GB2081989A

GB2081989A - DC-DC converter

Info

Publication number: GB2081989A
Application number: GB8025846A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-08-07
Filing date: 1980-08-07
Publication date: 1982-02-24
Also published as: GB2081989B; DE3130958A1

Abstract

In a DC-to-DC converter of the so-called fly-back type, the input DC drives a transistor (2) in a pulsed manner via a pulse width modulator (7). The transistor current flows in the primary of a transformer (4), and one secondary drives a filter plus rectifier unit (5) to give the DC output. Regulation is effected by a comparator (9) which compares the DC output with a reference, the result of the comparison exercising a control on the pulse width modulator (7). On initial switch-on, the transistor (2) is driven from a minimal pulse generator (1) via an OR gate (3) whose other input is driven from the pulse width modulator. To build up to the required output level, there is another secondary on the transformer whose output is used to control the pulse width modulator. It should be noted that the invention is also applicable to DC-to-DC converters of the so-called forward type. <IMAGE>

Description

SPECIFICATION DC-DC converter The present invention relates to DC-to-DC converters.

In a typical DC-to-DC converter the DC input drives an oscillator which feeds a transformer, which in turn transforms the voltage up or down as required. The transformer output is then rec-tified and filtered to give an output DC. The output voltage ;s set to the desired level by comparing that voltage or a voltage derived therefrom with a reference voltage. The result of this comparison is an error voltage, which controls the pulse width of the oscillator's output applied to the transformer. Thus as the output load increases or decreases, so do the pulse widths. Such a pulse width modulator is often controlled on start up to give a minimum pulse width which ramps up to the control pulse width, so reducing the initial currents.

Such an arrangement is often of the open loop type, and difficult to control, and it is well known that starting up problems are major contributors to switch mode failures. An object of the invention is to provide a relatively inexpensive DC-to-DC converter in which the above difficulties are minimised or overcome.

According to the invention, there is provided a DC-to-DC converter, which includes a switching transistor which is rendered conductive in a pulsed manner, the pulse width being dependent on the output voltage required, a pulse generator which supplies relatively narrow pulses to the transformer via an OR gate, which na-row pulses are effective to control the transistor on supply switch on, an error control loop including a comparator which compares the DC output of the converter or a parameter prorortional thereto with a reference condition, the result of the comparison being effective in normal operation of the converter to control a pulse width modulator which controls the width of pulses applied to the transistor via another input to the OR gate, and a further output from the converter from which a control is applied to the pulse width modulator to facilitate the control of the modulator to produce the pulse width appropriate to the desired output.

According to the invention there is also provided a DC-to-DC converter, which includes a switching transistor which is rendered conductive in a pulsed manner, the pulse width being dependent on the output voltage required, a pulse generator which supplies fixed-width narrow pulses to the transistor via an OR gate, which narrow pulses are effective Z control the ansistor on supply ssvitch- , a current transformer having a primary winding in which the current passed by the transistor flows, a first secondary winding from which the DC output of the converter is derived via a rectifier circuit, an error control loop including a comparator which compares said DC output or a parameter proportional thereto with a reference condition.The result of said comparison being effective in normal operation of the converter to control a pulse width modulator which controls the width of pulses applied to the transistor via another input to said OR gate, and a second secondary winding on the transformer from which a control is applied to the pulse width modulator on switch-on to facilitate the control of said modulator to produce the pulse width appropriate to said desired output.

An embodiment of the invention will now be described with reference to the accompanying drawing.

In the circuit shown schematically in the accompanying drawing, the input direct current, which may come from a battery such as a telephone exchange battery, drives a clock and minimum pulse generator 1, the output of which controls the switch transistor 2 via an OR gate 3. On switch-on, the generator 1 supplies narrow pulses each of which switches on the transistor 2 for a short duration. Hence voltages appropriate to the pulse width and to the transformation ratio of the transformer 4 are produced across the two secondary windings, one of which produces the output voltage V0 and the other auxiliary voltage VAUX. In both cases filters 5, 6 are provided, each of which includes a rectifier plus smoothing circuit.

The output from the lowermost secondary winding, that which produces VAUX, is applied to a pulse width modulator 7 whose output is applied via an isolation circuit 8 to the OR gate 3 and therefrom to the transistor 2. The combination of the error amplifier 9 and the pulse width modulator 7 will be relatively ineffective until the auxiliary voltage output is sufficiently generated. Thus start up is initially under the drive of the minimum pulse generator 4 in the block 1. Due to the action of the OR gate, the transistor thus stays on until the pulse width modulator output reverts to zero.

The normal control of the modulator 7 is from the error amplifier 9, which compares the value of V0 with a reference voltage VREF, derived from the initial voltage supply. Note that the voltage applied to the uppermost input of the error amplifier 9 may be the actual output voltage or a voltage proportional thereto, t - derived from a tapping on a potentiometer connected across the V0 output.

Thus on initial switch-on the system runs on the narrow pulses from the minimum pulse generator, and the initial broadening of the pulses from the pulse width modulator is effected under the control of the output from the lowermost secondary winding of the transformer 2. In due course as the voltage builds up the control via the error amplifier takes over.

Such an arrangement does not need the provision of an isolated start-up circuit, and the operation is fail safe. That is, difficulties which occur at switch-on will not cause the generation of excessive currents. Such a circuit can be achieved by purely semiconductor circuitry (apart from the transformer) so that a standard integrated circuit unit (or hybrid unit) could be produced for use in switched mode power supplies in general.

Although the invention has been described in its application to the so-called fly-back type of DC-DC converter it will be appreciated that it is also applicable to converters of the socalled forward type.

Claims

1. A DC-to-DC converter, which includes a switching transistor which is rendered conductive in a pulsed manner, the pulse width being dependent on the output voltage required, a pulse generator which supplies relatively narrow pulses to the transformer via an OR gate, which narrow pulses are effective to control the transistor on supply switch on, an error control loop including a comparator which compares the DC output of the converter or a parameter proportional thereto with a reference condition, the result of the comparison being effective in normal operation of the converter to control a pulse width modulator which controls the width of pulses applied to the transistor via another input to the OR gate, and a further output from the converter from which a control is applied to the pulse width modulator to facilitate the control of the modulator to produce the pulse width appropriate to the desired output.

2. A DC-to-DC converter, which includes a switching transistor which is rendered conductive in a pulsed manner, the pulse width being dependent on the output voltage required, a pulse generator which supplies fixed-width narrow pulses to the transistor via an OR gate, which narrow pulses are effective to control the transistor on supply switch-on, a current transformer having a primary winding in which the current passed by the transistor flows, a first secondary winding from which the DC output of the converter is derived via a rectifier circuit, an error control loop including a comparator which compares said DC output or parameter proportional thereto with a reference condition, the result of said comparison being effective in normal operation of the converter to control a pulse width modulator which controls the width of pulses applied to the transistor via another input to said OR gate, and a second secondary winding on the transformer from which a control is applied to the pulse width modulator on switch-on to facilitate the control of said modulator to produce the pulse width appropriate to said desired output.

3. A DC-to-DC converter substantially as described with reference to the accompanying drawing.