GB2152771A

GB2152771A - Power supply circuit

Info

Publication number: GB2152771A
Application number: GB08429627A
Authority: GB
Inventors: David Gurwicz
Original assignee: Nada Electronics Ltd
Current assignee: Nada Electronics Ltd
Priority date: 1983-12-16
Filing date: 1984-11-23
Publication date: 1985-08-07
Also published as: GB8333627D0; GB8429627D0; GB2152771B

Abstract

AC mains voltage is full-wave rectified and applied to a storage capcitor 22 providing input to an inverter 24. The voltage applied to the capacitor is maintained above the peak mains voltage by an additional voltage which is applied to the output of the rectifier 20. The additional voltage is produced by a second rectifier circuit 50 fed with a high frequency input from a transformer 42 having its primary 44 across the inverter output. The additional voltage is applied by a filter 76, 74. The signal from the transformer is modulated by PWM by the action of a control circuit 70 which governs the firing of the SCR's 52, 54. The control circuit uses error signals including a signal representing the difference between the actual current waveform and a predetermined waveform to ensure that the waveform of the current drawn from the mains 10 is as required e.g. sinusoidal and in phase with the mains voltage. <IMAGE>

Description

SPECIFICATION Power supply circuit The invention relates to power supply circuits.

Atypical example of a well-known power supply circuit is described by H. Knoll in his paper "3kW Switch Mode Power Supply Providing Sinusoidal Mains Current and Large Range of DC-Output" given atthe Second Annual European Power Conversion Conference September 3-5 1980 and published at pages 2.6-1 to 2.6-17 ofthe proceedings. The author describes the circuit as classical (pages 2.6-1 to 2.6-3) and goes on to describe various modified circuits intended to overcome drawbacks in the classical circuit.

Further discussion of similar circuits is given by F. J.

Burgum in his paper "Mains Pollution by Rectifier Systems and the significance of EN 50 0006" given at the same conference and published at pages 2.7-1 to 2.7-10 ofthe proceedings.

Further discussion is given by M. Herfurth in his paper "Switching Power Supplies for Electronic Devices with Sine Wave Current Consumption from the Line" given at the Third International PCI Conference on Power Conversion 1981 and published at pages 111 to 117 of the proceedings.

Those papers show that in the classical circuit a large storage capacitor is charged by a rectifier bridge and the peak currents drawn from AC mains can overload the mains and the control equipment and distortthe mains voltage waveform, an effect known as "mains pollution". The discussions include propos als for overcoming such drawbacks, including circuits which draw a substantially sinusoidal currentfrom the mains and in which the potential across the storage capacitor is raised above the peak mains value by means of a so-called boost converter. By controlling the boost voltage in response to the sinusoidally varying mains voltage, current can flow to the capacitorthroughouteach cycle of the mains voltage.

The object of the invention is to provide a power supply circuit in which the voltage applied to the storage capacitor is raised above the peak mains voltage withoutthe need for a separate converter.

A powersupply circuit, according to the invention, comprises input points, a storage capacitor, first rectifier means arranged to rectify an alternating current drawn by the circuit at the input points and to charge the storage capacitor, an inverter having its input connected to the storage capacitor, a transformer having a primary winding receiving the output of the inverter and having a secondary winding, second rectifier meansarrangedto rectifythe outputfrom the secondary winding, a filter arranged to apply the output voltage from the second rectifier means between an input point and the storage capacitor to maintain the voltage applied to the storage capacitor above the peak mains voltage of the same polarity and a control circuit which controls the operation of the second rectifier means.

Embodiments of powersupply circuits in accordance with the invention will now be described by way of example with reference to the accompanying drawings being four schematic circuit diagrams and a further diagram in which: Figure 1 shows a first embodiment of power supply circuit; Figure 2 shows a control circuit for use in the circuit shown in Figure 1; Figure 3 shows a modified second rectifier means for use in the circuit shown in Figure 1; Figure 4 shows a second embodiment of power supply circuit for energisation from a three-phase mains; and Figure 5 shows voltage waveforms arising at different points in the circuit shown in Figure 1.

Figures 1 and 2 show a circuit for energisation from a single-phase supply 10, connected to two input points 12,14, the circuit producing an AC output at two output points 16,18. The input points 12,14 are connected to input nodes of a first rectifier means 20, a bridge comprising semiconductor switch devices, such as silicon controlled rectifiers. The output nodes of the bridge are connected across a large storage capacitor 22, typicallythough not necessarily of electrolytic type. The capacitor 22 is connected across the input of an inverter24,which comprises two transistors 26,28 and two capacitors 30,32 providing an inverting centrepoint. The capacitors 30,32 are shunted by respective resistors 34,36.The transistors 26,28 are switched in correct inverting sequence by a drive circuit 38 connected to the transistor bases.

according to standard practice.

A high frequency transformer 42 has a primary winding 44 connected between the line joining the resistors 34,36 and the line joining the emitter of the transistor 26to the collector of the transistor 28 so as to receive the inverter output. The transformer 42 has first, second and third secondarywindings 46,47 and 48, respectively. The winding 46 provides a voltage used for boosting the voltage applied to the storage capacitor 22 as explained below. The winding 47 develops a signal which is modulated in the drive circuit 38 to vary the mark-space ratio and used to regulate either or both output voltage and current of the inverter 24. The winding 48 is connected to the output points 16,1 8, the outputfrom which may be subsequently rectified, as is commonly required for many applications.

The AC outputfrom the terminals Z, ZZ ofthe winding 46 is rectified by second rectifier means, a half-controlled bridge circuit 50 connected via the terminals Z, ZZto the winding 46. The circuit 50 consistsoftwo silicon controlled rectifiers 52, 54and two diodes 56,58. A choke 60 is optional depanding on the type of rectifier used. A high tension snubber circuit including a resistor 62 and capacitor 64 is also optional. Afree-wheel diode 66 allows each rectifier to recover during a null voltage period.

The firing sequence of the rectifiers 52,54 is controlled by a control circuit 70 described below with reference to Figure 2. A control signal for that circuit is derived from a resistive shunt72 interposed between The drawing(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

the rectifier bridge 20 and the storage capacitor 22.

The DC output voltage from the bridge 50 is added to the voltage output from bridge 20 by a filter made up of an inductance 74 and a capacitor 76 which is connected in series with, and between,the bridge 20 and the storage capacitor 22.

Figure 2 shows the main components ofthe control circuit 70 which are: a first error amplifier 80 whose inputs are a reference voltage at 82 and at 84 the voltage across the storage capacitor 22; a multiplier 86 whose inputs are the output 88 from the first error amplifier 80 and at 90 a predetermined current waveform shown at 92 by way of example in alternative forms; otherforms can be used if prefer red; a second error amplifier 94whose inputs arethe output 96 from the multiplier 86 and at 98 the signal from the shunt 72; and a firing angle control 100 whose input at 102 is the output from the second error amplifier 94.

The firing angle control 100 controls the firing of the rectifiers 52,54.

OPERATION The output of the inverter 24 is a high frequency rectangular voltage waveform imposed on the transformer primary winding 44 and an identical waveform appears acrosstheterminalsZ,ZZofthesecondary winding 46. That voltage is rectified by the bridge 50 and pulse-width modulated by the control circuit of Figure 2. The high frequency component is removed by the filter 74,76 so that the output from the rectifier bridge 50 has the waveform indicated at C in Figure 5.

The filter is arranged to apply that output between the rectifier bridge 20 and the storage capacitor 22so that the voltage is added to the voltage output ofthe bridge 20, which is shown at Bin Figure 5. The mains voltage is shown at A. Thetroughs ofthe waveform C coincide with the peaks ofthe rectified output B from the bridge 20 so that, when the voltages are added, a steady DC voltage results. The level of the voltage D applied to the storage capacitor22 is thus always higherthan the peak positive-going voltage of the mains, V (Fig. 5) by an amount dV chosen to suit output requirements.

The result is that distortion and pollution ofthe mains waveform is avoided and power is drawn from the mainsthroughoutthe alternating mains cycle, which allows much greaterflexibility in the overall design ofthe circuit to meet whatever duty it is to fulfil.

Also, the regulation of the output of the circuit can be more efficiently achieved by choice ofthe two control facilities in the drive circuit 38 and in the control circuit 70 at 82. By varying the reference voltage at 82 the pulse width modulation of the high frequency input at Z, ZZto the circuit 50 is varied, thus changing the peak voltage ofthewaveform C and hence the level of the voltage D.

The circuit shown in Figure2 meetstwo requirements namely (i) thatthewaveform ofthe current drawn by the supply circuit from the mains shall be of a predetermined form; and (ii) regulation ofthe output ofthe supply circuit additional to the regulation available by variation ofthe modulation of the signal from the winding 47.

Where the requirement (i) is that the current drawn shall be sinusoidal (as assumed in the discussion above), the predetermined current waveform 92 is a full-wave rectified sinusoidal waveform derived, for example, by rectification of the mains voltage. Alternatively, the waveform of the current drawn may be required to be rectangularortriangular. In such cases the predetermined waveform at 90 is correspondingly rectangular or triangular as indicated at 92. Such waveforms may be derived from a suitable generator, for example.

Adjustment of the input reference voltage at 82 gives the additional control of regulation of the output of the power supply circuit. This is available independently of the need to meet the requirement (i) since once the predetermined waveform at 90 has been chosen and applied to ensure that the current drawn is ofthe required shape and in the required phase relationship, if any, with the mains voltage, the magnitude ofthe voltage across the storage capacitor can be varied as required above the peak mains voltage level. Where mains pollution is to be avoided, the current drawn will not only be sinusoidal butwill also be in phase with the mains voltage. However, the invention is applicable also in cases where either no phase relationship our a predetermined phase difference is imposed.

Input 96 is a waveform exactly representing the rectified current required to be drawn from the bridge 20. The signal input at 98 exactly represents the current actually being drawn from the bridge 20. The firing angle control 100 operates to time the firing of the rectifiers 52,54 so as to reduce to zero the difference between those inputs 96 and 98. The resulting pulse-width modulation of the output of the bridge 50 determinesthe amplitude ofthe voltage represented at C.

Figure 3 shows a modified form of second rectifier means 100 but components corresponding to those shown in Figure 1 retain the same reference number.

The rectifier means 100 has two fully controlled bridges 102,104 each havingfoursilicon controlled rectifiersl06,108,110,112and114,116,118,120, respectively. The snubber circuit and diode shown in Figure 1 are dispensed with.The circuit shown in Figure 3 enables the polarity of the voltage applied to the storage capacitorto be varied, as well as the amplitude.

The circuit shown in Figure 3 can be arranged so that the outputvoltage is added tothe alternating mains voltage before it has been rectified. This is particularly advantageous where the power supply circuit is to be energised from three-phase mains. Figure4shows such an application in which there is such a second rectifier means 100 and a control circuit (similarto that shown in Figure 2) for each phase, only one phase being shown in detail, the othertwo being exactly similar.

In Figure4the input points are 212,214,216 connected to a three-phase AC supply 210. The phases have respective resistive shunts 218, each corresponding to the shunt 72 in Figure 1, connected between the input points and respective terminals 220. Each terminal 220 is one ofthe two output terminals 220,222 of a rectifierbridge circuit 100 ofthe kind shown in Figure 3.

The control circuitfor each phase consists of a first error amplifier 280, a multiplier 286, a second error amplifier 294 and a drive circuit 238. The drive circuit has two outputs at 240, 242 which control the rectifiers 106,108,110 and 112 in the bridge 102 and the rectifiers114,116,118and120inthebridge104, respectively, in the bridge circuit 100.

A variable reference voltage is applied to the first amplifier 280 at 282. The input 290 to the multiplier 286 is a predetermined current waveform which is a non-rectified sinusoidal waveform 292 e.g. taken directly from the respective phase where the waveform ofthe current drawn from the mains is required to remain sinusoidal and in phase with the mains voltage.

Alternatively, the waveform is a rectangular alternating waveform as indicated at 293; or is an alternating triangular waveform as at 295; or is of some other form. The main rectifier is now a three-phase bridge 226. Although only one pair of terminals Z, ZZ are shown in Figure4,thetransformer in the inverter section 24 has in fact two additional secondary windings whose terminals are connected to the othertwo second rectifier means 100.

The operation of the circuit as shown in Figure 4 is exactly analogous to that of the single-phase circuit described with reference to Figures 1 and 2. The input at 296 exactly represents the demanded phase current, including amplitude and polarity.

In a modification (not shown) the second rectifier means 50 or 100 consists of a diode bridge with an associated switching transistor. In another modification (notshown)the single transformer shown in Figure 2 is replaced by separate transformers having the respectivesecondarywindings. In another modification (not shown) the final output from the circuit can be taken from output points connected directly to the inverter output e.g. points atthe ends ofthe transformer primary.

Where it is not required to control the amplitude of the voltage added by the filter74,76 in Figure 2 or Figure 3 the error amplifier 80 or 280 in Figure 2 or Figure 4 is omitted and a constant signal is applied at the input88 or 288to the multiplier.

Where such control is required butthe shape of the waveform ofthe current drawn is not required to be maintained then the multiplier 86 or 286 and second error amplifier 94 or 294 are omitted and the output from the first amplifier is applied directly to the firing control input 102 or 302.

In a further modification (not shown) the voltage across the capacitor 22 can be used to supply a load connected to output points on either side of the capacitor, provided isolation of the load from the mains is not required.

Claims

1. A power supply circuit comprising input points, a storage capacitor, first rectifier means arranged to rectify an alternating current drawn by the circuit at the input points and to charge the storage capacitor, an inverter having its input connected to the storage capacitor, a transformer having a primary winding receiving the output of the inverter and having a secondary winding, second rectifier means arranged to rectify the output from the secondary winding, a filter arranged to app[ytheoutputvoltagefromthe second rectifier means between an input point and the storage capacitorto maintain the voltage applied to the storage capacitor above the peak mains voltage of the same polarity and a control circuit which controls the operation of the second rectifier means.

2. A circuit according to Claim 1, in which the transformer has a second secondary winding connected to output points for supplying a load.

3. A circuit according to Claim 1, in which a second transformer has a primary winding receiving the output ofthe inverter and has a secondary winding connected to output points for supplying a load.

4. A circuit according to any preceding claim, in which the filter is arranged between the first rectifier means and the storage capacitor to add said output voltage to the rectified voltage from the first rectifier means.

5. A circuit according to any claim of Claims 1 to 4, in which the filter is arranged between the input point and the first rectifier means to add said output voltage to the mains voltage.

6. A circuit according to Claim 1,2 or 3 for energisation from a three-phase mains, in which the first rectifier means is of three-phase configuration and in which there is a respective second rectifier means, a respective filter and a respective control circuit for each phase, and in which each filter is arranged between the respective phase input point and the first rectifier means to add the respective output voltage to the respective mains phase voltage.

7. A circuit according to any preceding claim, in which the, or each, control circuit comprises a multiplierwhich receives a constant input and an input representing a predetermined current waveform and an error emplifierwhich compares the current outputfrom the multiplier with actual current drawn from the storage capacitor and which produces an error signal used to control the second rectifier means.

8. A circuit according to any claim of Claims 1 to 6, in which the, or each, control circuit comprises an error amplifierwhich compares the voltage acrossthe storagecapacitorwith a referencevoltage and which produces an error signal used to control the first rectifier means and in which the reference voltage is variable so as at least to contribute to the regulation of the output ofthe power supply circuit.

9. A circuit according to any claim of Claims 1 to 6, in which the, or each, control circuit comprises a first erroramplifierwhich comparesthevoltageacrossthe storage capacitor with a reference voltage and which provides a first input to a multiplier, a predetermined currentwaveform being a second input to the multiplierwhich provides a first input to a second error amplifier, a second input to the second error amplifier representing the current drawn from the storage capacitorandthe outputfrom the second error amplifier being used to control the first rectifier means.

10. A circuit according to Claim 7 as dependent on Claim 4 or according to Claim 9 as dependent on Claim 4, in which the predetermined currentwaveform is a full-wave rectified sinusoidal waveform and in which the waveform of the current drawn from the mains is alternating, sinusoidal and in phase with the mains voltage.

11. A circuit according to Claim 7 as dependent on Claim 6, or according to Claim 9, as dependent on Claim 6, in which the predetermined current waveform is in the case of each phase alternating and sinusoidal and in which the waveform of each phase current drawn from the mains is alternating, sinusoidal and in phase with the respective mains phase voltage.

12. A power supply circuit according to Claim 1 substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

13. A power supply circuit according to Claim 1 substantially as hereinbefore described with reference to Figure 1, as modified with reference to Figure 3, and with reference to Figure 2 ofthe accompanying drawings.

14. A power supply circuit according to Claim 1 substantially as hereinbefore described with reference to Figure 1, as modified with reference to Figures 3 and 4 of the accompanying drawings.

15. A power supply circuit according to Claim 12, 13 or 14 substantially as hereinbefore described with reference to Figure 5 ofthe accompanying drawings.