Switch mode power supply

Details Switch mode power supply Switch mode power supply

1997-06-09

1998-11-03

Berhane, Adolf

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

363 97, H02M 3335

Patent

active

058318372

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Technical Field
The present invention relates to a switch mode power supply for operation at a range of frequencies.
2. Prior Art
Switch mode power supplies, or DC to DC convertors as they are sometimes referred to, are used in electrical appliances, such as television receivers and computer visual display units for example, to efficiently transfer electrical power from the domestic electricity mains supply to electrical circuitry of the appliances.
A typical current mode SMPS comprises a switch for alternately opening and closing a current path through the primary winding of a transformer in response to a square wave switching signal. In operation, a DC voltage is applied across the primary winding of the transformer. Electrical energy is transferred to a load connected to the secondary winding of the transformer by alternately opening and closing the switch as a function of the switching signal. The amount of electrical energy transferred to the load is a function of the duty cycle of the switch and the frequency of the switching signal. The duty cycle of the switch is the ratio of the time for which the switch is open relative to the time for which the switch is closed. The duty cycle thus corresponds to the mark-space ratio of the switching signal. An oscillator generates the switching signal. A pulse width modulator is connected to the output of the oscillator for varying the mark-space ratio of the switching signal to vary the duty cycle of the switch as a function of a feedback signal from the load. The feedback signal from the load completes a negative feedback loop enabling the controller to regulate the power supplied to the load in accordance with the power demanded by the load, thereby leading to efficient energy usage. The power available to the load is limited by sensing the current in the primary winding of the transformer and turning off the switch in the event of the peak primary current exceeding a predetermined threshold. Because power is proportional to the square of the current, this gives effective power limiting.
In many electrical appliances the switching signal is synchronised by the oscillator to a synchronisation signal in the interests of, for example, noise immunity. For example, in a raster-scanned cathode ray tube (CRT) display, such as a television receiver or a computer visual display unit, the switching signal is usually synchronised to the raster line scan signal of the display. There is an increasing requirement for CRT displays to operate within a range of different video synchronisation frequencies so that a range of different picture formats can be presented on the same display screen. It is therefore not unusual for such CRT displays to be required to operate in range of line scan frequencies extending from 31.5 kHz to 64 Khz, for example. However, the amount of power deliverable by a conventional switch mode power supply for such a display, the "power throughput", is directly proportional to the frequency of the switching signal for a given level of current in the primary winding. In a failure condition, such as a short circuit of the secondary windings, the power throughput can be excessive without necessarily invoking current limiting in the primary winding because of a high frequency of operation. Such excessive power throughput causes a temperature rise in the transformer. In the interests of safety, it is therefore desirable to base the design of such a conventional power supply on the maximum power level that the supply can deliver. Note for example that a typical SMPS which is designed to limit at 100 W at 31.5 kHz may not power limit below 200 W at 64 kHz. In particular, it is desirable in conventional power supplies to include a transformer which is sufficiently large to dissipate the power throughput under the above mentioned failure conditions. However, this a leads to an increase in transformer size for each increase in the frequency range of operation of the power supply.


SUMMARY OF INVENTION

In accordance with the pre

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