Linear voltage regulator

Abstract

A linear voltage regulator (140) comprises an input side (205) for connection to an input voltage; a control circuit (212) for providing a predetermined control voltage based on the input voltage; and an output side (210) for providing an output voltage. A linear control element (U3) between the input side (205) and the output side (210) is configured to provide the output voltage with respect to the control voltage. A capacitive voltage divider (220) is also provided, comprising a first (Cp1) and a second capacitor (C5, C6) connected in series with the input voltage, with a tap (225) of the capacitive voltage divider (220) located between the capacitors (Cp1, C5, C6) being connected to the output side (210).

Description

The present invention relates to a linear voltage regulator. In particular, the invention relates to a voltage regulator for use on board a motor vehicle.A motor vehicle comprises a voltage regulator configured to convert a higher DC voltage into a lower one. The higher voltage may be provided by a traction battery storing energy for operating an electric drive motor. The voltage of the battery may be several 100 V to over 1000 V. The lower voltage can be supplied to an on-board power supply system, for example.The voltage converter is usually designed as a switching DC voltage converter, for example as a charge pump or step-down converter. For operation, the switching DC-DC converter requires a supply voltage which can be obtained from the output voltage provided. However, the supply voltage is not yet present immediately after the switch-on, so that an additional linear voltage regulator is provided in order to provide the operating voltage at least during the switch-on phase.The linear voltage regulator operates as a series regulator and includes a transistor that controls current flow from an input side to an output side. The transistor is driven by means of a control circuit which comprises a constant voltage source. For vibration damping, a capacitor to ground can be provided on the output side.Immediately after switching on, the full input voltage is applied to such a linear voltage regulator via the transistor. Accordingly, a sufficiently voltage-proof type is to be selected for the transistor. Depending on the transistor used and the size of a capacitance connected to the output side, the predetermined output voltage is only reached after a certain time. A slow increase in the output voltage can delay the switching DC-DC converter from being put into operation. It is therefore advantageous to use a fast-starting voltage regulator which can preferably be constructed with cost-saving components.Upon closer observation, it is found that two disadvantages of a known linear voltage regulator cause each other. If the peak voltage across the control element is decreased, the rise time increases until the predetermined voltage is provided at the output. Conversely, the peak voltage increases as the rise time is shortened. Thus, with the existing technique, it is not possible to reduce the peak voltage and at the same time to shorten the rise time.An object on which the present invention is based is to provide an improved voltage regulator, in particular for applications on board a motor vehicle. This object is achieved by the subjects of the appended independent claims. Dependent claims indicate preferred embodiments.According to a first aspect of the present invention, a linear voltage regulator comprises an input side for connection to an input voltage; a control circuit for providing a predetermined control voltage based on the input voltage; and an output side for providing an output voltage. A linear control element between the input side and the output side is configured to provide the output voltage with respect to the control voltage. A capacitive voltage divider is also provided, comprising a first and a second capacitor connected in series at the input voltage, a tap of the capacitive voltage divider located between the capacitors being connected to the output side.At the moment of switching on, when an input voltage is applied to the input side, the capacitive voltage divider can derive a portion of the voltage applied across the linear control element and thus significantly reduce its voltage load. A component which is low in voltage-resistant compared to a conventional solution can be used for the control element. The voltage regulator may operate more robust or reliable, and material costs for the voltage regulator may be reduced. In addition, the capacitive voltage divider may help to charge the output side rapidly, so that a predetermined output voltage is reached earlier. A load supplied by the linear voltage regulator can be put into operation significantly more quickly. This may be important, for example, in an application on board a motor vehicle, where the linear voltage regulator may provide an operating voltage for a safety-relevant component.It is preferred that the linear control element comprises a transistor. The transistor can be embodied as a bipolar junction transistor (BJT) or as a field effect transistor (FET). In a preferred, simple embodiment, the control element is designed as an NPN-type bipolar transistor.The control circuit may include a zener diode. The zener diode can be installed in a general discrete control circuit which, in the simplest case, additionally requires only one resistor. The zener diode can also be contained in a discrete control circuit with current limiting or in a linear control circuit, which can be present in particular as an integrated circuit (IC). Zener diodes having different breakdown voltages can be readily and inexpensively available. A connection of a zener diode can be kept very simple, in particular if a bipolar transistor is used as the control element.A voltage present between the input side and the output side across the control element can be reduced with respect to the input voltage by a measure which depends on the ratio of the capacitances of the two capacitors of the capacitive voltage divider. It is preferred that the second capacitor connected to the output side is initially dimensioned for the operation of the linear voltage regulator independently of the first capacitor. The first capacitor, which together with the second forms the capacitive voltage divider, can then be dimensioned to fit it.It is preferred that the capacitive voltage divider is dimensioned such that the voltage at the tap is smaller than the output voltage. This is to apply in particular when the output voltage has already reached the predetermined value.In this case, a diode may be provided between the first capacitor connected to the input voltage and the tap. The diode allows current flow from the input side to the output side, but prevents current flow in the opposite direction. As a result, the capacitive voltage divider can be virtually shut off when the output voltage reaches a sufficiently high value. How far the output voltage must have approached the predetermined value in order to block the diode and to switch off the capacitive voltage divider can be determined by appropriately dimensioning the capacitors. A common value is about 80% of the predetermined voltage.It is further preferred that a capacitance of the first capacitor connected to the input voltage is as large as possible. In particular, the capacitance of the first capacitor may be greater than that of the second capacitor. The attainment of the predetermined voltage on the output side can thereby be expedited.In a further embodiment, a resistor is provided between the input side and the control element. All current passed through to the output side flows through the resistor, and preferably the control circuit is also connected to a terminal of the resistor remote from the input side. The resistor may contribute to reducing a current flowing through the voltage regulator when the voltage regulator is switched on or settling. However, by using the resistor, a delay until the output voltage reaches the predetermined value may be somewhat increased.A voltage regulator system includes a voltage regulator described herein followed by another linear voltage regulator. Such an arrangement is also referred to as a cascade of voltage regulators. As a result, a high input voltage, as can be provided, for example, by a traction battery on board a motor vehicle, can be reduced stepwise to a predetermined, lower voltage. The capacitive voltage divider of the first voltage regulator can also be used to speed up the further voltage regulator. Optionally, the further voltage regulator may also be equipped with a capacitive voltage divider in a manner described herein.A combined voltage regulator includes a switching DC-DC converter having a control voltage and a voltage regulator or voltage regulator system described herein for providing an operating voltage to the control circuit. The combined voltage regulator may be configured such that the operating voltage is provided by the voltage regulator or the voltage regulator system only until the operating voltage can be generated from an output voltage of the combined voltage regulator.The linear voltage regulator or the voltage regulator system can therefore be optimized for short-term operation. A typical operating time within the scope of a switch-on process of the combined voltage regulator is usually less than approximately 1 sec. The switching DC-DC converter can operate on the principle of a flux converter or a flyback converter. Optionally, the DC-DC converter can also operate according to the resonant converter principle. For example, the DC-to-DC converter can be designed as a charge pump, a step-down converter, a step-up converter, a double inverter, a flyback converter or a push-pull flux converter.Performance of the combined voltage regulator may depend to only a small extent on performance of an included linear voltage regulator. The linear voltage regulator can easily be adapted to requirements of the switching DC-DC converter or its control circuit.It is preferred that the combined voltage regulator is configured for use on board a motor vehicle. For this purpose, the combined voltage regulator can have a large ratio between an input voltage and an output voltage. The ratio may be, for example, in a range of about 15 to about 100. In particular, the combined voltage regulator, and thus also the linear voltage regulator comprised by it, can be configured for operation at an input voltage in the range of approximately 400 V, approximately 800 V or approximately 1100 V.An output voltage of the combined voltage regulator may comprise a low voltage and in particular be below approximately 50 V. In one specific embodiment, the voltage regulator is designed to reduce electrical energy from a traction battery of the motor vehicle for an on-board power supply system. The on-board power supply system can operate with a rated voltage of, for example, approximately 12 V, approximately 24 V or approximately 48 V.In yet another aspect of the present invention, a motor vehicle includes a combined voltage regulator described herein. The motor vehicle can be electrically drivable and comprise a traction battery which can have a high rated voltage, usually in the range of several 100 V. The motor vehicle preferably comprises a motorcycle, a passenger car, a truck or an omnibus.The invention will now be described in more detail with reference to the accompanying figures, in which: FIG. 1 shows a combined voltage regulator on board a motor vehicle; FIG. 2 shows a linear voltage regulator; FIG. 3 shows curves of linear voltage regulators; FIG. 4 shows a voltage regulator system with cascaded linear voltage regulators; and FIG. 5 shows curves on a voltage regulator systemis.FIG. 1 shows a combined voltage regulator 100 on board a motor vehicle 105. The motor vehicle 105 can comprise a traction battery 110, the electrical energy of which can be processed by means of an inverter 115 for an electric drive motor 120. A voltage of the traction battery 110 may be in the range of about 100 V or more. The combined voltage regulator 100 is configured to supply an on-board power supply system 125 with electrical energy from the traction battery 110. In this case, an open-circuit voltage of the on-board power supply system 125 is usually significantly lower than a voltage provided by the traction battery 110.As can be seen in more detail in a lower region of FIG. 1, the combined voltage regulator 100 comprises a switching DC-DC converter 130, for the control of which a control circuit 135 is provided. An input side (Vin), shown on the left, can be configured for connection to the traction battery 110 and an output side (Vout), shown on the right, can be configured for connection to the on-board power supply 125. In particular, the traction battery 110 may form Vsource.The DC-to-DC converter 130 is schematically implemented as a buck converter ("buck converter"). It includes a switching element SW, a diode D 1, an inductor L 1, and a capacitor C 1. The switching element SW is driven by the control circuit 135 by means of a signal PWM_GATE. An output voltage provided on the output side is divided by two resistors R 1, R 2 by means of a voltage divider in order to provide a feedback voltage FB, which can be compared by the control circuit 135 as an actual value with an internally generated setpoint value in order to adapt a frequency or a duty cycle of a control signal for the switching element SW such that the feedback voltage FB corresponds as far as possible to the reference value and thus the output voltage Vout corresponds as far as possible to a predetermined value.The control circuit 135 is supplied with an operating voltage during operation of the switching DC-DC converter 130, which can be generated on the basis of the output voltage Vout. This voltage may be provided to the control circuit 135 via a diode D 4; a capacitor C 3 may contribute to smoothing the operating voltage. If the output voltage Vout is not available, for example immediately with the provision of the input voltage Vin, the operating voltage can be provided to the control circuit 135 by means of a linear voltage regulator 140.An output voltage of the linear voltage regulator 140 can be fed to the control circuit 135 via a diode D 3. The higher of the output voltage Vout of the switching DC-DC converter 130 and the output voltage of the linear voltage regulator 140 is thus present as the operating voltage at the control circuit 135. The linear voltage regulator 140 may be deactivated by the control circuit 135, in particular when the operating voltage available to it indicates that the output voltage Vout of the switching DC-DC converter 130 is sufficiently stable.FIG. 2 shows an example embodiment of a linear voltage regulator 140. Reference numerals of the components illustrated herein are independent of reference numerals of other figures. Values or types shown for components are to be regarded as purely exemplary.On the left in FIG. 2, an input side 205 for connection to an input voltage Vin is shown, and on the right an output side 210 for providing an output voltage Vout is shown. A voltage source V 3 for providing the input voltage Vin can be formed in particular by the traction battery 110.A linear control element 212 is designed as a transistor U 3, in particular as a bipolar NPN transistor. A collector C is connected to the input side 205 via an optional resistor R 4 and an emitter E is connected to the output side 210. A control circuit 215 includes a voltage divider of a resistor R 3 and a zener diode D 4. The control circuit 215 is operated with an input-side voltage and provides a control voltage to a base B of the linear control element 212.Two capacitors C 5, C 6 are connected in parallel on the output side. Together with a capacitor Cp1 and an optional diode D5, the capacitors C5, C6 form a capacitive voltage divider 220 between the input side 205, possibly after the resistor R4, and ground. A tap 225 between the capacitors Cp1 and C5, C6 is connected to the output side 210. If diode D5 is provided, it is connected between capacitor Cp1 and tap 225 in the forward direction as shown.For the circuit shown in FIG. 2 by way of example, C p may be determined as follows:The effective precharge voltage on the output side is:The maximum voltage across the parallel capacitor Cp1 is:The peak value of the reflux voltage of the parallel diode Dp during operation is:When the input voltage is switched off: FIG. 3 shows exemplary curves of linear voltage regulators 140. For the sake of clarity, the curves are plotted in different diagrams with the same scaling. A time is plotted in the horizontal direction and a voltage is plotted in the vertical direction. The illustration illustrates the behavior of a linear voltage regulator 140 of the type of FIG. 2 with and without capacitive voltage dividers 220.A first voltage 305 is present across the control element 212 in the variant without capacitive voltage divider 220, and a second voltage 310 is present across the control element 212 with capacitive voltage divider 220. It can be seen that the second voltage 310 with approximately 7.2 V reaches a maximum value that is approximately half as great as the first voltage 305 with approximately 15 V. In addition, the second voltage 310 decays significantly more quickly and reaches a normal value of approximately 5 V already after 40 μs, while the first voltage 305 does so only after approximately 100 μs.A third voltage 315 relates to an input voltage for both variants of the linear voltage regulator 140. A first output voltage 320 is determined at voltage regulator 140 without capacitive voltage divider 220 and a second output voltage 325 is determined at a variant with voltage divider 220. After the switching-on, the first output voltage 320 requires about 100 μs in order to achieve the final value of about 10.5 V. The second output voltage 325 reaches a value of approximately 8 V already after approximately 1 μs, which corresponds to approximately 80% of the intended output voltage. The first output voltage 320 reaches this value only after approximately 70 μs.FIG. 4 shows a voltage regulator system 400 with two cascaded linear voltage regulators 140. Component names shown are customary in the art and are independent but analogous to those of other figures. Dimensions and component types shown are to be understood as exemplary.A first linear voltage regulator 140 shown on the left in FIG. 4 can be connected to an input voltage Vin Approximately 900 V. The first linear voltage regulator 140 comprises a capacitive voltage divider 220 proposed herein, the two capacitive elements of which are each formed by a plurality of capacitors Cp 1, Cp 2 and Cp 3, Cp 4, Cp 5, respectively. The first voltage regulator 140 illustratively provides an output voltage Vout1 420V. This voltage is determined by the breakdown voltage of the zener diode Dz 1.The downstream second linear voltage regulator 140 correspondingly converts the provided voltage into a second output voltage Vout2A, which is approximately 19 V. This value is defined by the second zener diode D 2. In the illustrated embodiment, the second linear voltage regulator 140 does not include a capacitive voltage divider 220. However, such may be added in another embodiment.FIG. 5 shows curves on a voltage regulator system 400 of the type of FIG. 4 Five diagrams 505 to 520 plot a time in each case in the horizontal direction and different voltages in the vertical direction. Identifiers for the individual curves refer to the figure. In this case, indices ending with 'a' indicate the proposed embodiment shown in FIG. 4 with a capacitive voltage divider 220 at the first voltage regulator 140, while indices ending with 'b' indicate a conventional embodiment without such a voltage divider 220.The first plot 505 shows input voltages of the voltage regulators 140. With and without capacitive voltage dividers 220, comparable curves result.The second 510 diagram shows a voltage V CE. applied across the switching element U 1 of the first voltage regulator 140. It can be seen that the voltage rises less quickly and far less far when the voltage divider 220 is used.The third diagram 515 shows an output voltage of the first voltage regulator 140. This increases more quickly when voltage divider 220 is used than in the prior art, reaches a usable value earlier and also reaches the final value earlier. The useful value is about 80% of the final value. Transitions between zero and the usable value, as well as between the usable and final values, are substantially linear. A characteristic of the output voltage at the conventional voltage regulator 140 is non-linear and adapts to the final value.The fourth diagram 520 shows a voltage V CE. applied across the switching element U 2 of the second voltage regulator 140. Maximum values of the voltages V CE with and without capacitive voltage dividers 220 are approximately the same size, but the voltage increases significantly more quickly and essentially linearly in sections in the embodiment with voltage divider 220.The fifth diagram 525 shows an output voltage of the second voltage regulator 140. Although no capacitive voltage divider 220 is used at the second voltage regulator 140, the output voltage rises more quickly than without capacitive voltage dividers at the first voltage regulator 140. The intended value of the output voltage is achieved with about 1 / 3 less delay after turn-on. A profile of the output voltage is substantially linear.Reference numeral 100 combined voltage regulator 105 motor vehicle 110 traction battery 115 inverter 120 electric drive motor 125 on-board power supply 130 switching DC voltage converter 135 control circuit 140 linear voltage regulator 205 input side 210 output side 212 linear control element 215 control circuit 220 capacitive voltage divider 225 tap 305 first voltage via control element without capacitive voltage divider 310 second voltage via control element with capacitive voltage divider 315 third voltage: input voltage for voltage regulator 320 first output voltage without capacitive voltage divider 325 second output voltage with capacitive voltage divider SW switching element D1, D3, D4 diode L1 coil C1, C3 capacitor R1, R2, R3 resistor FB feedback voltage (feedback) Vin input voltage Vout output voltage Vsource, V3 voltage source U3 linear control element C collector E emitter B base D4 zener diode 400 voltage regulator system 505 first diagram 510 second diagram 515 third diagram 520 fourth diagram 525 fifth diagram

Claims

A linear voltage regulator (140) comprising: an input side (205) for connection to an input voltage; a control circuit (212) for providing a predetermined control voltage based on the input voltage; an output side (210) for providing an output voltage; a linear control element (U3) between the input side (205) and the output side (210), wherein the control element (U3) is configured to provide the output voltage with respect to the control voltage; and a capacitive voltage divider (220) having a first (Cp1) and a second capacitor (C5, C6) connected in series at the input voltage; wherein a tap (225) of the capacitive voltage divider (220) connected between the capacitors (Cp1, C5, C6) is connected to the output side (210).The voltage regulator (140) of claim 1, wherein the linear control element (U3) comprises a transistor.The voltage regulator (140) of claim 1 or 2, wherein the control circuit (212) comprises a zener diode.The voltage regulator (140) of any preceding claim, wherein the capacitive voltage divider (220) is dimensioned such that the voltage at the tap (225) is less than the output voltage.The voltage regulator (140) of claim 4, wherein a diode is provided between the first capacitor (Cp1) connected to the input voltage and the tap (225).The voltage regulator (140) of any of the preceding claims, wherein a capacitance of the first capacitor (Cp1) connected to the input voltage is greater than a capacitance of the second capacitor (C5, C6) connected to the output side (210).The voltage regulator (140) of any preceding claim, further comprising a resistor (R1) between the input side (205) and the control element (U3).Voltage regulator system (400) comprising a voltage regulator (140) according to one of the preceding claims, followed by a further linear voltage regulator (140).A combined voltage regulator (140) comprising a switching DC-DC converter (130) having a control circuit (212) and a voltage regulator (140) according to any preceding claim for providing an operating voltage to the control circuit (212).A motor vehicle (105) comprising a combined voltage regulator (140) according to claim 9.

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