DE3912849C2 - - Google Patents

Description

The invention relates to a power supply device according to the Preamble of claim 1.

Such a power supply device is known from the patent specification DD 2 33 467 A1. This known power supply device contains a transformer with secondary components to form an output voltage U _a according to the forward converter principle. A voltage is also formed on the secondary side according to the flyback converter principle, but this is not a second output voltage that can be loaded by a consumer. This voltage is used only to regulate the single output voltage by acting on the primary magnetization monitoring circuit there, which controls the switch-on time of the switching transistor.

In addition to the flow converter, the literature also includes the Flyback converter and its mode of operation known, for. B from Maceck, Switching power supplies, motor controls and their special Components, Dr. Alfred Hüthig, Hüthig-Verlag, Heidelberg, 1982, pages 27-53. This includes circuits with one  primary-side controllable switches, as well as half-bridge circuits known with two controllable switches. Also the possibility of forming a so-called on the secondary side Multi-output converter to provide multiple outputs at which different voltages, e.g. B. 5 V and 12 V can be supplied. The regulation of these output voltages is only possible together with known circuits.

Due to tolerance regulations for the voltage or Power is supplied by certain consumers Demand for power supply devices, their output voltages can be regulated separately.

The object of the invention is therefore a power supply device to create the type mentioned, in which two There are outputs, both of which can be controlled separately are.

This task is accomplished with a power supply device Features of claim 1 solved.

Advantageous embodiments of the invention are the subclaims refer to.

With non-regulated output for flyback converter operation and Neglecting losses is the voltage time area on the primary side equal to the voltage time area of the secondary side. The following applies:

A _prim. = A _sek (at Ü = 1) (1)

Furthermore:

A _sek = U _Fly × t _Fly (2)

Here mean:

A _prim = primary voltage time area,
A _sek = secondary voltage time area,
U _Fly = fly-back voltage,
t _Fly = fly-back time.

From equation (2) it follows:

U _Fly = A _sek / t _Fly . (3)

It can be seen from equation (3) that the voltage U _{Fly can be} changed or controlled by changing the time t _Fly . There are two ways to do this:

It is possible on the secondary side in the flyback mode working output a controllable electronic Insert switch that connects the winding output to ground connects, and thus virtually shorts.

There is also the option of virtually short-circuiting the transformer on the primary side. For this purpose, two transistors are switched on in the primary circuit in such a way that one transistor is provided in front of and the other transistor behind the primary winding of the transformer. Both transistors are switched on or off simultaneously for clocking, and one of the two transistors is switched on to control the output voltage U _Fly or the time t _Fly , while the other transistor remains in the switched-off position.

Starting from the voltage U _Fly , the transistors practically causing the short-circuit of the secondary or primary winding are then controlled, the level of the voltage U _{Fly being able to be} regulated by the switch-on period or the start of the switch-on time.

Using the drawing, in which some embodiments the invention are shown, the invention and further advantageous refinements and improvements to the Invention will be explained and described in more detail.

It shows

Figs. 1 to 3 each shows a circuit arrangement in which the invention is implemented, and

FIGS. 4a to 4c three voltage-time diagrams, wherein a voltage-time diagram (Fig. 4a) shows the unregulated condition.

Reference is made to FIG. 1.

The circuit arrangement has a transformer 10 , the primary winding 11 of which is in a circuit which contains a rectifier diode 12 and a transistor 13 which is controlled by a suitable controller 14 . A smoothing capacitor 15 is connected to the connection point of the diode 12 with the primary winding 11 of the transformer 10 , one connection of which is connected to the cathode of the diode 12 and the other of which is connected to ground.

The secondary winding 15 a has a winding start marked by a point, a tap 26 a connected to ground, approximately in the middle of the secondary winding 15 a, and a further tap 24 arranged in front of the winding end.

The between the beginning of the winding and center tap 26 a of the secondary winding 15 a lying part of the winding forms together with a rectifier diode 18 , a freewheeling diode 20 , a choke coil 19 and a charging capacitor 21, the secondary part of a forward converter with an output 16 at which an output voltage U 1 is present.

The lying between the center tap 26 a and the further tap 24 winding part of the secondary winding 15 a forms, together with a connected to the tap 24 diode 25 and a capacitor 26, the secondary part of a flyback converter with an output 17 at which an output voltage U₂ is present .

The end of the secondary winding 15 a is connected to ground via a diode 22 and a short-circuit transistor 23 . This arrangement allows separate regulation of the flyback converter output voltage U₂ as will be explained.

The flow converter operation requires no further explanation, since the flow converter output voltage U 1 at the output 16 is formed in the usual way by controlling the primary-side transistor 13 .

The switching operation of the transistor 13 also forms the flyback converter output voltage U₂ at the output 17 in the conventional flyback converter operation. The output voltage U₂ is not firmly coupled to the voltage U₁, but can be influenced with the help of the secondary transistor 23 . This mode of operation is explained below with reference to FIGS. 4a to 4c.

FIG. 4a shows the converter operating without use of the secondary-side transistor 23. Transistor 23 is therefore permanently switched off. During the switch-on time t₁₃ of the primary-side transistor 13 , energy corresponding to the voltage time area A1 is stored in the transformer 10 in a known manner. In the flyback converter part, energy is delivered to the flyback converter output during the switch-off time t _{Fly of} the transistor 13 in accordance with the voltage time area A2. The output voltage U₂ (flyback converter part) results from the ratio of voltage time area A₂ / time period t _Fly .

The output voltage U₁ (flow converter part) is through determines the voltage time area A₁.

Figs. 4b and 4c show a transducer operating according to the invention, in which the secondary transistor 23 is switched.

If the transistor 23 is switched on at a time t _x in a circuit according to FIG. 1, the duration t _{Fly is} ended. The voltage level U₂ is changed depending on the time t _x . So if t _{Fly is made} smaller, U₂ becomes higher, and vice versa.

The embodiment of FIG. 2 is similar to that of Figure 1. The main difference is that the transistor 23 is connected to the connection point of the diode 25 to the terminal 17,. So that the flyback converter output winding not to ground but to the output terminal 17 takes place, whereby a curve of the voltage U _{Fly is achieved} according to FIG. 4b.

At point 24 , the secondary winding 15 a is divided. It is assumed for explanation that the entire secondary winding has fifteen turns, whereas the tapping takes place with twelve turns where 12 volts are drawn off. This data can of course be changed by suitable dimensioning of the secondary winding and the tap point and is therefore only an example.

If the transistor 23 is switched off, then only the 12-turn circuit is active and accordingly only 12 volts can be drawn off at the terminal 17 . If the 15-turn circuit is switched to 12 volts with transistor 23 switched on, this leads to the output of the 12-turn circuit leading to 9 volts in accordance with the turn ratio. This gives you a gradation of the voltage time area A₂ when you turn on the transistor; thus increases U₂ and because of the peak value formation by the capacitor 26 , this voltage remains practically constant even over the stepped gap. The current flows practically during the entire freewheeling phase, once from the 12-turn circuit and once from the 15-turn circuit. The current flow time into the capacitor 26 is thus extended, whereby the effective current that loads the capacitor 26 is reduced. This has no influence on the level of the voltage. Thus, by changing the time t _x at which the transistor 23 is turned on, the voltage U₂ is increased or decreased, that is to say regulated.

The voltage time areas A₁ and A₂ are the same and constant.

FIG. 3 shows a further embodiment of the invention.

The diode 12 and the smoothing capacitor 15 are located on the primary side of the transformer 10 . On both sides of the primary winding 11 , a transistor 30 and 31 are connected, of which the transistor 31 connects the primary winding 11 to ground. Furthermore, freewheeling diodes 32 are provided for transistor 30 and diode 33 for transistor 31 , the task of which will be explained below. One end of the secondary winding 15 a of the transformer 10 is connected to an output terminal 34 , whereas the other end of the secondary winding 15 a is connected to earth 37 via a diode 35 connected in the reverse direction and a choke 36 . The freewheeling diode 38 required for the forward converter operation is connected between the connection point of the diode 35 with the choke 36 on the one hand and the connecting terminal 34 on the other hand. At the connection point of the diode 35 with the corresponding end of the secondary winding 15 a branches off to a connecting branch 39 leading conductor branch 40 , in which a diode 41 is switched on; a smoothing capacitor 42 is connected between the connection point of the diode 41 with the connecting terminal 39 on the one hand and the connecting terminal 34 on the other hand; another smoothing capacitor 43 between terminal 34 and ground 37 is in series with capacitor 42 .

The mode of operation of the circuit arrangement according to FIG. 3 is as follows:

First, the two transistors 30 and 41 are closed. As soon as the transistors 31 and 30 become conductive, a voltage is delivered to the load or to the consumer via the inductor 36 , the diode 35 and the secondary winding, at which moment the forward converter operation is given. If the two transistors 30 and 31 go into the off position, that is to say in the blocking state, then the electromagnetic energy stored in the transformer 10 discharges via the line 40 to the connecting terminal 39 , so that the voltage taken off there corresponds to the so-called freewheeling voltage U _Fly ; in this case the power supply is operated in flyback mode. This is the case, which is shown in Fig. 4a.

There is now the possibility of regulating the voltage at the terminal 39 by bringing either the transistor 31 or the transistor 30 into a conductive state during flyback converter operation. In this case, the transformer is virtually short-circuited on the primary side via the freewheeling diodes 33 and 32, respectively. Fig. 4c shows this embodiment: After switching off the two transistors 30 and 31 , the transistor 31 is turned on at time t _x and in this way the time t _{Fly is} shortened and the voltage U₂ increased.

You can form the drive times of the transistors 30 and 31 via suitable regulator circuits so that the voltage U₂ is kept at a constant value. This regulation is known per se and as such is of little importance for the invention.

Claims (2)

1. Switched-mode power supply with a transformer, the primary winding of which is connected to a DC voltage source via at least one controllable switch, and to whose secondary winding the necessary components are connected for providing a regulated output voltage according to the forward converter principle, characterized in that

• a) to the secondary winding ( 15 a) of the transformer ( 10 ) also components ( 25, 26 ) are connected to form a further output voltage (U₂) after flyback operation, and that
• b) means ( 22, 23 ) are available for controlling this flyback converter output voltage (U₂) by changing the discharge time for a magnetic energy stored in the core of the transformer ( 10 ).

2. Power supply device according to claim 1, characterized in that a controllable switch (transistor 30, 31 ) are provided in series with the primary winding at the two ends thereof, one of which can be switched into conductive state during flyback operation during the blocking time of the other, wherein a free-wheeling diode ( 32, 33 ) is connected in parallel to a series connection of the primary winding ( 11 ) with one of the transistors ( 30, 31 ).