WO2012123140A2

WO2012123140A2 - Switched-mode power supply for a charger

Info

Publication number: WO2012123140A2
Application number: PCT/EP2012/050450
Authority: WO
Inventors: Juergen Mack
Original assignee: Robert Bosch Gmbh
Priority date: 2011-03-11
Filing date: 2012-01-12
Publication date: 2012-09-20
Also published as: WO2012123140A3; DE102011005415A1

Abstract

A switched-mode power supply for converting an input voltage into an output voltage in a charger comprises a switching device for switching through the input voltage in a clocked manner depending on a control signal, a transformer for transforming the input voltage switched through in a clocked manner, a rectifier for providing the output voltage on the basis of the transformed voltage, and a control device, which is designed to control the control signal both depending on the output current and depending on the output voltage.

Description

description

title

Switching power supply for a charger The invention relates to a switching power supply. In particular, the invention relates to a

Switching power supply for a charger for charging an electrical energy storage.

State of the art

An electrical device, such as a cordless screwdriver, has an electrical energy storage, which is to be charged from time to time by means of a charger. A first type of charger uses a transformer with sheet metal core fed directly from a supply network, with a rectifier and a smoothing capacitor connected to the secondary side of the transformer. Such a charger is simple, but provides only a low efficiency. In addition, such a charger has a load-dependent output characteristic, which is usually non-linear and in which the output voltage decreases with increasing output current. Conversely, the output current rises sharply when the output voltage is low, for example when the connected energy store is heavily discharged. In this case, a high performance can be implemented in the charger, which can lead to damage or destruction of a controller of the charger.

A second type of charger is based on a switching power supply (SNT), which has a much higher efficiency, and whose output characteristic is largely independent of the input voltage. The output current is limited to a maximum current and the output voltage to a maximum voltage. A circuit which is connected to the switching power supply to control the charging process of the energy store can thereby be designed or dimensioned professional. Due to the principle, however, there is the problem that the maximum output power can be taken at a point of the output characteristic at which the output voltage is maximum. The complexity, the size and, accordingly, the manufacturing price of the switching power supply is largely determined by the output power, so that such a switching power supply requires relatively high manufacturing costs.

The output voltage of the charger is determined by a cell voltage of the energy storage to be charged, therefore, to minimize the output power, usually the maximum output current of the switching power supply is limited. By this measure, however, the output current towards the end of the charging process is at a relatively high level, which is for example in the so-called IU charging method, which is used in lithium-ion or lead-acid batteries, unsuitable.

It is therefore an object of the invention to provide a suitable for a charger switching power supply with an improved output characteristics. The invention solves this problem by means of a switching power supply with the features of claim 1. Subclaims give preferred embodiments again.

DISCLOSURE OF THE INVENTION A switching power supply according to the invention for converting an input voltage into an output voltage in a charger comprises a switching device for clocking through the input voltage in response to a control signal, a transformer for transforming the clocked through input voltage, a rectifier for providing the output voltage on the basis of transformed voltage and a control device which is adapted to control the control signal both in dependence of the output current and in dependence of the output voltage.

This makes it possible to provide a switched-mode power supply with an output characteristic which ensures that a maximum output power does not coincide with an ner maximum output voltage of the switching power supply coincides. Rather, the output power over the output current can be kept substantially constant, so that the switching power supply can be advantageously dimensioned to this output power, while at the same time the output characteristic is adapted to the requirements of the charging of the energy storage. In particular, the IU charging method, which is preferably used with lithium-ion or lead-acid batteries, may be supported by this output characteristic. In the IU charging method, in a first phase, a charge of the energy store is made with a constant or limited charging current until the charging voltage has reached a predetermined value, and then during a second phase a charge having a constant or limited charging voltage until the charging voltage Charging current has reached another predetermined value.

Preferably, the control device is configured to control the control signal in such a way that the output voltage linearly decreases in a region above a predetermined output current. Such a linearly decreasing output characteristic of the switched-mode power supply can be generated with little effort and make the output power provided by the switched-mode power supply usable in a wide range of the output voltage or the output current. Below the predetermined output current, the output voltage is preferably constant.

Further preferably, the control device is adapted to control the control signal such that the output current is limited in a range below a predetermined output voltage to a predetermined value. Thereby, a further support of the IU charging process can take place without unnecessarily large dimensioning of the switching power supply.

In a preferred embodiment, a transistor is provided for varying the STEU ersignals in response to the output current, wherein a control input of the transistor is connected by means of a dipole with the output voltage to provide a variation of the control signal in response to the output voltage.

As a result, a cost-effective implementation of the generation of the described control signal can be carried out. The transistor can be used in a switching be already installed power supply of the prior art, so that the described output characteristic can be obtained by adding the dipole. A dimensioning of components of the switching power supply to the thus reduced maximum output power can be done in a known manner to save manufacturing costs.

In one embodiment, the two pole comprises a resistor. This may result in a nearly linear decrease of the output current with increasing output voltage.

The dipole may also include a zener diode. Wrd the switching power supply used, for example, to charge a battery whose terminal voltage increases with increasing charge, the charging current can be selectively reduced by the accumulator by means of the Zener diode from a predetermined terminal voltage.

In a preferred embodiment, a series arrangement of a Zener diode and a Wandsand can be used to reduce the output current beyond the described point with increasing output voltage. The degree of reduction of the output current can be influenced by changing the resistance and / or the series resistance.

In yet another embodiment, the two-terminal comprises a diode, wherein a similar situation arises as above with respect to a zener diode in the two-terminal. The Zener voltage is replaced by the forward voltage of the diode. When a diode is connected in series with a Zener diode, the voltage up to which the output current is constant can be increased by the amount of the forward voltage of the diode. The forward voltage of a common silicon diode is about 0.7 V.

In one embodiment, an optocoupler may be provided, which the transistor controls to galvanically separate the output voltage from the switching device. The driving of a light-emitting diode within the opto-coupler by the transistor can be carried out in an advantageously simple manner with a low proportionality error. In a particularly preferred embodiment, the switched-mode power supply comprises a switch-off device in order to terminate a charging process of an energy store connected to the output voltage as soon as the output voltage exceeds a predetermined threshold value. Such a shutdown device can be easily implemented, so that a charger, such as for lead accumulators, can be produced inexpensively on the basis of the switching power supply. A similar shutdown can take place in the case of a lithium accumulator, for example, as soon as the output current falls below a further predetermined threshold value.

A charger for an energy storage includes the switching power supply described. Preferably, the energy store has an increasing internal resistance with increasing cell voltage, so that the charging current through the energy store increases with increasing cell voltage. The charger can be easily and inexpensively sized for electrical power using the described switching power supply, which is sufficient for charging the energy storage.

The invention will now be described in more detail with reference to the attached figures, in which:

Figure 1 is a block diagram of a charger with an energy storage;

Figure 2 is a circuit diagram of a section of a switching power supply in the charger of Figure 1; and

FIG. 3 shows characteristic curves of the switched-mode power supply from FIG.

Detailed description of embodiments

FIG. 1 shows a block diagram 100 of a charging device 105 having an energy store 1 10. The energy store 110 is an electrically rechargeable accumulator. tor, preferably lithium-ion or lead-based. The energy storage device 1 10 is connected by means of a connection 1 15 with the charger 105.

The charging device 105 comprises a switched-mode power supply 120 and a charging circuit 125. The charging circuit 125 may be included in the energy storage 110 in another embodiment instead of the charging device 105. In yet another embodiment, the charging circuit 125 may also be omitted.

The switched-mode power supply 120 comprises a mains connection 130 which is brought out of the charger 105. The grid connection 130 can be connected to an AC power supply network, which provides an AC voltage of, for example, 230 V / 50 Hz or 1 10 V / 60 Hz. The mains connection 130 leads to a mains filter 135 of the switched-mode power supply 120. The mains filter 135 comprises a rectifier and a smoothing capacitor and optionally also an interference filter for suppressing interference voltages.

The line filter 135 is connected to a switching device 140 of the switching power supply 120. The switching device 140 usually comprises a power transistor, in particular a field effect transistor (FET). The switching device 140 operates in response to a control signal, which it receives via a control line 145. An output of the switching device 140 is connected to a transformer 150 of the switched-mode power supply 120, and the switching device 140, in response to the control signal, turns the rectified voltage through the line filter 135 stepwise to the transformer 150 so as to transform an AC-like voltage. In other embodiments, the voltage supplied to the transformer 150 by the switching device 140 may also have a different shape, in particular approximately sinusoidal or sinusoidal.

The voltage transformed by the transformer 150 is connected to a rectifier 155 of the switching power supply 120. The rectifier 155 optionally includes a smoothing capacitor. The rectified by the rectifier 155 voltage represents the output voltage of the switching power supply 120. The output voltage is connected to the charging circuit 125 and to a controller 160 of the switching power supply 120. The control device 160 generates on the basis of the output voltage and the output current, which are provided to the charging circuit 125 and the energy storage 110, a signal which is decoupled by means of an optocoupler 165 and forwarded to a monitoring device 170 of the switching power supply 120. The monitoring device 170 prepares this by the control device

160 provided signal and provides on the basis of the control signal on the control line 145 to the switching device 140 ready. The monitoring device 170 may further be configured to bring about a power reduction or disconnection of the switched-mode power supply 120, for example if an excess temperature in the region of the switched-mode power supply 120 is determined or a

Over or undervoltage of the voltage provided by the line filter 135 voltage is detected.

Figure 2 shows a circuit diagram 200 of a section of the switching power supply 120 in the charger 105 of Figure 1. The section shown covers a part of

Transformer 150, the rectifier 155, the controller 160 and a part of the optocoupler 165 from. The charging circuit 125 is not shown.

Connected to a secondary winding of the transformer 150 is a diode D1 whose second end is connected to a smoothing capacitor C1. The diode D1 and the smoothing capacitor C1 together form the rectifier 155 of FIG. 1. The second end of the diode D1 is connected to a positive terminal of the connection 15. A negative terminal of the connection 15 is connected to the second end of the smoothing capacitor C1 and the output winding of the transformer by means of a series resistor R3

150 connected. Between the terminals of the connection 115 is an output voltage UAUS of the switching power supply 120 at. A by a consumer which is connected to the junction 115, the output current IOUT flowing also flows through the series resistor R3, so that a voltage drop across the series resistor R3 voltage U ₃ is proportional to the output current I _{O ff.}

A resistor R1 leads from the positive terminal of the connection 1 15 to the light emitting diode of an opto-coupler IM whose second terminal is connected to the collector of an NPN transistor V1. The emitter of the transistor V1 is connected to ground. A resistor R1 limits a current through the light emitting diode of the optocoupler IM. If a positive voltage is applied to the base of the trans- applied sistor V1, the LED lights in the optocoupler IM and a not shown phototransistor of the optocoupler IM begins to conduct. In this case, a degree of the conductivity of the phototransistor of the opto-coupler IM is dependent on which voltage is applied to the base of the transistor V1. Within the monitoring device 170, the conductivity of the phototransistor of the

Optocoupler IM used to provide the control signal on the control line 145 in Figure 1. In this case, the control signal is generated in such a way that the switching device 140 is reduced to power given to the transformer 150 with increasing voltage at the base of the transistor V1.

The voltage U ₃ dropping across the series resistor R3 is coupled out by means of a resistor R2 and conducted to the base of the transistor V1. Accordingly, the power dissipated by the transformer 150 is reduced as the output current I _A usc increases.

In addition, the positive terminal of the connection 115 is connected to the base of the transistor V1 by means of a dipole Z. Depending on a characteristic of the dipole Z thus the power transmitted through the transformer 150 is reduced even when the output voltage U _A usc increases.

FIG. 3 shows a diagram 300 with characteristics of the switched-mode power supply 120 from FIG. 1. In a horizontal direction, the output current Uus and in a vertical direction the output voltage U _A us of the switched-mode power supply 120 of FIGS.

A first characteristic 305 results when the two-terminal Z of FIG. 2 is formed by a resistor R4. By coupling the output voltage U _A us to the base of the transistor V1 in Figure 2, the output voltage U _AU s decreases with increasing output current Uus from a predetermined maximum UMAX substantially. When the output current Uus reaches a predetermined maximum output current I _MA x, the output voltage U _AU s is a minimum output voltage U _M IN- How strong the slope of the first characteristic 305 is depends on how large the heat resistance R4 is and how large the Series resistance R3 is. A second characteristic 310 results when the two-terminal Z is formed by a series connection of a diode D2, a Zener diode ZD and the resistor R4. The order of series connection of the diode D2, the Zener diode ZD and the resistance R4 is arbitrary, wherein the forward direction of the diode D2 from the positive terminal of the connection 115 in the direction of the base of the transistor

V1 occurs and the Zener diode ZD is oriented in the opposite direction.

If the output current l _A us the switching power supply 120 between 0 and a first current, so the output voltage U _A us constant at the value U _M AX falls between the first current and a second, higher current l _2,

Curve 310 linear from. The steepness of the characteristic in this range depends on the resistance R4 and the series resistance R3. The second current I ₂ is so close to the maximum current I _M AX that a constant output current us between the second current I ₂ and I _M AX can be assumed. In this area, the output voltage U _A us linearly decreases.

The first current, from which a lowering of the output voltage U _A us occurs, is dependent on the sum of the breakdown voltages of the diode D2 and the Zener diode ZD. With a suitable choice of the Zener diode ZD, the diode D2 can also be omitted. The output voltage, which is the second

Current l ₂ adjusts, can be influenced by the breakdown voltages of the Zener diode ZD and the diode D2 and the values of the resistance R4, R3 and R2.

In a known switching power supply 120, a regulation of the output current Uus, as explained above with reference to Figure 2, by means of the series resistor R3 and the resistor R2. However, a regulation of the output voltage U _A usus is usually carried out to a constant size, which is usually derived by means of a Zener diode and optionally a voltage divider from the output voltage U _A us. A characteristic curve of a switching power supply of the prior art runs at an output current between 0 and 1 _MA x constant at U MAX and then drops steeply to 0 from. A maximum power of such a switching power supply results as a product of the maximum output current and the maximum output voltage.

The illustrated characteristics 305 and 310 of the switched-mode power supply 120 of FIGS. 1 and 2 remove a point at which a maximum power is generated by the switching power supply. power supply 120 is provided from a point at which the output voltage UAUS is maximum. Therefore, the maximum power of the switching power supply 120 is already at a smaller output current l _A us available. As a result, components of the switched-mode power supply 120 can be dimensioned to be smaller in terms of cost and the output characteristic curve 305, 310 can advantageously be adapted to the IU charging method for the energy store 110.

In one embodiment of the charger 105 of Figure 1, it is sufficient to form the charging circuit 125 to separate the energy storage device 1 10 from the output voltage UAUS as soon as the output voltage U _A us has reached a predetermined value. This value is determined on the basis of a cell voltage of the energy storage device 1 10.

A simple and gentle charging of the energy store 1 10 can be implemented in the manner described with minimal effort, since the components of the switching power supply 120 can be taken over substantially unchanged and only the additional dipole Z must be included.

Claims

A switched mode power supply (120) for converting an input voltage into an output voltage (UAUS) in a charger, comprising:

a switching device (140) for pulsed switching through of the input voltage in response to a control signal;

a transformer (150) for transforming the clocked on input voltage; and

a rectifier (155) for providing the output voltage based on the transformed voltage,

a control device (160) which is set up to control the control signal in dependence on the output current (l _A us),

characterized in that

the control device (160) is further configured to control the control signal in dependence on the output voltage (U _A us).

Switching power supply (120) according to claim 1, characterized in that the control device (160) is adapted to control the control signal such that the output voltage (U _A us) in a range above a predetermined output current () linearly decreases.

Switching power supply (120) according to claim 1 or 2, characterized in that the control device (160) is adapted to control the control signal such that the output current (I _M AX) in a range below a predetermined output voltage (U _A us) on is limited to a predetermined value.

Switching power supply (120) according to one of the preceding claims, characterized by a transistor (V1) for varying the control signal as a function of the output current (l _A us), wherein a control input of the transistor (V1) by means of a two-pole (Z) with the output voltage (U _AU s) is connected to provide a variation of the control signal in response to the output voltage (U _AU s). Switching power supply (120) according to claim 4, characterized in that the two-terminal (Z) comprises a resistor (R4).

Switching power supply (120) according to claim 4 or 5, characterized in that the two-terminal (Z) comprises a Zener diode (ZD).

Switching power supply (120) according to one of claims 4 to 6, characterized in that the two-terminal (Z) comprises a diode (D2).

Switched-mode power supply (120) according to one of the preceding claims, characterized in that an optocoupler (IM) is provided, which the transistor (V1) controls in order to galvanically separate the output voltage (U _A us) from the switching device (140). 9. Switching power supply (120) according to any one of the preceding claims, characterized in that a turn-off device (125) is provided to terminate a charging operation of the output voltage (U _A us) energy storage device (1 10), as soon as the output voltage (U _A us) exceeds a predetermined threshold.

10. Switching power supply (120) according to any one of the preceding claims, characterized in that a turn-off device (125) is provided to terminate a charging operation of the output voltage (U _A us) energy storage device (1 10), as soon as the output current (l _AU s) falls below a predetermined threshold.

1 1. Charger (100) for an energy store (1 10), wherein the charger (100) comprises a switching power supply (120) according to one of the preceding claims.