JP2009296216A - Switching drive circuit, and switching circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for reducing harmonic noise and suppressing an increase in switching loss.
A switching drive circuit 11 switches between a drain electrode and a source electrode of a transistor Tr10 temporally between a conductive state and a non-conductive state by switching a gate voltage Vg of the transistor Tr10. The switching drive circuit 11 includes a variable capacitance element 14 connected between the drain electrode D and the gate electrode G of the transistor Tr10. The capacitance of the variable capacitance element 14 is characterized by decreasing as the potential difference between the drain electrode D and the gate electrode G of the transistor Tr10 increases.
[Selection] Figure 1

Description

  The present invention relates to a switching drive circuit that switches a main electrode of a transistor between a conductive state and a non-conductive state in time by switching a gate voltage of the transistor. The present invention also relates to a switching circuit having the switching drive circuit.

  FIG. 7 shows a schematic circuit diagram of a conventional DC-DC converter circuit 100. The DC-DC converter circuit 100 includes a DC power supply V100, an inductance element L100, a diode D100, a capacitor C100, a load element R100, a transistor Tr100, and a switching drive circuit 101.

  Inductance element L100 and transistor Tr100 are connected in series to DC power supply V100. The load element R100 is connected in parallel to the transistor Tr100. The diode D100 is provided for preventing a backflow of current, and the capacitor C100 is provided for charge charging. The switching drive circuit 101 switches the time between the drain electrode D and the source electrode S of the transistor Tr100 between the conductive state and the nonconductive state by switching the gate voltage Vg of the transistor Tr100. The switching drive circuit 101 includes a control circuit 102 and a gate resistor 103. The switching drive circuit 101 generates a gate voltage Vg from the control voltage Vin generated by the control circuit 102 and applies it to the gate electrode G of the transistor Tr100.

  As shown in FIG. 8, in the DC-DC converter circuit 100, when the transistor Tr100 is turned on, a voltage is generated in the inductance element L100 in a direction that prevents a change in current. Next, when the transistor Tr100 is turned off, the impedance of the transistor Tr100 increases, and the current flowing through the inductance element L100 tends to decrease. At this time, a voltage having a direction shown in FIG. 9 is generated in the inductance element L100. Thereby, the load voltage applied to the load element R100 is the sum of the voltage of the DC power supply V100 and the voltage generated in the inductance element L100, and is boosted. The DC-DC converter circuit 100 boosts the voltage of the DC power supply V100 to generate a load voltage, and applies it to the load element R100.

  In such a DC-DC converter circuit 100, harmonic noise is generated by switching of the transistor Tr100. In order to suppress this harmonic noise, it is necessary to lengthen the change time during switching of the drain voltage Vd.

  In the conventional DC-DC converter circuit 100, the change time of the gate voltage Vg is lengthened by setting the resistance value Rg of the gate resistor 103 high, and thereby the change time at the time of switching of the drain voltage Vd is lengthened. . FIG. 10 shows how the drain voltage Vd changes. “T” indicates the time of one cycle of the drain voltage Vd. “Τ” indicates the pulse width and indicates the time during which the transistor Tr100 is off. As shown in FIG. 10, when the resistance value Rg of the gate resistor 103 is set high, the rise and fall of the gate voltage Vg change slowly, and the change time Δt of the gate voltage Vg becomes longer.

FIG. 11 shows the relationship between the frequency and amplitude (noise level) of the harmonic noise of the switching waveform of FIG. The harmonic noise in FIG. 11 is automatically determined from the switching waveform. This harmonic noise is divided into three regions. The first region I is a region composed of low-order harmonics, and the amplitude of the harmonics is constant regardless of the frequency. The second region is a region composed of relatively low-order harmonics and attenuates with a slope of −20 dB / dec as the frequency increases. The third region is a region composed of higher-order harmonics and attenuates with a slope of −40 dB / dec as the frequency increases. The frequencies f ₁ and f ₂ that divide the three regions are determined by the pulse width τ of the drain voltage Vd and the change time Δt of the rise or fall of the drain voltage Vd, and can be expressed by the following equations.

As shown in Equation 2, it is effective to increase the change time Δt of the drain voltage Vd in order to reduce higher-order harmonic noise. When the change time Δt of the drain voltage Vd is lengthened, the boundary (f ₂ ) between the second region II and the third region III shown in FIG. 11 is shifted to the left, and the harmonic noise in the third region III is reduced. The

JP 2004-187463 A JP 2004-242484 A JP 2007-228447 A

In the prior art, the resistance value of the gate resistance is increased in order to lengthen the change time Δt of the drain voltage Vd. However, when the resistance value of the gate resistance is increased, both the drain voltage Vd and the drain current Id change slowly, and the switching loss increases.
An object of this invention is to provide the technique which suppresses the increase in switching loss while reducing a harmonic noise.

  The inventors focused on increasing the gate-drain capacitance rather than increasing the resistance value of the gate resistance in order to lengthen the change time Δt of the drain voltage Vd. When the gate-drain capacitance is increased, only the change time of the drain voltage Vd is lengthened, and the change time of the drain current Id is not changed. That is, the technique disclosed in this specification increases the gate-drain capacitance, thereby lengthening only the change time of the drain voltage Vd and reducing harmonic noise caused by the voltage change. On the other hand, in the technique disclosed in this specification, the resistance value of the gate resistance can be reduced, so that the change time of the drain current Id is shortened. For this reason, the switching loss determined by the product of the drain voltage Vd and the drain current Id can be reduced. Further, the technology disclosed in this specification is characterized in that the gate-drain capacitance is configured with a variable capacitance that decreases as the drain voltage Vd increases. As a result, the change in the drain voltage Vd changes slowly at the beginning and changes sharply in the latter half (concave change). When the drain voltage Vd is changed in this way, the switching loss determined by the product of the drain voltage Vd and the drain current Id can be reduced while maintaining a long change time of the drain voltage Vd. The techniques disclosed in this specification can be summarized as follows: (1) Since the resistance value of the gate resistance can be reduced, the drain current Id changes sharply and switching loss can be reduced. (2) Gate By making the capacitance between the drains variable, the switching loss can be reduced while maintaining the change time of the drain voltage Vd at a value equivalent to that when the gate resistance is large. As a result, the technique disclosed in this specification can reduce harmonic noise and suppress an increase in switching loss.

  That is, a switching drive circuit that embodies the above technique includes a variable capacitance element connected between the high-voltage side electrode and the gate electrode of the transistor. The capacitance of the variable capacitance element is characterized by decreasing as the potential difference between the high-voltage side electrode and the gate electrode of the transistor increases. An insulated gate transistor is used as the transistor driven by the switching drive circuit. Insulated gate transistors include MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors). When the transistor is a MOSFET, the high-voltage side electrode is the drain electrode. When the transistor is an IGBT, the high-voltage side electrode is a collector electrode.

The variable capacitance element preferably has a diode connected between the high-voltage side electrode and the gate electrode of the transistor. The diode is characterized in that the anode is electrically connected to the gate electrode of the transistor and the cathode is electrically connected to the high-voltage side electrode of the transistor.
The diode has a characteristic that when the potential difference between the anode and the cathode increases, the depletion layer width of the pn junction increases and the junction capacitance decreases. The diode is an element suitable for the variable capacitance element disclosed in this specification.

  The diode used for the variable capacitance element is preferably a PIN diode. The voltage dependence of the junction capacitance of the PIN diode is larger than that of the PN diode due to the effect of the I layer. The junction capacitance of the PIN diode rapidly decreases as the potential difference between the anode and the cathode increases. For this reason, if a PIN diode is used for the variable capacitance element, switching loss can be greatly reduced.

The switching drive circuit disclosed in this specification preferably further includes a second diode connected between the high-voltage side electrode and the gate electrode of the transistor and connected in series to the diode. . The second diode is characterized in that the anode is electrically connected to the high voltage side electrode of the transistor and the cathode is electrically connected to the gate electrode of the transistor.
When the second diode is provided, current is prevented from flowing from the gate electrode of the transistor to the high-voltage side electrode, and the on / off control of the transistor is stabilized.

  The switching drive circuit disclosed in this specification reduces harmonic noise and suppresses an increase in switching loss.

(First embodiment)
FIG. 1 shows a schematic circuit diagram of the DC-DC converter circuit 10. The DC-DC converter circuit 10 includes a DC power supply V10, an inductance element L10, a diode D10, a capacitor C10, a load element R10, a transistor Tr10, and a switching drive circuit 11.

  Inductance element L10 and transistor Tr10 are connected in series to DC power supply V10. The load element R10 is connected in parallel to the transistor Tr10. The diode D10 is provided for preventing a backflow of current, and the capacitor C10 is provided for charge charging. The switching drive circuit 11 is for temporally switching between the drain electrode D and the source electrode S of the transistor Tr10 between a conductive state and a non-conductive state, and includes a control circuit 12, a gate resistor 13, and a variable capacitance element 14. It has. The control circuit 12 generates a control voltage Vin. The switching drive circuit 11 generates a gate voltage Vg from the control voltage Vin and applies it to the gate electrode G of the transistor Tr10.

  In the DC-DC converter circuit 10, when the transistor Tr10 is on, a voltage is generated in the inductance element L10 in a direction that prevents a change in current (the voltage drops from the DC power supply V10 side toward the transistor Tr10 side). ). Next, when the transistor Tr10 is turned off, the impedance of the transistor Tr10 increases, and the current flowing through the inductance element L10 tends to decrease. At this time, a voltage in the opposite direction to the above is generated in the inductance element L10 (the voltage rises from the DC power supply V10 side toward the transistor Tr10 side). Thereby, the load voltage applied to the load element R10 is the sum of the voltage of the DC power supply V10 and the voltage generated in the inductance element L10, and is boosted. The DC-DC converter circuit 10 boosts the voltage of the DC power supply V10 to generate a load voltage and applies it to the load element R10.

  The switching drive circuit 11 includes a variable capacitance element 14. The variable capacitance element 14 is connected between the drain electrode D and the gate electrode G of the transistor Tr10. FIG. 2 shows the relationship between the capacitance of the variable capacitance element 14 and the voltage. As shown in FIG. 2, the variable capacitance element 14 is characterized in that the capacitance Cgd decreases as the potential difference between both ends increases. In this example, the capacitance of the variable capacitance element 14 decreases as the gate-drain voltage increases. The initial capacitance (capacitance when the gate-drain voltage is 0) of the variable capacitance element 14 is the built-in capacitance between the gate and drain of the transistor Tr10 (capacitance caused by the gate insulating film and depletion layer capacitance spreading to the drain side). Much larger than the series capacity).

  FIG. 3 shows how the drain current Id and the drain voltage Vd change when the transistor Tr10 is switched from on to off (turned off). In the case of the DC-DC converter circuit shown in FIG. 1, the switching loss occurs mainly when the transistor Tr10 is turned off. Therefore, the characteristics of the present embodiment relating to the switching loss will be described using changes in the drain current Id and the drain voltage Vd when the transistor Tr10 is turned off. Also, the broken lines shown in FIG. 3 are the drain current Id and the drain voltage Vd in the case of the conventional structure, and are displayed together for comparison. Note that the conventional switching drive circuit is an example in which the resistance value of the gate resistor is greatly adjusted in order to reduce high-frequency noise.

First, in the switching drive circuit 11 of this embodiment, since the initial capacitance of the gate-drain capacitance is set large, the change time Δt of the drain voltage Vd is substantially the same as that of the conventional structure. For this reason, harmonic noise is substantially equivalent in this embodiment and the conventional structure.
In other words, the switching drive circuit 11 of the present embodiment does not increase the resistance value Rg of the gate resistor 13 as in the conventional structure in order to lengthen the change time of the gate voltage Vg, but does not increase the gate-drain capacitance Cgd. Has increased. The switching drive circuit 11 of the present embodiment increases the gate voltage Vg change time and the drain voltage Vd change time Δt by increasing the gate-drain capacitance Cgd. As a result, harmonic noise is reduced. Reduced.

  For this reason, in the switching drive circuit 11, the resistance value Rg of the gate resistor 13 is set smaller than that of the conventional structure. Thereby, in the transistor Tr10 driven by the switching drive circuit 11, the change in the drain current Id becomes steeper than that in the conventional structure, and the switching loss determined by the product of the drain voltage Vd and the drain current Id is reduced.

  Further, in the switching drive circuit 11, the gate-drain capacitance Cgd is a variable capacitance that decreases as the drain voltage Vd increases. Thereby, in the transistor Tr10 driven by the switching drive circuit 11, the drain voltage Vd is depressed downward. As a result, the switching drive circuit 11 can reduce the switching loss determined by the product of the drain voltage Vd and the drain current Id while maintaining a long change time of the drain voltage Vd.

(Second Embodiment)
FIG. 4 shows a schematic circuit diagram of the DC-DC converter circuit 20. The DC-DC converter circuit 20 is characterized in that the variable capacitance element of the switching drive circuit 21 is composed of a diode 24. The anode of the diode 24 is electrically connected to the gate electrode G of the transistor Tr10, and the cathode is electrically connected to the drain electrode D of the transistor Tr10 via a blocking diode 25 described later.

  The diode 24 has a characteristic that when the potential of the drain electrode D increases, the width of the depletion layer increases and the junction capacitance decreases. For this reason, the diode 24 is an element suitable for the variable capacitance element disclosed in this specification. Further, since the diode 24 is configured in a simple form, it can be incorporated in the semiconductor substrate on which the transistor Tr10 is formed. In addition, it can be incorporated in a composite IC or a discrete power MOS.

  The switching drive circuit 21 further includes a blocking diode 25 connected between the gate electrode G and the drain electrode D of the transistor Tr10. The blocking diode 25 is connected in series to the diode 24, the anode is electrically connected to the drain electrode D of the transistor Tr10, and the cathode is electrically connected to the gate electrode G of the transistor Tr10 via the diode 24. It is connected. The junction capacitance of the blocking diode 25 is larger than the junction capacitance of the diode 24.

  The blocking diode 25 blocks current from flowing from the gate electrode G to the drain electrode D of the transistor Tr10. For example, assume that the blocking diode 25 is not provided. In this case, when the control voltage Vin is high, the gate voltage Vg is divided by conduction between the gate electrode G and the drain electrode D of the transistor Tr10, so that the transistor Tr10 is repeatedly turned on and off. . The operation of the transistor Tr10 becomes unstable.

  On the other hand, when the blocking diode 25 is provided, it is possible to prevent a current from flowing from the gate electrode G to the drain electrode D of the transistor Tr10, so that the on / off control of the transistor Tr10 can be stabilized. .

  FIG. 5 shows the result of studying the effect when a diode is used for the variable capacitance element. FIG. 5A shows a conventional example in which no variable capacitor is provided and the gate resistance Rg is set to a large value of 1 kΩ. The built-in capacitance between the gate and drain of the transistor is 0.14 nF. FIG. 5B is a comparative example, in which a fixed-capacitance capacitor is provided between the drain electrode and the gate electrode. The capacitance of the capacitor is set to 3 nF, and the gate resistance Rg is set to 10Ω. FIG. 5C shows this embodiment, which is an example in which a pn diode is used as the diode 24. The initial capacitance of the junction capacitance of the diode 24 is 13 nF, and the gate resistance Rg is 10Ω. The junction capacitance of the blocking diode 25 is 50 nF. FIG. 5D also shows this embodiment, which is an example in which a PIN diode is used as the diode 24. The initial capacitance of the junction capacitance of the diode 24 is 13 nF, and the gate resistance Rg is 10Ω. The junction capacitance of the blocking diode 25 is 50 nF. FIG. 6 shows the voltage dependency of the junction capacitance of the PN diode and the PIN diode. As shown in FIG. 6, the junction capacitance of the PIN diode is more voltage dependent than the junction capacitance of the PN diode.

  In the comparative example shown in FIG. 5B, the gate resistance Rg is set smaller than that in the conventional example shown in FIG. 5A, so that the drain current Id changes more rapidly. Further, in the comparative example of FIG. 5B, since the gate-drain capacitance is large, the change time Δt of the drain voltage Vd is maintained equal to the conventional example of FIG. For this reason, the level of the harmonic noise in FIG. 5B is equivalent to the level of the harmonic noise in FIG. However, since the capacitance in FIG. 5B is fixed, the change in the drain voltage Vd changes linearly. In the comparative example of FIG. 5B, the increase in switching loss due to the linear change in the drain voltage Vd is larger than the decrease in switching loss due to the steep change in the drain current Id. Then, switching loss will increase. If the switching loss of the conventional example of FIG. 5A is “1”, it is confirmed that the switching loss of the comparative example of FIG. 5B increases to “1.06”.

  In the comparative example shown in FIG. 5C, the gate resistance Rg is set smaller than that in the conventional example shown in FIG. 5A, so that the drain current Id changes sharply. Further, since the gate-drain capacitance is variable, the change in the drain voltage Vd is recessed downward. The change in the drain voltage Vd in FIG. 5C is recessed below the conventional example in FIG. In the embodiment of FIG. 5C, the switching loss is reduced due to the decrease in switching loss due to the steep change in drain current Id and the decrease in switching loss due to the change in drain voltage Vd being depressed downward. To do. If the switching loss of the conventional example of FIG. 5A is “1”, it is confirmed that the switching loss of the comparative example of FIG. 5C is reduced to “0.90”.

  In the embodiment of FIG. 5D, since a PIN diode is used for the diode 24, the change of the drain voltage Vd is further depressed below that of the embodiment of FIG. For this reason, in the embodiment of FIG. 5D, the amount of decrease in switching loss is larger than that of the embodiment of FIG. As a result, it is confirmed that when the switching loss of the conventional example of FIG. 5A is “1”, the switching loss of the comparative example of FIG. 5D is reduced to “0.83”.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or the drawings can achieve a plurality of objects at the same time, and has technical usefulness by achieving one of the objects.

The circuit diagram of the DC-DC converter circuit of 1st Embodiment is shown. The voltage dependence of the capacity | capacitance of a variable capacitance element is shown. The state of change of drain current and drain voltage when the transistor of the first embodiment is switched from on to off is shown. The circuit diagram of the DC-DC converter circuit of 2nd Embodiment is shown. (A) A configuration of a conventional example and a state of changes in drain current and drain voltage are shown. (B) A configuration of a comparative example and a state of changes in drain current and drain voltage are shown. (C) The configuration of the embodiment in the case where the diode is a PN diode, and how the drain current and drain voltage change. (D) The configuration of the embodiment in the case where the diode is a PIN diode, and how the drain current and drain voltage change. The voltage dependence of the capacity | capacitance of a PN diode and a PIN diode is shown. The circuit diagram of the conventional DC-DC converter circuit is shown. An equivalent circuit diagram when a transistor is turned on in a conventional DC-DC converter circuit is shown. The equivalent circuit diagram when the transistor is OFF in the conventional DC-DC converter circuit is shown. The change of the gate voltage is shown. The relationship between the frequency and amplitude of harmonic noise during switching is shown.

Explanation of symbols

10, 20: Converter circuits 11, 21: Switching drive circuit 12: Control circuit 13: Gate resistor 14: Variable capacitance element 24: Diode 25: Blocking diode

Claims (5)

A switching drive circuit that switches between the main electrode of the transistor between the conductive state and the non-conductive state in time by switching the gate voltage of the transistor;
A variable capacitance element connected between the high-voltage side electrode and the gate electrode of the transistor;
The switching drive circuit characterized in that the capacitance of the variable capacitance element decreases as the potential difference between the high-voltage side electrode and the gate electrode of the transistor increases. The variable capacitance element has a diode connected between the high-voltage side electrode and the gate electrode of the transistor,
2. The switching drive circuit according to claim 1, wherein the diode has an anode electrically connected to a gate electrode of the transistor and a cathode electrically connected to a high-voltage side electrode of the transistor.   The switching drive circuit according to claim 2, wherein the diode is a PIN diode. A second diode connected between the high-voltage side electrode and the gate electrode of the transistor and connected in series to the diode;
4. The switching drive according to claim 2, wherein the second diode has an anode electrically connected to the high-voltage side electrode of the transistor and a cathode electrically connected to the gate electrode of the transistor. 5. circuit.   A switching circuit comprising the switching drive circuit according to claim 1 and a transistor driven by the switching drive circuit.

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