Automotive power at high frequencies

01 March 2007

Power supplies that operate at high frequencies are becoming a necessity in automotive applications. Yet converters that operate at high switching frequencies typically cannot handle high voltages

The increasing electronics content presents a challenge when converting an automobile’s battery to the voltages required by most automotive modules (e.g. 5V and 3.3V).

Performing this conversion with a linear regulator creates significant power dissipation, making thermal management both difficult and expensive. The higher power requirements of faster processors and ASICS have steered converting power from simple, low-cost, inefficient linear regulators to more complex but efficient switching converters.

Choosing the converter
The size of a switching converter depends on its switching frequency. Power inductors and capacitors become smaller with higher switching frequency. Also, a high-efficiency converter reduces power dissipation, thus eliminating bulky, expensive heatsinks. These advantages make the switching converter an obvious choice for power management of body electronics, infotainment, and engine control modules.

Higher switching frequencies increase power loss, however, partly offsetting the advantage of using a switching converter. The switching losses get markedly worse at higher input voltages because they are proportional to the square of the operating voltage. Furthermore, the high-voltage IC processes (40V or higher) needed to withstand over-voltage transients such as load dump cause losses to increase. High-voltage processes use relatively larger geometry and higher gate thickness. Longer channel length means longer propagation delays. Therefore, high voltage processes are inherently slow and can be very inefficient, as the transition losses increase due to the switch’s longer rise and fall times.

Stress conditions
Any over-voltage condition that persists longer than the thermal time constant of an electronic device can be considered a steady-state phenomenon. In these situations, the continuous power dissipation and resulting temperature rise are of primary concern.

The output set-point of the voltage regulator is typically about 13.5V. It is possible for the alternator voltage regulator to fail in such a way as to provide full field current irrespective of load or output voltage conditions. When this happens, voltage in excess of 13.5V may be applied to the entire system. Typically, an OEM’s failed-regulator test requirement is about 18V for one hour.

Jump-start point
Another over-voltage condition that is effectively steady-state is a double-battery jump-start. This condition typically occurs when a 24V system is used to jump-start a disabled vehicle. A typical OEM double-battery test requirement requires the application of 24V for two minutes. Certain engine management and safety-related systems are required to operate under these conditions.

Whenever current is interrupted, an over-voltage pulse will typically be produced. Due to the amplitudes and durations involved, filters, MOVs (metal oxide varistors), or transient voltage suppressors are required to suppress these over-voltage transients. The ISO 7637 standard defines four basic test pulses to address these inductive switching transient over-voltage conditions.

Suppression systems
The battery voltage cannot be fed directly to low-voltage, highperformance switching converters. Instead, transient voltage suppressors like MOVs and bypass capacitors, followed by traditional input voltage limiters, must be added between the battery and switching converter. These simple circuits are built around a P-channel MOSFET. The MOSFET operates in saturation when the input voltage VBAT is below the breakdown voltage of the zener Z2. During the input voltage transient, the MOSFET blocks voltages that are higher than the Z2 breakdown voltages. The disadvantages of this circuit are its number of components and the expense of the P-channel MOSFET.

Another approach involves an NPN transistor with its collector connected to the battery and its emitter connected to the downstream electronics. A Zener diode connected between the transistor’s base and ground clamps the base voltage, thus regulating the emitter voltage at VBE below the voltage VZ. Although this circuit is less costly than the MOSFET circuit, it is also less efficient (PLOSS = IIN*VBE). Furthermore, the drop across the transistor increases the minimum operating battery voltage, which is critical during cold crank.

A third possible solution includes an N-channel MOSFET for the blocking element. N-channel MOSFETs are cheaper than the P-channel variety. However, the circuitry to drive the gate is more complex, as the voltage on the gate must be higher than the voltage on the source. The MAX6398 includes an internal charge-pump to drive the external N-channel MOSFET. (See figure 2.) The MOSFET turns off completely when VBAT is above the set limit during the load dump; it remains off as long as VBAT remains above the set voltage. The MAX6398 controls the N-channel MOSFET to protect the high-performance power supply from automotive over-voltages. The MAX5073 2MHz two-output buck converter connected downstream reduces the size of the circuit. The strategy of combining a protector with a lowvoltage/high-frequency power supply saves space and cost compared to a high-voltage converter operating at a significantly lower frequency.

NITIN KALJE is director, corporate applications and GREG DYGERT is strategic applications engineer, Maxim Integrated Products

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