Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-01-22
2001-01-09
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S095000, C363S097000
Reexamination Certificate
active
06172883
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of switching power supplies, and more particularly, to a flyback regulator for providing a low voltage power supply and a high voltage power supply.
2. Description of the Related Art
Audio, graphics, and data processing requirements in portable devices such as laptop computers are constantly increasing. The increased processing capability is accompanied by increasing power dissipation and heat. There is a corresponding requirement for portable power supplies that are as energy and space efficient as possible. Temperature management is also a high priority, as elevated temperatures may adversely affect a device's reliability. To meet power requirements, the trend in the industry is to use a greater number of power supplies that provide lower voltages and increased current.
Portable electronic devices are typically capable of operating with regulated DC power supply from a portable, rechargeable battery pack, or from an AC power supply using an AC to DC adapter having one end that plugs into an electrical socket and another end that plugs into the device. The adapter supplies power to operate the device as well as to recharge the battery pack. AC adapter power conversion and battery recharging is accomplished most efficiently at high output voltage. To improve battery charging efficiency, it is therefore desirable to provide a high voltage signal to charge batteries.
Many portable electronic devices utilize Lithium icon (Li-ion) batteries which are capable of providing both high voltage and excellent capacity, resulting in relatively high energy density. The internal impedance of Li-ion batteries is very high, however, and the batteries are therefore more efficiently utilized in circuits; that require high voltage and low current. This characteristic runs contrary to the industry trend to use lower voltages and increased current.
Converting DC voltage supplied by the battery or AC adapter to regulated DC voltage supplied to the device is accomplished most efficiently in situations where there is a low input to output voltage differential. This characteristic also runs contrary to the industry trend to utilize lower operating voltage, however, since low operating voltages increase the differential between the input and the output voltage. The differential voltage depends on the voltage delivered by the battery or the AC adapter and is typically greatest when utilizing power supplied by the AC adapter. To improve conversion efficiency, it is therefore desirable to generate a low voltage input signal that may be converted to regulated DC voltage supplied to the device.
Re-chargers for portable batteries utilize switching regulators to regulate DC power input to the battery pack. Switching regulators are typically classified into different configurations or “topologies.” One such topology is the inverting or “flyback” regulator where a switch determines whether the voltage applied to an inductor is the input voltage, V
dc
, or zero. In this manner, the output voltage is a function of the average voltage applied to the inductor. The regulator controls; the turning ON and turning OFF of the switch in order to regulate the flow of power to the load. The switching regulator employs inductive energy storage elements to convert the switched current pulses into a steady load current. Power in a switching regulator is thus transmitted across the switch in discrete current pulses.
In order to generate a stream of current pulses, switching regulators typically include control circuitry to turn the switch on and off. The switch duty cycle, which controls the flow of power to the load, can be varied by a variety of methods. For example, the duty cycle can be varied by either (1) fixing the pulse stream frequency and varying the ON or OFF time of each pulse, or (2) fixing the ON or OFF time of each pulse and varying the pulse stream frequency. Which ever method is used to control the duty cycle, the switch in switching regulators is, either OFF, where no power is dissipated by the switch, or ON in a low impedance state, where a small amount of power is dissipated by the switch. This generally results in fairly efficient operation with regard to the average amount of power dissipated.
In view of the foregoing, it is desirable to provide an AC adapter utilizing a switching regulator that is capable of supplying a plurality of different voltages to efficiently meet the device's low voltage and high voltage requirements.
SUMMARY
In one embodiment, a switching regulator circuit for providing a plurality of regulated DC voltage power supplies is provided. The regulator circuit includes a primary inductive element coupled in series with a first switch to turn charging current flow through the primary inductive element ON and OFF, and a first secondary inductive element having a first end coupled to produce a first power source. The first secondary inductive element is magnetically coupled with the primary inductive element. A second secondary inductive element is also magnetically coupled with the primary inductive element and includes a second switch coupled to turn current flow through the second secondary inductive element ON and OFF. A control circuit is coupled to control the ON and OFF duty cycle of the second switch such that when the first switch is operated to induce magnetizing current in the secondary inductive elements by discharging the primary inductive element during the flyback interval. Current is forced to flow through the first secondary inductive element before the second secondary inductive element by keeping the second switch open until the energy requirements of the load connected to the first secondary inductive element are met. The second switch does not close until the flyback current is zero. Energy losses relative to the second switch are thus minimized since the second switch is operated when the magnitude of the current is either reduced from its beginning value or zero.
The present invention includes a ramped voltage generator circuit that outputs a ramp voltage signal for input to the control circuit. The magnitude of the ramp voltage signal decreases during each duty cycle of the first switch. The second switch is turned on when the value of the ramp voltage signal is less than or equal to a reference voltage. The second switch is turned off after the flyback interval is complete and before the start of the next flyback interval. The ramped voltage generator circuit includes a third switch coupled in parallel to the third inductive element, a capacitor coupled in parallel to the third switch, and a current sink coupled in parallel to the capacitor. The capacitor is pulled up to a charged level between flyback intervals and discharges during the flyback intervals to create the ramp voltage signal.
The control circuit includes an error amplifier for generating an error signal indicative of the voltage output by the second power source, a pulse width modulator coupled to receive the feedback signal and the ramp voltage signal. The pulse width modulator generates a driver signal that is input to a latch circuit that outputs the driver signal to the second switch.
Another embodiment of the present invention provides a method for generating a plurality of regulated DC voltage supplies utilizing a flyback regulator. The flyback regulator includes a primary inductive element magnetically coupled to a plurality of secondary inductive elements wherein one of the secondary inductive elements is coupled to a first switch. The method includes:
opening the first switch coupled with the one secondary inductive element;
charging the primary inductive element with a charge current over a controlled time period;
opening a second switch coupled to the primary inductive element, thereby reversing polarity of the inductive elements and causing magnetizing current to flow through another one of the secondary inductive elements;
generating a ramped voltage signal having a negative slope, and c
Cummings John
Kates Barry K.
Bertani Mary Jo
Dell USA L.P.
Patel Rajnikant B.
Skjerven Morrill & MacPherson LLP
Terrile Stephen A.
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