WO1993021683A1

WO1993021683A1 - Voltage transformer

Info

Publication number: WO1993021683A1
Application number: PCT/DE1993/000349
Authority: WO
Inventors: Harry Gaus
Original assignee: Industrie Automation Mikroelektronik Gmbh & Co.
Priority date: 1992-04-21
Filing date: 1993-04-17
Publication date: 1993-10-28
Also published as: DE4213096A1

Abstract

The voltage transformer described has a rectifier for an a.c. input voltage and a switching converter which is pulsed by a pulse-width-modulated control signal at a significantly higher frequency than the a.c. input voltage. The invention calls for the rectified a.c. input voltage to be fed to the switching converter without using an energy-storage device (reservoir capacitor) acting at the frequency of the a.c. input voltage. For pulse-width modulation of the control signal, a controller is fitted to which are fed (a) the value of the current (Iactual?) taken from the switching converter and (b) the value of an at least approximately sine?2-shaped reference signal (Iref?), the at least approximately sine?2-shaped signal being synchronized with the a.c. input voltage.

Description

Voltage converter

The invention relates to a voltage converter with a rectifier for an input AC voltage and a switching converter which is clocked with a pulse-width-modulated control signal at a frequency substantially higher than that of the input AC voltage.

Various voltage converters have become known for converting AC line voltage into any DC voltage. According to the prior art, voltage converters of the type mentioned have a rectifier and a large capacitor for removing and temporarily storing the energy removed from the network. The capacitor is also known as a DC link. The actual converter, for which many different circuit principles are known, is connected downstream of this intermediate circuit. Common to all is that the DC voltage is converted into a high-frequency AC voltage by at least one semiconductor circuit and this is then converted to a desired DC output voltage by means of general AC and control technology.

In the known voltage converters with the DC voltage intermediate circuit, however, there are various disadvantages. On the one hand, a large starting current flows when the converter is connected to the network. The other is Power factor small and capacitive. Finally, very high short-term current pulses are taken from the network. The two last-mentioned disadvantages mean that, given a current capacity of the network of, for example, 16 A _ff, only a small active power can be drawn. This limitation of the active power interferes, for example, with chargers for batteries on electric vehicles, so-called on-board chargers, in which as much energy as possible is to be loaded back into the battery.

Voltage converters have become known which draw an approximately sinusoidal current with a power factor of almost 1 from the mains (US Pat. No. 4,677,366). With these voltage converters, an active sine filter is connected upstream of the actual switching converter. This filter consists of a rectifier, an auxiliary capacitor that is much smaller than the usual charging capacitors, and a step-up converter that stores the sinusoidal current drawn in an intermediate circuit capacitor.

The active converter, which can be constructed according to one of the known circuit principles, is connected downstream of the active sine filter, which provides a constant voltage at its output which is higher than the peak voltage of the mains voltage. This known voltage converter thus draws an approximately sinusoidal current from the network with a favorable power factor, but two separately clocked stages with control devices and an intermediate circuit capacitor are required, which means a large space requirement and a heat sensitivity. In addition, as with the other known voltage converters with an intermediate circuit capacitor, a large starting current flows because no current limitation by the step-up converter is possible. Special start-up circuits are required to protect the rectifier or the mains fuse. The object of the present invention is to provide, while avoiding these further disadvantages, a voltage converter which is particularly suitable as a battery charger, preferably as a so-called on-board charger.

The voltage converter according to the invention is characterized in that the rectified input AC voltage can be fed to the switching converter without using an energy store (charging capacitor) which is effective at the frequency of the input AC voltage and that a controller is provided for pulse width modulation of the control signal, on the one hand the value of the switching converter withdrawn current (I.,) and on the other hand an at least approximately sinusoidal ² -shaped setpoint (I ,,) can be supplied, the at least approximately sinusoidal ² -shaped curve being synchronized with the AC input voltage.

The voltage converter according to the invention has the advantage that the current drawn from the mains is practically sinusoidal and in phase with the mains voltage, and is particularly suitable for battery chargers.

A further development of the invention enables control of the output voltage in that the setpoint is formed by a signal with an at least approximately sinusoidal ² -shaped curve, the amplitude of which is dependent on the output variable of a further regulator, and in that the further regulator, on the one hand, the output voltage of the switching converter and on the other hand, a voltage setpoint can be supplied.

Another development is that the output variable of the further controller is sampled at the time the output current passes through zero and the value obtained by the sampling is followed by the following Sampling is saved. This ensures when charging a battery over longer lines that the voltage of the battery is measured without the voltage drop in the lines.

Charging characteristics usually contain several sections with constant voltage and constant current of various sizes. According to an advantageous embodiment of this development, a limit value for the current can be set by limiting the output variable or the sampled and stored output variable of the further controller.

Another development of the invention consists in the fact that the at least approximately sinusoidal ^2- shaped course of the setpoint and possibly further variables are derived by a processor according to a suitable program. Suitable processors are known per se and can be implemented, for example, by microcomputers or microprocessors.

An advantageous embodiment of the voltage converter according to the invention, in which the switching converter comprises an isolating transformer with a primary winding and a secondary winding, consists in that the output variable of the regulator via means for electrically isolated

Signal transmission (transformer) is fed to a pulse width modulator for generating the control signal and that a signal derived from the mains alternating voltage is fed to an input of the processor for synchronization via further means for electrically isolated signal transmission (transformer). This ensures complete network separation. A good approximation of the input current to the desired sinusoidal shape results with a relatively simple generation of the profile of the current setpoint in that the current setpoint has an essentially trapezoidal profile.

According to another development, it is provided that the current setpoint takes the value 0 in a predetermined range around the zero crossing of the AC input voltage, preferably ± 30 °. This avoids undefined conditions in the area of the zero crossings.

A simple way of generating a trapezoidal current setpoint value is that a processor can control a number of analog switches which connect the setpoint input of the controller and the input of the integrator to fixed potentials during time intervals output by the processor.

Exemplary embodiments of the invention are shown in the drawing using several figures and are explained in more detail in the following description. It shows:

1 shows a first embodiment,

2 time diagrams of currents and voltages in the first embodiment with a sinusoidal ² current setpoint,

3 time diagrams in the first embodiment with a trapezoidal profile of the current setpoint,

4 time diagrams to illustrate the current flow angle in a known voltage converter, Fig. 5 shows a second embodiment and

Fig. 6 timing diagrams in the second embodiment.

Identical parts are provided with the same reference symbols in the figures.

In the first exemplary embodiment according to FIG. 1, the input AC voltage of, for example, 230 V _ff input terminals 1, 2 can be fed in. A rectifier circuit 3 follows, the outputs of which are connected to a capacitor 4. This capacitor is dimensioned such that it which can supply energy for one clock cycle at a time, but does not maintain a sufficient voltage over a period of the input AC voltage, the DC voltage thus pulsating is supplied to a switching converter 5. Since switching converters of this type are known per se, only those for understanding the invention are shown in FIG necessary parts of the switching converter 5 are shown, namely a power transistor 6, ^" a transformer 7 with a primary winding 8 and a secondary winding 9.

One end of the secondary winding 9 is connected to an output terminal 11 via a rectifier 10, while a current measuring resistor 13 is connected between the other end of the secondary winding 9 and another output terminal 12. A capacitor 14 is provided to suppress high-frequency voltage peaks.

For safety reasons, a potential separation is provided between the primary part and the secondary part of the switching converter, which is shown in FIG. 1 as a dashed line 15. A pulse width modulator 16, which generates a control signal for the power transistor 6 in that clock pulses, is used to control the power transistor 6 predetermined frequency can be modulated with a control voltage supplied to the pulse width modulator 16. The frequency of the clock pulses is, for example, 50 kHz and, depending on the circumstances, can also be significantly higher or lower.

A current regulator 17 is provided in order to achieve the greatest possible sinusoidal current load, the inputs of which are supplied as the actual value, the current measured by the current measuring resistor 13 and a setpoint value I -,. A further regulator 18, which forms a control loop which is superimposed on the current regulator, serves to regulate the output voltage. The regulator 18 is also called voltage regulator in the following. The output voltage U.. and a setpoint U ,, supplied. The output voltage of the voltage regulator 18 is guided to be explained later influences at 19 and 20 to a multiplier 21, multiplied by the aid of a sine ² -shaped voltage and the current controller 17 is fed as a current setpoint. To generate the voltage command value U - ,, and the sine ² -shaped waveform of the desired current value I - ,, a processor 22 is provided. This gives calculated instantaneous values in the form of pulse-width-modulated pulses PWM via an output 23, 24. The pulse-width-modulated pulses are each converted into a continuous voltage profile in an integration circuit 25, 26, U,, essentially forming a DC voltage, the change over time, for example according to the requirements of the respective charging process.

To synchronize the processor 22, a further rectifier circuit 28 is connected to the input terminals 1, 2 via a resistor, which operates a light-emitting diode 29 in a pulsating manner. Together with a phototransistor 30, this forms an optocoupler, from the pulses reach an input 31 of the processor 22 and synchronize it. Because of the required po ential separation, the output voltage of the current regulator 17 is also supplied to the control input of the pulse width modulator 16 via an optocoupler 32, 33.

Due to the action of the control loop formed by the current regulator 17, the pulse width modulator 16, the switching converter 5 and the current measuring resistor 13, the output current I. assumes the time profile of I ,,. It is assumed that the control loop is fast enough to follow the specification. In the case of the sinusoidal ^2- shaped specification sketched in FIG. 1 by the processor 22, the curve shape shown in FIG. 2, line a results. As already mentioned at the beginning, such a curve shape is quite suitable for charging batteries. The resulting fluctuations in the output voltage are shown in FIG. 2, line b.

To control the output voltage in the voltage converter according to FIG. 1, during the zero crossings of the AC input voltage, the output voltage of the voltage regulator 18 is sampled with the aid of the switch 19 (line c in FIG. 2) and stored in a capacitor 34 until the next sampling time. This ensures that the output voltage is measured when there is no current flow. When using longer supply lines between the terminals 11, 12 to the battery, the voltage actually present on the battery is measured.

Charging characteristics of batteries usually contain several sections with constant voltage or constant current of different sizes. The target voltage has already been mentioned been. The target current or its limit can be output by a further processor output 36 in the form of a pulse-width-modulated signal and then integrated at 37. A limiter circuit 20 is controlled with the output voltage of the integrator 37 in such a way that the size of the multiplicant generated by the voltage regulator 18 for the target current is arbitrarily limited.

Lines a to c of FIG. 2 have already been mentioned in connection with the explanation of FIG. 1. Line d represents the course of the input AC voltage U. (solid line) and the course of the likewise sinusoidal E Eininnggaannggss - WWeecchhsseellssttrroommss II .. ((ggeessttrriicchheellttee LLiinniiee)) ddaa: r. Fig. 2 also contains a mlattlematical derivation of the EEiinnggaannggssssttrroommss II .., which shows that the input current is sinusoidal.

In the case of voltage converters according to the invention which are implemented in practice, it may be expedient to deviate from a sinusoidal ^2- shaped output current. On the one hand, this can mean a reduction in effort. On the other hand, undefined states can be avoided by a current gap in the range of, for example, ± 30 ° around the zero crossing. For example, with the trapezoidal shape shown in FIG. 3, line b, good results can be achieved with regard to the current flow angle and the power factor. Such a trapezoidal shape can be produced in a simple manner by supplying an integrator with pulses with the shape shown in line a. Line c represents the input current in this case compared to a sinusoidal input current (dashed). The differences between the two current forms are hatched in line c. In comparison, the pulsating DC voltage U and the input current I in the case of a known voltage converter with a in FIG

DC link capacitor shown. As can be seen from the drawing, there is a current flow angle of only about 30 ° with a power factor of 0.6 to 0.7. The second exemplary embodiment is explained in the following with reference to FIGS. 5 and 6, wherein essentially differences from the exemplary embodiment according to FIG. 1 will be discussed.

The switching converter is in the exemplary embodiment according to FIG. 5 in a push-pull arrangement with so-called

Current mode control executed. A modulator 41 for controlling the power transistors 42, 43, a current measuring resistor 44, a transformer 45 and two rectifiers 46, 47 are shown as essential parts of the switching converter. The positive output is connected to the output terminal 11 via a choke 48, while the output terminal 12 is connected via the current measuring resistor 13 to the center tap of the secondary winding of the transformer 45. The use of the push-pull arrangement with so-called current mode control is particularly advantageous for the regulation of a large input voltage range. The input voltage of the switching converter can assume values from 0 V to approximately 350 V corresponding to a half-sine wave.

The small capacitor 4 is only intended to deliver the impulse flow in the microsecond range, but not to smooth the 100 Hz ripple. For a given output voltage, the voltage threshold above which energy can be drawn from the network depends only on the transformation ratio of the transformer and on the maximum duty cycle of the control signal generated by the modulator 41. The duty cycle of the push-pull converter is twice that of a single-ended converter. Therefore the push-pull converter is particularly well suited for an unsmoothed input voltage.

For the supply of the modulator 41, a voltage supply circuit 49 is connected to the rectifier circuit 3, which generates a smoothed and preferably also stabilized DC voltage in a manner known per se. However, since the current requirement for the modulator is quite low, the pulse-shaped current draw by the voltage supply circuit 49 is not disadvantageous.

5, a processor 50 is provided which has three inputs 51, 52, 53 for analog signals and three outputs 54, 55, 56 for binary signals. The secondary-side circuits including the processor 50 and the current regulator 17 are supplied with operating voltage U_ by a voltage supply circuit 57, the feeds not being shown in detail in FIG. 5. The voltage supply circuit 57 also generates a reference voltage U _f , which is fed to the input 53 of the processor 50.

The input 52 of the processor 50 receives, via a voltage divider 58, the output voltage of the voltage converter as the actual value U.,. After the voltage drop across the current measuring resistor 13 has been amplified with the aid of an amplifier 59, the current actual value I, an input of the current regulator 17 and the input 51 of the processor 50 are supplied.

Since the battery voltage - that is, the output voltage of the voltage converter - can only change relatively slowly, the voltage regulation in the exemplary embodiment according to FIG. 5 is in the form of a PI algorithm in the processor, for example executed. This algorithm therefore essentially replaces the circuit parts 18, 19, 20 and 21 in the

Embodiment according to Fig. 1. The result of

Voltage regulation affects the amplitude of the

Current setpoint isol, l,.

In the exemplary embodiment according to FIG. 5, three controllable switches 61, 62, 63 and an integrator 64 to 68 are provided to form an approximately sinusoidal ^2- shaped current setpoint. The controllable switches are controlled via the outputs 54 to 56 of the processor 50. The integrator consists of a differential amplifier 64, to whose right-inverting input a reference voltage of the size U / 2 is supplied. The inverting input is connected via a resistor 67, 68 to each of the controllable switches 62, 63 and via a capacitor 66 to the output. The output of the differential amplifier 64 is connected via a resistor 65 to the input of the current regulator 17 provided for the current setpoint. This input can be connected to ground via the controllable switch 61.

The interaction of the digital signals at the outputs 54 to 56 of the processor 50 with the controllable switches 61 to 63 and the integrator to form the approximately sinusoidal ² current setpoint is explained in more detail below with reference to FIG. 6. Line a shows the signal at the output 54 and thus the switching state of the switch 61, "1" being conductive and "0" being non-conductive. This sets I •, - to 0 during a phase angle from -30 ° to + 30 ° or 150 ° to 210 °.

The timing and width of the pulses of the signal at 54 are independent of the output value of the voltage regulator. Line b of FIG. 6 shows the output signal at 55 and, accordingly, the position of the switch 62. If the switch 62 is closed, the integrator becomes one Reference voltage U supplied, so that the output signal of the integrator drops linearly. During the closing time of switch 63, the integrator input is connected to ground, which causes a linear increase in the output voltage of the integrator. If both switches are not conductive, the output voltage of the integrator maintains the value reached in each case.

The time shift of the signals at 55 and 56 in the direction of the arrow reduces the amplitude of the curve shape shown in line d of FIG. 6. Line e shows the output current at full load, compared to the desired se sine ² -shaped profile. The approximation is pretty good.

Line f shows the resulting current consumption from the network, the phase shift being 0 - that is, no reactive current is drawn. The slight deviation from the ideal shape is meaningless. In comparison to a conventional voltage converter with a power factor (cos phi) of 0.6 to 0.7 and a current flow angle of approximately 30 °, the advantage of the voltage converter according to the invention is considerable.

If the gap around the zero crossing, in which energy is not drawn from the network, is to be narrowed further, the outlay on the transformer rises sharply due to an increasingly unfavorable transmission ratio and for radio interference suppression due to increasing pulse amplitudes. The curve shapes shown in lines e and f of FIG. 6 represent favorable compromises.

Claims

Expectations

1.Voltage converter with a rectifier for an input AC voltage and a switching converter which is clocked with a pulse-width-modulated control signal of a frequency substantially higher than that of the input AC voltage, characterized in that the switching converter has the rectified input AC voltage without using one at the frequency the input AC voltage effective energy storage (charging capacitor) can be supplied and that a controller is provided for pulse width modulation of the control signal, which on the one hand the value of the current drawn from the switching converter (I..) and on the other hand an at least approximately sinusoidal ² -shaped setpoint

(I ,,) can be supplied, the at least approximately sinusoidal ^2- shaped curve being synchronized with the input AC voltage.

2. Voltage converter according to claim 1, characterized in that the setpoint is formed by a signal with at least approximately sinusoidal ² -shaped course, the amplitude of which depends on the output variable of a further regulator, and that the further regulator on the one hand the output voltage of the switching converter and on the other hand a voltage setpoint can be supplied.

3. Voltage converter according to claim 2, characterized in that the output variable of the further controller is sampled at the time of the zero crossing of the output current and the value obtained by the sampling up to following scan is saved.

4. Voltage converter according to one of claims 2 or 3, characterized in that the output variable or the sampled and stored output variable of the further controller is limited.

5. Voltage converter according to one of the preceding claims, characterized in that the at least approximately sinusoidal ^2- shaped course of the setpoint and, if appropriate, further variables are derived from a processor according to a suitable program.

6. Voltage converter according to one of the preceding claims, in which the switching converter comprises an isolating transformer with a primary winding and a secondary winding, characterized in that the output variable of the controller is supplied via means for electrically isolated signal transmission (transformer) to a pulse width modulator for generating the control signal and that further means for electrically isolated signal transmission (transformer) a signal derived from the mains alternating voltage is fed to an input of the processor for synchronization.

7. Voltage converter according to claim 6, characterized in that the primary winding is connected to a push-pull circuit.

8. Voltage converter according to claim 6, characterized in that the primary winding is connected to a bridge circuit of four transistors.

9. Voltage converter according to one of claims 2 to 8, characterized in that the voltage regulator is implemented by a suitable processor program.

10. Voltage converter according to one of the preceding claims, characterized in that the current setpoint has approximately a sinusoidal ^2- shaped course.

11. Voltage converter according to one of claims 1 to 9, characterized in that the current setpoint has a substantially trapezoidal shape.

12. Voltage converter according to claim 11, characterized in that the current setpoint in a predetermined range around the zero crossing of the input AC voltage, preferably ± 30 °, assumes the value 0.

13. Voltage converter according to one of claims 11 or 12, characterized in that a plurality of analog switches can be controlled by a processor, which connect the setpoint input of the controller and the input of the integrator during time intervals output by the processor with fixed potentials.