Motor drivers contain protection and diagnostic functions

Today's typical midsize automobile already contains around 50 relatively small electric motors and more are being added all the time. Highly integrated, fast-switching motor control ICs with PWM support have the potential to reduce the energy consumption of these devices — and hence the associated CO2 emissions — significantly.

Infineon's new RoHS-compliant NovalithIC series supports PWM applications with frequencies of up to 25 kHz for motor drives operating with currents of up to 70 A.

The NovalithIC series combines three separate chips in one package: a p-channel MOSFET (high-side), an n-channel MOSFET (low-side) and a driver IC in a half-bridge configuration (see Fig. 1).

Fig. 1: Circuit diagram of a NovalithIC — the driver IC provides comprehensive protection and diagnostic functions

Featuring innovative chip-on-chip and chip-by-chip connection technologies, the various modules are suitable for motor drive applications such as air conditioning fans, seatbelt systems and fuel pumps. They are also ideal for a variety of industrial applications. The power switches use vertical MOS technologies for optimal on-state resistance. The p-channel high-side switch means that no charge pump is required. This improves EMC performance. Integrated functions such as logic level inputs, diagnostic functions, slew rate adjustment, dead time generation and a range of protective functions ensure straightforward connection to the microcontroller. The NovalithIC modules can also be combined for H-bridge configurations or to drive three-phase motors.

The power stage with the two DMOS switches can be operated at 25 kHz on both high and low sides, which enables active freewheeling. This avoids the power loss associated with the body diode in off-commutation mode, as off-commutation proceeds via the active high-side switch (p channel MOS) rather than via the body diode.

The modules provide comprehensive protective and diagnostic functionality integrated in the separate driver chip. The overvoltage protection (in the event of load dropout) switches the low-side MOSFET off and the high-side MOSFET on if the supply voltage reaches the corresponding value. This protective function has priority over the other protective functions such as undervoltage switch-off, overtemperature protection, current limiting and short-circuit protection.

The bridge current at both switches is measured for the purposes of current limiting. As soon as the forward current at one of the switches reaches the limit, the switch concerned is deactivated and the other switch is activated for fCLS . All changes at the input pin are ignored during this period of time, although the pin can still be used to switch off both MOSFETs. This provides low-resistance switched-mode current limiting for a typical inductive load such as a motor. This method of current limiting results in much lower power loss than solutions using a MOSFET with linear control. Current limiting can be maintained for a short period without exceeding the maximum permissible junction temperature.

The control and diagnostic functions of the driver IC include dead time generation, which is used to prevent the two MOSFETs switching simultaneously. The dead time is automatically aligned with the selected slew rate, which can also be adjusted. This is done by changing the switching speed of the MOSFETs using an external resistor in order to optimize EMC performance. A combined analog current measurement with status flag output is also provided at the status pin of the driver IC.

Reduced power consumption

The NovalithIC modules have a typical path on-state resistance of 16 m&#937, and a closed-circuit power consumption of just 7 A. Combined with the pulse-width modulation of up to 25 kHz on both MOSFETs, this makes it possible to reduce power consumption significantly in many applications in which motors have previously been switched statically via a relay or a MOSFET. The actuation of an air conditioning fan provides a good example. It is still common to find a MOSFET with linear control that converts between 80 and 130 W into waste heat as the starting resistor for the motor. Using a PWM fan with a NovalithIC can slash the power loss to about 50 W. This reduction of up to 80 W equates to 1.9 g CO2/km. The additional cost of this solution will hence be amortized after just six months or 8,000 km.

The NovalithIC series in the 'green' TO-263 package includes the BTN7930B for currents of up to 32 A (typical), the BTN7960B (up to 47 A) and the BTN7970B (up to 70 A). The BTN7971B, which is also designed for 70 A, has had its switching performance enhanced again, as a result of which switching losses can be reduced by up to 50 percent with the same PWM application and comparable current.

Johann Winter is head of product marketing for High-Power Control at Infineon

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