Electric power conversion systems – Current conversion – With voltage multiplication means
Reexamination Certificate
2000-06-15
2001-05-01
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With voltage multiplication means
C307S110000
Reexamination Certificate
active
06226194
ABSTRACT:
The invention relates to a DC/DC converter operating on the principle of a charge pump, comprising a first capacitor, the one electrode of which is connectable via a first MOSFET to the input of the converter and via a second MOSFET to ground and the other electrode of which is connectable via a third MOSFET to the input of the converter and via a fourth MOSFET to the output of the converter, a second capacitor connected between the output of the converter and ground, and a control circuit with an oscillator connected to the gates of the four MOSFETs and serving with the charge pump active to signal the second and third MOSFET ON in the charging phase of the charge pump and the first and fourth MOSFET ON in the discharge phase of the charge pump.
Many electronic circuits require in addition to the supply voltage further voltages sometimes at a level above that of the supply voltage. One low-cost, simple and compared to converters employing an inductor highly compact solution for making these further voltages available are voltage converters operating on the charge pump principle. Converters of this kind are described e.g. in the text book “The Art of Electronics” by Paul Horowitz, 2nd edition, Cambridge University Press, New York, 1991 on pages 377 to 379 thereof.
Horowitz also describes a simple DC/DC converter operating on the charge pump principle with which an output voltage is achievable corresponding roughly to twice the input voltage at the most. The basic circuit of the converter consists substantially of a capacitor and four controllable switches (e.g. MOSFETs), whereby one electrode of the capacitor is connectable via a first switch to the input voltage terminal of the converter and via the second switch to ground, and the other electrode of the capacitor is connectable via the third switch to the input voltage terminal and via the fourth switch to the output voltage terminal of the converter. The converter comprises further a clock oscillator which signals the switch so that in a first phase of a clock cycle, the charge phase, the second switch and the third switch are ON whilst the other switches are OFF so that the capacitor is charged to the input voltage, and in a second phase of a clock cycle, the discharge phase, the first switch and the fourth switch are ON whilst the other switches are OFF so that the charged capacitor is then connected in series to the input voltage, resulting in a voltage value at the smoothing capacitor located at the output of the circuit corresponding to roughly twice the input voltage. In actual practice the converter is regulated by a regulator mechanism to a fixed output voltage level between that of the input voltage and twice thereof depending on the requirements of the particular application.
Since DC/DC converters operating on the charge pump principle often need to be accommodated as an integrated circuit in miniature portable devices, e.g. cellular telephones, powered by a battery or storage battery, the intention in such applications is to produce the converter firstly as small as possible, i.e. taking up as little circuit area as possible, and secondly to maintain the power consumption of the converter as low as possible to relieve the load on the battery or storage battery.
One problem fundamental in this context is the starting phase of the DC/DC converter since in this case the converter draws a particularly high current from the input of the converter to charge not only the, as yet, empty capacitor of the charge pump but also the smoothing capacitor, likewise empty, at the output of the converter. In the converter as described by Horowitz the starting current is limited only by the input voltage and the ON resistance of the MOSFET switches. Since the entirety of the input voltage is thus applied to the switching transistors a heavy peak current materializes which is a considerable load on the battery when the device is battery-powered. In addition to this the ohmic losses occurring in this case are considerable.
Corresponding conditions occur likewise when the converter is short-circuited at its output, i.e. when the output terminal of the converter is grounded, here too, the entirety of the input voltage being applied to the switching transistors.
The company Maxim Integrated Products describes in its product catalog available in May 1999 at site http://www.maxim-ic.com a converter corresponding to the aforementioned DC/DC converter with the product designation MAX679 which additionally includes a separate circuit for the starting phase and the short-circuit case of the converter, serving to reduce the current during the starting phase and when the output of the converter is short-circuited.
The salient element of this circuit is a timer with the aid of which a starting time is defined. On time-out of the starting time the current for charging the charge pump capacitor is directed through a separate switch, the circuit area of which corresponds to {fraction (1/10)}th of that of the four switching transistors of the charge pump. In actual practice, since the circuits are designed integrated, no additional switch is installed, but instead simply 10% only of the circuit area of the corresponding switching transistor is used during the starting phase. To charge the output capacitor an additional MOSFET switch, identified P
4
in the document is made use of. In the starting phase this additional MOSFET connects the input of the converter to the output of the converter which connects the output capacitor.
However, the drawback of this circuit for the starting phase and the short-circuit case of the DC/DC converter is that it is relatively complicated and takes up a relatively large area of the circuit, it requiring a timer and an additionally “large” MOSFET which takes up a considerable proportion of the circuit “real estate”, since it is via this switch that the current for charging the capacitor at the output of the circuit needs to flow in the starting phase. In addition, with this configuration neither the starting current nor the short-circuit current permits particularly good control since they are dictated only by the ON resistance of the additional MOSFET.
It is thus the object of the present invention to provide a DC/DC converter of the aforementioned kind having a circuit for the starting phase and the short-circuit case which is configured so as to obviate the drawbacks as cited above.
In accordance with one aspect of the invention this object is achieved by a DC/DC converter as cited at the outset characterized by a fifth MOSFET, the drain of which is connected to the input of the converter, its source being connected via a current source to ground and its gate to the source as well as being connectable via a first controllable switch to the gate of the third MOSFET, a second controllable switch connected to the gate of the second MOSFET and a third controllable switch connected to the gate of the fourth MOSFET and a comparator, the one input of which is connected to the output of the converter and the other input of which is connected to a reference voltage, the comparator outputting a first control signal to the controllable switches and to the control circuit when the output voltage is smaller than the reference voltage, by means of which the first controllable switch is signalled ON, the second and third controllable switch being actuated so that the second MOSFET and the fourth MOSFET are signalled ON and the charge pump is deactivated, and then when the output voltage is larger than the reference voltage, outputs a second control signal to the controllable switches and to the control circuit by means of which the first controllable switch is signalled OFF, the second and third controllable switch actuated so that the second MOSFET and the fourth MOSFET are signalled OFF and the charge pump activated.
The achievement in accordance with the invention sufficies without an additional “large” switching transistor since now the current flowing in the starting phase for charging the two capacitors flows exclusively
Bayer Erich
Schmeller Hans
Kempler William B.
Telecky , Jr. Frederick J.
Texas Instruments Deutschland GmbH
Vu Bao Q.
Wong Peter S.
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