Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%

When powering automotive Electronic systems, it is challenging not only to meet high reliability requirements, but also to deal with relatively unstable battery voltages. The electronic and mechanical systems connected to the vehicle battery are heterogeneous and can cause large voltage excursions from the nominal 12 V supply. In fact, over a period of time, a 12 V supply can vary from C14 V to +35 V, with possible voltage spikes of +150 V to C220 V.

By David Megaw, Senior Design Engineer, Analog Devices

Introduction

When powering automotive electronic systems, it is challenging not only to meet high reliability requirements, but also to deal with relatively unstable battery voltages. The electronic and mechanical systems connected to the vehicle battery are heterogeneous and can cause large voltage excursions from the nominal 12 V supply. In fact, over a period of time, a 12 V supply can vary from C14 V to +35 V, with possible voltage spikes of +150 V to C220 V. Some of these surges and transients occur in daily use, others are due to malfunctions or human error. Regardless of the cause, the damage they cause to the car’s electronic systems is difficult to diagnose and expensive to repair.

Drawing on lessons learned over the past century, automakers have categorized electrical conditions and transients that can disrupt operation and cause damage. The International Organization for Standardization (ISO) compiles this industry knowledge into ISO 16750-2 and ISO 7367-2 specifications for road vehicles. The power supply used by an automotive electronic control unit (ECU) should at least be able to withstand these conditions without causing damage. As for critical systems, their functionality and tolerances must be maintained. This requires the power supply to be able to regulate the output voltage through transients to keep the ECU running. Ideally, a complete power solution requires no fuses, minimizes power consumption, and uses low quiescent current to keep the system on at all times without draining the battery.

The Situation Facing ISO 16750-2 Automotive Electronic Systems

Analog Devices publishes publications detailing the ISO 7367-2 and ISO 16750-2 specifications and how to use LTspice®Mock these specifications.1,2,3,4

In its most recent iteration, the ISO 7367-2 EMC specification addresses large amplitude (>100 V), short duration (150 ns to 2 ms) transients from relatively high impedance sources (2 Ω to 50 Ω) . These voltage spikes can often be eliminated using passive components. Figure 1 shows the defined ISO 7367-2 pulse 1, with the added 330 μF bypass capacitor. The capacitor reduces the spike amplitude from C150 V to C16 V, well within the range supported by the reverse battery protection circuit. ISO 7367-2 pulses 2a, 3a and 3b consume far less energy than pulse 1 and require less suppression capacitors.

Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 1. ISO 7367-2: Pulse 1 with and without 330 μF bypass capacitor.

ISO 16750-2 deals primarily with long pulses from low impedance sources. These transients cannot be easily filtered and often require the use of active regulator-based solutions. Some of the more challenging tests include: Load Dump (Test 4.6.4), Reverse Battery Connection (Test 4.7), Superimposed Alternating Voltage Test (Test 4.4), and Engine Start Conditions (Test 4.6.3). Figure 2 shows a view of these test pulses. The variability of the conditions shown in ISO 16750-2, coupled with the voltage and current requirements of the ECU, often requires a combination of these schemes to meet all requirements.

load dump

Load dump (ISO 16750-2: Test 4.6.4) is a severe transient overvoltage, simulating a situation where the battery is disconnected but the alternator is supplying a large amount of current. The peak voltage during load dump is classified as suppressed or unsuppressed, depending on whether or not avalanche diodes are used on the output of the 3-phase alternator. Suppressed load dump pulses are limited to 35 V, and unsuppressed pulse peaks range from 79 V to 101 V. In either case, it can take up to 400 ms to recover because of the large amount of electromagnetic energy stored in the alternator stator windings. While most car manufacturers use avalanche diodes, increasing reliability requirements have led some manufacturers to require that the peak load dump voltage of the ECU must be close to that of the unsuppressed condition.

One of the solutions to the load dump problem is to add transient voltage suppressor (TVS) diodes to locally clamp the ECU power supply. A more compact, tighter tolerance approach is to use an active surge suppressor such as the LTC4364, which linearly controls a series-connected N-channel MOSFET to clamp the maximum output voltage to a user-configured level (e.g., 27 V). Surge suppressors can help disconnect the output, support configurable current limit and undervoltage lockout, and can use back-to-back NFETs to provide reverse battery protection that is often required.

For linearly regulated power devices, such as surge suppressors, there is a concern that the N-channel MOSFET may dissipate a lot when limiting the output voltage during load dump, or limiting current during a shorted output. The safe operating area (SOA) limitation of a power MOSFET ultimately limits the maximum current a surge suppressor can deliver. It also gives a limit on how long regulation must be maintained (usually set using a configurable timer pin) before the N-channel MOSFET must be turned off to avoid damage. The limitations imposed by these SOAs become more severe as the operating voltage increases, making it more difficult to use surge suppressors in 24 V and 48 V systems.

A more scalable approach uses a buck regulator that operates from a 42 V input, such as the LT8640S. Unlike linear regulators, switching regulators do not have MOSFET SOA limitations, but they are obviously more complex. The efficiency of the buck regulator enables high current operation, and its top switch allows the output to be disconnected and supports current limiting. As for the buck regulator quiescent current problem, it has been solved by the latest generation of devices that consume only a few microamps and maintain regulation under no-load conditions. By using Silent Switcher®Technology and spread spectrum technology, the switching noise problem has also been greatly improved.

In addition, some buck regulators can operate at 100% duty cycle, keeping the top switch on continuously, transferring the input voltage to the output through the Inductor. During overvoltage or overcurrent conditions, switching operations are triggered to limit the output voltage or current, respectively. These buck regulators, such as the LTC7862, function as switching surge suppressors, enabling low noise, low loss operation while maintaining the reliability of switched mode power supplies.

Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 2. Overview of some of the more stringent ISO 16750-2 tests.

reverse voltage

A reverse voltage condition (also known as a reverse battery condition) occurs when a battery terminal or jumper is reversely connected due to an operator fault. The relevant ISO 16750-2 pulses (Test 4.7) repeatedly apply C14 V to the DUT for 60 seconds each. Some manufacturers have added their own dynamic versions of this test that initially power the device (e.g. VIN = 10.8 V).

A quick study of the data sheet reveals that there are very few IC designs that can accept reverse bias, where the absolute minimum pin voltage of the IC is generally limited to C0.3 V. Voltages below ground that exceed the voltage of one diode can cause additional current to flow through internal junctions, such as ESD protection devices and the body diodes of power MOSFETs. Polarized bypass capacitors such as aluminum electrolytics can also be damaged under reverse battery conditions.

Schottky diodes prevent reverse currents, but during normal operation, when forward currents are higher, this approach results in greater power dissipation. Figure 3 shows a simple protection scheme based on connecting P-channel MOSFETs in series. This scheme can reduce power loss, but at low input voltages (eg, engine start), this scheme may not be possible due to the device threshold voltage. Run smoothly. A more efficient approach is to use an ideal diode controller such as the LTC4376 to drive a series N-channel MOSFET that cuts off the input voltage at negative voltages. During normal operation, the ideal diode controller regulates the source-drain voltage of the N-channel MOSFET to 30 mV or less, reducing forward voltage drop and power dissipation by more than an order of magnitude (compared to Schottky diodes).

Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 3. Different approaches to solving difficult ISO 16750-2 tests.

Superimposed alternating voltage

The Superimposed Alternating Voltage Test (ISO 16750-2: Test 4.4) simulates the effect of the AC output of a car’s alternator. As the name suggests, a sinusoidal signal is superimposed on the battery track with a peak-to-peak amplitude of 1 V, 2 V, or 4 V, depending on the severity classification. The maximum input voltage is 16 V for all severity levels. The sine frequencies were arranged logarithmically, ranging from 50 Hz to 25 kHz, then back to 50 Hz over 120 seconds for a total of 5 repetitions.

This test will result in large resonant current and voltage swings below 25 kHz in any interconnected filter network. It can also cause problems with switching regulators, whose loop bandwidth limitations make it difficult to regulate with high frequency input signals. The solution is like an intermediate rectifier element, such as a power Schottky diode, but for reverse voltage protection, this is not a good solution to the problem.

In this case, an ideal diode controller cannot function as well as in reverse voltage protection applications because it cannot switch the N-channel MOSFET fast enough to keep pace with the input. The gate pull-up strength is one of the limiting factors, typically around 20 μA due to the internal charge pump. While an ideal diode controller can quickly turn off the MOSFET, it will turn on very slowly, making it unsuitable for rectification beyond very low frequencies.

A more appropriate approach is to use the LT8672 active rectifier controller, which can rapidly switch N-channel MOSFETs to rectify the input voltage at frequencies up to 100 kHz. An active rectifier controller is an ideal diode controller with two important additional components: a large charge storage boosted by the input voltage, and a robust gate driver that rapidly switches N-channel MOSFETs. Compared to using Schottky diodes, this approach can reduce power losses by more than 90%. The LT8672, like an ideal diode controller, also protects downstream circuits from reverse battery connections.

Startup condition

Engine cranking conditions (ISO 16750-2: Test 4.6.3) are extreme undervoltage transients, sometimes referred to as cold crank pulses, because at lower temperatures the worst battery voltage drop occurs. In particular, when the starter is activated, the 12 V battery voltage may drop to 8 V, 6 V, 4.5 V or 3 V immediately, depending on the severity classification (Class I, IV, II and III, respectively).

In some systems, low dropout (LDO) linear regulators or switching buck regulators are sufficient to support the supply rail against these transients, as long as the ECU voltage is below the minimum input voltage. For example, if the highest ECU output voltage is 5 V, and it must reach severity level IV (minimum input voltage of 6 V), then a regulator with a dropout of less than 1 V is sufficient. The zone with the lowest voltage at engine start only lasts 15 ms to 20 ms, so the rectifiers (Schottky diodes, ideal diode controllers, active rectifier controllers) after the large bypass capacitors may be able to withstand this part of the pulse, If the voltage headroom drops briefly below the regulator dropout.

However, if the ECU must support voltages higher than the minimum input voltage, a boost regulator is required. The boost regulator can effectively maintain a 12 V output voltage from an input below 3 V at high current levels. However, there is another problem with boost regulators: the diode path from input to output cannot be broken, so naturally the current is not limited at startup or short-circuit. To prevent current runaway, dedicated boost regulators such as the LTC3897 controller integrate a surge suppressor front end to support output disconnect and current limiting, as well as reverse voltage protection when back-to-back N-channel MOSFETs are used. This solution can address load dump, engine start, and reverse battery connection with a single IC, but the available current is limited by the SOA of the surge suppressor MOSFET.

A 4-switch buck-boost regulator removes this limitation by combining a synchronous buck regulator and a synchronous boost regulator with a shared inductor. This approach meets the requirements of load dump and engine cranking conditions without the current level or pulse duration being limited by the MOSFET SOA, while maintaining the ability to turn off the output and current limit.

The switching operation of a buck-boost regulator is determined by the relationship between the input and output voltages. If the input is much higher than the output, the boost top switch remains on and the buck power stage reduces the input. Likewise, if the input is much lower than the output, the buck top switch remains on and the boost power stage increases the output. If the input and output are approximately equal (between 10% and 25%), the buck and boost power stages are turned on simultaneously in an interleaved fashion. In this way, the efficiency of each switching region (buck, buck-boost, boost) can be individually maximized by limiting switching to only the MOSFETs required to regulate input voltages above, approximately equal to, or below the output.

ISO 16750-2 Solution Summary

Figure 3 summarizes the various solutions for handling load dump, reverse input voltage, superimposed AC voltage, and engine cranking conditions, as well as the advantages and disadvantages of each. Several key conclusions can be drawn:

A series N-channel MOSFET whose drain faces the input is extremely useful because it can be used to limit current and disconnect the output, whether it is used as a switch (for example, in a buck power stage) or as a linear control device (for example, in a wave surge suppressor).

When it comes to reverse input protection and superimposed alternating voltages, using an N-channel MOSFET as the rectifying component (source facing the input) can significantly reduce power losses and voltage drops (compared to using Schottky diodes).

Using a switch-mode power supply is more appropriate than a linear regulator because it eliminates reliability issues and output current limitations caused by SOA in power devices. It can infinitely adjust the input voltage limit, while linear regulators and passive solutions have inherent time constraints, which can complicate the design.

The boost regulator may or may not be required, depending on the classification of the starting conditions and the details of the ECU (what is the highest voltage it must provide).

If boost regulation is required, then a 4-switch buck-boost regulator incorporates the required qualities described above into a single device. It effectively regulates severe undervoltage and overvoltage transients at high current levels for extended duration. This makes it the most reliable and simple method from an application point of view, but its design complexity also increases. However, the typical 4-switch buck-boost regulator has some drawbacks. For one, reverse battery protection cannot be provided naturally, and additional circuitry must be used to address this.

The main problem with a 4-switch buck-boost regulator is that it spends a significant portion of its operating life in the lower-efficiency, higher-noise buck-boost switching region. When the input voltage is very close to the output voltage (VIN ~VOUT), all 4 N-channel MOSFETs are actively turned on to maintain regulation. Efficiency decreases as switching losses increase and maximum gate drive current is used. When both the buck and boost power stage hot loops are enabled, the regulator input and output currents are intermittent, radiated and conducted EMI performance in this area can be affected.

4-switch buck-boost regulators can regulate occasional large undervoltages and transient overvoltages, but require high quiescent current, reduced efficiency, and higher noise in the more common, conventional transition region.

Bandpass Operation Provides High Efficiency and EMI Performance Buck-Boost Region

The LT8210 is a 4-switch buck-boost DC/DC controller that can conventionally operate from a fixed output voltage and supports a new Pass-Thru™ mode of operation (Figure 4) that eliminates switching losses and EMI. Operating in the 2.8 V to 100 V range, the controller can regulate the most severe battery voltage drop during engine startup, as well as the peak amplitude of unchecked load dump. It itself provides C40 V reverse battery protection by adding a single N-channel MOSFET (DG in Figure 5).

Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 4. A buck-boost controller that supports bandpass mode solves many of the problems associated with automotive standard testing.

In bandpass mode, the output voltage is regulated to the edge of the voltage window when the input voltage is outside the window. The top and bottom of the window are configured through the FB2 and FB1 resistor dividers. When the input voltage is within this window, the top switches (A and D) remain on, passing the input voltage directly to the output. In the non-switching state, the total quiescent current of the LT8210 is reduced to tens of microamps. No switching means no EMI and switching losses, so efficiency is over 99.9%.

For those who want the best of both worlds, the LT8210 can be used, which can be switched between different operating modes by toggling the MODE1 and MODE2 pins. In other words, the LT8210 can operate as a conventional buck-boost regulator with a fixed output voltage (CCM, DCM, or Burst Mode™) under certain conditions, and then, when application conditions change, switch to Bandpass mode. This feature is very useful for always-on systems and start-stop applications.

Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 5. This 3 V to 100 V input buck-boost controller operates with an 8 V to 17 V bandpass output.

Bandpass performance

The bandpass solution shown in Figure 5 transfers the 8 V and 17 V input to the output in the window. When the input voltage is above the bandpass window, the LT8210 steps down this voltage to a regulated 17 V output. If the input drops below 8 V, the LT8210 boosts the output voltage to 8 V. If the current exceeds the inductor current limit or the set average current limit (via the IMON pin), the switching operation is triggered in the bandpass window as a protection feature to control the current.

Figure 6, Figure 7, and Figure 8 show the response of the LT8210 circuit to load dump, reverse voltage, and start-up condition tests, respectively. Figures 9 and 10 show the efficiency improvement achieved and the low current operation that can be achieved under the bandpass window (the efficiency at low current is surprising). Figure 11 shows the dynamic transition between bandpass mode and CCM operation. For an LTspice simulation of this circuit, and an accelerated version of the most stringent ISO 16750-2 test pulse, see: analog.com/media/en/simulation-models/LTspice-demo-circuits/LT8210_AutomotivePassThru.asc.

Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 6. Bandpass response to unsuppressed load dump.
Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 7. LT8210 response to reverse battery connection.
Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 8. Bandpass response to engine cold start.
Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 9. Efficiency of CCM and bandpass operation.
Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 10. In Bandpass Mode (VIN = 12 V), no load input current.
Provides power and protection for automotive electronic systems, with no switching noise and efficiency up to 99.9%
Figure 11. Dynamic transition between bandpass and CCM operation.

in conclusion

When designing power supplies for automotive electronic systems, the LT8210 4-switch buck-boost DC/DC controller provides an excellent solution with its 2.8 V to 100 V input operating range, built-in reverse battery protection, and its new bandpass mode of operation. plan. Bandpass mode improves buck-boost operation with zero switching noise, zero switching losses, and ultra-low quiescent current, while regulating the output to a user-configured window level rather than a fixed voltage. The output voltage minimum and maximum values ​​are tied to large transients such as load dump and during cold cranking, with no MOSFET SOA or current or time limitations due to linear conditions.

The new LT8210 control scheme supports clean and fast transients between different switching regions (boost, buck-boost, buck and non-switching), thus enabling regulation of large signals and high frequency AC voltages at the input. The LT8210 can switch and keep running between a bandpass mode of operation and a traditional fixed output voltage, buck-boost mode of operation (CCM, DCM or Burst mode) where the fixed output can be set to any voltage in the bandpass window (e.g. , in the 8 V to 16 V window, VOUT = 12 V). This flexibility enables the user to switch between bandpass and conventional buck-boost operation, taking advantage of the low noise, low IQ and high efficiency operation of bandpass mode for more accurate operation in CCM, DCM or Burst mode regulation and better transient response.

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