H-Bridges provide 'smart power' for automotive SoC applications: Part 2 - Architectures and circuit protection

With so called automatic "mixed decay" current shaping, design engineers can work around the tedious micro-stepping "forward," "slow-decay," "fast-decay," and "mixed-decay" current shaping to drive stepper motors for headlamp control (see Part 1 and a feature on the Advanced Front-Lighting System (AFS)). AMIS has designed a motor driver control IC family to fit these needs with integrated features.

The AMIS-3062x (block diagram below) offers a fully-automatic mixed-decay current drive for the automotive electronic module designer.

Power and protection
To implement an H-Bridge on silicon, different aspects have to be taken into account. Issues to be considered are the total power consumption of the switch as well as over-current and short-circuit detection and protection.

To protect the drivers and the motor against over-currents or short circuit conditions, an over-current protection circuit is required. In a previous figure (Part 1) of an H-Bridge circuit (reproduced below), the current of T2 and T4 is sensed via the sense switches T2S and T4S and compared with a threshold via comparators Comp3 and Comp4.

Due to the presence of a capacitor on the motor pins, the over-current detection circuit should be masked or filtered during the turn-on time of the switch—which is accomplished by the filter capacitors C1 and C2. To limit the current during the turn-on time, i.e. the time in which the over-current detection circuit is masked, transistors T5 and T6 are added to limit the current through the switches via an analog loop.

In case an over-current condition occurs, the magnitude of the over-current impacts how long it takes before the bridge gets deactivated. Normally this time is in the order of some microseconds. However, in case of a "soft" short, the time could be larger and is in the millisecond range. Hence, care should be taken how much energy can be dissipated in the switch for a given time (the so-called energy capability of the driver).

HVAC
To refer to an application other than the headlamp, consider a cabin ventilation unit of the heating, ventilation, and air conditioning system (HVAC). While the motor used in the headlamp case was a stepper motor driver, here a brushless DC motor will be considered.

The ventilator motor controller IC is used to control the airflow rate and temperature of the air supplied to the occupants. The brushless DC motor that controls the position of the flaps in the cabin air ducts is driven by an H-Bridge consisting of p-type high-side switches and n-type low-side switches. Both switches are typically 1Ω. The n-type switch is controlled via a PWM (pulse-width modulated) signal that is controlled by the on-chip microcontroller. The position of the flaps is controlled via a potentiometer mounted on the axis controlling each flap. The voltage over the potentiometer is measured via an on-chip ADC (analog-to-digital converter) and is a measure of the flap position.

It is clear that during the turn-on time of the switches, the over-current detection circuit may not be activated, because during this time we have an additional current superimposed on the motor current, which is needed to charge the capacitor across the motor pins. Therefore, the over-current detection is masked (or filtered) during the turn-on time of the switch. Because of that situation, the current flowing through the switches is limited via an analog limitation (T5 and T6 in the H-Bridge figure).

As soon as an over-current is detected, the corresponding low-side switch is turned off by hardware and the over-current detection is latched until the software releases the interrupt. The inrush current is controlled via software. With over-current, all motor current re-circulates through the internal bulk diodes of the high-side switches, and synchronous rectification is implemented on the high-side switches.