POWER CONVERSION DEVICE

A power conversion device includes an AC current sensor, an AC voltage sensor, a DC voltage sensor, and a control circuit. The control circuit includes an integration unit configured to integrate a difference between a predetermined overcurrent threshold and the output current detected by the AC current sensor and output an integration output, a multiplication unit configured to multiply the integration output outputted from the integration unit by the output voltage detected by the AC voltage sensor and output a multiplication output, a division unit configured to calculate and output a Duty ratio by dividing the multiplication output outputted from the multiplication unit by the DC voltage detected by the DC voltage sensor, and a voltage control unit configured to control the AC voltage according to the Duty ratio outputted from the division unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2022-036729 filed on Mar. 10, 2022, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a power conversion device.

BACKGROUND ART

As an in-vehicle inverter, there is known an inverter that includes a current limiter for protecting a semiconductor switching element from an overcurrent, and performs a control to broaden a limiter value of the current limiter only for a predetermined period from a time when a load is connected to an AC output of the inverter (for example, see Patent Literature 1). In this inverter, a load connection detection circuit detects a temporary drop in output voltage of the inverter or a temporary increase in various currents that occurs when a load is connected to the AC output of the inverter, and the limiter value of the current limiter is broaden only for a short time of about one second. Accordingly, it is intended to supply a current required for activation of the load connected to the AC output of the inverter and to protect the switching element of the inverter from an overcurrent.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

In the inverter described in Patent Literature 1, after a temporary change in output voltage or various currents of the inverter is detected, an operation of broadening the limiter value is terminated in a short time during which a temperature threshold of the switching element is not exceeded. Therefore, it is necessary to extremely shorten a time delay until a temporary change in output voltage or the like of the inverter is detected, and it is necessary to provide an expensive microcomputer having a high clock frequency in the load connection detection circuit. That is, in the inverter described in Patent Literature 1, in addition to a matter that providing the load connection detection circuit itself causes an increase in size and an increase in cost, there is a problem that providing the expensive microcomputer further increases the cost.

In view of the above circumstances, an object of the present invention is to provide a power conversion device capable of dealing with activation of a load connected to an AC output of an inverter circuit, and capable of achieving reductions in size and cost.

A power conversion device of the present invention includes an inverter circuit configured to convert a DC voltage into an AC voltage and output the AC voltage to a load; an AC current sensor configured to detect an AC current outputted from the inverter circuit; an AC voltage sensor configured to detect the AC voltage outputted from the inverter circuit; a DC voltage sensor configured to detect the DC voltage input to the inverter circuit; and a control circuit configured to control the AC voltage based on the AC current detected by the AC current sensor, the AC voltage detected by the AC voltage sensor, and the DC voltage detected by the DC voltage sensor. The control circuit includes an integration unit configured to integrate a difference between a predetermined overcurrent threshold and the AC current detected by the AC current sensor and output an integration output, a multiplication unit configured to multiply the integration output outputted from the integration unit by the AC voltage detected by the AC voltage sensor and output a multiplication output, a division unit configured to calculate and output a Duty ratio by dividing the multiplication output outputted from the multiplication unit by the DC voltage detected by the DC voltage sensor, and a voltage control unit configured to control the AC voltage according to the Duty ratio outputted from the division unit.

According to the present invention, it is possible to cope with activation of a load connected to an AC output of an inverter circuit, and to achieve reductions in size and cost.

DESCRIPTION OF EMBODIMENTS

The present invention will be described below in accordance with a preferred embodiment. Also, it should be noted that the present invention is not limited to the embodiment shown below, and can be appropriately modified without departing from the gist of the present invention. In addition, in the embodiment shown below, illustration and description of some configurations are omitted, but regarding details of the omitted techniques, publicly known or well-known techniques are appropriately applied as long as there is no contradiction with the contents described below.

FIG.1is a circuit diagram showing a power conversion device1according to an embodiment of the present invention. The power conversion device1shown in this circuit diagram has a function of a DC/AC converter, and converts a DC voltage outputted from a battery2into an AC voltage and outputs the AC voltage to a load3. For example, the power conversion device1converts a DC voltage outputted from the battery2of DC 400 V into an AC voltage of AC 100 V. It should be noted that it is also possible to provide the power conversion device1in an in-vehicle charger. In this case, by using the power conversion device1as a bidirectional AC/DC converter, it is possible to convert an AC voltage input from a system power supply (not shown) into a DC voltage to charge the battery2, and convert a DC voltage outputted from the battery2into an AC voltage and output the AC voltage to the load3.

The power conversion device1includes an inverter circuit10, a voltage sensor20for an output voltage AC_Ref, a current sensor30for an output current I_AC, a voltage sensor40for a DC link voltage V_DC_f, and a control circuit100. The inverter circuit10is a voltage-type full-bridge power factor correction (PFC) inverter, and includes semiconductor switching elements S1 to S4, a capacitor11, and an inductor12. It should be noted that the voltage-type full-bridge PFC inverter is an example, and a configuration of the inverter circuit10may be a configuration of another inverter circuit such as a current-type half-bridge inverter. When the inverter circuit10is not a bidirectional AC/DC converter, it is not essential that the inverter circuit10is a PFC circuit, and it is not essential that the inverter circuit10includes the inductor12.

Examples of the semiconductor switching elements S1 to S4 include a field effect transistor (FET) and an insulated gate bipolar transistor (IGBT). ON/OFF control of each of the semiconductor switching elements S1 to S4 is performed according to a pulse width modulation (PWM) signal outputted from the control circuit100.

Here, the output voltage AC_Ref is determined by a Duty ratio, which is an ON/OFF time ratio of each of the semiconductor switching elements S1 to S4. Although details will be described later, the control circuit100calculates this Duty ratio and generates a PWM signal according to the calculated Duty ratio.

Here, the inductor12and the semiconductor switching elements S1 to S4 may be thermally destroyed by an overcurrent. Therefore, although details will be described later, the control circuit100calculates the Duty ratio based on an overcurrent threshold AC_Limit, which is an upper limit value of the output current I_AC.

The voltage sensor20detects the output voltage AC_Ref of the inverter circuit10and outputs the output voltage AC_Ref to the control circuit100. In addition, the current sensor30detects the output current I_AC of the inverter circuit10and outputs the output current I_AC to the control circuit100. Further, the voltage sensor40detects the voltage (DC link voltage) V_DC_f of the capacitor11and outputs the voltage (DC link voltage) V_DC_f to the control circuit100.

FIG.2is a circuit diagram showing the control circuit100shown inFIG.1. As shown in this figure, the control circuit100calculates the Duty ratio of each of the semiconductor switching elements S1 to S4 based on a difference between the overcurrent threshold AC_Limit and an absolute value of the output current I_AC outputted from the current sensor30(seeFIG.1).

The control circuit100includes a comparator101, an integrator102, a multiplier103, a divider104, and a PWM generation circuit105. Further, the control circuit100includes absolute value circuits106and107. The output current I_AC outputted from the current sensor30(seeFIG.1) is input to the absolute value circuit106, and in the absolute value circuit106, the absolute value of the output current I_AC is calculated. The absolute value of the output current I_AC is input from the absolute value circuit106to the comparator101. In addition, the overcurrent threshold AC_Limit is input from a memory (not shown) to the comparator101.

The comparator101calculates a difference between the overcurrent threshold AC_Limit and the absolute value of the output current I_AC, and outputs the difference to the integrator102. When the absolute value of the output current I_AC is smaller than the overcurrent threshold AC­_Limit, the comparator101outputs a positive value. On the other hand, when the absolute value of the output current I_AC is larger than the overcurrent threshold AC_ Limit, the comparator101outputs a negative value.

The integrator102integrates the output value of the comparator101and outputs the result to the multiplier103. Here, the minimum value of the integration output of the integrator102is 0 (0%), and the maximum value of the integration output of the integrator102is 1 (100%). When the absolute value of the output current I_AC is smaller than the overcurrent threshold AC_Limit and the comparator101outputs a positive value, the integration output of the integrator102converges to the maximum value of 1 (100%). On the other hand, when the absolute value of the output current I_AC is larger than the overcurrent threshold AC_Limit and the comparator101outputs a negative value, the integration output of the integrator102fluctuates toward the minimum value of 0 (0%).

The output voltage AC_Ref outputted from the voltage sensor20(seeFIG.1) is input to the absolute value circuit107, and in the absolute value circuit107, an absolute value of the output voltage AC_Ref is calculated. The absolute value of the output voltage AC_Ref is outputted from the absolute value circuit107to the multiplier103. The multiplier103multiplies an output of the integrator102by the absolute value of the output voltage AC_Ref and outputs the result to the divider104.

The DC link voltage V_DC_f outputted from the voltage sensor40is input to the divider104. The divider104calculates the Duty ratio by dividing a product of the integration output of the integrator102and the absolute value of the output voltage AC_Ref by the DC link voltage V_DC_f, and outputs the calculated Duty ratio to the PWM generation circuit105.

The PWM generation circuit105generates a PWM signal for turning ON/OFF each of the semiconductor switching elements S1 to S4 at the Duty ratio outputted from the divider104. Here, a magnitude of the output voltage AC_Ref is determined according to the Duty ratio. For example, when the Duty ratio is the maximum value of 1 (100%), the output voltage AC_Ref and the DC link voltage V_DC_f match. On the other hand, as the Duty ratio approaches 0 (0%), the output voltage AC_Ref is decreased.

FIG.3is a waveform diagram showing relationships of the output current I_AC, the output voltage AC_Ref, the integration output of the integrator102, a resistance of the load3, and the Duty ratio when the load3is activated. As shown in this waveform diagram, when the load3is activated, the resistance of the load3is progressively increased from the minimum value to the maximum value. Then, after the load3is activated (after the resistance is increased to the maximum value), the resistance of the load3converges to the maximum value. It is assumed that in the load3, an activation current of a motor or the like is large, and thus overcurrent protection of the inverter circuit10is required when the load3is activated. It should be noted that the waveform diagram shown inFIG.3does not represent an activation current waveform of a motor or the like, but simulates an activation current waveform of a motor or the like as an example of a waveform in which an activation current is large.

Here, it is also assumed that since the resistance of the load3when the load3is activated is small as compared with that after the load3is activated, when voltage control of restricting the output voltage AC_Ref is not performed, the output current I_AC is increased and exceeds the overcurrent threshold AC_Limit. In contrast, in the present embodiment, when the load3is activated, if the output current I_AC exceeds the overcurrent threshold AC_Limit, the integration output of the integrator102fluctuates from the maximum value of 1 (100%) to the minimum value of 0 (0%). Accordingly, when the load3is activated, the Duty ratio outputted from the divider104is restricted to be small as compared with that when the integration output of the integrator102is the maximum value of 1 (100%). Therefore, by restricting the output voltage AC_Ref when the load3is activated as compared with that after the load3is activated, it is possible to restrict the output current I_AC when the load3is activated to the same magnitude as that after the load3is activated, and to achieve the overcurrent protection of the inverter circuit10when the load3is activated.

Then, as the resistance of the load3is progressively increased, the integration output of the integrator102gradually converges to the maximum value of 1 (100%), and the Duty ratio outputted from the divider104is increased to a predetermined value (for example, 30% to 50%). Here, the output voltage AC_Ref determined by the Duty ratio of the predetermined value is a value that allows the output current I_AC to be maintained at or below the overcurrent threshold AC_Limit.

As described above, in the power conversion device1according to the present embodiment, the control circuit100controls the output voltage AC_Ref based on the output current I_AC, the output voltage AC_Ref, and the DC link voltage V_DC_f. Accordingly, it is possible to activate the load3such as a motor connected to an AC output of the inverter circuit10while the overcurrent protection of the inverter circuit10can be achieved.

Specifically, in the control circuit100, the integrator102integrates a difference between the overcurrent threshold AC_Limit and the output current I_AC and outputs an integration output, and the multiplier103multiplies the integration output of the integrator102by the output voltage AC_Ref and outputs a multiplication output. Here, when the load3such as a motor is activated, the integration output of the integrator102fluctuates from the maximum value of 1 (100%) to the minimum value of 0 (0%), so that the multiplication output of the multiplier103is reduced as compared with that after the load3is activated.

Then, the divider104calculates and outputs the Duty ratio by dividing the multiplication output of the multiplier103by the DC link voltage V_DC_f. Here, when the load3such as a motor is activated, the Duty ratio is restricted to be small as compared with that after the load3is activated.

Then, the PWM generation circuit105controls the output voltage AC_Ref according to the Duty ratio outputted from the divider104. Here, when the load3such as a motor is activated, the output voltage AC_Ref is restricted to be small as compared with that after the load3is activated.

As described above, when the load3such as a motor is activated, the activation current can be supplied to the load3while the output current I_AC can be restricted to the same magnitude as that after the load3is activated. That is, it is possible to activate the load3such as a motor connected to the AC output of the inverter circuit10while the overcurrent protection of the inverter circuit10can be achieved.

Here, in the power conversion device1according to the present embodiment, it is possible to activate the load3such as a motor connected to the AC output of the inverter circuit10while the overcurrent protection of the inverter circuit10can be achieved without requiring a load connection detection circuit that detects the connection of the load3. Therefore, according to the power conversion device1of the present embodiment, it is possible to achieve reductions in cost and size as compared with a power conversion device that requires a load connection detection circuit.

In addition, in the power conversion device1of the present embodiment, it is possible to activate the load3such as a motor connected to the AC output of the inverter circuit10while the overcurrent protection of the inverter circuit10can be achieved without changing the overcurrent threshold AC_Limit between when the load3is activated and after the load3is activated. Therefore, by increasing the overcurrent threshold AC_Limit when the load3is activated, a reduction in cost can be achieve as compared with a power conversion device that requires an expensive microcomputer having a high clock frequency. In addition, by increasing the overcurrent threshold AC_Limit when the load3is activated, an effect is exerted that a wide load3can be activated regardless of restrictions of temperature conditions, as compared with a power conversion device that has difficulty in coping with activation of a motor or the like in a high-temperature environment.

Further, according to the power conversion device1of the present embodiment, in the control circuit100, the absolute value circuit106calculates and outputs the absolute value of the output current I_AC, and the comparator101calculates the difference between the overcurrent threshold AC_Limit and the absolute value of the output current I_AC and outputs the difference to the integrator102. Then, the integrator102integrates the difference between the overcurrent threshold AC_Limit and the absolute value of the output current I_AC and outputs the integration output. Accordingly, in a case that the absolute value of the output current I_AC exceeds the overcurrent threshold AC_Limit, the integration output fluctuates from the maximum value of 1 (100%) to the minimum value of 0 (0%). On the other hand, in case that the absolute value of the output current I_AC does not exceed the overcurrent threshold AC_ Limit, the integration output converges to the maximum value of 1 (100%).

In addition, in the power conversion device1of the present embodiment, in the control circuit100, the absolute value circuit107calculates and outputs the absolute value of the output voltage AC_Ref, and the multiplier103multiplies the integration output outputted from the integrator102by the absolute value of the output voltage AC_Ref, and outputs the multiplication output. Accordingly, it is possible to calculate the Duty ratio in the divider104.

Although the present invention has been described above based on the embodiment, the present invention is not limited to the above embodiment, and modifications may be made without departing from the gist of the present invention and publicly known or well-known techniques may be appropriately combined.