DC / DC converter for converting an input voltage into an output voltage

Abstract

A DC-DC converter (10) is proposed for converting an input voltage into an output voltage, wherein the DC-DC converter (10) comprises at least one transistor (20, 22), an analog subtraction circuit (40) for forming the difference between the output voltage of the DC-DC converter (10) and a reference voltage (34) and for outputting a subtraction signal based on the difference being formed, and a digital pulse width modulation circuit (50) for controlling the at least one transistor (20, 22) based on the subtraction signal.

Description

The invention relates to a DC / DC converter for converting an input voltage into an output voltage. State of the art Numerous DC-DC converters are known for converting an input voltage into an output voltage. These converters use switching transistors to generate the output voltage from the input voltage. Clock-based modules (triangle generator, current sensor, comparator, etc.) are employed. A disadvantage of this approach is that at high switching frequencies, e.g., above 2 MHz, the modules either no longer function fully or become very complex and expensive. This is particularly true for integrated or semi-integrated solutions with input voltages above 10 V. Disclosure of the invention The invention is based on the objective of demonstrating a DC-DC converter or a method for converting an input voltage into an output voltage using a DC-DC converter, which functions reliably at high and very high switching frequencies. This problem is solved by a DC / DC converter according to claim 1. In particular, the problem is solved by a DC-DC converter for converting an input voltage into an output voltage, wherein the DC-DC converter comprises at least one transistor, an analog subtractor circuit for forming the difference between the output voltage of the DC-DC converter and a reference voltage and for outputting a subtraction signal based on the difference being formed, and a digital pulse width modulation circuit for controlling the at least one transistor based on the subtraction signal. One advantage of this design is that the DC-DC converter operates reliably and efficiently even at high switching frequencies of at least one transistor, for example, above 2 MHz. Furthermore, the DC-DC converter is cost-effective and technically simple in design. In addition, the DC-DC converter exhibits high efficiency and low losses. Since the subtraction of the reference voltage and the output voltage is analogous, the pulse-width modulation circuit can be designed to be particularly simple and cost-effective. The task is also solved by an integrated electronic circuit comprising a DC-DC converter as described above. The problem is also solved by a method according to claim 11. In particular, the problem is also solved by a method for converting an input voltage into an output voltage using a DC-DC converter, especially using a DC-DC converter according to one of the preceding claims, wherein the method comprises the following steps: applying the input voltage to the DC-DC converter; calculating the difference between the output voltage of the DC-DC converter and a reference voltage using an analog subtraction circuit to output a subtraction signal based on the calculated difference; and controlling at least one transistor based on the subtraction signal using a digital pulse-width modulation circuit to generate the output voltage. An advantage of this method is that high switching frequencies can be achieved, making it efficient.Furthermore, a technically simple and cost-effective DC-DC converter can be used for this process. Another advantage is that, with the integrated solution, the inductor and capacitor typically remain external components. Therefore, increasing the switching frequency is beneficial for reducing costs, particularly for the inductor but also for the required capacitor. Put simply, doubling the switching frequency means that only half the inductor or coil is needed. According to one embodiment of the DC-DC converter, the pulse-width modulation circuit comprises an analog-to-digital converter for converting the subtraction signal into a digital duty signal and a pulse-width modulation device for switching the transistors based on the digital duty signal. An advantage of this is that the transistors can be switched particularly easily. Furthermore, the pulse-width modulation circuit is technically very simple and cost-effective. According to one embodiment of the DC-DC converter, the analog-to-digital converter includes a flash converter, or the analog-to-digital converter is itself a flash converter. The advantage of this is that the conversion of the analog reference signal into the digital duty signal can be performed particularly quickly and cost-effectively. This enables particularly high switching frequencies. According to one embodiment of the DC-DC converter, the flash converter is a 5-bit or 6-bit flash converter. An advantage of this is the technical simplicity of the DC-DC converter. The analog-to-digital converter (ADC) therefore does not require high precision and can thus be cost-effective. This distinguishes the DC-DC converter according to the invention from the prior art, where the ADC is typically located before the subtractor circuitry, necessitating high precision. Furthermore, the ADC does not need to be very fast. According to one embodiment of the DC-DC converter, the pulse-width modulation circuit, and in particular the pulse-width modulation component, comprises a ring oscillator. This allows for the simple generation of the circuit signal for switching the transistors. Furthermore, the ring oscillator is technically simple, reliable, and cost-effective. According to one embodiment of the DC-DC converter, the DC-DC converter further comprises a spreading circuit that is electrically connected to the pulse-width modulation (PWM) element, in particular the ring oscillator, such that the frequency of the PWM element, in particular the ring oscillator, can be varied. An advantage of this is that the emission of electromagnetic radiation can be spectrally spread or distributed (temporally) over a frequency range. This reduces electromagnetic interference from the ring oscillator and / or from the input of the inductor or coil switched by the converter. At the same time, however, the duty cycle or the frequency of the duty signal can remain unchanged. According to one embodiment of the DC-DC converter, the input of the DC-DC converter is connected to the pulse-width modulation (PWM) circuit such that the input voltage of the PWM circuit, and in particular the analog-to-digital converter (ADC), is provided as a reference value. An advantage of this is that the output voltage of the DC-DC converter, as in a voltage-feed-forward DC-DC converter according to the prior art, can be dependent on the input voltage. The maximum value of the ADC can be proportional to, or set to, the input voltage of the PWM circuit. In particular, the maximum value of the ADC can be a multiplicative of the input voltage. Consequently, the output signal of the PWM circuit is dependent on the input voltage. This speeds up the response to changes in the input voltage. According to one embodiment of the DC-DC converter, the DC-DC converter does not include a ramp generator. An advantage of this is that the DC-DC converter is technically very simple and cost-effective. According to one embodiment of the DC-DC converter, the DC-DC converter does not include a pulse generator. This results in a technically simple design and a small number of components. According to one embodiment of the method, the method further comprises the following steps: converting the subtraction signal into a digital duty signal; and switching the at least one transistor based on the digital duty signal. An advantage of this is that the transistors are switched in a technically simple manner. According to one embodiment of the method, the pulse-width modulation circuit generates the switching signals for the at least one transistor using a ring oscillator. An advantage of this is that the switching signals for switching the at least one transistor are technically simple to generate. According to one embodiment of the method, the method further comprises the following step: changing the frequency of the pulse-width modulation circuit, in particular the ring oscillator, over time. The advantage of this is that the electromagnetic radiation emitted by the DC-DC converter or the pulse-width modulation circuit is spectrally spread over a frequency range. This means that the maximum or peak of the electromagnetic interference is reduced, since the emitted power is distributed over several frequencies or a frequency range. The duty cycle or the frequency of the duty signal can remain unchanged, independent of the frequency of the pulse-width modulation device. According to one embodiment of the method, the method further comprises the following step: changing the maximum value of the analog-to-digital converter depending on the input voltage applied to the input of the DC-DC converter. An advantage of this method is that, as in a voltage-feed-forward DC-DC converter according to the prior art, the output voltage of the DC-DC converter can be, or is, dependent on the input voltage. This can particularly mean that the maximum value of the duty cycle signal output by the analog-to-digital converter depends on the input voltage applied to the input of the DC-DC converter. It is possible for the maximum value of the duty cycle signal to be proportional to a multiple of the input voltage. A pulse-width modulation circuit can be understood, in particular, as a circuit in which the electrical voltage alternates between two values. The pulse-width modulation circuit (PBM or PDM circuit or PWM circuit) can, in particular, comprise or be a pulse-length modulation circuit (PLM circuit) or a pulse-width modulation circuit (PBM circuit). The analog comparison circuit can in particular include a subtractor that compares the output voltage with a reference voltage and outputs the difference value as a comparison value. The analog-to-digital converter can, in particular, include or be an analog-to-digital converter or ADC (analog-to-digital converter). A flash converter can be understood to be, in particular, a single-stage parallel converter. In this case, a separate subtractor can be implemented for each possible output value, except for the largest output value. A 5-bit flash converter can be understood to be, in particular, a single-stage parallel converter with 2^5 - 1 = 31 comparators. A 6-bit flash converter can be understood to be, in particular, a single-stage parallel converter with 2^6 - 1 = 63 comparators. Such flash converters are particularly cost-effective. The term "ring oscillator" can refer in particular to a relaxation oscillator circuit based on the propagation delay of an odd number of inverting amplifier components. The ring oscillator can incorporate bipolar transistors in common-emitter configuration as inverters. A spreading circuit can be understood in particular as a circuit that changes the bias current and / or a supply voltage of the ring oscillator in order to change the frequency of the ring oscillator. The term "connected" or "connection" can be understood in particular as an electrical connection, i.e., a short circuit exists between two interconnected components or interconnected points of a circuit. The term "digital pulse width modulation circuit" can be understood to mean, in particular, a pulse width modulation circuit that (after converting the input signal into the digital pulse width modulation circuit into a digital signal) works with digital or discrete values / signals or processes / uses digital / discrete values. The invention is based on the idea of ​​replacing only a portion of the previously analog circuitry in a DC-DC converter with a digital circuit, while leaving the remaining analog circuitry unchanged. In other words, the invention is based on the idea of ​​implementing digitally the part of the DC-DC converter circuitry that generates the switching signal for the transistor(s) from the subtraction signal of the analog comparison. This allows for high switching frequencies, e.g., above 2 MHz, to be achieved at low cost. Preferred embodiments are set forth in the dependent claims. The invention is explained in more detail below with reference to drawings of exemplary embodiments. Figure 1 shows a schematic view of a first exemplary embodiment of the DC-DC converter according to the invention; Figure 2 shows a schematic view of a second exemplary embodiment of the DC-DC converter according to the invention. In the following description, the same reference numbers are used for identical and similarly functioning parts. Fig. 1 shows a schematic view of a first exemplary embodiment of the DC voltage converter 10 according to the invention. The DC-DC converter 10 converts an input voltage applied to input 30 into an output voltage applied to output 32. The DC signal is converted into a square wave signal. This square wave signal is generated by switching transistors 20 and 22. The inductor 24 (specifically the coil) smooths the square wave signal again, resulting in a DC voltage at output 32. The input voltage typically differs from the output voltage. In particular, the output voltage can be higher or lower than the input voltage. The DC voltage converter 10 comprises two transistors 20, 22, an inductor 24 (in particular a coil), an analog subtractor circuit 40, an analog-to-digital converter 55 and a digital pulse width modulation circuit 50. A capacitor 12 is connected to input 30 and ground. The input 30, to which the input voltage is present or can be applied, is connected to a first transistor 20, e.g., a MOSFET. The first transistor 20 can be an n-channel MOSFET. The input voltage is connected to the source of the first transistor 20. The drain of the first transistor 20 is connected to the inductor. The gate of the first transistor 20 is connected via a gate driver circuit / driver 59 to a pulse-width modulation circuit 50 or a pulse-width modulation element 58 of the pulse-width modulation circuit 50. The drain of the first transistor 20 is connected to the drain of the second transistor 22. The gate of the second transistor 22 is connected via the (gate) driver circuit / driver 59 to the pulse width modulation circuit 50 or the pulse width modulation element 58 of the pulse width modulation circuit 50. An inductor 24, e.g., a coil, is arranged or connected between transistors 20, 22 and the output 32 of the DC-DC converter 10. The right side of the inductor 24, or coil, which faces away from transistors 20, 22, is connected to output 32, where the output voltage is applied. The right side of the inductor 24 is connected to ground via a capacitor 16. Furthermore, the right side of the inductor 24, or coil, is connected to an input 30 of a subtractor circuit 40, or a subtractor. A first complex resistance (typically a combination of resistance and capacitance) 18 can be connected between the right side of the coil and the first input of the subtractor 40. Additionally, a second complex resistance (typically a combination of resistance and capacitance) 17 can be connected between the first input of the subtractor 40 and the output of the subtractor 40. A reference voltage 34 is connected to the second input of the subtractor 40 or the subtraction circuit. The subtractor circuit 40 compares the reference voltage 34, which is applied to the second input of the subtractor 40, with the voltage applied to the first input of the subtractor circuit 40. The subtractor 40 outputs as a subtraction signal the difference, or subtraction result, between the voltage at the first input of the subtractor 40, which is the output voltage of the DC-DC converter 10, and the reference voltage 34, which is applied to the second input of the subtractor 40. In other words, the output voltage is subtracted from the reference voltage 34, and the result is output as the subtraction signal. The comparison circuit is implemented analogously. The subtractor circuit 40 or the subtractor is designed analogously. The components between the subtractor 40 or the subtraction circuit and the transistors 20, 22 or the (gate) drivers of the transistors 20, 22 are digitally implemented. This means that a digital pulse-width modulation circuit 50 is present from the subtractor up to and including the respective transistors 20, 22. In the DC-DC converter 10, a significant part or a large part of the circuit still consists of analog components. The control of transistors 20 and 22 is implemented digitally. This allows high switching frequencies, e.g., above 2 MHz, to be achieved, while simultaneously making the DC-DC converter 10 cost-effective. The output of the subtractor 40 is connected to the input of the digital pulse width modulation circuit 50. This means that the digital pulse width modulation circuit 50 receives the subtraction signal from the subtractor 40. The digital pulse width modulation circuit 50 (DPWM circuit) comprises an analog-to-digital converter 55 and a digital pulse width modulation device 58 for controlling the two transistors 20, 22. The analog-to-digital converter 55 receives the subtraction signal from the subtractor 40 and converts the analog subtraction signal into a digital duty signal. The output of the analog-to-digital converter 55 is connected to the input of a pulse-width modulation device 58. The pulse-width modulation device 58 receives the digital duty signal from the analog-to-digital converter 55 and switches the two transistors 20 and 22 depending on the duty signal. Thus, depending on the output signal of the analog-to-digital converter, the first transistor 20 and the second transistor 22 are switched by the pulse-width modulation device 58. The output signal of the pulse-width modulation device 58, or the pulse-width modulation circuit 50, indicates the portion of the respective period during which the two transistors 20 and 22 are switched on or conducting. The pulse-width modulation circuit 50 therefore switches the transistors 20 and 22. The pulse-width modulation circuit 50 outputs a digital signal that switches transistors 20 and 22 on (conducting) and off (blocking). By switching transistors 20 and 22 on and off (conducting and blocking), a DC current or voltage is generated by the coil, which is output by the DC-DC converter 10 at its output 32. The analog-to-digital converter 55 does not need to exhibit high precision. Since the analog-to-digital converter 55 is located downstream of the subtractor circuit in the present invention, it has no direct influence on the accuracy of the output voltage. This is an important difference from the prior art, where an analog-to-digital converter is located directly at the output. The analog-to-digital converter 55 can be a so-called flash converter. In particular, the analog-to-digital converter 55 can be a 5-bit or a 6-bit flash converter. Thus, the analog-to-digital converter 55 can be cost-effective and fast. The pulse width modulation circuit 50, in particular the pulse width modulation component 58, can include or be a ring oscillator. The analog-to-digital converter 55, for example, outputs the duty signal to the ring oscillator. The input 30 of the DC-DC converter, or the input voltage of the DC-DC converter, can be connected to the analog-to-digital converter 55. The input voltage can determine or influence the maximum value of the output of the analog-to-digital converter 55. The input voltage of the DC-DC converter is, in effect, used as an input-voltage-dependent reference for the analog-to-digital converter 55. In this way, the input voltage dependency of the pulse-width modulation can be achieved with the DC-DC converter 10, as is typically the case with so-called voltage-feed-forward methods. This allows the DC-DC converter 10 to react quickly to changing input voltages and adjust the output voltage accordingly. Fig. 2 shows a schematic view of a second exemplary embodiment of the DC voltage converter 10 according to the invention. The second embodiment differs from the first embodiment by a spreading circuit 60, which is connected to the pulse width modulation circuit 50. The DC-DC converter 10 can include a spreading circuit 60. The spreading circuit 60 is connected to the pulse-width modulation circuit 50 in such a way that the oscillation frequency of the ring oscillator can be varied by means of the spreading circuit 60. In particular, the oscillation frequency of the ring oscillator can be varied over time by means of the spreading circuit 60. In this way, the emission of electromagnetic radiation is spread over a frequency range, for example around a fundamental frequency. Without changing the oscillation frequency of the ring oscillator by means of the spreading circuit 60, the oscillation frequency of the ring oscillator would remain unchanged, and consequently, the electromagnetic interference energy or interference power emitted by the ring oscillator would have a fixed frequency. By changing the frequency, the electromagnetic interference (EMI) of the DC-DC converter 10 is reduced, since the radiated power of the ring oscillator is distributed over a wide frequency range. Depending on where the signal is taken from the ring oscillator, i.e., which inverter is tapped, the resetting of the pulse width modulation signal is determined. The frequency of the duty signal remains constant, or rather, the duty cycle is independent of the frequency at which the pulse-width modulation element 58 operates, or independent of the frequency of the ring oscillator. This is an advantage over purely analog implementations in state-of-the-art DC-DC converters, where spreading the input frequency always affects the control loop of the DC-DC converter and thus potentially modulates the output voltage of the DC-DC converter unintentionally. For example, the frequency of the ring oscillator can be varied by ± 5% around a predetermined frequency, or fundamental frequency, or oscillation frequency of the ring oscillator by the spreading circuit 60. In this way, the maximum or peak of the electromagnetic radiation of the ring oscillator, as well as of the switching output (i.e., input 23 of coil 24), is reduced by approximately 6 dB to approximately 7 dB. This also applies to harmonics of the electromagnetic radiation of the ring oscillator, as well as of the switching output (i.e., input 23 of coil 24). In other words, the radiated power of the DC-DC converter 10 is, or can be, spread over a frequency spectrum by means of the spreading circuit 60. The unwanted radiated power of the DC-DC converter 10 is, so to speak, dispersed over several frequencies or a frequency range. In both the first and second embodiments of the DC-DC converter 10, the ramp generator and comparator, which are sensitive, clock-based components, are omitted compared to the prior art. These are replaced by the pulse-width modulation circuit 50 or the analog-to-digital converter 55 and the digital pulse-width modulation element 58, which, due to their digital processing, can be designed for high frequencies. The technical requirements for these two components, i.e., the analog-to-digital converter 55 and the pulse-width modulation element 58, are low. This results in a significant advantage over purely digital DC-DC converters, which require a very fast and accurate analog-to-digital converter 55. The digitization of the signal, or the signal itself, thus takes place in the DC-DC converter 10 according to the invention after error amplification or the calculation of the difference between the output voltage and the reference voltage 34. Therefore, the analog-to-digital converter 55 does not need to be very fast, since compensation already takes place in the subtractor 40, or the signal is reduced in frequency. A current measurement for regulating the output voltage is typically not required for the DC-DC converter 10. However, it is conceivable that a current measurement is performed or already present for the appropriate handling of fault conditions or for current limiting after switch-on. This would then intervene with a higher priority in the digital pulse-width modulation device 58. The following aspects are also disclosed here: 1) A DC-DC converter (10) for converting an input voltage into an output voltage, wherein the DC-DC converter (10) comprises at least one transistor (20, 22), an analog subtractor circuit (40) for calculating the difference between the output voltage of the DC-DC converter (10) and a reference voltage (34) and for outputting a subtractor signal based on the calculated difference, and a digital pulse-width modulation circuit (50) for controlling the at least one transistor (20, 22) based on the subtractor signal. 2) A DC-DC converter (10) according to aspect 1), wherein the pulse-width modulation circuit (50) comprises an analog-to-digital converter (55) for converting the subtractor signal into a digital duty signal and a pulse-width modulation device (58) for switching the transistors (20, 22) based on the digital duty signal. exhibits.3) DC-DC converter (10) according to aspect 2), wherein the analog-to-digital converter (55) comprises a flash converter or is a flash converter. 4) DC-DC converter (10) according to aspect 3), wherein the flash converter is a 5-bit flash converter or a 6-bit flash converter. 5) DC-DC converter (10) according to any of the preceding aspects, wherein the pulse-width modulation circuit (50), in particular the pulse-width modulation element (58), comprises a ring oscillator or is a ring oscillator. 6) DC-DC converter (10) according to any of aspects 2)-5), further comprising a spreading circuit (60) which is electrically connected to the pulse-width modulation element (58), in particular the ring oscillator, such that the frequency of the pulse-width modulation element (58), in particular the ring oscillator, is variable.7) DC-DC converter (10) according to any of the preceding aspects, wherein the input (30) of the DC-DC converter (10) is connected to the pulse-width modulation circuit (50) such that the input voltage of the pulse-width modulation circuit (50), in particular the analog-to-digital converter (55), is provided as a reference value. 8) DC-DC converter (10) according to any of the preceding aspects, wherein the DC-DC converter (10) does not include a ramp generator. 9) DC-DC converter (10) according to any of the preceding aspects, wherein the DC-DC converter (10) does not include a pulse generator. 10) Integrated electronic circuit comprising a DC-DC converter (10) according to any of the preceding aspects.11) Method for converting an input voltage into an output voltage using a DC-DC converter (10), in particular using a DC-DC converter (10) according to one of the preceding aspects, wherein the method comprises the following steps: applying the input voltage to the DC-DC converter (10); calculating the difference between the output voltage of the DC-DC converter (10) and a reference voltage (34) using an analog subtractor circuit to output a subtraction signal based on the calculated difference; and controlling at least one transistor (20, 22) based on the subtraction signal using a digital pulse-width modulation circuit (50) to generate the output voltage. 12) Method according to aspect 11), further comprising the following steps: converting the subtraction signal into a digital duty signal; and switching the at least one transistor (20, 22) based on the digital duty signal.13) Method according to aspect 11) or 12), wherein the pulse width modulation circuit (50) generates the switching signals for the at least one transistor in the middle of a ring oscillator. 14) Method according to one of aspects 11)-13), further comprising the following step: changing the frequency of the pulse width modulation circuit (50), in particular of the ring oscillator, over time. 15) Method according to one of aspects 11)-14), further comprising the following step: changing the maximum value of the analog-to-digital converter (55) depending on the input voltage applied to the input (30) of the DC-DC converter (10). Reference symbol list 10 DC-DC converter 12 Capacitor 16 Capacitor 17 First complex resistor 18 Second complex resistor 20 First transistor 22 Second transistor 23 Inductor input (coil) 24 Inductor (coil) 30 Input 32 Output 34 Reference voltage 40 Subtractor / subtractor circuit (analog) 50 Digital pulse-width modulation circuit 55 Analog-to-digital converter 58 Digital pulse-width modulation device 59 Driver circuit 60 Spreader circuit

Claims

DC-DC converter (10) for converting an input voltage into an output voltage, wherein the DC-DC converter (10) comprises at least one transistor (20, 22), an analog subtractor circuit (40) for forming the difference between the output voltage of the DC-DC converter (10) and a reference voltage (34) and for outputting a subtractor signal based on the difference formed, and a digital pulse-width modulation circuit (50) for controlling the at least one transistor (20, 22) based on the subtractor signal, wherein the pulse-width modulation circuit (50), in particular the pulse-width modulation element (58), comprises a ring oscillator or is a ring oscillator. DC voltage converter (10) according to claim 1, wherein the pulse width modulation circuit (50) comprises an analog-to-digital converter (55) for converting the subtraction signal into a digital duty signal and a pulse width modulation element (58) for switching the transistors (20, 22) on the basis of the digital duty signal. DC voltage converter (10) according to claim 2, wherein the analog-to-digital converter (55) comprises a flash converter or is a flash converter. DC voltage converter (10) according to claim 3, wherein the flash converter is a 5-bit flash converter or a 6-bit flash converter. DC voltage converter (10) according to one of claims 2-4, further comprising a spreading circuit (60) which is electrically connected to the pulse width modulation element (58), in particular the ring oscillator, in such a way that the frequency of the pulse width modulation element (58), in particular the ring oscillator, can be changed. DC voltage converter (10) according to one of the preceding claims, wherein the input (30) of the DC voltage converter (10) is connected to the pulse width modulation circuit (50) such that the input voltage of the pulse width modulation circuit (50), in particular the analog-to-digital converter (55), is provided as a reference value. DC-DC converter (10) according to one of the preceding claims, wherein the DC-DC converter (10) does not include a ramp generator. DC voltage converter (10) according to one of the preceding claims, wherein the DC voltage converter (10) does not include a pulse generator. Integrated electronic circuit comprising a DC / DC converter (10) according to any one of the preceding claims. A method for converting an input voltage into an output voltage using a DC-DC converter (10), in particular using a DC-DC converter (10) according to one of the preceding claims, wherein the method comprises the following steps: applying the input voltage to the DC-DC converter (10); calculating the difference between the output voltage of the DC-DC converter (10) and a reference voltage (34) using an analog subtractor circuit to output a subtraction signal based on the calculated difference; and controlling at least one transistor (20, 22) based on the subtraction signal using a digital pulse-width modulation circuit (50) to generate the output voltage, wherein the pulse-width modulation circuit (50) generates the switching signals for the at least one transistor using a ring oscillator. The method according to claim 10 further comprising the following steps: converting the subtraction signal into a digital duty signal; and switching the at least one transistor (20, 22) on the basis of the digital duty signal. Method according to claim 10 or 11, further comprising the following step: changing the frequency of the pulse width modulation circuit (50), in particular the ring oscillator, over time. Method according to one of claims 10-12, further comprising the following step: changing the maximum value of the analog-to-digital converter (55) depending on the input voltage applied to the input (30) of the DC voltage converter (10).

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