JP6631366B2 - Insulated power converter - Google Patents

Description

  The present invention relates to an insulated power converter that converts power input from a DC power supply by a transformer and supplies the converted power to a load.

  Conventionally, in a power converter that converts input power and supplies it to a load by the operation of a switching element (hereinafter referred to as a “switch”), an overcurrent abnormality due to a short circuit on the load side is detected and the switching operation of the switch is stopped. Techniques for doing so are known. For example, in the technology disclosed in Patent Document 1, a current detection resistor is provided in an input side current path of an inverter, and an overcurrent abnormality is detected based on an output of an amplification comparison circuit connected between terminals of the current detection resistor. ing.

JP 2008-17649 A

Although the prior art of Patent Document 1 relates to a non-insulated inverter, this technique is similarly applied to a configuration in which a short-circuit abnormality of a secondary load is detected by a current flowing through a primary side in an insulated power converter. Can be applied. In the prior art of Patent Document 1, although the amplification and comparison circuit is provided to lower the resistance value of the current detection resistor, the connection of the resistor to a path through which a large current flows remains unchanged, and a loss occurs. .
Therefore, there is a method using a current sensor to prevent the occurrence of loss. However, when a current sensor is used, the current detection delay increases, and it takes time to determine that an overcurrent is abnormal. As a result, the stopping of the switching operation is delayed, which may lead to destruction of the device.

  The present invention has been made in view of such a point, and an object of the present invention is to provide an insulated power converter that detects a short-circuit abnormality of a load quickly with low loss.

An insulated power conversion device according to the present invention includes a first primary winding (30) and a second primary winding (40), a first switch (SW1) and a second switch (SW2), and one or more secondary windings. It includes a winding (50), a capacitor (60), a drive signal generation unit (70), and a short circuit determination unit (801, 802, 803).
One end of each of the first primary winding and the second primary winding is connected to one electrode of the DC power supply (15), and forms a primary side of the transformer (20). The first switch and the second switch are connected between each other end of the first primary winding and the second primary winding and the other electrode of the DC power supply.
The DC power supply here includes a smoothing capacitor.

One or more secondary windings constitute the secondary side of the transformer.
A capacitor is connected across the secondary winding and, together with the leakage inductance (52) of the secondary winding, constitutes an LC resonant circuit as a "load".
The drive signal generation unit generates a drive signal that drives the first switch and the second switch to turn on / off complementarily.
Note that the capacitor is not limited to an independent element, and various components having a capacitance component or an aggregate of a plurality of components may be regarded as a capacitor.

Here, the voltage induced in the transformer when the first switch is turned off and the second switch is turned on and generated in the exciting inductances (31, 41) of the primary winding is defined as "exciting winding voltage (Vc)".
The short-circuit determination unit detects the excitation winding voltage based on a first switch voltage (Vsw1) that is a drain-source voltage of the first switch. Then, the short-circuit determination unit determines a load short-circuit abnormality based on the detected value of the exciting winding voltage.
In this specification, the term “short circuit” is not limited to the case where the resistance is completely 0 [Ω], but also includes any abnormality in which the resistance is extremely small as compared with a normal state and an overcurrent exceeding an allowable range flows. Strictly speaking, the term “drain-source voltage” as a term of a field-effect transistor is interpreted as including “collector-emitter voltage” in a bipolar transistor.

During normal operation, due to the resonance of the LC resonance circuit that occurs with the ON operation of the second switch, the excitation winding voltage monotonically changes in an S-shaped curve during the detection period that is the ON period of the second switch. On the other hand, when the load is short-circuited, the only component of the load is the leakage inductance of the secondary winding, and no LC resonance occurs. Therefore, the excitation winding voltage has a constant value during the detection period.
Further, when the first switch is turned off and the second switch is turned on, the excitation winding voltage can be detected based on the first switch voltage of the first switch that is turned off.

In the present invention, attention is paid to this point, and the short-circuit determination unit determines the short-circuit abnormality of the load based on the detected value of the exciting winding voltage. Specifically, the short-circuit determination unit determines that the load is abnormal when the excitation winding voltage monotonically changes during the detection period, and that the load is abnormal when the excitation winding voltage is constant. This eliminates the need to connect a resistor to a path through which a large current flows, so that the loss can be reduced as compared with the related art.
In addition, an overcurrent abnormality can be detected more quickly than a technique using a current sensor. Therefore, by immediately stopping the switching operation of the switch when an abnormality is detected, it is possible to appropriately prevent destruction of the device and adverse effects on peripheral devices and the like.

Specifically, the short-circuit determination unit may compare, for example, the time change rate of the exciting winding voltage with a predetermined change rate threshold.
Alternatively, the short-circuit determination unit sets a range including a constant value of the exciting winding voltage when the load is short-circuited as a specified range, and sets the number of times that the AD conversion value of the exciting winding voltage is within the specified range, or May be compared with respective thresholds.

FIG. 4 is a diagram illustrating a general configuration of the insulated power converter of each embodiment and a current path when (a) SW1 is on / SW2 off, and (b) when SW1 is off / SW2 is on. FIG. 7A is a diagram showing an equivalent circuit when the switch SW2 is on in a normal state, and FIG. FIG. 7A is a diagram showing an equivalent circuit when a switch SW2 is turned on when a load is short-circuited, and FIG. FIG. 1 is a configuration diagram of an insulated power converter according to a first embodiment. 4 is a time chart in a normal state according to the first embodiment. Time chart at the time of load short circuit according to the first embodiment The block diagram of the insulation type power converter of 2nd Embodiment. 9 is a time chart in a normal state according to the second embodiment. Time chart at the time of load short circuit according to the second embodiment The block diagram of the insulation type power converter of 3rd Embodiment. 9 is a time chart in a normal state according to the third embodiment. Time chart at the time of load short circuit according to the third embodiment 7A is a configuration diagram of a short-circuit determination according to a first comparative example, and FIG.

Hereinafter, an embodiment of an insulated power converter will be described with reference to the drawings. This insulated power converter is a device that converts power input from a DC power supply by a transformer and supplies the converted power to a load. In the present embodiment, a DC-AC converter that converts DC power into AC power will be exemplified.
[General configuration and operation of insulated power converter]
First, the general configuration and operation of the insulated power converter of the present embodiment will be described with reference to FIG.

As a general configuration, the insulated power conversion device 10 includes a transformer 20, two switches SW1 and SW2 provided on the primary side of the transformer 20, a capacitor 60 provided on the secondary side of the transformer 20, and a drive signal generation unit 70. And The primary side of the transformer 20 is configured by two primary windings 30 and 40, and the secondary side is configured by one secondary winding 50.
The switches SW1 and SW2 are semiconductor switching elements, for example, MOSFETs having a body diode are used. In addition, an IGBT or the like in which a freewheeling diode is connected in parallel may be used. In this case, the switch symbol in the drawing is read and interpreted. Of the two primary windings 30, 40, the switch connected to the first primary winding 30 is called a first switch SW1, and the switch connected to the second primary winding 40 is called a second switch SW2.

The center tap CT on the primary side is connected to the positive electrode of the battery 15 as a “DC power supply”. One end of the first primary winding 30 and the second primary winding 40 is connected to the center tap CT, and the other end is connected to the drains of the switches SW1 and SW2. The sources of the switches SW1 and SW2 are connected to the negative electrode of the battery 15.
The capacitors 60 are connected to both ends of the secondary winding 50. The capacitor 60 is not limited to an independent element, and various components having a capacitance component or an aggregate of a plurality of components may be regarded as a capacitor.
In the figure, Vin is the power supply voltage of the battery 15, and Vo is the output voltage applied between the electrodes of the capacitor 60.

The drive signal generation unit 70 generates a drive signal that drives the first switch SW1 and the second switch SW2 to turn on / off complementarily.
When the first switch SW1 is turned on and the second switch SW2 is turned off, as shown in FIG. 1A, the primary current flows through the first primary winding 30 and the first switch SW1. At this time, a secondary current flows in the secondary winding 50 in a direction opposite to the current flowing through the first primary winding 30.
When the second switch SW2 is turned on and the first switch SW1 is turned off, as shown in FIG. 1B, the primary current flows through the second primary winding 40 and the second switch SW2. At this time, a secondary current flows in the secondary winding 50 in a direction opposite to the current flowing through the second primary winding 40, that is, in a direction opposite to that in the case of FIG.

Thus, an alternating voltage of positive and negative is generated between the electrodes of the capacitor 60 by the on / off operation of the first switch SW1 and the second switch SW2.
A period in which the first switch SW1 and the second switch SW2 are turned on once each is defined as “one cycle” of the switching cycle. If the lengths of the ON periods of both switches SW1 and SW2 are set to be the same and the dead time during which both switches SW1 and SW2 are turned off at the same time is ignored, the length of the ON period of one switch becomes “two-minutes” of the switching cycle. One cycle ".

In the insulated power conversion device 10 having such a configuration, if the load on the secondary side is short-circuited and an overcurrent flows, the device may be destroyed. As noted in the section of "Means for Solving the Problem", the term "short circuit" in this specification is not limited to the case where the resistance is completely 0 [Ω], but the resistance becomes extremely small compared to the normal state. Includes general abnormalities in which an overcurrent exceeding the allowable range flows.
Here, a configuration of a short circuit determination according to a comparative example assumed based on the related art will be described with reference to FIG. In FIG. 13, the substantially same components as those in FIG. 1 of the present embodiment are denoted by the same reference numerals, and description thereof is omitted.

The first comparative example shown in FIG. 13A is one in which the technology of the non-insulated inverter disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2008-17649) is applied to an insulated power converter. In the device 108 of the first comparative example, the detection voltage of the current detection resistor R provided in the primary current path is amplified by the amplification / comparison circuit 808, and when the amplified voltage exceeds a threshold value, a short circuit abnormality is determined. By amplifying the detection voltage, the resistance value of the current detection resistor R can be reduced. However, a resistor is still connected to a path through which a large current flows, and a loss occurs.
Further, in a configuration in which an overcurrent is determined using a current sensor, loss can be prevented, but it takes time to determine an overcurrent abnormality. As a result, the stop of the switching operation is delayed, and the switch or the like may be broken.

  The second comparative example shown in FIG. 13B is based on the technology disclosed in JP-A-2001-72401. In the device 109 of the second comparative example, a detection winding 39 dedicated to abnormality detection is provided on the primary side of the transformer 29. Further, the short-circuit determination unit 809 connected to the detection winding 39 detects a short-circuit abnormality based on the voltage of the detection winding 39 on which information of the power supply voltage Vin and the output voltage Vo is reflected. However, in this configuration, it is necessary to provide the detection winding 39 dedicated to abnormality detection, and the device becomes large. In addition, the discharge characteristics are not effectively utilized, which is inefficient.

In contrast to such a comparative example, the present embodiment focuses on a voltage change that occurs with the on / off switching of the switches SW1 and SW2, does not require a dedicated winding, and can quickly detect a short-circuit abnormality of the load with low loss and low loss. This implements a detectable configuration.
Next, the principle of determining a short-circuit abnormality according to the present embodiment will be described with reference to FIGS.
Hereinafter, the present embodiment focuses on a voltage change in a state where the first switch SW1 is turned off and the second switch SW2 is turned on. The ON period of the second switch SW2 corresponding to a half of the switching period is referred to as a “detection period”.

  FIG. 2A shows an equivalent circuit when the second switch SW2 is on when the load is normal and FIG. 3A shows a short circuit in the load. The second switch SW2 in the ON state is considered to be equivalent to a conducting wire. A first switch voltage Vsw1, which is a drain-source voltage, is generated across the first switch SW1 in the off state. The current I1 flows through the second primary winding 40, and the current I3 flows through the secondary winding 50. Here, the turn ratio between the second primary winding 40 and the secondary winding 50 is “n: 1”. In FIG. 3, which shows the load shorted, the capacitor 60 is considered equivalent to a conductor.

In addition, each of the windings 30, 40, and 50 is schematically described separately as “winding components 31, 41, and 51 that generate excitation inductance” and “winding components 32,, and 52 that generate leakage inductance”. Hereinafter, the "winding components 31, 41, 51 that generate the excitation inductance" are omitted, and are referred to as "excitation inductances 31, 41, 51", and the "winding components 32, 42, 52 that generate the leakage inductance" are omitted. It is referred to as “leakage inductance 32, 42, 52”.
In a normal state, the leakage inductance 52 of the secondary winding 50 and the capacitor 60 form an LC resonance circuit on the secondary side, and the LC resonance voltage appears in the output voltage Vo.

Here, when the first switch SW1 is turned off and the second switch SW2 is turned on, the voltage induced in the transformer 20 and generated in the excitation inductances 31 and 41 of the primary windings 30 and 40 is defined as "excitation winding voltage Vc". I do. The excitation inductance 51 of the secondary winding 50 generates a voltage (Vc / n) of “1 / n” which is proportional to the turn ratio with respect to the excitation winding voltage Vc.
The meanings of the following symbols in FIGS. 2 and 3 are as follows.
L1: Primary side leakage inductance L3: Secondary side leakage inductance R2: Resistance of second switch SW2 and second primary winding 40 R3: Resistance of secondary winding 50

Based on the above physical quantities, the exciting winding voltage Vc is expressed by the following equation (1). The “R × I” term corresponds to the voltage drop due to the resistance.

Assuming that the resistances R2 and R3 are extremely small in Expression (1), the exciting winding voltage Vc is expressed by Expression (2).

In a normal state, an LC resonance voltage appears in the output voltage Vo which is the second term of the equation (2). Therefore, as shown in FIG. 2B, in the detection period in which the second switch SW2 is in the ON state, the exciting winding voltage Vc monotonically increases from the initial voltage S to the ultimate voltage F in a S-shaped curve.
On the other hand, when the load is short-circuited, the C component of the capacitor 60 is lost, and the only component of the load is the leakage inductance 52 of the secondary winding 50. That is, since no LC resonance occurs, the output voltage Vo becomes a constant voltage. Therefore, assuming that the power supply voltage Vin does not suddenly change, the excitation winding voltage Vc has a constant value K during the detection period as shown in FIG.

Therefore, during the detection period when the first switch SW1 is turned off and the second switch SW2 is turned on, it is normal if the exciting winding voltage Vc monotonically increases, and if the exciting winding voltage Vc is constant, it is normal. Can be determined to have occurred. Note that “monotonically increasing” is read as “monotonically decreasing” depending on the positive or negative definition of the exciting winding voltage Vc. Generally speaking, when the exciting winding voltage Vc changes “monotonically”, it is determined that the load is normal.
The insulated power converter 10 of the present embodiment is characterized by including a short-circuit determination unit that determines a short-circuit abnormality of a load based on the above-described determination principle, in addition to the general configuration illustrated in FIG. The short-circuit determination unit detects the excitation winding voltage Vc based on the first switch voltage Vsw1, and determines a load short-circuit abnormality based on the detected value of the excitation winding voltage Vc.

[Configuration of short-circuit determination unit]
Hereinafter, a plurality of embodiments according to the short-circuit determination unit will be sequentially described. As reference diagrams corresponding to the respective embodiments, a configuration diagram of a control block of a short-circuit determination unit and a time chart of signals sequentially processed by the short-circuit determination unit when a load is normal and when a short circuit is abnormal are attached.
In FIGS. 4, 7, and 10, which are configuration diagrams of each embodiment, the same reference numerals are given to substantially the same configuration as the configuration of the above-described embodiment, and the description is omitted. In each configuration diagram, the secondary-side capacitor 60 is shown in a form in which two capacitors 61 and 62 are connected in series. The capacitor 61 is connected in parallel with two Zener diodes 63 and 64 whose cathodes are connected to each other.

(1st Embodiment)
First, a first embodiment will be described with reference to FIGS.
As shown in FIG. 4, the short-circuit determination unit 801 of the first embodiment includes an excitation winding voltage detection (“Vc detection” in the figure) unit 81, an AD conversion unit 82, a time change rate calculation unit 83, and a threshold comparison unit 861. , And a SW-off signal hold unit 87.

Further, in the insulated power converter 10, an AND circuit 75 is provided between the drive signal generator 70 and the switches SW1 and SW2. When the drive signal for turning on the switch SW1 or SW2 is input from the drive signal generation unit 70 and the SW on signal is input from the SW off signal hold unit 87, the AND circuit 75 connects the gate of the switch SW1 or SW2 to the gate of the switch SW1 or SW2. Output drive signal. On the other hand, when the input of the SW-on signal from the SW-off signal hold unit 87 is interrupted, no drive signal is output to the switches SW1 and SW2 regardless of the drive signal from the drive signal generation unit 70.
In short, the short-circuit determination unit 801 outputs the SW-ON signal to the AND circuit 75 to drive the switches SW1 and SW2 when the load is normal, and shuts off the output of the SW-ON signal to the AND circuit 75 when the load is short-circuited. , Stop the switching operation of the switches SW1 and SW2.

Next, the operation of each control block in the short-circuit determination unit 801 will be described in order.
The excitation winding voltage detection unit 81 acquires the first switch voltage Vsw1, and detects the excitation winding voltage Vc based on the first switch voltage Vsw1. If the excitation winding voltage Vc is to be obtained accurately, it is necessary to calculate the excitation winding voltage Vc by using the information of the power supply voltage Vin according to the equation (3).
Vc = Vsw1-Vin (3)

  However, as described later, in the first embodiment, only the information on the time change rate of the exciting winding voltage Vc is used, and the overall offset need not be considered. Therefore, on the assumption that the power supply voltage Vin is stable at least during the detection period, the power supply voltage Vin may be considered as a constant. Therefore, substantially, the time change rate of the first switch voltage Vsw1 may be directly treated as the time change rate of the excitation winding voltage Vc.

The AD converter 82 AD-converts the analog value Vc_a of the excitation winding voltage at a predetermined conversion cycle, and outputs an AD converted value Vc_d. 5 and 6, the AD conversion timing in the detection period is represented by t1 to t9, and the period between the AD conversion timings is represented by P1 to P9.
As shown in FIGS. 5 and 6B, the AD conversion value Vc_d of the excitation winding voltage is displayed as a discrete digital value. Since the AD conversion value at time t10 at which the detection period elapses is not used for control, the point at time t10 is outlined.

The time change rate calculation unit 83 calculates the time change rate of the AD conversion value Vc_d of the excitation winding voltage, that is, the slope in FIGS. 5 and 6B.
The time rate of change α (i) appearing in the i-th (i ≧ 2) period Pi is expressed by the following equation (4), which is the difference between the current value Vc_d (i) and the previous value Vc_d (i−1) of the exciting winding voltage. The difference is calculated by dividing the difference by the conversion period Δt.

The threshold comparison unit 861 compares the time change rate α calculated by the time change rate calculation unit 83 with the change rate threshold α_th. As shown in FIGS. 5 and 6C, the change rate threshold α_th is set as the lower limit of the normal range for the time change α. When the time rate of change α is less than the rate of change threshold α_th, the threshold comparing unit 861 determines that the load has a short circuit abnormality and outputs a SW off signal.
Here, in the above example, the sign is defined so that the exciting winding voltage Vc has a positive value, but the definition of the positive and negative of the exciting winding voltage Vc may be changed as appropriate. Generally speaking, the threshold comparing unit 861 determines that the load is short-circuited when the absolute value of the time change rate α is less than the change rate threshold α_th.

In a normal state, as shown in FIG. 5B, the AD conversion value Vc_d of the exciting winding voltage monotonically increases from the initial voltage S to the ultimate voltage F in a S-shaped curve.
In FIG. 5C, for example, the change rate from time t1 to time t2 is reflected in the period P2, and the change rate from time t5 to time t6 is reflected in the period P6. That is, a value delayed by the conversion period Δt from the theoretical differential value is obtained as the time change rate α. The time change rate α becomes maximum near the center of the detection period, and decreases at the beginning and end of the detection period.

Here, the change rate threshold α_th is set to be larger than 0 and equal to or less than the minimum time change rate α reflected in the period P2. Therefore, the time change rate α does not become smaller than the change rate threshold α_th during the detection period.
In FIG. 5D, in the first period P1 during which the time rate of change α is not calculated, the SW ON signal is output unconditionally. Further, since the time change rate α after the period P2 is always equal to or more than the change rate threshold value α_th, the SW-on signal continues.

On the other hand, when the load is short-circuited, the AD conversion value Vc_d of the excitation winding voltage becomes constant at the value K as shown in FIG. Therefore, as shown in FIG. 6C, the time change rate α after the period P2 is always 0, and is smaller than the change rate threshold α_th.
Therefore, in FIG. 6D, the SW-OFF signal is output after the period P1 during which the SW-ON signal is output unconditionally.
The SW off signal hold unit 87 holds the SW off signal. As a result, the output cutoff state of the SW ON signal to the AND circuit 75 is maintained. Therefore, the switching operation of the switches SW1 and SW2 is stopped when a load short circuit occurs.

As described above, the short-circuit determination unit 801 of the first embodiment determines a load short-circuit abnormality based on whether or not the time change rate α of the AD conversion value Vc_d of the excitation winding voltage is 0.
The insulated power converter 10 of the present embodiment including the following second and third embodiments is provided with such a short-circuit determination unit 801 so that a resistor flows through a path where a large current flows as in the first comparative example. Since it is not necessary to connect the power supply, the loss can be reduced.
In addition, an overcurrent abnormality can be detected more quickly than a technique using a current sensor. Accordingly, by immediately stopping the switching operation of the switches SW1 and SW2 when an abnormality is detected, it is possible to appropriately prevent the destruction of the device and the adverse effects on peripheral devices and the like.
Furthermore, unlike the second comparative example, a winding dedicated to abnormality detection is not required, so that an increase in the size and cost of the device can be avoided.

(2nd Embodiment)
Next, a second embodiment will be described with reference to FIGS.
As shown in FIG. 7, the short-circuit determination unit 802 of the second embodiment is different from the short-circuit determination unit 801 of the first embodiment in that the excitation winding voltage detection unit 81, the AD conversion unit 82, and the SW off signal hold unit 87 Have in common. In addition, the short-circuit determination unit 802 includes a specified range comparison unit 84, a counter 852, and a threshold comparison unit 862 as unique configurations.

Further, the short-circuit determination unit 802 includes a power supply voltage (“Vin detection” in the figure) unit 91 that detects the power supply voltage Vin, and a specified range setting unit 92 that sets a specified range Rs. The specified range setting unit 92 variably sets a range including the constant value K of the exciting winding voltage Vc when the load is short-circuited as the specified range Rs according to the power supply voltage Vin. Specifically, as shown in FIG. 8B, the specified range Rs is set to an intermediate range between the initial voltage S and the attained voltage F.
Further, in the second embodiment, the excitation winding voltage detection unit 81 acquires the detected value of the power supply voltage Vin, and based on the power supply voltage Vin and the first switch voltage Vsw1, calculates the current excitation winding voltage by the equation (3). The voltage Vc may be calculated accurately.

The specified range comparison unit 84 of the second embodiment compares the AD conversion value Vc_d of the excitation winding voltage output from the AD conversion unit 82 with the specified range Rs set by the specified range setting unit 92, When the AD conversion value Vc_d of the voltage is within the specified range Rs, a hit signal is output discretely. The counter 852 counts the number Ns of times the hit signal has been output.
The threshold comparing unit 862 compares the number of times Ns counted by the counter 852 with the number of times threshold N_th. As shown in FIGS. 8 and 9C, the count threshold N_th is set as the upper limit of the normal range for the count Ns. When the number Ns of times that the AD conversion value Vc_d of the excitation winding voltage is within the specified range Rs exceeds the number threshold N_th, the threshold comparing unit 862 determines that the load is short-circuited and outputs the SW off signal. I do.

8 and 9, similarly to FIGS. 5 and 6 of the first embodiment, the AD conversion timing in the detection period is represented by t1 to t9, and the period between the AD conversion timings is represented by P1 to P9. In this example, it is assumed that the width of the specified range Rs is set relatively narrow.
In a normal state, as shown in FIG. 8B, the AD conversion value Vc_d of the excitation winding voltage slightly increases from the value of the immediately preceding AD conversion timing at all the AD conversion timings t1 to t9. I have. The AD conversion value Vc_d of the excitation winding voltage is outside the specified range Rs at times t1 to t5 and t7 to t9, and is within the specified range Rs only at time t6. Therefore, the hit signal is output once in the period P6 during which the comparison result at the time t6 is recognized.

  Therefore, as shown in FIG. 8C, the number of times Ns = 1 is counted in the period P6, and is maintained until time t10 when the detection period ends. The count Ns is reset at time t10 when the detection period has elapsed. Here, the count threshold N_th is set to a value larger than 1, and the count Ns does not exceed the count threshold N_th during the detection period. Therefore, the SW ON signal continues as shown in FIG.

On the other hand, when the load is short-circuited, as shown in FIG. 9B, the AD conversion value Vc_d of the excitation winding voltage becomes constant at the value K, and is within the specified range Rs at all the AD conversion timings from time t1 to t9. Therefore, the hit signal is output once each time during the entire period of P1 to P9.
Therefore, as shown in FIG. 9C, the count Ns increases stepwise in each period from P1 to P9, and is reset at time t10. In this example, the count Ns = 9 in the final period P9. If the count threshold N_th is set between 1 and 2, the count Ns exceeds the count threshold N_th in the period P2. Then, as shown in FIG. 9D, a SW off signal is output after the period P2.

The SW-off signal holding unit 87 holds the SW-off signal as in the case of the short-circuit determination unit 801 of the first embodiment. As a result, the output cutoff state of the SW ON signal to the AND circuit 75 is maintained. Therefore, the switching operation of the switches SW1 and SW2 is stopped when a load short circuit occurs.
As described above, in the second embodiment, the load short-circuit abnormality is determined by comparing the AD conversion value Vc_d of the excitation winding voltage with the specified range Rs. Therefore, the same effect as in the first embodiment can be obtained. In addition, since the specified range setting unit 92 variably sets the specified range Rs according to the detected value of the power supply voltage Vin, even when the power supply voltage Vin fluctuates, the load short-circuit abnormality can be reliably detected.

(Third embodiment)
Next, a third embodiment will be described with reference to FIGS.
As shown in FIG. 10, the short-circuit determination unit 803 of the third embodiment is different from the short-circuit determination unit 802 of the second embodiment in that the excitation winding voltage detection unit 81, the specified range comparison unit 84, and the SW off signal hold unit 87 , A power supply voltage detecting unit 91 and a specified range setting unit 92. The short-circuit determination unit 803 does not include the AD conversion unit 82, but includes a timer 853 and a threshold comparison unit 863 instead of the counter 852 and the threshold comparison unit 862.

  Similarly to the second embodiment, the specified range setting unit 92 variably sets a range including the constant value K of the exciting winding voltage Vc when the load is short-circuited as the specified range Rs according to the power supply voltage Vin. Specifically, as shown in FIG. 11A, the specified range Rs is set to an intermediate range between the initial voltage S and the attained voltage F. Note that, for the sake of matching with FIGS. 8 and 9, (b) in FIGS. 11 and 12 is omitted.

The specified range comparing unit 84 of the third embodiment compares the analog value Vc_a of the exciting winding voltage acquired from the exciting winding voltage detecting unit 81 with the specified range Rs set by the specified range setting unit 92, and sets the exciting winding value. When the analog value Vc_a of the line voltage is within the specified range Rs, a hit signal is continuously output. The timer 853 measures a time Ts during which the hit signal is output.
The threshold comparing unit 863 compares the time Ts measured by the timer 853 with the time threshold T_th. As shown in FIGS. 11 and 12C, the time threshold T_th is set as the upper limit of the normal range for the measurement time Ts. Then, when the time Ts during which the analog value Vc_a of the excitation winding voltage is within the specified range Rs exceeds the time threshold T_th, the threshold comparing unit 863 determines that the load has a short-circuit abnormality and outputs a SW off signal. .

In a normal state, as shown in FIG. 11A, the analog value Vc_a of the excitation winding voltage rises with time from the initial voltage S to the ultimate voltage F, and passes through the specified range Rs in a relatively short time Ts. A hit signal is output only for the passing time Ts, and is measured by the timer 853. When the detection period has elapsed, the measurement time Ts is reset.
As shown in FIG. 11C, the measurement time Ts at this time is equal to or less than the time threshold T_th. Therefore, the SW ON signal continues as shown in FIG.

On the other hand, when the load is short-circuited, as shown in FIG. 12A, the analog value Vc_a of the exciting winding voltage becomes constant at the value K and stays within the specified range Rs throughout the detection period. Therefore, a hit signal is always output during the detection period.
As shown in FIG. 12C, when the time Ts measured by the timer 853 exceeds the time threshold T_th at the time tx, the SW off signal is output after the time tx as shown in FIG. 12D. When the SW-off signal holding unit 87 holds the SW-off signal, the state of interrupting the output of the SW-on signal to the AND circuit 75 is maintained. Therefore, the switching operation of the switches SW1 and SW2 is stopped when a load short circuit occurs.
The third embodiment has the same effects as the second embodiment.

(Other embodiments)
(A) In the description of the short-circuit determination units 801 to 803 in the above-described embodiment, the processing for eliminating erroneous determination due to a detection error, noise, or the like is omitted. In reality, even though the load is normal, the time conversion rate α of the first embodiment may temporarily fall below the threshold value due to disturbance such as noise, or the excitation windings of the second and third embodiments may be different. It is assumed that the voltage values Vc_d and Vc_a may temporarily enter the specified range Rs. Conventionally, as a technique for eliminating such erroneous determinations and improving detection reliability, for example, a method of determining an abnormality when the state of the tentative determination has continued for a predetermined number of times or for a predetermined time has been known.
Therefore, a configuration in which those well-known techniques are combined with the configuration of the short-circuit determination unit of the present embodiment described above is also interpreted as being included in the technical scope of the present invention.

  (B) The short-circuit determination units 802 and 803 of the second and third embodiments have the power supply voltage detection unit 91 and the specified range setting unit 92, and variably set the specified range Rs according to the detected value of the power supply voltage Vin. I do. On the other hand, for example, when the fluctuation range of the power supply voltage Vin is relatively narrow or predictable, the fixed specified range Rs is set to include the upper and lower limits of the constant value K corresponding to the fluctuation range of the power supply voltage Vin. You may. Thereby, the configuration of the short-circuit determination unit can be simplified.

  (C) When the short-circuit determination unit 801-803 of the above-described embodiment determines that the load has a short-circuit abnormality, it shuts off the output of the SW ON signal to the AND circuit 75 and stops the switching operation of the switches SW1 and SW2. . In another embodiment, for example, a power supply relay provided on the battery 15 side may be forcibly cut off as a measure in the event of a load short-circuit abnormality.

  (D) In the above embodiment, a DC-AC converter that includes one secondary winding 50, supplies alternating current flowing through the secondary winding 50 without rectification, and outputs AC power to a load is exemplified. I have. In another embodiment, the present invention is applied to a push-pull DC-DC converter that includes a plurality of secondary windings, alternately rectifies and flows current flowing through each secondary winding, and outputs DC power to a load. May be applied.

(E) In addition to the first primary winding 30 and the second primary winding 40, a third or more primary winding may be provided as a primary winding of the insulated power converter. If two of the windings have the same configuration as described above and their operation is not hindered by the third or more windings, they are interpreted as being included in the scope of the present invention.
As described above, the present invention is not limited to the above embodiment, and can be implemented in various forms without departing from the spirit of the invention.

10 ... insulated power converter,
15 ... battery (DC power supply),
20 ... transformer,
30 ... first primary winding, 40 ... second primary winding,
50 ... secondary winding,
60 ... condenser,
801, 802, 803...
SW1 ... first switch, SW2 ... second switch.

Claims (6)

A first primary winding (30) and a second primary winding (40), both ends of which are connected to one electrode of a DC power supply (15) and constitute a primary side of the transformer (20);
A first switch (SW1) and a second switch (SW2) connected between each other end of the first primary winding and the second primary winding and the other electrode of the DC power supply;
One or more secondary windings (50) constituting a secondary side of the transformer;
A capacitor (60) connected to both ends of the secondary winding and constituting an LC resonance circuit as a load together with a leakage inductance (52) of the secondary winding;
A drive signal generator (70) for generating a drive signal for driving the first switch and the second switch to turn on / off in a complementary manner;
When the first switch is turned off and the second switch is turned on, the exciting winding voltage (Vc), which is induced in the transformer and is generated in the exciting inductance (31, 41) of the primary winding, is applied to the second winding. A short-circuit determination unit (801, 802,..., 802, 802) that detects based on a first switch voltage (Vsw1), which is a drain-source voltage of one switch, and determines short-circuit abnormality of the load based on a detected value of the exciting winding voltage. 803)
An insulated power converter comprising: The short-circuit determination unit (801)
2. The insulated power converter according to claim 1, wherein when the absolute value of the time change rate of the excitation winding voltage is less than a predetermined change rate threshold value (α_th), the load is determined to be abnormally short-circuited. The short-circuit determination unit (802)
A range including a constant value of the exciting winding voltage at the time of load short circuit is set as a specified range,
The excitation winding voltage is A / D converted at a predetermined conversion cycle, and when the number of times that the A / D conversion value is within the specified range exceeds a predetermined number threshold (N_th), it is determined that the load has a short circuit abnormality. The insulated power converter according to claim 1. The short-circuit determination unit (803)
A range including a constant value of the exciting winding voltage at the time of load short circuit is set as a specified range,
2. The insulated power converter according to claim 1, wherein when the excitation winding voltage is within the specified range exceeds a predetermined time threshold (T_th), the load is determined to be abnormally short-circuited. The short-circuit determination units (802, 803)
The insulated power converter according to claim 3, wherein the specified range is variably set according to a power supply voltage (Vin) of the DC power supply. When the load is determined to be abnormal short circuit by the short circuit determination unit,
The insulated power converter according to any one of claims 1 to 5, wherein the switching operation of the first switch and the second switch is stopped.

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