JP2011223678A - Power conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power conversion device which allows a switching element to be used in up to the vicinity of a limit temperature without losing a purpose of preventing the thermal breakage of the switching element.SOLUTION: The power conversion device comprises a rectifier circuit 2, an inverter 3, a current detector 6, a gate driving circuit 8, a temperature detector 7 which detects a temperature of a neighboring part neighboring a heat generation part of the switching element, control means 9 which gives a reference gate signal to the gate driving circuit 8, and protection means 10. The protection means 10 includes: means for calculating a heat value of the switching element in accordance with a load current and an output frequency; means for calculating a junction temperature of the switching element and a temperature of the neighboring part from the heat value, and thermal resistances and thermal time constants of respective parts; and means for correcting the junction temperature of the switching element in accordance with the calculated temperature of the neighboring part and a temperature difference detected by the temperature detector 7. The protection means 10 controls the gate driving circuit 8 or the control means 9 so that the corrected junction temperature of the switching element does not exceed a temperature limit value.

Description

  The present invention relates to a power conversion device that performs variable speed control of an electric motor, and more particularly to a power conversion device having a protection function against thermal destruction of a switching element.

  A conventional power converter for variable speed control of a general motor converts an AC input of a commercial frequency power system power source to DC by a rectifier circuit, and variable frequency and variable voltage by an inverter through a DC smoothing circuit. The motor is driven by converting to AC.

  A normal motor is connected to a speed sensor, and detects the magnetic pole position or speed of the motor for feedback control. A deviation between the speed feedback signal and the speed reference is obtained by a speed controller to calculate a desired torque reference, while a magnetic flux reference is obtained from the speed feedback signal, and a torque current reference is obtained from the torque reference and the magnetic flux reference.

  Further, a current sensor is attached to the output of the inverter, and a torque current feedback signal is obtained based on this current feedback signal. The voltage reference is obtained by inputting the deviation between the torque current reference and the torque current feedback signal to the current controller, and the voltage reference is input to the PWM modulator and given to the gate drive circuit, and each switching element constituting the inverter Drive.

  Power converters such as inverters and choppers perform desired power conversion by turning on and off switching elements such as IGBTs (Insulated Gate Bipolar Transistors), and power loss occurs in the operation of the switching elements. The main power loss includes steady ON loss, switching loss, and the like. The loss increases the temperature of the switching element. If the junction temperature of the switching element exceeds a predetermined value, thermal damage occurs.

  For this reason, in such a switching element, the switching element is controlled so as not to exceed a preset load current that detects the load current and prevents thermal damage in order to protect the switching element from thermal damage.

  By the way, when the switching element of the inverter is operated at a variable voltage and variable frequency, the temperature condition of the switching element is deteriorated in a low frequency region.

  The switching element is controlled to be turned on / off for inverter operation, and the switching element becomes conductive in the on state. When the operation is performed at a low frequency, the current conduction period of one time of the switching element becomes long, and the temperature rise of the switching element is increased as compared with the operation at a high frequency. As a result, the switching element generates more heat in low frequency operation than in high frequency operation.

  For this reason, the temperature rise of the switching element is calculated considering not only the output current of the inverter but also its output frequency, and the output current of the inverter is set so that the allowable limit value of the junction temperature of the switching element does not exceed this calculated value. A control method has been proposed (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 09-233832 (page 2-4, FIG. 1)

  According to the technique disclosed in Patent Document 1, it is possible to consider a change in temperature rise of the switching element due to a change in the output frequency of the inverter. However, when the operating condition changes or the calculation condition is actually If there is a difference between the calculated element junction temperature setting value and the actual junction temperature, there is a risk that the switching element capability may be suppressed more than necessary.

  The present invention has been made in view of the above, and an object of the present invention is to provide a power conversion device that can use the switching element to near the limit temperature without losing the purpose of preventing thermal damage of the switching element. To do.

In order to achieve the above object, a power converter of the present invention includes a rectifier circuit that converts an AC voltage of an AC power source into a DC voltage, and converts the DC voltage obtained by the rectifier circuit into an AC voltage of variable voltage and variable frequency. An inverter supplied to the load, a current detector that detects a load current flowing into the load, a gate drive circuit that supplies a gate pulse to the switching element that constitutes the inverter, and a proximity that is close to the heat generating portion of the switching element A temperature detector for detecting the temperature of the part; a control means for supplying a reference gate signal to the gate drive circuit; and a protection means for performing a protection operation so that the switching element is not operated exceeding an operating limit temperature. The protection means calculates the amount of heat generated by the switching element from the load current detected by the current detector and the output frequency of the inverter. Means for calculating the junction temperature of the switching element and the temperature of the adjacent part from the calculated calorific value, preset thermal resistance and thermal time constant of each part, and the calculated temperature of the adjacent part And a means for correcting the junction temperature of the switching element according to the magnitude of the temperature difference detected by the temperature detector, so that the corrected junction temperature of the switching element does not exceed the temperature limit value. Further, the gate driving circuit or the control means is controlled.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the power converter device which can use a switching element to the limit temperature vicinity, without losing the objective of the thermal damage prevention of a switching element.

The block block diagram of the power converter device which concerns on Example 1 of this invention. FIG. 3 is an internal configuration diagram of a protection circuit according to the first embodiment. The block block diagram of the power converter device which concerns on Example 2 of this invention. FIG. 6 is an internal configuration diagram of a protection circuit in Embodiment 2.

  Embodiments of the present invention will be described below with reference to the drawings.

  Hereinafter, the power converter concerning Example 1 of the present invention is explained with reference to FIG.1 and FIG.2.

  FIG. 1 is a block diagram of a power conversion apparatus according to Embodiment 1 of the present invention. The AC voltage supplied from the AC power supply 1 is converted into a DC voltage by the rectifier circuit 2, and the ripple voltage included in the DC voltage is absorbed by the smoothing capacitor 3. The inverter 4 converts this DC voltage into an AC voltage again and drives the AC motor 5 as a load.

  A load current is detected by a current detector 6 installed between the inverter 4 and the AC motor 5, and is supplied to a protection circuit 10 described later in detail. The inverter 4 uses a switching element in its main circuit. The temperature around the switching element is detected by the temperature detector 7 and supplied to the protection circuit 10.

An on / off signal is given from the gate drive circuit 8 to the gate of the switching element constituting the inverter 4. A reference pulse signal is given from the control circuit 9 to the gate drive circuit 8. Although illustration of the internal configuration of the control circuit 9 is omitted, for example,
Based on the speed command of AC electric motor 5, it has the composition which has the circuit which determines the output voltage according to the output frequency of inverter 4 and the output frequency inside. In this case, since the output frequency information is obtained by the control circuit 9, this signal is also given to the protection circuit 10.

  Next, the internal configuration of the protection circuit 10 will be described with reference to FIG. FIG. 2 is a block diagram showing the internal configuration of the protection circuit 10, which is expressed using some functional blocks.

  First, the generated loss of the switching element is calculated by the element heat loss calculation 11 from the value of the load current and its frequency. In this case, the output frequency is an index for setting the time for which the loss generated in the same switching nest continues.

  Next, the temperature of each part is calculated in each part temperature calculation 12 based on the calculated loss. In this case, the temperature of the reference part, for example, the temperature of the air-cooled heat sink is made constant, and the thermal resistance and thermal time constant from the previously stored heat sink to the case of the switching element, the thermal resistance and thermal time constant from the case to the junction Calculate the temperature rise of the case and junction. And the temperature of a case and a junction is calculated | required by adding these temperature rise values to the temperature (for example, 40 degree | times) of a heat sink. This calculation may be performed at an appropriate sampling interval, but the temperature can be calculated relatively easily by appropriately setting the duration of the load current obtained from the output frequency.

  Next, the calculated temperature Tc of the case is compared with the actual temperature Ta obtained by the temperature detector 7. In order to perform the calculation under the worst condition that is normally assumed, Tc> Ta is usually satisfied, and the difference ΔT is an allowance. In the temperature correction calculation 13, the difference ΔT is calculated, and a correction rate C corresponding to the difference ΔT is obtained. The correction rate C is, for example, C = (Ta + ΔT) / Ta. Then, in the junction temperature estimation 14, if the junction temperature calculated in each part temperature calculation 12 is multiplied by C, it becomes possible to obtain a corrected junction temperature that is a junction temperature that is closer to the actual.

  Next, the corrected junction temperature is compared with the limit temperature set by the temperature limit setting 15 (for example, 120 degrees) by the comparison circuit 16. When the corrected junction temperature becomes equal to or higher than the limit temperature, the comparison circuit 16 outputs a protection command to the gate drive circuit 8 and stops the gate signal.

  As the temperature detector 7 in the above, for example, a contact type thermistor or thermocouple can be used. Thereby, the influence of noise generated by the switching operation of the switching element can be avoided. Although it has been described that the case temperature of the switching element is measured because it is difficult to directly measure the junction temperature of the switching element, it is important that the measurement is performed as close to the chip of the switching element as possible.

  FIG. 3 is a block diagram of the power conversion apparatus according to the second embodiment of the present invention. About each part of this Example 2, the same part as each part of the power converter device which concerns on Example 1 of this invention of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The second embodiment is different from the first embodiment in that the control circuit 9A has a current limiting circuit 92, and the current limiting value of the current limiting circuit 92 is changed by the output of the protection circuit 10A. It is.

  In FIG. 3, the current control circuit 91 outputs a voltage reference so that the deviation between the current reference command and the load current detected by the current detector 6 is minimized. The current limiting circuit 92 limits the voltage reference so that the load current does not exceed a predetermined value. In the drawing, the current control circuit 91 is shown separately from the current limiting circuit 92 for simplicity, but actually operates as a unit. The PWM circuit 93 supplies the gate drive circuit 8 with a reference gate signal that is pulse-width modulated in accordance with a given voltage reference.

  FIG. 4 is an internal configuration diagram of the protection circuit 10A. This protection circuit 10A differs from the protection circuit 10 of the first embodiment in that the junction temperature corrected by the junction temperature estimation 14 is obtained, and then the load current limit value is obtained by the current limit control circuit 17, and this value is obtained as the current limit. This is a configuration provided to the circuit 91. The current limit control circuit 17 calculates the load current value when the junction temperature reaches a limit temperature (for example, 120 degrees) according to the current load current and the corrected junction temperature value. If this estimation is performed at a predetermined sampling cycle, not only can the update be performed in consideration of changes in the surrounding environment at each sampling cycle, but the junction temperature can be reduced even if the calculation of the load current limit value is not necessarily strict. This is practical because the estimation accuracy improves as the temperature approaches the limit temperature. It is also possible to combine with the first embodiment while leaving the protection command of the first embodiment, and when the load current suddenly increases and exceeds the load current limit value set one sampling period before, Similarly to the protection command, the gate signal may be stopped.

DESCRIPTION OF SYMBOLS 1 ... AC power source 2 ... Rectification circuit 3 ... Smoothing capacitor 4 ... Inverter 5 ... AC motor 6 ... Current detector 7 ... Temperature detector 8 ... Gate drive circuit 9, 9A ... Control circuit 10, 10A ... Protection circuit 11 ... Element heat Loss calculation 12 ... Temperature calculation for each part 13 ... Temperature correction calculation 14 ... Junction temperature estimation 15 ... Temperature limit setting 16 ... Comparison circuit 17 ... Current limit control circuit 91 ... Current control circuit 92 ... Current limit circuit 93 ... PWM circuit

Claims (5)

A rectifier circuit that converts the AC voltage of the AC power source into a DC voltage;
An inverter that converts the DC voltage obtained by the rectifier circuit into an AC voltage having a variable voltage and a variable frequency and supplies the AC voltage to a load;
A current detector for detecting a load current flowing into the load;
A gate drive circuit for supplying a gate pulse to the switching elements constituting the inverter;
A temperature detector for detecting the temperature of a proximity part close to the heat generating part of the switching element;
Control means for providing a reference gate signal to the gate driving circuit;
Protective means for performing a protective operation so that the switching element is not operated exceeding the operating limit temperature,
The protective means is
Means for calculating the amount of heat generated by the switching element from the load current detected by the current detector and the output frequency of the inverter;
Means for calculating the junction temperature of the switching element and the temperature of the adjacent portion from the calculated calorific value and preset thermal resistance and thermal time constant of each part;
Means for correcting the junction temperature of the switching element according to the magnitude of the difference between the calculated temperature of the adjacent portion and the temperature detected by the temperature detector;
The power conversion device according to claim 1, wherein the gate drive circuit or the control means is controlled so that the corrected junction temperature of the switching element does not exceed a temperature limit value. The protective means is
The power converter according to claim 1, wherein when the corrected junction temperature of the switching element exceeds a temperature limit value, a gate stop command is output to the gate drive circuit. The control means includes
Current control means for controlling the reference gate signal so as to minimize a deviation between a current reference command and the load current;
Current limiting means for controlling the reference gate signal so that the load current does not exceed a predetermined current limit value;
The power converter according to claim 1, wherein the protection unit changes the current limit value in accordance with a difference between a corrected junction temperature of the switching element and a temperature limit value. The control means includes
The power converter according to claim 3, wherein when the load current exceeds the current limit value, a gate stop command is output to the gate drive circuit.   The power converter according to any one of claims 1 to 4, wherein the proximity portion is a case of the switching element.

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