EP2668715A1

EP2668715A1 - Method for regulating a buck/boost converter

Info

Publication number: EP2668715A1
Application number: EP11785629.4A
Authority: EP
Inventors: Jalal Hallak
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2011-01-25
Filing date: 2011-11-09
Publication date: 2013-12-04
Also published as: RU2013139169A; CN103314515A; US20130307508A1; EP2479878A1; EP2479878B1; WO2012100853A1; US9112403B2; CN103314515B

Abstract

The method involves operating buck-boost converter in a batch mode with quasi resonant switching. The inductor current (iL) or current through down converter switching element (S1) is compared with a reference current (Iref) to amplify control signal (Ureg), so that generation time of reference current before a turn-off pulse of pulse-width modulated switching signal (SigS1) is reached by amplified control signal (Uv) and pulse-width modulated switching signal (SigS2). An independent claim is included for buck-boost converter.

Description

Method for controlling a low boost converter

The invention relates to a method for controlling a clocked low-boost converter, wherein a buck clock switching element is driven with a common clock frequency with a first pulse width modulated switching signal and a boost converter switching element with a second

pulse width modulated switching signal is driven, for converting an input voltage into a regulated

Output voltage, wherein a regulator signal of a

Output voltage regulator for generating the

pulse width modulated switching signals is used. Furthermore, the invention relates to a switching converter for

Implementation of the procedure.

So-called low-boost converter have been known for a long time. These are essentially to switching converter, which alternately or in a transition region at the same time in the manner of a boost converter and in the manner of a

Buck converter work. In a corresponding

Circuitry a common throttle and a common input filter and a common output filter are provided. In general, the buck converter switching element is connected between the input voltage and the first terminal of the throttle. The reference potential of the input voltage is connected on the one hand to the reference potential of the output voltage and on the other hand also connected via a first diode to the first terminal of the throttle. The boost converter switching element switches the reference potential to the second terminal of the reactor. This second connection of the throttle is also connected via a second diode with the connection of

Output voltage connected (Fig. 1). The diodes can also be designed as a synchronous switch. If the input voltage exceeds the output voltage,

the switching converter works as a buck converter. As soon as the input voltage drops below the output voltage, a change takes place into the boost converter mode (FIG. 2).

The two switching elements are usually by means of

pulse width modulated switching signals controlled. To form these switching signals, usually a control signal of an output voltage regulator is superimposed on a sawtooth or triangular signal. For example, DE 43 06 070 C1 discloses a low boost converter, wherein two pulse width modulated signals are generated with only one regulator signal. A sawtooth signal is offset from the second sawtooth signal by a value equal to the amplitude. Thus, with increasing controller output signal is the

Duty cycle of the first pulse width modulated switching signal for driving the buck converter 100% percent before the second switching signal provides the boost converter first switching pulses.

In order to compensate for voltage drops in the components of the switching converter, methods are also known in which the two sawtooth signals are shifted by a value equal to the amplitude minus a correction value. A

Such superposition of Sägezahnsignale to realize an intersecting operation of the high and

Buck converter is known from US 6 166 527 A.

From WO 2009/033924 A2 is known during a

Transition from buck converter mode to

Hochsetzstellerbetrieb impose the Hochsetzer switching element an earlier clocking. This will be another

Pulse width modulated switching signal generated, which dictates an early clocking in the transition.

A trouble-free transition between the two modes is usually only possible if the switching elements of the Low boost converter to be switched in a continuous mode. The common choke always remains magnetically charged and there is none

Resonant vibrations, which usually occur after a successful demagnetization of the throttle.

In a discontinuous mode, the respective

Switching element switched on after the throttle

is demagnetized. So there is a dead time. According to the prior art so-called quasi-resonant switching (valley-switching) is sought in a discontinuous mode. In this case, the switching element is turned on when the oscillating at a resonant frequency voltage at the switched-off switching element has a minimum. In this way, turn-on losses are kept low.

In a generic low-boost converter can

indefinite resonant events within existing ones

Oscillating circuits lead to faults. Such disturbances affect, for example, increased losses, because no quasi-resonant switching can take place. In addition, occurring disturbances can lead to unwanted noise emissions.

The invention has for its object to provide an improvement over the prior art for a method of the type mentioned. In particular, a should

trouble-free operation can be realized in a discontinuous mode. Another object is to describe a buck-boost converter for carrying out the method.

According to the invention this object is achieved by a method according to claim 1 and a buck-boost converter according to

Claim 7. Improvements are to be taken from the dependent claims. Here, the low boost converter is in one

operated with quasi-resonant switching discontinuous mode, wherein the inductor current or the current detected by the buck converter switching element and compared with a reference current, wherein the regulator signal to the extent

is amplified, in which the achievement of the reference current in time before a turn-off of the first

Pulse width modulated switching signal occurs and wherein by means of the amplified control signal, the second

pulse width modulated switching signal is generated.

The described interventions in the control have the consequence that after a Drosselentmagnetisierung always a

unique resonant state exists, which allows a trouble-free quasi-resonant switching. A sloping

Input voltage initially leads to an increase in the

Controller signal. As a result, amplification of the regulator signal causes the boost converter switching element to begin to clock sooner, when the input voltage of the

Approximates output voltage. When superimposing with a constant Sägezahn- or triangular signal done

Overlaps, which are the on and off times

determine. The overlaps occur earlier than with the non-amplified control signal. The non-amplified regulator signal is further used to generate the first pulse width modulated switching signal for the buck converter switching signal.

Earlier activation of the boost converter causes the output voltage regulator to provide a lower regulator signal to slew the output voltage to a desired setpoint. The buck converter switching element continues to clock while the input voltage falls below the output voltage. This continued forced shutdown of the

Buck converter switching element causes a

parasitic capacitor of this buck converter switching element continues to charge. The parasitic capacitor forms together with the reactor inductance and a parasitic capacitor of the boost converter switching element a

Resonant circuit. The voltage minima occurring at the switching elements are used to switch the low boost converter quasi-resonant.

Without the specified measure, the transition from

Buck converter operation for boost converter operation

Undefined condition arise. Depending on if that

Decrement switching element just shuts off during a switching cycle or already remains on continuously, the state of charge of the parasitic

Change the capacitor of the buck converter switching element continuously. That would have one clock cycle to clock cycle

different resonance behavior of the resonant circuit for

Episode. A quesiräsonantes switching would not be possible.

It is advantageous if as reference current that current is specified which is just required to charge a parasitic capacitor of the buck converter switching element to the value of the input voltage. The intervention in the regulation remains to a minimum, without affecting the desired effect. Furthermore, it is advantageous if by means of a

Comparing the inductor current or the current through the buck converter switching element with the reference current

Comparative signal is formed, which is a

Comparison pulse has when the inductor current or the current through the buck converter switching element, the value of the

Reference current reached. Such a signal can be generated by means of a simple circuit structure.

An advantageous development of the method provides that by means of a phase evaluation of the first

pulse width modulated switching signal and the comparison signal, a phase difference signal is formed, that while the Phase difference signal is equal to zero, if within a switching cycle, the comparison pulse of the comparison signal in time after the turn-off pulse of the first

pulse width modulated switching signal occurs and that the phase difference signal proportional to the temporal

Difference between comparison pulse of the comparison signal and turn-off pulse of the first pulse-width modulated switching signal increases when within a switching cycle of the

Comparison pulse of the comparison signal before the time

Shutdown pulse of the first pulse width modulated switching signal occurs. The phase difference signal is directly to

Gain of the controller signal usable.

In particular, it is favorable to realize the amplification in such a way that the phase difference signal and the

Regulator signal are added to generate a boosted controller signal.

A further improvement in the transition from buck converter to boost converter operation is given when the Hochsetzer- switching element is driven in such a way that when approaching the falling input voltage to a smaller output voltage, the boost converter switching element with

starts rising clocking before the input voltage is equal to the output voltage. In this way, at each transition from the buck converter to the

Hochsetzer operation ensures that the Hochsetzer- switching element begins to clock with very short turn-on, when the output voltage to the

Input voltage approximates. This is done independently of the course of the second pulse width modulated switching signal, which is usually generated by superposition of a sawtooth or triangular signal with the controller signal. This measure results in that the profile of the current in the inductor of the step-up converter runs continuously in the transition phases from the step-down converter to the step-up converter mode. The inventive step-up converter for carrying out one of the methods described above provides that a control signal of an output voltage regulator together with a sawtooth or triangular signal a first

Pulse width modulator is supplied, at the output of the first pulse width modulated switching signal for driving the Tiefsetz switching element is applied and wherein a order in the

Substantially an amplitude of the sawtooth or triangular signal offset further sawtooth or triangular signal is fed to a second pulse width modulator, at whose output the second pulse width modulated switching signal is applied to drive the Hochsetzer switching element. Here are the

Inductor current or the current through the buck converter switching element and a reference current to a comparator

supplied, wherein the comparison signal at the output of the

Comparator and the first pulse width modulated switching signal are supplied to a phase evaluation module, wherein the

Phase difference signal applied to the output of the phase evaluation module and wherein the with the phase difference signal

amplified control signal is supplied to the second pulse width modulator. Thus, a simple structure is against, which is feasible both with discrete components as well as by means of an integrated circuit.

Advantageously, the phase evaluation module is as

Function module designed a microcontroller. Alternatively, a flip-flop circuit is connected downstream

Provide smoothing stage.

To generate the second pulse-width-modulated switching signal, it is advantageous if the phase difference signal and the regulator signal are fed to an adder and if the amplified applied to the output of the adder

Control signal is supplied to the second pulse width modulator. For a smooth transition from Tiefsetzsteller- to boost converter operation, it is also advantageous if a third pulse width modulator is provided, the sawtooth or triangular signal and an output signal of a

Transition controller are supplied and which is connected together with the second pulse width modulator via an OR circuit with the boost converter switching element. By means of the output signal of the transition controller, the third pulse width modulator generates independently of the controller signal

Pulse width modulated signal, which can clock the Hochsetzer- switching element earlier in the transition region from the buck converter to the boost converter operation, as would happen as a result of the second pulse width modulated switching signal.

The invention will now be described by way of example with reference to the accompanying drawings. In a schematic representation:

Fig. 1 Deep-boost converter according to the prior art Fig. 2 voltage waveform of a known Tief- Hochsetzstellers

Fig. 3 block diagram of an exemplary Tief- Hochsetzstellers without transition controller

Fig. 4 course of the switching signals over time without

Intervention in the scheme

Fig. 5 Course of the switching signals over time with intervention in the scheme

Fig. 6 voltage waveform of a Tief- Hochsetzstellers invention

Fig. 7 block diagram of an exemplary Tief- Hochsetzstellers with transition controller

Fig. 8 waveforms of a low boost converter after

Fig. 7 A known power section of a low boost converter comprises a throttle L, which is connected via a buck converter switching element Sl to an input voltage U _I. An output voltage U _0UT and the input voltage U _I are related to the same reference potential. There are

usually an input capacitor Ci and a

Output capacitor Co provided. Between the

Reference potential and a connection between inductor L and buck converter switching element Sl is a first diode Dl

connected. This is conducting in buck converter mode when the buck converter switching element Sl turns off.

On the output side, the inductor L is connected to the positive output of the low boost converter via a second diode D2

connected. A connection between the inductor L and the second diode D2 is connected via a boost converter switching element S2 to the

Reference potential switched. The two switching elements Sl, S2 each have a parasitic capacitor C _s i, C _S2 and are controlled by means of a pulse width modulator PWM control. To regulate the output voltage U _OUT , the pulse width modulator PWM control is supplied with both the output voltage U _OUT and the input voltage U _IN (FIG. 1).

As soon as the input voltage U _I drops below the output voltage U _OU , there is a transition from the step-down operation T to the step-up operation H (FIG. 2).

In a discontinuous operation form the parasitic capacitors C _s i, C _S2 with the inductance of the inductor L a resonant circuit. To avoid that during the transition from buck converter T to

Booster operation H an undefined

Resonance vibration behavior occurs, takes place

inventive intervention in the controller. A

exemplary embodiment is shown in Fig. 3. Of the

Power section of the low boost converter remains unchanged. In addition to the output voltage U _0UT is a control part of the current through the inductor L, followed by inductor current i _L called, fed. Specifically, this inductor current i _{L is} compared in a comparator Komp with a reference current I _Re f. By means of simple measurements, the value of the reference current I _Re f is determined in such a way that the parasitic capacitor Csi of the buck converter switching element Sl after each switching off of the buck converter switching element Sl to the voltage of

Input capacitor Ci is charged. Alternative to

Inductor current i _L , the current through the buck converter switching element Sl can be used to the appropriate

AbschaltZeitpunkt the buck converter switching element Sl to

determine.

In a transition region between buck converter and

Step-up operation increases the inductor current i _L , as long as the buck converter switching element Sl and the boost converter switching element S2 are turned on. After the boost converter switching element S2 switches off, the inductor current i _L drops. In this case, the time is detected at which the inductor current i _{L reaches} the value of the reference current I _Re f. This detection takes place by means of a at the output of the comparator Komp

appended comparison signal Sig _K om _P - The signal state of this comparison signal Sig _K om _P is for example equal to one when the inductor current is i _L to the reference current I _re f, and equal to zero when the inductor current i _{L becomes} lower than the

Reference current I _Re f is (Fig. 4, Fig. 5). A change from one to zero thus signals the achievement of the reference current

An output voltage regulator REG compares the

Output voltage U ₀ UT with a nominal value SOLL. The regulation will be described below by way of example for the case that a falling input voltage UI drops below the output voltage UOU. Without control intervention also sinks

Output voltage U ₀ UT from. To counteract this output voltage drop, the output voltage regulator REG provides a higher regulator signal U _RE G (FIG. 8). The regulator signal U _RE G is in a first pulse width modulator PWM1 a first Sawtooth signal U _{Saw of} a signal generator GEN superimposed. The resulting pulse width modulated switching signal Sig _s i is used to control the buck converter switching element Sl. The sawtooth signal U _Sä gl is also a peak detector PD for detecting a peak signal of the sawtooth signal U _Sä gl supplied. In an adder, the ramp signal U _Sä gl and the peak signal is added to an offset sawtooth signal U _Säg 2, which is supplied to a second pulse width modulator PWM2 for generating the second pulse width modulated switching signal Sig _S2. In this case, the offset sawtooth signal U _Sä g2 is the amplified signal controller U superimposed _v. The same functionality is achieved when the signal generator GEN outputs a triangular signal instead of a sawtooth signal. The extent of the control signal amplification depends on how long the period between the time at which the

Reactor current i _{L reaches} the value of the reference current I _Re f, and the AbschaltZeitpunkt the buck converter switching element Sl. If the shutdown time is before reaching the

Reference current I _Re f is, there is no gain of the control signal U _RE G -

Conveniently, the amplified control signal U _v by adding the control signal U _RE G with a

Phase difference _signal U _P h _aS e formed.

FIGS. 4 and 5 show the formation of the

Phase difference _signal U _PhaS e · In diagrams are the

Throttle current i _L , the first pulse width modulated switching signal Sig _s i, the second pulse width modulated switching signal Sig _S 2, the comparison signal Sig _K om _P and the phase difference _signal U _P h _aS e shown over the time t. By comparing the

Throttling current i _L with the reference current I _Re f is the

Comparison signal Sig _K om _P formed, as described above. Occurs during a switching cycle a falling edge of the

Comparison signal Sig _K om _P after the turn-off of the first pulse width modulated switching signal Sig _s i, this remains

Phase difference _signal U _P h _aS e is equal to zero (FIG. 4).

On the other hand, the phase difference _signal U _P h _aS e is not equal to zero when, during a switching cycle, a falling edge of the comparison signal Sig _K om _P occurs before the switch-off pulse of the first pulse-width-modulated switching signal Sig _s i. The longer the time span between the falling edge of the

Comparing signal Sig _K om _P and the turn-off _pulse , the greater the phase difference _signal U _P h _aS e (Fig. 5). Accordingly, the phase difference _signal U _P h _aS e indicates how much later the buck converter switching element Sl shuts off, based on the _instant at which the inductor current i _{L reaches} the value of the reference current I _Re f. The formation of the phase difference signal is realized in a simple manner by means of a flip-flop circuit and a downstream smoothing stage.

Alternatively, the formation of the phase difference signal U _P h _ase can be done by means of a program which is implemented in a microcontroller.

The intervention in the regulation described below

prevents too late switching off the buck converter switching element Sl, so that there is always enough residual current to charge the parasitic capacitor C _s i. As soon as a phase difference signal U _P h _ase non-zero occurs, an amplification of the control signal U _RE G- means of the amplified control signal U _v is the second

pulse width modulated switching signal Sig _S 2 formed. During the transition from buck converter to boost converter operation, the boosted regulator signal U _v causes the boost converter switching element S2 to clock sooner and with a longer duty cycle than unboosted

Regulator signal U _RE G would be the case. This increases the

Output voltage U ₀ UT and the output voltage regulator REG provides a lower control signal U _RE G to the

Output voltage U ₀ UT again to the setpoint nominal. As a result of the lower regulator signal U _REG shortens the duty cycle of the buck converter switching element Sl. Per

Switching cycle takes place, the switch-off pulse earlier and the

Time interval to the falling edge of the comparison signal

Sig _K0 mp shortens until both pulses are synchronized. This control loop causes the phase difference signal Up _h ase again goes to zero.

The described intervention in the regulation causes the transition from the buck converter operation T to

Boost mode H already starts when the

Input voltage U _I drops below a voltage U _T / _H , which is above the output voltage U _0UT (Fig. 6). In addition, also during the boost converter operation H, switching off of the buck converter switching element S1 continues.

In Fig. 7, a circuit construction is indicated, which during a transition from the buck converter operation T for

Boost converter operation H additionally causes a forced early clocking of the boost converter switching element S2, independently of the output signal of the second pulse width modulator PWM2. The corresponding signal curves are shown in FIG. 8.

On the second pulse width modulator PWM2 is unchanged to a second pulse-width-modulated output signal PWM2 OUT, which consists of the other, offset sawtooth signal U _Sä g2 and the increased controller signal U _v is formed. This

Output signal PWM2 _0UT is no longer directly available as a second pulse width modulated switching signal Sig _S2 . The formation of the first pulse width modulated

Switch signal Sig _s i remains unchanged. In addition, a third pulse width modulator PWM3, the sawtooth signal U _Sä gl and an output signal U _UK of a transition controller UK are supplied.

The transitional controller UK is, for example, one

Analog circuit or a digital signal processor, the one Function of the output signal U _UK over the difference between input voltage U _I and output voltage U _0UT maps.

For example, an output signal U _{UK is} generated when the input voltage U _I approaches a lower output voltage U _OUT and the difference between the input voltage U _I and the output voltage U _{0UT drops} below a limit value. The third pulse width modulator PWM gives a third

pulse width modulated output PWM3 ₀ U _T off, which is synchronous to the second pulse width modulated output PWM2 ₀ U _T. However, the switch-on pulses with rising duty cycle already occur when the

Input voltage U _{IN of} the output voltage U approaches _0UT and the second pulse width modulated output PWM2 ₀ U _T still has no pulses.

From the second pulse width modulated output signal PWM2 _0UT and the third pulse width modulated output signal PWM3 _0UT OR is the second by means of an OR circuit

Pulse width modulated switching signal Sig _S2 for driving the Hochsetzer switching element S2 formed.

This switching signal Sig _S2 corresponds to the third pulse-width-modulated output signal PWM3 ₀ uiv until the duration of the switching pulses are shorter than ene of the second pulse-width-modulated output signal PWM2 _0UT running synchronously. From this time, the second corresponds

pulse width modulated switching signal Sig _S2 the second

pulse width modulated output signal PWM2 _0UT -

The third pulse width modulated output signal PWM3 _0UT is therefore only used to drive the Hochsetzer switching element S2 when the difference between the input voltage U _IN and output voltage U _0UT below the limit of

Difference drops and the second pulse width modulated

Output signal PWM2 _0UT still provides no turn-on _times , which ene the third pulse width modulated output signal PWM3 ₀ U _T exceed. The activation of the boost converter switching element S2 under

Inclusion of the transitional control UK is thus only in the short phases of the transition from buck converter to boost converter operation, without the regulation continues to

influence .

Claims

claims

1. A method for controlling a clocked Tief- Hochsetzstellers, wherein at a common clock frequency, a buck converter switching element (Sl) with a first

pulse width modulated switching signal (Sig _s i) is controlled and a boost converter switching element (S2) is controlled with a second pulse width modulated switching signal (Sig _S2 ), for converting an input voltage (Ui _N ) into a regulated output voltage (U _0UT ) I wherein a control signal ( U _REG ) of an output voltage regulator (REG) for generating the

pulse-width modulated switching signals (Sig _s i, Sig _S2 ) is used, characterized in that the buck-boost converter is operated in a discontinuous mode with quasi-resonant switching, that the inductor current (i _L ) or the current detected by the buck converter switching element (Sl) and with a reference current (I _Ref ) is compared that the regulator signal (U _REG ) is amplified to the extent that the achievement of the reference current (I _Ref ) in time before a turn-off pulse of the first pulse width modulated

Switching signal (Sig _s i) occurs and that by means of the

amplified control signal (U _v ) the second

pulse width modulated switching signal (Sig _S2 ) is generated.

2. The method according to claim 1, characterized in that as reference current (I _Ref ) that current is given, which is required to a parasitic capacitor of the buck converter switching element (Sl) to the value of

Input voltage (Ui _N ) to charge.

3. The method according to claim 1 or 2, characterized

characterized in that by means of a comparison of

Inductor current (i _L ) or the current through the buck converter switching element (Sl) with the reference current (I _Re f) a

Comparison signal (Sig _K0 mp) is formed, which is a

Comparative pulse when the inductor current (i _L ) or the current through the buck converter switching element (Sl) reaches the value of the reference current (I _Re f).

4. The method according to claim 3, characterized in that by means of a phase evaluation (PhB) of the first

pulse width modulated switching signal (Sig _s i) and the

Comparison _signal (Sig _K om _P ) is a phase difference _signal (U _P h _aS e) is formed, that while the phase difference _signal (U _P h _aS e) is equal to zero, if within a switching cycle of the

Comparison pulse of the comparison signal (Sig _K om _P ) in time after the turn-off of the first pulse width modulated

Switching signal (Sig _s i) occurs and that the

Phase difference _signal (U _PhaS e) increases in proportion to the time difference between comparison pulse of the comparison signal (Sig _K0 mp) and turn-off pulse of the first pulse-width modulated switching signal (Sigsi), if within one

Switching cycle of the comparison pulse of the comparison signal (Sig _K om _P ) in time before the turn-off of the first

pulse width modulated switching signal (Sigsi) occurs.

5. The method according to claim 4, characterized in that the phase difference _signal (U _PhaS e) and the regulator signal (U _REG ) for generating an amplified control signal (U _V ) are added.

6. The method according to any one of claims 1 to 5, characterized in that the boost converter switching element (S2) is driven in such a way that at an approach of the falling input voltage (Um) to a smaller

Output voltage (U _0UT ), the _boost converter switching element (S2) begins to clock with increasing duty cycle, before the input voltage (Ui _N ) is equal to the output voltage (U _0UT ).

7. buck-boost converter for performing a method according to any one of claims 1 to 6, wherein a regulator signal

(U _REG ) of an output voltage regulator (REG) together with a sawtooth or triangular signal (U _Sag l) a first Pulse width modulator (PWM1) is supplied to the output of a first pulse width modulated switching signal (Sig _s i) for driving a bucking switching element (Sl) is applied and wherein a substantially one amplitude of the sawtooth or triangular signal (U _Sä gl) offset further sawtooth - or triangular _signal (U _Säg 2) a second pulse width modulator (PWM2) is supplied to the output of a second

Pulsweitenmoduliertes switching signal (Sig _S 2) for driving a Hochsetzer switching element (S2) is applied, characterized in that the inductor current (i _L ) or the current through the buck converter switching element (Sl) and a reference current (I _Re f) a comparator ( Komp) are fed to that

Comparison signal (Sig _Ko mp) at the output of the comparator (Komp) and the first pulse width modulated switching signal (Sig _s i) are fed to a phase evaluation module (PhB) that a

Phase difference _signal (U _P h _aS e) at the output of the

Phase _{evaluation module} (PhB) is applied and that with the phase difference _signal (U _P h _aS e) amplified control signal (U _RE G) is fed to the second pulse width modulator (PWM2).

8. low boost converter according to claim 7, characterized

in that the phase evaluation module (PhB) is designed as a functional module of a microcontroller.

9. buck-boost converter according to claim 7 or 8, characterized in that the phase difference signal (Up _ha se) and the regulator signal (U _REG ) are fed to a summing element and that applied to the output of the addition element

amplified control signal (U _v ) is supplied to the second pulse width modulator (PWM2).

10. buck-boost converter according to one of claims 7 to 9, characterized in that a third

Pulse width modulator (PWM3) is provided to which the _Sägezahn- or triangular _signal (U _Säg l) and an output signal (U _Kon t) of a transitional controller (UK) are fed and which

together with the second pulse width modulator (PWM2) via an OR circuit (OR) is connected to the boost converter switching element (S2).