WO2000033451A1

WO2000033451A1 - Dc/dc converter with multiple inputs

Info

Publication number: WO2000033451A1
Application number: PCT/DK1999/000636
Authority: WO
Inventors: Bjarne Jensen
Original assignee: Bjarne Jensen
Priority date: 1998-12-01
Filing date: 1999-11-19
Publication date: 2000-06-08
Also published as: DK174329B1; AU1263900A; DK199801581A

Abstract

The present invention relates to a converting unit for converting two or more DC voltage levels from the input of the converting unit to a DC voltage on the output of the converting unit, wherein the converting unit comprises controllable switch means capable of connecting and disconnecting the individual DC input voltage level to form an oscillation signal, and wherein the converting unit comprises filtering means for low-pass filtering of the oscillation signal to form the DC voltage on the output of the converting unit. The filtering means comprise a self-induction and a capacitance, and the self-induction is positioned between the switch means and the output so that when the oscillation signal changes from a first voltage level to a second lower voltage level, the self-induction obtains energy from the energy source with the second voltage level because of the stored magnetic energy of the self-induction.

Description

DC/DC CONVERTER WITH MULTIPLE INPUTS

The present invention relates to a converting unit for converting two or more DC voltage levels from the input of the converting unit to a DC voltage on the output of the converting unit, wherein the converting unit comprises controllable switch means capable of connecting and disconnecting the individual DC input voltage level to form an oscillation signal, and wherein the converting unit comprises filtering means for low-pass filtering of the oscillation signal to form the DC voltage on the output of the converting unit. The invention also relates to a method, described in claims 7-9, of converting two or more DC voltage levels to a DC voltage.

In connection with the use of voltage sources supplied by energy sources with varying energy levels, such as e.g. alternative energy sources, such as solar cell panels, it is necessary to use a supplementary voltage supply in or- der to be able to supply a permanent constant voltage level .

One way of providing this supplement is to use exclusively the one or the other supply depending on the en- ergy level of the supply. This, however, means that the energy from the alternative energy source is not utilized optimally. Thus, it is interesting to use a converting unit which gives a more optimum utilization of the energy in the alternative energy source .

The publication GB 2 080 639 describes a switching method for voltage step-up by means of a resonance circuit consisting of inductances and capacitors. This document, however, just describes uptake of energy from a voltage source. The publications JP 7 163 144 A, US 3 769 571 and US 5 781 419 describe DC-DC converting units which can convert several DC input voltage levels to one DC voltage via transformers. This conversion method is relatively expen- sive inter alia because of the transformers. In addition, the conversion method involves a need for a large number of components, which results in a low efficiency because of energy losses in the components.

The invention provides a converting unit which solves the above-mentioned problems .

This is achieved according to the invention in that the filtering means of the converting unit comprise a self- induction and a capacitance, and that the self-induction is positioned between the switch means and the output so that, when the oscillation signal changes from a first voltage level to a second lower voltage level, the self- induction obtains energy from the energy source with the second voltage level because of the stored magnetic energy of the self-induction. It is hereby possible to use a self-induction which is relatively inexpensive with respect to the transformers. Furthermore, this gives a simple solution which requires relatively few components, thereby providing a greater efficiency.

In a special embodiment, the unit comprises a control loop which, on the basis of a comparison between a value of the output voltage and a reference value, can form an output voltage corresponding to the reference value, by changing the duty cycle of the square-wave signal. It is hereby possible to form an output voltage which is stable even though the DC input voltage levels vary.

In a further embodiment, the converting unit comprises switching means capable of switching between first and second DC input voltage levels, so that the first voltage level may be disconnected and the second voltage level be connected to the self-induction via the switch means, said switching means being a relay in one embodiment. It is hereby possible to use just the one voltage level in the event that it originates from an alternative energy source which has enough energy to maintain the output voltage alone.

In an embodiment, the switching means comprise detecting means for detecting the first and second input voltage levels, and means for automatic switching depending on whether the input voltage levels are higher or lower than the desired output voltage. Thus, the switching may take place automatically.

In a special embodiment, the converting unit comprises a step-up unit, e.g. an SMPS DC-DC converting unit of the step-up type, which, when one of the voltage levels originates from an alternative energy source, can draw maximum energy from it. This gives an optimum utilization of the alternative energy source.

In a special embodiment, the alternative energy source and the step-up unit have interposed between them a switch control unit, where the unit can compare the voltage level of the energy source with a permissible minimum level and disconnect the energy source if the voltage level of the energy source drops below the permissible minimum level.

A further object of the invention is to provide a method of converting two or more DC voltage levels to a DC voltage. This is achieved as described in the characterizing portion of claim 7. Preferred embodiments of the method are defined in claims 8-10.

The invention will be explained more fully below with reference to the figures, in which

fig. 1 shows a basic sketch of the converting unit,

fig. 2 shows a basic diagram of the structure of the converting unit,

fig. 3A shows a diagram of a classic converting unit which uses step-down,

fig. 3B shows a diagram of the converting unit for converting several voltage levels according to the invention,

fig. 4A shows a diagram of the converting unit with a control loop for converting several voltage levels according to the invention,

fig. 4B shows a curve of the voltage level measured at a point A on the converting unit shown in figure 4A,

fig. 5A shows a diagram of a special embodiment,

fig. 5B shows a curve of the voltage level measured at a point A on the converting unit shown in figure 5A,

fig. 6 shows the converting unit, wherein a step-up circuit is used in connection with the voltage level V2. An embodiment of the invention will be described below.

Fig. 1 shows a converting unit 11 which is connected to 2 voltage sources for supplying a load. These voltage sources have different DC voltage levels VI and V2 , and the converting unit 11 converts them into one stabilized output voltage Vos .

Fig. 2 shows a basic sketch of how the DC voltage levels Vl and V2 are converted into one DC output voltage Vo . Use is made of a duty cycle controller 21 which forms a square-wave signal with controllable duty cycle. The controller 21 is connected to a switch 22 which switches be- tween access to the voltage level VI and V2 depending on the square-wave signal. Thus, a signal will be formed at the output of the switch which switches between the voltage levels VI and V2. This square-wave signal is low-pass filtered with a low-pass filter 23 in such a manner that a DC output voltage Vo is generated. The output voltage Vo thus depends on the size of VI and V2 and on the duty cycle of the square-wave signal . The output voltage before the filtering is compared with a reference value Vref in 24, which may e.g. be an error amplifier, and the resulting signal from 24 is then used for regulating the duty cycle of the square-wave signal. The output voltage is used before it is filtered because of the phase shift which takes place in the low-pass filter 23. Thus, a loop is provided for forming and maintaining a stabilized out- put voltage Vos .

A detailed description of the structure of the converting unit will be given below. For the purpose of understanding this, the principle will be explained in a classic SMPS DC-DC converting unit of the step-down type, illustrated in figure 3A, which can convert a higher voltage level VI into a lower output voltage Vo . Use is made of a switch which is composed of a PWM transistor 31 which connects and disconnects, respectively, the connection to VI depending on the control signal, and a signal is formed at the point A which oscillates between 0 and VI. This signal is low-pass filtered in a filter composed of an inductance 32 and a capacitance 33, thereby forming a DC output voltage Vo across the load 34.

The classic converting unit operates in that when the signal at the point A has the voltage level VI, magnetic energy is stored in the inductance 32 so that the current through the inductance 32 is stabilized slowly. Since the magnetic energy cannot be released instantaneously, the current through the inductance 32 cannot change instantaneously, and it will thus be attempted to maintain the current via the diode 35 which is connected to earth. Hereby a DC output voltage Vo with a lower voltage level than VI may be formed by controlling the length of the connecting and disconnecting periods of the PWM transistor.

Fig. 3B shows a converting unit according to the invention built on the basis of the principle from the classic SMPS DC-DC converting unit of the step-down type, but which can use 2 voltage levels VI and V2 , where V2 is lower than Vo. The diode 35 is here connected to the lower voltage level V2. Thereby, energy will be drawn from V2 in the interrupted periods of the PWM transistor because of the inductance 32. Thus, a signal will be obtained at the point A which switches between VI and V2 , and a DC output voltage Vo depending on VI and V 2 will be obtained.

Figure 4A shows a diagram of an embodiment of the converting unit 1 with feedback for maintaining a constant output voltage Vo . This embodiment is particularly relevant in connection with varying voltage levels VI and V2. A value of the output voltage at the point A across a feedback resistor RF is measured and is compared with a reference value in a comparator 41. This comparison results in an error which is used in a duty cycle controller 42 for adaptation of the connected and disconnected periods of the PWM transistor. The reason why the output voltage Vo is measured before it is low-pass filtered, is that a phase shift takes place in the low-pass filter consisting of the self-induction 43 and the capacitance 44. It is noted that a connection is maintained between the point A and earth across a diode 45 for the case where V2 is out of operation.

Figure 4B illustrates the voltage at the point A for various values of the voltage level V2. In the illustrated example, VI has a voltage level of 70V, and V2 varies from 0 - 50V in jumps of 10V. It will be seen that the proportion between the connected and disconnected periods changes when V2 increases, so that an increasing amount of energy from V2 is used. It is noted that the signal is approximately constant after the low-pass filtering.

Figure 5A illustrates another embodiment of the invention. In this embodiment, a switch in the form of a relay 51 is provided. In the case where V2 is higher than VI and VI is higher than the desired output voltage, the re- lay 51 changes position. VI is thus disconnected and exclusively V2 is used via the PWM transistor 52, the proportion between the connected and disconnected periods depending on the voltage level V2. The output voltage Vo across the load 53 thus remains stable. In a special em- bodiment, the switch device is arranged such that it comprises means for detecting VI and V2 , and the relay 51 will be in the shown position when V2 is detected to be greater than VI and in the opposite position when V2 is detected to be smaller than VI.

Figure 5B shows the voltage at the point A when V2 is greater than VI, and the relay 51 thereby replaces VI by V2. V2 drops from 90V - 65V in jumps of 5V. When V2 decreases, the connected period of the PWM transistor increases relatively to the disconnected period because of the control loop. It is noted that the signal is approximately constant after low-pass filtering.

Figure 6 shows an embodiment using a step-up unit which may e.g. be an SMPS DC-DC converting unit of the step-up type, in connection with the one energy source with the voltage level V2. In case that the load needs a voltage and a current which do not utilize the energy from the energy source with the voltage level V2 optimally, it is of interest to increase the supplied energy, thereby get- ting a better utilization of the available power of the energy source. To avoid undue loading of the energy source, a switch control unit 61 is provided in front of the step-up unit, monitoring the voltage of the output voltage of the energy source by voltage distribution across the resistances 62 and 63. The unit operates in that it compares the output voltage of the energy source with a value for a permissible minimum voltage. If the output voltage drops below this, the switch opens, thereby interrupting the connection to the step-up part.

The step-up part operates in that a switch, in this case a PWM transistor 64, is controlled by a step-up controller 65 which alternatively turns the PWM transistor 64 on and off. When the transistor 64 is turned on, the self- induction 66 is connected to earth, and the current through it induces a great magnetic energy. When the PWM transistor 64 is turned off, the self-induction 66 is connected to earth via a diode 67 and a capacitance 68. It is attempted to maintain the current in the self- induction 66, which causes a voltage to be created across the capacitance 68 which is greater than V2. To prevent the voltage from getting greater than necessary, it is measured by using voltage distribution. The value is compared in 69 with a reference value which represents the desired output voltage. If the measured output voltage Vo is greater than the reference value, the step-up controller 65 and thereby the PWM transistor 64 are interrupted, and the capacitance 68 is discharged to the voltage level V2. The step-up controller 65 restarts when the output voltage Vo has dropped to a value which is lower than the reference voltage.

The invention is particularly useful in connection with alternative energy sources such as solar energy, it being of interest to utilize the alternative energy source op- timally.

It should be stressed that the present invention is not restricted to the embodiments shown, and it will also be appreciated that the invention may be incorporated in or be applied in connection with existing structures or features .

Claims

P a t e n t C 1 a l m s

1. A converting unit for converting two or more DC volt- age levels from the inputs of the converting unit into one DC voltage on the output of the converting unit, wherein the converting unit comprises controllable switch means capable of connecting and disconnecting the individual DC input voltage level to form an oscillation sig- nal, and wherein the converting unit comprises filtering means for low-pass filtering of the oscillation signal to form the DC voltage on the output of the converting unit, c h a r a c t e r i z e d in that the filtering means comprise a self-induction and a capacitance, and that the self-induction is positioned between the switch means and the output so that when the oscillation signal changes from a first voltage level to a second lower voltage level, the self-induction obtains energy from the energy source with the second voltage level because of the stored magnetic energy of the self-induction.

2. A converting unit according to claim 1, c h a r a c t e r i z e d in that the unit comprises a control loop which, on the basis of a comparison between a value of the output voltage and a reference value, can form an output voltage corresponding to the reference value, by changing the duty cycle of the signal which connects and disconnects the individual DC input voltage level.

3. A converting unit according to claims 1-2, c h a r a c t e r i z e d in that it comprises switching means capable of switching between first and second DC input voltage levels, so that the first voltage level can be disconnected and the second voltage level can be con- nected to the self-induction via the switch means, said switching means being a relay in an embodiment.

4. Switching means according to claim 3, c h a r a c t e r i z e d by comprising detecting means for detecting the first and second input voltage levels, and means for automatic switching depending on whether the input voltage levels are higher or lower than the desired output voltage.

5. A converting unit according to claims 1-4, c h a r - a c t e r i z e d in that the converting unit comprises a step-up unit, e.g. an SMPS DC-DC converting unit of the step-up type, which, when one of the voltage levels originates from an alternative energy source, can draw maximum energy from it.

6. A converting unit according to claims 1-5, c h a r a c t e r i z e d in that the alternative energy source and the step-up unit have interposed between them a switch control unit, where the unit can compare the volt- age level of the energy source with a permissible minimum level and disconnect the energy source if the voltage level of the energy source drops below the permissible minimum level .

7. A method of converting two or more DC voltage levels into a DC voltage, wherein an oscillation signal is formed by connecting and disconnecting the individual DC input voltage level by using switch means, and the oscillation signal is low-pass filtered to form the DC volt- age, c h a r a c t e r i z e d in that the filtering takes place with a self-induction and a capacitance, and that the self-induction is positioned between the switch means and the output so that when the oscillation signal changes from a first voltage level to a second lower voltage level, the self-induction obtains energy from the energy source with the second voltage level because of the stored magnetic energy of the self-induction.

8. A method according to claim 7, c h a r a c t e r - i z e d in that the unit comprises a control loop which, on the basis of a comparison between a value of the output voltage and a reference value, forms an output voltage corresponding to the reference value, by changing the duty cycle of the signal which connects and disconnects the individual DC input voltage level.

9. A method according to claims 7-8, c h a r a c t e r i z e d in that the size of the first and second DC input voltage levels is detected, and depending on whether the DC input voltage levels are higher or lower than the desired output voltage switching is performed between the first and second DC input voltage levels, so that the highest DC input voltage level is connected to the self- induction via the switch means .

10. A method according to claims 7-9, c h a r a c t e r i z e d in that a switch control unit is positioned in connection with a step-up unit, positioned in connection with an alternative energy source, and that the voltage level of the energy source is compared with a permissible minimum level and is disconnected if this is exceeded.