In the manufacture of portable battery powered devices, operation time is a critical factor affecting marketability. Devices are routinely compared to competing devices on the basis of how long a given battery allows a user to operate the device. As a result, great efforts have been undertaken to reduce to power consumption of such devices. These efforts have produced a substantial body of technology for increasing the efficiency of battery powered devices.
Among the areas of primary concern are operating voltage, switching speed, regulator efficiency, and leakage current in semiconductors. For example, it had been a standard for some time that semiconductor logic devices operated at a 5 volt level. Routinely, however, manufacturers are instead preferring 3 volt, and in many cases 1 volt, logic systems. Lower voltage reduces the power dissipation and the effect of voltage induced leakage current. The logic systems depend on transistors to switch from one logic level to another. These switching transistors draw virtually negligible amounts of current when switched from one logic level to another. However, many components, such as microprocessors, have millions of such switching transistors, and they are operated in a way such that many of them may be making millions of transitions per second. The cumulative effect of these transitions results in a substantial current flow through these components. The focus on switching speed and efficiency has reduced the current drain for these components.
While these efforts have reduced the power required from the battery, electric energy recovery has, in comparison, been neglected in portable devices. For the most part this is justified. Reducing the power used by a device reduces the amount of energy that could be recovered, and in addition, since most of the electric energy is converted to heat, it is thermodynamically impossible, or at least very impractical, to recover. Still, some opportunity exists in certain devices.
Many of these devices include motors. Examples include cordless power tools, portable computers, and CD players. A control circuit controls operation of the motor by opening and dosing a control switch connected between the motor and the battery. If motor speed is to be regulated, then a current and/or voltage regulator circuit is also placed in series with the control switch. However, when the motor is tuned off, the momentum of the motor tends to keep the motor rotating, and causes the motor to generate a transient charge. In addition, the inductive windings of the motor are energized at the time the control switch is opened, and this energy must be released before the current through the windings can cease. The voltage generated by the transient has the opposite polarity of that applied to the motor to make it spin. When a charged inductive element, like a motor winding, is abruptly disconnected from a power source, a very large negative voltage transient is generated to dissipate the energy stored therein. As is well known in mechanical switch systems, this transient event can produce an arc between the switch contacts. In the case of a motor system having kinetic energy, the transient is both slightly larger and prolonged.
Transients can easily damage a semiconductor circuit in the absence of safeguards. For example, a transistor, used to control motor activation, which is connected directly to the motor is susceptible to damage from a transient event. The typical method of dealing with them is to place a capacitor paralleled with a resistor, and both in series with a diode, across the motor. This circuit is well known and widely used in the art, and is commonly referred to as a snubber circuit. This acts to cushion the electrical shock, and limit the negative voltage level experienced during a transient event. However, this simply results in converting electric energy to heat by dissipating energy through the resistor. It does not provide for reuse of the generated charge, which could be of benefit in a portable device since there is a finite amount of charge available from the battery.
Therefore there exists a need for a circuit in a portable battery powered device for handling transient charge generation, and there exists a need for reusing that charge.