A SINGLE LEVEL TIME-SHIFTED REFERENCE DC/DC CONVERTER

Abstract: A DC/DC converter (1), comprising inductive electrical energy st orage means (L), switching means (S1, S2) and control means (6). The control means (6) are arranged for selectively operating the switching means (S1, S2) for transferring an amount of electrical energy from the energy storage means (L) to an output of said DC/DC converter (1), for providing a desired output voltage (Vout) in accordance with a two-state switching cycle by comparing the output voltage (Vout) with a reference voltage (Vref) at each transition from one state to another of the two-state switching cycle.

Description: A single level time-shifted reference DC/DC converter The present invention relates to a DC/DC converter, comprising inductive electrical energy storage means, switching means and control means, wherein the control means are arranged for selectively operating the switching means for transferring an amount of electrical energy from the energy storage means to an output of the DC/DC converter, for providing a desired output voltage. A DC/DC converter of this type is known from International Patent Application WO 95/34121 in the name of applicant. In practice, a DC/DC converter of the above type can be operated in a continuous or PWM (Pulse Width Modulation) mode, wherein electrical energy is continuously stored in the energy storage means, or in a discontinuous or PFM (Pulse Frequency Modulation) mode, wherein the energy storage means may become completely discharged. In a single output DC/DC converter operated in PWM mode, a typical switching cycle comprises a first phase wherein energy is stored in the energy storage means and a second phase wherein energy is transferred from the energy storage means to the output of the converter. For controlling the output voltage of the known DC/DC converter a voltage window is required, comprising an upper output voltage reference level and a lower output voltage reference level. Controlling the output voltage using such a voltage window causes a random low frequency ripple in the output voltage, equal to the switching frequency of the converter, and a spread in the output voltage. These effects can be reduced by reducing the voltage window, however this reduction is limited due to process spreading in voltage comparators used for comparing the output voltage and the reference voltage. The ripple in the output voltage is caused by the charging and uncharging of the energy storage means, and the load connected to the output of the converter, such as a smoothing capacitor connected across the output terminals of the converter. It is an object of the present invention to provide a DC/DC converter of the above-mentioned type, using a single voltage reference level for controlling the output voltage, and a single comparator means for measuring and comparing the output voltage and the reference voltage. This object is solved in a DC/DC converter according to the present invention, in that the control means are configured for operatively controlling the switching means for transferring electrical energy in accordance with a two-state switching cycle, by comparing the output voltage with a reference voltage at each transition from one state to another of the two-state switching cycle. The present invention advantageously uses the voltage ripple in the output voltage as a "voltage window" by sampling the output voltage at different well-defined moments in time, i.e. the state transitions of the converter. In a further embodiment of the DC/DC converter according to the invention, wherein the control means are arranged for controlling the switching means such that, if the output voltage is below the reference voltage the energy storage means are charged, if the output voltage is higher than the reference voltage the energy storage means are discharged, and if the output voltage equals the reference voltage the energy storage means are neither charged nor discharged. In a preferred embodiment of the invention, the DC/DC converter comprises: - first and second input terminals for receiving an input voltage Vin; - first and second output terminals for providing an output voltage Vout; - a coil, having first and second connection ends, wherein the first connection end connects to the first input terminal; - first switching means operatively connected to provide a conduction path from the second connection end of the coil to the second input terminal; - second switching means operatively connected to provide a conduction path from the second connection end of the coil to the first output terminal; - diode means parallel connected to the second switching means and providing a conduction path from the second connection end of the coil to the first output terminal; - capacitor means connected between the first and second output terminals; - control means arranged for operatively switching the first and second switching means into their closed or conductive state and their open or non-conductive state, - comparator means, configured for comparing the output voltage with the reference voltage providing a control signal, and - first latching means operatively connected for receiving the control signal of the comparator means at a first sample moment at a first state transition of the converter, and - second latching means operatively connected for receiving the control signal of the comparator means at a second sample moment at a subsequent second state transition of the converter, and - wherein the first and second latching means provide control signals for controlling the control means. Preferably, switching means constructed as MOS (Metallic Oxide Semiconductor) transistor means are used, having their control terminal (gate) connected with the control means for controlling the conductive or non-conductive state of the transistors. The DC/DC converter according to the invention is of particular advantage if applied in a portable electronic appliance, such as but not limit to battery powered appliances. The invention will now be described in more detail with reference to the accompanying drawings showing a DC/DC up-con verier, wherein: Figure 1 shows a circuit diagram of a single output DC/DC up-converter operated in accordance with the present invention. Figure 2 shows, in a graphic representation, the output voltage measurement with a voltage window, in accordance with the prior art. Figure 3 shows, in a graphic representation, the output voltage measurement with a single level time-shifted reference in accordance with the present invention. Figure 4 shows a circuit diagram of the control means for operating the DC/DC converter shown in Figure 1, in accordance with the present invention. Figure 1 shows, by way of example, a DC/DC up-converter 1 operated in accordance with the present invention, and having a single output. The converter 1 comprises inductive electrical energy storage means taking the form of a coil L and first switching means SI, series connected between a first input terminal 2 and a second input terminal 3. The connection of the coil L and the first switching means SI connects via second switching means S2 to a first output terminal 4. A diode D is parallel connected with the second switching means S2 and provides a current conductive path from the first input terminal 2 to the first output terminal 4. The second input terminal 3 and a second output terminal 5 connect through a common conductive path, for example the earth or mass of an electronic appliance. A smoothing capacitor C connects between the first and second output terminals 4, 5. The converter 1 is operated to provide a controlled or regulated output voltage Vout at the output terminals 4, 5 in response to an input voltage Nin at the input terminals 2, 3. To this end a controller or control means 6 are provided for operating the first and second switching means SI and S2 in accordance with a switching sequence, such that the output voltage Nout is higher than the input voltage VI. The control or the first and second switching means SI and S2 is schematically indicated by arrows 7, 8, respectively. Comparator means 15 are provided, having a first input terminal 9 connected to the output of the converter 1, for comparing the output voltage Nout with a reference voltage Nref applied at a second input terminal 10 of the comparator means 15. An output of the comparator means 15 connects to an input of the control means 6, as schematically indicated by arrow 11. Figure 2 illustrates the output voltage Nout against the time t of a DC/DC up- converter known from International Patent Application WO 95/34121, wherein a reference voltage window is used for controlling the output voltage Nout, comprising an upper output voltage reference level Nrefh and a lower output voltage reference level Nrefl, schematically indicated by dashed lines in Figure 2. In a typical switching cycle in a Pulse Width Modulation (PWM) conversion mode, during a first phase Φl energy is stored or built up in a coil L, whereas in a second phase Φ2 of the PWM switching cycle, the stored energy is delivered to the output terminals 4, 5 of the converter 1. In the first phase Φl, the first switching means SI are closed, that is in a current conductive state, while the second switching means S2 are open, that is in a non- current conductive state. During the first phase Φl current flows only through the coil L storing electrical energy therein. During this phase, the current I through the coil L increases. In the second phase Φ2 the first switching means SI are open and the second switching means S2 are closed. In this phase, the current I through the coil drops because energy is delivered to a load 12 connected across the output terminals 4, 5 of the converter 1. This results in a ripple 20 in the output voltage, as shown in Figure 2. The frequency of the ripple 20 equals the switching frequency of the converter 1, having a period equal to the length in time of the phases Φl and Φ2, as indicated in Figure 2. The ripple 20 in the output voltage is also caused by the charging and uncharging of the output capacitor C and the current through the equivalent series resistance (ESR) of this capacitor C. The size of the ripple 20 is directly related to the coil current I and the ESR. As shown in Figure 2, the amount of electrical energy stored in the coil L is controlled such that the output voltage Vout remains within the reference voltage window defined by Vrefh and Vrefl. Besides the complication of providing dual reference voltage sources, for setting the voltage window and, accordingly a more complicated comparator circuit, the output voltage Vout will have a relatively large spread, which can be hardly reduced by reducing the voltage window due to process limitations in the comparators. Figure 3 shows, in a graphic representation, output voltage measurements with a single level time-shifted reference in accordance with the present invention, wherein the output voltage Vout is sampled at defined moments in time, that is at each state transition of the converter 1, i.e. a transition from the first phase Φl to the second phase Φ2 and a transition of the second phase Φ2 to the first phase Φl. In Figure 3, the sampling moments are indicated by arrows 21-29. Again, the output voltage Vout has a ripple 30 with a frequency which equals the switching frequency of the converter, however, the ripple 30 is essentially reduced in amplitude compared to the ripple 20 by the control algorithm according to the present invention by sampling the output voltage as indicated above. Figure 4 shows an implementation of the single level time-shifted reference principle according to the invention, using digital control means 6 and first and second latching means 16, 17, respectively, producing digital output signals VH and VL which are inputted in the digital control means 6. The first latching means 16 are sampled at the transition of the first phase Φl to the second phase Φ2 of the conversion cycle, indicated by Vmin, and the second latching means 17 are sampled at the transition of the second phase Φ2 to the first phase Φl of a conversion cycle, indicated by Vmax. In other words, the first latching means 16 are sampled at the end of the period in which energy is stored in the energy storage means such as the coil L of the converter circuit shown in Figure 1, whereas the second latching means 17 are sampled at the end of a conversion cycle, that is wherein energy is transferred from the energy storage means to the output of converter 1. A comparator means 15 provides a control signal based on the result of a comparison of the output voltage Vout and a single reference voltage level Vf, respectively provided at the input terminals 9, 10 of the comparator means 15. The resulting control signal is fed to the first latching means 16 and, through inverter means 18, to the second latching means 17. The truth table for the circuit shown in Figure 4 is: Truth table. The measurements with the comparator 15 at the sample moments Vmin and Vmax result into digital signals VH and VL. If the measured output voltage Vout equals the reference voltage, this results in VH = 0 and VL = 0, such that the controller 6 takes no action. When VH = 1, the output voltage Vout is above the reference voltage Vref, as a result of which the control means 6 reduce the current through the coil L which, eventually, results in a lower output voltage Vout. If VL = 1, the output voltage Vout is too low, such that the controller has to increase the coil current in order to provide a larger amount of electrical energy to the output of the converter 1, eventually resulting in a higher output voltage. Those skilled in the art will appreciate that the circuit implementation of the control algorithm according to the invention as shown in Figure 4, is just an examplary embodiment. The controle algorithm according to the invention can be realized in a number of different circuits, however, a digital control using latching means 16, 17, or equivalent sampling means, is preferred. In the DC/DC up-converter shown in Figure 1, the second switching means S2 are optional, and are used to increase the power conversion efficiency of the DC/DC converter. The DC/DC converter operating in according with the principle of the present invention may be used with or in an electronic appliance, such as a portable electronic appliance 13 using a battery as a voltage power source, Further, the DC/DC converter may be arranged to form a separate power supply 14, schematically indicated by dashed lines in Figure 1.

Claims: CLA S: 1. A DC/DC converter (1), comprising inductive electrical energy storage means (L), switching means (SI, S2) and control means (6), wherein said control means (6) are arranged for selectively operating said switching means (SI, S2) for transferring an amount of electrical energy from said energy storage means (L) to an output of said DC/DC converter (1), for providing a desired output voltage (Vout), characterized in that said control means (6) are configured for operatively controlling said switching means (SI, S2) for transferring electrical energy in accordance with a two-state switching cycle by comparing said output voltage (Vout) with a reference voltage (Vref) at each transition from one state to another of said two-state switching cycle. 2. A DC/DC converter (1) according to claim 1, wherein said control means (6) are arranged for controlling said switching means (SI, S2) such that if said output voltage (Vout) is below said reference voltage (Vref) said energy storage means are charged, if said output voltage (Vout) is higher than said reference voltage (Vref) said energy storage means are discharged, and if said output voltage (Vout) equals said reference voltage (Vref) said energy storage means are neither charged nor discharged. 3. A DC/DC converter (1) according to claim 1 or 2, wherein said control menas (6) are arranged for controlling said output voltage (Vout) in a Pulse Width Modulation (PWM) mode switching cycle. 4. A DC/DC converter (1) according to any of the previous claims, comprising: - first and second input terminals (2, 3) for receiving an input voltage Vin; - first and second output terminals (4, 5) for providing an output voltage Vout; - a coil (L), having first and second connection ends, wherein said first connection end connects to said first input terminal (2); - first switching means (SI) operatively connected to provide a conduction path from said second connection end of said coil (L) to said second input terminal (3); - second switching means (S2) operatively connected to provide a conduction path from said second connection end of said coil (L) to said first output terminal (4); - diode means (D) parallel connected to said second switching means (S2), and providing a conduction path from said second connection end of said coil to said first output terminal (4); - capacitor means (C) connected between said first and second output terminals (4, 5); - control means (6) arranged for operatively switching said first and second switching means (SI, S2) into their closed or conductive state and their open or non- conductive state - comparator means (15), configured for comparing said output voltage (Vout) with said reference voltage (Vref) providing a control signal, and - first latching means (16) operatively connected for receiving said control signal of said comparator means (15) at a first sample moment (Vmin) at a first state transition of said converter (1), and - second latching means (17) operatively connected for receiving said control signal of said comparator means (15) at a second sample moment (Vmax) at a subsequent second state transition of said converter (1), and - wherein said first and second latching means (16, 17) provide control signals for controlling said control means (6). 5. A DC/DC converter (1) according to claim 4, wherein said latching means (16, 17) provide digital control signals, and wherein said control means (6) comprise digital circuitry for controlling said switching means (SI, S2). 6. A DC/DC converter (1) according to claim 4 or 5, further comprising inverter means (18), operatively arranged for providing an inverted control signal of said comparator means (15) to said second latching means (17). 7. A DC/DC converter (1) according to claim 4, 5 or 6, wherein said switching means (SI, S2) comprise semiconductor switching means, in particular MOS (Metallic Oxide Semiconductor) transistor means. 8. A DC/DC converter (1) according to any of the claims 4-7, wherein said control means (6) are configured for operating in an up conversion mode providing an output voltage (Vout) at its output terminals (4, 5) which is higher than an input voltage (Vin) at its input terminals (2, 3). 9. A power supply (14) comprising a DC/DC converter (1) according to any of the previous claims. 10. A portable electronic appliance (13) comprising a DC/DC converter (1) according to any of the previous claims.

Inventors: HART CORNELIS M, SLUIJS FERDINAND J, HART, CORNELIS, M., SLUIJS, FERDINAND, J.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H02M
IPC codes: H02M3