Here’s a different way to drive IGBTs
Most volume users of IGBT modules view the design of the gate drive circuit as a core competence, but now there is a a different approach to the design and deployment of power switching electronics, writes Mark Snook, chief technology officer at Amantys
The insulated gate bipolar transistor (IGBT) has been around for about 30 years as a commercially available product, and has been through several generations of successively improved performance and usability.
They are used in diverse range of power conversion applications, from lower voltage domestic appliances through hybrid electric vehicles to trains and ships at the high voltage end of the range.
Each generation has delivered steadily improved performance with faster switching, greater ruggedness and reduced vulnerability to failure. These advances have led to wider deployment and are enabling new potential applications such as solid state transformers and high voltage DC (HVDC), or renewable power generation both in wind turbines and solar farms.
However, in contrast to most other areas of electronics, this is a domain in which architectures have remained almost exclusively analogue, with only limited deployment of digital control techniques. Systems are built around IGBT modules that still require a bespoke “gate driver” circuit to operate the switch safely and effectively.
Even if an application employs a module available from more than a single power transistor manufacturer, the gate drive needed to operate apparently compatible modules must be uniquely matched to each, and needs to be evaluated, characterised and configured for that specific combination of module and drive.
Most volume users of IGBT modules view the design of the gate drive circuit as a core competence, and therefore only around a third of the market today is served by commercially available gate drives – the greater quantity are served with in-house design and supply.
Until now, this status quo has remained unchallenged, but now there is a a different approach to the design and deployment of power switching electronics.
At every stage of the power industry from generation, through transmission and distribution to consumption, there is intense pressure both to reduce carbon emissions through greater efficiency, and to provide greater output to match the demands of a growing global population: now there’s a need for change.
After three decades of delivering system performance hampered by the limitations of the core technology, the combined challenges of reliability, efficiency, simplicity and control mean that there is also now an appetite for a fresh approach to power electronics.
Problems of dual sourcing and design
In the development phase, the variations in gate drive requirements for between different IGBT module manufacturers oblige the OEM to choose between being sole-sourced for key components in the supply chain, or qualifying two sets of module and gate drive vendor combinations.
Product supply shortages in recent years have left supply chain managers reluctant to expose themselves to single-source risk for such critical components. But even if they approve dual-sourced solutions, the OEM still has to run two systems in manufacture, since neither module nor gate drive offers a drop-in replacement.
In addition, the system switching characteristics have to be set at the design stage, and cannot be varied in system with changing environmental and load characteristics.
This therefore puts pressure on extensive characterisation of the IGBT module in the development phase to define those parameters for the load and duty cycles of the specific application.
Parallel sourcing has further implications for engineering resources: power electronics engineering skills are in short supply, and those with extensive experience are very much in demand. The need to design, test and qualify two similar systems simply adds an unwanted extra burden on scarce resources that could be better deployed to create greater competitive advantage.
Design complexity and device characteristics
In high voltage applications, the challenges facing a designer are particularly acute. The design must ensure sufficient creepage and clearance for good isolation; there are increased inductive effects with higher current, plus it’s more difficult to get enough charge into the device quickly, and there’s the perpetual trade-off between faster switching speed for conversion efficiency to balance against reduced noise and EMC radiation.
OEMs are frustrated that apparently compatible IGBTs have different performance and drive characteristics, and there isn’t enough consistency between different modules. In addition, each application demands different performance parameters and this takes expensive re-characterisation to define the safe operating area of the IGBT between frequency, temperature and gate resistor settings.
During the development process, any change of gate resistor settings means the cold plate assembly has to be disassembled and rebuilt – a time-consuming and non-productive effort. There’s little reuse between dual designs in development, but when it comes to qualification and certification, the work and cost is simply doubled.
The consequence is that many volume users of IGBT modules take the view, perhaps reluctantly, that the design of the gate drive is a core competence that has to be kept in house. Up to now, this has limited the available open market for gate drive products, but with the arrival of more advanced drives for all, this is set to change.
Field failures are costly in any system, and this is especially so in power electronics. An IGBT failure on a wind farm could lose several days of production – usually in times of peak wind availability – and this is particularly acute for an offshore installation.
Two factors are central in power module failure – the total number of switching cycles and the thermal stress of the module in ope ration.
Unfortunately though, there is no available mechanism for fault indication, fault diagnosis, or failure prognosis: the system is generally on, off, or faulty.
In the case where the power module fails catastrophically, the damage is such that there’s nothing left to indicate what caused the failure. Was it a problem with the IGBT itself, or was it driven outside its specified safe operating area, or did the gate drive fail unsafe?
The term “black box” in this context tends to mean exactly that – there’s not much left to indicate what happened!
The first stage in delivering better power switching solutions is to build products that that look and feel familiar, but already break some long-established practices.
For example, the Amantys Power Drive can simplify design by operating a series of different manufacturers’ IGBT modules through one single drive in a simple programmable configuration.
Power monitoring and control
Exporting performance data from of a high voltage inverter is difficult because of isolation and noise, so even achieving a basic level of monitoring is a challenge.
Our approach is to extend the familiar format of existing gate drives with the addition of extra circuitry to provide simple monitoring and control signals.
During development, the gate resistor settings can be modified and refined in system via the fibre optic link, and so saving the need to disassemble the cold plate assembly many times over. This eliminates laborious assembly and disassembly giving a major benefit in development time and flexibility.
Since the drive and module are already characterised, the manufacturer no longer needs to experiment to determine safe operating areas.
The gate drive integrates additional circuitry to monitor and sense a series of key power transistor and gate drive characteristics. Key parameters tracked include drive temperature cycling and number of switching operations to give a leading indicator of potential system faults or failures.
In the event of a fault, the data log gives instant information on the events that preceded the fault.
Longer-term, an intelligent power switch integrates a tightly coupled power transistor, gate drive and communications processor in a single power module with controlled feedback of power switching for optimum system performance.
This allows the designer to use a standard building block to create and configure the power conversion architecture needed, regardless of the underlying power transistor technology, controlling the switching over a standard digital interface.