6 seconds! Who will protect the extreme response when a fault occurs?
Automotive electrification has driven electronic fuses "eFuse" to replace mechanical relays and fuses for more compact and efficient solutions. The NIV3071 eFuse protects downstream circuitry from overcurrent, overtemperature and short-to-ground events and provides a fault indicator through the open-drain FAULT pin. The device features four integrated high-side channels that can be controlled independently via the EN pin or can be connected in parallel for larger loads. The device features configurable current limit and on-time to support a variety of loads.
NIV3071 in Automotive Applications
There are several advantages to using the NIV3071 in automotive applications. The device comes in a 6x5mm package, significantly reducing the required circuit board area compared to traditional solutions such as mechanical relays and fuses. In contrast to these traditional solutions, eFuse does not require replacement in the event of a failure because its protection features will protect both the device and the load, enabling a distributed zone control architecture throughout the vehicle. The device is available in two versions: latched and auto-retry. If a fault occurs, the latching device latches until a command is sent by toggling the EN pin or power cycling. The auto-retry device will wait 3ms, and if it enters thermal shutdown protection, will wait enough time to cool the chip before trying to turn it back on again.
Another advantage of the NIV3071 compared to traditional fuses is its very fast short-circuit response time of only 6 seconds in the event of a fault. This reduces peak current and power consumption, respectively, compared to the conventional fuse shown in Figure 1, which is beneficial to the peak current and power rating of any associated harness. Additionally, the fast response time prevents input supply voltage dips, protecting any safety-critical loads as well as other loads using the same power supply, as shown in Figure 2.
Figure 1. Blade fuse on left, PTC in center, NIV3071 eFuse on right
Figure 2. Multiple eFuses on the same power supply
The NIV3071 consists of four integrated channels that can be driven independently or connected in parallel to provide more load current, as shown in Figure 3. Inputs can be powered by different power supplies or a common power supply. With this flexibility, the NIV3071 can easily support 48V and 12V loads from the same device. This is very advantageous in automotive applications because both 48V and 12V loads in the ECU (Electronic Control Unit) can be protected with a single device.
Figure 3. Configuration of NIV3071
This device should be used with a regulated power supply. Since this device has UVLO functionality and no reverse current protection, it is not recommended for use in other automotive applications that require cold cranking and load dump (load dump).
Dynamic features and considerations
NIV3071 has a controlled conduction function, which can be described by the conduction delay time (tDLY(On)) and the conduction time (tRAMP(On)), as shown in Figure 4:
Figure 4. Turn-on timing of NIV3071
On-delay time is defined as the time between the EN pin reaching 90% of its maximum value and the output reaching 10% of its nominal voltage. On-time is defined as the time between 10% and 90% of the nominal voltage at the output. If there are any momentary voltage spikes on the EN pin, or there is a slight delay in the microcontroller sending the correct logic signal on power-up, the turn-on delay time is used as a deglitch filter to ensure the load is safely started at the expected turn-on time .
The on-time function is configurable with an external capacitor. To use the on-time function correctly, the load type must be considered. While for capacitive loads, increasing the on-time helps reduce inrush current spikes, for resistive loads, the current consumed will continue to increase as the output voltage rises. Since this DC current and the extended output on-time will cause a voltage gradient across the device, the device will dissipate significant power and may enter thermal shutdown to protect the chip before the device fully conducts. This depends on the input voltage, output voltage on-time, load current profile and ambient temperature.
Inrush current control
The NIV3071 and other eFuses from ON Semiconductor have an inrush current control feature that limits the peak current that occurs when capacitive loads are turned on. This function is controlled by an external pin, which can be left open circuit, or a smaller value ceramic capacitor can be used between the DVDT pin and ground. By adding capacitance to this pin, the output on-time can be extended, thereby reducing the peak current, which can be expressed as:
The following relationship can be used to control the peak current at a given Cload:
On-Time vs. dv/dt Pin Capacitance, TJ = -40°C
Figure 5. On-time vs. dv/dt pin capacitance
Using the above equation and Figure 5, the user can set the maximum inrush current when capacitive loads are turned on. Figures 6 and 7 below provide a test case:
Figure 6. Inrush current control test case
Figure 7. Measurement results of the test case
Large capacitive loads and Schottky diodes
For larger capacitive loads (and low current DC loads), a Schottky diode can be placed between the output and input, as shown in Figure 8, to protect the eFuse's body diode from excessive Effect of reverse current. Once the input voltage drops below UVLO, the device shuts down, which creates a reverse voltage across the device as the output capacitor maintains the output voltage until it is fully discharged. At the same time, the negative voltage biases the Schottky diode, providing a path for discharge around the device, as shown in the measurement results in Figure 9.
Figure 8: Using Schottky diodes to prevent reverse current conditions
Figure 9. Measuring the current flowing through the device and the external Schottky diode
Turn off inductive loads
During a short-to-ground or overcurrent fault event at the output, the NIV3071 quickly shuts down to protect both the device and downstream circuitry. If there is significant inductance in the power path (such as a cable), rapid shutdown will cause a voltage spike that exceeds the rated voltage of the device. To mitigate these voltage spikes, several options are available. A TVS diode can be placed between the input and ground, and a Schottky diode can be placed in parallel with the load as a freewheeling diode. In addition, RC snubber circuits can be used in parallel with the load. If the expected inductance in a specific application exceeds 5uH, the use of an external Schottky diode or buffer is highly recommended.
As the electrification trend in the automotive market continues to escalate and 48V systems become more and more popular, NIV3071 has many advantages in automotive applications.
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