Soft starting driver for piezoelectric device

A driver includes a boost converter, a pulse width modulator controlling the boost converter, and a timer controlling the pulse width modulator. The timer, such as a digital counter, causes the pulse width modulator to produce narrow pulses unless or until the end of a period is reached, at which point the pulse width modulator is not controlled by the timer.

BACKGROUND

This invention relates to a voltage boost circuit and, in particular, to a boost circuit that limits starting current to a load, such as a piezoelectric driver.

A piezoelectric actuator requires high voltage, greater than typical battery voltages of 1.5 to 12.6 volts. A “high” voltage is 20-200 volts, with 100-120 volts currently being a typical drive voltage. Some line driven power supplies for actuators provide as much as 1000 volts. Producing high voltage from a battery is more difficult. As noted in U.S. Pat. No. 7,468,573 (Dai et al.), the high voltage required “to drive piezoelectric actuators in today's small electronic devices is undesirable.” The solution proposed in the '573 patent is to use two pulses of “lower” voltage instead of a single pulse at high voltage. The “lower” voltage is not disclosed. Single layer actuators generally require a higher voltage than multilayer actuators. Multilayer actuators have the advantage of providing greater feedback force than single layer actuators.

Thus, there is a need for a battery powered driver, that is, a single chip power supply, for piezoelectric devices. A voltage boost circuit can be used to convert the low voltage from a battery to a higher voltage for the driver. In a boost converter, the energy stored in an inductor is supplied to a capacitor as pulses of current at high voltage.

FIG. 1is a schematic of a basic boost converter well known in the art; e.g. see U.S. Pat. No. 3,913,000 (Cardwell, Jr.) or U.S. Pat. No. 4,527,096 (Kindlmann). Inductor11and transistor12are connected in series between supply13and ground. When transistor12turns on (conducts), current flows through inductor11, storing energy in the magnetic field generated by the inductor. Current through inductor11increases quickly, depending upon battery voltage, inductance, internal resistances, and the on-resistance of transistor12. When transistor12shuts off, the magnetic field collapses at a rate determined by the turn-off characteristic of transistor12. The rate of collapse is quite rapid, much more rapid than the rate at which the field increases. The voltage across inductor11is proportional to the rate at which the field collapses. Voltages of one hundred volts or more are possible. Thus, a low voltage is converted into a high voltage.

When transistor12shuts off, the voltage at junction15is substantially higher than the voltage on capacitor14and current flows through diode16, which is forward biased. Each pulse of current charges capacitor14a little and the charge on the capacitor increases incrementally. At some point, the voltage on capacitor14will be greater than the supply voltage. Diode16prevents current from flowing to supply13from capacitor14.

A problem with the converter shown inFIG. 1is that, when capacitor14is not charged, the voltage across diode16is maximum and current is limited by the internal resistance of the inductor. Adding resistance to reduce current reduces the efficiency of the circuit during normal operation. A high current results in a high voltage that can damage piezoelectric or other devices powered by the converter. The high current also puts a significant load on the low voltage battery powering the boost circuit.

It is known in the art that pulse width, i.e. the period during which transistor12conducts, affects current (as long as the inductor does not saturate). Over the years, the circuit ofFIG. 1has been embellished with various feedback loops, some of which modulate pulse width; e.g., U.S. Pat. Nos. 7,106,036 (Collins) and 7,129,679 (Inaba et al.) The '679 patent discloses that gradually changing duty cycle during startup gradually increases the output voltage from the converter. The gradual change is accomplished by a closed loop feedback circuit that significantly increases the cost, complexity, and power consumption of the converter.

In view of the foregoing, it is therefore an object of the invention to provide a soft starting, high voltage driver for piezoelectric devices.

Another object of the invention is to minimize power drain by single chip, battery powered drivers.

A further object of the invention is to limit peak current in a boost converter, thereby preventing saturation of the inductor, minimizing power consumption, and avoiding damage to loads.

Another object of the invention is to provide a simple, soft start mechanism for a boost converter.

A further object of the invention is to provide an open loop, soft start converter.

SUMMARY OF THE INVENTION

The foregoing objects are achieved in the invention in which a driver includes a boost converter, a pulse width modulator controlling the boost converter, and a timer controlling the pulse width modulator. The timer causes the pulse width modulator to produce narrow pulses unless or until the end of a period is reached, at which point the pulse width modulator is not controlled by the timer but by other means. The timer is preferably a digital counter coupled to a source of clock signals in the driver.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one aspect of the invention, illustrated inFIG. 2, an arbitrary number of narrow pulses21are applied to gate18(FIG. 1) of transistor12when the converter is first turned on or after a reset. The pulses are narrow in the sense that the field induced in inductor11is well below saturation.

After the arbitrary number of pulses, wider pulses22are applied to gate18(FIG. 1) of transistor12and continue to be applied until the driver is shut off or is reset. Pulses21provide less charge per pulse than pulses22, thereby providing reduced initial current to capacitor14(FIG. 1). The voltage across capacitor14increases, as indicated by curve23, but can not reach normal operating voltage. Only after the pulse width is increased does the output voltage from the converter reach operating level24, as indicated by curve25. Without the soft start, peak current can be twice the steady state current. With the soft start, peak current is less than the steady state current.

In one embodiment of the invention, the gate duty cycle was fifty percent during start-up. The duty cycle remained at this value for 128 clock cycles, then the duty cycle increased to an optimal value for steady state. Optimal performance with the particular circuit used was achieved with a duty cycle of seventy-five to ninety percent. The clock rate was 130 kHz to 175 kHz. This corresponds to a start-up time of 0.985 to 0.731 milliseconds.

A clock rate in this range of frequencies enables one to use inductors that are physically small and less expensive. The inductors used in one embodiment of the invention had inductances of 33 μH and 68 μH. Current increases with inductance and decreases with frequency. Smaller inductors can be used but are more difficult to make with commercially acceptable precision or cost.

FIG. 3is a block diagram of a driver constructed in accordance with a preferred embodiment of the invention. Register or counter31has each stage thereof coupled to NAND gate32. As pulses from clock33are counted, the bit pattern in the register will eventually become all ones (11111111), e.g. 7 FH in hexadecimal notation or 127 in decimal notation. AND gate34prevents further counting after the maximum count is reached, until the system is reset. Power on is functionally the same as a reset.

While the output from NAND gate32is high (logic 1), pulse width modulator35produces narrow pulses21(FIG. 2). When the bit pattern in register31is all ones, the output from NAND gate32will switch from high to low (logic 0), causing pulse width modulator35to produce wider pulses22(FIG. 2). Thus, initially, converter41produces a lower voltage than the normal operating voltage and the supply voltage for amplifier42, which drives piezoelectric device43, is less than normal. Thus, excess voltage is prevented from reaching piezoelectric device43. When normal operating voltage is achieved, excess voltage cannot occur.

When the output from NAND gate32goes low, it releases control of pulse width modulator35to other inputs, such as inputs37and38. These inputs can control, for example, frequency and pulse width, and be coupled to suitable loops for voltage regulation or other needs. Generally, pulse width will increase, as indicated inFIG. 2, but this is not to imply a limit on control inputs37and38, which may, momentarily at least, cause the pulse width to be smaller than pulses21(FIG. 2).

Circuits for pulse width modulation are well known in the art. Pulse width can be determined by a variety of circuits. For example, the count in a counter can represent pulse width and the output from NAND circuit32can control one bit of such a counter.

The invention thus provides a soft starting driver for piezoelectric devices requiring a boost converter for operating from a battery. The driver limits starting current and minimizes power drain. Peak current is limited, thereby preventing saturation of the inductor or damage to loads. Open loop control provides a simple, soft start mechanism for a boost converter. The invention is easily implemented in a single integrated circuit using existing libraries for counters, logic, and pulse width modulating circuits, combined in accordance with the invention.

Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, the specific values given are by way of example only. The use of a NAND gate (negative logic) does not mean that positive logic could not be used instead. Other types of counters, with different peripheral logic, can be used instead; e.g. using a carry bit to indicate the end of start-up. Any number can be used to indicate end of start-up, e.g. 43 H, with suitable peripheral logic for sensing that number.

The invention also can be used with single output inverters such as described in U.S. Pat. No. 5,313,141 (Kimball). The pulse width modulator can use the same clock signal as the counter or some other clock signal.