JP2003244935A - Electrically insulating switching element drive circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrically insulating switching element drive circuit which can directly drive, by high frequency, a large-power switching element requiring considerable power for its drive. <P>SOLUTION: This drive circuit transmits pulse-modulated AC power from its primary side to its secondary side using a coreless transformer 4 in driving a switching element, and resonates it with a resonance circuit where the leakage inductance of the coreless transformer 4 is made to be an inductance component. The circuit rectifies the AC power and drives the switching element. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrically insulating switching element drive circuit.

[0002]

2. Description of the Related Art In a DC-DC converter and various inverter circuits, it is common to drive a reference potential different from the reference potential of the input circuit system. The most extreme example of such an application is to drive the high-side element of the inverter circuit as the potential of the output terminal of this inverter circuit,
In this case, a control voltage based on the output terminal of the inverter circuit (the connection point between the high-side element and the low-side element of the inverter circuit) must be applied to the control terminal of the high-side element. Further, not only the high side element but also the low side element of the inverter circuit is often driven with a reference voltage completely different from that of the input side circuit system as a reference. In addition, a circuit system in which the power supply voltage has an absolute potential (or reference potential) different from that of the input side circuit system is required not only for the above-mentioned inverter circuit but also for various applications.

In such a circuit system, at the time of transmitting a signal from the input side circuit to at least the first switching element of the output side circuit, at least only the driving power of the first switching element can be supplied from the input side circuit. It's easy.

In order to realize this, a transformer insulation type electrically insulating switching element drive circuit has been conventionally known. In this circuit, the input side circuit oscillates to form an AC voltage type signal, which is AC transmitted to the output side circuit through a transformer, which is rectified by the output side circuit and rectified high energy switching signal voltage. Therefore, a method of intermittently connecting the first switching element (hereinafter, also referred to as a transformer insulation type switching element drive circuit) is often used.

However, in the conventional transformer insulation type switching element drive circuit, the winding of the coil is not easy because the core of the transformer is small, and the switching element has a high frequency although power accompanying transmission of a signal with a low intermittent frequency is possible. There was a problem that it could not be driven.
This is because, in the conventional transformer insulation type switching element drive circuit, the hysteresis loss of the transformer core is significantly increased due to the increase of the frequency, and most of the high frequency AC power input to the transformer is attenuated by the heating of the core. This is because the leakage inductance reduces the transmission efficiency.

As a conventional electrically isolated switching element drive circuit, in addition to the transformer isolated switching element drive circuit described above, there is a method of electrically isolating input and output by using a piezo transformer (piezo coupler). Since the drive frequency of the piezo element is low, it is difficult to drive the switching element on the output side at a high frequency.

Further, as a conventional electrically isolated switching element drive circuit, a photocoupler using an LED (light emitting diode) and a photodiode (PD) is used to perform input / output electrical insulation and transmit a signal associated with power. Although a method may be considered, a large chip area is required for the LED (light emitting diode) and the photodiode (PD) in order to transmit the power necessary for directly driving the switching element on the output side. Moreover, it was not easy to realize a circuit for boosting low-voltage power, and it was very difficult to put it into practical use.

The present invention has been made in view of the above problems, and an object thereof is to provide an electrically isolated switching element drive circuit capable of directly driving a high power switching element which requires a considerable amount of power for driving at high frequency. There is.

[0009]

According to a first aspect of the present invention, there is provided an electrically isolated switching element driving circuit, comprising: an oscillating circuit for converting a binary signal voltage for intermittently driving a switching element into an AC voltage having a predetermined fundamental frequency. A leakage transformer in which an AC voltage output from the AC conversion circuit is applied to a primary coil; and a resonance circuit that receives the output voltage of a secondary coil of the leakage transformer and resonates with the fundamental frequency as a resonance frequency, It is characterized by comprising a waveform shaping circuit for rectifying the AC voltage output from the resonance circuit, converting it into a binary signal voltage and applying it between the reference potential main terminal and the control terminal of the switching element.

A resonance circuit having a fundamental frequency as a resonance frequency is a resonance circuit in which the resonance frequency is set to 0.8 to 1.2 times, and more preferably 0.95 to 1.05 times the fundamental frequency. And Alternatively, it means a resonance circuit that generates a secondary voltage larger than the theoretical value of the secondary voltage determined by the input voltage of this leakage transformer and its winding ratio. The capacitors mentioned here mean all capacitors that are arranged on the secondary circuit side and that form a resonant circuit together with the leakage inductance of the leakage transformer and the parasitic capacitance of the secondary coil. The leakage transformer here is a term that distinguishes it from a closed magnetic circuit type transformer, and means that the electromagnetic coupling coefficient between the primary coil and the secondary coil is 50% or less. The above-mentioned closed magnetic circuit type transformer means a transformer having a core structure having no air gap in the magnetic circuit or having only an air gap at the joint portion of a plurality of partial cores forming the closed magnetic circuit, and the leakage A transformer is a transformer that has a large air magnetic path in a closed magnetic circuit and has a very large leakage inductance of both coils.

The transformer-isolated switching element drive circuit of the secondary side resonance type leakage transformer structure of the present invention is characterized by the resonance between the large leakage inductance (leakage inductance) of the secondary coil of the transformer and the capacitor.
More specifically, due to the resonance phenomenon including the inductance-capacitance distributed constant circuit configured by the inductance of the secondary coil of the transformer and the parasitic capacitance (between each turn) and the leakage inductance of the secondary coil of the transformer. The output voltage loss can be reduced, and the effective electromagnetic coupling efficiency of the transformer can be dramatically improved despite the leakage transformer structure.

As a result, the switching element electrically insulated from the voltage system on the input side can be switched at high speed by the power supplied from the input side by using the leakage transformer which is small and light and easy to wind the coil. The type switching element drive circuit can be configured simply and at low cost.

Further, due to the resonance, the output voltage of the transformer can be increased without setting the winding ratio of the transformer to a large value (for example, even if it is set to 1: 1). .

According to a second aspect of the present invention, in the electrically insulating switching element drive circuit according to the first aspect, the leakage transformer comprises a core restaurant. With this configuration, since the core is not used at all, it is possible to increase the frequency without considering the loss increase of the core (for example, ferrite core) due to the hysteresis loss. By increasing the frequency, it is possible to achieve high-speed response of the switching element. In addition, since the core restaurant coil and the resonance capacitor can be downsized by increasing the frequency, it is possible to significantly downsize the device. As a result of the above, it is possible to significantly reduce the size and weight of the electrically isolated switching element drive circuit.

In particular, when the core restaurant is adopted, the use of the resonance circuit is particularly important when it is necessary to apply an output voltage having an amplitude larger than the input voltage of the core restaurant to the switching element. That is,
The increase in output voltage amplitude due to the adoption of this resonance circuit is effective in improving the electromagnetic coupling efficiency by changing the turns ratio of the core restaurant.
A winding number ratio close to 1 can be adopted, and power transmission efficiency can be improved.

According to a third aspect of the present invention, in the electrically isolated switching element drive circuit according to the second aspect, the waveform shaping circuit includes a rectifying circuit for rectifying an AC voltage output from the resonant circuit, and the rectifying circuit. Since it has a comparison circuit for converting the output rectified voltage into a binary signal voltage and applying it between the reference potential main terminal and the control terminal of the switching element, a sharp edge is formed in the switching element. Pulse voltage can be applied, and the switching loss in the transient state of the switching element can be reduced.

The comparison circuit is particularly important when the rectification circuit has a smoothing capacitor (or a peak hold capacitor) that is charged through a rectifying diode. That is, in this circuit configuration, since the coreless capacitor can quickly charge the smoothing capacitor through the rectifying diode when driving the switching element, the switching element can be driven at high speed. However, in this case, even if the charging of the smoothing capacitor through the diode for rectification is stopped when the switching element is turned off, the electric charge accumulated in the smoothing capacitor does not immediately disappear in the core restaurant, so that the smoothing capacitor It takes a long time to turn off the switching element due to the voltage drop, and transient power loss and heat generation of the switching element during this period increase. This problem can be solved by using this comparison circuit. In addition, it is preferable that the comparison circuit typically compares the voltage of a predetermined portion between the smoothing capacitor and the secondary coil or the partial voltage thereof with the partial voltage of the smoothing capacitor. A comparator is generally used as the comparison circuit. For example, a voltage of a predetermined portion between the smoothing capacitor and the secondary coil or its divided voltage is applied to the gate of a MOS transistor to which a predetermined threshold voltage is applied. You may use on and off.

According to a fourth aspect of the present invention, in the electrically insulating switching element drive circuit according to the second aspect, the waveform shaping circuit rectifies the AC voltage output from the resonance circuit to use the reference potential of the switching element. It is characterized by comprising a rectifying circuit applied between the main terminal and the control terminal. In this aspect, since the comparison circuit is not used, a problem occurs in high-speed driving of the switching element, but when the switching element is driven at a low speed, an electrically isolated switching element drive circuit can be realized by a simple circuit without any problem. .

According to a fifth aspect of the present invention, in the electrically isolated switching element drive circuit according to the second aspect, the input side circuit and the output side circuit are mounted on the primary coil and the secondary coil of the core restaurant. It is characterized in that each of them is composed of an insulating layer on the circuit board or a spiral printed coil laminated in the thickness direction with the circuit board sandwiched therebetween, so that the core restaurant can be easily mounted on the circuit board. .

According to a sixth aspect of the present invention, in the electrically insulating switching element drive circuit according to the fifth aspect, a constant voltage is applied with low impedance around the primary coil and the secondary coil of the core restaurant. Since the voltage conductor region is formed, it is possible to improve the electromagnetic coupling cutoff property with the outside, and it is possible to increase the parasitic capacitance in the distributed constant circuit of the secondary coil of the core restaurant and improve its resonance. can do.

According to a seventh aspect of the present invention, in the electrically insulating switching element drive circuit according to the fifth aspect, the inner end portion of the spiral print coil has a surface of the circuit board through a hole formed in the circuit board. Since it is connected to the via-hole conductor that is drawn out to the outside of the other spiral print coil that is formed in (1), it is possible to simplify the lead-out structure of the depression side end of the spiral print coil.

According to an eighth aspect of the present invention, in the electrically insulating switching element drive circuit according to the seventh aspect, the other spiral print coil is formed so as to be recessed inward at a portion close to the via hole conductor. With this feature, the via-hole conductor can be easily pulled out to the outside of another spiral print coil, and the wiring structure can be simplified.

According to a ninth aspect of the present invention, in the electrically insulating switching element drive circuit according to the eighth aspect, the via-hole conductor is formed at a corner of the other spiral print coil formed in a rectangular shape. In order to avoid the via hole conductor while suppressing the decrease in the electromagnetic coupling coefficient of both coils, it is possible to prevent the turn length of the other spiral print coil from increasing and reduce the resistance loss of the coil. You can

According to a tenth aspect of the present invention, in the electrically insulated switching element drive circuit according to the second aspect, the input side circuit and the output side circuit are mounted on the primary coil and the secondary coil of the core restaurant. And a magnetic sheet disposed on at least one surface side of the spiral print coils, the spiral print coils being coaxially wound around the circuit board. Accordingly, it is possible to increase the inductance and the electromagnetic coupling coefficient while suppressing the increase in the structure and the process.

According to the eleventh aspect of the invention, in the electrically insulating switching element drive circuit according to the second aspect, 80% or more of the conductor layers forming the both coils are located at the same position in the main surface direction of the circuit board (mutually). It is arranged in the overlapping position). By doing so, the electromagnetic coupling coefficient between the turns of both coils can be improved, so that the power transmission efficiency of the core restaurant can be maximized.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of an electrically insulating switching element drive circuit of the present invention will be described below with reference to the drawings.

[0027]

[Embodiment 1] An electrically insulating switching element drive circuit of this embodiment will be described below with reference to FIG. (Circuit configuration) 1 is an oscillation circuit, 2 is a modulation circuit, 3 is a current amplification circuit, 4 is a coreless circuit, 5 is a rectification circuit, 6 is a comparison circuit, 7 is a current amplification circuit, and 8 is a constant voltage diode for voltage regulation. Is.

The oscillation circuit 1 is a known circuit that outputs a high frequency pulse voltage (fundamental frequency 9 MHz) having a phase difference of 180 degrees and a pulse width / pulse period of about 50%. Gates 21, 22,
Also, through the complementary emitter follower circuits 31 and 32 forming the bridge-type current amplifier circuit 3, a high frequency pulse voltage is applied to both ends of the series circuit of the primary coil 41 of the core restaurant 4 and the capacitor Co only during a period in which a switching element (not shown) is turned on. Apply. As a result, the primary coil 4
A high frequency pulse voltage having a peak value of about 5 V is applied to both ends of 1. The series capacitor Co is for equivalently reducing the leakage inductance of the primary coil 41 of the core restaurant 4, and constitutes a series resonance circuit with the primary coil 41. The series capacitor Co can be omitted.

The core restaurant 4 is a transformer having no core, 41 is a primary coil and 42 is a secondary coil.

The rectifier circuit 5 has a voltage doubler detection circuit (voltage doubler rectifier circuit) including a capacitor C1 and diodes D1 and D2, a capacitor C2, a diode D3, capacitors C3 and C4, and a resistor R1. In this embodiment, the capacitor C1 is about pF and the capacitor C2 is about
pF, capacitor C3 is about pF, capacitor C
4 is set to about pF.

The secondary coil 42 of the core restaurant 4 is
The above-mentioned capacitors connected to the secondary coil 42, particularly the capacitors C1 and C2, form a resonance circuit that resonates at the fundamental frequency. Particularly in this embodiment, the capacitor C1 constitutes a resonance circuit which resonates at the above-mentioned fundamental frequency together with the leakage inductance of the secondary coil 42 (more accurately, the impedance of the distributed constant circuit of the secondary coil 42) and the capacitor C2. There is. The capacitor C1 functions as a basic component of a voltage doubler rectifier circuit described later, and since the voltage drop of the capacitor C1 and the voltage drop of the leakage inductance of the secondary coil 42 are different in phase from each other by 180 degrees AC, the capacitor C1 is
1 effectively reduces the voltage drop of the leakage inductance to improve the electromagnetic transmission efficiency and voltage loss of the core restaurant 4. The capacitor C1 and the leakage inductance as the distributed constant circuit may resonate at the fundamental frequency, and the capacitor C2 and the leakage inductance as the distributed constant circuit may resonate at the fundamental frequency.

The diode D1 applies a power supply voltage that has been double-voltage rectified to a comparison circuit and a current amplification circuit 7 which will be described later,
The capacitor C3 constitutes a smoothing capacitor (peak hold capacitor) for stably supplying this power supply voltage.

The anode voltage of the diode D1 is applied to the capacitor C4 and the resistor R1. This anode voltage has a peak value of the diode D1 in the positive half-wave period.
The pulse voltage is larger than the peak voltage output from the cathode by the amount of the forward voltage drop. Diode D3
It is possible to omit the capacitor C4.

The rectifier circuit 5 includes diodes D1, D2, D
3, a smoothing capacitor C3, and a voltage dividing capacitor C4. The diode D1 is supplied with power from one end of the secondary coil 41 through the capacitor C1 to charge the capacitor C3, and the capacitor C3 applies a power supply voltage to the subsequent circuit.

The comparison circuit 6 is composed of a comparator 61 and a voltage dividing circuit formed by resistors R2 and R3.
The divided voltage of the voltage doubler rectified voltage output from the diode D1 is compared with the voltage of the resistor R1. When the AND gate 2 is cut off by the control voltage Vs, the voltage of the resistor R1 rapidly decreases, the output voltage of the comparator 61 becomes low level, and the output voltage of the comparator 61 passes through the complementary emitter follower circuit forming the current amplifier circuit 7. The gate electrode voltage of the switching element (not shown) is rapidly lowered. R4 is a pull-up resistor that pulls up the input voltage of the current amplifier circuit 7 to a high level.

FIG. 2 shows measured waveforms of the input voltage and the output voltage obtained by this circuit. The power supply voltage applied to the current amplification circuit 3 is 5V. Due to the effect of the above resonance circuit, the output voltage is voltage-amplified exceeding the limit voltage 10V of the voltage doubler rectifier circuit, and even in applications requiring a high voltage, the output voltage is large without changing the winding ratio of the core restaurant 4. There is an advantage that the output voltage can be applied to the switching element. The rectifier circuit and the resonance circuit can be replaced with other known circuits having a rectification effect and a resonance effect.

Details of the core restaurant 4 will be described below with reference to FIGS.

In FIG. 3, reference numeral 100 denotes a printed circuit board on which the circuit of FIG. 1 is mounted, and a primary coil 41 is printed on the front surface and a secondary coil 42 is printed on the back surface. As shown in FIG. 4, both coils 41 and 42 are spiral print coils having a turn ratio of 1, and the same turn of both coils 41 and 42 is the printed circuit board 1
00 are formed so as to completely overlap each other in the surface direction. As a result, the turns of both coils 41, 42 are electromagnetically coupled to each other favorably.
Core restaurant 4 with improved electromagnetic coupling coefficient of 1 and 42
The power transmission efficiency can be improved. (Additional explanation) In addition, the parasitic capacitance of core restaurant 4 is
It exists between each turn of the primary coil, between the primary coil and ground, between each turn of the primary coil and each turn of the secondary coil, between each turn of the secondary coil, and between the secondary coil and ground. High frequency analysis of such a complicated circuit is not easy, but the resonance point may be obtained by an experiment.

[0039]

Second Embodiment Another embodiment of the core restaurant 4 will be described below with reference to FIG.

In this embodiment, both coils 41 and 42 are formed in a substantially square shape, and when the printed circuit board 100 is viewed from above, one corner portion 410 of the primary coil 41 is recessed into a square shape. Secondary coil 4 facing diagonally
One of the corners 420 of No. 2 has a square depression.

One end of the primary coil 41 is drawn out to the back surface side of the printed circuit board 100 through a through hole 411 provided in the printed circuit board 100. The through hole 411 is located outside the corner portion 420 of the secondary coil 42. Therefore, wiring can be performed without straddling both coils 41 and 42. Similarly, one end of the secondary coil 42 has a through hole 42 formed in the printed circuit board 100.
Although it is drawn out to the surface side of the printed circuit board 100 through 1, the through hole 421 is located outside the corner portion 410 of the primary coil 41, so that wiring can be performed without straddling both coils 41 and 42.

[0042]

Third Embodiment Another embodiment of the core restaurant 4 will be described below with reference to FIG.

In this embodiment, the core restaurant 4 is a two-output type, and a pair of secondary coils 4 are provided on the back side of the printed circuit board 10 along each turn of the primary coil 41.
200 and 4201 are arranged. In this way, a pair of secondary voltages electrically isolated from each other (see FIG. 7) can be obtained, and thus the pair of secondary voltages is used to form the high side element (upper arm element) of the inverter circuit. The first switching element and the second switching element forming the low-side element (lower arm element) of this inverter circuit can be operated in reverse with respect to potentials different from each other. This circuit is shown in FIG. However, in FIG. 8, since the pair of secondary coils 4201 and 4202 output voltage in the same direction, the pair of comparators 611 and 61
The input voltages of each pair of No. 2 are made opposite to each other so that their outputs have opposite phases.

[0044]

Eighth Embodiment Another embodiment of the core restaurant 4 will be described below with reference to FIG.

In this embodiment, four spiral print coils 401 to 4 are provided on the multilayer wiring printed circuit board 1000.
04 is arranged. These four spiral print coils 401 to 404 can be used to generate an antiphase output voltage and to connect in series or in parallel.

[0046]

[Embodiment 9] Another embodiment of the core restaurant 4 is shown in FIG.
Will be described below.

In this embodiment, two spiral printed coils 4 are provided inside the multilayer printed circuit board 1000.
1 and 42 are laminated as a primary coil and a secondary coil, and further, these spiral printed coils 41 and 42 are covered to cover the front and back surfaces of the multilayer printed circuit board 1000.
The grounded copper foils 1003 and 1004 for electromagnetic shielding are arranged to reduce the emission of electromagnetic noise from the core restaurant 4 to other core restaurants and external circuits. In this embodiment, this copper foil 100 for electromagnetic shielding is used.
3, 1004 and the parasitic capacitance formed between the secondary coil 42 can also be expected to reduce the voltage drop of the leakage inductance of the secondary coil 42 and improve the electromagnetic transmission efficiency of the core restaurant 4. . (Modification) In each of the above embodiments, the resonance frequency of the resonance circuit on the secondary coil side of the core restaurant is the basic frequency of the pulse voltage supplied from the primary side, but instead of the secondary coil side of the core restaurant, The resonance frequency of the resonance circuit may be triple the fundamental frequency of the pulse voltage supplied from the primary side. This is because the pulse voltage contains many triple harmonic components. (Modification) A resonance capacitor may be provided on the primary coil side of the core restaurant 4 to resonate with the leakage inductance of the primary coil 41 of the core restaurant 4.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a circuit configuration of a first embodiment.

FIG. 2 is a measured voltage waveform diagram showing input / output characteristics of the circuit of FIG.

FIG. 3 is a schematic side view showing the core restaurant of FIG.

FIG. 4 is a schematic plan view showing the spiral print coil of FIG.

FIG. 5 is a schematic plan view showing another embodiment of the spiral print coil of FIG.

6 is a schematic plan view showing another embodiment of the spiral print coil of FIG.

7 is a measured voltage waveform diagram showing the input / output characteristics of a circuit using the spiral print coil of FIG.

8 is a circuit diagram showing a configuration of a circuit using the spiral print coil of FIG.

9 is a schematic plan view showing another embodiment of the spiral print coil of FIG.

FIG. 10 is a schematic plan view showing another embodiment of the spiral print coil of FIG.

[Explanation of symbols]

1 Oscillation circuit 4 Coreless transformer (leakage transformer, resonance circuit) C1, C2 Capacitor (resonance circuit) 5 Waveform shaping circuit

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toshihiko Sugiura             1-1, Showa-cho, Kariya city, Aichi stock market             Inside the company DENSO (72) Inventor Shigeo Hirashima             1-1, Showa-cho, Kariya city, Aichi stock market             Inside the company DENSO (72) Inventor Yuji Hayashi             14 Iwatani Shimohakaku-cho, Nishio-shi, Aichi Stock Association             Company Japan Auto Parts Research Institute F-term (reference) 5H740 BA11 JA01 JB01 KK03

Claims (11)

[Claims] 1. An oscillating circuit for converting a binary signal voltage for intermittently driving a switching element into an alternating voltage having a predetermined fundamental frequency, and an alternating voltage output from the alternating circuit is applied to a primary coil. A leakage transformer, a resonance circuit that receives the output voltage of a secondary coil of the leakage transformer and resonates with the fundamental frequency as a resonance frequency, and an AC voltage output from the resonance circuit is rectified and converted into a binary signal voltage. And a waveform shaping circuit applied between the control potential terminal and the reference potential main terminal of the switching element, and an electrically isolated switching element drive circuit. 2. The electrically isolated switching element drive circuit according to claim 1, wherein the leakage transformer comprises a core restaurant. 3. The electrically isolated switching element drive circuit according to claim 2, wherein the waveform shaping circuit rectifies an AC voltage output from the resonance circuit, and a rectified voltage output from the rectification circuit. Is converted into a binary signal voltage and is applied between the reference potential main terminal and the control terminal of the switching element, and an electrically isolated switching element drive circuit. 4. The electrically isolated switching element drive circuit according to claim 2, wherein the waveform shaping circuit rectifies an AC voltage output from the resonance circuit to provide a reference potential main terminal and a control terminal of the switching element. An electrically isolated switching element drive circuit comprising a rectifying circuit applied between the electric isolation type switching element drive circuit and the drive circuit. 5. The electrically isolated switching element drive circuit according to claim 2, wherein the primary coil and the secondary coil of the core restaurant are insulated from a circuit board on which the input side circuit and the output side circuit are mounted. An electrically isolated switching element drive circuit, each of which is composed of a spiral print coil laminated in the thickness direction with the layers or the circuit board sandwiched therebetween. 6. The electrically isolated switching element drive circuit according to claim 5, wherein a constant voltage conductor region, to which a constant voltage is applied with low impedance, is formed around the primary coil and the secondary coil of the core restaurant. An electrically isolated switching element drive circuit characterized by the following. 7. The electrically insulating switching element drive circuit according to claim 5, wherein the inner end of the spiral print coil is formed on the surface of the circuit board through a hole formed in the circuit board. 2. An electrically isolated switching element drive circuit, which is connected to a via-hole conductor drawn outside the spiral print coil. 8. The electrically insulating switching element drive circuit according to claim 7, wherein the other spiral print coil is formed so as to be recessed inward at a portion close to the via-hole conductor. Electrically isolated switching element drive circuit. 9. The electrically insulating switching element drive circuit according to claim 8, wherein the via-hole conductor is formed at a corner portion of the other spiral print coil formed in a rectangular shape. Insulated switching element drive circuit. 10. The electrically insulating switching element drive circuit according to claim 2, wherein the primary coil and the secondary coil of the coreless transformer are mutually provided on a circuit board on which the input side circuit and the output side circuit are mounted. An electrically-insulated switching element drive circuit comprising a spiral print coil wound coaxially, and a magnetic sheet is disposed on at least one surface side of both spiral print coils. 11. The electrically insulating switching element drive circuit according to claim 2, wherein 80% or more of the conductor layers forming the both coils are arranged at the same position in the main surface direction of the circuit board. A characteristic electrically isolated switching element drive circuit.