AU599579B2

AU599579B2 - Dc chopped power supply from an ac voltage

Info

Publication number: AU599579B2
Application number: AU51440/85A
Authority: AU
Inventors: Daniel Jean-Louis Simon
Original assignee: Sagem SA
Current assignee: Sagem SA
Priority date: 1984-12-19
Filing date: 1985-12-18
Publication date: 1990-07-26
Anticipated expiration: 2005-12-18
Also published as: DE3544955C2; FR2575008B1; DE3544955A1; AU5144085A; IN164607B; FR2575008A1

Description

COMQMONIEALTH OF PtJSTr1PTF; 9 df PATENTS ACT1, ?'952 1.y /2) CONPLETE SPECIFICATION

(ORIGINAL)

FOP, OFFICE USE; Short Title:.

Int. C14 Application 'Nunber: Lodged: 9- Complete Spec if ic aLion-Lockred: Acceptedi Lan~sed.

Published: Priority-.

ained wade kmalcr Su :tin 49 Uft's (MucT ior Related Art.

TO BE COMPLETED BY TAPPTICIITT Name of Applicant; Address cf Applicant; Ac'L-ual Inventor2 Address for- Service, SOCIETE D'APPLICATIONS GENERALES D'ELECTRICITE ET DE ME CANI(.LJE S A G E M 6, Avernue d'Iena, 75783 Par-is Cedex 16,

FRANCE

Daniel Jean-Louis SIMON ARTHJUR S. CAVT ri CO. Patent and Trade Mark Attorneys, 1 7,fred Street, Sydney, New South W,4ales, Australia, 2000.

Complete Specification for the invention entitleO: DC CHOPPED POWER SUPPLY FROM AN AC VOLTAGE The following statement is a full descriptioit of thiq invention, including the best method of performuing it known to me.- ASC-4

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la The present invention relates to a chopped power supply comprising means for rectifying an AC mains voltage, a high frequency insulating transformer with primary and secondary windings of the same polarity, connected, at the primary, to means for rectifying the voltage of the mains and, at the secondary, to at least one load, in the primaty circuit of the transfomer, means for interrupting the voltage thereof, i.e. chopping it and, in the secondary of the transformers, means for chopping its voltage, means for smoothing the chopped voltage of the secondary and means for regulating the o«o 0 DC voltage at the terminals of the load adapted for comparing the chopped and smoothe- voltage of the secondary of the transformer with a reference voltage and controlling this chopped and smoothed voltage.

Such a DC power supply from an AC voltage using an AC-DC converter is described more particularly in French patent 2 374 768, with as regulation means, means adapted for controlling the chopping means of the primary and, as means for chopping the secondary, rectifying means.

Thi, power supply may be used in all sorts of apparatus, such for example as television sets, type writers, teleprinters.

Should the mains voltage undergo large variations, it has an efficiency which remains satisfp tory and it also accepts considerable variations of load.

The voltage at the terminals of the load is regulated and so held constant, by controlling the rate of conduction of the chopping means of the primary adapted for keeping the integral of the secondary voltage constant from one period to another. Considering the switching speed of chopping means at present available on the market, any regulated voltage may nevertheless be obtained satisfactorily with an appropriate number of turns of the windings of the high frequency pulse transformer. It is then, it is true, an isolating, but also regulating transformer.

Considering the identical polarity of the windings of the transformer, the means for chopping and rectifying the secondary voltage conduct when the chopping means of the primary also are conducting. The power supply in question is a direct energy transfer supply.

In the case where this power supply, of the above mentioned type, must feed a second load, or even several others, a second secondary winding of the transformer may be provided, having a number of turns which is still appropriate, and a circuit identical to the first one at the terminals of this second secondary transformer winding, and a good correctly regulated load voltage may still be obtained.

But if, for example, the second secondary circuit of the transformer serves for supplying with power a teleprinter printer head with very large load variations which, when it is not printing, places this second secondary in open circuit, it may happen, precisely, in this case that the choppe 1 and rectified secondary voltage persists at the terminals of the smoothing means which can no longer then play their role, the capacitor which they generally comprise no longer being able to discharge into the so called free wheel diode which they also generally comprise.

To overcome this disadvantage, either a residual load, or a Zener diode is provided for maintaining the output current above a threshold and thus preventing the output voltage from rising beyond a substantially rectilinear voltage level.

Unfortunately, correction of this disadvantage induces a cezond one. In fact, and assuming that the regulation in the first secondary circuit is quite satisfactory, regulation in the second variable load secondary circuit, because of the presence of this residual resistance or this Zener diode, cannot be satisfactory as well.

In fact, the Zener diode has a certain tolerance which prevents the coupling defect between the secondary winding of the variable load circuit and the primary winding of the transformer from being compensated for.

1 As for the residual resistance, its value must be able to vary so that the output voltage of the circuit may be stabilized. In addition, the tolerance in regulation of the second load circuit is increased because of the coupling of the two secondary windings of the transformer.

Recourse is then had to one or other of two solutions without mentioning that which consists in providing several complete power supplies. First of all, a secondary regulator may be provided in the second secondary circuit in question.

But if it is a question of a linear regulator, the power efficiency of the whole will be lowered. And if it is a question of a chopped type regulator, like the first regulator mentioned above, comprising an oscillator, the multiplicity of these oscillators will increase the cost and space occupied by the power supply.

Several converters may also be provided in parallel across the output of the means rectifying the mains voltage.

;ut in this case, if the respective chopping frequencies of the converters are different, beat problems will arise.

Besides the drawbacks of the power supply of the above mentioned type which have just been mentioned, it should be further -oted that the means controlling the meats chopping the primary voltage often use photocouplers, whose cost is high, or isolating transformers, correct coupling of which is difficult to provide, and that the coupling of several secondary windings to the primary of the pulse transformer is just as difficult to provide.

Secondary regulators are therefore all the more indispensable.

To overcome these drawbacks and at least to avoid all these secondary regulators, the Japanese patent L2 1-6-7-1-8-9- proposes a power supply in which the chopping means of the primary of the transformer have a fixed conduction rate and the means chopping the voltage of the secondary of the transformer and the regulation means are adapted so that these latter control the former.

In such a _-ower supply, the feed back loop of the secondary towards the primary is eliminated. However, ^r- 1 it the means for chopping the prinary voltage comprise a transistor which must switch a relatively high current at a relatively high voltag. The result is a lowering of the overall efficiency of the power supply and it may be necessary, in such a power supply, to provide networks assisting the switching of this transistor. In addition, the accuracy of such a power supply is low for the voltage of the secondary, adjusted rather than servo-controlled by controlling the conduction rate of the chopping means of the secondary, risks deviating from the desired value.

The present invention overcomes the preceding disadvantages.

For this, the present invention provides a chopped power supply comprising; means for rectifying an AC voltage, a high frequency isolating transformer with primary and secondary windings of the same polarity, the primary winding connected to the means Sfor rectifying to comprise a primary circuit, the primary circuit also including primary chopping means in series with the primary winding for chopping the voltage across the primary winding and having a fixed conduction rate, and the secondary winding connected to at least one load, the, or each of said at least one load, and the secondary winding comprising at least one secondary circuit, the, or each of said at least one, secondary circuit including in series with the respective load: 00 0 o a r.

secondary means for chopping the voltage across the load; 00 a .0 0 (ii) means for smoothing the chopped voltage across the load; and (iii) means for regulating the DC voltage across the terminal of the load adapted for comparing the voltage across the load with a reference voltage and controlling the secondary chopping means, characterised by the regulation means comprising a flip flop adapted for -r 4a controlling the secondary chopping means, which enables the chopping means on the rising fronts of the pulses of voltage across the load and disables it at the moment when the voltage across the load exceeds the reference voltage, the ratio between the number of turns of the primary and secondary windirigs of the transformer being so that the secondary chopping means is put in its open state earlier than the primary chopping means.

The chopping means of the secondary may advantageously 0, 0 0120k/AJG C -II I comprise an NPN or PNP type transistor, a thyristor, a switch with two stable states (GTO), or any other component with on or off operation.

In the power supply of the invention, the accuracy of the regulation no longer depends on the coupling in the transformer, but only on the accuracy of the reference voltage, on the accuracy of the comparison means and on the reaction speed of the regulation loop.

The power supply also has the advantage that several regulated load circuits delivering different voltages may be disposed in parallel across the secondary winding, without for all that having to increase the number of oscillators, a single one being provided, as in the preferred embodiment of the power supply, in the chopping means of the primary for driving a chopping transistor.

The invention will be better understood from the following description of the preferred embodiment of the power supply of the invention, with reference to the accompanying drawings in which: Figure 1 shows the circuit diagram of the regulated power supply of the invention, with a single load circuit; Figure 2 shows the diagram of the power supply of the invention with two load circuits; Figures 3 show the curves of the primary voltage and current and of the switching losses of the transistor driving the transformer of the power supply of the invention; Figures 4 show curves similar to those of Figures 3, but with advanced disabling of thie secondary transistors, and Figures 5 show curves similar to those of Figures 4 but with delayed disabling of the secondary transistors.

In Figure 1, the DC power supply of a load 1 is provided from the AC voltage of an available mains network 2. I. comprises a high frequency isolating transformer 3 -it may for example be of 30 kHz- with high mutual inductance, low leak inductance coil and primary 4 and secondary 6 windings of the same polarity. The primary 4 of transformer 3 is connected to the output of a conventional diode rectifier bridge 6, whose input is connected to mains 2.

The voltage of mains 2 rectified and applied to the primary 4 of the transformer is chopped by means of a fixed frequency oscillator 7 controlling, by its base, a chopping or switching transistor 8, so with fixed conduction rate, in series with the primary winding 4. The voltage rectified by bridge 6 is filtered by a capacitor 9.

The secondary 5 of transformer 3 is connected to load 1 through a chopping switch 10 and a smoothing circuit 11 in series. The smoothing circuit 11 comprises an induction coil 12 in series with switch 10, between a capacitor 13 and a so called free wheel protection diode 14, in parallel, diode 14, which shunts the secondary winding 5 and switch 10 in series, being disposed so as to let through only the discharge current of cappcitor 13, in parallel across load 1, when switch 10 is open.

In parallel across load 1 is connected a divider bridge comprising resistors 15, 16 of predetermined values for extracting a part of the output DC voltage 16 and comparing it with a reference voltage Vref in a comparator 17, receiving at a negative input this output voltage portion and, at a positive input, the reference voltage Vref* The output comparator 17 is connected to the reset input (RAZ) 18 of a flip flop 19 whose input is connected to the input of switch 10 and whose outpuL 21 controls switching of switch 10. The bridge 15, 16 the comparator 17 and flip flop 19 form a regulation loop 22. On a rising front of the pulses, or square waves, of the voltage at the terminals of the secondary winding of transformer 3, which pulses are generated by the switching transistor 8 of primary 4, the flip flop 19 changes state so as to close switch 10 before again changing state for opening switch 10 as soon as the extracted portion of the output voltage of the terminals of load 1 exceeds the reference voltage Vref.

7 The chopping, switching function of the switch is advantageously provided by a transistor 10, here of PNP type (Figure So that this transistor 10 is not damaged by the negative half waves of the voltage at the terminals of a secondary 5 of transformer 3, a protection diode 23 (Figure 2) is disposed in series between the secondary 5 and transistor 10. In the case where the switching function of the load circuit is provided for example by a thyristor, the protection diode would no longer be necessary. The enabled state of transistor 10 corresponds to closure of the switch and the disabled state to opening, when the energy required is transferred into coil 12.

In the case where several loads are to be supplied at different voltages, such for example as in a teleprinter comprising in particular microprocessors, a motor, a printing head, as many identical circuits are connected in parallel across the secondary winding 5 of transformer 3, but delivering different voltages, with their regulation loop, as there are loads to be supplied. In Figure 2, two load circuits have been shown with their similar elements bearing the same references as in Figure 1, those of the second circuit being designated by a prime accent. These two circuits are independent of each other and stabilization of the output voltage of one has no influence on that of the other.

The efficiency of the load circuit of Figure 1 or of one of the circuits of Figure 2 depends greatly on the switching losses of transistor 8 of the primary circuit.

Let I be the current flow ing in the primary winding 4 of transformer 3, and comprising the sum of the secondary currents brought back to the primary, and V the voltage of the terminals of transistor 8, shown in Figures 3A, 3B, as a function of time t. The switching losses in transistor 8 are illustrated by the signal P in Figure 3C, corresponding to the multiplication of signals I and V.

It is a question of not inconsiderable pulse losses, which 8 occur at each change of state t i (closure), ti+ 1 (opening or disabling), of transistor 8.

If the ratio between the number of turns of the primary winding 4 and the number of turns of the secondary winding 5 of transformer 3 is determined so that the switches or transistors 10, 10' are systematically disabled before transistor 8, and this is precisely what the regulation seeks to achieve, taking into account the coupling between the windings, and in accordance with the curves of Figures 4A, 4B, 4C, the primary current I' is already almost zero when transistor 8 is disabled and when voltage P' bugins to increase, which consi' 1 .ibly reduces the losses on opening P' of transistor 8, the primary current I' beginning to drop, before the opening of transistor 8, a first time on opening t of one of the two transistors 10, 10' and 0 2 a second time on opening t of the other of the two transistors 10, If, in addition, and using conventional delay means in the secondary circuits, the action of flip flop 19 controlling the enabled state or conduction of transistors is delayed with respect to transistor 8 and in accordance with the curves of Figures 5A, 5B, 5C, the voltaqe V" being already almost zero when transistor 8 is enabled and when the primary current I" reaches an appreciable value, the closure losses are considerably reduced, since the primary current I" only begins to increase, after closure of transistor 8, a first time on closure t F of one of the two transistors 10, 10' and a second time on closure t 2 F of the other of the two transistors 10, Thus, with advanced disabling and retarded enabling of transistors 10, 10' with respect to those of transistor 8, respectively, the switching losses become negligible.

Still in this case, the networks for assisting switching of transistor 8 when the rectified voltage of mains 2 is high may be omitted.

The power supply regulation which has just been described is of great simplicity and may be integrated ft 73 in a power chip carrier which is no larger than a linear regulator chip carrier.

Furthermore, with the regulation circuit which has just been described protection circuits may be associated, for example current limitation protection, protection against over voltages, temperature protection, etc.

Finally, a positive voltage power supply has been desc.7ibed and shown but the invention would apply just as well to the supply of negative DC voltage loads. It would be sufficient for example, for that, to reverse the terminals of the protection diodes 23, 23', those of the free wheel diodes 14, 14' and of the smoothing capacitors 13, 13' and to provide NPN chopping transistors.

Claims

1. A chopped power supply comprising; means for rectifying an AC voltage, a high frequency isolating transformer with primary and secondary windings of the same polarity, the primary winding connected to the means for rectifying to comprise a primary circuit, the primary circuit also including primary chopping meai.s in series with the primary winding for chopping the voltage across the primary winding and having a fixed conduction rate, and the secondary winding connected to at least one load, the, or each of said at least one load, and the secondary winding comprising at least one secondary circuit, the, or each of said at least one, secondary circuit including in series with the respective load: secondary means for chopping the voltage across the load; (ii) means for smoothing the chopped voltage across the load; and (iii) means lor regulating the DC voltage across the terminal of the load adapted for comparing the voltage across the load with a reference voltage and controlling the secondary chopping means, characterised by the regulation means comprising a flip flop adapted for controlling the secondary chopping means, which enables the chopping means on the rising fronts of the pulses of voltage across the load and disables it at the moment when the voltage across the load exceeds the reference voltage, the ratio between the number of turns of the primary and secondary windings of the transformer being so that the secondary chopping means is put in its open state earlier than the primary choppi.,g means.

2. The power supply according to claim 1, wherein means are provided for delaying the action of the flip flop controlling the enabled state of the secondary chopping means.

3. The power supply according to of claim 1 or claim 2, wherein the flip flop is connected by a reset input to the 11 output of a comparator receiving, on its inputs, the load voltage and the reference voltage.

4. The power sup,,y according to any one of claims 1 to 3, wherein the primary chopping means comprise a fixed frequency oscillator driving a component with on and off operation in series with the primary winding of the transformer.

The power supply according to any one of claims 1 to 4, wherein the secondary chopping means comprise a component with on and off operation.

6. The power supply according to claim 5, wherein said component is a thyristor.

7. The power supply according to claim 5, wherein said component is a transistor in series with a protection diode.

8. A power supply, substantially as herein described with reference to the drawings. DATED this 5th day of April, 1990. SOCIETE D'APPLICATIONS GENERALES D'ELECTRICITE ET DE MECANIQUE S A G E M By Its Patent Attorneys ARTHUR S. CAVE CO. 0120k/AJG