Many known AC-to-DC power converter circuits, also commonly referred to as switch mode power supplies, typically have three principal sections that perform distinct functions: input rectification, voltage reservoir (i.e., energy storage), and DC-to-DC conversion. The circuitry that performs input rectification changes the bidirectional input voltage from an AC line into rectified input voltage with current that flows only in one direction. Voltage reservoir circuitry is typically utilized to smooth out voltage fluctuations, also known as ripple voltage, of the rectified input voltage. The voltage reservoir circuitry provides a low ripple DC voltage to the input of the DC-to-DC conversion circuitry. DC-to-DC conversion is typically required to deliver a suitable output voltage needed to power electronic devices.
The energy storage function is typically performed by one or more capacitors coupled across the rectifier bridge. These capacitors are commonly referred to as a bulk capacitance. The bulk capacitance is charged by the rectified input current, thereby storing energy in the form of a voltage. The voltage on the bulk capacitance is typically too large and fluctuating to power the delicate circuitry of electronic equipment. Therefore, a DC-to-DC conversion circuit is utilized to convert the voltage across the bulk capacitance into a stable, low output voltage capable of powering electronic devices with high efficiency.
The rectified voltage on the bulk capacitance typically has a ripple that varies between a crest value and a valley value at a frequency that in a full bridge rectification is double the AC line frequency (e.g., 120 Hz). The crest value is defined by the peak voltage of the AC input voltage. The valley value is determined by the discharge rate of the bulk capacitor which is a function of the capacitance value and the amount of power demanded by the DC-to-DC converter. The ripple voltage amplitude is the difference between the crest and valley voltage levels. Persons of skill in the art understand that the DC-to-DC power converter needs a minimum rectified voltage at its input to produce the desired output power. Consequently, the ripple voltage amplitude should not exceed a certain value at the minimum AC input voltage specified for the power supply.
A power supply is typically required to provide its normally regulated output voltage for a short time (frequently referred to as the holdup time) after the AC input line voltage is removed. During the holdup time, the voltage input to the DC-to-DC power converter is provided entirely by the stored energy in the bulk capacitance, which is proportional to the bulk capacitance value and the square of the voltage on the bulk capacitance.
The individual capacitors that make up the bulk capacitance are typically selected to meet several requirements that are influenced by the intended use of the power supply. The physical size of the power supply is influenced by the value and the voltage rating (the maximum voltage that an individual capacitor can reliably withstand) of the capacitors that constitute the bulk capacitance. The cost of the bulk capacitance is also a significant part of the total cost of the power supply. For a given value of capacitance, a higher voltage rating translates to higher cost. The voltage rating is selected for reliable operation at the maximum AC input voltage of the power supply, whereas the capacitance of the individual capacitors is selected based on the minimum specified AC input voltage of the power supply.
The total bulk capacitance value is typically selected to ensure that the minimum input voltage required for proper DC-to-DC conversion is not reached when the power supply is operating with the minimum specified AC input line voltage. Typically, AC-to-DC power supplies are designed to operate from a wide range of AC input line voltages, e.g., between 85 volts AC and 265 volts AC. Consequently, the bulk capacitance usually includes a physically large capacitor, which provides a high capacitance value required by the minimum AC input voltage requirement of the DC-to-DC power convertor, as well having a high voltage rating (e.g., 400 V) that exceeds the rectified maximum AC input line voltage.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.