GB2110011A

GB2110011A - DC-DC converter

Info

Publication number: GB2110011A
Application number: GB08134634A
Authority: GB
Inventors: Malcolm Goodman
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-11-17
Filing date: 1981-11-17
Publication date: 1983-06-08
Also published as: DE3241256A1; GB2110011B

Abstract

In a DC-DC converter a switching transistor (T) is so controlled as to drive current pulses through the primary of power transformer (Tx1), which has a number of secondaries one per DC output required. Each such secondary supplies its output via a rectifier and a smoothing filter. The control and protection circuitry needs an isolated circuit for producing a sense signal. This is effected by a sensing transformer (Tx2) whose primary is in series with the primary of the power transformer (Tx1). The sense signal is derived from the secondary of this transformer (Tx2). To ensure that it is derived only from the main output it is necessary to cancel out components in the primary current due to the other outputs. To do this, further windings of the same transformer (Tx2) are placed in series with the other secondaries of the power transformer (Tx1), they being so poled and so dimensioned that the sense signal is derived only from the main output. <IMAGE>

Description

SPECIFICATION DC-DC converter This invention relates to DC-DC converters.

In such converters the protection and control circuitry is often driven from a sense signal derived from the converter 5 output.

Where the converter has two or more DC outputs it is necessary to be able to derive this sense signal from a selected one of the outputs, without any interference from the other output or outputs.

A DC-DC converter, in which the input direct current powers a switching tranistor so controlled as to cause pulses of current to flow in the primary winding of a power transformer, in which the power transformer has two or more secondary windings, each of which feeds an output via a rectifier circuit, in which a sensing transformer has its primary winding in series with the primary winding of the power transformer and is used to supply from its secondary winding a sense signal for use in the control and/or protection circuitry of the converter, in which since the same current flows in both of said primary windings the current induced into the secondary winding of the sensing transformer from its said primary winding has components due to the output currents from all of the secondary windings of the power transformer, so that to produce a sense signal derived from one only of the outputs it is necessary to eliminate the effects of the components due to the other of said outputs, and in which to cancel out each such unwanted components a further winding of the sensing transformer is connected in series with the secondary of the power transformer appropriate to the component to becancelled out, the winding being so proportioned and so poled as to effect said cancellations.

An embodiment of the invention will now be described with reference to the accompanying drawing, which shows the sense signal generating circuitry for a DC-DC converter.

The DC-DC converter to which the arrangement described is applied includes a switching transistor T whose on-off ratio is controlled, or modulated, by the converter's control circuitry in accordance with the load current it has to provide. The converter also has protection elements which are also controlled from the output so as to switch the converter off when a fault which could lead to the passing of an excessive current.

To provide the necessary information for the control and protection circuits, it is necessary to produce from the outputs of the converter one or more sense signals, which may be AC or DC depending on the nature of the control and protection circuitry. When a sense signal is derived via a transformer in a converter with two or more outputs, the sense signal is influenced by the current conditions of all outputs. This is a nuisance where a sense signal influenced by any one output is needed. The circuit described herein is intended to overcome this difficulty.

In the drawing, there is in series with the emitter-collector path of the transistor T the primary windings of the power transformer Tx1 and of a sensing transformer Tx2 from which an isolated sense signal is obtained.

The latter winding may be only a single turn winding. For each required DC output the power transformer Tx1 has a secondary winding, and three such secondaries are shown, which respectively drive a main high current output and two auxiliary outputs. Each such secondary winding drives its output via a rectifier circuit with a choke input filter.

The sense transformer Tx2 has a secondary winding with a resistor R1 across it, which supplies a sense signal Vm from a diode D, to which is connected a resistor Rm. With a transformer having several secondary windings, the primary current is influenced by the current conditions of all of those secondaries.

Thus the current induced into the secondary of Tx2 is also influenced by conditions in all three outputs of the converter. In this case the sense signal is required to be dependent on the main high current output, and to be as far as possible uninfluenced by the auxiliary outputs. To achieve this, windings of Tx2 are included in series with the power transformer secondaries of the auxiliary outputs. Hence the currents in these circuits cause components to be induced into the sense transformer secondary circuit. The windings are so poled that these components are subtracted from the current induced into that circuit from the primary winding of the sense transformer Then by suitable proportioning of the windings and by suitable poling thereof, the desired cancellation is achieved. In the drawing the poling of these windings is indicated in conventional manner by dots.

1. A DC-DC converter, in which the input direct current powers a switching tranistor so controlled as to cause pulses of current to flow in the primary winding of a power transformer, in which the power transformer has two or more secondary windings, each of which feeds an output via a rectifier circuit, in which a sensing tranfsormer has its primary winding in series with the primary winding of the power transformer and is used to supply from its secondary winding a sense signal for use in the control and/or protection circuitry of the converter, in which since the same current flows in both of said primary windings the current induced into the secondary winding of the sensing transformer from its said primary winding has components due to the output currents from all of the secondary

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION DC-DC converter This invention relates to DC-DC converters. In such converters the protection and control circuitry is often driven from a sense signal derived from the converter 5 output. Where the converter has two or more DC outputs it is necessary to be able to derive this sense signal from a selected one of the outputs, without any interference from the other output or outputs. A DC-DC converter, in which the input direct current powers a switching tranistor so controlled as to cause pulses of current to flow in the primary winding of a power transformer, in which the power transformer has two or more secondary windings, each of which feeds an output via a rectifier circuit, in which a sensing transformer has its primary winding in series with the primary winding of the power transformer and is used to supply from its secondary winding a sense signal for use in the control and/or protection circuitry of the converter, in which since the same current flows in both of said primary windings the current induced into the secondary winding of the sensing transformer from its said primary winding has components due to the output currents from all of the secondary windings of the power transformer, so that to produce a sense signal derived from one only of the outputs it is necessary to eliminate the effects of the components due to the other of said outputs, and in which to cancel out each such unwanted components a further winding of the sensing transformer is connected in series with the secondary of the power transformer appropriate to the component to becancelled out, the winding being so proportioned and so poled as to effect said cancellations. An embodiment of the invention will now be described with reference to the accompanying drawing, which shows the sense signal generating circuitry for a DC-DC converter. The DC-DC converter to which the arrangement described is applied includes a switching transistor T whose on-off ratio is controlled, or modulated, by the converter's control circuitry in accordance with the load current it has to provide. The converter also has protection elements which are also controlled from the output so as to switch the converter off when a fault which could lead to the passing of an excessive current. To provide the necessary information for the control and protection circuits, it is necessary to produce from the outputs of the converter one or more sense signals, which may be AC or DC depending on the nature of the control and protection circuitry. When a sense signal is derived via a transformer in a converter with two or more outputs, the sense signal is influenced by the current conditions of all outputs. This is a nuisance where a sense signal influenced by any one output is needed. The circuit described herein is intended to overcome this difficulty. In the drawing, there is in series with the emitter-collector path of the transistor T the primary windings of the power transformer Tx1 and of a sensing transformer Tx2 from which an isolated sense signal is obtained. The latter winding may be only a single turn winding. For each required DC output the power transformer Tx1 has a secondary winding, and three such secondaries are shown, which respectively drive a main high current output and two auxiliary outputs. Each such secondary winding drives its output via a rectifier circuit with a choke input filter. The sense transformer Tx2 has a secondary winding with a resistor R1 across it, which supplies a sense signal Vm from a diode D, to which is connected a resistor Rm. With a transformer having several secondary windings, the primary current is influenced by the current conditions of all of those secondaries. Thus the current induced into the secondary of Tx2 is also influenced by conditions in all three outputs of the converter. In this case the sense signal is required to be dependent on the main high current output, and to be as far as possible uninfluenced by the auxiliary outputs. To achieve this, windings of Tx2 are included in series with the power transformer secondaries of the auxiliary outputs. Hence the currents in these circuits cause components to be induced into the sense transformer secondary circuit. The windings are so poled that these components are subtracted from the current induced into that circuit from the primary winding of the sense transformer Then by suitable proportioning of the windings and by suitable poling thereof, the desired cancellation is achieved. In the drawing the poling of these windings is indicated in conventional manner by dots. CLAIMS

1. A DC-DC converter, in which the input direct current powers a switching tranistor so controlled as to cause pulses of current to flow in the primary winding of a power transformer, in which the power transformer has two or more secondary windings, each of which feeds an output via a rectifier circuit, in which a sensing tranfsormer has its primary winding in series with the primary winding of the power transformer and is used to supply from its secondary winding a sense signal for use in the control and/or protection circuitry of the converter, in which since the same current flows in both of said primary windings the current induced into the secondary winding of the sensing transformer from its said primary winding has components due to the output currents from all of the secondary windings of the power transformer, so that to produce a sense signal derived from one only of the outputs it is necessary to eliminate the effects of the components due to the other of said outputs, and in which to cancel out each such unwanted components a further winding of the sensing transformer is connected in series with the secondary of the power transformer appropriate to the component to be cancelled out, the winding being so proportioned and so poled as to effect said cancellations.

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