Design of automotive electronic protection circuit
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The automotive environment is very demanding for electronics: any circuit connected to the 12V supply must operate within the nominal voltage range of 9V to 16V, and other pressing issues include load dump, cold crank, reverse battery, dual battery boost, spikes, noise, and an extremely wide temperature range. During load dump, the output voltage of the alternator quickly rises to 60V or higher; cold crank refers to starting the car when it is cold, which causes the battery voltage to drop to 6V or less; reverse battery is caused by carelessly reversing the polarity of the cables when activating a dead battery. Many towing trucks are equipped with two 12V batteries connected in series to help start a car with a dead battery in cold weather. This will increase the voltage range of the electrical system to 28V until the car is started and the towing truck driver disconnects the jumper cables.
Considering that the automotive electrical system consists of high-current motors, relays, solenoids, lights, and constantly chattering switch contacts, it is not surprising that spikes and noise are present. In addition, the alternator is a three-phase motor regulated by chopper excitation, which sometimes charges the battery with very high current. Therefore, circuit design for working in the automotive environment is especially important to accommodate the high input voltage circuits generated during load dump and dual battery boost situations.
Passive protection circuit
A passive protection network for automotive electronics is shown in Figure 1. Circuits similar to this are widely used to protect various systems connected to the car's 12V bus. This network protects against damage from high voltage spikes, sustained overvoltage, reverse battery, and excessive current draw. The current protection in Figure 1 is obvious, and fuse F1 will melt if the load current exceeds 1A for a long time. D1 combines with F1 to prevent damage from reverse battery connection, and a large current flows through forward-biased D1 and blows the fuse. Electrolytic capacitors have an interesting characteristic at about 150% of their rated voltage: As the terminal voltage increases, this capacitor consumes more and more current, and in the case of C1, it acts as a clamp (ultimately blowing the fuse) as the input continues to rise. The voltage when the dual battery boost is about 28V, which will not blow the fuse because C1's 25V rating is high enough and the additional current draw is small. The inductor adds a small resistance to limit the peak fault current and the slew rate of the input transient, thereby helping C1 clamp when there is a spike.
The main disadvantage of the passive network is that it relies on blown fuses to protect against damage from overcurrent, overvoltage, and reverse battery. Another disadvantage is that it relies on electrolytic capacitors to achieve clamping. As such capacitors age, the electrolyte dries out and the characteristic of increasing the equivalent series resistance (ESR) is lost, which impairs the clamping effect. Sometimes a large Zener diode is used in D1 to help this capacitor do its job. Active circuits have been designed to overcome these disadvantages.
Active Circuit
Figure 2 shows an active solution for shielding sensitive circuits from the volatile 12V automotive system. The LT1641 is used to drive the input N-channel MOSFET, and the passive solution above does not have this additional protection: First, the LT1641 disconnects the load when the input is below 9V to prevent system failure at low input voltage and reduce the opportunity for the system to provide valuable current to non-critical loads during startup or when the charging system fails; second, the LT1641 gradually increases the output voltage when power is first applied to soft-start the load; third, the output is protected from overload and short circuit by current limiting and timed circuit breakers. If a current fault occurs, the circuit breaker automatically re-attempts to establish a connection at a rate of 1 to 2Hz. The tolerance of the fuse on the upstream line of the protection circuit can be set so that it does not melt when a current fault occurs on the downstream line of the LT1641; finally, the circuit shown in Figure 2 isolates the overvoltage condition that occurs at the input and provides a clamped output so that the load circuit can continue to operate normally when an overvoltage occurs.
Under normal conditions with a 12V input, the LT1641 charges the gate of the MOSFET to approximately 20V to fully boost the voltage of the MOSFET and provide power to the load. The 27V Zener diode D1 is connected across the gate to ground, but has no effect in the 9 to 16V operating voltage range. As the input rises above 16V, the LT1641 continues to charge the gate of the MOSFET in an attempt to keep the MOSFET fully turned on. If the input rises too high, the Zener diode clamps the gate of the MOSFET and limits the output voltage to approximately 24V. The LT1641 itself can handle voltages up to 100V at its input and is not affected by the gate clamp action. The gate clamp circuit is much more accurate than that of a passive solution and can be easily adjusted to meet the load requirements simply by selecting a D1 with the appropriate breakdown voltage.
The circuit of Figure 2 works well for load currents up to about 1A, but for higher load currents, the circuit of Figure 3 is recommended to prevent excessive power dissipation in the MOSFET. Excessive power dissipation is a risk if the overvoltage condition persists, such as when the electrical system is powered by two batteries in series for longer than is normally required, or when the current is slowly ramping up after a load dump and the MOSFET is small. The output is sampled by D1 and D2, and if the input exceeds 16.7V, a signal is fed back to the "SENSE" pin to stabilize the output at 16.7V. The regulation here is more precise than that of the circuit of Figure 1 and can be easily customized to meet the needs of the load by choosing the appropriate Zener diode.
Figure 1: Passive protection network characterized by simplicity
Figure 2: An overvoltage transient protector clamps the output at around 24V and disconnects if the input drops below 9V
The total power dissipation is limited by the “TIMER” pin, which records the total time the MOSFET has been regulating the output. If the overvoltage condition persists for more than 15ms, the LT1641 shuts down and allows the MOSFET to stop regulating the output. After about half a second, the circuit attempts to restart. This restart cycle continues until the overvoltage condition disappears and normal operation resumes. The method for handling an overcurrent fault is the same as that described in Figure 2.
Reverse battery protection
Reverse battery protection can be added to the circuits shown in Figure 2 or Figure 3 by simply adding a series diode.
Figure 3: Adjusting the clamp voltage to clamp when the input surges, protecting the MOSFET from excessive power dissipation
In most cases, a normal pn diode will suffice, but if the forward voltage drop is important, a Schottky diode can be chosen. In critical applications where the power dissipation in the isolation diode is unacceptable, the simple circuit shown in Figure 4 can solve the problem.
Figure 4: Reverse battery protection for Figures 2 and 3
Under normal operating conditions, the body diode of MOSFET Q2 is forward biased and delivers power to the LT1641. When the LT1641 turns on, the gate of Q2 is driven, turning it fully on. If the input is reversed, the emitter of Q3 is pulled down below ground, Q3 turns on, pulling the gate of Q2 down and keeping it close to the source level of Q2. In this case, Q2 remains off and isolates the reverse input from reaching the LT1641 and the load circuit. Microampere current flows through the 1MΩ resistor to the "GATE" pin of the LT1641.
High Voltage LDO as Voltage Limiter
Buck regulators with a maximum input voltage rating of 25V or less, such as the LT1616 , are generally not considered for automotive applications. However, if combined with a low dropout (LDO) linear regulator such as the LT3012B / LT3013B , the disadvantage in input voltage can be easily overcome. This small size and high efficiency combination, shown in Figure 5, can provide a 3.3V output in an automotive environment.
Figure 5: LT3013B used as a voltage limiter
The LT3013B has a wide input voltage range of 4V to 80V and integrated reverse battery protection, eliminating the need for special voltage limiting or clamping circuits, thus saving cost and board area. When operating at moderate load currents, the efficiency of the LDO regulator is approximately equal to V OUT /V IN . If V OUT is much lower than V IN , the efficiency of the LDO decreases. For example, when stepping down a 12V input to a 3.3V output, the efficiency is only 28%.
In Figure 5, higher efficiency is achieved by operating the LT3013B in low dropout over the normal input voltage range. In this case, the LT3013B output voltage is set to 24V. With the output voltage of the LDO only 400mV below V IN , it powers the LT1616 buck regulator with 97% efficiency , right in the middle of the normal operating voltage range. Under load dump conditions, V IN can quickly rise as high as 80V, but when V IN exceeds 24.4V, the LT3013B will regulate its output and effectively “clamp” it to 24V, which is well within the rated voltage range of the LT1616 switch. If V IN rises above 24.4V, the efficiency of the LDO will drop, but this condition is short-lived and has no adverse consequences.
The LT1616 converts the limited output of the LT3013B to 3.3V. The efficiency of this switch is about 80% at a 12V input. During a cold crank, the car voltage may drop to 5V. In this case, the input voltage to the LT1616 is 4.6V, which is well within its operating voltage range. The LT3013B LDO regulator combined with the LT1616 switch provides a stable 3.3V output over the wide operating voltage range typical of 12V automotive electrical systems without sacrificing efficiency.
A more integrated solution is the LT3437. The LT3437 is a 200kHz monolithic step-down regulator with an input voltage range of 3.3V to 80V. Its low quiescent current of 100uA at no load is a must for today's always-on systems. A low-cost diode can be connected in series with the input of the LT3437 to provide reverse battery protection.
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