Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Patent
1994-09-27
1997-04-29
Krishnan, Aditya
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
H02J 710
Patent
active
056255403
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an electronic switched-mode power supply for supplying current from an input voltage source to an accumulator and to an electrical load adapted to be switched on, comprising a self-oscillating flyback converter having a transformer whose primary winding, in series arrangement with the collector-emitter circuit of a first transistor and an emitter resistor, is connected in parallel with the input voltage source, and whose secondary winding is connected in series with the accumulator and a first diode, with the base of the first transistor being connected to the secondary winding through a feedback circuit in addition to being coupled to a control circuit inhibiting conduction of the first transistor when the voltage at the accumulator has reached a set point, and enabling it to conduct again after the voltage has dropped below this set point or a predetermined lower value.
A switched-mode power supply of this type is known, for example, from EP-0 162 341 B1 or from prior German patent application No. P 41 22 544.9-32. In both circuit arrangements, the trickle charging current (continuous charging current) after the accumulator is fully charged amounts to about 10% of the charging current initially flowing when the accumulator is depleted.
In some types of accumulator as, for example, nickel hydride accumulators, even a low trickle charging current has a detrimental effect on the service life. Small household appliances such as electric shavers or electric toothbrushes are often kept in wall mounts through which the accumulator is recharged. Such appliances are accordingly permanently connected to the input voltage source (the line), so that in conventional switched-mode power supplies with self-oscillating flyback converters a continuous charging current (trickle charging current) flows to the accumulator.
It is an object of the present invention to prolong the service life of the accumulator of such appliances.
This object is accomplished in that an interrupting device is provided turning the first transistor off for a prolonged period when, with the switched-mode power supply in the pulsating mode, the ratio of oscillation off-time to the duration of oscillation bursts exceeds a predetermined magnitude.
The magnitude of this ratio is advantageously selected such as to correspond to the normally flowing trickle charging current. It is thereby ensured that the accumulator is fully charged before the first transistor is turned off for a prolonged period.
The interrupting device is configured such as to enable the first transistor to conduct again not until after the switched-mode power supply was previously disconnected from the input voltage source (power plug pulled or removed from the wall mount), or until the electrical load was switched on (motor power on), or until the accumulator voltage has dropped below its set point significantly, meaning, for example, that only a half charge remains in the accumulator.
Advantageously, the interrupting device is configured in such a manner that a capacitor connected to the base of a turn-off transistor is charged during oscillation off-time of the flyback converter, discharging during the occurrence of oscillation bursts, whereby the turn-off transistor is rendered conducting from a specified ratio of pulse spaces to pulses, turning the first transistor off for a prolonged period.
Further features will become apparent from the other subclaims.
Embodiments of the present invention will be described in more detail in the following with reference to the accompanying drawings. In the drawings,
FIG. 1 is a schematic circuit diagram of an electronic switched-mode power supply incorporating an interrupting device:
FIG. 2 is a first detailed embodiment with reference to the circuit arrangement described in German patent application No. P 41 22 544.9-32;
FIG. 3 is a second detailed embodiment with reference to the same circuit arrangement;
FIG. 4 is a graphical representation of the charging current in % of the initial current during charging of an
REFERENCES:
patent: 4965506 (1990-10-01), Algra et al.
Braun Aktiengesellschaft
Krishnan Aditya
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