Interface circuits such as MOSFET/IGBT gate drivers often don’t get much respect or attention. After all, these components – which are “analog” in their performance despite their apparent digital outcome – don’t seem to do much: some level translation, some signal boosting, adding some extra current sink/source capability, managing slew rates, and perhaps protecting the source from load faults and the reverse. There’s the misguided perception that there’s not a lot of “intelligent” value added by these devices to the system.
Of course, that’s not the case, as there really are few, if any, insignificant components in a circuit; if there were, they would have been deleted or commoditized a long time ago by the pressures of reducing cost, space, and dissipation. Still, there’s a lack of respect and attention that they get that is the result of these misconceptions.
Fortunately, those who provide MOSFETS and IGBTs, or their drivers, as well as those who have to design them into circuits, know better – or soon do. In many cases, vendors of these power devices suggest well-matched drivers from another vendor with whom they work closely; in other cases, these power-device vendors have drivers in their own portfolio which are a good fit.
Even so, that suggested “best” driver may not be the one to choose. Every driver – as with other components – is a compromise among various technical, cost, and other factors, with one or more of them having higher priority in a particular design. As a result, a designer may have to choose a different driver, or enhance the suggested one. For this reason, looking at detailed papers on driver selection and design is often a necessary “evil” in the design cycle.
As a result, driver vendors are introducing devices which implement strategies that go beyond the basic functions. For example, some gate drivers from Texas Instruments include features they call IDRIVE and TDRIVE (see “Understanding IDRIVE and TDRIVE in TI Motor Gate Drivers”). IDRIVE provides the ability to dynamically adjust the gate driver’s output-drive current, which enables control of the MOSFET VDS slew rate, Figure 1. (MOSFET VDS slew rate is important because it is a key parameter for achieving optimal switching efficiency and minimal parasitic effects.)
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Managing and adjusting slew rate of the driver output, here done by the DRV8305, offers the potential for improved performance across several key MOSFET parameters. (Image source: Texas Instruments)
The TDRIVE function automatically determines the appropriate amount of dead time regardless of the inputs, MOSFETs, or other system conditions, determining when one MOSFET has been disabled and the other can be enabled. It monitors the MOSFET’s VGS, and enables insertion of the optimal amount of dead time even when system parameters change as the motor drive operates. The result is carefully managed turn on/off for greater efficiency, but without the dangers of shoot-through, which is one of the nastiest misfortunes that can happen to a MOSFET bridge topology, Figure 2.
Shoot-through occurs when both the upper and lower MOSFETs are on-times overlap, even for a very brief period with the result of a short from power to ground as well as possible power-device damage; device turn-on/off timing must allow for a brief “dead time” when neither device is on, in order to avoid the possibility of this happening. (Image source: Texas Instruments)
Other vendors, of course, are also looking at how more analog and simple logic circuitry can be added to the gate driver. Nearly all have written detailed tutorials and application notes about the issues associated with gate drivers, see References for just a few of the many available resources.
Before you just drop in the suggested driver, it’s worth taking the time to investigate this “humble” component and ask:
- What resources does the vendor of the MOSEFT/IGBT offer for choosing a driver?
- How well does this driver meet the many conflicting goals and priorities I have in my specific design?
- Is there a driver available, other than the recommended one, which is a better fit for my application?
- Is there a driver from an independent vendor (who does not make MOSFETs/IGBTs) that might be a better choice?
All this seems obvious, and perhaps like a lot of extra work for this basic interface component. However, given the many requirements placed on the power devices and their drive circuitry, it’s worth some time to investigate. After all, the right driver can make the difference between a pretty good drive circuit and a really good one, which in turn may make the difference between a decent product and a stand-out one.
1 Texas Instruments, SLVA714A, Understanding IDRIVE and TDRIVE in TI Motor Gate Drivers
2 IXYS Corp., Application Note 401, MOSFET/IGBT Drivers Theory and Applications
3 Fairchild/ON Semiconductor, AN-6069, Application Review and Comparative Evaluation of Low-Side Gate Drivers
4 Vishay/Siliconix, Application Note AN608A, Power MOSFET Basics: Understanding Gate Charge and Using it to Assess Switching Performance
5 Infineon/International Rectifier, AN-937, Gate Drive Characteristics and Requirements for HEXFET Power MOSFETs
6 Microchip Technology, Inc., AN898, Determining MOSFET Driver Needs for Motor Drive Applications
Drivers are driving me crazy
Where’s your inefficiency?