GB2238635A

GB2238635A - A regulated power supply

Info

Publication number: GB2238635A
Application number: GB8926774A
Authority: GB
Inventors: Toshihiko Yamashita
Original assignee: UNIQEY
Current assignee: UNIQEY
Priority date: 1989-11-27
Filing date: 1989-11-27
Publication date: 1991-06-05
Anticipated expiration: 2009-11-27
Also published as: GB2238635B; GB8926774D0; DE9016271U1

Abstract

A power source capable of providing a constant DC output voltage when supplied with one of a range of AC input voltages has a transformer 12 providing a number of outputs 22, 24, 26, 28 of differing voltage, each proportional to the input voltage. Each output is rectified 30, and connected in series to a maximum voltage regulator 32 with no capacitive smoothing. The outputs from each regulator 32 are connected to a common output 36 to give the constant DC voltage. Each voltage regulator 32 has a set regulating voltage which is one of a discrete range of voltage steps close to the required substantially constant DC voltage, such that, as the input voltage V varies, the regulating paths A, B, C, D are switched so that current is not only drawn from the one regulator. <IMAGE>

Description

ELECTRICAL POWER SOURCES This invention relates to electrical power sources.

As explained in our United States Patent No. 4 712 171, many pieces of domestic electrical equipment need to be capable of working with different mains supply voltages, the most common of which are the 110 volts supply existing, for example, in the United States of America and the 220 volts supply existing, for example, in Europe.

However, in some countries the standard voltage is slightly different from these standard voltages whilst in other countries the supply itself may have a rather erratic and variable value.

To meet the differences between a 110 volts and a 220 volts supply, equipment such as electric shavers may use a simple resistor which can be manually switched into and out of circuit to dissipate the excess of the larger voltage as heat. Alternatively a transformer can be used where a large power supply is concerned but it is still necessary to adjust the tap position manually to match the supply and in many cases adjustment for the relatively small changes from the two standard voltages is not provided for. Also no account can be taken of erratic changes in voltage.

According to the invention described in that Patent there is provided an electrical power source capable of accepting an input within a range of AC input voltages and providing a substantially constant DC output supply, comprising converter means for providing from the AC input supply a number of outputs of differing voltages each proportional to the input voltage, each of these outputs being connected respectively to one of a number of parallel circuit lines each of which includes a rectifier, a maximum voltage regulator and non-return current device, the outputs of these lines being connected to a common output intended to provide the substantially constant DC voltage supply, each voltage regulator having a set regulating voltage which is one of a discrete range of voltage steps close to the required substantially constant DC voltage, the lowest of the steps being no greater than the voltage reached for the line connected to the highest output from the converter means when the input supply is at the low end of the range of AC voltages, the next of the steps being greater than the lowest and being reached for the line connected to the next highest output from the converter means, and, if there are more than two parallel circuit lines, the next step being greater and being reached for the line connected to the next highest output from the converter means and so on, so as to cover the range of possible input AC voltages.

Such a power supply has been found to work well and can be very simple to make and in addition can occupy a relatively small space It can, therefore, fit easily within many domestic electrical appliances. Also the supply automatically accepts any AC voltage within the designed range of AC input voltage and gives a substantially constant DC output. Further the power source accepts automatically any erratic and fluctuating changes in the input voltage substantially without affecting the output DC voltage.

In the arrangement shown in Figure 1 of that Patent, the differing output voltages are kept reasonably constant by the presence of the capacitors 42. Thus, the circuit quickly reaches a substantially stable operating condition where, for a particular AC mains input supply, the voltages on the lines A to D are substantially constant. Therefore for a particular AC mains supply the current is drawn from one only of the lines A to D. This has disadvantages in requiring each line to be capable of carrying a continuous current.

We have now found that by eliminating these capacitors the output voltages vary with the AC mains input and the circuit will draw instantaneously from whichever line A to D is appropriate.

According to the present invention, therefore, there is provided an electrical power source capable of accepting an input within a range of AC input voltages and providing a substantially constant DC output supply, comprising converter means for providing from the AC input supply a number of outputs of differing voltages each instantaneously proportional to the AC input voltage, each of these outputs being directly connected respectively to one of a number of parallel circuit lines with substantially no capacitive smoothing of the output from the converter means, rectifier means in series with the convertor means to rectify the AC output, each circuit line including a maximum voltage regulator and non-return current device, the outputs of these lines being connected to a common output intended to provide the substantially constant DC voltage supply, each voltage regulator having a set regulating voltage which is one of a discrete range of voltage steps close to the required substantially constant DC voltage, the lowest of the steps being no greater than the voltage reached for the line connected to the highest output from the converter means when the input supply is at the low end of the range of AC voltages, the next of the steps being greater than the lowest and being reached for the line connected to the next highest output from the converter means, and, if there are more than two parallel circuit lines, the next step being greater and being reached for the line connected to the next highest output from the converter means and so on, so as to cover the range of possible input AC voltages.

By the term "substantially no capacitive smoothing we mean that no deliberate capacitance is included in or across the connection to limit or smooth the rate of voltage change supplied from the convertor means, although it is recognised that all components and even wiring connections have some small inherent capacitance which cannot be avoided.

The advantage of such a power supply is that the voltage applied to each regulator will follow closely the rectified instantaneous output and, so except when the instantaneous voltage is greater than the regulating voltage, the instantaneous power output will approximate the product of the instantaneous voltage and the current.

The power supply according to the present invention can still be very simple to make and in addition can occupy a relatively small space. It has, therefore, the advantages of the power supply described in our above noted Patent.

Typically there will be four parallel circuit lines to cover an input AC voltage range from 80 to 260 volts.

In order to improve the stability of the output DC voltage, the common output can be fed through another maximum voltage regulator having a set output voltage which is lower than any of the set values of those in the parallel circuit lines. In this way even though there will be small voltage changes as a result of current being drawn from one or other of the parallel circuit lines depending on the input voltage, the final output voltage can be very stable. In some cases, however, this further step may not be necessary because the relatively small differences in output from the various parallel circuit lines may insignificant to the piece of equipment receiving the DC output.

In order to smooth the power output from the additional voltage regulator, a smoothing capacitor can be provided.

As a further simplification, in the power supply according to the present invention the rectifier means could be a single diode in series with the convertor means rather than a diode in series with each regulator.

An important advantage of the invention is that the power source has low high-frequency noise and so a low risk of radio interference.

The invention also has advantages in a saving of components compared with the circuit shown in our earlier US Patent noted above.

A power source according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of one form of power source according to the invention; Figure 1A is a detail showing a modified embodiment; Figure 1B is a detail showing another modified embodiment; Figure 1C is a detail showing a further modified embodiment; and Figures 2 and 3 are graphs illustrating the operation of a power source according to the invention at differing AC mains input voltages.

The power source 10 shown in Figure 1 includes a transformer 12 which has a primary 14 and secondary 16.

The primary 14 receives an input AC voltage V across the terminals 18 and 20. The secondary 16 has a number of tappings providing outputs at points 22, 24, 26 and 28.

Each of the points 22 to 28 is connected to one of the parallel lines A, B, C and D. Each line includes a a voltage regulator 32 and a diode 34 to prevent return current flow. The lines A to D all join at a common point 36 which provides a substantially constant DC voltage output v at the output 38.

In series with the secondary 16 is a diode 30 which acts as a half-wave rectifier. Alternatively a separate diode could be provided in each of the lines A to D. Further full-wave rectification can be provided by adopting the modification shown in Figure 1A.

The connections between the secondary 16 and the regulators 32 in each of the lines A to D is with no smoothing capacitors, ie. no intentional smoothing capacitors are provided across any of the lines A to D.

The transformer 14 could be replaced by a resistor with tappings to provide voltages corresponding to those at the points 22 to 28.

Each voltage regulator 32 comprises a transistor 50 whose base is held at a particular voltage by a resistor 52 and a common Zenner diode 53, and if appropriate one or more diodes 54. In this way depending upon the voltage supplied to its base, each transistor 50 will give an output DC voltage at its emitter which reaches a maximum predetermined level when the voltage on its collector reaches a certain value and will not exceed this even when the voltage applied to the collector exceeds that said certain value.

It will be seen that the voltage which is applied to the base of the respective transistors 50 varies from one of the lines A to D to another. Indeed the arrangement is such that the voltage at the base in line A is lower than that at the base in line B, the voltage at the base in line B is lower than that at the base in line C and that the voltage at the base in line C is lower than that at the base in line D.

The AC voltage V which is supplied to the primary 14 of the transformer 12 is within a predetermined ranges and as an example it is convenient to design the power supply so that it will accept AC voltages within the range of 80 to 260 volts. This will then embrace the full range of commonly existing mains supply voltages and in particular the American 110 voltage and European 220 voltage.

Reference will now be made to Figure 2 and 3 to illustrate the operation of the power source 10 during a half cycle of operation of the AC mains supply.

Referring to Figure 2, the half-wave outputs from the transformer 12 at the points 22 to 28, i.e. the instantaneous voltages on the lines A to D, respectively are shown and marked VA to VD The voltage levels Va to Vd are the maximum preset voltage levels passed by the transistors 50 in the lines A to D respectively. By way of example, these could be say 5.1 volts for Va, 5.2 volts for Vb, 5.3 volts for Vc and 5.4 volts for Vd. In this way the maximum variation in the output voltage v is from 5.1 to 5.4 volts, depending upon the instantaneous values of the voltages VA to VD.

The resulting output voltage v at the point 38 is shown by the thick line 70. As can be seen, during each cycle the current at the output 38 is drawn from the line A to D which at that instant has the highest voltage appearing after its respective diode 34, eg. the line A at the point 71 at a voltage below 5.1 volts since no other line is at such a high voltage, the line A at the point 72, the line B at the point 74 at 5.2 volts, the line C at the point 76 at 5.3 volts, the line D at the point 78 at 5.4 volts, and so on.

In the example shown in Figure 2, the AC mains input is relatively high, eg. 220 VAC. In the example shown in Figure 3, the mains input voltage is lower, eg.

110 VAC. Now the voltage level reached by the lines A to D are lower but the selection to draw the output current occurs in just the same way with the current being drawn from whichever output line A to D has the highest instantaneous voltage.

In both examples, however, it will be noted that switching from one line A to D occurs frequently during each cycle.

In many cases this variation in v from 5.1 to 5.4 volts depending upon the voltage V is sufficiently constant for the piece of equipment with which the power source is associated. However, if a more constant supply is required, then a further voltage regulating step may be required. This is achieved by passing the output from the point 34 to another voltage regulator 64, which can be identical with the regulators 32. However, the set voltage for this regulator must be lower than that for the lowest of the lines A to D, namely line A. In the example above, if the set maximum voltage of the regulator 64 is 5.0 volts, then the small variation from 5.1 to 5.4 can be avoided and the resulting final DC outputs voltage u at an output 66 will be substantially constant at 5.0 volts. In addition, a smoothing capacitor 68 could additionally or alternatively be provided.

The precise construction for the voltage regulators 32 and 64 is not critical. For example as shown in Figure 1B each regulator may have its own Zenner diode 68a having the appropriate threshold voltage or as shown in Figure 1C each regulator may include a Darlington pair arrangement.

Although the diodes 34 are shown, they will often not be necessary since the transistors 50 will act as diodes to prevent return current flows along the lines A to D.

Claims

WHAT WE CLAIM IS:

1. An electrical power source capable of accepting an input within a range of AC input voltages and providing a substantially constant DC output supply, comprising converter means for providing from the AC input supply a number of outputs of differing voltages each instantaneously proportional to the input AC voltage, each of these outputs being connected respectively to one of a number of parallel circuit lines with substantially no capacitive smoothing of the output from the converter means, rectifier means in series with the convertor means to rectify the AC output, each circuit line including a maximum voltage regulator and non-return current device, the outputs of these lines being connected to a common output intended to provide the substantially constant DC voltage supply, each voltage regulator having a set regulating voltage which is one of a discrete range of voltage steps close to the required substantially constant DC voltage, the lowest of the steps being no greater than the voltage reached for the line connected to the highest output from the converter means when the input supply is at the low end of the range of AC voltages, the next of the steps being greater than the lowest and being reached for the line connected to the next highest output from the converter means, and, if there are more than two parallel circuit lines, the next step being greater and being reached for the line connected to the next highest output from the converter means and so on, so as to cover the whole range of possible input AC voltages.

2. An electrical power source as claimed in Claim 1 in which the converter means comprises å transformer with a number of output taps corresponding to the number of parallel circuit lines.

3. An electrical power source as claimed in Claim 1 or Claim 2 in which there are four parallel circuit lines to cover a range of input voltages of from 80 to 260 volts.

4. An electrical power source as claimed in any preceding claim in which a Zenner diode acts as the maximum voltage regulator.

5. An electrical power source as claimed in any preceding claim in which the diode rectifier stage is provided in series with the convertor means.

6. An electrical power source as claimed in any preceding claim in which the DC voltage from the common output of the parallel circuit lines is fed through another maximum voltage regulator having a set output voltage which is lower than the lowest of the set values of the parallel circuit lines.

7. An electrical power source as claimed in any preceding claim in which capacitor means are provided to smooth the power output from the common output of the parallel circuit lines.

8. An electrical power source substantially as herein described with reference to the accompanying drawings.