete ICs to power modules, is aimed at helping the development of optimized system solutions for hybrid and electric vehicles. In order for the electric motor to provide directly or indirectly traction energy to the vehicle, an inverter is used to convert the high-voltage battery dc voltage into ac signals driving the motor. Typically, an inverter is made of a high power IGBT module controlled by a smart logic operating in the low voltage (typically 12V) battery domain. Between the microcontroller and the IGBT switch stands a gate driver whose main role is to control optimally and safely the switching behaviour of the IGBT transistor. Infineon's 1ED020I12FA, 1ED020I12FTA and 2ED020I12FA automotive gate drivers have been specifically designed for this purpose.
The microcontroller and the high-power stage need to be electrically insulated. Infineon has developed the Coreless Transformer Technology (CLT), which enables galvanic insulation up to 6kV (according to EN50178). The main idea of the coreless transformer technology is to integrate the two coils of a transformer into an integrated circuit. Data transfer is enabled bi-directionally in an inductive way. CLT offers multiple advantages in comparison to other technologies. First, it does not show the degradation over life time which is typically seen with optocouplers. Secondly, with appropriate design and packaging measures, it shows high immunity to electro-magnetic interferences and transients. Finally, it can be easily implemented within standard chip production processes, which leads to a reduction of system costs in comparison for example to discrete approaches. Moreover, while a discrete transformer needs a core to direct the magnetic flux the coils in an IC can be placed close enough to save the core. Infineon EiceDRIVER™ automotive family are all based on the CLT technology. The 1ED020I12FA (Figure 1) is an advanced IGBT gate driver. It can source or sink up to 2A which makes it suitable to drive directly smaller MOSFET and IGBT power modules (up to 100A typically). Control and safety relevant functions are included in order to increase the reliability of the whole system. The driver consists of two galvanic separated parts: the input chip can be directly connected to a standard 5V DSP or microcontroller with CMOS standard I/Os, the output chip being connected to the high voltage side. The rail-to-rail driver output enables the user to clamp easily the IGBTs gate voltage during short circuit conditions, while at the same time limiting the power losses in the device.
The 1ED020I12FA has implemented several internal end external protection features. To ensure the correct switching of IGBTs, the device is equipped with separate under voltage lockout monitors for the primary and secondary sides. If the power supply voltage of either side drops below a given threshold, the IGBT is brought into a safe off-state. The PWM signals at the input are then ignored until the failing supply reaches a safe operating level again. In case of supply failure, a fast notification is sent to the microcontroller via the READY pin. Internally, the integrity of the signal pace across the CLT is strengthened by several mechanisms. In order to limit the effect of single disturbances, the PWM signal commands (both ON and OFF) are resent at periodic intervals of about 500ns. A watchdog function is also used to monitor automatically the signal transmission across the CLT. If a failure is detected, the IGBT is switched off and the READY output reports an internal error to the microcontroller. Externally, a desaturation protection (DESAT) ensures the protection of the IGBT in case of short circuit. The DESAT functionality monitors the voltage across the IGBT. When the voltage at the DESAT pin exceeds a threshold, the IGBT gate voltage is forced to low. Moreover, the FAULT output is activated, allowing a fast notification to the microcontroller. A programmable blanking time is used to suppress spikes and noise introduce by IGBT switching. Blanking time is controlled by a highly precise internal current source and an external capacitor. The internal active Miller clamp function prevents the occurrence of parasitic turn-on effects during high dV/dt situations (for example when the other transistor of the leg switches on). During turn off, the gate voltage is monitored and the clamp output is activated automatically in order to provide a low impedance path to the parasitic current flowing through the Miller capacitance. The 1ED020I12FTA is a further evolution as it provides an additional functionality: the Two Level Turn-Off (Figure 2). With this feature, the gate driver switches the IGBT off using two steps, i.e. by forcing the gate voltage to an intermediate value. Both plateau value and hold time are configurable. This way, too fast current variations can be avoided and therefore the collector-emitter voltage of the IGBT can be kept below the allowed maximum value.
The 2ED020I12FA goes one step further towards function integration since it provides high and low side isolated referenced outputs. The device consists of two galvanic separated drivers. The device also includes a DESAT protection with a FAULT status output for each of the drivers. Two READY status outputs reports if the device is supplied and operates correctly. Samples of the 1ED020I12FA and 1ED020I12FTA can be order in PG-DSO-20-55 Package. Samples of the 2ED020I12FA in PG-DSO-36/32-1 Package will be available in September 2010.
Over years, the general trend for electronic automotive systems has always been to go for more integration: the exponential increase of computational power of microcontrollers leads to the progressive replacement of hardware functions into software; digitalization leads to the introduction of smart sensors with enhanced functional and diagnostic capability. This general trend is also expected for the drivers, which are mainly relying today on analog technology. The realization of smart and cost-effective drivers, integrating a high share of digitalized functions and meeting functional safety requirements as per SIL and ASIL levels, will be one of the most exciting challenges the high-power automotive electronic manufacturers will have to face in the coming years. www.infineon.com