Power and protect automotive electronic systems with no switching noise and up to 99.9% efficiency

Introduction

Powering automotive electronic systems presents challenges in addition to high reliability requirements and the need to cope with relatively unstable battery voltages. The variability of the electronic and mechanical systems connected to the vehicle battery can cause large voltage excursions from the nominal 12 V supply. In fact, over a period of time, the 12 V supply can vary from –14 V to +35 V, and can experience voltage spikes of +150 V to –220 V. Some of these surges and transients occur in everyday use, while others are caused by malfunctions or human error. Regardless of the cause, the damage they cause to automotive electronic systems can be difficult to diagnose and expensive to repair.

Over the past century, automotive manufacturers have classified electronic conditions and transients that can interfere with operation and cause damage. The International Organization for Standardization (ISO) compiled this industry knowledge into the ISO 16750-2 and ISO 7367-2 specifications for road vehicles. Power supplies used in automotive electronic control units (ECUs) should at least be able to withstand these conditions without causing damage. For critical systems, functionality and tolerances must be maintained. This requires a power supply that can regulate the output voltage through transients to keep the ECU running. Ideally, a complete power solution would eliminate the need for fuses, minimize power consumption, and use low quiescent current to allow the system to always be on without draining the battery.

Figure 1. ISO 7367-2: Pulse 1 with and without a 330 μF bypass capacitor.

ISO 16750-2 Status of Automotive Electronic Systems

Analog Devices has published several publications detailing the ISO 7367-2 and ISO 16750-2 specifications and how to simulate them using LTspice®. 1,2,3,4

In its most recent iteration, the ISO 7367-2 electromagnetic compatibility specification focuses on 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 ISO 7367-2 Pulse 1 as defined, with the addition of a 330 μF bypass capacitor. The capacitor reduces the spike amplitude from –150 V to –16 V, well within the range supported by the reverse battery protection circuit. ISO 7367-2 Pulses 2a, 3a, and 3b consume much less energy than Pulse 1 and require less suppression capacitance.

ISO 16750-2 focuses on long pulses from low impedance sources. These transients cannot be easily filtered and typically require an active solution based on a voltage regulator. Some of the more challenging tests include: load dump (test 4.6.4), reverse battery (test 4.7), superimposed alternating voltage testing (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, combined with the voltage and current requirements of the ECU, often requires a combination of these approaches to meet all requirements.

Figure 2. Overview of some of the more stringent ISO 16750-2 tests.

Load Dump

Load dump (ISO 16750-2: Test 4.6.4) is a severe transient overvoltage that simulates a situation where the battery is disconnected but the alternator is supplying a large amount of current. The peak voltage during a load dump is classified as suppressed or unsuppressed, depending on whether avalanche diodes are used at the output of the 3-phase alternator. Suppressed load dump pulses are limited to 35 V, while unsuppressed pulse peaks range from 79 V to 101 V. In either case, recovery may take 400 ms due to the large amount of electromagnetic energy stored in the alternator stator windings. Although most car manufacturers use avalanche diodes, increasing reliability requirements have led some manufacturers to require the peak load dump voltage of the ECU to be close to the voltage in the unsuppressed case.

One solution to the load dump problem is to add a transient voltage suppressor (TVS) diode to locally clamp the ECU supply. A more compact and tighter tolerance approach is to use an active surge stopper, such as the LTC4364, which linearly controls a series N-channel MOSFET to clamp the maximum output voltage to a user-configured level (e.g., 27 V). The surge stopper can help disconnect the output, supports configurable current limit and undervoltage lockout, and can provide the commonly required reverse battery protection using back-to-back NFETs.

For linear regulated power devices, such as surge stoppers, the concern is that the N-channel MOSFET may dissipate significant power when limiting the output voltage during a load dump, or limiting the current during a shorted output. The safe operating area (SOA) limitations of the power MOSFET ultimately limit the maximum current that the surge stopper can source. It also places a limit on how long regulation must be maintained (usually set using a configurable timer pin) before the N-channel MOSFET must turn off to avoid damage. These SOA-induced limitations become more severe as the operating voltage increases, making surge stoppers more difficult to use in 24 V and 48 V systems.

A more scalable approach uses a buck regulator that can operate from a 42 V input, such as the LT8640S. A switching regulator does not have the MOSFET SOA limitations of a linear regulator, but is obviously more complex. The efficiency of a buck regulator allows for high current operation, and its top switch allows the output to be disconnected and allows for current limiting. The buck regulator quiescent current issue has been addressed with the latest generation of devices that consume only a few microamps and maintain regulation under no-load conditions. Switching noise issues have also been greatly improved by using Silent Switcher® technology and spread spectrum techniques.

In addition, some buck regulators can operate at 100% duty cycle, ensuring that the top switch is continuously on, transferring the input voltage to the output through the inductor. In overvoltage or overcurrent conditions, switching action is triggered to limit the output voltage or current respectively. These buck regulators (such as the LTC7862) act as switching surge stoppers, achieving low noise and low loss operation while maintaining the reliability of the switch mode power supply.

Figure 3. Different approaches to solving difficult ISO 16750-2 tests.

Reverse voltage

A reverse voltage condition (also called a reverse battery condition) occurs when the battery terminals or jumpers are connected in reverse due to operator error. The relevant ISO 16750-2 pulse (test 4.7) repeatedly applies –14 V to the DUT for 60 seconds. Some manufacturers have added their own dynamic version of this test, initially powering the device (for example, VIN = 10.8 V) before suddenly applying reverse bias (–4 V).

A quick study of data sheets reveals that few IC designs can tolerate reverse bias, with the absolute minimum pin voltage for ICs typically limited to –0.3 V. Voltages more than a diode below ground can cause excess current to flow through internal junctions, such as ESD protection devices and the body diode of power MOSFETs. Polarized bypass capacitors, such as aluminum electrolytics, can also be damaged under reverse battery conditions.

Schottky diodes can prevent reverse current flow, but this approach results in greater power dissipation during normal operation when forward current is higher. Figure 3 shows a simple protection scheme based on a series P-channel MOSFET, which can reduce power losses, but may not work well at low input voltages (for example, engine starting) due to the device threshold voltage. A more effective 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 the forward voltage drop and power dissipation by more than an order of magnitude (compared to a Schottky diode).

Figure 4. A buck-boost controller with pass-through mode solves many of the problems associated with automotive standards testing.

Superimposed alternating voltage

The superimposed alternating voltage test (ISO 16750-2: Test 4.4) simulates the effects of the AC output of a vehicle's alternator. As the name implies, a sinusoidal signal is superimposed on the battery rail with a peak-to-peak amplitude of 1 V, 2 V, or 4 V, depending on the severity classification. For all severity levels, the maximum input voltage is 16 V. The sinusoidal frequency is ranged logarithmically from 50 Hz to 25 kHz, then back to 50 Hz over 120 seconds, repeated a total of 5 times.

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

In this case, an ideal diode controller cannot function as well as in a reverse voltage protection application because it cannot switch the N-channel MOSFET on and off quickly enough to keep pace with the input. The gate pull-up strength is a limiting factor, typically limited to about 20 μA by the internal charge pump. While the ideal diode controller can turn off the MOSFET quickly, it turns on very slowly, making it unsuitable for rectification except at very low frequencies.

A more appropriate approach is to use the LT8672 active rectifier controller, which can quickly 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 additions: a large charge reservoir boosted by the input voltage and a powerful gate driver that quickly switches the N-channel MOSFET. This approach can reduce power losses by more than 90% compared to using Schottky diodes. The LT8672 also protects downstream circuitry from reverse battery effects, just like an ideal diode controller.