"Magic Smoke" is an informal term used to refer to corrosive fumes produced by overheated electronic circuits and components. This phrase is spoken by almost every electrical engineer - usually when they forget to adjust the supply voltage level or accidentally short the voltage rail to the logic pin.
The technical cause of the incident was electrical overstress (EOS); although human error is a common source, there are other possible causes and more subtle. Some of the most common issues posted to the TI E2ETM Community Motor Drive Forum (including those beginning with "My motor drive has stopped working properly", "My motor drive is damaged" and "My motor is no longer spinning" The reason behind the problem is often EOS. Below I will briefly explain what EOS is and list several common sources that can cause EOS in a motor drive system. In the next article, the author will discuss several ways to help prevent EOS in the motor drive system.
EOS is thermal damage that occurs when an electronic device is subjected to a current or voltage that exceeds the limits specified by the device. Thermal damage is generally caused by excessive heat generated in an EOS event (a large current across a resistor). This high temperature can cause damage to the materials used to build the integrated circuit, causing its operation to be destroyed or permanently altered.
Now that you know what EOS is, how do you know what the limits are? To do this, you must carefully review the Absolute Maximum Ratings table in the electronic product data sheet (see Table 1, which is from the DRV8701 product manual). The absolute maximum ratings are such specifications - permanent damage will occur beyond them. In the product data sheet, the absolute maximum ratings are different from the recommended operating conditions; if the specifications exceed the recommended operating conditions, the device can continue to operate, but only in specifications outside the recommended operating conditions of the product specification. An example of a violation of Table 1 is: If a transient event on the power rail, the VM power pin reaches 50V...
Table 1: Absolute Maximum Ratings Table for DRV8701
So, what are some common root causes of EOS in a motor drive system?
Power supply overvoltage
One of the most common sources of EOS is the overvoltage event on the power supply input of the device. The power supply overvoltage can be caused by the motor regenerative energy (as outlined in the previous article, see Figure 1 for an example) or external events of the system (such as component failure). Understanding the root cause of an overvoltage event requires monitoring the system power rail under all possible internal and external operating conditions.
Figure 1: Power supply overvoltage transients
Switching transient
Another common source of EOS phenomena in motor drive systems is the voltage transients associated with switching of power metal oxide semiconductor field effect transistors (MOSFETs). In an ideal half-bridge switching system, the voltage will alternate between VGROUND and VSUPPLY (Figure 2). But in the real world, parasitic phenomena in power MOSFETs and printed circuit board (PCB) layouts can cause voltage transients—the voltage will become lower than VGROUND or higher than VSUPPLY (Figure 3).
Figure 2: The ideal half-bridge driver
Figure 3: Half-bridge driver with parasitic phenomena
MOSFET overcurrent
The last EOS event I will mention is related to the overcurrent of the power MOSFET. TI's integrated motor drivers feature overcurrent and overtemperature protection to prevent EOS in overcurrent conditions. I discussed this in detail in a previous article, but for systems that use gate drivers (with external power MOSFETs), you must be careful not to violate the MOSFET safe operating area regulations. The power MOSFET product specification usually includes a Safe Operating Area (SOA) plot (shown in Figure 4). Excessive current in the power MOSFET will eventually result in thermal damage to the device or its package.
Figure 4: CSD18540Q5B maximum safe working area
In the next article, the author will discuss some common solutions that prevent EOS from appearing in motor drive systems. These solutions range from external protection components to simple design considerations. If you would like me to address a specific issue related to EOS, please post a comment below or ask questions in the TI E2E Community Motor Drive Forum.
Other resources:
Read other blogs in this series (specifically for the most common problems on the motor drive forum).
Are you looking for other suggestions to prevent EOS incidents? Then find the tips in this "Engineer It" video.
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