1. Field of the Invention
The present invention relates generally to integrated circuit power converters for providing an AC power output, and more specifically, to circuit and method for managing startup transients in the AC output.
2. Background of the Invention
AC output switching power converters are currently in widespread use. In particular switching power converters are commonly used in automotive amplifiers and other amplifiers in which a high power output is produced from an available DC voltage that is too low to deliver the desired power to a fixed impedance load. Switching power converter amplifiers, sometimes referred to as Class D amplifiers, also have higher efficiency than linear amplifiers, making them well suited for battery driven applications and applications where power dissipation in the form of heat is a problem, such as very high power professional audio applications, as in concert halls.
Another application of AC power converters is to provide an AC power source for operating line powered devices when only battery power is available. AC power inverters are in common use for this application and provide for operation of line-powered equipment in automobiles and other locations where only DC battery power is available.
Switching AC power converters can either be single ended (half-bridge) or double-ended (full-bridge). The full-bridge configuration is commonly used, as the half-bridge configuration requires a DC blocking capacitor if the average DC voltage of the output stage is not the same voltage as the load return connection. In typical automotive applications, the power supply is single-ended, and therefore the operating point for a 12VDC system is 6V and not ground. However, single-ended output stages are more space efficient in that only two output stage devices are required per channel and the on-resistance of the devices can be twice that of the corresponding full-bridge design, as only one device is in series with the load at a time.
In the half-bridge circuit at startup, without special startup circuitry, the converter switching stage must generate pulses (or switch one driver on continuously) to drive the output upward from zero volts to the midpoint of the power supply or other operating point, causing a transient to appear on the output of the converter. In amplifier applications, this transient is undesirable, as it is heard through speakers as a “pop” or “click” and in extreme cases may cause damage to the load.
Therefore, startup circuits are employed in AC output power converters that are capacitively-coupled, so that the transient is not produced or is minimized. One such circuit provides a resistor divider with equal value resistors connected between the power supply rails and the output of the power converter. At startup, the converter power stage is held off by a timing circuit and the resistor divider slowly charges the output terminal to the DC operating point voltage. The resistor divider must typically be provided outside of an integrated circuit power amplifier, and thus increases parts count and cost of the circuit.
The above-mentioned resistor divider technique requires either precision resistors, or the circuit will generate a DC offset at the output of the amplifier which yields another transient when the power converter start. If there is no DC feedback from the output, the offset will remain at the output during operation, affecting performance of the amplifier. The resistor technique is also unsuited for automotive applications in which the current drawn through the resistors is continuously present even if the amplifier is producing no output signal. In general, the presence of the resistor divider lowers the power efficiency of the amplifier. The resistance of the divider must be reasonably low in order to keep the startup time short, so the lower resistance leads to high sink currents through the divider.
An alternative technique that overcomes the drawbacks of the resistor divider and that can be integrated within the converter integrated circuit is described in U.S. Pat. No. 6,538,590 to Gaboriau, et al. A second output driver stage connected to the output of the converter drive electronics is connected via a resistor to the output terminal, essentially in parallel with the output drivers. By disabling the primary output drivers until a predetermined time has passed, the second output driver provides a lower current drive level that more gradually raises the voltage at the output terminal to the power supply midpoint. However, the technique disclosed therein does not so much control the startup transient, but rather reduces the output drive level during the unstable period of startup in the switching output stage.
Therefore, it would be desirable to provide a method and system for controlling startup transients in a capacitively coupled switching power converter that does not require continuous current after startup or components external to an integrated circuit.