KR101090701B1 - Method for controlling DC-DC converter - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method of a DC-DC converter, and more particularly, to a control method for improving reliability and ensuring durability of a DC-DC converter by preventing excessive heat generation of a switching element in an extremely low load state. In order to achieve the above object, the step of sensing the output current of the DC-DC converter during the operation of the DC-DC converter and monitoring the operating state of the DC-DC converter; Comparing the output current of the sensed DC-DC converter with a reference current for determining an extremely low load state; If the output current of the DC-DC converter is less than the reference current, determining an extremely low load state of the DC-DC converter and determining whether a predetermined PWM off prohibition condition is applied from an operating state of the DC-DC converter; Stopping the output of the DC-DC converter while keeping the switching element of the DC-DC converter off by stopping the PWM for on / off control of the switching element when the PWM off prohibition condition is not satisfied; Thereafter determining whether it corresponds to a predetermined PWM off mode releasing condition from the discharge state of the auxiliary battery linked to the output terminal of the DC-DC converter; A method of controlling a DC-DC converter is disclosed, including: restarting the DC-DC converter by restarting the PWM when the PWM mode release condition is satisfied.

Description

Control method for DC-DC converters {Method for controlling DC-DC converter}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method of a DC-DC converter. The present invention relates to a control of a low voltage DC-DC converter that provides power to an auxiliary battery and electric load in an electric vehicle such as a hybrid vehicle or a fuel cell vehicle. It is about a method.

Engine electric devices (starters, ignition devices, charging devices) and lighting devices are generally used as electric devices of automobiles. However, as the vehicle has become more electronically controlled, most systems including chassis electric devices have become electronic. .

Various electric appliances such as lamps, audio, heaters, and air conditioners installed in automobiles are supplied with power from the battery when the vehicle is stopped, and from the generator when driving the vehicle. Capacity is used.

Recently, with the development of the information technology industry, a variety of new technologies aimed at increasing the convenience of automobiles are being applied to vehicles, and the development of new technologies that can make the most of the current automobile system is expected to continue.

As is well known, a low voltage DC-DC converter (Low Voltage DC) is used to charge a 12V battery (secondary battery) and supply electric power to a 12V electric load in an electric vehicle such as a hybrid vehicle (HEV), a fuel cell vehicle, a fuel cell hybrid vehicle, and the like. -DC Converter, LDC) is installed.

The low-voltage DC-DC converter, which acts as a generator of a normal gasoline vehicle, supplies a 12V voltage by turning down the high voltage of the main battery (usually a high voltage battery of 144V or more), and 12V of the high voltage (DC) of regenerative energy by the main battery or the driving motor. Convert to DC to charge auxiliary battery (12V battery) or to supply electric load.

The low-voltage DC-DC converter of the electric vehicle employs a full-bridge type power circuit structure for power conversion, and typically includes an input filter unit and a switching element including an input inductor and an input capacitor. It includes a switching unit, a transformer (transformer) for performing power conversion including a transformer, a rectifier (diode) including a diode, an output filter unit (smoothing) including an output inductor and an output capacitor. do.

Among them, the switching unit includes four full-bridge semiconductor switches (MOSFETs, etc.) as the switching elements, and each switch in the full-bridge structure is turned on / off by a PWM signal applied from a PWM generator in the converter controller. .

The PWM generator is a component that generates a PWM signal for the switching operation of each switch to perform a switching control for each switch, PWM IC for generating four PWM signals to turn on / off each switch at a predetermined period, and And a switching driving circuit unit for applying a PWM signal transmitted from the PWM IC to a corresponding switch.

In the above-described DC-DC converter, each switch is operated by applying a full bridge type phase shift control method as a switching method, and a phase shifted PWM signal applied by the PWM generator to the gate terminal of each switch. The on and off timing of each switch is controlled by the.

On the other hand, in DC-DC converter, in order to reduce the switching loss by turning on the voltage of the FET, which is a switching element, in order to reduce the voltage while supplying the main battery power to the electric load, it is highly efficient. .

In general, the charging power of the DC-DC converter is determined according to the operation of the electric load and the state of charge of the 12V battery, which is an auxiliary battery. In order to achieve high efficiency in the load region, the ZVS is not performed in the low load region.

However, since it is difficult to technically perform ZVS in the entire load region as described above, excessive heat generation occurs in the FET under extremely low load (for example, a load of 10% or less of the rating and a load of about 10A or less).

In other words, in the extremely low load region, overheating occurs intensively in the FET which is a switching element in the DC-DC converter, which is not ZVS, resulting in deterioration of the durability and reliability of the converter.

In fact, when the voltage of the 12V battery is higher than the output terminal voltage of the converter in the vehicle state, and there is no output current of the converter or the vehicle demand load itself is low, the above phenomenon occurs and the durability of the converter is greatly reduced.

Therefore, the present invention has been invented to solve the above problems, and by judging the very low load state of the DC-DC converter effectively prevents excessive heating and damage of the switching element in the converter in the very low load state, and the reliability and Its purpose is to provide a method that can further increase durability.

In order to achieve the above object, the present invention includes the steps of sensing the output current of the DC-DC converter during the operation of the DC-DC converter and monitoring the operating state of the DC-DC converter; Comparing the output current of the sensed DC-DC converter with a reference current for determining an extremely low load state; If the output current of the DC-DC converter is less than the reference current, determining an extremely low load state of the DC-DC converter and determining whether a predetermined PWM off prohibition condition is applied from an operating state of the DC-DC converter; Stopping the output of the DC-DC converter while keeping the switching element of the DC-DC converter off by stopping the PWM for on / off control of the switching element when the PWM off prohibition condition is not satisfied; Thereafter determining whether it corresponds to a predetermined PWM off mode releasing condition from the discharge state of the auxiliary battery linked to the output terminal of the DC-DC converter; And restarting the PWM to restart the DC-DC converter when the PWM mode canceling condition is satisfied.

According to a preferred embodiment of the present invention, if the failure of the DC-DC converter is detected or the abnormal operation state of the DC-DC converter is detected in the PWM off prohibition condition determination step, it is determined that the PWM off prohibition condition corresponds.

In addition, the output terminal voltage of the converter during the operation of the DC-DC converter is sensed, and in the step of determining the PWM off mode release condition, the output terminal voltage of the converter is characterized in that compared with the reference voltage for the PWM off mode release determination.

In addition, when the output terminal voltage of the converter is less than the reference voltage, it is determined that the PWM off mode release condition is satisfied.

In addition, the reference voltage is characterized in that the reference value which is variable according to the output voltage command for controlling the output of the DC-DC converter.

In addition, the reference voltage is characterized in that the output voltage command (V _cmd )-α '(α> 0) having a predetermined voltage difference to prevent the over-discharge of the auxiliary battery.

Accordingly, according to the control method of the present invention, if the extremely low load state is determined based on the output current of the DC-DC converter, the PWM of the DC-DC converter is stopped (PWM OFF, switching blocking, switching element OFF) By stopping the output, it is possible to effectively prevent overheating and damage of the switching element in the converter under extremely low load, and to increase the reliability and durability of the converter.

As a result, it is possible to stably supply electric power to the electric load of the vehicle, and there is an advantage of saving the power of the main battery by operating the DC-DC converter in an extremely low load state.

In addition, it is possible to solve the heating problem of the converter simply by improving the control logic without installing additional hardware for heat dissipation of the switching element, which is a heating element inside the DC-DC converter, and reducing the cost of A / S by increasing the durability and reliability of the converter. Can be provided.

1 is a schematic diagram showing a power system of an electric vehicle in which a control process according to the present invention is performed.
2 is a flowchart illustrating a control process of the DC-DC converter according to the present invention.
3 is a diagram illustrating a PWM on / off state and a converter output terminal voltage state in a control process of a DC-DC converter according to the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings, which will be readily apparent to those skilled in the art to which the present invention pertains.

1 is a schematic diagram illustrating a power system of an electric vehicle in which a control process according to the present invention is performed, FIG. 2 is a flowchart illustrating a control process of a DC-DC converter according to the present invention, and FIG. 3 is a DC according to the present invention. The figure shows the PWM on / off state and the converter output stage voltage during the control of the DC converter.

The present invention is to improve the heat generation problem of the converter appearing at very low load in an electric vehicle such as a hybrid vehicle or a fuel cell vehicle equipped with a low voltage DC-DC converter (LDC), The main purpose is to prevent overheating of the FET to improve the reliability of the converter and at the same time ensure durability.

In the present invention, when the output load value of the DC-DC converter, that is, the output current (I _DC ) of the converter is less than the preset reference current (I _min ) for the determination of the extremely low load state, enters the PWM off mode to turn off the PWM of the converter. And stops the output of the DC-DC converter to prevent overheating of the FET.

When the PWM is off, the PWM signal for switching control is not applied, so the FET, which is the main heating component of the DC-DC converter, is kept off.

However, in case of abnormal operation mode such as detecting DC-DC converter failure such as disconnection / short circuit of current sensor, failure of switching element, or limiting output of DC-DC converter by other protection function, PWM OFF mode Excluded from entry conditions.

The release of the PWM off mode is determined by the feedback voltage of the converter output stage (converter output stage voltage) (V _OUT ) connected to the auxiliary battery.In the PWM off mode, the feedback voltage (V _OUT ) of the converter output stage drops to release the PWM off mode. If the voltage falls below the reference voltage (V _min ) for judgment, that is, the charge current to the 12V battery, which is a secondary battery, is sufficiently large (I _DC > I _min ), but the 12V battery is at a voltage level that can prevent overdischarge of the 12V battery. If is a voltage discharged state, confirm the release of PWM off mode and restart the DC-DC converter.

The control process of the present invention will be described in more detail as follows.

First, referring to FIG. 1, the electric power system of an electric vehicle includes a main battery (high voltage battery) 20 and a main battery 20 that provide power for driving a motor for driving a vehicle, that is, a driving motor. The DC-DC converter 30 for charging the auxiliary battery (12V battery) 40 or supplying the electric load 50 by stepping down the high voltage of the converter, a converter controller for controlling the operation of the DC-DC converter 30. (Converter controller) 31, a secondary battery 40 that is charged with the power supplied from the main battery 20 and provides electric power to the electric load 50 of the vehicle, the main battery 20 and the auxiliary battery 40 supplies necessary power while repeating charging / discharging while driving a vehicle.

The converter controller 31 controls the operation of the DC-DC converter 30 according to an operation command applied from an upper controller (for example, a hybrid control unit (HCU)), for example, an output from the upper controller 10. DC-DC converter by turning off DC-DC converter 30 by receiving stop command or generating and outputting PWM signal for switching control based on output voltage command V _cmd applied from upper controller 10 The output of 30 is controlled.

The output voltage command V _cmd applied from the upper controller 10 to the converter controller 31 varies depending on the driving state (child / acceleration / deceleration / constant speed) of the vehicle during the operation of the DC-DC converter 30.

In addition, the converter controller 31 performs a failure diagnosis function for detecting a failure of the converter 30, such as a disconnection / short circuit of the converter current sensor 33 or a failure of the switching element, and also restricts the output of the converter as necessary. do.

As an example of a method of detecting a failure of the DC-DC converter 30, a method of detecting whether a voltage corresponding to the output voltage command V _cmd by detecting the feedback voltage V _OUT of the converter output terminal may be applied. The converter output terminal voltage V _OUT and the output current V _DC are detected through the voltage sensor 32 and the current sensor 33 positioned at the output terminal of the converter 30.

In FIG. 1, reference numeral V _LB denotes a voltage of an auxiliary battery (12V battery).

Next, a control process according to the present invention will be described with reference to FIG. 2. First, a voltage sensor installed at the converter output terminal is provided with a converter controller 31 during vehicle driving and operation of a DC-DC converter (denoted by reference numeral 30 in FIG. 1). The converter output terminal voltage V _OUT and the output current I _DC detected through the 32 and the current sensor 33 are input, and the operating state of the converter 30 is monitored (S11 and S12).

Subsequently, it is determined whether the DC-DC converter 30 is in a very low load state. At this time, the converter controller 31 sets the output current I _DC of the DC-DC converter 30 detected by the current sensor 33 in advance. If it is less than the reference current I _min , it is determined as an extremely low load state.

Here, as shown in FIG. 2, when the output current I _DC of the DC-DC converter 30 is kept below the reference current I _min for a set time (S13, S14), it is determined to be in an extremely low load state. It is preferable (S15).

If it is determined that the ultra-low load state as described above, it is determined whether the operating state of the DC-DC converter 30 corresponds to a predetermined PWM off prohibition condition (S16, S17), DC-DC according to a conventional failure diagnosis algorithm In case of abnormal operation state, such as when a failure state of the converter is detected, when normal control of the DC-DC converter is impossible, when the output limitation of the DC-DC converter is made, or when an output stop command is applied from the host controller. Entry into the PWM off mode is prohibited.

On the other hand, after determining the extremely low load state, in the normal operation mode other than the state of receiving the output stop command without the fault state and the output limit state of the DC-DC converter 30, the system enters the PWM off mode ( S18).

In the PWM off mode, the DC-DC converter 30 is turned off by the converter controller 31. In this case, the output of the PWM signal is interrupted and the PWM is turned off. As a result, each switching element (FET) is kept off. The output of the DC converter is stopped.

As described above, the reference current I _min , which is set for the determination of the extremely low load state, may be referred to as the current reference value that is set to determine the area where zero voltage switching (hereinafter referred to as ZVS) is performed. Since the extremely low load region of 10% or less of the region is not ZVS, the reference current I _min may be set to a current value of 10% of the rating (for example, about 10A, which is 10% of the rating).

If the DC-DC converter is operated under extremely low load without ZVS, the overheating of the switching element FET will be concentrated, resulting in failure of the DC-DC converter such as damage to the FET.

Therefore, when the ultra low load state without ZVS is determined by comparing the output current of the DC-DC converter with the reference current, and determined as the ultra low load state, the output of the PWM and DC-DC converters for the on / off control of the FET is performed. By stopping and keeping the FET off, it prevents the FET from overheating.

On the other hand, in order to release the PWM and DC-DC converter output is stopped in the PWM off mode, it is necessary to determine whether to meet the predetermined PWM off mode release conditions (S19), PWM off mode release is the auxiliary link to the converter output stage It is determined from the discharge state of the battery (12V battery) 40.

That is, the release of the PWM off mode is determined based on the converter output terminal voltage V _OUT detected through the voltage sensor 32 as the voltage fed back to the DC-DC converter 30.

At this time, in the two PWM and DC-DC converter output stop state there is be a secondary battery 40, the voltage of the converter output (V _OUT) is lowered as the discharge according to the load requirements, and thus voltage of the converter output (V _OUT) is based on When the voltage falls below V _min , the DC-DC converter is operated in the normal operation mode by releasing the PWM off mode, restarting the DC-DC converter by restarting the PWM for switching control of the switching element in the DC-DC converter. .

When the PWM off mode is released, the converter controller 30 generates a PWM signal based on the output voltage command V _cmd input from the upper controller 10 as in the usual case, thereby turning on / off the FET as a switching element. The control is normally performed, and the output corresponding to the output voltage command V _cmd is made in the DC-DC converter 30.

In determining the release of the PWM off mode, when the release is determined by the output current I _DC of the DC-DC converter 30, the release and entry of the PWM off mode are repeated according to the load transition characteristic and the state of charge of the battery. The control of the converter 30 may become unstable.

Accordingly, the output terminal voltage V _OUT of the converter 30 is set by setting the lowest voltage value at which the auxiliary battery, that is, the 12V battery 40, to prevent over discharge is set as the reference voltage V _min for the release determination of the PWM off mode. ) Becomes less than the reference voltage V _min (Vout <Vmin), the PWM off mode is released, and then the normal output of the converter 30 is made in the PWM off mode.

In a preferred embodiment, the output voltage command V _cmd applied from the upper controller 10 to the converter controller 31 varies within a specific range (eg, 12.8 to 14.7 V) depending on the running state of the vehicle during the operation of the converter. Therefore, a reference voltage V _{min that} is variable according to the output voltage command V _cmd of the upper controller is used as a reference value for determining the release of the PWM off mode.

That is, the lowest voltage value of the reference voltage (V _min) is "output voltage command (V _cmd) - α 'is varied in accordance with an output voltage command (V _cmd) is to such that the (α> 0, for example, α = 0.8 V).

As described above, the minimum voltage value is different from the output voltage command V _cmd of the upper controller 10 so that the output current I _DC of the converter 30 is the reference current (based on ZVS performance) (I _min ) 12V battery 40, which is an auxiliary battery, is discharged in a sufficiently larger state, but the minimum voltage value (reference voltage) V _min is varied according to the output voltage command V _cmd and the minimum voltage value is at least. The voltage of 12V or more is prevented to prevent overdischarge of the 12V battery and to prevent unnecessary charge current increase when the PWM off mode is released.

In this way, according to the DC-DC converter control method of the present invention, in order to prevent overheating of the switching element, as shown in FIG. 3, an extremely low load state in which ZSV of the DC-DC converter is not achieved (I _DC < I _min ) enters the PWM off mode to temporarily stop the PWM for on / off control of the switching element (PWM OFF), thereby keeping the switching element off and at the same time stopping the output of the DC-DC converter. This prevents overheating of the switching element in the extremely low load state.

In this very low load region where ZVS is not achieved, switching off PWM prevents the output of the DC-DC converter, thereby preventing switching losses. Therefore, the secondary battery temporarily supplies the power supply instead of the DC-DC converter. do.

Then, when the voltage fed back to the DC-DC converter, that is, the voltage at the output terminal of the converter falls and falls below the reference voltage (the minimum voltage at which the secondary battery can be prevented from overdischarging), the PWM off mode is canceled and the PWM control is resumed to resume the DC-DC. The converter will be restarted.

After determining the discharge state of the auxiliary battery based on the voltage (V _OUT ) fed back to the DC-DC converter, when the PWM off mode is released (V _out <V _min ), the output voltage of the upper controller becomes PWM ON again. According to the command, the operation of the DC-DC converter is controlled. In the PWM ON state, the output voltage of the converter is increased again.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the scope of the present invention is not limited to the disclosed exemplary embodiments. Modified forms are also included within the scope of the present invention.

10: upper controller (HCU) 20: main battery (high voltage battery)
30: DC-DC converter 31: converter controller
32: voltage sensor 33: current sensor
40: auxiliary battery (12V battery) 50: electric load
I _DC : Converter output current V _OUT : Converter output voltage
I _min : reference current V _min : reference voltage
V _LB : Battery voltage V _cmd : Output voltage command

Claims (6)

Sensing the output current of the DC-DC converter during operation of the DC-DC converter and monitoring the operating state of the DC-DC converter;
Comparing the output current of the sensed DC-DC converter with a reference current for determining an extremely low load state;
If the output current of the DC-DC converter is less than the reference current, determining an extremely low load state of the DC-DC converter and determining whether a predetermined PWM off prohibition condition is applied from an operating state of the DC-DC converter;
Stopping the output of the DC-DC converter while keeping the switching element of the DC-DC converter off by stopping the PWM for on / off control of the switching element when the PWM off prohibition condition is not satisfied;
Thereafter determining whether it corresponds to a predetermined PWM off mode releasing condition from the discharge state of the auxiliary battery linked to the output terminal of the DC-DC converter;
Restarting the DC-DC converter by restarting the PWM when the PWM mode releasing condition is satisfied;
DC-DC converter control method comprising a.
The method according to claim 1,
The control method of the DC-DC converter characterized in that it is determined that the PWM off prohibition conditions when the failure of the DC-DC converter or the abnormal operation state of the DC-DC converter is detected in the PWM off prohibition condition determination step. .
The method according to claim 1,
Sense the output voltage of the converter during the operation of the DC-DC converter,
And controlling the output voltage of the converter to the reference voltage for determining the PWM off mode release.
The method according to claim 3,
And if the output terminal voltage of the converter is less than the reference voltage, determining that the PWM off mode releasing condition is satisfied.
The method according to claim 3 or 4,
And the reference voltage is a reference value which varies according to an output voltage command for controlling the output of the DC-DC converter.
The method according to claim 5,
The reference voltage of the DC-DC converter, characterized in that the output voltage command (V _cmd )-α (α> 0) that has a predetermined voltage difference to prevent the over-discharge of the auxiliary battery is prevented Control method.

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