1. Field of the Invention
The invention relates generally to the field of electrical switches and more particularly to electrical switches used in switched-mode DC-DC converters.
2. Discussion of the Related Art
Electrical loads are often required to be powered by one or more direct current (DC) voltages at regulated values that are derived from an available DC voltage source having a different and often unregulated value. For example, electrical equipment in a spacecraft is often powered by a single unregulated DC voltage source, for example, a battery bank and/or solar panel, whereas the various pieces of electrical equipment in the spacecraft may require different regulated voltages.
Similarly, modern electrical devices such as, for example, computers, cellular phones, personal digital assistants (PDAs) and the like, include electrical components that are required to be powered by regulated DC supply voltages of specific values, when the only electrical power available may be from a different DC voltage source having a voltage level different from the DC supply voltage or voltages desired. Further, the available DC voltage source may be substantially unregulated. For example, in a desk-top computer, an unregulated DC voltage is typically derived from the alternating current (AC) mains by rectification and crude filtering to produce a voltage source having an average DC value with a substantial ripple corresponding to the mains AC frequency. This unregulated voltage is typically different in magnitude from the voltage or voltages required to power the various components in the computer. In another example, in battery operated devices, such as lap-top computers, cellular phones or PDAs, the voltage supplied by the battery may vary substantially over time, and it may be of a different value than the voltage or voltages required to power the individual components of the device.
DC-DC converters are used in such situations to supply one or more regulated supply voltages from an unregulated voltage source. In such DC-DC converters, of great concern is conversion efficiency, which is defined as the ratio of converter output power to input power.
Switched-mode DC-DC converters are often used in such applications, as they provide improved efficiency over dissipative conversion methods. In such switched-mode DC-DC converters, an unregulated input voltage is converted into a periodic pulse waveform that has an average value which varies with the ratio of the pulse width to the pulse period. The average value of the pulse waveform may be extracted using filtering techniques, typically including the use of passive filtering components such as capacitors and inductors.
As a practical matter, the use of a high switching frequency in a switched-mode DC-DC converter is desirable because it permits a reduction in the size and weight of the passive filtering components. Switching frequencies in excess of 500 kHz in switched-mode DC-DC converters are common, and the use of large field effect transistors (FETs) as the switching element in the DC-DC converters has facilitated the increase in switching frequency. However, as the switching frequency increases, switching losses during the transition of the switch from off to on and from on to off also increase. This is due to the fact that during these transitions, the current passing through the switch and the voltage across the switch both have positive values resulting in a positive voltage-times-current (VI) product, and thus power dissipation. These are known as switch transition losses and are undesirable because they degrade conversion efficiency.
Present approaches used in an attempt to reduce the switch transition losses in switched-mode DC-DC converters include the creation of special low gate resistance FETs, however, this requires a semiconductor manufacturing technology change. Another approach is the use of complex negative biasing on the gate of the FET during switch transitions in order to extract gate charge faster thus improving switch transition speed and reducing transition losses. However, this approach requires the use of complicated gate drive circuitry. Yet another approach in an attempt to reduce losses is to employ multiple discrete FETs in parallel with a common gate drive in order to reduce the on resistance of the parallel combination. However, this approach does not reduce switching times and does not reduce switch transition losses. Finally, switch transition losses may be reduced simply by reducing the switching frequency resulting in fewer switch transitions during a given time period. However, this necessarily results in the undesirable increase in size, weight and cost of the passive filtering components used in the converter.
Thus each of these prior attempts to reduce the switch transition losses has its drawbacks, and it would be preferable to reduce switch transition losses in DC-DC converters without changing the switch manufacturing technology, without the use of complex gate biasing techniques, and without reducing switching frequency.
There is a need for the following embodiments. Of course, the invention is not limited to these embodiments.
One embodiment of the invention is a DC-DC converter for converting an unregulated input voltage into at least one regulated output voltage, the converter having an inductor, a capacitor coupled to the inductor, a rectifier coupled to the inductor and capacitor; and a controllable electrical switch, coupled to the inductor the capacitor and the rectifier, the electrical switch including a first FET having gate, drain and source terminals, and having a first switching time, and a second FET having a gate terminal, a drain terminal coupled to the drain terminal of the first FET, a source terminal coupled to the source terminal of the second FET, a switching time of the first FET being less than a switching time of the second FET. Different switching times may be realized by use of FETs with different channel areas, different gate areas, different gate capacitance, or different gate drive circuits, or a combination of one or more of these features.
In accordance with another embodiment of the invention, an electrical switch includes a first FET having gate, drain and source terminals, and having a switching time, and a second FET having a gate terminal, a drain terminal coupled to the drain terminal of the first FET, a source terminal coupled to the source terminal of the first FET, the switching time of the first FET being less than the switching time of the second FET.
Yet another embodiment of the invention includes a method of operating a switched-mode DC-DC converter, having an inductor, a capacitor coupled to the inductor, a rectifier coupled to the capacitor and inductor, and first and second parallel-connected FETs coupled to the inductor the capacitor and the rectifier, a switching time of the first FET being less than a switching time of the second FET. The method comprises turning the first FET on before turning the second FET on during a switch on transition, and turning the second FET off before turning the first FET off during a switch off transition.
These, and other, embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating various embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions and/or rearrangements may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements.