EP1297613A1

EP1297613A1 - Switched-mode power supply

Info

Publication number: EP1297613A1
Application number: EP01923374A
Authority: EP
Inventors: Wolfgang Croce; Günther Danhofer
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-04-12
Filing date: 2001-04-12
Publication date: 2003-04-02
Also published as: US20030169027A1; AU2001250145A1; WO2001080411A1

Abstract

The invention relates to a switched-mode power supply, comprising an input circuit (1) for periodically switching an input voltage (U>e<) or an input current (I>e<) on and off with a switching frequency (f>s<), a transmission circuit (2) connected thereto and an output circuit (3) which is connected to said transmission circuit and to which a load (4) can be connected. The aim of the invention is provide a switched power supply that is smaller and more economical than conventional switched mode power supplies. To this end, the invention provides that the transmission circuit (2) is formed by a bandpass (7) consisting of at least one capacitor (C) and at least one inductor (L), the resonant frequency (f>o<) of said bandpass lying outside, especially above, the switching frequency (f>s<) of the input circuit (1). A transformer is no longer necessary and so the disadvantages associated with the use thereof are avoided. At least one capacitor is provided in each branch of the LC bandpass (7), in series with the rest of the circuit, in order to produce a galvanic separation.

Description

Switching Power Supply

The invention relates to a switching power supply comprising an input circuit for periodically switching an input voltage or an input current with a switching frequency on and off, a transmission circuit connected to it and an output circuit connected to this, to which a load can be connected.

Power supplies that derive one or more DC or AC voltages of the appropriate size from the AC network are required for the supply of electronic devices. In conventional power supplies, the voltage ratio and the mostly required galvanic isolation from the network are taken over by a transformer, which have a relatively large volume and weight and relatively high losses with respect to the entire circuit. With the help of switching power supplies, the mains voltage is rectified and "chopped" with a relatively high frequency. By increasing the operating frequency, the disadvantages of conventional power supplies with low-frequency transformers can be greatly reduced. Due to the higher operating frequency, smaller components can be used which have smaller absolute losses. This results in switched-mode power supplies which have a significantly lower volume and a significantly lower weight than conventional power supplies. The switching power supplies are also used to convert DC or AC voltages into DC or AC voltages (from DC or AC to DC or AC).

Apart from the fact that galvanic isolation can be achieved with the help of the transformers in the switching power supplies, these components have a number of disadvantages. The transformers cause high losses and when they are used, the risk of saturation caused by asymmetrical control must be prevented by appropriate, sometimes complex circuits and processes. Compared to today's miniaturized electronic components, transformers are still relatively large and heavy, even with switching power supplies, and are also relatively expensive to manufacture. A further disadvantage would be the inevitable leakage inductances, which, among other things, cause overvoltages and thus impermissible loads on components of the switched-mode power supply or closed circuits can come. There is an unstoppable trend towards higher frequencies in electronic circuits. When using transformers, however, an increase in the magnetization frequency equivalent to the semiconductors cannot be expected due to physical conditions, since the white areas must be aligned in the magnetic field (for example, rotation losses of the magnetic material are caused by this). In contrast, new materials and new manufacturing methods in the

Semiconductor technology a much faster rise to ever higher cut-off frequencies can be determined.

Known circuits usually also have disadvantages with regard to short-circuit safety or overcurrent safety, since these safety measures can only be achieved with additional circuits or circuit parts or have to be dispensed with.

From WO 94/06260 AI a ballast for a gas discharge lamp is known, which contains a flyback converter and a downstream inverter and no transformer. To reduce the switching losses and the interference radiated back into the network, the flyback converter contains a series and parallel resonance circuit in combination, which serve as an energy buffer. The switching losses of the circuit breaker of the ballast are reduced by switching the circuit breaker at a time when the current through the circuit breaker is minimal. However, galvanic isolation as well as short-circuit and overcurrent protection cannot be achieved with this circuit.

The object of the present invention is to develop a switching power supply with which a reduction of the disadvantages mentioned above can be achieved. In particular, the circuit should be characterized by a smaller size, lower costs and a higher level of security compared to conventional switching power supplies.

The object of the invention is achieved in that the transmission circuit is formed by a bandpass (hereinafter referred to as LC bandpass) made up of at least one capacitance and at least one inductor, the resonance frequency of which lies outside the switching frequency of the input circuit. To a certain extent, the transmission of the GangsSchaltung "chopped" signal uses a frequency-selective circuit instead of a transformer. The LC bandpass causes a peak current limitation through the sum of the impedances. A higher efficiency can be achieved with the circuit according to the invention, since the losses of the capacitors used are smaller compared to those of a transformer and the absolute losses of the inductors used are also smaller due to the smaller size. The costs for the capacitors and coils of the bandpass filter used according to the invention are significantly lower than the manufacturing costs of a transformer. The disadvantageous leakage inductances in the coil, which is smaller for the same power compared to the transformer, are also smaller.

If the LC bandpass is dimensioned so that its resonance frequency is above the switching frequency of the input circuit, the turn-off losses can also be reduced. If the capacitors are largely charged before the next switching cycle and thus almost no current flows, the components of the switching stage, usually transistors, can be switched off in an almost de-energized state, as a result of which the switch-off losses almost disappear. However, it takes time to charge the capacitors, which is reflected in a lower maximum operating frequency and thus a lower transferable power. A compromise must therefore be made between the switch-off losses and the maximum operating frequency. If the LC bandpass is dimensioned so that its resonance frequency is below the switching frequency of the input circuit, the turn-off losses cannot be reduced.

According to a further feature of the invention, the LC bandpass consists of a series connection of at least one capacitor and at least one coil. This represents the simplest and therefore also the cheapest implementation of the circuit according to the invention. The capacitor and the coil can of course be constructed from several individual components.

A symmetrical arrangement is achieved if the LC bandpass consists of two series circuits, each arranged in parallel between the input and the output of the transmission circuit, of at least one capacitor and at least one coil, the values for the or each capacitor and the or each coil of each series circuit in the are essentially the same. Thereby on the one hand the component load is reduced and on the other hand galvanic isolation can be achieved. It is a frequency-selective galvanic isolation due to the high-pass effect of the capacitors. This is comparable to commercially available sheath current filters for antenna systems to eliminate earth loops, in which a coupling capacity is used in the sheath of the antenna cable. In this case, the energy transfer takes place through the electrostatic field of the capacitors.

If, in the above case, the value of the resulting capacitor of each series connection of the LC bandpass is less than or equal to 10nF, the circuit can be used for electrical isolation without the use of a transformer, which meets the usual legal safety requirements. The limit value of IOnF for the coupling capacity between the primary and secondary side can be found, for example, in relevant standards for medical-technical devices. Due to the small amounts of charge due to the small capacity, the arrangement meets the requirements for electrical isolation. From an operating frequency of a few kilohertz, in contrast to such capacitors, a transformer is considerably larger, more expensive and more lossy.

Further advantages can be achieved if at least one coil of the LC bandpass has an inductance which is variable as a function of time or of the current. In particular, the use of a so-called saturation coil, which has a high inductance at the time of switch-on and then a very low inductance, namely the saturation inductance, is advantageous because it delays the rise in current at the start of a switching process and thereby the switches, usually transistors in the switching stage be switched on in the de-energized state as possible, whereby the switching losses are reduced. After the switching process, the coil saturates and allows the entire current to flow. The saturation coil is dimensioned by suitable selection of the magnetic core material, the core volume and the number of turns. The use of a saturation coil in series with a thyristor can be found, for example, in DE 33 34 794 AI.

An increase in operational reliability through the most complete possible separation between the input side and the output side of the circuit can be achieved if the input A low-pass filter is arranged on the side of the input circuit, the cut-off frequency of which is substantially below the resonance frequency of the LC bandpass. The cut-off frequency of the low-pass filter should be so far below the resonance frequency of the band-pass filter that the attenuation of the transfer function in the blocking region between the low-pass filter and the bandpass filter is as large as possible. In combination with the bandpass filter of the switching power supply according to the invention, the transmission of primary-side high-frequency interference frequencies and transients to the secondary side and thus to the load can be avoided. The combination of the low pass and the band pass and their dimensioning means that "independent" energy transfer from the primary side to the secondary side cannot take place in any frequency range. For example, high-frequency network disturbances that are in the pass band's pass band can be effectively attenuated by the low-pass filter. The energy transmission is made possible by the switching frequency of the input circuit, by means of which the input signal is "raised" in frequency in the pass band of the LC band pass. Without the low-pass filter on the input side, interference frequencies on the input side, which are in the pass band of the LC band pass, could be transmitted to the secondary side, where they could damage the load or the subsequent circuits and endanger people and lead to inadmissible energy transfers.

The features of the invention are explained in more detail with the aid of the attached figures, which show schematic diagrams for explaining the invention and a preferred embodiment of a switching power supply according to the invention.

In it show:

1 shows the basic block diagram of a switching power supply,

2 shows the usual design of the transmission device of a switching power supply in the form of a transformer,

3 shows the embodiment according to the invention of the transmission device of a switching power supply in the form of an LC band pass,

4 shows the basic course of the impedance of an LC bandpass as a function of the frequency,

5 shows the circuit diagram of an advantageous embodiment of a switching power supply according to the invention,

Fig. 6 shows the basic transfer function of the circuit of FIG. 5 as a function of frequency, and Fig. 7a to 7c, the time profiles of a switching current through a saturation reactor and its inductance during a switch-on process.

In Fig. 1 is a basic block diagram of a _S chaltnetzteils is reproduced. After any transformers or rectifiers (not shown), there is an input signal in the form of an input voltage U _e or an input current I _e which is "chopped" in an input circuit 1. For this purpose, the input circuit 1 is connected to a control circuit 5, in which the frequency with which the input signal is "chopped" is generated or fixed. The quantity supplied by the input circuit 1 is transformed or transmitted into a corresponding quantity in a transmission circuit 2. As a rule, the transmission circuit 2 consists of a transformer (see FIG. 2). The electrical circuit 3 is then further processed in the output circuit 3, for example rectification and screening, before the signal is applied to the respective load 4. A control of the switched-mode power supply can also take place via the control circuit 5, so that a specific output voltage U _a or a specific output current I _a or a desired output power P _a occurs at the load 4 independently of the input signal. As indicated, the load 4 can also be variable.

FIG. 2 shows the conventional case of using a transformer 6 as a transmission circuit 2 of a switched-mode power supply according to FIG. 1. With the aid of the transformer 6, a primary voltage U _P or a primary current I _{P is converted} into a secondary voltage Us or a secondary current I _s .

3 shows the embodiment of the transmission circuit 2 according to the invention of the switching power supply in the form of an LC band pass 7. In the embodiment shown, the LC bandpass 7 consists of two series connections, each with a capacitor C and a coil L, each of the same size. In the simplest version, the transmission circuit 2 consists of a series connection of a capacitor C and a coil L. LC bandpasses of a higher order or series or parallel connections of inductors or capacitors for current or voltage distribution are also possible. According to the invention, the values for the capacitors C and coils L are determined such that the resonance frequency fo determined by the capacitors C and the coils L. of the bandpass lies outside the switching frequency f _{s of} the input circuit 1 of the switching power supply. The implementation of the transmission circuit 2 according to the invention can also be viewed as a series resonant circuit which is operated outside its resonance frequency f ₀ , resulting in a frequency-dependent impedance of the transmission circuit 2.

By implementing the transmission circuit 2 according to the invention in the form of an LC band pass 7, the transformer that is usually used can be avoided. This also eliminates the disadvantages of a transformer, such as high losses, large volume, high weight and high manufacturing costs. 3 consists of two series connections, each with a capacitive and an inductive reactance. The coil L limits the peak current when switched on. The charging capacitor C, on the other hand, determines the transferable energy, that is, after the capacitor C has been fully charged, further transmission of the energy is prevented. This combination of C and L in series connection is therefore absolutely necessary. The coil L causes the current to start up smoothly and thus limits the switch-on losses. Compared to a transformer, the capacitor C and the coil L have significantly lower losses. In particular at high frequencies, the advantages of the circuit according to the invention over the use of a transformer become particularly clear. If the component values in each series connection are essentially the same size, the component load is minimized. An asymmetrical arrangement causes different component loads, but can also be an advantage. For example, a saturation coil can only be provided in one branch, by means of which the switch-on losses can be reduced. Instead of two series connections of a capacitor C and a coil L, one would theoretically also be sufficient, but then the advantage of galvanic isolation would not be connected.

The implementation of the transmission circuit 2 according to the invention is a series resonant circuit, the overall impedance of which has a real profile as a function of the frequency in accordance with FIG. 4. At a resonance frequency f ₀ , the impedance is minimal, theoretically even zero in the lossless case. However, it is the objective _A not pplication inn to a series resonant circuit in the conventional _S because a vibration not desired and by the external wiring is not possible. Rather, the series connection of the capacitor C and the coil L is operated outside the resonance frequency fo, so that the impedance can be controlled by changing the frequency f. The circuit can be viewed in conjunction with the load resistor as a frequency-controlled voltage divider. It is advantageous if the operating frequency is chosen below the resonance frequency f ₀ . A variation in the output power can be achieved by varying the switching frequency f _s in a specific frequency range fsi to fs2.

5 shows an advantageous embodiment of a switching power supply according to the invention. A low-pass filter 8 is arranged on the input side, which suppresses higher-frequency interference. Then follows the input circuit 1 consisting of a rectifier and the chopper, which is controlled by a control circuit 5. The transmission circuit 2 consists of an LC bandpass formed from two series circuits each of a capacitor C and an inductance L. After an output circuit 3, which in this case is formed by a rectifier and a low-pass filter, a load 4 is connected on the output side.

FIG. 6 shows the basic transfer function of the circuit according to FIG. 5 as a function of the frequency f. The cut-off frequency f _{G of} the low-pass filter 8 is substantially below the resonance frequency f _{0 of} the LC band-pass filter of the transmission circuit 2, so that undesired interference in the blocking region of the low-pass filter 8 is sufficiently damped.

7a to 7c show the time profiles of a switching current through a saturation inductor and its inductance during a switch-on process. 7a shows a switch-on process, for example the base current of a transistor as an electronic switch. 7b shows the corresponding time profile of the current I (t) through a saturation coil L (t) and in FIG. 7c the inductance L (t) of the saturation coil L (t) as a function of time t during the switch-on process. After switching on, the current rises very slowly due to the relatively high inductance of the coil L (t). By dimensioning the coil L (t) accordingly can be achieved that at a precisely defined current I _s , given by the operating voltage and the already elapsed switch-on time, the coil L (t) saturates. The area of the core saturation is characterized in that the magnetic flux cannot be significantly increased despite the increase in the current in the coil L (t). In the area of saturation, almost all elementary magnets of the core material are aligned in the preferred direction. In the area of saturation, the inductive resistance of the winding drops, as a result of which only the undesirable ohmic component of the resistance limits the current in the winding. Therefore, the inductance of the coil L (t) drops to a minimum value L _min . This is mainly determined by the number of turns and the core material of the coil L (t). The current I (t), on the other hand, now increases more rapidly to its maximum value I _max limited by the load. The dimensioning of the coil L (t) is preferably carried out by suitable selection of the magnetic core material, the number of turns and the core volume. These parameters influence not only the time t _s at which the coil L (t) goes into saturation, but also the behavior of how the transition to saturation takes place, ie for example the steepness of the current increase in the area of the saturation of the coil L (t ).

Claims

Claims:

Switching power supply comprising an input circuit (1) for periodically switching an input voltage (U _e ) or an input current (I _e ) with a switching frequency (fs) on, a transmission circuit (2) connected to it and an output circuit connected to it ( 3), to which a load (4) can be connected, characterized in that the transmission circuit (2) is formed by a bandpass (7) made up of at least one capacitance (C) and at least one inductance (L), the resonance frequency of which (f ₀ ) lies outside the switching frequency (fs) of the input circuit (1).

2. Switching power supply according to claim 1, characterized in that the resonance frequency (fo) of the LC band pass (7) is above the switching frequency (fs) of the input circuit (1).

3. Switching power supply according to claim 1 or 2, characterized in that the LC bandpass (7) consists of a series connection of at least one capacitor (C) and at least one coil (L).

4. Switching power supply according to one of claims 1 to 3, characterized in that the LC bandpass (7) from two series circuits arranged in parallel between the input and the output of the transmission circuit (2) in each case at least one capacitor (C) and at least one coil ( L), the values for the or each capacitor (C) and the or each coil (L) of each series connection being substantially the same.

5. Switching power supply according to claim 4, characterized in that the value of the resulting capacitor (C) of each series circuit of the LC bandpass filter (7) is less than or equal to 10nF.

6. Switching power supply according to one of claims 1 to 4, characterized in that at least one coil (L) of the LC band pass (7) has an inductance (L (t)) which is variable as a function of time or of the current.

7. Switching power supply according to one of claims 1 to 6, characterized in that on the input side of the input circuit (1) a low-pass filter (8) is arranged, the cutoff frequency (fo) of which is substantially below the resonance frequency (fo) of the LC bandpass (7).