Common Protection Functions of an Intelligent Power Device (IPD)
In safety-critical applications, like in automotive designs, reliability is key. Here are a few ways IPDs can guard against issues related to current, heat, and more.
An intelligent power devices (IPDs) is a semiconductor power switch with various diagnostic and failure detection features that allow us to reliably provide power to different types of loads.
The modern IPD can offer a few important features, including overcurrent protection, thermal shutdown, open load detection, and the ability to dissipate the demagnetization energy stored in an inductive load. These protection functions can minimize the risk and severity of failures (for example in automotive applications) even in the event of the processing unit malfunction.
Depiction of the differences between an IPD and a MOSFET. Image used courtesy of ROHM Semiconductor
In this article, we’ll first introduce a new family of IPDs from ROHM Semiconductor and then briefly discuss the common protection functions of an IPD and how they can help us to have a more reliable system.
ROHM’s New High-Side Switch ICs
ROHM Semiconductor recently announced a new family of AEC-Q100 qualified IPDs designed for automotive applications. In addition to more common protection functions, such as overcurrent protection; thermal shutdown; open load detection; and under-voltage lockout, the new IPDs allow users to set an adjustable overcurrent mask time to achieve the optimum protection for a given load.
The new devices are available in both 1- and 2-channel variants with RDS(on) ratings of 45 mΩ, 70 mΩ, and 90 mΩ. The company claims that the new IPDs can be used for driving different resistive, inductive, and capacitive loads in automotive applications.
IPDs can set an adjustable upper limit to the current that flows through the load. This feature allows us to protect both the switch and the load from excessive current. The overcurrent protection feature relies on an accurate current sensing mechanism.
High-performance IPDs can sense the output current with an accuracy in the range of ±3 %. While we can use discrete components to add the current limiting feature to a simple MOSFET switch, it can be very challenging for a discrete solution to achieve the accuracy of an IPD that relies on an integrated current sensing circuitry.
The ability to limit the output current finds use in many applications. For example, in an automotive seat heater, an IPD can be used to limit the maximum current and consequently, the maximum heat that is generated. In this particular application, a large amount of current even for a short period of time can damage the system and even cause dangerous situations for the user.
Another application in which the current limiting feature becomes vitally important is driving large capacitive loads. Charging a large capacitive load can draw inrush currents as high as 100 A. In addition to causing damage to the load and switch, a large transient current can lead to supply voltage droop, which can harm other circuits in the system that are connected to the same power supply.
With an IPD, we can limit the output current to safely drive a capacitive load. The following figure shows three of the capacitive load examples in the automotive applications.
Application diagram of automotive capacitive load driving. Image used courtesy of Texas Instruments
An IPD will shut down as its junction temperature crosses a certain threshold. Note that overcurrent protection discussed above cannot prevent the device from overheating because the dissipated power is related to both the current and the voltage drop across the switch. Even with a limited current value, we can have a high voltage drop, and consequently, an unsafe power dissipation.
IPDs can have two different thermal shutdown mechanisms: absolute thermal shutdown and relative thermal shutdown (sometimes referred to as ΔTj protection). With the absolute thermal shutdown, the device turns off as its temperature goes beyond a typical safe level of about 150°C.
However, the relative thermal shutdown measures the temperature difference between the device and the ambient temperature. The relative thermal shutdown will be triggered if the temperature difference goes above a typical threshold value of about 120°C.
How can relative thermal shutdown be helpful?
A large temperature difference between the switch and the surrounding environment can be an indication of an unreasonably rapid increase in the device temperature. In fact, the device temperature rises so rapidly that it cannot reach a thermal equilibrium with its surrounding environment.
In this case, the relative thermal shutdown disables the output to prevent an imminent failure. Besides, when a short-circuit occurs with an inductive load, the relative thermal shutdown allows us to more reliably dissipate the demagnetization energy of the load.
Open Load Detection
With open load detection, we can implement diagnostic features that can recognize broken wires and faulty modules (open-circuit failures).
IPDs can have two different types of open load detection. Some of them can detect an open-circuit failure only when the output transistor is off. As shown below, these IPDs rely on a pull-up resistor between the output and the power supply.
The above circuit monitors output voltage when the output MOSFET is off, allowing it to detect an open load. Image used courtesy of Toshiba
If a load is present, the output voltage will be below a certain value. However, with an open failure, the output will be very close to the supply voltage.
The second open load detection mechanism is based on accurately measuring the output current (when the switch is on). If the load current goes below a certain level, an open failure situation will be communicated back to the processing unit. This requires information about the normal current value that should flow through the load when there is no open failure.
An example application that can benefit from this diagnostic feature is driving an LED load that consists of multiple LED strings in parallel. If the connection to one of these LED strings is broken, the IPD will sense a change in the output current and signal an open load situation.
The Ability to Dissipate the Demagnetization Energy
When driving inductive loads, such as relays, motors, and solenoids, magnetic energy will be stored in the load. When turning an inductive load off, the stored magnetic energy can create transient voltages as high as hundreds of volts that can severely damage the drive circuitry.
The IPD should have an efficient clamping circuitry that keeps the voltage across the switch below a certain threshold and safely dissipates the stored magnetic energy.