CN108108573B - IGBT power module junction temperature dynamic prediction method - Google Patents

Abstract

The invention provides a junction temperature dynamic prediction method for an IGBT power module, which solves the problems of excessive derating use and unreasonable thermal design for avoiding failure caused by overhigh junction temperature and overlarge fluctuation of the IGBT power module at present. According to the running state of the motor, dynamic analysis of circuit parameters including modulation ratio, output current, output voltage, output frequency and the like is carried out, and the analysis value is input into a junction temperature calculation model considering electrothermal coupling, so that dynamic junction temperature prediction under the working condition is realized.

Description

IGBT power module junction temperature dynamic prediction method

Technical Field

The invention relates to the field of junction temperature prediction of IGBT modules, in particular to a dynamic junction temperature prediction method of an IGBT power module in a three-phase inverter system under working condition application.

Background

Due to the kHz switching frequency characteristic of the IGBT (insulated gate transistor) power module, great heat loss can be generated during operation, the temperature (junction temperature) at the PN junction of a chip is increased and fluctuated, and the module can be failed in serious cases. The existing countermeasure is to derate the IGBT module, or to match the radiator with larger mass and volume to the inversion system where the IGBT module is located to fully ensure the heat radiation. However, in the above measures, the excessive derating may reduce the application range of the power module, and the unreasonable heat dissipation design may also result in increased system weight and wasted occupied space. Therefore, accurate junction temperature prediction is of great significance in determining the safety limit of the IGBT power module, improving the application range and reliability and carrying out reasonable thermal design.

The electric field and the temperature field in the IGBT power module are mutually coupled, meanwhile, the operation conditions of related circuit parameters under different application occasions also change in real time, the influence of electrothermal coupling is usually ignored by the existing IGBT junction temperature prediction model, or the existing IGBT junction temperature prediction model can only be used for junction temperature prediction at a specific working point, and the IGBT power module still has defects in the aspect of working condition application. Therefore, the dynamic junction temperature prediction of the IGBT power module under the working condition application is necessary.

Disclosure of Invention

Aiming at the technical problems in the prior art, the invention provides a junction temperature dynamic prediction method for an IGBT power module, which specifically comprises the following steps:

step 1, obtaining the torque and the rotating speed of the motor at a first working point according to the running state of the motor;

step 2, establishing a motor working point analytical model; and (3) inputting the torque and the rotating speed obtained in the step (1) into the established motor working point analytical model to obtain a dq axis current voltage value, and further obtaining the three-phase current voltage and the switching signal output by the inverter. Storing grid resistance of a driving end, switching frequency and DC end direct current bus voltage information;

step 3, establishing a loss calculation model of the IGBT power module; setting initial temperatures of an IGBT and an anti-parallel diode FWD in the power module, and inputting the parameters in the step 2 into the established loss calculation model to obtain loss values of the IGBT and the anti-parallel diode FWD;

step 4, establishing a thermal resistance network model of the IGBT power module; inputting the loss value obtained in the step 3 into the thermal resistance network model to obtain junction temperature corresponding to the current motor working point;

and 5, inputting the junction temperature feedback obtained in the step 4 into a loss calculation model of the IGBT power module to realize dynamic junction temperature prediction under the application of working conditions.

Further, the establishing of the motor operating point analytical model in the step 2 specifically includes:

in the case of a surface-mounted motor, the inductances of d and q axes are the same and the same as the phase inductance, so the electromagnetic torque T is the same_emIs expressed as follows

Wherein p is the logarithm of the pole, #_fIs a permanent magnet flux linkage i_qIs the q-axis current;

alternatively, a motor constant k is adopted_tAnd i_qManner of representing electromagnetic torque:

judging whether the motor is in a non-weak magnetic region or a weak magnetic region: in the non-weak magnetic region, id is 0, and the amplitude of the output phase voltage at this time is represented as:

in the weak magnetic region, the amplitude of the output phase voltage is expressed as follows:

after id and iq are obtained, A, B, C three-phase output under the corresponding torque and rotating speed is obtained through constant amplitude transformation, wherein Park transformation is applied to the change of the dq axial alpha beta rotation coordinate system, and the conversion relation is as follows:

wherein θ is a phase angle;

the transformation of the α β rotating coordinate system into three-phase A, B, C is a Clark transformation, with the following transformation relationships:

wherein i_A、i_B、i_CRespectively, for each phase current.

The model can realize the state analysis of circuit parameters of the module such as output voltage, current, frequency, switching signals and the like under different load working conditions.

Further, the establishing of the loss calculation model of the IGBT power module in step 3 specifically includes:

loss P of IGBT power module_ModuleThe method comprises the following steps: on-state loss P generated during IGBT operation_{IGBT_con}And turn-on loss P in the transient state of the switch_{IGBT_on}Turn-off loss P_{IGBT_off}(ii) a On-state loss P of FWD in operation_{FWD_con}And reverse recovery loss P_{FWD_re}：

P_Module＝P_{IGBT_con}+P_{IGBT_on}+P_{IGBT_off}+P_{FWD_con}+P_{FWD_re}

On-state voltage drop V when IGBT and FWD are conducted_CEAnd V_DFrom respective threshold voltages V_CEO、V_DOAnd an on-resistance R_ch、R_dThe pressure drop generated is composed of two parts and is related to the actual temperature T, and the relation is expressed as follows₀Is a reference temperature, I_CAnd I_DCurrent through IGBT and FWD, respectively, wherein b_TM and b_DAre all temperature-related terms that can be fitted by a curve.

V_D(T)＝V_DO(T₀)+b_D·(T-T₀)+R_d(T)·I_D ²

Calculating to obtain the on-state loss, wherein D_TAnd D_DFor the duty cycle of IGBT and FWD within a unit switching period:

P_{IGBT_con}＝(V_CEO(T)·I_C+R_ch(T)I_C ²)·D_T

P_{FWD_con}＝(V_DO(T)I_D+R_d(T)I_D ²)·D_D

at a switching frequency f_swTime, turn-on power loss P of IGBT_{IGBT_on}Turn off power loss P_{IGBT_off}And reverse recovery power loss P of FWD_{FWD_re}Can be expressed as follows:

P_{IGBT_on}＝f_sw·E_{IGBT_on}

P_{IGBT_off}＝f_sw·E_{IGBT_off}

P_{FWD_re}＝f_sw·E_{FWD_re}；

wherein, the first and the second end of the pipe are connected with each other,

wherein E is_{IGBT_on}Turn-on energy loss for IGBT, E_{IGBT_off}Turn-off energy loss for IGBT, E_{FWD_re}For the reverse recovery energy loss of FWD, a_on、b_on、c_on、a_off、b_off、c_off、a_re、b_re、c_reAs fitting constant, k_on、k_off、k_reAs a temperature-dependent term, E_on(R_g)、E_off(R_g)、E_off(R_g) And E_on(R_rated)、E_off(R_rated)、E_off(R_rated) Respectively corresponding on and off energy consumption of IGBT, reverse recovery energy consumption of FWD, V under actual gate resistance and reference gate resistance_{DC_rated}Is a reference dc bus value.

Further, the establishing of the thermal resistance network model of the IGBT power module in step 4 specifically includes:

the model is based on the following assumptions:

(1) neglecting the effects of thermal radiation and thermal convection, the form of heat transfer within the module is thermal conduction:

(2) due to the filling of the heat insulating silica gel, the heat transfer path is from the chip to the substrate:

(3) the influence of the thermal coupling between the chips is ignored:

(4) neglecting the local temperature difference of a single chip, and adopting a centralized parameter method;

and establishing a fourth-order Foster thermal resistance network model based on the assumptions.

Further, the junction temperature corresponding to the current motor operating point is calculated by the following method:

let the bottom surface temperature of the copper substrate be constant T_CThe thermal resistance of IGBT and FWD from respective chips to bottom case is R_{jc_IGBT}And R_{jc_FWD}Junction temperature T of IGBT and anti-parallel diode FWD_{j_IGBT}、

T_{j_FWD}Can be expressed as follows:

T_{j_IGBT}＝T_c+R_{jc_IGBT}·P_IGBT

T_{j_FWD}＝T_c+R_{jc_FWD}·P_FWD

wherein, P_IGBTIncluding P_{IGBT_con}、P_{IGBT_on}And P_{IGBT_off}，P_FWDIncluding P_{FWD_con}And P_{FWD_re}。

The method provided by the invention solves the problems of excessive derating use and unreasonable thermal design for avoiding failure caused by overhigh junction temperature and overlarge fluctuation of the IGBT power module at present. According to the running state of the motor, dynamic analysis of circuit parameters including modulation ratio, output current, output voltage, output frequency and the like is carried out, and the analysis value is input into a junction temperature calculation model considering electrothermal coupling, so that dynamic junction temperature prediction under the working condition is realized. There are numerous non-obvious advantages over the prior art.

Drawings

FIG. 1 is a schematic flow chart of a method provided by the present invention

FIG. 2 is a schematic diagram of a half-bridge structure of an IGBT power module

FIG. 3 is a schematic diagram of the switching transient of an IGBT

FIG. 4 is an equivalent schematic diagram of an internal package of an IGBT power module

FIG. 5 is a schematic diagram of the SPWM bipolar modulation principle (regular sampling method)

FIG. 6 is a Foster equivalent thermal resistance network model

FIG. 7 shows the judgment logic of weak magnetic area and non-weak magnetic area of the motor

FIG. 8 is a schematic diagram of switching signals under SWPM modulation

Detailed Description

The technical scheme of the invention is further explained in detail by combining the attached drawings.

As shown in fig. 1, the present invention provides a method for dynamically predicting junction temperature of an IGBT power module, which specifically includes the following steps:

step 1, obtaining the torque and the rotating speed of the motor at a first working point according to the running state of the motor;

step 2, establishing a motor working point analytical model; and (3) inputting the torque and the rotating speed obtained in the step (1) into the established motor working point analytical model to obtain a dq axis current voltage value, and further obtaining the three-phase current voltage and the switching signal output by the inverter. Storing grid resistance of a driving end, switching frequency and DC end direct current bus voltage information;

step 3, establishing a loss calculation model of the IGBT power module; setting initial temperatures of an IGBT and an anti-parallel diode FWD in the power module, and inputting the parameters in the step 2 into the established loss calculation model to obtain loss values of the IGBT and the anti-parallel diode FWD;

step 4, establishing a thermal resistance network model of the IGBT power module; inputting the loss value obtained in the step 3 into the thermal resistance network model to obtain junction temperature corresponding to the current motor working point;

and 5, inputting the junction temperature feedback obtained in the step 4 into a loss calculation model of the IGBT power module to realize dynamic junction temperature prediction under the application of working conditions.

In a preferred embodiment of the present application, the establishing an analytic model of the operating point of the motor in step 2 specifically includes:

in the case of a surface-mounted motor, the inductances of d and q axes are the same and the same as the phase inductance, so the electromagnetic torque T is the same_emIs expressed as follows

Wherein p is the logarithm of the pole, #_fIs a permanent magnet flux linkage i_qIs the q-axis current;

alternatively, a motor constant k is adopted_tAnd i_qManner of representing electromagnetic torque:

judging whether the motor is in a non-weak magnetic region or a weak magnetic region, as shown in fig. 7: in the non-weak magnetic region, the control requirement can be met by adopting id equal to 0. But when the voltage reaches the limit u_limIf the motor is to be operated at a higher rotation speed, the exciting current needs to be reduced, that is, the field weakening control is performed, and even if id becomes negative, the amplitude of the maximum phase voltage that can be output by the inverter under the SPWM modulation is 0.5U_DC。

Therefore, if the motor is in the non-flux weakening region, id is 0, and the amplitude of the output phase voltage at this time can be represented as:

and if the weak magnetic region has been entered, the amplitude of the output phase voltage is expressed as follows:

therefore, when performing field weakening, it is necessary to perform discrimination according to a certain logic. At any given rotational speed and torque, the id can be calculated when the weak magnetic area is entered. Subsequently, in conjunction with the weak magnetic determination, if the weak magnetic region is not present, it means that id is 0, and the calculated value is reset to 0, and if the weak magnetic region is present, the calculation result is kept unchanged according to the previous experiment.

After id and iq are obtained, the output of three-phase A, B, C at the corresponding torque and rotating speed can be obtained through constant amplitude conversion. The change of the dq axial alpha beta rotation coordinate system is Park transformation, and the conversion formula is as follows:

the transformation of α β into three-phase A, B, C is a Clark transformation, and the transformation formula is as follows.

Taking phase A as an example, the phase voltage U is set_ATriangular carrier U_triangleThe signal output 1 represents that the upper bridge arm is switched on and the lower bridge arm is switched off; and 0 represents that the lower bridge arm is switched on and the upper bridge arm is switched off. Taking phase a as an example, the output rule of the switching signal is as follows: u shape_A>U_triangleWhen the voltage is equal to the voltage of the upper bridge arm, the switching signal is output 1, the upper bridge arm is conducted and is combined with the phase voltage i_AJudging whether the IGBT or the same bridge arm FWD works; when U is turned_A<U_triangleWhen the switching signal is output to be 0, the lower bridge arm is conducted, and the corresponding IGBT and FWD work judgment is also carried out through i_AThe positive and negative judgment of (1) is opposite to the upper bridge arm condition. The switching signal output under the judgment is schematically shown as the following:

the model can realize the state analysis of circuit parameters of the module such as output voltage, current, frequency, switching signals and the like under different load working conditions.

In a preferred embodiment of the present application, the establishing a loss calculation model of the IGBT power module in step 3 specifically includes:

the IGBT power module generally includes an IGBT and an anti-parallel diode FWD, and thus, a power module having a typical half-bridge structure as shown in fig. 2 is formed, including upper and lower arms. Loss P of IGBT power module_ModuleThe method comprises the following steps: the transient process of the IGBT is shown in FIG. 3, and the on-state loss P is generated during the operation_{IGBT_con}And turn-on loss P in the transient state of the switch_{IGBT_on}Turn-off loss P_{IGBT_off}(ii) a On-state loss P of FWD in operation_{FWD_con}And reverse recovery loss P_{FWD_rr}：

P_Module＝P_{IGBT_con}+P_{IGBT_on}+P_{IGBT_off}+P_{FWD_con}+P_{FWD_re}

On-state voltage drop V when IGBT and FWD are conducted_CEAnd V_DFrom respective threshold voltages V_CEO、V_DOAnd an on-resistance R_ch、R_dThe resulting pressure drop is two-part and related to the temperature T, which is expressed as follows, T0 being the reference temperature, I_CAnd I_DCurrent through IGBT and FWD, respectively, wherein b_TM and b_DAre all temperature-related terms that can be fitted by a curve.

V_D(T)＝V_DO(T₀)+b_D·(T-T₀)+R_d(T)·I_D ²

Calculating to obtain the on-state loss, wherein D_TAnd D_DFor the duty cycle of IGBT and FWD within a unit switching period:

P_{IGBT_con}＝(V_CEO(T)·I_C+R_ch(T)I_C ²)·D_T

P_{FWD_con}＝(V_DO(T)I_D+R_d(T)I_D ²)·D_D

the turn-on process of the IGBT includes a turn-on delay t_d(on)And the current rises by t_riVoltage drop t_fvThree stages, the turn-on energy E generated by the IGBT when turning on once_{IGBT_on}Is represented as follows:

wherein, V_DCIs a DC bus voltage, I_RMReverse recovery of peak current for diodes

The turn-off process of the IGBT is changed from an on state to a forward blocking state. The process includes a voltage rise t_rvCurrent drop t_fiAnd a tail t_tailThree stages. Turn-off energy E generated by IGBT every time of turn-off_{IGBT_off}Is shown as follows, I_tailIs the tail current.

Where Δ V is the additional voltage spike.

Because the time of each stage in the switching process is difficult to determine, the voltage and current change rate is subjected to linear approximation treatment, and the IGBT switching energy loss and the direct-current bus voltage V can be obtained_DCIn a linear relationship with the collector current I_CTo be quadratic, the above equation can be transformed to:

similarly, the reverse recovery loss of a diode can be expressed as follows:

in SPWM bipolar modulation as shown in FIGS. 5 and 8, the IGBT operates in the positive half-cycle of the current and the FWD operates in the negative half-cycle of the current. The pulse width delta of the IGBT and FWD device of the upper bridge arm in the k modulation wave period can be obtained through geometric similarity relation, wherein T is shown as follows₀For modulating the wave period, T_CIs a switching cycle.

Therefore, the conduction loss P of the IGBT and the FWD of the upper bridge arm under the SPWM modulation can be obtained_{IGBT_con}And P_{FWD_con}The on-state power loss in the k-th modulation wave period is:

at a switching frequency f_swTime, turn-on power loss P of IGBT_{IGBT_on}Turn off power loss P_{IGBT_off}And reverse recovery power loss P of FWD_{FWD_re}Can be expressed as follows:

P_{IGBT_on}＝f_sw·E_{IGBT_on}

P_{IGBT_off}＝f_sw·E_{IGBT_off}

P_{FWD_re}＝f_sw·E_{FWD_re}。

in a preferred embodiment of the present application, the establishing a thermal resistance network model of the IGBT power module in step 4 specifically includes: as shown in fig. 4, the model is based on the following assumptions:

(1) neglecting the effects of thermal radiation and thermal convection, the form of heat transfer within the module is thermal conduction:

(2) due to the filling of the insulating silica gel, the heat transfer path is from the chip to the substrate:

(3) the influence of the thermal coupling between the chips is ignored:

(4) the local temperature difference of a single chip is ignored, and a centralized parameter method is adopted.

Each layer of the IGBT module can be considered as a thin flat wall with isotropic material. And the Bi number is very small, so that the unsteady state analysis can be carried out by using a centralized parameter method. The one-dimensional unsteady thermal conduction equation can be expressed as follows:

where ρ is the material density, C_PIs its heat capacity value. The differential equations describing the physical process of one-dimensional heat conduction have the same form as the system of equations for electrical conduction. Therefore, the thermal problem can be converted into the electrical problem through the electric-thermal analogy, namely, the thermal resistance is analogized to resistance, the thermal capacitance is analogized to capacitance, and the power is analogized to current. The thermal impedance value of the system changing along with time can be expressed as a simple analytical formula, wherein tau in the expression is a thermal time constant, and the formula shows that the value of the thermal resistance R, the thermal capacity C determines the response of the system to the step function of the power loss.

τ_i＝R_i·C_i

The parameters matched with the model can be obtained by fitting the transient thermal impedance curve, and a specific calculation formula is shown as follows, wherein n is the order of fitting. This resulted in a fourth order Foster thermal resistance network model, as shown in FIG. 6.

In a preferred embodiment of the present application, the junction temperature corresponding to the current motor operating point is calculated by:

for a single IGBT power module, the bottom surface temperature of the copper substrate is set to be constant T_CThe thermal resistance of IGBT and FWD from respective chips to bottom case is R_{jc_IGBT}And R_{jc_FWD}Junction temperature T of IGBT and anti-parallel diode FWD_{j_IGBT}、T_{j_FWD}Can be expressed as follows:

T_{j_IGBT}＝T_c+R_{jc_IGBT}·P_IGBT

T_{j_FWD}＝T_c+R_{jc_FWD}·P_FWD

wherein, P_IGBTIncluding P_{IGBT_con}、P_{IGBT_on}And P_{IGBT_off}，P_FWDIncluding P_{FWD_con}And P_{FWD_re}。

Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims (4)

1. A junction temperature dynamic prediction method for an IGBT power module is characterized by comprising the following steps: the method specifically comprises the following steps:

step 1, obtaining the torque and the rotating speed of the motor at a first working point according to the running state of the motor;

step 2, establishing a motor working point analytical model; inputting the torque and the rotating speed obtained in the step 1 into the established motor working point analytical model to obtain a dq axis current voltage value, and further obtaining an inverter output three-phase current voltage and a switching signal; storing grid resistance of a driving end, switching frequency and DC end direct current bus voltage information;

step 3, establishing a loss calculation model of the IGBT power module; setting initial temperatures of an IGBT and an anti-parallel diode FWD in the power module, and inputting all parameters in the step 2 into the established loss calculation model to obtain loss values of the IGBT and the anti-parallel diode FWD; the method specifically comprises the following steps:

loss P of IGBT power module_ModuleThe method comprises the following steps: on-state loss P generated during IGBT operation_{IGBT_con}And turn-on loss P in the transient state of the switch_{IGBT_on}Turn-off loss P_{IGBT_off}(ii) a On-state loss P of FWD in operation_{FWD_con}And reverse recovery loss P_{FWD_re}：

P_Module＝P_{IGBT_con}+P_{IGBT_on}+P_{IGBT_off}+P_{FWD_con}+P_{FWD_re}

On-state voltage drop V when IGBT and FWD are conducted_CEAnd V_DFrom respective threshold voltages V_CEO、V_DOAnd an on-resistance R_ch、R_dThe generated pressure drop is composed of two parts and is in phase with the actual temperature T of the temperatureThe relationship is expressed as follows:

V_D(T)＝V_DO(T₀)+b_D·(T-T₀)+R_d(T)·I_D ²

in the formula, T₀Is a reference temperature, I_CAnd I_DCurrent through IGBT and FWD, respectively, wherein b_TM and b_DAll are temperature-related terms fitted by a curve;

calculating to obtain the on-state loss, wherein D_TAnd D_DFor the duty cycle of IGBT and FWD within a unit switching period:

P_{IGBT_con}＝(V_CEO(T)·I_C+R_ch(T)I_C ²)·D_T

P_{FWD_con}＝(V_DO(T)I_D+R_d(T)I_D ²)·D_D

at a switching frequency f_swTime, turn-on power loss P of IGBT_{IGBT_on}Turn off power loss P_{IGBT_off}And reverse recovery power loss P of FWD_{FWD_re}Is represented as follows:

P_{IGBT_on}＝f_sw·E_{IGBT_on}

P_{IGBT_off}＝f_sw·E_{IGBT_off}

P_{FWD_re}＝f_sw·E_{FWD_re}；

wherein the content of the first and second substances,

wherein E is_{IGBT_on}Turn-on energy loss for IGBT, E_{IGBT_off}For turn-off energy loss, E, of the IGBT_{FWD_re}For the reverse recovery energy loss of FWD, a_on、b_on、c_on、a_off、b_off、c_off、a_re、b_re、c_reAs fitting constant, k_on、k_off、k_reAs a temperature-dependent term, E_on(R_g)、E_off(R_g)、E_off(R_g) And E_on(R_rated)、E_off(R_rated)、E_off(R_rated) Respectively corresponding on and off energy consumption of IGBT, reverse recovery energy consumption of FWD, V under actual gate resistance and reference gate resistance_{DC_rated}Is a reference dc bus value;

step 4, establishing a thermal resistance network model of the IGBT power module; inputting the loss value obtained in the step 3 into the thermal resistance network model to obtain junction temperature corresponding to the current motor working point;

and 5, inputting the junction temperature feedback obtained in the step 4 into a loss calculation model of the IGBT power module to realize dynamic junction temperature prediction under the application of working conditions.

2. The method of claim 1, wherein: the establishing of the motor working point analytical model in the step 2 specifically includes:

for surface-mounted motors, the d-axis inductance L_dAnd q-axis inductance L_qSame and equal to the phase inductance, so its electromagnetic torque T_emThe expression is as follows:

wherein p is the logarithm of the pole, #_fIs a permanent magnet flux linkage i_qIs the q-axis current;

alternatively, the motor constant K is adopted_tAnd i_qManner of representing electromagnetic torque:

judging whether the motor is in a non-weak magnetic region or a weak magnetic region: in non-weakly magnetic regions by using i_dThe amplitude of the output phase voltage at this time is represented as follows, where ω is the electrical angular velocity:

in the flux weakening region, the amplitude of the output phase voltage is represented as follows, where V_DCFor dc bus voltage:

after id and iq are obtained, A, B, C three-phase output under the corresponding torque and rotating speed is obtained through constant amplitude transformation, wherein Park transformation is applied to the change of the dq axial alpha beta rotation coordinate system, and the conversion relation is as follows:

wherein θ is a phase angle;

the transformation of the α β rotating coordinate system into three-phase A, B, C is a Clark transformation, with the following transformation relationships:

wherein i_A、i_B、i_CEach phase current.

3. The method of claim 2, wherein: further, the establishing of the thermal resistance network model of the IGBT power module in step 4 specifically includes: the model is based on the following assumptions:

(1) neglecting the effects of thermal radiation and thermal convection, the form of heat transfer within the module is thermal conduction:

(2) due to the filling of the insulating silica gel, the heat transfer path is from the chip to the substrate:

(3) the influence of the thermal coupling between the chips is ignored:

(4) neglecting the local temperature difference of a single chip and adopting a centralized parameter method;

and establishing a fourth-order Foster thermal resistance network model based on the assumptions.

4. The method of claim 3, wherein: the junction temperature corresponding to the current motor working point is calculated in the following way: let the bottom surface temperature of the copper substrate be constant T_CThe thermal resistance of IGBT and FWD from respective chips to bottom case is R_{jc_IGBT}And R_{jc_FWD}Junction temperature T of IGBT and anti-parallel diode FWD_{j_IGBT}、T_{j_FWD}Respectively, as follows:

T_{j_IGBT}＝T_c+R_{jc_IGBT}·P_IGBT

T_{j_FWD}＝T_c+R_{jc_FWD}·P_FWD

wherein, P_IGBTIncluding P_{IGBT_con}、P_{IGBT_on}And P_{IGBT_off}，P_FWDIncluding P_{FWD_con}And P_{FWD_re}。

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