Reverse Battery Protection Scheme for Automotive Applications

Abstract

Many self-contained critical electronic systems and subsystems, especially in automotive applications, use a 12 V or 48 V storage battery for the primary power source. To prevent extensive damage during operational life, these systems require the design-in of systems that—at a certain voltage level—provide protection from voltage polarity reversal. This article discusses a method that provides a very low insertion loss at low cost to protect automotive systems from accidental reversal of battery polarity.

Introduction

When a system receives power from a battery that has the potential to become reverse-polarized, such as in automotive motor-driver applications, protection schemes are required that prevent reverse voltage from being applied to system components such as the gate driver, MOSFET bridge, and motor combination.

Battery polarity reversal is most likely to occur during routine servicing, battery replacement, or emergency start using an external power source.

Automotive systems that require a peak operating current of less than 10 A are usually protected by inserting a suitably rated diode in series with the power supply. For automotive systems that require higher peak and operating currents that are much higher, power MOSFET bridges can be used to drive motors.

MOSFETs used in a bridge carry most of the current supplied to the system and feature very low on-resistance, which minimizes self-heating and power loss in the motor load. Hence, a protection scheme that features very low power loss is required. Another feature of this scheme is that, if battery polarity is reversed, body diodes in each MOSFET become forward biased. If the reverse polarity voltage is greater than approximately 2 V, this condition leads to current rise limited only by the diode characteristics.

Reverse Battery Protection Circuit

A simplified block diagram of reverse battery protection systems using the charge pump voltage, V_CP, to drive reverse protection circuitry is shown in Figure 1. The voltage source, V_CP, is referenced to V_BB and provides the gate voltage required to turn on the Q₁ MOSFET. During typical operation, the Q₁ MOSFET carries the full load current to the subsystem. This provides a very-low-resistance current path to minimize insertion losses when operating with typical polarity and an adequate voltage-blocking rating when battery polarity is reversed.

It should also be noted that the source and drain connections to the Q₁ MOSFET are intentionally reversed to ensure that there is a current path through the body diode of the Q₁ MOSFET, making it possible for control circuitry within the motor gate driver to start to operate during a power-up sequence. The Q₂ MOSFET is a P-channel small-signal MOSFET and is used to control the gate of Q₁ in correct and reverse battery conditions. Both Q₁ and Q₂ must be correctly rated for the full peak reverse battery voltage.

The voltage, V_CP, is either produced by the gate driver itself or circuitry closely associated with it. This means that V_CP is not present when V_BB = 0 V, or when a reverse polarity is applied. V_CP also takes some time to rise to a final stable value after V_BB is applied. From the V_CP supplies, resistor R_G—with minimum value of 100 kΩ ±1%—controls the gate-to-source voltage of Q₁.

Circuit Operation With V_CP

Correct Battery Polarity

The position of impedance Z_L is shown in Figure 2. Z_L provides a current path from V_BB to ground via the power bridge and the motor under all conditions.

The sequence of events during start-up is:

• With V_BAT = 0 V, current does not flow in Z_D and Z_L, and V_CP = 0 V.

• V_BAT, with the correct polarity, is applied to the gate driver.

• Current flows through the forward-biased body diode of Q₁ and the gate driver, causing it to execute a power-on-reset cycle and initialize.

• As the gate driver progresses through its various control states, it causes the value of V_CP referenced to V_BB to increase from zero and to raise the voltage on the source of Q₂, such that it becomes more positive than V_BB. This action increases the gate-to-source voltage on Q₂.

• As V_CP rises above the conduction threshold of Q₂, it starts to turn on, and current flowing through it produces a voltage across the R_G resistor. This voltage appears across the gate and source of Q₁, and it also starts to turn on.

• As Q₁ starts to conduct, the main supply current from V_BAT is shared between its body diode and source-to-drain path.

• At sufficiently high values of V_CP, the on-resistance of Q₁ becomes sufficiently low that the source-to-drain voltage falls below the forward voltage of the body diode in Q₁, and conduction ceases. All current then flows between the source and drain of Q₁ to produce an insertion loss. 

Reverse Battery Polarity

The reverse battery polarity condition is shown in Figure 3. In this condition, V_CP is always 0 V because the gate driver never achieves an operating condition.

The sequence of events during start-up is:

• As V_BAT increases in a negative direction, the 0 V V_GS condition persists on the gates of Q₁ and Q₂, and both transistors remain turned off.

• Because Q₁ and Q₂ remain turned off, all applied reverse voltage appears across the source and drain connections of Q₁ and Q₂. As a result, voltage across the gate driver and Z_L remain at 0 V.

• When V_BAT is in the reverse polarity condition, current does not flow through the gate driver.

• This condition is stable until the source-to-drain voltage breakdown rating of either Q₁ or Q₂ is exceeded by the reverse voltage on V_BAT.

Circuit Operation with Internal Charge Pump

Many products feature an output from an internal regulator charge pump that runs and switches continuously. The signal of the internal regulator charge pump, C_P1, may be used to pump a circuit that creates a voltage supply equivalent to the V_CP presented in Figure 1, Figure 2, and Figure 3. Operation of the circuit, as illustrated in Figure 4 under both correct and reverse voltage polarity conditions, closely follows the earlier description in the Circuit Operation with V_CP section.

Circuit Operation with Bootstrap Capacitor

Some products have neither an internal charge pump to provide the V_CP voltage nor an internal regulator charge pump to provide continuous running and switching. Instead, they use bootstrap capacitors to create the gate voltage required by the high-side power MOSFETs in the main output power bridge. Bootstrap capacitors are connected between bootstrap supply terminals C_A, C_B, and C_C and corresponding reference pins S_A, S_B, and S_C, as shown in Figure 5. When the system operates with a positive polarity on V_BAT, the operation of the bootstrap capacitors, C_BOOTA, C_BOOTB and C_BOOTC, typically results in the voltage on the C_A, C_B, and C_C terminals of the gate driver switching higher than the V_BB voltage when any high-side driver in the power bridge is commanded to turn on. Diodes D₁, D₂, and D₃ (see Figure 5) ensure that the peak voltage from any of the C_A, C_B, and C_C terminals is delivered to the source of the Q₂ MOSFET, as presented in Figure 5. The Q₂ MOSFET operates as a typical on-switch, unless the voltage on terminals C_A, C_B, and C_C is kept below the turn-on threshold of the Q₂ MOSFET. Therefore, the peak voltage from C_A, C_B, or C_C is applied to the gate of the Q₁ MOSFET. The gate-to-source capacitance of the Q₁ MOSFET can store charge from the bootstrap capacitors and produce a DC voltage for the gate of the Q₁ MOSFET. Using the scheme described here, a voltage supply equivalent to the V_CP voltage presented in Figure 1 has been created. Operation of the circuit, as illustrated in Figure 5 under the correct and reverse voltage polarity conditions, closely follows the earlier description in the Circuit Operation with V_CP section.

Experimental Results

The performance of the reverse battery protection circuit presented in Figure 1 was experimentally verified on the bench. Several experiments were performed on the bench using an Allegro AMT49107 gate driver, which has an internally designed charge pump terminal, V_CP. The test bench contains a dual DC power supply, commercially available 1 GHz scope, and Allegro AMT49107 demonstration board, which has a built-in reverse battery protection circuit. The parameters of the Q₁ and Q₂ transistors employed are presented in Table 1.

Experimental tests were performed by applying stepped voltage changes—from 0 V to 12 V and 0 V to –12 V—on the battery under test and observing the performance of the reverse battery protection circuit by monitoring the V_BAT, V_BB, and V_CP signals, as presented in Figure 6 and Figure 7. The performance of the reverse battery protection with the correct polarity is shown in Figure 6: During power-up, the initial system current is supplied to V_BB through the forward-biased source-to-drain diode of transistor Q₁ until V_CP voltage exceeds the threshold voltage of Q₁ and turns it on. The behavior of the reverse battery circuit with reverse polarity is shown in Figure 7: When the battery voltage is reversed, the voltage between V_CP and V_BB is zero, the gate-to-source voltage on Q₁ is zero, and its source-to-drain diode becomes reverse biased. In this condition, Q₁ blocks current flow to V_BB, and the voltage between V_BB and ground remains at approximately 0 V.

Summary

The reverse battery protection schemes described in this article can be used with Allegro gate drivers for automotive applications that employ gate drivers that have an internal charge pump terminal or an internal regulator charge pump terminal, or that use bootstrap capacitors. 

A list of the most prevalent Allegro gate drivers in automotive applications is provided in Table 2 along with the corresponding reverse battery protection techniques described in this article. Experimental results demonstrate that these gate drivers can be protected against the reverse battery polarity that can occur by changing the battery of a vehicle or during routine maintenance work on electronic systems.

Table 2: Allegro Gate Drivers and Reverse Battery Protection