WO2000031862A2

WO2000031862A2 - Method for controlling a rectifier circuit and corresponding rectifier circuit

Info

Publication number: WO2000031862A2
Application number: PCT/DK1999/000651
Authority: WO
Inventors: Nils-Ole Harvest; Ole Bachmann
Original assignee: Danfoss Compressors Gmbh
Priority date: 1998-11-26
Filing date: 1999-11-25
Publication date: 2000-06-02
Also published as: AU1375500A; WO2000031862A3; DE19854567A1

Abstract

The invention relates to a method for controlling a rectifier circuit and to a corresponding rectifier circuit. Said rectifier circuit has a rectifier (2) in series with an inductor (4), a diode (5) and a load (6). A capacitor (7) is arranged parallel to the load (6) and a controlled switch (8) is arranged parallel to the series connection of the diode (5) and load (6). Said switch is connected to a control device (9) which produces a control signal using the voltage (U3) through the load. The aim of the invention is to improved the efficiency. The control device (9) has a phase displacement device (12) which effects a phase displacement of the alternating component of the voltage (U3) through the load in the order of magnitude of 270 degrees. This allows the direct voltage signal that is used for obtaining a control signal for the switch (8) to be superposed by an alternating voltage signal whose frequency corresponds to the frequency of the ripple at the output of the rectifier (2) and which has a phase difference in relation to the ripple that is less than a predetermined measure.

Description

Method of controlling a rectifier circuit and rectifier circuit

The invention relates to a method for controlling a rectifier circuit with a rectifier and a load, in which at least one switch for controlled maintenance of a current flow through the rectifier is actuated and the signal necessary for actuation is generated with the aid of a DC voltage signal which is generated from the voltage above the Load is determined. The invention further relates to a rectifier circuit with a rectifier in series with an inductor, a diode and a load, in which a capacitor is arranged in parallel with the load and a controlled switch is connected in parallel with the series connection of diode and load, which switch is connected to a control device which generates a control signal with the aid of the voltage across the load, and a rectifier circuit with a rectifier which is arranged in parallel with a capacitance and with a load and whose input is connected to an inductor, a switch arrangement being connected to the inductor, which has at least one controlled switch which is connected to a control device which generates a control signal using the voltage across the load.

Such a method and such a rectifier circuit are known from EP 0 669 703 A2.

With such a rectifier circuit one tries to keep the effects of the load on the rectifier on the supply network small. If you do without the switch, there is a risk that the Electricity generates impulsive effects on the supply network. The switch, on the other hand, ensures that a certain current flow through the rectifier and the subsequent inductance is always maintained. This has the advantage that the repercussions on the supply network are kept small with a view to increasing the harmonic content. The network load with harmonics drops.

In the known case, the voltage across the load, ie a smoothed DC voltage, is used to generate a control signal for the switch. This DC voltage is divided and then compared with a reference voltage. The difference between the reference voltage and the voltage applied to the load or the corresponding part is a criterion for when the switch must be actuated. At the input of the rectifier there is a current profile that is approximately trapezoidal, i.e. the current remains constant outside of a rising edge and a falling edge. This reduces the harmonics in the current and increases the efficiency.

The object of the invention is to further improve the efficiency.

This object is achieved in a method of the type mentioned at the outset by superimposing an AC voltage signal on the DC voltage signal, the frequency of which corresponds to the frequency of the ripple at the output of the rectifier and which has a phase difference to the ripple that is smaller than a predetermined measure .

For easier understanding, the stationary case is first considered, in which there is no large changes. The voltage signal used to control the switch is therefore constant. In the same way, the current at the input of the rectifier is then constant except for short sections at the beginning and end of each half-wave. If one superimposes an alternating voltage signal on the direct voltage obtained from the load voltage, the frequency of which corresponds to the ripple at the output of the rectifier and the phase position of which also, if not exactly, corresponds to the phase position of the ripple at the output of the rectifier, then one obtains one Control signal which has a ripple in synchronism with the ripple of the rectifier output voltage. If one processes such a signal, for example, in the same way as is known from EP 0 669 703 A2, then a current profile is obtained which is no longer constant over a half-wave, but in the middle of each half-wave or Half period is "bulged", that is, it has an arcuate shape that is more closely approximated to a sinusoidal shape than the rectangular or trapezoidal shape. This gives the current through the inductance a greater similarity to the sinusoidal shape. The result is an even closer approximation to a power factor of 1, ie the efficiency of the circuit is further improved.

The alternating voltage signal is preferably obtained with the aid of the ripple. If one uses the ripple directly to obtain the AC signal, one ensures that the AC signal has the same frequency as the ripple. The ripple has a double frequency for a full-wave rectifier and the same frequency for a half-wave rectifier as the supply system. In addition, if the AC signal is generated from the ripple is won, a corresponding phase relationship is certain, so that the effort in further processing is kept low.

The AC voltage signal is preferably obtained with the aid of the voltage across the load. In this way, the load voltage is used twice, namely on the one hand to obtain a criterion for when the switch should be actuated and on the other hand to obtain the AC voltage signal. A second voltage tap is therefore no longer necessary.

The voltage across the load is preferably inverted and subjected to a further phase shift by 90 degrees. The load is usually connected in parallel with a capacitance, which contributes to smoothing the output voltage of the rectifier. This capacitance, for example a capacitor, leads to a phase shift of the ripple by 90 degrees. If you invert the voltage across the load, there is a further phase shift of 180 degrees. With a further phase shift of approximately 90 degrees, a phase shift of 360 degrees is achieved overall, or in other words, the phase positions match. The deviation from one period is in any case irrelevant in the stationary case. In the case of - load changes, there are usually no major negative effects to fear.

The phase shift is preferably determined by a

Filtering achieved. A filter usually has a phase shift. You can now set the cutoff or cutoff frequency of the filter so that exactly the desired phase shift is achieved. This is possible because the frequency of the ripple is known. An RC filter is preferably used for filtering. This is a relatively simple configuration that can be implemented with a few components.

In an advantageous embodiment it is provided that the switch is kept open in predetermined operating states. This configuration also leads to an improvement in the efficiency if no AC voltage signal is superimposed on the DC voltage signal for controlling the switch. If the switch is kept open, no current flows "unused" past the load. The efficiency increases accordingly.

A negative reaction of the harmonics to the mains current is not to be feared in particular if the predetermined operating state is defined by falling below a predetermined load. In this case, the amplitude of the current is low. Since the amplitudes of the harmonics are related to the current amplitude, a lowering of the current amplitude also results in a lowering of the harmonic amplitudes. In normal operation, in particular, when a starting process has been completed, power factor correction can often be dispensed with. For example, if the rectifier circuit is used to supply a compressor in a small refrigerator, much more power is required when starting the machine than in steady-state or continuous operation. This also applies if the load is not generated directly by the motor, but by an inverter, which in turn drives the motor with frequency control.

The problem is solved in a rectifier circuit of the type mentioned at the outset in that the control device has a phase shift device which causes a phase shift of the alternating component of the voltage over the load in the order of 270 degrees.

As explained above in connection with the method, the capacitor or the capacitance, which is connected in parallel with the load, already causes a phase rotation of the ripple by 90 degrees. If you add another 270 degrees, you get a phase shift of 360 degrees or - in steady operation - a phase shift of zero. The phase positions do not have to match exactly here. It is sufficient if a predetermined difference, for example 15 degrees, is not exceeded. Within such limits, the current profile of a sinusoidal shape is similar enough to keep the harmonic content low. This procedure is largely independent of the specific type of rectifier circuit used.

The phase shift device preferably has an inverter and a filter. The inverter causes a phase shift of 180 degrees. The filter is used for the 90 degrees still required. This can be achieved in a simple manner in that the cutoff frequency or the cutoff frequency of the filter is matched to the frequency of the ripple or the alternating component of the voltage across the load in such a way that the corresponding phase shift results.

The filter is preferably designed as an RC filter. Such a filter can be conveniently accommodated, for example, in the feedback of an amplifier that is used as an inverter. In a preferred embodiment it is provided that a load measuring device is provided which deactivates the switch when the load falls below a predetermined value. This load measuring device can also be used without the phase shift device. It has the effect that when the current amplitudes and thus also the amplitudes of the harmonics of the current are small, the power factor correction is switched off.

The invention is described below with reference to a preferred embodiment in connection with the drawing. Show here:

1 is a schematic representation of a circuit arrangement,

Fig. 2 different curves to illustrate current and voltage profiles and

Fig. 3 is a schematic representation of a modified circuit arrangement.

1 shows a circuit arrangement 1 with a rectifier 2, which in the present case is designed as a full-wave rectifier. The rectifier 2 is fed by a network 3, for example an AC network with a frequency of 50 Hz and a voltage U1 of, for example, 230 V. The current of the network 3 is accordingly an AC II.

The output of the rectifier 2 is connected to a load 6 via a coil 4 or a corresponding inductor and a diode 5. A capacitor 7 or another capacitance is arranged parallel to the load 6. The load 6 can be designed more or less arbitrarily. It can also be formed, for example, by an inverter, which in turn feeds an electric motor in a frequency-controlled manner, wherein the motor can be used, for example, to drive a compressor of a small refrigerator.

Accordingly, a voltage U2 and a current 12 result at the output of the rectifier 2.

Without additional measures, the current would charge the capacitor 7 up to the voltage U2. Since the diode 5 only allows the current to pass when the voltage U2 is greater than the voltage U3 across the load or the capacitor 7, there is no uniform flow of the current 12 without additional measures. Rather, this only becomes in the middle of each Half wave flow, which can have negative effects on the mains current II. There are then undesirably high harmonic contents.

In order to counteract this phenomenon, a switch 8 is arranged in parallel to the series connection of diode 5 and the parallel connection of load 6 and capacitor 7, which is controlled by means of a control device 9, i.e. the control device 9 opens and closes the switch. This makes it possible to maintain a current flow even in periods of time in which the voltage U2 is less than the voltage U3. This approach is basically over

EP 0 669 703 A2 known. Such a circuit is also referred to as a "boost converter" or "boost converter".

The control device 9 has a current detection device 10, which is equipped with an evaluation device 11 connected is. The current detection device 10 detects the current through the switch 8.

Furthermore, the control device 9 has a voltage detection device 12 which determines the voltage U3. The voltage detection device has a voltage divider 13, which is formed, for example, by two ohmic resistors 14, 15. Accordingly, a voltage U4 is present at the center tap 16 of the voltage divider 13 and is supplied to the inverting input of an amplifier 17. The non-inverting input of the amplifier 17 is connected to a reference voltage source 18, which emits a DC voltage. A voltage U5 is then established between the inverting input and the non-inverting input of the amplifier 17.

The output 19 of the amplifier 17 is connected to the inverting input via a parallel circuit comprising a resistor R and a capacitor C. Accordingly, the voltage detection device 12 not only has an inverter, but also an RC filter. At the output 19 of the voltage detection device 12, which is identical to the output of the amplifier 17, a voltage U6 results which is fed to a comparator device 20. The comparator device 20 compares the output voltage U7 of the current evaluation device 11 with the voltage U6 of the voltage detection device 12. When the voltage U7 has risen to the value of the voltage U6, the switch is opened. The switch can be closed again a predetermined time after the switch has been opened.

2 shows individual voltage and current profiles in different representations. 2a shows the line voltage U1 and the line current II over a full period, which is 20 ms at a frequency of 50 Hz. In Fig. 2a, the output voltage U2 of the rectifier 2 is also entered, which corresponds to Ul in the first half period.

Figure 2b shows the voltage U3, i.e. the voltage across the load 6. It can be seen that the voltage U3 is a DC voltage with a ripple which has a frequency twice as high as the voltage U1. The ripple of the voltage U3 is phase-shifted by approximately 90 degrees compared to the ripple of the voltage U2, which is not shown in any more detail. This phase shift results in particular from the capacitance 7.

Fig. 2c now shows the voltage U5, i.e. the difference between the voltage of the reference voltage source 18 and the voltage U3. This difference is shifted by 180 degrees with respect to the ripple of the voltage U3. 2c shows the voltage U6 at the output 19 of the voltage detection device 12. Because of the RC element, the voltage U6 is again 90 degrees out of phase with the voltage U5. The voltage U6 is thus in phase with the voltage U2 at the output of the rectifier 2.

If the voltage U6 is now used to control the switch 8, then the mains current II takes on a shape which, from a rectangular block or trapezoidal block, as is known from EP 0 669 703 A2, more closely approximates a sinusoidal shape, as is the case with this 2a can be seen. It is not absolutely necessary here that the current II is exactly in phase with the voltage Ul. Accordingly, it is not necessary derlich that the voltage U6 is exactly in phase with the voltage U2. Smaller deviations of 15 degrees, for example, are entirely permissible. However, by approximating the current II to a sinusoidal curve, the harmonic content of the current II can be further reduced.

A further current detection device 21 is arranged in the line between the load 6 and the rectifier 2. If the current detector 21 determines that the load drops below a predetermined value, then the switch 8 can be deactivated, i.e. it remains permanently open. This has the advantage that no electricity is wasted. The resulting higher harmonic content of the mains current II can be accepted, because with low amplitudes of the mains current II the amplitudes of the harmonics also remain correspondingly small.

Fig. 3 shows an alternative embodiment in which the same parts as in Fig. 1 are provided with the same reference numerals and corresponding parts with deleted reference numerals.

1 had the coil 4 arranged behind the rectifier 2. 3, the coil 4 'is arranged in front of the rectifier, which can be formed, for example, by two diodes D1, D2.

Accordingly, the switch 8 has been replaced by two switches 8 ', 8 ", which are also controlled by the control device 9', in the same way as the switch 8 in the circuit arrangement according to FIG. 1. 3, the rectifier and the step-up converter are therefore combined. This requires fewer components and a more compact design. In addition, the advantages of this circuit are that there are fewer line losses. In the circuit arrangement according to FIG. 3 there are only two diode sections in the rectifier, and the diode 5 is omitted. Furthermore, the coil 4 'can now be designed for alternating current. A design for direct current with ripple, as in the embodiment according to FIG. 1, is not necessary.

In addition, the switches 8 ', 8 "are connected to diodes Da, Db in parallel. Switches with diodes are available on the market as ready-made MOSFETs. These diodes are often already in the form of parasitic diodes.

Only very slight changes are necessary for the control device 9 '. For example, the comparator device 20 should now have two outputs in order to be able to control both switches 8 ¹ , 8 ". The two switches 8 ', 8" can be clocked synchronously. Alternatively, sequential control is also possible: If the first half-wave D1 is made conductive, the switch 8 'is switched. In the other half-wave, which acts on D2, the switch 8 "is clocked. For example, the current sensor 21 'can be used as the current sensor.

Claims

claims

1. A method for controlling a rectifier circuit with a rectifier and a load, in which at least one switch for the controlled maintenance of a current flow through the rectifier is actuated and the signal required for actuation is generated with the aid of a DC voltage signal which is determined from the voltage across the load is characterized in that an alternating voltage signal is superimposed on the direct voltage signal, the frequency of which coincides with the frequency of the ripple at the output of the rectifier and which has a phase difference to the ripple which is smaller than a predetermined measure.

2. The method according to claim 1, characterized in that the AC voltage signal is obtained with the aid of the ripple.

3. The method according to claim 2, characterized in that the AC voltage signal is obtained with the aid of the voltage across the load.

Method according to Claim 3, characterized in that the voltage across the load is inverted and subjected to a further phase shift by 90 degrees.

Method according to claim 4, characterized in that the phase shift is achieved by filtering.

6. The method according to claim 5, characterized in that an RC filter is used for filtering.

7. The method according to the preamble of claim 1 or one of claims 1 to 6, characterized in that the switch is kept open at predetermined operating states.

8. The method according to claim 7, characterized in that the predetermined operating state is defined by falling below a predetermined load.

9. Rectifier circuit with a rectifier in series with an inductor, a diode and one

Load, in which a capacitance is arranged parallel to the load and a controlled switch is arranged parallel to the series connection of diode and load, which is connected to a control device which generates a control signal with the aid of the voltage across the load, characterized in that the control device (9 ) has a phase shift device (12) which effects a phase shift of the alternating component of the voltage (U3) over the load in the order of 270 degrees.

10. Rectifier circuit with a rectifier, which is arranged in parallel with a capacitance and a load, and whose input is connected to an inductor, a switch arrangement being connected to the inductor, which has at least one controlled switch which is connected to a control device is connected, which generates a control signal using the voltage across the load, characterized in that the control device (9 ') has a phase shift device (12) which causes a phase shift of the alternating component of the voltage (U3) over the load in the order of 270 degrees.

11. The circuit according to claim 9 or 10, characterized in that the phase shift device (12) has an inverter (17) and a filter (RC).

12. Circuit according to claim 10, characterized in that the filter is designed as an RC filter.

13. Circuit according to one of claims 9 to 12, characterized in that a diode (Da, Db) is arranged in parallel with the switch (8, 8 ', 8 ").

14. Circuit according to the preamble of claim 9 or 10 or one of claims 9 to 12, characterized in that a load measuring device

(21) is provided which deactivates the switch (8) when the load falls below a predetermined level.