EP0729667A1

EP0729667A1 - Switched mode power supply

Info

Publication number: EP0729667A1
Application number: EP95928601A
Authority: EP
Inventors: Manfred Albach; Thomas DÜRBAUM
Original assignee: Philips Corporate Intellectual Property GmbH; Philips Patentverwaltung GmbH; Koninklijke Philips Electronics NV; Philips Electronics NV
Current assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips NV
Priority date: 1994-09-09
Filing date: 1995-09-05
Publication date: 1996-09-04
Also published as: US5729447A; WO1996008071A1; JPH09505460A

Abstract

The invention concerns a switched mode power supply having a transformer (Lp, N) to whose primary winding electrical energy from an energy source (Uin) can be fed at periodically repeated time intervals (t0 to t1). The transformer comprises at least one secondary winding via which the energy can be taken from the transformer (Lp, N) and fed through a rectifier arrangement (D1 to D4) to a storage inductance (Lo) and a load impedance (Co, RL). The energy output is increased with low expenditure on circuitry as a result of the fact that the rectifier arrangement (D1 to D4) has three current paths of which the first is arranged for feeding the energy to the storage inductance (Lo) and to the load impedance (Co, RL) during said time intervals (t0 to t1); the second current path is arranged to demagnetize the transformer (Lp, N) via its secondary winding and the load impedance (Co, RL); and the third current path is arranged to demagnetize the storage inductance (Lo) via the load impedance (Co, RL). The design of the second and third current paths is such that the transformer (Lp, N) and the storage inductance (Lo) are demagnetized simultaneously outside said time intervals (t0 to t1) and uninfluenced by each other.

Description

Switching power supply

The invention relates to a switching power supply with a transformer, the primary winding of which, at periodically recurring time intervals, electrical energy can be supplied from an energy source and which has at least one secondary winding, via which the energy can be taken from the transformer and supplied to a storage inductance and a load impedance by means of a rectifier arrangement.

In particular, the invention relates to such a switching power supply for generating one or more DC output voltages which are electrically isolated from an input voltage.

Such circuit arrangements can be implemented with the help of an active switching element. The various embodiments as forward converters or flyback converters are from the book "Switching Power Supplies - Fundamentals, Design, Circuit Examples" by J. Wüstehube et al. (Expert Verlag) known. In these circuits, energy transfer from the primary side (input source) to the secondary side (consumer, load) takes place only during the on-time of the active switching element (flux converter) or only during the off-time of the active switching element

(Flyback converter) instead. A major disadvantage of this circuit arrangement is the inefficient use of the magnetic components.

A circuit arrangement which allows energy transmission during the on and off duration of the active switching element is known from the article "A Dual Mode Forward / Flyback Converter" by JN Park and TR Zaloum (PESC 1982, 3). The circuit arrangement proposed there requires two separate secondary windings per desired output voltage. This poses in particular With a small available winding space or with a large number of desired output voltages, this is a decisive disadvantage.

A switching regulator with a switch is known from US Pat. No. 3,740,639, the line status of which is controlled as a function of the voltage across a load. This switching regulator contains a transformer, the primary winding of which is connected to the switch in a circuit. A rectifier and an inductive memory are connected in a circuit to the secondary winding, with a diode leading from one end of the secondary winding to a connection of the load. This diode is intended to transfer energy from the secondary winding to the load in periods when the switch is not turned on.

It has been shown, however, that in the switching regulator according to US Pat. No. 3,740,639, depending on the dimensions of the transformer and the inductive energy store and on the size and course of the currents in the switching regulator, optimum energy transfer to the load is not possible.

The invention has for its object to achieve the advantage of energy transfer during the on and off duration of the active switching element with only a single secondary winding per desired output voltage and thereby optimal energy transfer, i.e. to achieve an improved efficiency, regardless of the dimensioning of the elements of the switching regulator.

According to the invention, this object is achieved in a switched-mode power supply of the generic type in that the rectifier arrangement has three current paths, of which a first one for supplying the energy for storage inductance and for load impedance during the said time intervals, a second one for demagnetizing the transformer via its secondary winding and the Load impedance and the third is designed to demagnetize the storage inductance via the load impedance, the second and third current paths being designed in such a way that the demagnetization of the transformer and the storage inductance take place outside the time intervals mentioned and at the same time and are unaffected by one another.

In the invention, starting from the secondary winding, two separate current paths (power flow paths) are made possible with the aid of rectifier elements (diodes) in order to switch an active switching element during the on and off time through which the primary winding in the periodically recurring

Time intervals the electrical energy is supplied to transmit this energy to the output of the switching power supply and thus to the load impedance. In the first current path (power path), which is available for energy transmission during the on-period of the active switching element, ie during the above-mentioned time intervals, the storage inductance is used on the one hand to limit the current increase in the secondary winding and on the other hand to set the energy transferred in this switching state (mode). Freewheeling for the storage inductance is made possible by the rectifier arrangement (diode network) via the third current path formed by it. The energy transmission during the switch-off time of the active switching element, ie outside the specified time intervals, takes place analogously to the flyback converter from the energy temporarily stored in the transformer. In the circuit arrangement according to the invention, the third current path for the free running of the storage inductance is independent of the second current path for the energy transfer from the transformer into the load impedance during the switch-off time. Thus, regardless of the operating state of the second current path, the energy stored in the storage inductance can be delivered to the load impedance with optimum efficiency via the third current path. The invention is based on the knowledge that in the circuit arrangements known from the prior art according to US Pat. No. 3,740,639, the demagnetization of the inductances of the transformer and of the inductive memory does not take place independently of one another. In particular, this latter prior art shows that only the larger of the two currents in the secondary winding of the transformer and the inductive memory during the switch-off time leads to an energy transfer into the load. In the end, the efficiency of this switching regulator is arbitrarily dependent on the dimensioning just selected and the operating state.

The present invention, on the other hand, enables an optimal energy yield with little circuit complexity.

In a preferred embodiment, the switching power supply according to the invention is designed such that the rectifier arrangement comprises a bridge circuit with four rectifier elements arranged in each branch of the bridge circuit in the manner of a Graetz bridge, the input connections of which are connected to the secondary winding and the output connections of which are connected to the load impedance, the Rectifier elements of a first and a second of the branches form the first current path and the remaining rectifier elements are in their locked state during the time intervals mentioned, the rectifier elements of the third and fourth branches of the rectifier arrangement form the second current path and in the first branch of the bridge circuit in Row with the rectifier element, the memory inductance is arranged such that the first and fourth branches of the bridge circuit form the third current path.

This results in a simple and compact circuit structure in which the rectifier elements of the first and fourth branches have a double function in that they each have a part of the first and second Current paths together also form the third current path. In a modification of this circuit configuration it can be provided that the rectifier arrangement comprises a fifth branch with a rectifier element, that the memory inductance in the first branch of the bridge circuit is connected on one side to the adjacent output connection and that the fifth branch is the series circuit comprising the fourth branch and the rectifier element of the bridged the first branch. This fifth branch relieves the rectifier elements of the first and fourth branches of the bridge circuit from the demagnetizing current of the storage inductance. This can offer advantages when dimensioning the rectifier elements.

The switching power supply according to the invention can also advantageously be designed in such a way that the secondary winding has a tap to which the second branch of the bridge circuit is guided, the connection of the first and fourth branches forming the first input connection and the third branch to the second input connection is led. By means of this tapping, the transformation ratio of the transformer for the energy transmission during the said time intervals, i.e. increased during the duty cycle of the active switching element.

Another modification of the switching power supply according to the invention is such that the secondary winding has a tap to which the third branch of the bridge circuit is led, the connection of the first and fourth branches forming the first input connection and the second branch to the second input connection is led. With such a connection of the tapping of the secondary winding, the transmission ratio for the energy transmission is increased during the switch-off period of the active switching element, ie between two of the mentioned time intervals. The switching power supply according to the invention can have one or more secondary windings with the circuitry described. In the simplest form, only one simple secondary winding is required for each galvanically isolated DC output voltage. For each DC output voltage, the ratio of the energy transferred during the switch-on times (named time intervals) to that during the switch-off times (periods between the named time intervals) can be set by the ratio of the primary inductance of the transformer to the inductance of the respective secondary windings. In addition, outputs for electrically isolated DC output voltages can also be connected to the transformer of the switching power supply according to the invention, which are supplied with energy in a simple, known manner as flyback converter arrangements only during the switch-off times or as known flux converter arrangements only during the switch-on times of the active switching element.

The invention is explained in more detail below with the aid of the drawings. Show it

1 shows a first embodiment,

2 shows an embodiment with an alternative possibility of realizing the freewheel for the output inductance and possible forms of realization of further output voltages, FIG. 3 shows a possible realization of a larger transmission ratio for the

4 a possible realization of a larger transmission ratio for the energy transfer during the on-time,

5 diagrams for explaining the Fig. 1st

In Fig. 1 is an energy source, which essentially outputs a DC voltage Uin, with the primary winding L _{p of} a transformer with the transformation ratio N and a transistor S, which forms the active switching element, in one Circuit arranged. In a manner known per se, the transistor S is alternately switched into its conductive or its blocked state via a control circuit (“control”).

A secondary winding of the transformer is connected to a rectifier arrangement, which comprises four rectifier elements (diodes) D1, D2, D3 and D4 in each branch of a bridge circuit. An input connection formed by the connection between the first rectifier element D1 and the fourth rectifier element D4 is connected to one end of the secondary winding, a second input connection of the rectifier arrangement, formed by the connection of the third and second rectifier elements D3 and D2, is also connected connected to the other end of the secondary winding of the transformer. A memory inductance L _{0 is connected} in series with the first rectifier element D1, the load-side connection thereof in connection with the third rectifier element D3 forms a first output connection of the rectifier arrangement. The second output connection of the rectifier arrangement is represented by the connection between the second and fourth rectifier elements D2 and D4. Between the two output connections of the rectifier arrangement, the parallel connection of a load resistor R _L and a smoothing capacitor C _{0 is} arranged as the load impedance.

The mode of operation of the circuit can be explained most simply using the exemplary embodiment from FIG. 1 with the aid of the associated current and voltage curves in FIG. 5. The diagrams in FIG. 5 show the basic courses as an example for the case that both the transformer and the output coil work discontinuously. For the purpose of simplification, an embodiment with only one output voltage is considered here.

The transistor S is switched on by the control circuit at the time to and only switched off again at the time t1. During this duty cycle tl-to - o -

the voltage uT at the transistor is zero and the total input voltage Uin is at the primary side of the transformer. According to the selected transmission ratio N, the voltage Uin / N is found on the secondary side of the transformer.

The current through the transistor iT is composed of two parts. The first part corresponds to the magnetizing current 1 ^ of the transformer. This magnetizing current L _^ increases during the duty cycle according to the relationship:

"* 'Lp

linear on. Here Lp is the inductance of the primary winding of the transformer.

The second component corresponds to the current related to the primary side, which flows on the secondary side of the transformer (provided the transmission ratio is selected appropriately: N <Uin / Uo). This current on the secondary side flows along the current path Dl-Lo-Co // RL-D2. It also rises linearly because the fixed differential voltage Uin / N-Uo is present at the output inductance Lo. In the time interval to to tl, the currents iDl and iD2 therefore run according to:

iDl (t) = iD2 (t) = Uin I N-Uo Lo

These two current components correspond to the two amounts of energy described above. The magnetizing current characterizes the energy stored in the transformer, which is transmitted during the switch-off period. The current on the secondary side is a measure of the energy delivered directly to the secondary side during the on-time tl-to.

The currents through the diodes D3 and D4 are zero because they are reverse-polarized due to the voltage across the secondary winding. At time t1, the transistor is switched off via the control circuit and thus the current through the transistor is zero. The energy stored in the transformer is released on the secondary side along the current path D3-Co / RL-D4. Since D3 and D4 are now conducting, the output voltage Uo is present at the secondary winding. This transforms with the transmission ratio on the primary side, so that a voltage Uin + N • Uo lies across the transistor.

The current forms in the time interval tl to t2 are composed of two parts. The first part results from the demagnetization of the output inductance Lo. The freewheeling branch for this inductance is provided by the diodes D1 and D4. During the period tl to t2, a voltage of -Uo is present across the output inductance Lo and the current through Lo (which corresponds to iDl) decreases linearly, according to:

iDl (t) -iDl {tl) - g (t -tl)

The second part is generated by the demagnetization of the transformer and runs through the diodes D3 and D4. This current also decreases linearly:

iD3 (t) = N wn N ^• Uo (t -tl) Lp

The current through the diode D4 results from the superimposition of the currents of the diodes D1 and D3, since it is involved in both demagnetization processes. The current through the diode D2 becomes zero at the time point tl, since this is polarized in the reverse direction by the change in sign of the voltage on the secondary winding. In the example given here, the current through the diode Dl to point t2 becomes zero and Dl begins to block. The demagnetization of the transformer has not yet ended, so that the current iD3 continues to decrease linearly. Since iDl is zero from time point t2, the current through diode D4 in the time interval t2 to t3 corresponds to the current through diode D3.

At point t3, the current through the diodes D3 and D4 also becomes zero for the discontinuous case described here. As a result, the voltage across the secondary winding also becomes zero, so that in the remaining time interval t3 to t4 the voltage across the transistor is equal to the input voltage. Point t4 marks the end of the period under review, which is repeated from here.

The output voltage Uo is kept constant by varying the duty cycle (duty cycle) in the event of fluctuations in the load or the input voltage. This control of the duty cycle can be implemented with standard control circuits.

2 shows an alternative possibility of realizing the freewheeling for the output inductance. In this case the diode D5 shown in broken lines provides the freewheeling path, i.e. it carries the current of the output inductance in the time interval tl to t2. Diode D 1 blocks together with diode D2

Time tl at which the transistor is switched off. Since diode D4 is no longer in the freewheeling path, the current profile of D4 corresponds to that of D3, i.e. the current loads of Dl and in particular of D4 decrease (at the expense of another power diode).

3 and 4 show embodiments which make it possible to work with different transmission ratios by a simple tap. FIG. 3 shows the arrangement for a larger transmission ratio during the switch-off period, FIG. 4 shows the arrangement for a larger transmission ratio during the switch-on period. The arrangement according to FIG. 3 only differs from FIG. 4 characterized in that the third rectifier element D3 is no longer connected to the second rectifier element D2, but to the tap of the secondary winding of the transformer. In FIG. 4 the second rectifier element D2 is instead connected to the tap of the secondary winding of the transformer instead of the third rectifier element D3.

The transformer enables the generation of several galvanically isolated DC output voltages. There are various ways to generate such output voltages. The first option is to generate additional output voltages using the same secondary sound system.

The output power can be redistributed for each output (in parts of the power transmitted during the switch-on period or during the switch-off period). This division is set by the output inductance used in each case (see FIG. 2a).

The other options can also be read directly. These are the two 100% limit cases, i.e. energy is only transferred during the switch-on period or only during the switch-off period. The first case corresponds to a circuit on the secondary side analogous to the forward converter (see FIG. 2b), the second to a circuit analogous to the flyback converter (see FIG. 2c)).

Claims

PATENT CLAIMS

1. Switched-mode power supply with a transformer (L _p , N), the primary winding of which, at periodically recurring time intervals (tO to tl), can be supplied with electrical energy from an energy source (U) and which has at least one secondary winding via which the energy passes to the transformer (L _p , N) removable and through a rectifier arrangement

(Dl to D4) can be supplied to a storage inductance (L ₀ ) and a load impedance (C ₀ , RJ), characterized in that the rectifier arrangement (Dl to D4) has three current paths, one of which - a first one for supplying the energy to the storage inductance ( L ") and to

Load impedance (C ₀ , R during said time intervals (tO to tl), a second for demagnetizing the transformer (L _p , N) via its secondary winding and the load impedance (C ₀ , R and the third for demagnetizing the storage inductance (L ₀ ) is set up via the load impedance (C ", R _L ), the second and third current paths being designed in such a way that the demagnetizations of the transformer (L _p , N) and the storage inductance (L ₀ ) outside the time intervals mentioned (t0 to tl) and take place simultaneously and are unaffected by each other.

2. Switching power supply according to claim 1, characterized. that the rectifier arrangement (Dl to D4) comprises a bridge circuit with four rectifier elements (Dl, D2, D3 and D4) arranged in one branch of the bridge circuit in the manner of a Graetz bridge, the input connections (connections Dl, D4 and D2, D3 ) with the secondary winding and its Output connections (connections L ₀ , D3 or D2, D4) with the load impedance (C ",

R _L ) are connected, the rectifier elements (Dl, D2) of a first and a second of the branches forming the first current path and the other rectifier elements (D3, D4) being in their blocked state during the said time intervals (tO to tl), the rectifier elements (D3, D4) of the third and fourth branches of the rectifier arrangement form the second current path and the memory inductance (L _Q ) is arranged in the first branch of the bridge circuit in series with the rectifier element (Dl), such that the first and fourth Branch (Dl, D4) of the bridge circuit form the third current path.

3. Switching power supply according to claim 2, characterized. that the rectifier arrangement (Dl to D4) has a fifth branch with a

Rectifier element (D5) comprises that the memory inductance (L ₀ ) in the first branch (Dl, L ₀ ) of the bridge circuit is connected on one side to the adjacent output terminal (at D3) and that the fifth branch (D5) is the series circuit comprising the fourth branch and rectifier element bridged the first branch (Dl, D4).

4. Switching power supply according to claim 2 or 3, characterized. that the secondary winding has a tap to which the second branch (D2) of the bridge circuit is guided, the connection of the first and fourth branches (D1, D4) forming the first input connection and the third branch (D3) being connected to the second input connection is.

5. Switching power supply according to claim 2 or 3, characterized. that the secondary winding has a tap to which the third branch (D3) of the bridge circuit is guided, the connection of the first and the fourth Branch (Dl, D4) forms the first input connection and the second branch (D2) is led to the second input connection.