DC / DC converter

Abstract

The DC / DC converter (10) is an asynchronous DC / DC converter. A DC / DC converter is provided with an inductor (L1), a switching element (Q1), a diode (D1), and a controller (5) that controls the switching element (Q1). The switching element is connected in series with the inductor. The diode is connected between the power lines (PL1A, PL1B) and the headlight (1). The controller controls the switching element such that a switching frequency of the switching element is higher than an upper limit frequency of a predetermined frequency band. The diode includes a first capacitor and a second capacitor. The first capacitor is a capacitor in a rectification path through which a forward current of the diode flows. The second capacitor is a capacitor of a current path formed in parallel with the rectification path.

Description

Technical Field

[0001] The present disclosure relates to a DC / DC converter, and in particular to an asynchronous DC / DC converter. Background Art

[0002] International Publication No. 2015 / 186404 (Patent Document 1) discloses a multi-phase DC / DC converter. The DC / DC converter includes a plurality of (N) converter units. Each converter unit is an asynchronous converter including an inductor, a switching element, and a diode. In order to prevent the noise generated by the switch (hereinafter referred to as "switching noise") from interfering with the signal of the broadcast band (e.g., the AM broadcast band), the product of the switching frequency F of the switching element of each converter and the number of operating units N is set to be outside the range of the above-mentioned broadcast band.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: International Publication No. 2015 / 186404 Summary of the invention

[0006] When the switching frequency of the switching element of each DC / DC converter is set to be lower than a predetermined frequency band (for example, the above-mentioned broadcast frequency band), even if the switching frequency is set in such a way that the product of the switching frequency F and the number of operating units N is outside the range of the broadcast frequency band, the frequency of the higher harmonics of the switching frequency F is sometimes within the frequency band of the above-mentioned broadcast frequency band. As a result, there is a possibility that the switching noise interferes with the signal of the above-mentioned broadcast frequency band. Therefore, from the perspective of reducing noise, it is considered to set the switching frequency F higher than the upper limit frequency of the above-mentioned broadcast frequency band. However, when the switching frequency F is set higher than the above-mentioned upper limit frequency, there is a possibility that the power conversion efficiency of the DC / DC converter is significantly reduced due to the increase in switching loss.

[0007] The present disclosure has been made to solve the above-mentioned problem, and an object of the present disclosure is to prevent noise from interfering with a signal with a frequency within a predetermined frequency band in an asynchronous DC / DC converter, while suppressing a decrease in power conversion efficiency.

[0008] The DC / DC converter disclosed in the present invention is an asynchronous DC / DC converter. The DC / DC converter includes an inductor, a switching element, a diode, and a controller for controlling the switching element. The switching element is connected in series with the inductor. The diode is connected to the inductor and the switching element. The controller controls the switching element in such a way that the switching frequency of the switching element is higher than the upper limit frequency of a predetermined frequency band. The diode includes a first capacitor and a second capacitor. The first capacitor is the capacitor of the rectifier path through which the forward current of the diode flows. The second capacitor is the capacitor of the current path formed in parallel with the rectifier path.

[0009] According to the present disclosure, in an asynchronous DC / DC converter, it is possible to suppress a decrease in power conversion efficiency while avoiding interference of noise with a signal with a frequency within a predetermined frequency band. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a schematic diagram of a vehicle shown as an example of a device including a DC / DC converter according to the embodiment.

[0011] Figure 2 It is a diagram showing an example of the structure of a DC / DC converter.

[0012] Figure 3 FIG. 1 is a diagram showing an example of the structure of a load of a DC / DC converter.

[0013] Figure 4 It is shown Figure 2 A diagram showing an example of the structure of a diode.

[0014] Figure 5 is a diagram showing an equivalent circuit of a diode.

[0015] Figure 6 This is a diagram showing an example of transition of the voltage between the drain and the source when the switching element is turned on.

[0016] Figure 7 This is a diagram showing an example of the relationship between the total loss of the DC / DC converter and the switching frequency of the switching element.

[0017] Figure 8 This is a diagram showing an example of the relationship between the total loss difference of the DC / DC converter and the switching frequency of the switching element.

[0018] Fig. 9 This is a diagram for explaining an example of a frequency spectrum of noise generated due to a switching operation of a switching element of a DC / DC converter.

[0019] Fig.10 FIG. 1 is a diagram showing an example of the configuration of a DC / DC converter in a modified example. DETAILED DESCRIPTION

[0020] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and their description will not be repeated.

[0021] Figure 1 2 is a schematic diagram of a vehicle shown as an example of a device including a DC / DC converter according to the embodiment. Figure 1The vehicle 100 includes a load LOAD, a DC / DC converter 10 , a battery 20 , and a radio 25 .

[0022] The load LOAD includes a headlight 1 . The headlight 1 illuminates the front in the direction in which the vehicle 100 travels.

[0023] The DC / DC converter 10 is connected to the load LOAD and the battery 20, and receives power from the battery 20 to drive the load LOAD. In the present embodiment, the DC / DC converter 10 is a boost converter, and the driving voltage of the load LOAD is 30V to 70V. The battery 20 is used as a power source for the DC / DC converter 10. The voltage of the battery 20 is, for example, 12V. The vehicle 100 is an electric vehicle such as a gasoline vehicle, an electric vehicle, or a hybrid vehicle. In the case of an electric vehicle or a hybrid vehicle, the battery 20 is equivalent to an auxiliary battery.

[0024] The connection relationship between the DC / DC converter 10 and the battery 20 and other components is not limited to Figure 1 As an example, the DC / DC converter 10 and the battery 20 may be arranged at a position close to the headlight 1 (eg, in the engine room).

[0025] The radio 25 is configured to receive a broadcast signal BS1 having a frequency within an AM broadcast band, for example, and generate the sound of an AM broadcast program. Similarly, the radio 25 is also configured to receive a broadcast signal BS2 having a frequency within an FM broadcast band, and generate the sound of an FM broadcast program. In this embodiment, the AM broadcast band is set to a frequency band of 0.52 MHz to 1.71 MHz, for example, and the FM broadcast band is set to a frequency band of 76 MHz to 108 MHz, for example.

[0026] Figure 2 1 is a diagram showing an example of the structure of the DC / DC converter 10. Figure 2 The DC / DC converter 10 is an asynchronous DC / DC converter, and includes a switching element Q1 , an inductor L1 , a diode D1 , a capacitor C1 , and a controller 5 .

[0027] The inductor L1 is connected between the positive node of the battery 20 and the switching element Q1. Various components may be provided between the positive node of the battery 20 and the inductor L1. The various components include, for example, a harness for supplying power from the battery 20, a protection circuit for protecting the DC / DC converter 10 when the battery 20 is mistakenly connected to the reverse polarity, or a filter circuit for reducing noise and voltage fluctuations.

[0028] In this example, the switching element Q1 is a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), but it may also be an IGBT (Insulated Gate Bipolar Transistor) or the like. The switching element Q1 is connected in series with the inductor L1. That is, the switching element Q1 is connected between the inductor L1 and the negative electrode node of the battery 20. For example, a wiring harness for power supply from the battery 20 may be provided between the negative electrode node of the battery 20 and the switching element Q1. The voltage between the drain and source of the switching element Q1 is represented as a voltage Vds.

[0029] The diode D1 is connected between the power lines PL1A, PL1B and the load LOAD. A step-down converter (described later) or a harness for supplying power to the headlight 1 may be provided between the diode D1 and the headlight 1 of the load LOAD. Each of the power lines PL1A, PL1B is provided between the inductor L1 and the switching element Q1, and is connected to the anode of the diode D1 via the power line PL1C. Each of the power lines PL1A, PL1B, PL1C may be replaced by a circuit formed by a copper foil pattern or the like formed on a circuit board.

[0030] The diode D1 is a trench Schottky diode. The forward voltage (voltage drop) of the diode D1 is represented by the forward voltage Vf. When the driving voltage of the load LOAD is 30V to 70V, the withstand voltage of the diode D1 for the reverse voltage is, for example, 40V to 100V. The capacitor C1 is provided to smooth the output voltage Vout.

[0031] The controller 5 outputs a pulse modulation signal such as PWM (Pulse width modulation) as a control signal CS via a gate drive circuit (not shown). The control signal CS is input to the gate terminal of the switching element Q1 and is used for switching control. The switching frequency of the switching element Q1 is determined according to the carrier frequency of the PWM control. The controller 5 controls the output voltage Vout of the DC / DC converter 10 by controlling the on / off duty ratio of the switching element Q1.

[0032] When the switching element Q1 is turned on, the input current Iin of the DC / DC converter 10 flows into the switching element Q1 via the inductor L1. During the on-period of the switching element Q1, the inductor L1 accumulates energy of the input current Iin supplied from the battery 20. The amount of accumulated energy varies depending on the on / off duty ratio of the switching element Q1.

[0033] On the other hand, when the switching element Q1 is turned off, the input current Iin flows into the diode D1 via the inductor L1. During the off period of the switching element Q1, in addition to the input current Iin, the inductor L1 also supplies the energy accumulated during the on period of the switching element Q1 to the capacitor C1 and the load LOAD via the diode D1. As a result, the input voltage Vin of the DC / DC converter 10 is boosted to the output voltage Vout (Vout≧Vin).

[0034] Thus, the DC / DC converter 10 boosts the input voltage Vin supplied from the battery 20 and supplies the boosted voltage (Vout) and power to the load LOAD, thereby driving the load LOAD. In this example, the input voltage Vin is 12V, and the output voltage Vout is 30V to 70V.

[0035] Figure 3 1 is a diagram showing an example of the structure of the load LOAD of the DC / DC converter 10. Figure 3 The load LOAD includes an LED (Light Emitting Diode) array 3 disposed in the headlight 1, and a buck converter 2 that supplies a predetermined current to the LED array 3. The buck converter 2 may be disposed on the same circuit substrate as the DC / DC converter 10, or may be disposed together with the LED array 3 in the headlight 1. In the present embodiment, a set of the buck converter 2 and the LED array 3 is also represented as a unit 23.

[0036] The LED array 3 includes LEDs 3A to 3Z. In the present embodiment, the LEDs 3A to 3Z are connected in series in the LED array 3. The brightness of each of the LEDs 3A to 3Z is determined by the current Id supplied to the LED array 3.

[0037] Typically, the number of LEDs 3A to 3Z in the LED array 3 is large. For example, when the number of LEDs 3A to 3Z is 8 and a current Id of about 1A to 1.5A is supplied, the forward voltage of the LED array 3 (i.e., the sum of the forward voltages of the LEDs 3A to 3Z) is typically about 24V to 27V. Therefore, in most cases, the voltage applied to the LED array 3 is higher than the voltage of the battery 20 (e.g., 12V). Therefore, the DC / DC converter 10 connected to the battery 20 is typically a step-up converter.

[0038] The buck converter 2 steps down the output voltage Vout of the DC / DC converter 10 and supplies the stepped-down voltage to the LED array 3. The buck converter 2 uses a current sensing resistor to monitor the output current of the buck converter 2, that is, the current Id of the LED array 3, and controls the stepped-down voltage so that the monitored value is consistent with a predetermined target current value (e.g., 1 A). Thus, a constant current Id is supplied to the LED array 3.

[0039] exist Figure 3 In the case of the load structure shown, the DC / DC converter 10 controls the boosted output voltage (Vout) to be constant. In the DC / DC converter 10, the controller 5 monitors the boosted output voltage (Vout) and controls the output voltage Vout in such a way that the monitored voltage value is consistent with a predetermined target voltage value (30V to 70V in the case of this embodiment). Thus, a constant output voltage Vout is supplied to the load.

[0040] The controller 5 may be an analog circuit including an operational amplifier and a comparator, or a digital circuit including an analog-to-digital converter (ADC), an FPGA (Field Programmable Gate Array), a CPU (Central Processing Unit), or an MCU (Micro Controller Unit). Alternatively, the controller 5 may include both the analog circuit and the digital circuit.

[0041] The controller 5 may monitor the current of the inductor L1 or the switching element Q1 of the DC / DC converter 10 (the current between the drain and the source in the case of a MOSFET) using a current sensing resistor and use the monitored current as a comparison triangle wave for pulse modulation (current mode control method).

[0042] In addition, the load LOAD may be configured to include a plurality of cells 23 provided in parallel. Some or all of the step-down converters 2 in the plurality of cells 23 may be arranged on the same circuit board as the DC / DC converter 10 .

[0043] Alternatively, the buck converter 2 may be removed. When the buck converter 2 is removed, the DC / DC converter 10 supplies a constant current Id to the LED array 3. In this case, the DC / DC converter 10 uses a current sensing resistor to monitor the output current of the DC / DC converter 10, that is, the current Id of the LED array 3, and controls the boost output voltage (Vout) in such a way that the monitored current value is consistent with a predetermined target current value (e.g., 1A). Thus, a constant current Id is supplied to the LED array 3.

[0044] Figure 4 It is shown Figure 2 In the following description, reference is made to the following examples of the structure of the diode D1. Figure 2 . Reference Figure 4 The X direction and the Y direction are perpendicular to each other and are perpendicular to the Z direction. The Z direction is the direction in which the forward current of the diode D1 flows, that is, the direction from the anode to the cathode of the diode D1. Figure 4 This is a cross-sectional view of the diode D1 as viewed from the Y direction.

[0045] The diode D1 is different from a planar diode and has a groove portion 70 extending in the Z direction. Figure 4 , but in fact, the diode D1 has a plurality of groove portions 70 periodically arranged in the X direction. It is assumed that the groove portion 70 has a width Lx in the X direction, a width Ly in the Y direction, and a depth Lz in the Z direction. That is, the shape of the groove portion 70 is represented by the width Lx, Ly, and the depth Lz.

[0046] The diode D1 includes an anode electrode layer 40 , a metal layer 50 , a polysilicon layer 55 , an insulating layer 60 , a semiconductor layer 30 , and a cathode electrode layer 90 .

[0047] Anode electrode layer 40 is connected to inductor L1 and switching element Q1 via power line PL1C. Metal layer 50 is formed between anode electrode layer 40 and semiconductor layer 30 , and a Schottky junction is formed at Schottky junction portion JP of semiconductor layer 30 and metal layer 50 .

[0048] The polysilicon layer 55 is formed of polysilicon which is a conductive material and fills the trench portion 70 . The insulating layer 60 is formed between the polysilicon layer 55 and the semiconductor layer 30 and fills the trench portion 70 .

[0049] In this embodiment, the semiconductor layer 30 is formed of silicon (Si), but may also be formed of a wide bandgap semiconductor such as silicon carbide (SiC) or gallium nitride (GaN). The semiconductor layer 30 includes an n-type drift layer 80 and an n+ substrate layer 85 .

[0050] The n-type drift layer 80 is formed between the metal layer 50 and the n+ substrate layer 85. The equipotential lines in the n-type drift layer 80 are indicated by equipotential lines EPL.

[0051] The n+ substrate layer 85 is formed so as to be adjacent to the cathode electrode layer 90. The cathode electrode layer 90 is connected to the load LOAD.

[0052] In addition, unlike this example, the electric field in the n-type drift layer of the planar Schottky diode becomes stronger as it approaches the Schottky junction (the interval between equipotential lines becomes narrower), and is non-uniform. Therefore, the reverse voltage near the Schottky junction increases, and the breakdown voltage becomes lower. As a result, in the planar diode, the leakage current tends to increase compared to the trench diode D1 having the same degree of forward voltage Vf.

[0053] On the other hand, in the present embodiment, the diode D1 has a groove portion 70, which functions in the same manner as a field plate. As a result, the intervals between the equipotential lines EPL are substantially uniform. Therefore, the increase in the electric field strength near the Schottky junction JP is suppressed, so the breakdown voltage increases. As a result, the trench-type diode D1 is easier to reduce leakage current than a planar diode having the same degree of forward voltage Vf.

[0054] Figure 5 is a diagram showing an equivalent circuit of the diode D1. Figure 5 , the equivalent circuit of the diode D1 includes a rectification path P1 and a current path P2.

[0055] The rectifier path P1 is a path through which a forward current flows from the anode electrode layer 40 to the cathode electrode layer 90 when a forward voltage is applied to the diode D1. The capacitance component (parasitic capacitance) of the rectifier path P1 is represented by the capacitor Cd1. The resistance component of the rectifier path P1 is represented by the resistor Rd1 and the resistor Rd2 shared with the current path P2. Each of the capacitor Cd1 and the resistor Rd1 varies depending on the direction and size of the applied voltage of the diode D1. When a forward voltage is applied to the diode D1, the resistance value of the resistor Rd1 decreases. When a reverse voltage is applied to the diode D1, the resistance value of the resistor Rd1 increases, but on the other hand, its value is limited, so the above-mentioned leakage current occurs.

[0056] The current path P2 is electrically connected in parallel with the rectification path P1. The capacitance component (parasitic capacitance) of the current path P2 is represented by the capacitance Cs1. The capacitance Cs1 is configured to make the electric field distribution in the diode D1 uniform (i.e., to make Figure 4 The resistance component of the current path P2 is represented by the resistor Rs1 and the resistor Rd2 shared with the rectification path P1.

[0057] The capacitor Cs1 and the resistor Rs1 are the capacitance and the resistor of the groove portion 70 of the diode D1, respectively, and are elements inherent to the groove-type diode D1. That is, the planar Schottky diode does not have a capacitance component equivalent to the capacitor Cs1 and a resistance component equivalent to the resistor Rs1, while the diode D1 includes the capacitor Cs1 and the resistor Rs1. For example, the capacitor Cs1 and the resistor Rs1 are determined according to the shape of the groove portion 70, the conductivity of the polysilicon layer 55, and the physical properties of the material of the insulating layer 60.

[0058] Refer again Figure 1 as well as Figure 2 The switching frequency of the switching element Q1 is preferably determined so that the frequency and its higher harmonic frequencies do not interfere with the signal of the frequency band of the broadcast signal BS1 (AM broadcast signal). As described above, from the perspective of reducing noise, the switching frequency is preferably higher than the upper limit frequency of the AM broadcast band.

[0059] In the present embodiment, the AM broadcast band is determined as a predetermined prohibited frequency band, and the controller 5 controls the switching element Q1 so that the switching frequency becomes higher than the upper limit frequency (1.71 MHz in this example) of the AM broadcast band.

[0060] Conventionally, when the DC / DC converter driving the vehicle headlight is a boost converter connected to a battery and the switching frequency of the switching element Q1 is set higher than the upper limit frequency, there is a problem that the power conversion efficiency of the DC / DC converter 10 is significantly reduced due to the increase in switching loss. As a result, it is difficult to use a switching frequency higher than the upper limit frequency.

[0061] For the purpose of reducing switching loss, it is also considered that the switching element Q1 is preferably formed of a MOSFET of a wide-bandgap semiconductor such as silicon carbide (SiC) or gallium nitride (GaN).

[0062] However, in a boost converter driving a vehicle headlight, if the switching frequency is set higher than the upper limit frequency of the prohibited frequency band, it is completely insufficient from the perspective of reducing losses. Furthermore, if the specifications such as the withstand voltage of the MOSFET are not optimal or the reliability against failure is difficult to ensure, there is also the problem that the cost may increase significantly.

[0063] The inventors have found that in the DC / DC converter 10 connected to the battery 20 and having a boost function for driving the headlight 1, even when the controller 5 controls the switching element Q1 at a switching frequency higher than 1.71 MHz, it is possible to suppress an excessive decrease in the power conversion efficiency of the DC / DC converter 10. This point will be described in detail below.

[0064] Figure 61 is a diagram showing an example of the change in the drain-source voltage Vds when the switching element Q1 is switched from the off state to the on state. In this example, the switching frequency is 1.8 MHz or more (for example, 1.8 MHz, 1.9 MHz, 2.0 MHz, 2.1 MHz, or 2.2 MHz). In the following description, reference is made to Figure 2 .

[0065] Reference Figure 6 , and the dotted line 200 represents the transition of the voltage Vds in the comparative example. The comparative example is different from the present embodiment in that a planar diode having a forward voltage Vf of the same degree is used instead of the trench-type diode D1 in the DC / DC converter. As described above, in both the present embodiment and the comparative example, the semiconductor layer 30 serving as the diode D1 is formed of silicon (Si).

[0066] Time td1 is the falling edge delay time of the voltage Vds in the comparative example. Time tf1 is the falling edge time of the voltage Vds in the comparative example. The falling edge delay time of the voltage Vds refers to the time from when the switch element Q1 is turned on to when the voltage Vds substantially starts to drop sharply with a certain slope. The falling edge time of the voltage Vds refers to the time from when the voltage Vds substantially starts to drop sharply with a certain slope after the switch element Q1 is turned on to when the voltage Vds first reaches the voltage V0 (reference voltage).

[0067] The solid line 250 shows the transition of the voltage Vds in the present embodiment. In the present embodiment, as described above, the DC / DC converter 10 uses a trench-type diode D1 having a trench portion 70. The time td2 is the falling edge delay time of the voltage Vds in the present embodiment. The time tf2 is the falling edge time of the voltage Vds in the present embodiment.

[0068] The time td2 is shorter than the time td1. The time difference tdif between the time td2 and the time td1 in the present embodiment is of the order of several ns or less. The longer the time td1 is, the longer the time difference tdif is, for example, 1 ns, so that the power efficiency of the DC / DC converter of the comparative example decreases according to an exponential function. On the other hand, in the present embodiment, the time td2 is short, so when the time td2 becomes shorter even at the order of, for example, sub-nanoseconds, the power efficiency of the DC / DC converter 10 of the present embodiment is greatly improved.

[0069] Refer again Figure 5The parasitic capacitance between the anode and cathode of the diode D1 of the present embodiment is the sum of the capacitance Cd1 of the rectification path P1 and the capacitance Cs1 of the current path P2. This total capacitance is the same as or greater than the parasitic capacitance of the planar diode of the comparative example. The capacitance Cd1 of the rectification path P1 of the diode D1 of the present embodiment is smaller than the parasitic capacitance of the planar diode of the comparative example. That is, the reverse recovery time of the diode D1 of the present embodiment is shorter than the reverse recovery time of the diode of the comparative example. When the reverse recovery time is short, the power supply efficiency of the DC / DC converter can be improved. In addition to being caused by the short reverse recovery time mentioned above, the power supply efficiency of the DC / DC converter 10 of the present embodiment is also greatly improved due to the generation mechanism of the time difference tdif. The following describes the generation mechanism.

[0070] Refer again Figure 6 , the time difference tdif depends greatly on the resonant frequency of the voltage Vds immediately after the switching element Q1 is turned on. The resonant frequency of the voltage Vds depends greatly on the parasitic capacitance and parasitic inductance on the current path formed by the switching element Q1, the capacitor C1, and the diode D1 of the DC / DC converter 10. The resonant frequency also depends greatly on the output voltage Vout of the DC / DC converter 10, the output power Pout, the time when the switching element Q1 switches from the off state to the on state, the switching frequency, etc.

[0071] In the DC / DC converter 10 (step-up converter in this example) driving the headlight 1, the output voltage Vout is typically about 30V to 70V, and the output power Pout is about 20W to 200W. In the DC / DC converter 10 involved in this embodiment driven under this condition, the switching frequency of the switching element Q1 is set to be higher than 1.71MHz (preferably 1.8MHz or more), and a trench diode D1 is used, so the resonance frequency of the voltage Vds is higher than that of the comparative example. As a result, the time td2 in this embodiment is shortened, and the time difference tdif occurs, so the power supply efficiency of the DC / DC converter 10 is improved.

[0072] The resonant frequency of the voltage Vds also affects the fall time of the voltage Vds described above. As a result, the time tf2 in the present embodiment is shorter than the time tf1 in the comparative example. Further, in the present embodiment, the time T2 (= td2 + tf2) required for the voltage Vds to first reach the voltage V0 (reference voltage) from the time when the switching element Q1 is turned on is significantly shorter than the time T1 (= td1 + tf1) required in the comparative example (T2 << T1). As a result, in the present embodiment, the switching loss in the switching element Q1 can be significantly reduced. Therefore, even if the controller 5 controls the switching element Q1 at a switching frequency higher than 1.71 MHz (for example, 1.8 MHz or higher), it is possible to suppress an excessive decrease in the power conversion efficiency of the DC / DC converter 10 while avoiding noise interference with the broadcast signal BS1.

[0073] Figure 7 FIG. is an example showing the relationship between the total loss of the DC / DC converter 10 and the switching frequency of the switching element Q1. The total loss of the DC / DC converter 10 is the sum of the losses of the inductor L1, the switching element Q1, the diode D1, and the capacitor C1 ( Figure 2 ).

[0074] Refer to Figure 7 , and the dashed line 300 and the solid line 350 respectively represent the relationship between the total loss Loss and the switching frequency f in the comparative example and the present embodiment. Let the total loss difference ΔLoss be the difference between the total loss Loss (dashed line 300) in the comparative example and the total loss Loss (solid line 350) in the present embodiment.

[0075] Figure 8 FIG. is an example showing the relationship between the total loss difference ΔLoss of the DC / DC converter and the switching frequency f. Figure 8 The relationship of Figure 7 is based on the relationship of Figure 8 . Refer to

[0076] At frequencies lower than the AM broadcast band (frequencies lower than 0.52 MHz), the total loss difference ΔLoss is not so significant. In the AM broadcast band, the total loss difference ΔLoss starts to become significant as the switching frequency f increases. At frequencies higher than the AM broadcast band (for example, 2.0 MHz), the total loss difference ΔLoss becomes more significant, and the advantage of the present embodiment over the comparative example becomes more prominent. That is, the total loss Loss of the present embodiment is significantly reduced compared to the comparative example.

[0077] The MOSFET as the switching element Q1 may be a semiconductor formed of silicon and its total charge (Qg) may be as small as about 15 nC or less. As a result, the DC / DC converter 10 can be manufactured more cheaply and with improved reliability against failures. Furthermore, the power conversion efficiency of the DC / DC converter 10 can be suppressed from being excessively reduced while preventing noise from interfering with the broadcast signal BS1.

[0078] In order to prevent noise from interfering with the signal in the FM broadcast band, the switching frequency f is set to at least be higher than 1.71 MHz (preferably 1.8 MHz or higher) and 10.85 MHz or lower. This point will be described in detail later.

[0079] Fig. 9 1 is a diagram for explaining an example of a frequency spectrum of noise generated due to the switching operation of the switching element Q1 of the DC / DC converter 10. Fig. 9 , the vertical axis represents the noise intensity I (maximum value is Imax), and the horizontal axis represents the noise frequency component fn. The scale of the horizontal axis may not be consistent with the scale of the actual frequency component.

[0080] The noise frequency component fn is a frequency component of noise generated by turning on / off the switching element Q1 in the DC / DC converter 10. In this example, it is assumed that the noise frequency component fn is the input current Iin ( Figure 2 ), but may also be a frequency component of radiation noise generated from the DC / DC converter 10.

[0081] Line 400 represents the spectrum of the noise frequency component fn. This spectrum is derived using the Fast Fourier Transformation (FFT) of the input current Iin. This spectrum can also be derived using the FFT of the noise terminal voltage obtained from the LISN (Line Impedance Stabilization Network) provided between the battery 20 and the DC / DC converter 10. The frequency band Δf is the half-amplitude band in the above spectrum (in other words, the half-amplitude band of the peak frequency fp1 corresponding to the switching frequency f).

[0082] In this example, the switching frequency f is determined in such a way that the condition (first condition) that the lower limit frequency f1 of the frequency band Δf is higher than the upper limit frequency (1.71 MHz) of the AM frequency band is satisfied. That is, the controller 5 controls the switching element Q1 in such a way that the lower limit frequency f1 is higher than the upper limit frequency of the AM frequency band. Thus, it is possible to more reliably avoid noise interfering with the signal of the AM frequency band while suppressing the reduction in power conversion efficiency. As an example, when the frequency band Δf is narrow, the switching frequency f is determined in such a way that the lower limit frequency f1 of the frequency band Δf becomes 1.8 MHz or more.

[0083] The width of the frequency band Δf varies depending on the waveform of the voltage Vds during the operation of the switching element Q1. An example of a switching frequency f for which the first condition is reliably satisfied is 2.0 MHz. The switching frequency f for which the first condition is satisfied may be 2.0 MHz or more, for example, 2.1 MHz or 2.2 MHz.

[0084] Alternatively, the switching frequency f may be determined so that the condition (second condition) that the frequency obtained by subtracting 3σ from the peak frequency fp1 is higher than 1.71 MHz is satisfied. σ is the standard deviation of the spectrum. For example, the value of the switching frequency f that satisfies the first or second condition is determined in advance by experiments.

[0085] When the switching frequency f is determined so that the first or second condition is satisfied, it is possible to more reliably prevent noise interference caused by the switching operation of the switching element Q1 from affecting the broadcast signal BS1 and suppress a decrease in power conversion efficiency.

[0086] It is preferred that the noise generated in the DC / DC converter does not interfere with the broadcast signal BS1 and the broadcast signal BS2 (FM broadcast signal). Therefore, the controller 5 may control the switching element Q1 in such a manner that, in addition to the first condition that the peak frequency fp1, i.e., the switching frequency f, is higher than the upper limit frequency (1.71 MHz) of the AM broadcast band, the condition (third condition) that the nth-order harmonic component (frequency component fpn (n≧2)) of the peak frequency fp1 is not included in the FM broadcast band is satisfied.

[0087] In practice, when the 7th harmonic (n=7) is set to be lower than the lower limit frequency of the FM band (76MHz in this embodiment), noise interference with the signal of the FM band can be basically avoided. Therefore, the controller 5 can also control the switching element in a manner that the 7th harmonic component is not included in the FM broadcast band (more specifically, in a manner that is lower than the lower limit frequency of the FM broadcast band). Specifically, the switching frequency f can also be set to a range higher than 1.71MHz and below 10.85 (≒76 / 7)MHz. More preferably, the switching frequency f is set to a range of greater than 1.8MHz and less than 10.85MHz. More preferably, the switching frequency f can also be set to a range of greater than 2.0MHz and less than 10.85MHz in a manner that makes the first condition also true.

[0088] In order to more reliably prevent noise from interfering with the signal of the FM broadcast band of the n-order harmonic component, the 14th-order harmonic (n=14) may also be set to be lower than 76MHz. Therefore, the controller 5 may also control the switching element in a manner that the 14th-order harmonic component is not included in the FM broadcast band (more specifically, in a manner that is lower than the lower limit frequency of the FM broadcast band). Specifically, the switching frequency f may also be set to be higher than 1.71MHz and less than 5.42 (≒76 / 14)MHz. More preferably, the switching frequency f is set to be within the range of greater than 1.8MHz and less than 10.85MHz. More preferably, the switching frequency f may also be set to be within the range of greater than 2.0MHz and less than 5.42MHz in a manner that the first condition is also satisfied.

[0089] In addition, the AM broadcast band corresponds to an example of the “first prohibited frequency band” of the present disclosure, and the FM broadcast band corresponds to an example of the “second prohibited frequency band” of the present disclosure.

[0090] When the switching frequency f is set so that the third condition is satisfied, the noise interference caused by the switching operation of the switching element Q1 can be prevented from interfering with the broadcast signal BS2. Therefore, the DC / DC converter 10 can drive the headlight 1 while suppressing the reduction of the power conversion efficiency while preventing the noise caused by the switching operation from interfering with either of the broadcast signals BS1 and BS2.

[0091] The controller 5 may control the switching element Q1 in such a manner that, in addition to the above-mentioned first condition, the intensity I of the seventh harmonic (or the fourteenth harmonic) is substantially zero in the FM broadcast band (the fourth condition). The intensity I being substantially zero means, for example, that the intensity I is greater than zero and less than a predetermined positive value. The predetermined positive value is determined so that when the intensity I is equal to the positive value, the influence of the switching noise on the broadcast signal BS2 can be ignored from a practical point of view. As an example of a predetermined positive value, the limit value of the intensity I determined in the EMC (Electromagnetic Compatibility) specification can be cited.

[0092] As described above, according to this embodiment, even if the controller 5 controls the switching element Q1 at a switching frequency f higher than the upper limit frequency of the AM broadcast band, it is possible to prevent noise from interfering with the broadcast signal BS1 while suppressing an excessive decrease in power conversion efficiency.

[0093] Furthermore, unlike the case where the switching frequency is lower than the AM broadcast band, it is possible to avoid the situation where the higher harmonic components of the switching frequency f interfere with the broadcast signal BS1. Therefore, for example, there is no need to add an input noise filter or the like between the battery 20 and the DC / DC converter 10. In addition, in the present embodiment where the switching frequency f is higher than the above-mentioned case, the inductance value of the inductor L1 and the resistance value of the DC component can be reduced. Thus, the miniaturization of the inductor L1 and the reduction of the heat generation can be achieved at the same time. As a result, the manufacturing cost of the DC / DC converter 10 can be reduced.

[0094] The diode D1 of this embodiment is a Schottky diode, so it is suitable for high-speed switching. Furthermore, as described above, the leakage current is easy to reduce, so it can operate stably even when the temperature rises. By setting it as such a structure, high-speed switching at a switching frequency f higher than the upper limit frequency of the AM broadcast band can be properly performed. Thus, even if the switching frequency f is higher than the upper limit frequency of the AM broadcast band, the DC / DC converter 10 can operate properly.

[0095] The semiconductor layer 30 of the diode D1 of the present embodiment is formed of silicon (Si). Therefore, as the diode D1, a product with the best reverse voltage can be easily selected or manufactured. Therefore, the miniaturization and low cost of the diode D1 can be achieved at the same time, and further, the reliability of the diode D1 against failures can also be improved. As a result, the DC / DC converter 10 can be manufactured at a low cost while being miniaturized and highly reliable. Furthermore, the withstand voltage of the diode D1 is several 10V to 100V, so it is suitable for the driving voltage (30V to 70V) of the headlight 1 of the vehicle 100.

[0096] The DC / DC converter 10 is mounted on the vehicle 100 and connected to the battery 20. The input voltage Vin corresponding to the voltage of the battery 20 is typically 12V, but sometimes drops to about 6V in an instant. Therefore, the DC / DC converter 10 receives a large input current Iin from the battery 20 in order to drive the headlight 1. According to the present embodiment, even when such a large input current Iin is supplied to the DC / DC converter 10, the headlight 1 can be driven while avoiding noise interference with the broadcast signal BS1 (and the broadcast signal BS2) and suppressing a decrease in power conversion efficiency.

[0097] Variation example.

[0098] In the above embodiment, the DC / DC converter 10 is a boost converter, but a buck-boost converter may be used instead.

[0099] Fig.10 1 is a diagram showing an example of the structure of a DC / DC converter in this modification. Fig.10 The DC / DC converter 10A is an asynchronous buck-boost converter configured to boost an input voltage Vin to an output voltage Vout. The DC / DC converter 10A includes a switching element Q11 , an inductor L11 , a diode D11 , a capacitor C11 , and a controller 5 .

[0100] The switching element Q11 is connected in series with the inductor L11 and is controlled by the controller 5. The diode D11 is a trench-type diode ( Figure 4 ), connected between power lines PL11A, PL11B and a load LOAD. Power lines PL11A, PL11B are provided between inductor L11 and switching element Q11.

[0101] In the present embodiment, the number of LEDs 3A to 3Z is large and the voltage applied to the LED array 3 is higher than the voltage of the battery 20. That is, the DC / DC converter 10A operates in the step-up mode. In addition, when the number of LEDs 3A to 3Z is small and the voltage applied to the LED array 3 is lower than the voltage of the battery 20, the DC / DC converter 10A may operate in the step-down mode.

[0102] Even in the DC / DC converter 10A, the resonance frequency of the voltage Vds immediately after the switching element Q11 is turned on is higher than the resonance frequency when a planar diode is used as the diode D11. As a result, when the switching frequency is set higher than 1.71 MHz in the DC / DC converter 10A, the same advantages as those of the above embodiment (advantages of the trench diode D11) are obtained. The resonance frequency of the voltage Vds is determined by the parasitic capacitance and parasitic inductance on the current path formed by the switching element Q11, the capacitor C11, the diode D11, and the input capacitor (not shown), the output voltage Vout, the output power Pout, and the switching frequency.

[0103] As described above, similarly to the above-described embodiment, it is possible to prevent noise from interfering with the broadcast signal BS1 (and the broadcast signal BS2 ) while suppressing a decrease in power conversion efficiency.

[0104] In addition, the DC / DC converter 10A may also be a Cuk converter, a Zeta converter, or a Sepic converter (Single Ended Primary Inductor Converter).

[0105] Other variations.

[0106] Refer again Figure 3 In the case where the number of LEDs 3A to 3Z is small and the voltage applied to the LED array 3 is lower than the voltage of the battery 20, the DC / DC converter 10 may be replaced by an asynchronous buck converter. Figure 3 The constant current controlled buck converter 2 shown may also be an asynchronous converter. In this case, the above-mentioned embodiment or its modified example may also be applied to the buck converter 2. Specifically, the switching frequency of the switching element of the buck converter 2 may be set higher than 1.71 MHz, and a trench diode may be used as the diode of the buck converter 2.

[0107] In the above embodiment, the semiconductor layer 30 is formed of silicon, but may be formed of a semiconductor different from silicon (for example, SiC or GaN).

[0108] The DC / DC converter 10 may also be used for purposes other than driving the headlight 1 of the vehicle 100. As long as the parameters such as the voltage, current, power, or switching frequency of each part of the DC / DC converter 10 are the same as the parameters for driving the headlight 1 of the vehicle 100, the same advantages as those of the above-mentioned embodiment and its modified examples can be obtained.

[0109] The “second prohibited frequency band” of the present disclosure is not limited to the FM broadcast band, and may be, for example, a frequency band (26.5 MHz to 29.7 MHz) broadcasted by a Citizens Band Radio (CB) radio.

[0110] The embodiments disclosed herein are to be considered in all respects as illustrative only and not restrictive. The scope of the present disclosure is indicated by the claims rather than the above description, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0111] (Explanation of symbols)

[0112] 1: headlight; 5: controller; 10, 10A: DC / DC converter; 20: battery; 25: radio; 100: vehicle; C1, C11: storage battery; Cd1, Cs1: capacitor; D1, D11: diode; L1, L11: inductor; LOAD: load; P1: rectification path; P2: current path; Q1, Q11: switching elements.

Claims

1. An asynchronous DC / DC converter, comprising: Inductors; a switching element connected in series with the inductor; a diode connected to the inductor and the switching element; and a controller, controlling the switching element, The controller controls the switching element so that the switching frequency of the switching element is higher than an upper limit frequency of a predetermined frequency band. The diode comprises: The first capacitor is a capacitor of a rectifier path through which the forward current of the diode flows; as well as The second capacitor is a capacitor of a current path formed in parallel with the rectification path.

2. The DC / DC converter according to claim 1, wherein: The diode is a trench diode, The second capacitance is the capacitance of the trench portion of the diode.

3. The DC / DC converter according to claim 1 or 2, wherein: The predetermined frequency band is an AM broadcasting band.

4. The DC / DC converter according to claim 1 or 2, wherein: The upper frequency limit is 1.71 MHz.

5. The DC / DC converter according to any one of claims 1 to 4, wherein: The controller controls the switching element so that a lower limit frequency of a half-amplitude band of a spectrum of noise generated due to a switching operation of the switching element becomes higher than the upper limit frequency.

6. The DC / DC converter according to any one of claims 1 to 4, wherein: The switching frequency is in the range higher than 1.71 MHz and lower than 10.85 MHz.

7. The DC / DC converter according to claim 6, wherein: The switching frequency is in the range of above 2.0 MHz and below 10.85 MHz.

8. The DC / DC converter according to any one of claims 1 to 7, wherein: The diode is a Schottky diode.

9. The DC / DC converter according to any one of claims 1 to 8, wherein: The diode comprises a semiconductor layer, The semiconductor layer is formed of silicon.

10. The DC / DC converter according to any one of claims 1 to 9, wherein: The inductor is connected between the positive node of the power supply of the DC / DC converter and the switching element. The switch element is connected between the inductor and the negative node of the power supply, The anode of the diode is connected to the inductor and the switching element.

11. The DC / DC converter according to claim 10, wherein: The DC / DC converter is configured to be mounted on a vehicle. The power source is a battery of the vehicle, The load of the DC / DC converter is configured to include a headlight of the vehicle.

12. The DC / DC converter according to any one of claims 1 to 11, wherein: The controller controls the switching element so as to prevent the seventh-order harmonic component of the switching frequency from being included in a second frequency band higher than a first frequency band that is the predetermined frequency band.

13. The DC / DC converter according to any one of claims 1 to 11, wherein: The controller controls the switching element so as to prevent a 14th-order harmonic component of the switching frequency from being included in a second frequency band higher than a first frequency band that is the predetermined frequency band.

14. The DC / DC converter according to claim 12 or 13, wherein: The second frequency band is an FM broadcast band.

15. The DC / DC converter according to claim 12 or 13, wherein: The second frequency band includes a frequency band greater than or equal to 76 MHz and less than or equal to 108 MHz.

Images