Everything is powered by auxiliary motors, sometimes independently from the primary motor if efficiency demands it. Requirements range from drawing a few watts to pushing the car battery to its limit. Motors also need to be small and cheap. Figure 1 shows a number of components of motor control systems. Micronas' HVC4223F is the market leader in terms of the level of integration. Discrete constructions with separate ICs such as voltage controllers, microcontrollers, bus transceivers and more are at the other bottom end of the integration scale.
In the mid-range segment, Infineon's new TLE956x motor system ICs combine the functions of the mid-range system basis chips of the TLE94xx (Lite SBC) and TLE926x (mid-range + SBC) series with two additional functions: an operational amplifier for measuring current using shunt resistors and gate drivers for controlling N-channel MOSFETs. Then there are outputs for anti-reverse polarity MOSFETs and up to four high-side switches for any desired purpose.
The motor system ICs are available in two versions: one to control DC motors with up to four half-bridge drivers, one for operation with BLDC motors with drivers for a six-pulse bridge.
Microcontrollers and MOSFETs must be connected externally to the TLE956x motor system ICs; see the block diagram (Figure 2). Depending on the version of the TLE956x family, the CAN FD transceiver offers partial networking via wake-up pattern (WUP) or wake-up frame (WUF).
Creating a Control Device with Autosar
Compared to the derivatives of the TLE985x/6x/7x embedded power IC family, the motor system ICs offer flexibility in terms of CPU architecture and greater scalability in terms of processor performance, memory expansion and bus integration (CAN FD). This allows users to keep using their preferred microcontroller, saving them time and money that they would otherwise need to invest in learning another architecture and in new development tools. Thanks to the extra memory of an external microcontroller and the TLE956x, it is possible to create a control device using a memory-hungry operating system such as Autosar.
Do not disturb!
However, placing the output transistors in the output stage has two undesirable effects. Firstly, each switching operation causes switching losses, as the transition slopes are finitely steep. These losses increase the system temperature. Also, the steeper the transition slopes, the more electrical interference signals there are. The cable-related proportion of interference can be mitigated using an EMI filter in the power cable, but the filter components do cause extra costs.
The aim is therefore to adjust the transition slope in such a way that the switching operation just about passes the EMC test. A resistor/diode network in the gate cable also often limits the control current. In conjunction with the gate capacitance of the MOSFET, this creates an RC circuit with a time constant that limits the voltage slope. However, this does generate additional cost for the resistors and the diode. When switching to another MOSFET with a different gate capacitance, the resistor network also needs to be adapted.
Adaptive MOSFET Gate Control
The TLE956x solves the problem more elegantly by having a digital control loop update the actual transition slope steepness values based on the defined set point value. The control value is the current from the gate drivers. This concept, referred to as "Adaptive Gate Control", eliminates the need for the resistor/diode network in the gate cables, which also removes this cost factor, eliminating any issues with switching to MOSFETs with other gate capacitance values. The controller adjusts the gate currents automatically to produce the configured slope steepness.
Further Reducing Switching Losses with Active Freewheeling
When operating DC motors in H-bridges, a high-side MOSFET is often switched on statically and the clock cycle of the diagonally positioned low-side MOSFET is regulated using pulse width modulation. If this active MOSFET is switched off using a PWM cycle, the motor current commutates through the body diode of the MOSFET in the same branch above.
Active freewheeling involves the half-bridge of the active MOSFET following a push-pull circuit design. Triggering the MOSFET causes the freewheeling current to commutate through its channel instead of its body diode. If RDS,on is low, this results in reduced loss compared to body diode commutation.
Brakes Even in Sleep Mode
Electric motors, especially those with permanent magnets, can also serve as generators-for example if someone applies manual force to open or close a motorized trunk door. Reverse polarity protection (diode or untriggered MOSFET) in the control device ensure that the generated energy cannot be fed back into the vehicle battery. Instead, the voltage is increased in the intermediate circuit using the bypass capacitor connected in parallel to the MOSFETs. In a worst-case scenario, the generated energy can cause a fault due to overvoltage.
Type TLE956x motor system ICs solve this by means of a braking effect achieved using short-circuiting, where all low-side MOSFETs are switched on at the same time. This briefly short-circuits the armature/rotor to enable
- the generated energy to dissipate as heat by means of the effective resistance of the shorting circuit and
- to apply a mechanical counterforce (braking effect).
The braking function also works in sleep mode with the TLE956x, which keeps power consumption low. There are two variations of this: a braking force that is varied depending on the intermediate circuit voltage, and continuous braking regardless of the intermediate circuit voltage.
Now is the right time to start developing using the TLE956x family of components. It is at the leading edge of technology, commercially competitive and is at the start of its lifecycle. This is a perfect storm promising an end product that offers superb value for money and long availability-for example for a seat adjuster, sunroof controller, belt tensioner, handbrake, window regulators or motorized trunk, as well as BLDC applications such as pumps, fans and sunroofs.
Arduino-compatible evaluation boards for the TLE956x provide an entry-level solution for development (Figure 3 & 4). Suitable controller boards with automotive microcontrollers from Infineon include the Aurix TC275 Shield Buddy (Rutronik Order No. TOOL4294) and the Aurix TC375 Shield Buddy (Rutronik Order No. TOOL4354).
Find components at www.rutronik24.com.
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