Abstract:
A disconnect device for a switched-mode power supply, including an activation device for a first transistor for generating a transformable voltage. The device includes a second transistor of the PNP type and a third transistor of the NPN type, the base of the second transistor being connected to the collector of the third transistor and the base of the third transistor being connected to the collector of the second transistor. The emitter of the third transistor is connected to ground. The emitter of the second transistor is connected to a control voltage terminal of the activation device, the control voltage terminal being configured for suppressing the generation of the voltage by the first transistor, if the control voltage terminal is connected to ground, so that the generation of the voltage is suppressed if the base of the third transistor is acted upon by a voltage exceeding a predetermined threshold value.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a circuit for providing a supply voltage. In particular, the present invention relates to an electronic circuit for providing a supply voltage for charging an electrical energy store. 
       BACKGROUND INFORMATION 
       [0002]    For charging electrical energy stores, for example, NiCd or NiMH batteries, a charging device is used, which is operated on a supply voltage, and a charging voltage is provided to the energy store, which ensures that it is charged. To safeguard both the energy store as well as the charging device against extraordinary operating conditions, frequently only a very simple safety device is used, which operates according to the so-called “crowbar” principle. In this connection, a component of the charging device is deliberately destroyed when an extraordinary operating condition occurs, so that the charging device is disconnected and brought into a safe operating mode. The component to be destroyed may include a dedicated fuse. However, a component, for example, a diode or a transistor, which fills another function during the normal operation of the charging device, may be selectively destroyed by excessive voltage or excessive current. 
         [0003]    In this case, it is believed to be disadvantageous that the charging device cannot easily be put back into operation after the destruction of the component. Furthermore, it is believed that it is not possible to verify that the disconnect mechanism is functional, for example, in the context of quality assurance. Thus, it is believed that there is always some uncertainty as to whether the disconnect mechanism is able to fulfill its function at all. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the exemplary embodiments and/or exemplary methods of the present invention is therefore to provide an indestructible disconnect mechanism for a switched-mode power supply. 
         [0005]    The exemplary embodiments and/or exemplary methods of the present invention may achieve this objective using a circuit having the features described herein. Further descriptions herein describe advantageous specific embodiments. 
         [0006]    A disconnect device according to the present invention for a switched-mode power supply, the switched-mode power supply including an activation device for a first transistor for generating a transformable voltage, includes a second transistor of the PNP type and a third transistor of the NPN type, the base of the second transistor being connected to the collector of the third transistor and the base of the third transistor being connected to the collector of the second transistor. Furthermore, the emitter of the third transistor is connected to ground and the emitter of the second transistor is connected to a control voltage terminal of the activation device, the control voltage terminal being set up for suppressing the generation of the voltage by the first transistor if the control voltage terminal is connected to ground, so that the generation of the voltage is suppressed if the base of the third transistor is acted upon by a voltage which exceeds a predetermined threshold value. 
         [0007]    The described disconnect device made up of two transistors disconnects the switched-mode power supply reliably and effectively via the control voltage terminal when the base of the third transistor is connected to an adequately high cut-off voltage. In this case, the control voltage terminal also continues to be connected to ground when the cut-off voltage drops again. Only when the voltage present on the disconnect device has decayed sufficiently, for example, because the switched-mode power supply is disconnected, is the control voltage terminal separated from ground, making it possible for the switched-mode power supply to be switched on again. This forces the switched-mode power supply to be disconnected longer, which may increase the reliability of the switched-mode power supply and a consumer connected to it. For example, the disconnection may be initiated by a detected overtemperature, and the disconnection may last long enough that the element in question cools down to the extent that the switched-mode power supply may be operated again. 
         [0008]    In a first specific embodiment, the control voltage terminal includes a control terminal of the first transistor. As a result, the disconnection may occur in any type of primary clocked switched-mode power supply, in particular those which are configured as self-oscillators. 
         [0009]    In a second specific embodiment, the control voltage terminal includes a control terminal of a voltage source for providing an operating voltage for the activation device for the first transistor. As a result, a function of the activation device may be utilized, which stops the activation of the first transistor in the event of an undervoltage. As a result, the disconnect device may also be used in a switched-mode power supply which uses an integrated circuit as an activation device which has no dedicated input for disconnection and which also includes the first transistor in one specific embodiment. 
         [0010]    A first capacitor may be connected between the emitter and the base of the PNP transistor and/or a second capacitor may be connected between the base and the emitter of the NPN transistor. As a result, the disconnect device may in each case be more resistant to interfering impulses. 
         [0011]    A storage capacitor may be provided between the emitter of the second transistor and the emitter of the third transistor. As a result, a disconnection time may be extended, so that a disconnection of the switched-mode power supply will last for at least a predetermined time. 
         [0012]    In one specific embodiment, an optocoupler is provided to separate one potential of the turn-off pulse from the disconnect device. This makes it possible to ensure operating reliability, for example, in a mains-operated charging device. 
         [0013]    The exemplary embodiments and/or exemplary methods of the present invention will now be described in greater detail with reference to the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a block diagram of a device having a switched-mode power supply. 
           [0015]      FIG. 2  shows a circuit in the device from  FIG. 1 . 
           [0016]      FIG. 3  shows a variation of the circuit of  FIG. 2 . 
           [0017]      FIG. 4  shows another specific embodiment of a circuit in the device from  FIG. 1 . 
           [0018]      FIG. 5  shows a variation of the circuit of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a schematic block diagram of a device  100  and an energy store  105  which is connectable to device  100 . Energy store  105  is a battery, for example, based on lithium ions or nickel metal hydride. Device  100  is a charging device for energy store  105 . 
         [0020]    Device  100  includes a mains connection  110 , a rectifier  150 , a switching device  155 , a transformer  160 , another rectifier  165 , a control amplifier  170 , an optocoupler  175  and a controller  180  having a disconnect device  130 . 
         [0021]    Mains connection  110  is used for connecting to a mains voltage U N  of an energy supply network, in particular an alternating voltage network having 110 V/60 Hz or 230 V/50 Hz. Mains connection  110  is connected to rectifier  150 . Rectifier  150  filters and rectifies mains voltage U N  and, on this basis, provides intermediate circuit voltage U ZK , which is a direct voltage. 
         [0022]    Intermediate circuit voltage U ZK  feeds controller  180  including disconnect device  130  as well as switching device  155 . Switching device  155  converts intermediate circuit voltage U ZK  into a voltage which is transformable by transformer  160  and provides the converted voltage to transformer  160 . The provided voltage may have a rectangular, step, trapezoidal, sinusoidal or other form processable by transformer  160 . In this case, typically, a frequency is used which is above the frequency of mains voltage U N , for example, 50 kHz through 200 kHz, in particular 100 kHz. Transformer  160  transforms the converted voltage, which rectifier  165  converts into output voltage U aus , which is provided to charge controller  135 . 
         [0023]    One output of charge controller  135  is led through to a first charging connection  140  of charging device  100 ; a second charging connection  145  is connected directly to rectifier  165 . Energy store  105  is connectable to charging device  100  with the aid of corresponding charging connections  140 ,  145  in order to be charged on charging device  100 . In one specific embodiment, charge controller  135  has a disconnect function in the event of an undervoltage. If output voltage U aus  drops below a predetermined value, charge controller  135  stops the charging of energy store  105 . In one specific embodiment, energy store  105  is charged at a constant voltage. In this case, charge controller  135  may be omitted and charging voltage U L  is provided by output voltage U aus . In other specific embodiments, multiple elements  120  through  135  may be integrated together into a single component. 
         [0024]    Output voltage U aus  is monitored by control amplifier  170 , which provides a signal as a function of output voltage U aus  that is provided to controller  180  with the aid of optocoupler  175 . Based on the provided signal, controller  180  generates a control signal for switching device  155  in order to regulate the direct voltage generated by rectifier  165  to a predetermined voltage or the current provided by rectifier  165  to a predetermined current. 
         [0025]    Disconnect device  130  is set up to be triggered with the aid of a cut-off voltage U trig . If disconnect device  130  is triggered, it intervenes in the function of controller  180  in such a way that switching device  155  is no longer able to switch through, so that the energy transfer via transformer  160  is disrupted and output voltage U aus  is cut off. Cut-off voltage U trig  may be provided on the basis of, for example, an overvoltage, an undervoltage, an overcurrent, an undercurrent, an overtemperature and/or an undertemperature of any element on charging device  100 . 
         [0026]    In one specific embodiment, controller  180  is able to conduct a control voltage, provided by switching device  155  through controller  180 , directly to ground. In another specific embodiment, controller  180  includes a voltage source  120  for providing an operating voltage for controller  180 , and disconnect device  130  influences voltage source  120  in such a way that the provided operating voltage drops to such an extent that an undervoltage protection of controller  180  is activated, which interrupts the control voltage provided by switching device  155 . In this case, a part of controller  180  in the form of an integrated circuit (IC) may be present and may be configured to be integrated with switching device  155 . In this case, voltage source  120  for the operating voltage of integrated circuit  180  is not integrated and may be influenced by disconnect device  130 . 
         [0027]      FIG. 2  shows a circuit  200  in charging device  100  from  FIG. 1 . Circuit  200  represents voltage source  120  and disconnect device  130  in controller  180  in  FIG. 1 . The specific embodiment of disconnect device  130  shown in  FIG. 2  is in particular suitable when the activation of switching device  155  is carried out with the aid of a controller  180  configured as an integrated circuit. To cut switched-mode power supply  100  off, voltage source  120  is cut off so that remaining controller  180  detects an undervoltage and halts the activation of the switching device  155 . 
         [0028]    Voltage source  120  in  FIG. 2  is essentially formed by an NPN transistor T 1 , a Zener diode D 2  and a resistor R 2 . Disconnect device  130  is made up of transistors T 2  and T 3 , capacitors C 2  and C 3 , and a resistor R 3 . 
         [0029]    Intermediate circuit voltage U ZK  is connected to the collector of NPN transistor T 1  via a resistor R 1  as an auxiliary voltage U hilf . From the collector of NPN transistor T 1 , a capacitor C 1 , which may have an electrolytic capacitor having a high storage capacity, is connected to ground. 
         [0030]    The emitter of NPN transistor T 1  provides supply voltage Ucc. The provision takes place as a function of a control voltage Ust, which is present on the base of NPN transistor T 1 , here denoted as control voltage terminal  205 . In order to generate a suitable control voltage U St , resistor R 2  is connected to ground in series with a Zener diode D 2  from the collector of NPN transistor T 1 . A predetermined voltage drops across Zener diode D 2  as long as auxiliary voltage U hilf  exceeds the predetermined voltage across the series circuit made up of resistor R 2  and Zener diode D 2 . The base of NPN transistor T 1  and control voltage terminal  205  is connected between resistor R 2  and Zener diode D 2 . 
         [0031]    NPN transistor T 1  is generally operated with the aid of control voltage U St  in saturation, i.e., no limitation or regulation of supply voltage Ucc occurs. If NPN transistor T 1  departs from this working point, a power loss within NPN transistor T 1  is converted into heat. With the aid of diode D 1 , an auxiliary voltage U aux  is coupled to the collector of transistor T 1 . The amount of auxiliary voltage U aux  is generally ascertained empirically, and for reasons of safety, is selected around a predetermined amount of a few volts above the empirically ascertained voltage. A suitable selection of Zener diode D 2  makes it possible to influence a working point of NPN transistor T 1  with regard to a limiting behavior. This working point may be shifted by the circuit around transistors T 2  and T 3 , so that NPN transistor T 1  is increasingly limited after trigger voltage U trig  has risen above the predetermined threshold value. 
         [0032]    The components of disconnect device  130  are connected between the base of NPN transistor T 1  and ground. Transistor T 2  is a PNP transistor, while transistor T 3  is an NPN transistor. The base of transistor T 2  is connected to the collector of transistor T 3  and the base of transistor T 3  is connected to the collector of transistor T 2 . The emitter of transistor T 2  leads to the base of NPN transistor T 1 ; the emitter of transistor T 3  is connected to ground. Capacitors C 2  and C 3  are situated between the base and the emitter of transistor T 2  and transistor T 3 . Cut-off voltage U trig  is coupled to the base of transistor T 3  or to the collector of transistor T 2  with the aid of resistor R 3 , resistor R 3  being used to limit the current through the base-emitter path of transistor T 3 . 
         [0033]    During normal operation of circuit  200 , both transistors T 2  and T 3  block. If cut-off voltage U trig  exceeds a predetermined value, transistor T 3  switches through and its collector-emitter path becomes conductive. This causes the base of transistor T 2  to be connected to ground, so that transistor T 2  also switches through and its collector-emitter path becomes conductive. This causes control voltage U St , which is present on the emitter of transistor T 2 , to be conducted through to the collector of transistor T 2  and thus to the base of transistor T 3 , so that transistor T 3  remains in the conductive state, irrespective of whether cut-off voltage U trig  again drops below the predetermined value or not. 
         [0034]    While transistors T 2  and T 3  are conductive, a current flows in parallel with Zener diode D 2 , so that voltage source  120  is detuned in such a way that control voltage U St  drops, causing NPN transistor T 1  to block and supply voltage Ucc collapses to near 0. 
         [0035]    Capacitors C 2  and C 3  of disconnect device  130  ensure that when circuit  200  is switched on, i.e., if intermediate circuit voltage U ZK  (and auxiliary voltage U aux  rise from 0, the two transistors T 2  and T 3  initially remain in the non-conductive state if cut-off voltage U trig  is below the predetermined value at this point in time. 
         [0036]    In order to bring transistors T 2  and T 3  of triggered disconnect device  130  back to a non-conductive state, it is necessary to reduce control voltage U St , from which the two transistors T 2  and T 3  are fed, to 0. For this purpose, intermediate circuit voltage U ZK  is cut off, for example, by separating charging device  100  on mains connection  110  from the energy supply network, or by preventing a main switch of device  100  from providing intermediate circuit voltage U ZK . Transistors T 2  and T 3  remain in the conductive state until capacitor C 1 , which is connected in parallel to auxiliary voltage U hilf,  and capacitor C 4 , which is connected in parallel to the intermediate circuit voltage U ZK , are discharged. The discharge process of capacitor C 4  may last from a few seconds to several minutes. If during the discharge time, intermediate circuit voltage U ZK  is provided again, transistors T 2  and T 3  remain in the conductive state and supply voltage Ucc remains cut off. 
         [0037]      FIG. 3  shows a variation of circuit  200  from  FIG. 1 . In this case, cut-off voltage U trig  is decoupled from transistors T 2  and T 3  with the aid of an optocoupler U 1 . Optocoupler U 1  includes a light-emitting diode that is controllable with the aid of cut-off voltage U trig . The light-emitting diode acts on a phototransistor of optocoupler U 1  until the phototransistor is activated and a conductive connection exists on its collector-emitter path. The emitter and the collector of the phototransistor are led through on optocoupler U 1 , the emitter being connected to the base of transistor T 3  and the collector via resistor R 3  being connected to auxiliary voltage U hilf  on the collector of NPN transistor T 1 . If the light-emitting diode in optocoupler U 1  lights up, the base of NPN transistor T 3  is connected to a potential that is derived from U hilf  and activates transistor T 3 . The remaining function of transistors T 2  and T 3  is described above with reference to  FIG. 2 . The use of optocoupler U 1  electrically isolates cut-off voltage U trig  from the rest of circuit  200 . 
         [0038]      FIG. 4  shows another specific embodiment of a circuit  200  in the device from  FIG. 1 . Circuit  200  represents disconnect device  130  in controller  180  in  FIG. 1 ; also shown are a terminal to a component of controller  180 , which is not shown, and a FET transistor T 11 , which represents switching device  155  in  FIG. 1 . A gate terminal of FET transistor T 11  forms control voltage terminal  205  in this case. FET transistor T 11  may be a power transistor, through which flows a large portion of electrical power provided by switched-mode power supply  110 . In particular, FET transistor T 11  may be a MOSFET. In another specific embodiment, a thyristor or another electronic switching element may also be used in place of FET transistor T 11 . 
         [0039]    The specific embodiment of disconnect device  130  shown in  FIG. 4  may be used for disconnecting FET transistor T 11  and it may be preferable if a different possibility for influencing controller  180  is not present. In order to switch off switched-mode power supply  100 , the control terminal of FET transistor T 11  is connected to ground, so that no more voltage is provided to transformer  160  and the transfer of energy through transformer  160  is stopped. 
         [0040]    The remaining components shown correspond essentially to the components used in the specific embodiment of  FIG. 2 ; however, they were marked with a preceding numeral 1 for the sake of clarity. In particular, shown disconnect device  130 , made up of transistors T 12  and T 13  as well as capacitors C 12  and C 13  and resistor R 13 , corresponds to disconnect device  130  in  FIG. 2  made up of transistors T 2 , T 3  as well as capacitors C 2 , C 3  and resistor R 3 . 
         [0041]    The control voltage on control voltage terminal  205  is provided from intermediate circuit voltage U ZK  by a voltage divider with the aid of resistors R 11  and R 12 . The emitter of PNP transistor T 12  is connected to control voltage terminal  205  of FET transistor T 11  with the aid of diode D 11 . A capacitor similar to C 1  from  FIG. 2  is not provided in the represented specific embodiment. 
         [0042]    In a corresponding manner as described above with respect to the specific embodiment of  FIG. 2 , the control voltage on control voltage terminal  205  of FET transistor T 11  is lowered by disconnect device  130  if cut-off voltage U trig  exceeds a predetermined value. This essentially switches off FET transistor T 11 , so that no more voltage is provided to transformer  160  and the transfer of energy through transformer  160  is stopped. The reduction is maintained, even if cut-off voltage U trig  drops below the predetermined value. To cancel the reduction, intermediate voltage U ZK  must initially be cut off. 
         [0043]      FIG. 5  shows a variation of circuit  200  from  FIG. 4 . In a manner similar to that which was explained above with reference to  FIG. 3 , an optocoupler U 11  corresponding to optocoupler U 1  is provided in order to isolate cut-off voltage U trig  electrically from the remaining elements of circuit  200 .