A simple and in-depth understanding of the challenges of high-side drivers in automotive applications

With the development of automotive electronic technology, the demands for electrification, lightweighting and intelligence have driven the large-scale application of automotive-grade high-side drivers (HSD) in body load driving.

In the field of automotive applications, high-side drivers are mainly used to drive and switch loads such as car lights, valves, pumps, and motors, and monitor short circuits and open circuits, currents, and voltages during the switching process to protect and diagnose the loads. At the same time, the high-side driver integrates a clamping shutdown function to support the energy processing capabilities of the switch, without the need for a freewheeling current recycling path, thereby reducing design difficulty, reducing battery energy consumption, and saving system costs.

Currently, regarding high-side drivers, the automotive industry is mainly concerned with their characteristics when driving resistive, capacitive, and inductive loads.

Among the three major load types, the simplest is the resistive load (such as PTC, seat heating). Its load characteristics are relatively stable, and it tests the on-resistance of the high-side driver. The lower the internal resistance of the high-side driver, the larger the resistive load it can carry and the higher the rated current.

Capacitive loads will generate large surge currents when starting. Taking halogen lamp loads as an example, the load characteristics and surge current of vehicle lamps are usually described by three parameters: IDC, IINRUSH and tLAMP−ON. IDC defines the current consumption in the steady state, IINRUSH is the initial surge current, and the time constant tLAMP−ON describes the transition time to reach the steady state. It is generally believed that IINRUSH is 10 times IDC. When the drive current drops to less than half of IINRUSH, the vehicle lamp reaches the on state, and this time is defined as tLAMP−ON. If the high-side driver has short-circuit protection and start-up retry due to surge current, tLAMP−ON is defined as the time required from the start to the last start-up retry. In vehicle lamp design, it should be ensured that tLAMP−ON does not exceed 30ms[1]. The surge current is mainly affected by the filament temperature. The worst case basically occurs at -40℃, and the typical case is at ambient temperature (+25℃). However, the actual operating current is often much smaller than the surge current, so the current limiting protection design for capacitive loads is a challenge.

The most complex is the inductive load. Common inductive loads in automotive electronic systems include: actuators in transmission control module (TCU) applications, such as motors and solenoid valves; actuators in body control module (BCM), such as wipers, relays, fans, water pumps, oil pumps, etc., which also exhibit inductive characteristics. When the high-side driver is shutting down the inductive load, it needs to maintain the current flow direction of the inductive load through freewheeling protection. However, if the voltage polarity across the load suddenly reverses, the high-side driver output will instantly generate hundreds of volts of negative voltage. Since the magnitude of the negative voltage during shutdown is positively correlated with the demagnetization energy in the inductive load, the MOSFET DS end inside the high-side driver will be subjected to a huge reverse voltage. If no clamping measures are taken, the MOSFET will be at risk of being damaged [2]. At the same time, whether the demagnetization dissipation energy generated by the instantaneous shutdown is within the tolerance range of the high-voltage side device also determines whether the demagnetization shutdown will burn the high-side driver.

So what features does a good high-side driver chip need to have in order to cope with these load driving challenges? Usually, in addition to normal switching and driving capabilities, automotive applications are mainly evaluated from the perspective of protection functions and load diagnosis. Typical evaluation items are shown in Table 1.

Table 1 Typical evaluation items for high-side drivers

Does realizing the above-mentioned switch driving, functional protection and fault diagnosis functions make it a complete automotive-grade high-side driver chip?

The operating environment of automotive application systems is complex and harsh. To ensure that automotive chips can operate without failure for a long time, all emergency and extreme situations must be considered when designing the system. These include load dumping, cold start, reverse polarity of the battery, short circuit to ground, loss of ground, loss of power, double battery cross-connection, spike clamping, and extreme operating temperatures. At the same time, automotive-grade chips require a longer service life. Most chips must maintain safe and reliable operation for more than 10 years as the car is put into use. In addition, the fault tolerance rate requirements are also higher. For DPPM (defective products per million), consumer-grade chips require no more than 500 defects, while automotive-grade chips must be controlled to no more than 10 defects.

Before automotive components are put into mass production, they often have to undergo a series of rigorous reliability tests to ensure that product reliability meets automotive regulations. Currently, the industry’s commonly used automotive system certification standards include the functional safety standard ISO 26262, the quality management system certification IATF16949, and the reliability standard AEC-Q series certification [3]. Common automotive system and chip EMC tests are shown in Table 2 [4].

Table 2 Vehicle EMC test and standards

It can be seen that in order to cope with the challenges of harsh environments in automotive applications, automotive-grade high-side driver chips must not only integrate many functions such as switch driving, functional protection and fault diagnosis, but also comply with the above-mentioned various standard certifications. Therefore, an automotive-grade chip is stronger and more reliable than an industrial-grade or consumer-grade chip.

To make automotive chips, you should keep automotive regulations in mind first

The high-side driver SGM42202Q/3Q series launched by Shengbang Microelectronics has a wide voltage input range of 4.5V to 36V, a low on-resistance of 75mΩ, a maximum current limit of 22A, and can be configured with multiple current limit steps (2.5A/5A/10A/15A/22A) according to application requirements, and a built-in overcurrent shielding time setting pin. The device is implemented in a single chip and is widely used in automotive BCM modules, ECU units and other systems.

The low on-resistance and adjustable current limit gear characteristics of SGM42203Q can switch and drive various resistive loads in automotive systems. By changing the external capacitor and resistor constants, the inrush current protection time and the steady-state current limit setting value can be freely adjusted. This can be used in automotive systems to adjust the time from transient current to steady-state current, thereby starting capacitive loads such as car lights faster.

As shown in Figure 1 and Figure 2, the high-side driver integrates a 60V clamping circuit. Compared with the use of a freewheeling diode clamping shutdown, the 60V clamping voltage greatly shortens the demagnetization shutdown time tDEMAG. In some applications such as injector drive, PWM control valve, etc., when there are strict requirements on the shutdown time, it can also be easily dealt with; as shown in Figure 3 and Figure 4, when facing the 300mH inductive load drive shutdown, the measured VCLAMP voltage is 60V, and the shutdown demagnetization time tDEMAG is 9.2ms. According to the engineering approximate calculation formula (1), the demagnetization energy EAS can be calculated to be 276mJ, and the actual shutdown measurement of SGM42203Q The demagnetization energy EAS is 262.5mJ, which is close to the theoretical value. This also provides a certain demagnetization dissipation capability when driving an inductive load. When driving a load whose shutdown dissipation energy is within the tolerance range of the high-side driver, there is no need to increase the cost of designing an external clamp.

Through the multi-function CS pin, SGM42203Q integrates diagnostic and current detection output functions, which can not only perform real-time current sampling during operation, but also output VSENSEH high-level error in time when a fault is triggered and notify the control unit.

Figure 1 Inductive load turn-off clamp

Figure 2 Inductive load turns off freewheeling diode clamping

Figure 3 Inductive load 1A shutdown

Figure 4: Demagnetization energy dissipation when inductive load is turned off

Note 1: The test conditions of Figures 1 to 4 are VCC = 24V, TJ = +25℃, and a single pulse turns off a 300mH inductive load.

Realizing basic protection functions is only the first step. Shengbang’s requirements for automotive-grade high-side driver chips go far beyond this.

1: Automotive EMI/EMC test standard ISO7637-2

As the automotive electrical system often works under high temperature and vibration conditions, the environment is very complex and harsh, and electrical system failures may often occur, such as alternator overvoltage, disconnection of the connection system, etc. In order to verify the impact of transient conduction interference along the power supply on the high-side driver, Shengbang tested the performance of SGM42203Q in different combinations such as no-load/load under 12V and 24V battery systems in accordance with ISO 7637-2 standard. Figures 5 to 10 show that the high-side driver switch function of SGM42203Q is normal when simulating P1 negative pulse, P2a positive pulse, P3a negative pulse, P3b positive pulse, P4 reverse voltage and P5b load dump pulse.