TI Power Switch Design Tips 3 - Multiphase Boost Converter Powers Car Audio Amplifiers

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Author: Brian King and Dave Parks, Texas Instruments (TI) Applications Engineer

Car audio amplifiers typically use a boost converter to generate a battery output voltage of 18 V to 28 V (or higher). In these high-power applications of 100W and above, large boost inductors, multiple levels of output capacitors, parallel MOSFETs, and diodes are required. Dividing the power stage into multiple parallel phases reduces the stress on many power components, speeding up response to load changes such as those of the subwoofer, and improving system efficiency.

Finding a pulse width modulation controller (PWM) that can be used with a 2-phase boost converter is relatively easy. Most dual-channel interleaved offline controllers or push-pull controllers can be used to directly drive two boost MOSFETs out of phase. However, in a 4-phase solution, the choice of controller is more limited. Fortunately, some multiphase buck controllers can be easily retrofitted for use in 4-phase boost converters.

Figure 1 shows a 4-phase, 300W boost supply using TI's TPS40090 multiphase buck controller designed to handle 500W peak bursts that typically occur in audio applications. Typically, in a multi-phase buck configuration, the controller balances the power of each phase by sensing the average current in the output inductor. In contrast, in a multiphase boost structure, the sense of current is made in a resistor mounted on the source of each FET.

By balancing the peak current in each FET, the multiphase controller distributes power evenly across all boost phases. The gate drive signal from the controller is at a logic level, so each phase requires a MOSFET driver. In this design, a dual MOSFET driver (eg UCC27324) can be used to reduce the number of components.

By applying a current limit to each phase, the multiphase controller protects the controller from overload conditions. Audio applications have a transient peak power requirement that is much higher than the average output power. The current limit must be set high enough to meet these peak power requirements.

The external undervoltage lockout (UVLO) circuit also provides another level of protection that prevents the system from operating at low battery voltages. When the battery voltage drops, the boost supply will attempt to provide as much input current as possible, which can cause a sharp drop in battery voltage when the battery is depleted. This situation can cause damage to the battery, and in the worst case, the battery can be scrapped. The simple and low-cost UVLO circuit consists of a reference circuit, a two-channel comparator, and several resistors (not shown).

In this design, all four phases are switched at 500 kHz and are 90 degree sync. Figure 2 shows the drain-source voltage waveforms for all four phases. The ripple current from each phase is summed at the input and output, while they partially cancel each other at the input and output. This also reduces the ac ripple current of the input and output capacitors. In addition, when the integrated ripple current is 2 MHz, the phase frequency is four times that of a single.

Due to the reduced ripple current and higher frequency, the input and output capacitance is much smaller in a multiphase solution compared to a single phase solution. A more efficient switching frequency also allows the converter to respond more quickly to changes in load current.

The power stage for all individual phases consists of an inductor, MOSFET, Schottky diode, and current sense resistor. For example, one pin of L2, Q2, R8, and D3 forms a phase. Compared to single-phase solutions, it consumes less power and has a wider distribution area, simplifying thermal management. Reduced current and power rating provide a wider range of off-the-shelf inductor, FET and diode options. With a 300-W load, this 4-phase design has 94% efficiency, resulting in losses below 20W.

High efficiency makes this design a small form factor package. For lighter load power requirements, this design can be reduced to 2 phases. In that case, a push-pull or interleaved forward PWM controller can be used instead of the TPS40090. For some high power loads, power components can be added to handle the increased current and power consumption. After determining the proper size, the four phases should be sufficient to support any high-power audio application.