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It is somewhat similar to the fairy tales of Cinderella or the Ugly Duckling: for many years, processors of all types, sizes and speeds have been glamorous items of general media attention, as well as large investments in research and development. At the same time, power supplies – mostly silicon-based MOSFET as well as IGBT – were clearly underestimated and faded into the background as part of a supposedly boring energy niche.
Things have certainly changed in the last few years, as power supplies, especially those based on gallium nitride and silicon carbide processes, are receiving a lot of attention and investment. It is not practical here to provide specific amounts in dollars, as it is so difficult to measure; there are many legitimate ways to address the specifics and many ways to interpret this attention. However, there is no doubt that power supplies are where much of the visible activity is currently happening, powered by electric vehicles, green initiatives and many other factors.
But power schemes don’t just live on their power supplies. The gate driver, which is usually a separate device for all but smaller power supplies, plays a key role in power supply management, ensuring that it meets its performance potential and ensuring that the operating limit associated with fluctuations in voltage, current, on / off speeds, parasitic inductance and capacitance, etc. are observed and monitored.
In fact, making the gate driver a close and suitable partner for the switching power supply is an important part of the design process. The right choice can make this power supply meet the target specifications, while a suboptimal mismatch can negate much of its potential and even damage or destroy it. As a result, gate drivers have their own sets of application notes and design guidelines; some are device-specific, while others are more general (see References).
However, there is another, somewhat silent partner in the connection that is behind the scenes and easily ignored until the end of the cycle: the driver’s own power supply to the gate, almost always a DC / DC converter. While the power supply has its own power rails through the load, the gate driver needs its own rails, which must be tailored to the specifics of the driver / power supply combination (Figure 1).
Why worry about the power source of the gate driver?
It reminded me of this obvious but easily overlooked reality when I came across the app note “Gate Drive Application Notes: IGBT / MOSFET / SiC / GaN DC-DC Converters”By Murata Power Systems. This note discusses some of the driver considerations and how they affect the driver’s DC / DC converter, which acts as its power supply.
The first point is that in many circuit topologies, such as the widely used bridge device, the driver and its DC / DC converter must be galvanically isolated from the ground together with a high power device; in some cases both high and low side devices need to be isolated.
But this is just the beginning. It also turns out that the bipolar DC / DC power converter may not have to be symmetrical, but it will work effectively with a negative bus driver with a lower voltage than the positive one. The need for only a reduced negative output voltage provides greater flexibility in designing or defining the power management capabilities of the DC / DC converter. For example, among DC / DC Murata pairing, there are 20-V / –5-V and 18-V / –2.5-V units, in addition to the more common symmetric bipolar values.
As Murata notes, “While 0 V at the gate may be sufficient for many devices, the negative voltage is usually between –5 V and –10 V allows fast switching controlled by a gate resistor. Any inductance of the emitter L between the switch and the reference driver [Point X in Figure 2] causes the opposite gate emitter voltage when the switch is turned off. Although the inductance may be small, only 5 nH would produce 5V at a di / dt of 1000 A / μs, which is not uncommon (5 nH is only a few millimeters of cable connection.) Appropriate negative drive ensures that the gate- emitter is always actually zero or less. “
There are also considerations regarding the transient response of the DC / DC converter, as the load represented by the driver suddenly changes as it drives and turns off the power supply. Murata’s note also makes it clear: “IGBT / MOSFETs should not be actively driven by PWM signals as long as the drive voltage rails are at the correct values. However, because the DC / DC drive of the port is powered or reduced, there may be a transient state where the devices can be driven even when the PWM signal is inactive, resulting in interruption and damage. Therefore, the DC / DC outputs must behave well when switching on and off the power with monotonic increase and decrease. ” Yes, we knew that, but was this aspect taken into account? The note points out some concerns when choosing the right DC / DC converter for the gate driver.
There are times when I am anxiously concerned that the multidimensional requirements placed on engineers will not lead to a reasonably good final design, as there are too many conflicting expectations in the face of inevitable trade-offs. And yet, despite my concerns, I see very, very good projects that somehow manage to cover, to an acceptable or even required level, the many electrical, environmental, regulatory, thermal, efficiency, cost, production and other overlapping requirements to them.
The attributes of the DC / DC converter of the device-driver are another set of these challenges with many degrees of freedom to juggle. It is unfortunate that the general public does not appreciate this and probably thinks it is easy. That is why I find it particularly worrying when politicians determine the form and speed of progress towards a commendable goal. If only it were that simple.
What is your experience with exposing gate drivers and especially their power converters? Has the performance of the converter even become an unexpected limiting factor or problem, even when the driver and power supply have been properly combined?
- Texas Instruments. “Fundamentals of MOSFET and IGBT port driver schemes”
- IR / Infineon. “Gate Drive Features and Requirements for HEXFET Power MOSFET”, Application Note AN-937.
- Texas Instruments. “Common Mode Transitional Immunity (CMTI) for UCC2122x Isolated Port Drivers”
- “Drive circuits for power MOSFET and IGBT”Application Note AN524.
Power supply for electric vehicles