CROSS REFERENCE TO RELATED APPLICATION

This application is a National Stage of International Application No. PCT/JP2018/039349 filed Oct. 23, 2018, claiming priority based on Japanese Patent Application No. 2018-053924 filed Mar. 22, 2018.

TECHNICAL FIELD

The present application relates to a power conversion device.

BACKGROUND ART

In the field of vehicle-mounted power supply systems, a two-battery type vehicle power supply device wherein a vehicle-mounted power supply system is configured of two batteries of differing voltages is used in a hybrid vehicle or a start-stop vehicle. The two-battery type vehicle power supply device is such that power to a high power load is supplied from a high voltage battery of several tens to several hundreds of volts, and power to a normal low voltage electrical load is supplied from an existing low voltage battery (an auxiliary battery, hereafter called a low voltage battery), such as a lead battery, of between ten and twenty volts. The high voltage battery is charged by a high voltage generating device, and the high voltage battery or the generating device is configured so as to supply power to the low voltage battery or the low voltage electrical load via a step-down DC-DC converter.

A semiconductor switching element is incorporated in the step-down DC-DC converter, and in order to supply power at a load system power supply voltage appropriate to charging the low voltage battery, feedback control of the semiconductor switching element is carried out so that an output voltage converges to a predetermined target value.

This kind of DC-DC converter is such that managing a temperature of the incorporated semiconductor switching element is important, and when, according to information relating to a detected temperature of the semiconductor switching element, the temperature of the semiconductor switching element enters an overheating region near a predetermined stopping temperature, overheating of the semiconductor switching element is suppressed by limiting an output current of the DC-DC converter, preventing the temperature of the semiconductor switching element from rising as far as the stopping temperature, and an operation of the semiconductor switching element is stopped when the temperature reaches the stopping temperature.

Also, it has been proposed that a power saving control causing an output voltage of a DC-DC converter to decrease should be executed when an output current becomes greater than a preset threshold in a power conversion device (Patent Literature 1). This is to protect a part included in the DC-DC converter from an effect of heat by limiting the output voltage when the output current becomes greater than the threshold.

CITATION LIST

Patent Literature

Patent Literature 1: JP-A-2011-87407

SUMMARY OF INVENTION

Technical Problem

A DC-DC converter having droop characteristics is such that an output voltage is limited in an overcurrent region, because of which a semiconductor switching element thereof reaching a stopping temperature can be prevented, and charging of a low voltage battery can be continued even when a state wherein the output of the DC-DC converter is limited is continued. Because of this, a problem wherein an engine starting performance of the low voltage battery worsens can be restricted.

However, when a temperature of cooling water that cools the DC-DC converter differs depending on a vehicle type, or when the cooling water temperature rises abruptly (for example, as an effect of heat generated when another component using the same cooling water as the DC-DC converter operates), there is concern that when a cooler abnormality occurs (for example, a water pump failure, water draining away due to a pipe crack, or the like, when a cooling method of the cooler is a water cooling method), a part will fail when relying only on output voltage limitation.

The present application, having been contrived in order to resolve the heretofore described problem, has an object of obtaining a power conversion device such that the power conversion device includes a temperature detector that detects a temperature of a DC-DC converter, and a realization of a reduction in size and a reduction in cost is achieved by restricting an increase in specifications of a constituent component of the power conversion device, while appropriately protecting the constituent component from overheating. Also, the present application has an object of providing a power conversion device including a control method such that a low voltage battery can be appropriately charged without an operation stopping, even when the temperature detector fails.

Solution to Problem

A power conversion device according to the present application is characterized by including a DC-DC converter that reduces and outputs an input power, a current detector that detects a value of a current output by the DC-DC converter, a temperature detector that detects a temperature of the DC-DC converter, and a controller that sets current values at which a control using droop characteristics is started in a multiple of stages in accordance with temperature values, and carries out the control by switching the setting value in accordance with the temperature detected by the temperature detector.

Advantageous Effects of Invention

According to the power conversion device disclosed in the present application, a constituent part of the power conversion device can be appropriately protected from overheating by a current value at which a control using droop characteristics is started (a start current value) being appropriately changed in accordance with a state of a temperature of a DC-DC converter, and the DC-DC converter can output by an operation being continued as far as a temperature state slightly before needing to stop due to overheating, because of which a total energy balance is efficient.

