SmartFETs with high sides have gained popularity because of their simplicity and high degree of protection. SmartFETs can be used in a variety of automotive applications, just like standard MOSFETs. The control circuitry that is built into high-side SmartFET devices is what sets them apart. The control circuitry monitors the output current and temperature of the device and provides passive protection against voltage transients. The combination of passive and active protection features provides a robust application solution that extends the life of both the device and the load it protects.
onsemi now has a range of high-side SmartFETs from 45m to 160m. These devices can be used to switch different loads such as solenoids and bulbs. Table1 shows that the device name is indicative of the RDS At at 25degC. Below is a complete list of all products:
Table 1: The Complete Family of onsemi High-Side SmartFETs
onsemi’s devices are housed in an SO8 box. This allows for a compact footprint and high power. The family pinout allows for one pinout to be used for different application loads. You can swap out any device you need depending on the current requirement for that application. These devices can drive 12V automotive grounded loads, and offer protection and diagnostic capabilities. Advanced protection features include active inrush current management and over-temperature shut down with automatic restart. The Current Sense pin is a dedicated analog current monitor that provides precise fault indications of short circuit to ground, short to battery, and ON/OFF state open load detection. An active-high Current Sense Delay pin (NCV84160) or an active high Current Sense Enable Pin (all parts of the family) can disable or enable all diagnostic and current sense functions.
Switch loads is the “end requirement” for a high-side SmartFET. There are many options available on the market to achieve this end. For example, relays have been used in the automotive industry for many years to switch different loads, particularly those that require high current activation. There has been a steady reduction in the size and weight of automotive components and assemblies. This has led to a shift from relays to more compact semiconductor switches. These are smaller and less noisy and can be used for electromagnetic interference and noise immunity.
The high-side SmartFETs are now the most popular configuration for SmartFETs in the automotive market. They have replaced the simpler, lower-side SmartFET. Figure1 illustrates a high-side and low-side SmartFET configuration. A high-side SmartFET’s load connects to ground via a switched connection, while a low-side SmartFET connects to power supply via a switched connection. The SmartFET is usually housed in an ECU or control unit. The length of the load line is the length of the cable that connects the load and the pin connector on an ECU. This load line can belong to any type of load and where it is located in the vehicle. It also increases the chance that the load line will shorten to the chassis ground, which could cause severe stress for the load in a SmartFET configuration. This makes the high side SmartFET preferred for load switching.
Figure 1: High-Side Versus Low-side Switch in an Application (Source: onsemi)
The figure 2 shows the top-level block diagram, pin configuration and the NCV84xxx high side SmartFET family by onsemi. The high-side SmartFET, which is actually an NMOS FET with a Regulated Char Pump, draws the gate voltage high enough to drive the load.
The input (IN) pin controls the charge pump control logic and turns on the FET. The Current Sense Enable pin (CS_EN), enables or disables the Current Sense feature. The Current Sense pin allows for proportional load current sensing and feed back to the microcontroller to provide real-time feedback. Multiplexed, this pin reports analog fault events that are easily distinguished from normal operation. This allows the user to sense the output current and fault conditions in real time. The voltage (VD), pin connects directly with the power supply or battery, while the OUT pin connects the load. The ground (GND), pin simply refers to the device GND.
Figure 2: Block Diagram and Pin Configuration of NCV84xxx (Source: onsemi)
The NCV84xxx SmartFET family of devices offers the following protection features:
- Over Voltage Protection protects the entire device clamping VD-GND for voltages > 41V
- Undervoltage protection, in case of a low battery voltage, turns the device off and waits until the battery voltage has risen high enough to operate the Regulated Charge Pump to run the FET properly.
- Current Limit (see Figure 3 below) limits the current in case of a short-circuit or in-rush event to prevent damage. The current will stay limited until the internal die temperature of the device exceeds the overtemperature point and will turn itself off for protection until it has cooled down enough. This feature is excellent for driving bulb loads that require a high initial inrush current, and it also limits the amount of stress on the die from high power and temperature swings.
- Overtemperature and Power Protection with Auto-Restart protect the device from overheating due to high power dissipation and excessive ambient temperature rise. If overtemperature protection is activated, the device will shut itself off until it cools enough and auto-retry, assuming the input is “high.”
- OFF-State Open Load Detection alerts the microcontroller that the connection to the load has been lost in the Off-State before turning the input “high.”
- Output Clamping for Inductive Load Switching during an inductive discharge event, the output clamp will safely turn the FET on to handle the inductive discharge current.
Figure 3: How TJ Progresses During Short to GND/Overload (Source: onsemi)
For in-depth information on how the high-side SmartFETs operate, including protection functions, current sensing, etc., please refer to onsemi’s application note—High-Side SmartFET with Analog Current Sense Application Note.