HK1117650B - Dc/dc converter - Google Patents

Description

DC/DC converter

Technical Field

The present invention relates to a DC/DC converter, and is particularly useful when a predetermined DC output voltage is obtained by alternately turning on and off a main switching device and an auxiliary switching device.

Background

DC/DC converters are widely used for obtaining a predetermined DC output voltage by stepping down or stepping up the output voltage of a DC power supply, and are widely used in power supply circuits of cellular phones, for example. The DC/DC converter converts a DC input voltage into a predetermined DC output voltage by turning on and off a switching element to control a period of turning on and off at that time. The switching element here is generally a MOSFET.

The DC/DC converter is provided with a coil. Therefore, while the switching element is turned off, a closed circuit for discharging the electric energy stored in the coil needs to be formed, and the closed circuit has been formed using a circulating current diode in the past.

However, when a circulating diode is used, since the forward voltage drop thereof is large, there is a problem that the efficiency of the DC/DC converter is low due to the power consumption of this portion.

It has been proposed to use a DC/DC converter in which a MOSFET as a switching element is used in place of the aforementioned circulating current diode, and the aforementioned forward voltage drop is reduced by the switching function of the MOSFET itself. This is because the voltage loss due to the on-resistance of the MOSFET is small compared to the forward voltage of the circulating diode, and the loss thereof is reduced accordingly.

However, in a DC/DC converter in which a circulating diode is replaced with a switching element, two switching elements formed of MOSFETs are generally connected in series with each other. That is, a switching element (hereinafter, referred to as a main switching element) for converting the output voltage to a desired value, and a switching element (hereinafter, referred to as an auxiliary switching element) for discharging the electric energy stored in the aforementioned coil during the period in which the main switching element is turned off are connected in series with each other. For example, a step-down DC/DC converter employs a configuration in which a DC output voltage is extracted from a contact point of two switching elements via a coil.

However, in such a DC/DC converter, when the auxiliary switching element is switched to the off state in a mode in which the main switching element is in the off state and the auxiliary switching element is in the on state, the following harmful phenomena tend to occur: the current based on the electric energy stored in the coil flows into the DC power supply through the parasitic diode of the main switching element.

In order to avoid such a harmful phenomenon, in such a DC/DC converter according to the related art, the auxiliary switching element is switched to the off state by monitoring the coil current flowing in the auxiliary switching element and detecting a point of time when the coil current becomes zero. This is because the detrimental phenomenon occurs after the current becomes zero.

Therefore, in the DC/DC converter according to the related art having the main switching element and the auxiliary switching element and alternately switching them on and off to obtain a predetermined DC output voltage, a current detection circuit for detecting the coil current (particularly, the direction thereof) is provided. Such a current detection circuit can be realized by using the on resistance of the auxiliary switch or by connecting a current detection resistor in series in advance and comparing and monitoring the voltage across the resistor by a comparator. That is, when the voltage across the resistor is zero, it can be detected that the coil current to be detected is zero.

As a disclosure of a DC/DC converter having a main switching element and an auxiliary switching element and alternately switching them on and off to obtain a DC output voltage of a predetermined voltage, there is the following patent document.

Patent document 1: utility model registration No. 2555245.

Patent document 2: japanese patent application No. 3637904.

Disclosure of Invention

However, since the detection circuit uses a resistor for detecting an extremely small current value and an on-resistance of the switching element as the current detection device, the detection voltage level is extremely low, and it is necessary to quickly detect a time point at which the current becomes zero at high speed and with high accuracy.

Therefore, when the comparator as described above is included, the comparator requires a high-speed and high-precision device, which is not only expensive, but also requires a high-speed and high-precision current detection circuit, which requires a large drive current, and thus becomes an obstacle to achieving the miniaturization and high efficiency of the DC/DC converter. When a resistor is connected in series with the switching element as the current detection circuit, the resistor itself causes power loss, which hinders high efficiency. Further, since the detected voltage level is low, it is difficult to respond at high speed, and the change due to the residual error of the comparator and the response delay make up the unstable operation. This is a fatal disadvantage particularly when the DC/DC converter is implemented as an IC chip.

The present invention has been made in view of the above-mentioned conventional techniques, and an object of the present invention is to provide a DC/DC converter which can remove a harmful phenomenon caused by polarity inversion of current flowing in a coil by alternately turning on and off main and auxiliary switching devices to obtain a desired DC output voltage, and which is advantageous for miniaturization and cost reduction including high efficiency and IC chip formation.

In order to achieve the above object, the present invention is characterized by the following points.

1. A DC/DC converter is provided with two switching devices connected in series and a coil connected to a contact of the two switching devices, and converts a DC input voltage into a predetermined DC output voltage and supplies power to a load through the coil; the DC/DC converter is characterized in that: the control device controls the on time of the auxiliary switching device in the following period according to the time when the contact voltage of the two switching devices reaches a predetermined threshold after the auxiliary switching device is turned off, when the switching device turned on when the coil stores the electric energy is used as a main switching device and the switching device turned on when the coil releases the electric energy stored to the output side is used as an auxiliary switching device.

2. The DC/DC converter according to claim 1, characterized in that: the control device controls the on time of the auxiliary switch device in the subsequent cycle in a manner that the polarity of the current in the coil is not reversed.

3. The DC/DC converter according to claim 1 or 2, characterized in that: the turn-off time of the auxiliary switching device in the next cycle is controlled based on the potential stored in the capacitor between the contact voltages of the two switching devices reaching a predetermined threshold after the auxiliary switching device is turned off.

4. The DC/DC converter according to the above 3, characterized in that: the potential stored in the capacitor is controlled to a predetermined value only before the auxiliary switching device is turned off in each cycle.

5. The DC/DC converter according to any one of the above 1 to 4, characterized in that: is either a buck converter or a boost converter.

6. The DC/DC converter according to any one of the above 1 to 4, characterized in that: the control unit adopts one of a PWM system, a PFM system, or both the PWM and PFM systems to obtain the switching pulse of the main switching device.

(effect of the invention)

In the present invention, in order to detect the contact voltage of the two switching devices from the off state of the auxiliary switching device, the off time of the auxiliary switching device in the subsequent cycle is controlled so as not to reverse the polarity of the coil current in accordance with the time until the set threshold is reached, and therefore, low power consumption can be achieved without requiring a high-speed response.

Drawings

Fig. 1 is a circuit diagram of a DC/DC converter of a buck converter and a control circuit thereof according to an embodiment of the present invention.

Fig. 2 is a circuit diagram illustrating a specific configuration of the synchronization unit in the control unit shown in fig. 1.

Fig. 3 is a waveform diagram showing a time chart of signals of each part based on the control unit shown in fig. 1 (in the case of expanding the on period of the auxiliary switching element S2 in the discontinuous system).

Fig. 4 is a waveform diagram showing a time chart of signals from each part of the control unit shown in fig. 1 (in the case of a load reduction in the discontinuous system).

Fig. 5 is a waveform diagram showing a time chart (in the case of the continuous system) of signals of respective portions based on the control unit shown in fig. 1.

Fig. 6 is a waveform diagram showing a timing chart of signals based on respective portions in the case where the main switching element S1 is controlled by the PFM method.

Fig. 7 is a circuit diagram of a DC/DC converter of the boost converter and a control circuit thereof according to the embodiment of the present invention.

(identification number in the figure)

S1, S11, a main switching element, S2, S12, an auxiliary switching element, CO, a smoothing capacitor, L1, a coil, A, PWM signal generator, B, a buffer, C, a switching signal control section, 1, an error amplifier, 2, 17, a comparator, 15, a single SHOT (1-SHOT) unit, 16, a synchronizing unit, 18, 19, an RS flip-flop, 20, 21, 22, a constant current power supply, 11, 12, 13, a constant current, S3, S4, S5, a switching element, S1_ G, S11_ G, a switching pulse, S2_ G, S12_ G, a switching pulse, VLx, a voltage signal, 1Lx, a coil current, S20 _ on _ B, a switching pulse, V _ S2chg, V _ S2on, a voltage, S2_ DRV _1SHOT, single SHOT (1-SHOT) pulse, T _ S2_ SYNC, a diff _ S2 signal, and a diff _ S2 signal.

Detailed Description

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Fig. 1 is a circuit diagram of a step-down DC/DC converter and a control circuit thereof according to an embodiment of the present invention.

In the DC/DC converter according TO the present embodiment, the main switching element S1 formed of a MOSFET having a parasitic diode D1 connected in parallel is connected in reverse parallel, and the auxiliary switching element S2 formed of a MOSFET connected in series with the coil L1 and having a parasitic diode D2 connected in parallel TO the junction between the main switching element S1 and the coil L1 is connected in parallel, so that the DC output voltage Vout can be obtained via the coil L1 and the output terminal TO. Here, the main switching element S1 is a switching device that is turned on when electric energy is stored in the coil L1, the auxiliary switching element S2 is a switching device that is turned on when energy stored in the coil L1 is released to the output terminal, and the parasitic diode D2 functions as a circulating diode.

In such a DC/DC converter, the control unit controls the main switching element S1 and the auxiliary switching element S2 TO be alternately turned on and off, and controls the on time of the main switching element S1 at this time TO step down the DC output voltage (the DC input voltage of the DC/DC converter) of the DC power supply (not shown) and obtain a predetermined DC output voltage Vout from the output terminal TO. That is, the dc output voltage Vout is defined by the on time (operating state) of the main switching element S1. In addition, the electric energy stored in the coil L1 during the on period of the main switching element S1 may be discharged by circulating through the auxiliary switching element S2 and its parasitic diode D2 during the off period of the main switching element S1. CO in the figure is a smoothing capacitor for the dc output voltage Vout.

The control unit for controlling the on/off of the main switching element S1 and the auxiliary switching element S2 includes a PWM signal generator a, a buffer B, and a switching signal control unit C, and is configured together with the main switching element S1 and the auxiliary switching element S2.

In the PWM signal generator a, the dc output voltage Vout is divided by resistors R1 and R2 and a capacitor C1 and applied to the error amplifier 1. The error amplifier 1 also has a reference voltage VREF applied thereto. As a result, an error signal S21 is obtained. In the comparator 2, the error signal S21 is compared with the triangular wave S22 generated by the oscillator OSC, and the PWM signal S23 is obtained as an output signal thereof.

The PWM signal S23 reaches the buffer B via the buffer amplifier 3 and the inverter 4. The buffer B is composed of two nor circuits 5, 6 and 8 inverter circuits 7 to 14, and alternately turns on and off the main switching element S1 and the auxiliary switching element S2 in accordance with the PWM signal S23.

In the present embodiment, since the main switching element S1 is a P-channel element, it is turned on when the output signal S1_ G of the buffer B is in the L state, and since the auxiliary switching element S2 is an N-channel element, it is turned on when the output signal S2_ G of the buffer B is in the H state. The buffer B is obviously not limited to the configuration of fig. 1.

The switching signal controller C detects a time when the voltage signal VLx at the contact of the main switching element S1 and the auxiliary switching element S2 reaches a predetermined threshold value, and controls the off time of the auxiliary switching element S2 in the subsequent cycle so that the polarity of the coil current ILx in the coil L1 is not inverted. Therefore, the coil current ILx is set to the direction indicated by the arrow in the drawing as the positive direction.

Here, the switching signal control unit C according to the present embodiment includes a single-strike unit 15 and a synchronization unit 16. The single impact unit 15 generates a single impact pulse S2_ DRV _1SHOT lasting for a predetermined time by forming an H state by the rising edge of the switching pulse S2_ G of the auxiliary switching element S2 being turned on. The synchronization unit 16 generates a pulse signal S2_ SYNC that defines the timing of turning on and off the auxiliary switching element S2, particularly the turning-off timing, simultaneously with the input of the voltage signal VLx.

Fig. 2 is a circuit diagram illustrating a specific configuration of the synchronization unit 16. In the figure, 17 is a comparator, 18, 19 are RS flip-flop circuits, 20, 21 are constant current power supplies, 23, 24 are nand circuits, 25 are inverters, and S3, S4, S5 are switching elements.

As shown in the figure, the RS flip-flop circuit 18 can be reset by the output signal of the comparator 17 by setting the rising edge of the switching pulse S2_ G which turns on the auxiliary switching element S2 to form the rising edge of the 1SHOT pulse S2_ DRV _1SHOT in the H state. In this example, since the QB output (inverted output) of the RS flip-flop circuit 18 is used, when the rising edge of the output signal at the time of reset of the comparator 17 is in the H state, the rising edge of the pulse signal S2_ SYNC is in the L state. As a result, the auxiliary switching element S2 is turned off.

The RS flip-flop circuit 19 may also be set by the rising edge of the aforementioned 1 transmission pulse S2_ DRV _1 SHOT. In addition, the reset of the RS flip-flop circuit 19 may be performed at a point in time when the voltage signal VLx reaches a prescribed threshold value. The threshold value here is not particularly limited as long as it is a parameter that reflects the polarity inversion of the coil current ILx. For example, the logic level (the intermediate potential between the operating voltage and GND), the threshold value (about 0.7V) of the MOSFET, the GND level (OV) and the like of the RS flip-flop circuit 19 are preferable, and these are easily detectable values.

While the switching element S3 is in the off state, the capacitor CS2on can be charged gradually with the constant current I1 supplied from the constant current power supply 20, and the voltage V _ S2on is applied to the non-inverting input terminal of the comparator 17. When the switching pulse S2on _ B is in the H state, in other words, when the main switching element S1 is in the on state, the switching element S3 is in the on state for a period in which the blanking pulse is increased, and the charging of the capacitor CS2on by the constant current power supply 20 is interrupted.

The capacitor CS2chg is charged gradually with the constant current I2 supplied from the constant current power supply 21 while the switching element S4 is in the on state and the switching element S5 is in the off state, and the voltage V _ S2chg is applied to the inverting input terminal of the comparator 17. That is, the switching element S4 is turned on from the time point when the pulse signal S2_ SYNC is in the L state, that is, when the auxiliary switching element S2 is turned off, to the time point when the voltage signal VLx reaches the threshold value, and the capacitor CS2chg is charged by the constant current power supply 21. In the present embodiment, the desired object is achieved by reflecting the period from the time point when the auxiliary switching element S2 is turned off to the time point when the voltage signal VLx reaches the threshold value in the subsequent cycle.

In addition, the switching element S5 forms an on state during the 1 fire pulse S2_ DRV _1SHOT being in the H state, eliminating the charge that charges the capacitor CS2chg with the constant current 13 of the constant current power supply 22. As a result, the voltage V _ S2chg drops a little. This means that the comparison reference voltage applied to the inverting input terminal of the comparator 17 drops by a corresponding portion.

The overall operation will be described below with reference to waveform diagrams based on time charts of signals of respective portions of the control unit shown in fig. 1.

Fig. 3 is a waveform diagram of waveforms of respective portions in a case where the on time (tsync) of the auxiliary switching element S2 is short and the off time is long during the current non-continuous period. The current discontinuous period herein refers to a period from the time point OmA starts, when the coil current ILx is turned off, to the time point OmA at the instant when the main switching element S1 is turned on in 1 cycle P formed by the PWM signal S3 (see fig. 1). In other words, it is the case during which the coil current ILx is formed OmA within 1 period P.

As shown in fig. 3, the switching pulse S1_ G turns on the main switching element S1, which is a P-type MOSFET, when it becomes the L state, and turns off when it becomes the H state. The switching pulse S2_ G turns on the auxiliary switching element S2 as an N-type MOSFET when it becomes the H state, and turns off when it becomes the L state. Thus, the main switching element S1 and the auxiliary switching element S2 are alternately turned on.

As a result, the coil current ILx increases from the time point when the main switching element S1 is turned on, reaches a peak at the time point when the main switching element S1 is turned off, and then decreases.

In addition, the voltage signal VLx is controlled such that the time tdif until the voltage signal VLx reaches the predetermined threshold after the auxiliary switching element S2 is turned off is reflected in the auxiliary switching element on time (tsync) in the next period P. That is, in order to make the time after the auxiliary switch element S2 is turned on (tsync) in the previous cycle P and the time after the voltage signal VLx reaches the predetermined threshold value (tdif) after the auxiliary switch S2 is turned off the same in the next cycle P, the tdif is shortened while the tsync is extended. If this point is explained in detail, the following is the case.

1. As described above, the switching pulse S2on _ B is turned on for a period in which the off pulse period is increased while the main switching element S1 is turned on, and the charging of the capacitor CSon by the constant current power supply 20 is interrupted. Therefore, since the switching pulse S2on _ B is in the L state while the auxiliary switching element S2 is on, the switching element S3 is in the off state, and the capacitor CS2on is charged.

2. As a result, the voltage V _ S2on increases linearly before the rising edge of the switching pulse S2on _ B reaches the H state.

3. The 1-SHOT pulse S2_ DRV _1SHOT rises in synchronization with the rising edge of the switching pulse S2_ G.

4. The pulse signal T _ S2_ DIF is a signal based on the QB output (inverting output) of the RS flip-flop circuit 19 set by the 1-fire pulse S2_ DRV _1SHOT and the pulse signal S2_ SYNC.

Therefore, the L state is formed within a period from the time point when the auxiliary switching element S2 is turned off until the voltage signal VLx reaches the predetermined threshold value, that is, within a time tdif. As a result, the switching element S4 is turned on.

5. While the switching element S4 is in the on state, that is, as a result of the capacitor CS2chg being charged for the time tdif, the voltage V _ S2chg increases linearly, and the increase in the voltage V _ S2chg reflects the time tdif.

While the 1-SHOT pulse S2_ DRV _1SHOT rises, continuing the H state, the switching element S5 is brought into the on state. Therefore, the electric charge stored in the capacitor CS2chg during this time is eliminated by the constant current 13 of the constant current power supply 22. As a result, the voltage V _ S2chg is gradually decreased in the H period of the 1SHOT pulse S2_ DRV _1 SHOT.

6. Since the voltage V _ S2on is input to the non-inverting input terminal of the comparator 17 and the voltage V _ S2chg is input to the inverting terminal of the comparator 17, the voltage V _ S2on decreases the pulse signal S2_ SYNC at the time point when the voltage V _ S2chg crosses the voltage V _ S2chg after being increased. That is, the timing at which the auxiliary switching element S2 changes from the on state to the off state is defined. Thus, the pulse signal S2_ SYNC is formed, and the switching pulse S2_ G having the same waveform as the pulse signal S2_ SYNC is formed.

As shown in fig. 3, in the present embodiment, in order to reflect the time tdif in the on time (tsync) of the auxiliary switching element S2 in the next cycle P, the voltage V _ S2on and the voltage V _ S2chg are increased at the same rate. That is, the following conditions are satisfied.

On-time of S2 Δ Tsync CS2on × Δ V _ S2chg/I1 (1)

ΔV_S2chg＝I2×tdif/CS2chg (2)

The formula (3) is derived according to the formula (1) and the formula (2)

ΔTsync＝CS2on/I1×12/CSchg×tdif...(3)

Here, if I1 is 12 and CS2on is CS2chg, Δ Tsync is tdif. A similar relationship is generally established when I1: 12 is n: 1, and when CS2 on: CSchg is 1: n.

The above is the case when the on time (tsync) of the auxiliary switching element S2 is extended, and the case when narrowing is performed will be described below with reference to fig. 4. The graph is a waveform diagram showing a time chart of signals of each part based on the control unit shown in fig. 1, similarly to fig. 3. Here, portions different from those in fig. 3 are mainly described, and description thereof will be omitted for overlapping portions.

In this case, since the load is shifted to a light load, the on period of the switching pulse S1_ G is shortened, and the peak value of the coil current ILx is also lowered. At this time, in order to maintain the on time of the switching pulse S2_ G before 1 cycle P, the polarity of the coil current ILx is inverted in the next cycle P. When the coil current ILx is reversed, the time from the time point when the auxiliary switching element S2 is turned off until the voltage signal VLx reaches the predetermined threshold value does not exist. Therefore, the pulse signal T _ S2_ DIF becomes short.

Here, it is important to make the voltage V _ S2chg gradually lower by turning on the switching element S5 during H of the 1SHOT pulse S2_ DRV _1SHOT to eliminate the charge stored in the capacitor CS2 chg. That is, since a decrease in the voltage V _ S2chg means a decrease in the reference comparison voltage of the comparator 17, the point of time at which the increasing voltage V _ Non and the voltage V _ S2chg cross, namely, advance in synchronization with the rising edge of the auxiliary switching element S2. Therefore, the auxiliary switching element S2, which is defined by the time point at which the voltage V _ Non and the voltage V _ S2chg cross, temporally moves forward every 1 cycle P after the falling time point of the off state. In this way, the on time of the auxiliary switching element S2 gradually becomes shorter, and the charging and discharging of the voltage V _ S2chg are balanced at the point in time.

The above function is expressed by a mathematical formula as follows.

On-time Δ Tsync of S2

＝CS2on×(ΔV_S2chgI3＊S2_DRV_1SHOT/CS2chg)/I1...(4)

ΔV_S2chg＝I2×tdif/CS2chg...(5)

From the above formulae (4) and (5), the formula (6) can be derived

ΔTsync＝CS2on×(I2×tdif/CS2chgI3＊S2_DRV_1SHOT/CS2chg)/I1...(6)

Here, if I1 ═ I2 ═ I3 and CS2on ═ CS2chg are set, Δ Tsync is tdifS2_ DRV _1 SHOT.

It is the actual case that with I3, the 1-SHOT pulse S2_ DRV _1SHOT becomes short only during H Δ Tsync.

In the above embodiment, the case where the 1-SHOT pulse S2_ DRV _1SHOT control pulse signal T _ SYNC is used has been described, but is not limited thereto. The same effect can be obtained by a simple method such as a method of erasing for a long period of time using a minute constant current.

Fig. 5 is a waveform diagram of waveforms of respective portions in the current continuation period. The current continuation period referred to herein is a period in which the main switching element S1 is turned on at the instant of 1 cycle P forming the PWM signal 3 (see fig. 1), and the coil current ILx starts from OmA or more and does not return to OmA after being turned off. In other words, the coil current (load current) ILx in 1 cycle P does not have a period OmA.

As shown in fig. 5, in the current continuation period, the intersection time point of the voltage V _ S2on and the voltage V _ S2chg is the same in each period P. Therefore, the same time tdif as that of the previous cycle P can be secured in the next cycle P. Therefore, the same waveform signal can be repeated in each period P.

Fig. 6 is a waveform diagram based on a time chart of signals of respective portions when the main switching element S1 is controlled in the PFM manner. The PFM method mentioned here corresponds to the PWM method and the operating state of the switching pulse S1_ G in each period of the light-weight control of the load, and is performed at the frequency of the light-weight control of the load. Specifically, a PFM signal generator is configured to replace the PWM signal generator in the circuit shown in fig. 1. The PFM signal generator divides the dc output voltage Vout of fig. 1 by resistors R1 and R2 and a capacitor C1 and applies the divided voltage to the error amplifier 1, and controls the frequency of the PFM signal based on an error signal S21 obtained by comparing the error amplifier 1 with a reference voltage VREF.

In the PFM method, the pulse signal S2_ SYNC falls at the time point when the voltage V _ S2on crosses the voltage V _ S2chg, and the switching pulse S2_ G falls. As a result, the auxiliary switching element S2 is turned off. That is, as in the case of the circuit shown in fig. 1, the time tdif in the next period P can be defined by the time tdif until the voltage signal VLx reaches the predetermined threshold value from the time when the auxiliary switching element S2 is turned on.

Further, while having two functions of the PWM method of controlling the pulse width and the PFM method of controlling the pulse frequency in accordance with the error signal S21, a configuration may be adopted in which the output signal is obtained by appropriately switching the two methods. In this case, the PFM method is used for light load, and the PWM method is used for heavy load.

The above embodiment is the case of the step-down DC/DC converter, but is not limited thereto. The same applies to other modes such as step-up, reverse, step-up and step-down, Cuk, Zeta, Sepic, forward, flyback, and the like.

Fig. 7 is a circuit diagram of a boost converter DC/DC converter and a control circuit thereof according to an embodiment of the present invention. Because of the boost converter, the coil current ILx flowing in the coil L1 is in the opposite direction, and the relationship of the main switching element S11 and the auxiliary switching element S12 are reversed compared to the buck converter shown in fig. 1. That is, the main switching element S11 is formed of the same N-type MOSFET as the auxiliary switching element S2 of fig. 1, and the auxiliary switching element S12 is formed of the same P-type MOSFET as the main switching element S1 of fig. 1. And the switching pulse S11_ G forms the same waveform as the switching pulse S2_ G of fig. 1, and the switching pulse S12_ G forms the same waveform as the switching pulse S1_ G of fig. 1.

The rest of the constitution is the same as that of FIG. 1. Therefore, the same portions are denoted by the same reference numerals, and redundant description is omitted.

Claims (6)

1. A DC/DC converter is provided with two switching devices connected in series and a coil connected to a contact of the two switching devices, and converts a DC input voltage into a predetermined DC output voltage and supplies power to a load through the coil; the DC/DC converter is characterized in that: the control device controls the on time of the auxiliary switching device in the following period according to the time from the turning-off of the auxiliary switching device to the time when the contact voltage of the two switching devices reaches a predetermined threshold value.

2. The DC/DC converter according to claim 1, wherein: the control device controls the on time of the auxiliary switch device in the subsequent cycle in a manner that the polarity of the current in the coil is not reversed.

3. The DC/DC converter according to claim 1, wherein: the turn-off time of the auxiliary switching device in the next cycle is controlled based on the amount of charge stored in the capacitor during a period from the turn-off of the auxiliary switching device to when the contact voltage of the two switching devices reaches a predetermined threshold.

4. The DC/DC converter according to claim 3, wherein: the amount of charge stored in the capacitor is controlled to a predetermined value only before the auxiliary switching device is turned off in each cycle.

5. The DC/DC converter according to any one of claims 1 to 4, wherein: the DC/DC converter is either a buck converter or a boost converter.

6. The DC/DC converter according to any one of claims 1 to 4, wherein: the control device adopts a PWM system, a PFM system, or a combination thereof to obtain the switching pulse of the main switching device.