An increasing number of high-power DC motors are used in many automotive applications: the need, from carmakers, to reduce car weight impacted also on the motors' size. They are smaller and lighter compared to the previous ones but, as a result, to offer a suitable torque, they must work with higher (nominal and in-rush) currents. On top of this requirement, the trend to add more and more control and diagnostics on the motors brings a new challenge: how to drive those 'current thirsty' motors providing a further sophisticated control without making the module maker's life much harderμ
For most of those applications, from power-train applications (new start-stop applications, valves control, 4WD-2WD transfer case) and safety ones (seat belt tensioner, ABS) to body modules (sun-roof, window lift, seat adjustment, tail-gate and sliding door), a semiconductor based solution has become the standard: it combines superior diagnostics capability with optimized space usage that makes the solid state solution the most suitable for lightweight systems. What about thermal handling in case of such big current values? Internal junction temperature, for an automotive device, must not exceed 150-175°C: this represents a typical range that embedded protection circuitry limits, in order to protect silicon from thermal failure.
A typical power h-bridge device that can be found in the market, having MOSFETS as power outputs, offers 14-16mohm per leg: the bridge can be implemented as 2xhalf bridges or a full bridge in one package. They typically come with a current limitation, needed to protect the motor and device itself, of around 30-40A. So, how do they perform when driving some of the previously examined applications? We're going to analyze two of them: Seat belt tensioner and Recliner seat motor.
Starting with the seat belt tensioner application, fig.1 shows the typical current profile during a complete activation.
Fig.1 Typical Seat Belt Tensioner Current Profile
Four different activation regions can be identified in the above current profile:
• Motor activated, with the full power for the first 2ms as shown in the zoom window
• After 2ms the current is kept below 30A through PWM control for around 500ms
• The current in the motor is limited to 4A for 2.5s.
• The motor is switched off.
With such a current profile, the typical H-Bridge device with a minimum current limitation around 30A is not able to handle the starting full power phase. It tries to limit the current to 30A going into the saturation region with very high power dissipation. Under this condition, the junction temperature increases quickly, reaching the thermal shutdown threshold in a few hundred μs. Figure 2 shows the device thermal behavior of the first 800μs; it is enough to trigger the thermal shutdown event.
Fig.2: Seat Belt Tensioner " H-Bridge Thermal Junction Behavior
Indeed, starting from the ambient temperature of 85°C and due to the high power dissipation caused by the 30A current limitation, the junction temperature increases quickly, reaching the thermal shutdown protection in 600μs. It is obvious that an integrated H-Bridge device with a current limitation in the range 30...40A is not compatible with the seat belt tensioner application.
Another application that we are going to analyze is the seat recliner. As opposed to the previous case, this application, during the motor start-up, requires a peak current that is compatible with the examined H-Bridge current limitation. It is also requested that the device must be able to handle at least 10 consecutive activations. In a worst case condition, the typical current profile consists of a peak (around 28A) with a duration of a few milliseconds, then the current is limited to 8A for 5 seconds. A gap of 0.5 seconds must be considered between two consecutive activations.
Figure 3 shows the device's thermal behavior during five consecutive activations
Still with the examined H-Bridge, starting from the ambient temperature of 85°C, the thermal shutdown protection is triggered in only four consecutive activations. Since the device is not able to satisfy the minimum number of consecutive activations required from the application, we can conclude by saying that the typical H-Bridge devices are not fully compatible with the seat recliner requirements.