Driving Automotive Power Supplies to Higher Frequencies

Sep 1, 2006 12:00 PM
By Nitin Kalje, Senior Scientist, Maxim Integrated Products, Sunnyvale, Calif., and Greg Dygert, Str

Transient OV Conditions

The majority of transient OV conditions in an automobile are due to switching inductive loads. Examples of such loads include the starter motor, fuel pump, window motors, relay coils, solenoids, ignition components and distributed circuit inductances. Whenever current is interrupted in these inductive loads, an OV pulse will typically be produced. Due to the amplitudes and durations involved, filters, metal oxide varistors (MOVs) or transient voltage suppressors are required for suppressing these types of OV transients. A description of pulses based on the ISO 7637 standard, shown in the table, is as follows:

Pulse 1 is a negative-going repetitive pulse ranging from -80 V to -150 V in amplitude with a duration of 1 ms to 140 ms. The source impedance is typically on the order of 5 Ω to 25 Ω. Pulse 2 is a positive-going repetitive pulse ranging from 75 V to 150 V with a typical duration of 50 s. The source impedance is typically 2 Ω to 10 Ω. Pulse 3a is a series of negative pulses that are on the order of -150 V and 100 ns. Pulse 3b is a series of positive pulses on the order of 100 V and 100 ns. The impedance of the signal source is typically 50 Ω.

Pulse 5, also known as a load dump, is a condition that occurs when an alternator is supplying high current to a discharged battery and the battery is suddenly disconnected. Since the alternator is a magnetic device, the sudden reduction in stator current induces a high voltage at the alternator output to maintain the energy of the system. The duration of this transient is based on the electrical time constant of the alternator field circuit and regulator response time.

Due to conditions described earlier, the battery voltage cannot be fed directly to the low-voltage, high-performance switching converters. Transient voltage suppressors like MOVs and bypass capacitors, followed by traditional input voltage limiters are used. These circuits are simple and built around the p-channel MOSFET (Fig. 1). The p-channel MOSFET must be rated at 50 V or 100 V, depending on the voltage transients expected at the input voltage (V_BAT). The 12-V Zener diode Z1 limits the gate-to-source voltage of the MOSFET below the V_GSMAX. The MOSFET operates in saturation when V_BAT is below the breakdown voltage of the Zener Z2. During the input voltage transient, the MOSFET blocks the voltages higher than the Z2 breakdown voltages. The disadvantage is an expensive p-channel MOSFET and too many components around it.

Another approach is to use the npn transistor with collector connected to the “plus” terminal of the battery and the emitter to the downstream electronics. A clamping device (V_Z) then clamps the npn base voltage, which regulates the emitter voltage at V_BE below the V_Z. It's a lower-cost but inefficient (P_LOSS = I_IN × V_BE) solution. The drop also increases minimum operating battery voltage, which is especially critical through cold crank. The third possible solution is using an n-channel MOSFET. N-channel MOSFETs are widely available, cheaper and may be used as a blocking element. However, the gate drive is more complicated because it needs to be higher than the source voltage. The MAX6398 includes an internal charge pump to drive an external n-channel MOSFET (Fig. 2).

Fig. 2 shows the implementation of an n-channel MOSFET switch as a blocking device. The MOSFET can be completely turned off as soon as V_BAT increases above the set limit during the load dump. The MOSFET remains off as long as the V_BAT remains above the set voltage. The MAX6398 controls the n-channel MOSFET to protect the high-performance power supply from the automotive OV events, such as double-battery jump-starting and load dumping. The MAX5073, a 2-MHz, two-output compact buck converter is connected downstream.

As depicted in Fig. 3, the MAX6398 effectively blocks automotive load-dump pulses and regulates the voltage seen by low-voltage, high-performance electronics. The strategy of using a combination of protector and low-voltage, high-frequency power electronics saves space and cost compared to the high-voltage solutions operating at significantly lower frequencies.

More on Buck Converters

• Buck-Converter Design Demystified
• Optimizing Voltage Selection in Buck Converters
• Power Conversion Synthesis Part 1: Buck Converter Design
• Improving Efficiency in Synchronous Buck Converters

> Buck Converter Archive 

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