In megawatt-scale applications, there are three basic criteria that are instrumental to the success of a product: costs, level of serviceability, and system reliability. Given the significance of the IGBT driver integration here, a new generation of drivers is set to boost IGBT performance and lower costs at the same time.
Inverter controllers have to incorporate various parameters for efficient inverter control, the main requirement being sensor signal accuracy and dynamic response characteristics. The typical input parameters here are DC link voltage, output current and heatsink or module baseplate temperature. These signals are usually measured using DC link voltage sensors, AC-side current sensors and NTC sensors on the module substrate. Another important factor is that the signals have to be galvanically isolated before being converted into digital signals for processing. This setup calls for a separate power supply for the sensors and the evaluation circuits as well as appropriate wiring.An alternative to this rather complex solution is the IGBT driver Skyper Prime, which uses integrated sensing, the DC link voltages and the integrated module NTC being measured directly at module level. Both signals are galvanically isolated before being transmitted to the primary side for processing by the controller as digital PWM signals. The accuracy of the DC-side measurements is lower than 1.5% across the entire measurement chain, which is 25% better than in existing systems. Additional A/D conversion or level settings which would decrease the accuracy are not needed. The capture compare unit of the controller enables the PWM temperature and voltage signals to be processed directly. Separate signal conditioning boards are not required and the accuracy and dynamic response characteristics can be fully utilised. The result is a far more efficient life cycle assessment and optimised maintenance concept planning, reducing maintenance costs significantly.
More power thanks to efficient paralleling
Achieving symmetric current distribution is one of the biggest challenges when the IGBT modules in an inverter are connected in parallel to achieve the current flow direction needed in megawatt applications. IGBT chip tolerances, delay times and the gate voltages of the IGTB driver, as well as the load connections or cooling system can all affect current symmetry. In high-power applications with correspondingly large inverters, achieving current symmetry is a very challenging task. There are numerous ways of optimising current symmetry, one option being to use an additional AC inductance at the load output of the inverter. The problem here, however, is that this inductance mainly offsets the diode tolerances rather than the IGBT tolerances. A more effective approach is to optimise the individual factors that affect current symmetry. To do so, the individual tolerances of all the sub-components are reduced to a minimum, including the driver electronics, which has to deliver accurate drive signals across the entire temperature range. One key factor affecting current symmetry is the IGBT gate drive voltage. Temperature-based fluctuations in this voltage result in different switching times. Stabilized gate voltages solve this problem and ensure that the gate drive voltages remain constant irrespective of the customer-side voltage supply. This ensures that for the IGBTs connected in parallel no significant derating occurs across the entire temperature range.
Signal skew and its influence
Another key factor is the signal skew, which describes the difference in time between two rising edges. In an efficient circuit design for parallel connected IGBT modules, the skew is less than 50 ns. The main challenge here is ensuring that these differences in time are kept to a minimum without reducing EMC at the same time. The shorter the signal propagation delay, the more restricted the filter possibilities are. Given the sluggishness of IGBTs, propagation delays of less than 1 μs are therefore only required in certain specialised applications. For the propagation delays, however, the temperature-dependent tolerance has to be as low as possible. To ensure this is the case, the mixed-signal ASICs in the SKYPER Prime use, for example, digital input filters that remain stable across the entire temperature range. As a result, overload across the individual IGBT switches caused by asymmetric current distribution is prevented, as is any adverse impact on service life. Besides reduced tolerances, a symmetric gate driver board layout also offers the possibility of optimising synchronous switching times. What is important here is that split switching signals travel the same path, not only with regard to signal propagation delay but also owing to the different signal coupling behaviour in different gate driver circuit layouts. In parallel configurations in hard-switching conditions, parallel error and signal logic must also be possible. Error reaction must also be quick and not reliant on the external controller.
SOA optimisation for optimum output power
The Safe Operating Area (SOA) can be defined as the voltage and current conditions over which the device can be expected to operate without self-damage. The main limiting factor for this is overvoltage that occurs during IGBT turn-off. This voltage mustn’t exceed the maximum collector-emitter voltage of the IGBTs. Overvoltage is caused by parasitic inductance in the DC link, module inductance, coupling behaviour in the gate path, and the operating parameters of the inverter. The most critical scenario here is under overcurrent conditions. To ensure the driver electronics can fully utilise the available power of the IGBT module, overvoltages have to be reduced to a minimum both during normal switching operations and under short-circuit conditions. The most effective way of doing so is to use different gate-drive parameters in both cases. That means using different gate paths or output stages. The different behaviour of the chips at different temperatures has a huge impact on the size of the overvoltage. Tracking the chip characteristic across the entire temperature range enables the gate resistances and protective settings to be optimised, meaning the IGBTs can operate far more efficiently without additional control loops or clamping circuits. Active clamping is a rather widespread means of suppressing overvoltages after they occur. The additional losses and oscillations that result from this, however, can lead to unexpected system behaviour. Clamping diode tolerances also limit the effective DC link voltage and oscillations can result in unforeseen error conditions. A new alternative is to combine advanced short-circuit detection with SOA-optimised gate resistances. This would result in 5 percent more available power than in existing solutions. New IGBT drivers for megawatt inverters such as such as the Skyper Prime driver boast numerous advantages. Thanks to the integrated sense signals, overall system costs are up to 30 percent lower. The high level of accuracy and dynamic response of the voltage and temperature sensors are the key to efficient overall system monitoring, resulting in optimised maintenance planning and prolonged service life. Even in parallel configurations, the stable switching behaviour and the signal interface deliver maximum output power for high-power modules such as Semitrans 10.