VOLTAGE CONVERSION METHOD

Abstract

Method for a voltage converter, wherein the method comprises in each of successive control cycles: switching on a first electronic switch (1) connected in series with a primary winding (21) of a transformer (2) for a duty period (TON1); premagnetizing the transformer (2) for a premagnetization period (TON2) before the first electronic switch (1) is switched on; wherein a first delay time (TDEL1) exists between an end of the premagnetization period (TON2) and a start of the duty period (TON1), wherein the premagnetization period (TON2) is set based on information about the voltage level of an input voltage (Vin) supplied to the voltage converter, depending on the voltage level of the input voltage (VIN).

Description

[0001] Embodiments of the present invention relate to a voltage conversion method, in particular a method for operating a flyback converter in discontinuous conduction mode (DCM) and a voltage converter.

[0002] Switching-mode power supplies (SMPS) are widely used for power conversion in automotive, industrial, and consumer electronics applications. A flyback converter is a specific type of switching-mode power supply that incorporates a transformer with a primary and a secondary winding that have opposite winding directions. An electronic switch is connected in series with the primary winding. When the electronic switch is closed, the transformer is magnetized, and when the electronic switch is open, it is demagnetized. Magnetizing the transformer involves storing energy within it, and demagnetizing the transformer involves transferring the stored energy to the primary winding and any load connected to it.

[0003] A flyback converter can be operated in discontinuous conducting mode (DCM). In this mode, there is a delay between the time the transformer is completely demagnetized and the time the electronic switch turns on again. During this delay, parasitic oscillations of the voltage across the electronic switch can occur. In DCM, the flyback converter can also be operated in quasi-resonant mode, where the electronic switch turns on when the voltage across the switch is at its minimum. When the flyback converter is operated in quasi-resonant mode, the switching frequency of the electronic switch varies, whereas operating the electronic switch at a fixed frequency in a flyback converter can lead to increased switching losses.Nevertheless, there may be scenarios where it is desirable to operate the flyback converter in the DCM at a fixed frequency.

[0004] Therefore, there is a need for a method to operate a flyback converter in the DCM at a fixed frequency and with low switching losses.

[0005] One embodiment relates to a method for a voltage converter. The method comprises, in each of successive drive cycles, switching on a first electronic switch connected in series with a primary winding of a transformer for a specified duty period, and, prior to switching on the first electronic switch, premagnetizing the transformer for a premagnetization period. In this method, there is an initial delay between the end of the premagnetization period and the beginning of the duty period.

[0006] One embodiment relates to a voltage converter. The voltage converter comprises a transformer having a primary winding, a first electronic switch connected in series with the primary winding, and a control circuit. The control circuit is configured to switch on the first electronic switch for a duty cycle during each of a plurality of successive control cycles. Before the first electronic switch is switched on, this causes the transformer to be premagnetized for a premagnetization period and generates a first delay time between the end of the premagnetization period and the beginning of the duty cycle.

[0007] The following are examples of implementation with reference to the drawings. The drawings serve to illustrate certain principles, so only the aspects necessary for understanding these principles are shown. The drawings are not to scale. In the drawings, the same reference numerals denote the same features.

[0008] Fig. Figure 1 shows a flyback converter according to an exemplary embodiment;

[0009] Fig. Figure 2 shows time-series diagrams illustrating the operation of a flyback converter in a quasi-resonant operating mode;

[0010] Fig. Figure 3 shows time-series diagrams illustrating one type of operation of a flyback converter at a given frequency;

[0011] Fig. Figure 4 shows an embodiment of a flyback converter which has means according to an embodiment for premagnetizing a transformer in the flyback converter;

[0012] Fig. 5 shows a modification of the in Fig. 4 of the shown barrier converters;

[0013] Fig. Figure 6 shows an embodiment of a flyback converter which includes means according to an embodiment for premagnetizing a transformer in the flyback converter; and

[0014] Fig. Figure 7 shows an embodiment of a flyback converter which has means according to an embodiment for premagnetizing a transformer in the flyback converter.

[0015] The following detailed description refers to the accompanying drawings. The drawings form part of the description and illustrate, by way of specific embodiments, how the invention can be implemented. It is understood that the features of the various embodiments described herein can be combined with one another, unless expressly stated otherwise.

[0016] Fig. Figure 1 shows a voltage converter (switching power supply, SMPS) according to an exemplary embodiment. The one in Fig. The voltage converter shown in section 1 has a flyback converter topology and will be referred to as a flyback converter below. The flyback converter has one input with a first input node and a second input node, which are used to receive an input voltage V. IN be designed, and have an output with a first output node and a second output node, which are used to provide an output voltage V OUT are trained. A load Z (in Fig. (1 shown using dashed lines) the output voltage V OUT or an output current I OUT , which are available at the output. The flyback converter contains a transformer. 2 with a primary winding 2 1 and a secondary winding 2 2, which are connected to the primary winding 21 is magnetically coupled. The primary winding 2 1 and the secondary winding 2 Two have opposite winding directions. An electronic switch 1 is connected to the primary winding 2 1 connected in series, the series connection with the primary winding 2 1 and the electronic switch 1 is connected between the first and second input nodes to determine the input voltage V IN to obtain.

[0017] A capacitor is optional. 51 , which is subsequently referred to as the input capacitor, is connected between the input nodes of the input. The input capacitor 51 can help reduce ripple in the input voltage V IN to filter. According to one embodiment, the input voltage V IN a direct current (DC) voltage. This input voltage V IN can be achieved through a rectifier circuit 10 (in Fig. 1 shown using dashed lines) from an alternating voltage (AC voltage) V AC The input voltage is referenced to a first ground node GND1, and the output voltage is referenced to a second ground node GND2.

[0018] Referring to Fig. 1 indicates the barrier converter 1 also a rectifier circuit 3 on, which is between the second winding 2 2 and the output is connected. In the case of the Fig. In the embodiment shown in 1, this rectifier circuit 3 a series circuit with a rectifier element 31 , for example, a diode, and a capacitor. This series connection is related to the secondary winding. 2 2 connected in parallel, and the output voltage VOUT is across the capacitor 32 available. However, this is merely an example implementation of the rectifier circuit.3 Other implementations of the rectifier circuit 3 Implementations that include an additional throttle, for example, can be used just as well.

[0019] A control circuit 4 is trained to operate the electronic switch 1 based on a feedback circuit (control loop) 61 , 62 received feedback signal S FB to control. The feedback circuit can act as a filter. 61 feature that receives the output voltage VOUT, and a transmitter 62 . In the Fig. In the embodiment shown in 1, the filter is located 61 on the secondary side of the transformer, and the transmitter 62 transmits an output signal of the filter 61 from the secondary side to the primary side, with an output signal from the transmitter 62 the control circuit 4received feedback signal S FB is. The filter 61 is designed to generate an error signal and a reference signal from the output voltage, and the feedback signal S FB to generate based on the error signal. This is generally known, so no further detailed explanation is necessary in this regard. According to one embodiment, the filter has 61 a proportional (P) characteristic, a proportional-integral (PI) characteristic, a proportional-integral-differential (PID) characteristic. According to a further embodiment (not shown), the position of the filter is 61 and the transmitter 62 in the feedback loop, so that the transmitter 62 a signal that determines the output voltage V OUTrepresents, transmits from the secondary side to the primary side, and the filter receives the signal transmitted by the transmitter and the feedback signal S FB generated. In the illustrated embodiment, the transmitter 62 An optocoupler is used. However, this is merely an example. Other transmitters suitable for transmitting a signal across a potential barrier can be used just as well. Examples of such transmitters include a transmitter with a transformer, such as a coreless transformer.

[0020] The control circuit 4 is trained to operate the electronic switch 1 to operate in a pulse-width modulated (PWM) manner. According to one embodiment, the electronic switch is a transistor. In the Fig. In the embodiment shown in Figure 1, the transistor is a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), specifically an n-channel MOSFET. However, this is merely an example. Other types of transistors, such as an IGBT (Insulated Gate Bipolar Transistor), a JFET (Junction Field-Effect Transistor), a BJT (Bipolar Junction Transistor), or a p-channel MOSFET, can be used just as well.

[0021] A mode of operation of the in Fig. The blocking converter shown in point 1 is described below with reference to Fig. 2 explained. Fig. Figure 2 shows a time-history diagram of a load section voltage V DS over a load section of the electronic switch 2 , an auxiliary voltage V AUX over an auxiliary winding 2 3 of the transformer, a control signal S1, which the electronic switch 1 from the control circuit 4 refers to a load current I DSthrough the electronic switch 1 , and a magnetization M TR of the transformer 1 . In the Fig. 1 MOSFET shown 1 is the load section voltage V DS the drain-source voltage, and the load current I DS This is the drain-source current. The drive signal S1 is received by a gate node of the MOSFET. 1 The control signal S1 can be received from a first signal level that triggers the electronic switch. 1 switches on, and a second signal level that activates the electronic switch 1 switches off, exhibiting a level. The first level is subsequently referred to as the switch-on level and the second level as the switch-off level. For illustrative purposes only, the switch-on level of the control signal S1 is shown in the Fig. The high signal level is shown in the embodiment 2, and the switch-off level is shown as a low level.

[0022] Referring to Fig. 2. The operation of the flyback converter comprises a multitude of successive control cycles, with the control circuit being activated in each control cycle. 4 the electronic switch 1 for a duty cycle T ON1 switches on and after the switch-on period T ON1 the electronic switch 1 for a switching-off duration T OFF switches off. In the case of the Fig. In the embodiment shown in Figure 2, one of these control cycles begins at time t1 and ends at time t4, which is when the next control cycle begins. During the duty cycle T ON1 The input voltage V causes IN , that the load current I DS through the primary winding 2 1 and the electronic switch 1 flows, with a current level of the load current I DS during the switch-on time T ON1 increases. This increasing load current I DS increases with increasing magnetization MTR of the transformer 2 This magnetization is accompanied by a magnetic storage of energy in the transformer. 2 (more precisely, in an air gap of the transformer) 2 ) is connected, with the stored energy increasing when the load current I DS increases. During the duty cycle T ON1 is the load section voltage V DS of the electronic switch 1 essentially zero, and a voltage across the primary winding 2 1 is essentially equal to the input voltage V IN . In the Fig. In the exemplary embodiment shown in 1, the auxiliary winding has 2 3 and the primary winding 2 1. Opposite winding direction. In this case, a voltage level of the auxiliary voltage V. AUX given by V AUX = –(N AUX / N 21 )·V 21 (1) where N AUXthe number of turns of the auxiliary winding 2 3 is, N 21 the number of turns of the primary winding 2 1 is, and V 21 The voltage across the primary winding. During the duty cycle –T ON1 is the voltage level of the auxiliary voltage V AUX thus N AUX / N 21 ·V IN .

[0023] If the electronic switch 1 when it switches off, the transformer will be switched on 2 stored energy to the secondary winding 2 2. The rectifier circuit 3 or transfer the load Z. This causes the transformer to 2 is demagnetized. Fig. 2 denotes T DEMAG a duration during which the transformer 2 The magnetization process occurs, meaning that energy is transferred to the secondary side of the transformer. During this time period T DEMAG, which is also referred to below as demagnetization time, is the load section voltage V DS essentially equal to the input voltage V IN plus a reflected voltage (V) REFLECT The reaction voltage V REFLECT is essentially given by V REFLECT = N1 / N2·(V OUT + V3) (2), where N1 is the number of turns of the primary winding 2 1 is, N2 is the number of turns of the secondary winding. 2 2 is, and V3 is the voltage across the rectifier circuit. 3 The voltage V3 across the rectifier circuit is... 3 depends on the current level of a current I 22 through the secondary winding 2 2 off. This current I 22 decreases during the demagnetization period T DEMAG , so that the reaction voltage V REFLECT reduced and at the end of the demagnetization period T DEMAG N1 / N2·VOUT reached.

[0024] In quasi-resonant operating mode, there is a delay time between a time t3, at which the transformer has been completely demagnetized, and a time at which the next drive cycle begins, that is, when the electronic switch 1 is switched on again. During this time, the load line voltage V oscillates. DS This stems from a parasitic resonant circuit affecting the primary winding. 2 1 and a parasitic capacitance of the electronic switch 1 This parasitic capacitance can contribute to the load path of the electronic switch. 1 parallel capacity included. In the case of the Fig. In the embodiment shown in Figure 1, such a parasitic capacitance is represented by a capacitor connected in parallel to the load path. In quasi-resonant operating mode, the control circuit switches 4the electronic switch 1 one, if the load section voltage V DS , after the transformer 2 was demagnetized, a minimum V DSmin1 achieved. However, the voltage level of the minimum voltage V can be reached. DSmin1 The switching losses can be relatively high, resulting in relatively high switching losses. These switching losses include, for example, losses associated with the discharge of the parasitic capacitance of the electronic switch. 1 These losses are higher the higher the voltage level of the load section voltage V. DS at time t1 of switching on the electronic switch 1 Furthermore, the switching frequency changes in quasi-resonant operating mode and depends, among other things, on the duty cycle T. ON1However, there are applications where such a change in switching frequency is undesirable, and where it is desirable to operate the flyback converter at a predetermined frequency. The predetermined frequency can be fixed, or it can depend on the power consumption of the load Z.

[0025] Fig. Figure 3 shows time history diagrams of a procedure used for switching the electronic switch. 1 with a predetermined frequency and offers low switching losses. Fig. Figure 3 shows time-series diagrams of the load section voltage V DS , the auxiliary voltage V AUX , of the control signal S1 of the electronic switch 1 , of the load current I DS of the electronic switch 1 and the magnetization M TR of the transformer 2 The magnetization represents the magnetic flux in a core (not shown) of the transformer. 2or the flux density.

[0026] Referring to Fig. 3 includes the procedure in each control cycle, before the electronic switch is turned on. 1 for the duty cycle T ON1 , the pre-magnetization of the transformer 2 for a premagnetization period T ON2 and, after premagnetizing the transformer 2 , for an initial delay time T DEL1 before switching on the electronic switch 1 to wait. Pre-magnetizing the transformer 2 Can switching on one of the electronic switches 1 This includes various other electronic switches. Examples of such additional electronic switches are explained in more detail below. The following section describes the electronic switch 1 , which belongs to the primary winding 2The first electronic switch is connected in series, and the next electronic switch is used to control the transformer. 2 The pre-magnetized component is referred to as a second electronic switch. A control signal S2 for controlling this second electronic switch is also included. Fig. Figure 3 shows that in this embodiment, a high level represents a turn-on level that activates the second electronic switch to premagnetize the transformer, and a low level represents a turn-off level that deactivates the second switch. Premagnetizing the transformer 2 includes magnetizing the transformer 2 such that the load current I DS immediately after switching on the first electronic switch 1 in one of the Fig. The current flows in the opposite direction to that shown in point 1. The load current flows in this opposite direction until the transformer... 2 was demagnetized. That is, during the premagnetization period T ON2 preserved magnetization M TR possesses a polarity that corresponds to the polarity of the magnetization M TR at the end of the duty cycle T ON1 is opposite. That is, the magnetization M TR Its polarity changes during the on-time T. ON1 . In Fig. 3 denotes t11 a time at which the magnetization M TR zero and its polarity changes. The following describes the period during the premagnetization time T. ON2 preserved magnetization (the magnetic flux) M TR The magnetization after time t11 is referred to as negative magnetization, and the magnetization after time t11 is referred to as positive magnetization. The negative magnetization of the transformer 2includes the magnetic storage of energy in the transformer 2 After the premagnetization period T ON2 , that is, during the first delay time T DEL1 between the premagnetization period T ON2 and the duty cycle T ON1 , causes the transformer 2 and the parasitic capacitance of the electronic switch 1 stored energy causes an oscillation of the load line voltage V DS of the electronic switch. Due to the fact that the transformer is negatively magnetized, the amplitude of this oscillation is higher than the amplitude of the oscillation at the end of the previously explained demagnetization period T. DEMAG Therefore, a voltage minimum V DSmin2 , that the load section voltage V DS during the first delay time T DEL1 reached, lower than the minimum V DSmin1 , that the load section voltage V DSafter the end of the demagnetization period T DEMAG achieved. According to one embodiment, the first delay time T DEL1 chosen so that the first electronic switch 1 switches on when the load section voltage V DS the minimum V DSmin2 reached. This first delay time T DEL1 is controlled by the control circuit 4 controlled.

[0027] At the beginning of the duty period T ON1 will be in the transformer 2 during the demagnetization period T ON2 stored energy at the input or the input capacitor 51 transferred. The duration for which this energy is transferred to the input is represented by the duration for which the load current I DS at the in Fig. The time-series diagram shown in point 3 is negative. The transformer was demagnetized when the load current I DS At time t11 zero is reached (and the magnetization MTR Zero is reached). After time t11, the transformer 2 positively magnetized until the first electronic switch 1 at time t2 it switches off. After the on-time Ton1, the energy stored in the transformer is released during the demagnetization time T. DEMAG , as explained above, to the secondary winding 2 2. The rectifier circuit 3 and transfer the load Z.

[0028] At the in Fig. In the 3 methods shown, the switching losses are lower than in the quasi-resonant operating mode (see Fig. 2), since the first electronic switch 1 it switches at a lower load section voltage, namely at V DSmin2 compared to V DSmin1 in quasi-resonant operating mode. Premagnetizing the transformer. 2 This is not associated with significant losses, since the energy required to premagnetize the transformer 2is used at the beginning of the duty cycle T ON1 to the input or the input capacitor 51 is returned. The level of the second minimum V DSmin2 depends, among other things, on the demagnetization time T. ON2 from, whereby the voltage level of the minimum V DSmin2 decreased if the demagnetization time T ON2 increases. According to one embodiment, the demagnetization time T ON2 set so that the voltage level of the second minimum V DSmin2 greater than zero. The electronic switch 1 It can exhibit a parasitic capacitance that increases when the load section voltage V DS It decreases. Therefore, the [value] increases during the premagnetization period T. ON2 in the transformer 2 Energy to be stored, required to overcome the parasitic capacity in the first delay time T DEL1The rate of discharge is disproportionate, the lower the desired voltage level of the second minimum V. DSmin2 is. According to one embodiment, the demagnetization time T is ON2 such that the voltage level of the minimum V DSmin2 5 V or higher, 10 V or higher, or 20 V or higher. According to one embodiment, the minimum is less than 50 V.

[0029] The premagnetization period T ON2 can be done at any time after the demagnetization period T DEMAG begin. That is, there is no need to specify the demagnetization time T. ON2 at a specific phase angle of the parasitic oscillation of the load section voltage V DS to begin, which after the demagnetization period T DEMAG occurs. Therefore, the transformer 2 to be premagnetized at a predetermined (fixed) frequency. That is, a duration T between the start of the premagnetization period T ON2in a control cycle and the beginning of the premagnetization period T ON2 in a subsequent control cycle it can be constant. For example, if the premagnetization time T ON2 is essentially the same in each control cycle and the first delay time T DEL1 If the switching frequency of the first electronic switch is essentially the same in each control cycle, then 1 equal to the predetermined (fixed) frequency f = 1 / T with which the transformer is premagnetized. Therefore, this can be in Fig. 3 methods shown for the flyback converter for operating the electronic switch 1 with a fixed frequency. "Fixed frequency operation" means that the switching frequency during operation of the flyback converter is essentially fixed, independent of the load's power consumption. According to another embodiment, the controller changes 4The switching frequency is based on the power consumption of the load Z, whereby the switching frequency can decrease if the power consumption increases. The power consumption of the load Z is determined by the feedback signal S. FB represented. The power consumption of the load is essentially equal to the instantaneous output power of the flyback converter.

[0030] The duty cycle T ON1 of the first electronic switch 1 can be done in a conventional way depending on the feedback signal S FB and are therefore controlled depending on the power consumption of the load Z. If the power consumption of the load Z increases, the duty cycle T will be increased. ON1 longer, consequently the demagnetization time T DEMAG longer and the second delay time T DEL2 It becomes shorter. If the power consumption of the load Z increases, the duty cycle T becomes shorter. ON1 shorter, consequently the demagnetization time T becomesDEMAG shorter and the second delay time T DEL2 It will take longer.

[0031] In Fig. 3 denotes T OSC the duration of the parasitic oscillation of the load section voltage V DS , which after the demagnetization period T DEMAG occurs. This duration T OSC is essentially equal to the duration of the parasitic oscillations during the first delay time T DEL1 , which causes the load section voltage V DS falls away. Referring to Fig. 3 can be the first delay time T DEL1 essentially a quarter of the oscillation period, that is T OSC / 4. This oscillation period T OSC depends on the voltage level of the input voltage V IN Therefore, the first delay time T DEL1 according to one embodiment depending on a voltage level of the input voltage V IN set. Since the voltage level of the input voltage V INSince the voltage level is usually constant or changes slowly during the operation of the flyback converter, so that it remains constant over a large number of successive control cycles, this impairs the setting of the first delay time T. DEL1 depending on the input voltage V IN the specified frequency operation of the first electronic switch 1 No. The same applies to the setting of the premagnetization time T. ON2 depending on the input voltage V IN According to one embodiment, the premagnetization time T ON2 set so that it becomes longer when the voltage level of the input voltage V IN increases.

[0032] According to one embodiment, the method includes measuring the oscillation period T. OSC during the second delay time T DEL2 a control cycle and the setting of the first delay duration T DEL1during one or more successive control cycles based on this measurement. Since the oscillation period T OSC Referring to the above regarding the voltage level of the input voltage V IN If the voltage level is either constant or changes slowly, it may be sufficient to determine the oscillation period T. OSC not to be measured at every tax cycle, but every few tax cycles.

[0033] The oscillation period T OSC can be based on the auxiliary voltage V AUX can be measured. For example, the oscillation period T can be measured. OSC determining those times at which the auxiliary voltage V AUX zero, and measuring a duration between a time t31, at which the auxiliary voltage V AUX zero is reached for the first time, and at a time t32, at which the auxiliary voltage V AUXZero is reached for the third time. According to a further embodiment, the time difference between two successive points in time at which the auxiliary voltage V is reached is measured. AUX Zero is reached. This time corresponds to half the oscillation period T. OSC , that is, T OSC / 2. Based on this, the oscillation period T can be determined. OSC will be calculated.

[0034] The in Fig. The three methods shown can be implemented in different ways. According to one embodiment, the premagnetization periods T begin ON2 with a predetermined frequency. Referring to the above, the predetermined frequency can be fixed or depend on the power consumption of the load. A clock signal can be used to start these biasing periods T. ON2 to determine with a predetermined frequency. In Fig. 3 denotes t01 and t4 as the times at which the premagnetization periods begin. The premagnetization period T ON2 , the first delay time T DEL1 and the duty cycle T ON1 As explained above, the demagnetization time T can be set (calculated). DEMAG and the second delay time T DEL2 depend on the duty cycle T ON1 and adjust themselves automatically according to T DEL2 + T DEMAG = T – (T ON1 + T DEL1 + T ON2 ) (3).

[0035] According to a further embodiment, the duty cycles T begin ON1 with a predetermined frequency, which can be fixed or depend on the power consumption of the load. That is, the control circuit 4 switches the first electronic switch 1with a predetermined frequency. A clock signal can be used to determine the times at which the on-times T are activated. ON1 begin, that is, to which the electronic switch 1 switches on. In the figure, t1 and t5 denote the times at which the switch-on durations T ON1 begin. The duty cycles T ON1 , the premagnetization times T ON2 and the first delay time T DEL1 These parameters can, in turn, be set (calculated) as explained above. Based on these parameters and the duration T of a control cycle, the times at which the premagnetization periods T are calculated are determined. ON2 begin. Referring to Fig. 3 is a duration between the start of a duty cycle T ON1 and the beginning of the next premagnetization period T ON2 the duty cycle T ON1 plus the demagnetization time T DEMAG plus the second delay time T DEL2(T ON1 + T DEMAG + T DEL2 This duration can easily be derived from the duration T of a control cycle T, the premagnetization duration T, as follows: ON2 and the first delay time T DEL1 to be calculated: T ON1 + T DEMAG + T DEL2 = T – (T ON2 + T DEL1 ) (4).

[0036] According to yet another embodiment, the duration T of a control cycle or the switching frequency (which is 1 / T) is determined by setting (calculating) the second delay time T. DEL2 discontinued. Referring to Fig. 3 is a control cycle duration T given by T = (T ON1 + T DEMAG + T DEL2 + T ON2 + T DEL1 ) (5a), where the duty time T ON1 , the premagnetization period T ON2 and the first delay time T DEL1 as mentioned above, they can be set (calculated). The demagnetization time TDEMAG is based on the duty cycle T ON1 automatically. Thus, a desired duration T of a control cycle can be set, provided that the parameters T ON1 , T ON2 and T DEL1 be set up and T DEMAG automatically adjusts itself by setting the second delay time T DEL2 can be set. Therefore, no clock signal is required to start the duty cycle T. ON1 or the premagnetization period T ON2 determined. Based on equation (5a), the second delay time T can be DEL2 as follows: T DEL2 = T – (T ON1 + T DEMAG + T ON2 + T DEL1 ) (5b).

[0037] The desired control cycle duration T can be fixed or depend on the power consumption of the load. In the latter case, the control cycle duration T can increase if the power consumption decreases, thus reducing the switching frequency.

[0038] Setting the second delay time T DEL2 and thus, setting the control cycle duration T (the switching frequency) based on equations (5a) and (5b) requires that the demagnetization duration T DEMAG is measured (recorded). Measuring the demagnetization time can be measured as the duration between the end of the duty cycle T and the end of the demagnetization period. ON1 The time t2 and the time t3 at which the transformer was demagnetized are included. Time t2 is the time at which the control signal S1 switches to an off level. Furthermore, time t2 is given by the time at which the premagnetization period T ON2begins, plus the premagnetization period T ON2 , the first delay duration T DEL1 and the duty cycle T ON1 The latter are set (calculated) as explained above. A control cycle begins at the start of the premagnetization period T. ON2 as through the second delay time T DEL2 certainly.

[0039] According to one embodiment, the control cycle duration T is increased by setting a third delay time T DEL2 'Discontinued. Referring to Fig. 3 is the third delay duration T DEL2 ' equal to the second delay duration T DEL2 minus one quarter of the oscillation period T OSC . That means, T DEL2 ' = T DEL2 – T OSC / 4 (6).

[0040] Based on equations (5b) and (6), the third delay time T can be determined. DEL2 ' can be calculated as follows: T DEL2 ' = T – (T ON1 + TDEMAG + (T OSC / 4) + T ON2 + T DEL1 ) (7).

[0041] In other words, the control cycle duration T can be adjusted by setting the third delay time T. DEL2 ' be hired, provided that T ON1 , T ON2 and T DEL1 as explained above and T DEMAG and T OSC / 4 adjust automatically. Setting the second delay time T DEL2 and therefore, setting the control cycle duration T (the switching frequency) based on equation (7) requires that the demagnetization duration T DEMAG plus a quarter T OSC / 4 of an oscillation period T OSC is measured. Measuring this can be used to measure the end of the duty cycle T. ON1 to include time t2, and detecting when the auxiliary voltage V AUX for the first time after the end of the duty cycle T ON1 Zero crosses. In Fig. 3 denotes t31 the time at which the auxiliary voltage V AUX for the first time after the end of the duty cycle T ON1 Zero crosses.

[0042] Fig. Figure 4 shows an embodiment of a flyback converter designed to convert the in Fig. to carry out the 3 procedures shown. The one in Fig. The four blocking converters shown are based on the one in Fig. 1 shown flyback converter and additionally features a supply circuit 7 up, which with the auxiliary winding 2 3 coupled and designed to supply a voltage V CC to generate which is controlled by the control circuit 4 is obtained. The supply circuit 7 contains a series circuit with a rectifier element 71 1 such as a diode, and a capacitor 72 , which is part of the auxiliary winding 2 3 are connected in parallel. A second electronic switch 71 2 is for the rectifier element71 1 connected in parallel. The supply voltage V CC is above the capacitor 72 available... The second electronic switch 71 2 receives from the control circuit 4 A second control signal S2. Referring to the above, this control signal S2 determines the premagnetization time of the transformer. 2 .

[0043] One possibility for operating the in Fig. The blocking converter shown in section 4 is described with reference to the information in Fig. The three time-course diagrams shown are explained. During the demagnetization period T DEMAG Energy is not only transferred to the secondary winding 2 2. The rectifier circuit 3 and transfer the load Z, but via the auxiliary winding 2 3 and the rectifier element 71 1 also to the capacitor 72 the power supply circuit 7 During the premagnetization period T ON2 closes the control circuit4 the second electronic switch 71 2. This causes the transformer to be premagnetized, whereby the magnets in the transformer 2 Energy stored during the premagnetization period T ON2 through the supply capacitor 72 the power supply circuit 7 is provided.

[0044] Optionally, the flyback converter assigns a connection to the auxiliary winding. 2 Three voltage dividers connected in parallel. In this configuration, the control circuit 4 not the auxiliary voltage V AUX but one for auxiliary voltage V AUX proportional signal S AUX supplied. Optionally, the control circuit receives 4 the input voltage V IN via a resistance 53 The control unit 4 reference input voltage V IN can be used to control the control circuit 4 to supply before the electronic switch1 is switched on for the first time, that is, before the flyback converter is put into operation. Additionally, the control circuit can 4 the information about the voltage level of the input voltage V IN to use to determine the demagnetization time T ON2 or the first delay time T DEL1 to control. Alternatively, the first delay duration T is used. DEL1 by measuring the oscillation period T OSC set.

[0045] The control circuit 4 It can be implemented using a dedicated analog circuit or using hardware and software. According to one embodiment, the control circuit includes a microprocessor or microcontroller running software configured to control the operation with reference to Fig. 3 procedures explained.

[0046] The control circuit 4can operate in Current Mode (CM). In this configuration, the control circuit refers 4 a voltage V52 from a shunt resistor connected to the first electronic switch 1 connected in series. This voltage V52 is proportional to the load current I. DS In this configuration, the control circuit 4 trained to determine the duty cycle T ON1 based on the voltage V52 and the feedback signal S FB to adjust. According to another configuration, the control circuit 4 trained to determine the duty cycle T ON1 solely based on the feedback signal S FB to calculate.

[0047] The in Fig. 5 rectifier elements shown 71 1 and the parallel switch 71 2 can be implemented by a MOSFET that contains an internal body diode. Fig. Figure 5 shows an embodiment of a flyback converter in which a MOSFET 71 , as in Fig. 5 shown, the rectifier element 71 1 and the electronic switch 71 2 replaced. This MOSFET 71 is controlled by the second control signal S2. In the case of the Fig. The supply circuit of the 5 flyback converters shown is included. 7 another capacitor 73 and another rectifier element 74 In this configuration, a series circuit with the second electronic switch is required. 71 and the further capacitor 73 to the auxiliary winding 2 3 connected in parallel, and a series connection with the further rectifier element 74 and the supply capacitor 72 is to the further capacitor 73 connected in parallel. In this configuration, the supply capacitor 72 and the other capacitor 73during the demagnetization period T DEMAG charged, with the electronic switch 71 during the premagnetization period T ON2 only the additional capacitor 73 discharges. The rectifier element 74 prevents the supply capacitor 72 is discharged. With this design, premagnetizing the transformer cannot lead to changes in the supply voltage V. CC lead.

[0048] Fig. Figure 6 shows another embodiment of a flyback converter. This embodiment differs from the one in Figure 6. Fig. 5 shown embodiment by the fact that the supply circuit 7 another aid development 2 4 contains. This additional auxiliary winding is inductively connected to the primary winding. 2 1, the secondary winding 2 2 and the auxiliary winding 2 3 coupled and has the same winding direction as the auxiliary winding2 3. In this configuration, the second electronic switch 71 between the further aid development 2 4 and circuit node of the supply capacitor 72 or the further capacitor 73 , which are facing away from those circuit nodes where these capacitors are located 72 , 73 with the further rectifier element 74 are connected, connected.

[0049] Fig. Figure 7 shows another embodiment of a flyback converter. In this embodiment, the supply circuit corresponds to... 7 the in Fig. 4 supply circuit shown, wherein the electronic switch 71 1 and the rectifier element 71 2 only by a rectifier element 74 are replaced. In this embodiment, the secondary-side rectifier circuit contains 3 a second electronic switch 33, which belongs to the rectifier element 32 is connected in parallel. This second electronic switch 33 can be via a transmitter 9 through the control circuit 4 can be controlled. In this configuration, the energy required to pre-magnetize the transformer is supplied by the output capacitor. 31 provided when the second switch 33 is switched on during the pre-magnetization period.

[0050] Although various exemplary embodiments of the invention have been disclosed, it is clear to those skilled in the art that various changes and modifications can be made to realize some of the advantages of the invention. It is obvious to the average person skilled in the art that other components performing the same functions can expediently replace the original components. It should be noted that features explained with reference to a specific figure can be combined with features of other figures, even where this is not expressly stated. Furthermore, the methods according to the invention can be achieved either in pure software implementations operating with the corresponding processor instructions or in hybrid implementations operating with a combination of hardware logic and software logic to obtain the same results.

Claims

[1] Method in a voltage converter, wherein the method has in each of successive control cycles: Switching on a first electronic switch, which is connected in series with a primary winding of a transformer, for a switching duration; Pre-magnetizing the transformer for a pre-magnetization period before the first electronic switch is turned on; where there is an initial delay time between the end of the premagnetization period and the beginning of the switch-on period. [2] Method according to claim 1, wherein the premagnetizing of the transformer comprises switching on a second electronic switch different from the first electronic switch during the premagnetizing period. [3] Method according to claim 1, wherein the premagnetizing of the transformer during successive control cycles comprises premagnetizing the transformer at a predetermined frequency. [4] Method according to claim 3, wherein the predetermined frequency is determined by a clock signal. [5] Method according to claim 3, wherein the predetermined frequency is a fixed frequency. [6] Method according to claim 3, wherein the predetermined frequency depends on an instantaneous output power of the voltage converter. [7] Method according to claim 6, wherein the predetermined frequency increases when the instantaneous output power increases. [8] Method according to one of the preceding claims, wherein switching on the first electronic switch for the on-time comprises switching on the first electronic switch at a predetermined frequency. [9] Method according to claim 8, wherein the predetermined frequency is determined by a clock signal. [10] The method according to claim 8, further comprising: Calculating the start of the biasing period based on a reciprocal of the given frequency, the on-time, the biasing period, and the first delay time. [11] A method according to any of the preceding claims, further comprising: Setting a frequency at which the biasing periods begin in successive control cycles by setting a further delay time after a demagnetization period and a start of the biasing period. [12] A method according to one of the preceding claims, further comprising: Obtaining an input voltage through the voltage converter, Adjusting the bias duration depending on the voltage level of the input voltage. [13] Method according to claim 12, wherein adjusting the premagnetization time comprises increasing the premagnetization time when the level of the input voltage increases. [14] A method according to any of the preceding claims, further comprising: Obtaining an input voltage through the voltage converter, Setting the delay duration depending on a voltage level of the input voltage. [15] Method according to claim 14, wherein setting the delay duration comprises increasing the delay when the level of the input voltage increases. [16] A method according to any of the preceding claims, further comprising: Demagnetizing the transformer during a demagnetization period following the switch-on time; Detect, in a control cycle, an oscillation period of a voltage oscillation across the electronic switch after the demagnetization period; and Setting the first delay time in a control cycle after the previous control cycle based on the detected oscillation duration. [17] Method according to claim 16, wherein the detection of the oscillation period comprises evaluating a voltage across an auxiliary winding of the transformer or a signal dependent thereon. [18] Method according to claim 17, wherein the evaluation of the voltage or the signal dependent thereon comprises the detection of zero crossings of the voltage or the signal dependent thereon. [19] Method according to any one of claims 2 to 18, wherein the voltage converter comprises a control circuit configured to control the first electronic switch and the second electronic switch and a supply circuit coupled to an auxiliary winding of the transformer, and configured to supply a supply voltage to the control circuit, and the second electronic switch is coupled between a storage element in the supply circuit and the auxiliary winding. [20] Method according to claim 19, wherein a voltage across the auxiliary winding or a signal derived therefrom is supplied to the control circuit. [21] Method according to claim 19, the voltage converter has an additional auxiliary winding, and wherein a voltage across the auxiliary winding or a signal derived from it is supplied to the control circuit. [22] Method according to any one of claims 2 to 21, wherein the transformer has a secondary winding and the voltage converter has a rectifier circuit coupled to the secondary winding, and the second electronic switch is coupled between a storage element in the rectifier circuit and the secondary winding. [23] A method according to any of the preceding claims, further comprising: Providing an output signal through the voltage converter; Generating a feedback signal based on a signal level of the output signal; and Setting the duty cycle based on the feedback signal. [24] Method according to claim 23, wherein the output signal is a voltage and current of the voltage converter. [25] Method according to any of the preceding claims, wherein the electronic switch comprises a transistor. [26] Voltage converter which features: a transformer that has a primary winding; a first electronic switch connected in series with the primary winding; and a control circuit designed to, in each of a plurality of successive control cycles, switch on the first electronic switch for a duty cycle prior to the switching on of the first electronic switch, in order to cause the transformer to be premagnetized for a premagnetization period, and to create an initial delay time between the end of the premagnetization period and the start of the switch-on period. [27] Voltage transformer according to claim 26, wherein the control circuit is configured to switch on a second electronic switch different from the first electronic switch during the premagnetization period in order to cause the transformer to be premagnetized. [28] Voltage converter according to claim 27, wherein the voltage converter further comprises: an auxiliary winding of the transformer; and a supply circuit designed to supply a supply voltage to the control circuit, wherein the supply circuit includes at least one charge storage device and the second electronic switch, wherein the second electronic switch is coupled between the charge storage device and the auxiliary winding. [29] Voltage converter according to claim 27, wherein the voltage converter further comprises: a secondary winding of the transformer; and a rectifier circuit that is coupled between the secondary winding and an output of the voltage converter, where the second electronic switch in the rectifier circuit is coupled between the secondary winding and the output.