Root cause analysis of electrical overstress of motor
I often see questions about the motor in the forums, such as "My motor drive has stopped working normally", "My motor drive is damaged" and "My motor is no longer spinning". The root cause behind it is often EOS. In the following we will briefly talk about what EOS is, and enumerate several common sources of EOS in the motor drive system.
First, the definition of EOS
The generalized electrical stress (EOS) refers to the current or voltage stress that the component is subjected to exceeds its maximum allowable range. Over-current stress is one of the factors that affect optocoupler devices.
Second, EOS regulatory limits
We understand what EOS is, so how can we know what the limit is? To achieve this, we 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 specifications for absolute maximum ratings are such that they will be permanently damaged. 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...
Third, the common roots of EOS
What are some common root causes of EOS in motor drive systems?
1, power supply overvoltage
One of the most common sources of EOS is the overvoltage event on the power supply input of the device. Power supply overvoltage can be caused by motor regenerative energy 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.
2, switching transients
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. But in reality, parasitic phenomena in power MOSFETs and printed circuit board (PCB) layouts can cause voltage transients - voltages will become lower than VGROUND or higher than VSUPPLY
3, MOSFET overcurrent
TI's integrated motor drivers feature overcurrent and overtemperature protection to prevent EOS in overcurrent conditions. For systems using gate drivers (with external power MOSFETs), care must be taken not to violate the MOSFET safe operating area regulations. Power MOSFET product specifications typically include a Safe Operating Area (SOA) plot. Excessive current in the power MOSFET will ultimately result in thermal damage to the device or its package.
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