Many electronic devices, for example computer peripherals, notebooks, portable devices appertaining to entertainment electronics or musical instruments do not have a built-in power supply unit, but rather are supplied with a low-voltage DC voltage via an adapter. Although the device itself usually has an on/off switch, the latter is situated downstream of the power supply unit, so that, even when the load is switched off, current is taken by the power supply unit as long as this is connected to the electrical network.
At the present time, legislators are endeavoring to reduce the current consumption of such power supply units in the standby mode, that is to say when the load is switched off, from at the present time about 2 W to 0.5 W and then further to 0.1 W, which makes considerable requirements of the control circuits used in the power supply units since, even in the standby mode, the power supply unit is intended to output a regulated output voltage and, in particular, is intended to be able to momentarily output sufficient power after the load is switched off.
In order to provide drive signals for a switch in a switched-mode power supply, a control signal dependent on the output voltage provided is fed to the drive circuit. In the case of flyback converter switched-mode power supplies, in which the switch is connected in series with the primary coil of a transformer and in which output terminals for providing the output voltage are coupled to the secondary winding of the transformer, the feedback of the control signal on the secondary side to the drive circuit arranged on the primary side necessitates potential isolation, which is usually ensured by means of an optocoupler. It is precisely when a connected load has a low power consumption that such optocouplers contribute considerably to the power consumption of the switched-mode power supply and thus to the power loss, as is explained briefly below with reference to a switched-mode power supply according to the prior art.
FIG. 1 shows an example of a conventional flyback converter switched-mode power supply which is described for example in Köstner/Möschwitzer: “Elektronische Schaltungen” [Electronic circuits], Karl Hanser Verlag Munich, 1993, pages 286 to 288.
The power supply unit houses input terminals EK1, EK2 for applying a power supply voltage Un, and a rectifier arrangement—coupled to the input terminals EK1, EK2 with a bridge rectifier BG and an input capacitor Cin, across which a rectified voltage Uin is present. In order to convert said rectified input voltage Uin into a regulated output voltage Uout available at output terminals AK1, AK2, a converter with a transformer TR is available, in the primary coil Lp of which is connected in series with a switch T formed as a semiconductor switch, the input voltage Uin being present across this series circuit. A secondary coil Ls of the transformer TR is inductively coupled to the primary coil Lp, said secondary coil Ls being connected to the output terminals AK1, AK2 via a rectifier arrangement D1, C2. The power consumption of this power supply unit and thus the output voltage Uout are dependent on the duty ratio of a drive signal S3 which comprises a sequence of drive pulses and drives the semiconductor switch T. In the case of a fixedly clocked switched-mode power supply, in which the switch T is closed at regular time intervals, the power consumption rises as the duty cycle rises, the duty cycle representing the ratio between switch-on duration of the semiconductor switch T and the drive periods determined by the interval between two switch-on instants.
The drive signal S3 is made available by a drive circuit 12, to which a control signal S2 dependent on the output voltage Uout is fed. In order to generate the control signal, a controller 10 is provided on the secondary side, which controller is connected to the output terminals AK1, AK2 and provides a control signal S1, which is transmitted to the primary side by means of an optocoupler 11. In this case, the control signal S1 on the secondary side and the control signal S2 on the primary side are ideally proportional to one another. The controller 10 has for example an integral action (I controller) or a proportional-integral action (PI controller)
Owing to the relatively simple interconnection of the controller, it is customary to operate the optocoupler 11 with a negative control sense, i.e. to generate a control signal S1 or S2 which is all the greater, the lower the power that is to be output to the output terminals AK1, AK2. In the standby mode, that is to say in the case of an open circuit at the output terminals AK1, AK2 or when a load Z illustrated by dashed lines in FIG. 1 has only a very low power consumption, the control signal S1 is thus particularly large and the current consumption of the optocoupler 11 is thus particularly high.
For the voltage supply of the drive circuit 12, a supply voltage Vcc is generated internally, said supply voltage being provided for example by an auxiliary coil (not specifically illustrated) coupled to the primary coil Lp with a rectifier circuit connected downstream, as is described for example in Köstner/Möschwitzer, loc. cit. In this case, the supply voltage Vcc must be large enough to be able to ensure driving of the semiconductor switch T even in the standby mode. A reduction of the supply voltage Vcc, which would also result in a reduction of the power loss of the optocoupler 11, is thereby limited.
A known drive circuit for driving the semiconductor switch in a switched-mode power supply is a drive circuit of the ICE2AS01 type, from the Applicant. A switched-mode power supply in accordance with FIG. 1 is likewise elucidated in the data sheet of this drive circuit. In the case of this drive circuit, the maximum optocoupler current is about 1.5 mA, which, given an internally generated supply voltage Vcc of 15 V, results in a power loss brought about by the optocoupler of 22.5 mW.
The Applicant's control circuit of the TDA 16850 type uses control with a positive control sense, so that, when a load has a low power consumption, a small control signal is transmitted to the drive circuit via the optocoupler. However, this is associated with higher complexity of the circuit, in particular of the controller 10.
EP 0 585 789 B1 discloses using the optocoupler current of the control loop for the current supply of the control circuit in order thereby to reduce the number of terminals of the drive circuit and to be able to accommodate the circuit in a cost-effective TO-220 housing with just three terminal pins. In the case of this circuit, however, the optocoupler must be dimensioned such that a current sufficient for supplying the control circuit always flows through said optocoupler. In the case of a negative control sense, the optocoupler current in the standby mode is in this case a multiple of the current consumption required for the control circuit. A low standby power consumption is thus not possible in the case of this circuit.
It is an aim of the present invention to provide a drive circuit for a switch in a switched-mode power supply which has a low power consumption in the standby mode.