DE3041202A1 - DEVICE FOR VOLTAGE REGULATION IN DC CURRENT NETWORKS - Google Patents

Description

DIpI. Ing. P «t * r Ott ·. ₇₀₃₃ Harrenberg (Kuppingen) Patent attorney Ό "SifeletraQe 7

Telephone (070 32) 31999

1517 / ot / zz
Aug 18, 1980

Robert Bosch GmbH, 7000 Stuttgart 1

Device for voltage regulation in direct current on-board networks

State of the art

The invention is based on a device for voltage regulation for direct current on-board networks of mobile units, motor vehicles, and the like according to the genre of the main claim. It is known, power generators, preferably three-phase generators in motor vehicles o. The like. In spite of the high demands placed on them by regulating the excitation current supplied to them so that the generator output voltage can be influenced is maintained at a substantially constant desired level. The excitation current and thus the Excitation field in the rotor of the generator is controlled as a function of the voltage generated in the generator in such a way that despite considerable changes

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licher speed between idle and full load and despite considerable load fluctuations on the generator, the generator terminal voltage remains constant up to the maximum current. The mechanical single-contact or multi-contact regulators are known as regulators; Usually, electronic transistor regulators are now mainly used, which regulate the generator voltage by periodically weakening the excitation current S _1, usually periodically switching the generator on and off, since the voltage dsm product of speed and excitation current generated in this is essentially the same.

Problems arise in this context, for example, in the field of motor vehicle electrics in winter or in city traffic, because at When cold, the internal resistance of the battery increases considerably, so that its cold start performance drops sharply. There are also battery charging problems at low speeds. You could go through at least Temporary increased charging voltage if the alternator is designed accordingly and therefore sufficiently available Alternator output force a sufficient charging current even in cold weather, but the voltage of the on-board network can be adjusted with consideration for voltage-sensitive consumers, for example the incandescent lamps used, cannot be increased at will.

It is the result of future increasing demands on the quality of the Supply voltage in Bcrdnetzen the need for a central, powerful voltage regulator, which is able to with variable Input voltage despite considerable, flowing through the voltage regulator Power to provide an output voltage that is as constant as possible.

ORIGINAL INSPECTED

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Advantages of the invention

The device according to the invention with the characterizing features of the main claim has the advantage that it is also possible, in the case of on-board networks, initially in a very general manner, to have an extremely constant output voltage to be generated at high power consumption when the input voltage fluctuates between significant limit values. So could one 12 volt system, which is given here as an example, the input voltage lies between the voltage values of 10 ... 18 volts, while the output voltage is kept at a constant 13.8 volts.

This makes it possible to adjust the alternator voltage, for example to be adjusted according to the requirements of the battery and to the previously existing galvanic connection between the battery and the vehicle electrical system to waive, which is then taken over by the controller according to the invention - possibly as a second controller - which supplies the vehicle electrical system voltage holds at its target value with different input voltages and different loads. It is of course possible to make uncritical ones Consumers, such as the rear window heater and the like based purely on resistance heating, from the on-board network that is kept constant decouple and assign this to the consumers who need such a constant on-board network voltage or thereupon with a special long service life, such as lamps or other electronic consumers.

It is advantageous that the very precise voltage regulator according to the invention can be produced easily and inexpensively and also with existing ones

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Plants can be used. It manages to stand out from the stare To release the constraints of a vehicle electrical system voltage that is as constant as possible for only a few voltage-sensitive consumers, and for example the very different charging power requirements of the battery, for example correspond to. Today's vehicle electrical systems basically represent a compromise, for example, between the charging power requirement of the battery on the one hand On the other hand, the voltage requirement in the incandescent lamp used is as constant as possible.

The measures listed in the subclaims are advantageous Developments and improvements of those specified in the main claim Device possible. It is particularly advantageous to distinguish between upward regulation on the one hand and downward regulation on the other hand, an economy circuit embodiment such that a reduction in the semiconductor switching power and thus the power loss can also be achieved.

drawing

Embodiments of the invention are shown in the drawing and are explained in more detail in the description below. Show it Fig. 1 shows a circuit example of a flyback converter circuit such as Is already known in and of itself, the Fig. La and Ib curve profiles the input and output current of the flyback converter shown in Fig. 1, FIG. 2 shows a first exemplary embodiment of a voltage regulating circuit according to the invention as a flyback converter economy circuit, and FIG

ORIGINAL INSPECTED

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3 to 6 further embodiments of flyback converter economy circuits, Finally, FIGS. 7 and 8 show possible embodiments of trigger members for controlling the in each of the invention Voltage regulator circuit existing, at least one semiconductor.

Description of the exemplary embodiments

For a better understanding of the FIGS. 2, 3, 4, 5 and 6 The exemplary embodiments according to the invention shown are briefly referred to below first of all to those shown in FIG. 1, which are known per se Basic circuit of a flyback converter received and its use with reference to keeping an on-board network output voltage constant examined more closely. If one puts the already described above, for example The control task is to keep an input voltage between 10 volts and 18 volts constant to an output voltage of for example 13.8 volts, then the switching transistor Tr of the flyback converter shown in Fig. 1 must for at least a voltage of

u = u + u / ü (1)

T emax a '

and a stream of

i_ = 2 * (ü + u / u.) 'i (2)

T ^v a 'emin a'

be measured. These symbols are also in the current curves of Fig. la and Ib, where T + T = T the Ar-

1 Ci

is the operating period of the flyback converter.

The flyback converter shown in Fig. 1 consists of the actual converter or transformer Tr with a primary winding Wl and a secondary winding W 2 with the transformation ratio ü = w2 / wl. In

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A semiconductor switching element is arranged in series with the primary winding Wl, preferably a transistor Tr; however, other suitable semiconductor switching elements such as thyristors can also be used in a suitable form and control can be used. The structure of the one shown in FIG Flyback converter is completed by an input capacitor Ce parallel to the series connection of the primary winding with the semiconductor switching element, at which the input voltage u is applied. On the output side, the secondary winding of the converter Tr is in series with one Diode D and parallel to both an output capacitor Ca, across which the output voltage u drops. How such a flyback converter works is as shown in the illustration of Fig. La and Ib; the current increases during the switch-on period T of the semiconductor switching element i through the primary winding Wl up to the peak current i on; after Blocking the transistor Tr, a voltage is induced in the secondary winding of the converter, which leads to a corresponding output current i leads with the peak current value i. Input current i and output

current i are arithmetic mean values of the peak current, a

The switching capacity of the transistor Tr is obtained by linking the relationships (1) and (2) to:

Ct

ρ = u_.i = 2-u 'i «(1 + u / u.) + 2'i - (ü-u + _ L_) (3)

* T TT aa emax 'emin' a ^v emax TFü Γ ''

emm

According to the assumption, the relationships u = const as well ^{as u} emin " ^u e ~ ^u emax

For given voltages and currents, ü is chosen so that the switching power p _{T is} minimal:

opt '

1 . · U

emm emax

ORIGINAL INSPECTED

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If the product u * i is denoted as the throughput p “and leads

aa L)

the term c = u / u. as an abbreviation, then you get if

emax 'emin "

one substitutes the relation (4) in relation (3) ^{= 2} 'VV ^{(1 + C +}

The required switching capacity is then with constant input voltage, thus for c = 1.0 8 times the throughput; with the numerical values given above for the example of a 12 volt system results in c = 1, 4 and the switching capacity increases to 11 times. This leads to considerable problems with higher throughputs in an on-board network of a mobile unit, at around 300 watts as the medium one Value of the output power of the voltage regulator are required, and significant power dissipation if it is possible to use semiconductor switching elements to make available with such high switching capacities.

The embodiment shown in Fig. 2 reduces the required Performance is drastic and still ensures a high-precision voltage regulator that can be used inexpensively in any on-board network can be. Galvanic isolation between input and output is not provided as this is not required. The flyback converter is designated in Fig. 2 with 20; it has a primary winding 6 and a secondary winding 5. The primary winding 6 is in series with a first semiconductor switching element 2 and thus forms the input circuit with the input capacitor 21. In series with the secondary winding 5 is the rectifying diode 3 already present in the basic circuit diagram of the flyback converter of FIG. 1; but there is still a second free-wheeling diode 4 is provided, which connects the other terminal of the secondary winding 5 to ground or the second pole of the circuit and the in

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the same direction of flow as the rectifying diode 3 is polarized. In a The connection point of the special embodiment of the present invention is then still corresponding to the illustration in FIG Secondary winding 5 with the freewheeling diode 4 via a second semiconductor switching element 1 connected to the input terminal 23 of the voltage control voltage of FIG. 2, which when applied to an on-board network for example, the positive generator output voltage is supplied. The busbar 24 then carries the negative pole. The function of the first embodiment of a fiction ge MAe powerful The voltage regulator is then as follows. If the input voltage is greater than the output voltage, then the circuit works on the principle of down regulation and the switching transistor 2 is permanently blocked. The switching transistor 1 conducts and blocks alternately. During its switch-on time t, the current flowing through this transistor increases 1, the secondary winding 5 of the flyback converter 20 and the diode 3 flows, as is also shown in Fig. La. During the blocking period t of the switching transistor 1 is the further current flow forced by the winding inductance of the secondary winding 5 via the freewheeling diode 4 instead of via the transistor 1, but no more energy is supplied from the input side.

The level of the output voltage then results from the relative switch-on ratio to:

u * e . u (6)

^a τη- ^e

τη

e s

In this position, the circuit works on the principle of a so-called Buck converter. However, it is usually in the flowing over

ORIGINAL INSPECTED

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A3

output, the input voltage is less than the output voltage, then the basic principle of upward regulation results and the switching transistor 1 is then initially switched on continuously. The switching transistor 2 conducts and blocks alternately, with a power being transmitted to the output side by means of a transformer, which, however, is only a fraction of the throughput p _n that a flyback converter in the full circuit according to FIG. 1 would have to transmit. The power transmitted by the circuit of FIG. 2 in the form of a transformer during the upward regulation is then only ρ = (u - u) · i.

3. 6 3.

The following information makes some statements with reference to the in the case of downward or upward regulation by the individual switching transistors switching capacities to be taken over.

In the downward regulation, the switching transistor 1 takes on the following Switching capacity:

P_, = u -i. k, (7)

Tl emax a

where the constant k differs from its maximum value k = 2 by increasing it the switching frequency can be reduced to near its minimum value k = 1. The switching capacity of the switching transistor 2 has the upward regulation the following size:

P "= (u. + ^U a" ^U emin ) · 2 · (ü + ^U a " ^U emin ) · i (8) ι ζ emin" ——————— ^

ü u

emin

Introduce the following abbreviations

a = emax and b = emin

u u

a a

then the total switching capacity results

P _T -P _T1 + P _T2 - 2-i _a . _V [2 ₊ ψ 2b ₊ b. (U ₊ \ Λ \ - D ² )] · (9)

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For given voltages and currents, ü is selected again so that the switching power p _{T is} minimal

"opt ■ E - '^<10)

Using the throughput power already defined, ρ is obtained then, by inserting relationship (10) into equation (9), the following specification for the ratio of the passage to be switched Services

When using an economy circuit according to the invention based on the flyback converter principle, this is reduced for what is specified above Numerical example for a 12 volt system, the required transistor switching capacity to at least 4, 8 times, ideally to 3.5 times the throughput compared to the one calculated above 11 times the amount.

Another embodiment of a voltage regulator according to the invention for vehicle electrical systems is shown in Fig. 3, in which an additional reduction in the Tr to si disturbing switching capacity can still be achieved, if it is assumed that the transition between upward and downward regulation occurs only rarely and then comparatively slowly. First of all, however, it must be pointed out that the switching transistors used in each case are connected to their input or control electrodes are supplied with appropriate drive or trigger pulse sequences, which in their duty cycle and possibly in their frequency so adjusted and related to the desired ratio of the input voltages to the output voltages that the Can achieve desired voltage constant maintenance. An execution

ORIGINAL INSPECTED

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example of such a circuit that generates the drive pulse trains is then shown in Fig. 7 and will be explained in more detail below.

In the embodiment of FIG. 3, only one switching transistor is used l 'is required, with changeover contacts also being provided which are expediently controlled by a relay 7 with three, possibly with four contacts 8, 9, 10 and 12.

In the case of downward regulation, the relay 7 connects via the then closed Contact 8 the output terminal of the switching transistor Γ directly to the secondary winding 5 of the transformer or converter 20, the duty cycle of the switching transistor 1 'then supplied Control pulse sequence determines the exact control ratio between input voltage and constant output voltage.

In the case of up regulation, the output terminal becomes the same Transistor via the then closed switching contact 9 with the primary winding 6 of the converter 20 connected, at the same time a then closed switching contact 10 of the relay 7 a connection between the input voltage at input 23 with the connection point of the Secondary winding 5 and the freewheeling diode 4 produces. The relay 7 is via a trigger circuit 11 that controls the level of the input circuit detects, controls and picks up in the case of downward regulation, namely when the input voltage exceeds a certain predetermined level, where the transition from up to down regulation must take place. The trigger circuit 11 therefore has a reference

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Voltage circuit that is compared internally with that of the circuit applied input voltage; corresponding to the output voltage of a comparator used for this comparison, for example The relay 7 is then activated.

In this circuit according to FIG. 3, the required switching capacity for the only remaining transistor 1 'is reduced to the larger value ρ or p ™, which is calculated according to the above-mentioned relationships (7) and (8): P _T1 ^{= a} 'P _D * ^k

= 8- (lb) -p _D (13)

The switching capacity ρ is for the mostly applicable case of

J. Ct

8 · (] -b)> aJc decisive. In this numerical example given above, redu ^ee , ^{ideally F}

if the required transistor power is 2.2 times the throughput, Compared to the flyback converter described at the beginning, this results in a saving of 80% of the transistor switching capacity when fully switched can be achieved.

As a further advantage of this voltage regulator shown in FIG. 3 Relay switching is to be seen as the measure that, in the case of upward regulation, the power loss of transistor 1 - compared with Fig. 2 - is saved overall, since this is replaced by the contact 10 in FIG. The power loss can thereby be total reduce it by about half.

© RldfNAL INSPECTED

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In addition, another relay, the relay 7 associated switching contact 12, the diode 3, which is no longer required in the case of downward regulation, can be bridged, so that here too there is still a reduction in loss is achieved.

According to a further embodiment of the present invention it is also possible in a modification according to the illustration 4, a flyback converter voltage regulator can be used particularly advantageously when an upward regulation alone is in the eye must be taken and taken into account. In this variant of a flyback converter economy circuit shown in FIG. 4 are omitted with reference to the Representation of Fig. 2 still the free-wheeling diode 4 and the switching transistor 1 shown there, so that with constant galvanic connection of the Voltage at the input and the secondary winding 5 via the connecting line 25 only the switching transistor 24 is provided, which is connected to the primary winding 6 of the converter or transformer 20 is. The transistor power required by this modified circuit is shown in relation (13) above and also shows a clear reduction here.

In the embodiment of FIG. 5, there is the possibility in another variant to switch between full circuit and economy circuit of the flyback converter voltage regulator, depending on whether the output voltage smaller - down regulation - or larger - up regulation - than the input voltage.

In this embodiment, the secondary winding 5 of the transformer is located 20 via a changeover contact 14 either on the lower one

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At

Ground rail or on a connecting line 25 ', which the secondary winding 5 connects directly to the input voltage of the circuit, as shown in FIG. 4. The changeover contact is actuated by a relay 13, which can be controlled by a trigger circuit identical to the embodiment of FIG. 3 11, which is able to switch between the downward and the upward regulation to distinguish. The same purpose is achieved in this way as with the circuit in FIG. 4, but it is also possible to regulate downwards. The required Transistor switching capacity increases here in accordance with relation (5) with c = 1 to 8 times the throughput; Therefore however, the logic circuit required for driving the switching transistor 26 is simpler in this case, since it is always the same Scheme is regulated.

In the embodiment of FIG. 6, the one previously described is used Possibility to switch between full circuit and economy circuit of the flyback converter voltage regulator through the thyristor 30 and the diode 4 reached, the combination of which replaces the changeover contact 14 of the relay 11 in FIG. This variant is advantageous where the change occurs frequently and / or rapidly between up and down regulation.

In this embodiment, the secondary winding is 5 of the converter 20 on the one hand via the diode 4 with the lower ground rail, on the other hand via the thyristor 30 with the input rail carrying potential 23 connected in accordance with the illustration in FIG. 6. The thyristor is activated by a trigger circuit ll 'in the case of upward regulation ignited continuously.

INSPECTED

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It goes without saying that the economic optimum in the circuits shown in FIGS. 3, 4, 5 and 6 depends on the required throughput and the predefined curve of the input voltage and is therefore to be determined accordingly in each case.

The following is the possible structure of a switching transistor controlling Logic circuit with reference to the representation in FIGS. 7 and 8 explained briefly according to structure and mode of operation.

The logic circuit required for driving the at least one switching transistor 1, 2, 1 ', 24, 26 according to FIG downstream modulator 31 is varied according to the control signal of the controller, a trigger circuit 32 for controlling the switching transistors of the above-mentioned embodiments and a comparator 11 for controlling the switching relay 13 (see FIG. 5). The regulator receives at its input 28a via the adjustable voltage divider 33a, 33b the output voltage u to be regulated to a constant value

el

and compares it with the constant nominal voltage at its input 28b, which is used in a known manner as a reference voltage using at least one switching element which supplies a constant voltage value such as a Zener diode 29 from the between u

and u variable input voltage u is obtained.

The function is such that a possible differential voltage at the inputs 28a and 28b proportionally amplified and / or by the controller 28

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integrated and fed as a continuously variable manipulated variable to the modulator module 31, which the duty cycle of a at its input 31b from a generator 27 supplied square-wave voltage corresponding to the manipulated variable at its other input 31a changes. Generators that have a control pulse train at their output with a changing duty cycle depending on a changing one generate analog input voltage are known per se. A duty cycle is understood to mean, for example, in the case of a square-wave pulse train the switch-on duration related to the period duration, so that with the same switch-on and switch-off times, the duty cycle has the value l / 2.

The square wave voltage with variable duty cycle is then one Trigger circuit 32, which can have general amplifier form, fed and in suitable voltages and currents for controlling the respective Adjusting transistors 1, 2, 1 ', 24 and 26 implemented in such a way that the voltage difference present at the controller 28 becomes zero. Here, the circuit block 32 'shown in FIGS. 7 and 8 represents the The entirety of the voltage regulating circuit according to the invention, which has already been comprehensively explained in FIGS. 1 to 6.

The comparator 11 has an adjustable input at its input 11a

Voltage divider 34a, 34b also between u. and u vari-

^c ° emm emax

able input voltage u and compares this voltage with the

Reference voltage of the switching element present at the other input 11b 29. The comparison voltage can of course also be generated in a separate reference circuit assigned only to the comparator 11. the Switching threshold of the comparator is set, for example, at the adjustable voltage divider 34a, 34b to u -Au = u, where Λ u and a. the inevitable voltage losses in the switching transistors l ',

ORIGINAL INSPECTED

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24, 26 and in the diode 3 designated. At the output of the comparator there is then an absolute signal which only indicates whether im If u - ^ u> u downwards or if u -Δ u ^ u upwards must be, and which can be used directly or after suitable amplification to control the relay 7 or 13. In the case The comparator can be dispensed with if only the upward regulation according to FIG. 4 is used.

Is the changeover contact 14 of the relay 13, for example by the Thyristor 30 in Fig. 6 replaced, so instead of the comparator 11 is a comparator 11 'with an inverted output signal for controlling the Gate connection of the thyristor to be used.

In the case of control via 2 transistors 1 and 2, as shown in Fig. 2, the outputs of the comparator 11 and the modulator 31 can, for example, as shown in FIG. 8 via 3 logic gates 35, and 37 be connected to the drive circuits 32a and 32b of the two transistors 1 and 2. This, as already described above, achieved that in the case of upward regulation at logic level "0" of the comparator 11, the transistor 2 in the predetermined by the modulator 31 Way is cyclically switched on and off while transistor 1 is continuously conducting at the same time, while in the case of down regulation at logic level "1" of the comparator 11, the transistor is cyclically switched on and off while the transistor 2 is permanently off. For this purpose, the modulator 31 and the Comparator 11, which can be identical to the corresponding circuit elements of FIG. 7, each followed by an AND gate 35, 36,

JIM

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which are also driven crosswise by the modulator and the comparator are. The AND circuit 35 operates directly via a trigger circuit 32a on the switching transistor 2, during the AND circuit 36 is followed by a function block 37 which, in the case of the one shown Embodiment only allows positive signals to pass. The switching transistor 1 is then controlled via a further trigger circuit 32b. The AND gate 35 and the function block 37 are otherwise with the negated output signal of the comparator 11 controlled.

ORIGINAL INSPECTED

Claims (10)

Dipl. Ing.Peter Ott 7033 Herrenberg (Kuppingen) Patent attorney Eifeletraße 7 Telephone (070 32) 31999 1517 / ot / zz
Aug 18, 1980 Robert Bosch GmbH, 7000 Stuttgart 1 Claims / 1.! Device for voltage regulation in direct current on-board networks of mobile units, motor vehicles, trains, etc., with a rotary-driven generator, preferably a three-phase generator, and a battery storing electrical energy, characterized in that a flyback converter circuit consisting of a converter (20) with primary and secondary windings (6, 5) and a given winding ratio (ü) and at least one active semiconductor switching element (1, 2, 1 ', 24, 26) are provided that the input of the flyback converter circuit connected to the variable, rectified generator output voltage and its constant output voltage to the vehicle electrical system are and that at least one connecting line (1, 10, 25, 25 ') is provided, which bypassing the flyback converter primary winding (6) connects the input area to the secondary winding area. 2. Apparatus according to claim 1, characterized in that a flyback converter circuit is combined with the circuit of a buck converter in such a way that one terminal of the secondary winding (5) of the converter via a freewheeling diode (4) to the circuit zero point and equal I5l7 / ot / zz Aug 18, 1980 - 2 - timely via a first semiconductor switching element (transistor: 1) with the the variable input voltage (u) leading input is connected that the same input terminal via a further semiconductor switching element (Transistor 2) is connected to the primary winding (6) of the converter (20) and that in each of the two control directions (downward regulation, Upward regulation) one of the two semiconductor switching elements alternately conducts and blocks and the other is either permanently blocked or is permanently switched on (Fig. 2). 3. Apparatus according to claim 2, characterized in that the secondary winding (5) of the converter (20) is connected to the output via a diode (3) polarized in the same direction as the freewheeling diode (4) and that a control logic circuit (27, 28) is provided which each periodically switching semiconductor switching element a drive pulse train with such a duty cycle that the desired constant output voltage (u) with variable input voltage (u) results. 4. Apparatus according to claim 1, characterized in that one as a function to the ratio of the input voltage to the output voltage (u, u) switching relay (7) is provided which, depending on the direction of regulation (upward regulation, downward regulation), the secondary winding (5) of the converter (20) via a closed switching contact (10, 8) either directly to the potential-carrying input connection (23) or via a switching transistor (1 ") connects, with the direct connection of the switching transistor (lO via a switching ORIGINAL INSPECTED 1517 / ot / zz Aug 18, 1980 - 3 - contact (9) of the relay (7) with the primary winding connection of the converter (20) is connected, with double use of the switching transistor (I ') which is only present once (Fig. 3). 5. Apparatus according to claim 4, characterized in that the changeover relay (7) has a further switching contact (12) which bridges the diode (3) in the output circuit when the circuit works as a buck converter (switching transistor Y is with secondary winding 5 tied together). 6. Apparatus according to claim 1, characterized in that when single Upward regulation of the secondary winding connection is directly connected to the input connection (23) which is at input potential, where the circuit zero point for input and output side is common (4) (Fig. 4). 7. Apparatus according to claim 1, characterized in that one with the primary winding (6) of the converter (20) connected switching transformer (26) is provided as well as a changeover relay (13) with a changeover contact, which alternatively, depending on the direction of regulation (downward regulation, Upward regulation) the secondary winding connection via a changeover contact (14) either with the circuit zero point or directly with an input connection line (25 ') connects (Fig. 5). 8. Apparatus according to claim 1, characterized in that a combination is provided from a freewheeling diode (4 ') and a thyristor (30), the thyristor (30) b ·! effective freewheeling diode (4 ') blocked and for upward regulation the ignited tran- V 1 I t_ ν »/ 1517 / ot / zz Aug 18, 1980 - 4 - Sistor (30) supplies the input potential of the secondary winding (5) of the flyback converter with the freewheeling diode blocked at the same time (Fig. 6). 9. Device according to one or more of claims 1 to 8, characterized in that for controlling the switching transistors with a drive pulse train whose duty cycle is variable, a logic circuit is provided which has a frequency generator (27) and a setpoint value of the constant output voltage and the output actual voltage (u) is supplied. 10. Apparatus according to claim 9, characterized in that the logic circuit a regulator (28), which supplies a constant voltage value from the variable input voltage to a regulator Switching element obtained from output setpoint and the output actual voltage is supplied that the controller (28) is followed by a modulator (31) which controls the sequence of control pulses supplied to it by the generator (27) influenced in their duty cycle according to the output voltage of the controller (28) and that the modulator (31) via a trigger circuit (32) is connected to the respective switching element. ORIGINAL INSPECTED