GB2158275A - DC-to-DC converter - Google Patents

Abstract

A DC-to-DC converter which provides a highly stabilized output (not shown), also provides a relatively high voltage output which need not be as highly stabilized as the other output. This is done by using a booster technique in which the voltage from one converter output is added to the DC input voltage VIN. In the present arrangement, stability of that output is improved and power consumption reduced by giving the smoothing choke an extra winding (L2) coupled to the smoothing winding (L1). This winding (L2) is returned via a diode (D1) to the zero voltage rail. With proper proportioning of the windings (L1 and L2) the above objects are attained. <IMAGE>

Description

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

Where a DC-to-DC converter is used, as is the case, inter alia, in telephone line interface circuitry, there is required in addition to the stabilized voltages needed for the logic circuitry a relatively high voltage. This latter voltage can be semi-stabilized, i.e. stabilized to a lower level than the other voltages.

Where these voltages come from a single converter system. we have an arrangement such as that shown in Fig. 1, which only shows as much of the circuit as is relevant to the present discussion. Here one output transformer TF, via two secondaries each with its rectification and smoothing elements feeds the high, semi-stabilized, output voltage V, and the other (usually lower), stabilized voltage V2.

In this case a feedback connection (not shown) from the stabilized output varies the pulse-width flowing in the transformer primary in such a wayu as to semi-stabilize the high voltage, if forward converter techniques are used.

However, all the power is delivered by the windings of the main transformer of the converter. In one case this involves a high voltage of 63 volts, at 3 amp, i.e. 189 watts. If the high voltage does not have to be isolated from the input direct voltage, then the power from the converter can be reduced by using boost techniques-see Fig. 2. Here the output V2 is the stabilized output, while the output from the other rectifier circuitry is Vb, which is added to the input voltage VIN. Thus the 63 volts needed is made up from the sum of VIN and Vb. Hence any variations from VIN is seen as the same variation at the output.

To overcome such a variation, a conventional method is to use a secondary loop as shown in Fig. 3 to stabilize the output. Unfortunately the contents of the box labelled CON TROL are relatively expensive, and it is an object of the invention to provide a circuit which is more cost-effective.

According to the invention there is provided a DC-to-DC converter which provides at least one stablized DC output and a relatively high voltage DC output, in which the high voltage output is provided by generating a booster direct voltage which is added to the input direct voltage, in which the booster voltage output has a second winding on its smoothing choke one end of which is connected to the output and of that choke, in which the other end of the second winding is connected via a diode to the zero voltage side of the input voltage, and in which the relative polarizations of the windings on said smoothing choke are such, and the turns ratio of the windings are such, and the orientations of the rectifier is such, that the booster voltage is stabilized to an extent which may be less than the extent of stabilization of the stabilized DC output.

An embodiment of the invention will now be described with reference to Fig. 4, which shows as much of a DC-to-DC converter embodying the invention as is necessary to explain the invention.

In Fig. 4, in which the circuitry associated with the stabilized outpt is not shown, we see that the smoothing choke has two windings L1 and 12, both on the same core. Further, the reservoir capacitor C1 is returned to the zero voltage "rail" of the input supply VIN. As usual when using the booster technique, the booster output V, is connected in series with VIN by the connection shown. One end of L2 is connected to the "output" end of L1 and the other is connected via a diode D1 to the zero voltage "rail".

The effect of this circuit is that variations in the input voltage VIN vary the energy taken from the choke on the fly-back stroke. This varies the flux level in the choke, which causes the output voltage across C1 to vary inversely with VIN. With correct dimensioning of the choke windings, both of which are on the same core, the output voltage is stabilized to a much better order than the variation of VIN, provided that the voltage developed across L2 in the fly-back stroke falls below 0 volts input voltage before the voltage across L1 drops below the voltage at point A. This allows L2 to conduct in the fly-back stroke rather than L1.

1. A DC-to-DC converter which provides at least one stabilized DC output and a relatively high voltage DC output, in which the high voltage output is provided by generating a booster direct voltage which is added to the input direct voltage, in which the booster voltage output has a second winding on its smoothing choke one end of which is connected to the output end of that choke, in which the other end of the second winding is connected via a diode to the zero voltage side of the input voltage, and in which the relative polarizations of the windings on said smoothing choke are such, and the turns ratio of the windings are such, and the orientations of the rectifier is such, that the booster voltage is stabilized to an extent which may be less than the extent of stabilization of the stabilized DC output.

2. A DC-to-DC converter, substantially as described with reference to Fig. 4 of the accompanying drawings.

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

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION DC-to-DC converters This invention relates to DC-to-DC converters. Where a DC-to-DC converter is used, as is the case, inter alia, in telephone line interface circuitry, there is required in addition to the stabilized voltages needed for the logic circuitry a relatively high voltage. This latter voltage can be semi-stabilized, i.e. stabilized to a lower level than the other voltages. Where these voltages come from a single converter system. we have an arrangement such as that shown in Fig. 1, which only shows as much of the circuit as is relevant to the present discussion. Here one output transformer TF, via two secondaries each with its rectification and smoothing elements feeds the high, semi-stabilized, output voltage V, and the other (usually lower), stabilized voltage V2. In this case a feedback connection (not shown) from the stabilized output varies the pulse-width flowing in the transformer primary in such a wayu as to semi-stabilize the high voltage, if forward converter techniques are used. However, all the power is delivered by the windings of the main transformer of the converter. In one case this involves a high voltage of 63 volts, at 3 amp, i.e. 189 watts. If the high voltage does not have to be isolated from the input direct voltage, then the power from the converter can be reduced by using boost techniques-see Fig. 2. Here the output V2 is the stabilized output, while the output from the other rectifier circuitry is Vb, which is added to the input voltage VIN. Thus the 63 volts needed is made up from the sum of VIN and Vb. Hence any variations from VIN is seen as the same variation at the output. To overcome such a variation, a conventional method is to use a secondary loop as shown in Fig. 3 to stabilize the output. Unfortunately the contents of the box labelled CON TROL are relatively expensive, and it is an object of the invention to provide a circuit which is more cost-effective. According to the invention there is provided a DC-to-DC converter which provides at least one stablized DC output and a relatively high voltage DC output, in which the high voltage output is provided by generating a booster direct voltage which is added to the input direct voltage, in which the booster voltage output has a second winding on its smoothing choke one end of which is connected to the output and of that choke, in which the other end of the second winding is connected via a diode to the zero voltage side of the input voltage, and in which the relative polarizations of the windings on said smoothing choke are such, and the turns ratio of the windings are such, and the orientations of the rectifier is such, that the booster voltage is stabilized to an extent which may be less than the extent of stabilization of the stabilized DC output. An embodiment of the invention will now be described with reference to Fig. 4, which shows as much of a DC-to-DC converter embodying the invention as is necessary to explain the invention. In Fig. 4, in which the circuitry associated with the stabilized outpt is not shown, we see that the smoothing choke has two windings L1 and 12, both on the same core. Further, the reservoir capacitor C1 is returned to the zero voltage "rail" of the input supply VIN. As usual when using the booster technique, the booster output V, is connected in series with VIN by the connection shown. One end of L2 is connected to the "output" end of L1 and the other is connected via a diode D1 to the zero voltage "rail". The effect of this circuit is that variations in the input voltage VIN vary the energy taken from the choke on the fly-back stroke. This varies the flux level in the choke, which causes the output voltage across C1 to vary inversely with VIN. With correct dimensioning of the choke windings, both of which are on the same core, the output voltage is stabilized to a much better order than the variation of VIN, provided that the voltage developed across L2 in the fly-back stroke falls below 0 volts input voltage before the voltage across L1 drops below the voltage at point A. This allows L2 to conduct in the fly-back stroke rather than L1. CLAIMS

1. A DC-to-DC converter which provides at least one stabilized DC output and a relatively high voltage DC output, in which the high voltage output is provided by generating a booster direct voltage which is added to the input direct voltage, in which the booster voltage output has a second winding on its smoothing choke one end of which is connected to the output end of that choke, in which the other end of the second winding is connected via a diode to the zero voltage side of the input voltage, and in which the relative polarizations of the windings on said smoothing choke are such, and the turns ratio of the windings are such, and the orientations of the rectifier is such, that the booster voltage is stabilized to an extent which may be less than the extent of stabilization of the stabilized DC output.

2. A DC-to-DC converter, substantially as described with reference to Fig. 4 of the accompanying drawings.