Nicholas Oborny
There’s a saying that great engineers do not just discover problems, they solve problems.
In my last post for this series on top FAQs, I explained exactly what electrical overstress (EOS) is, how it affects integrated circuit (IC) components, and several common sources of EOS in motor-drive systems (based on the saying above, that is the “problem”). One of my colleagues also covered supply pumping, a common source for EOS. But what steps can you take to prevent EOS (the “solution”)? In this post, I’ll walk through some of the more common methods that system designers can use to prevent or protect against EOS in motor-drive systems.
In lieu of investigating elaborate protection schemes and devices, one of the most common methods is to design a system with sufficient operating margin.
I’ll give a simple example. An AC/DC converter generates a 24 V power supply for a simple brushed DC motor-drive system. The DRV8701 H-bridge gate driver and CSD18509Q5B power MOSFET drives a brushed DC motor and are supplied directly from the 24 V.
In an ideal case, the 24 V supply is exactly that: 24 V. In the real world, the 24 V varies due to different load conditions, parasitics and regeneration currents from the motor. Being able to understand the supply-voltage variation allows you to select components with proper voltage ratings. It is not uncommon to see up to 2x margin on component ratings in motor-drive systems. In Figure 1, the DRV8701 supports up to 45 V and the CSD18509Q5B supports up to 40V to tolerate variation and transients in the supply voltage.
Another common method is what I call “throwing” bulk capacitance at the issue. This can be as unsophisticated as it sounds or as complex as creating a detailed simulation that accounts for parasitics and motor response using different bulk-capacitance sizing. Large ceramic or electrolytic (bulk) capacitors provide a local repository for charge. The large capacitor allows the system to provide the charge required in a load step or source the charge generated during motor regeneration without excess voltage transient. Figure 2 is an example of local bulk capacitance.
Almost all motor-drive systems are designed with some amount of bulk capacitance, although the size depends on the specific system. Some factors are:
If the previous two methods still don’t provide enough protection from EOS, transient voltage suppressor (TVS) diodes are the next most commonly used protection scheme. TVS diodes operate by shunting excess current when the induced voltage exceeds the breakdown level of the TVS. The TVS will automatically reset when the overvoltage condition goes away. It’s important to understand several parameters of the TVS, including:
You should also be well aware of the parasitic capacitance and inductance of the device, which might limit its effectiveness.
Some of the common downsides associated with TVS protection schemes are related to their size and cost. The size of the TVS directly relates to the amount of energy that must be removed from the system during the voltage transient.
These are just some of the more common methods of protecting against EOS; a multitude of other methods exist. If you have an EOS solutions or experiences to share or topics you’d like to see in future blogs, please log in to comment below. If you have specific question, you can search or ask the Motor Application Team on the E2E™ Community Motor Drive forums.
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2023, Texas Instruments Incorporated