RU2240573C1 - Express method for measuring body transfer heat resistance of power semiconductor devices - Google Patents

Abstract

FIELD: measuring equipment engineering.

SUBSTANCE: method includes heating semiconductor crystal by letting constant current I₀ having given amplitude through it, during heating, measuring value of its heat-sensitive parameter, as which straight fall of voltage on crystal U_f is used, and concurrently temperature T_b of device body is measured in selected point. These values are recorded, by receiving them dependent on time t. Heating of semiconductor crystal is stopped after reaching temperature T_c having given value and in mode of natural cooling when feeding to crystal short measuring impulses of current having amplitude I₀ and off-duty ratio not affecting heat balance of device, values of temperature-sensitive parameter and body base temperature are recorded, receiving dependencies U_f(t) and T_b(t) on cooling interval. Length of cooling interval is selected on basis of unconditional realization of t>>3τ, where τ - maximum heat constant of device construction, moment of dynamic balance t₁ is determined at heating interval and on basis of received dependencies heat resistance of body transfer is calculated in current point t₁.

EFFECT: shorter measurement time, lower device resources cost, higher percent of accepted products during measurement.

2 cl, 3 tbl, 2 dwg

Description

The invention relates to a technique for measuring the thermal parameters of components of power electronics, in particular power semiconductor devices, including the structures of IGBT, MOSFET transistors and inverse FRD diodes, and can be used to control the quality of power electronics products and to evaluate their temperature reserves.

Thermal resistance (R _Тп-к ) of a semiconductor device is defined as the ratio of the difference between the effective transition temperature (Т _П ) and the base temperature of the case (Т _К ) at a controlled point to the power dissipation (Р) of the device in steady-state thermal mode, when the measured temperature does not change relation to the environment. Temperature-sensitive electrical parameters of the crystal are used as temperature sensors (thermometers) for indirect measurement of its temperature.

There is a method [1] for measuring the thermal resistance of the junction-case of semiconductor diodes, in which heating pulses of constant amplitude current I _m are supplied to the controlled diode, between which a constant initial current is passed through the diode and the change in the forward voltage θ of the diode and heating power P _m are measured . In this case, the supply of heating current pulses to the controlled diode is carried out in such a way that the reciprocal of the duty cycle of the heating current pulses Q ^-1 = τ _and · f _sl , where τ _and is the duration of the heating current pulses, and f _sl - their repetition rate, increase linear law with constant slope S _Q. The values obtained determine the thermal resistance by the expression

where K _T is the temperature coefficient of the direct voltage of the diode during the flow of a constant initial current.

The disadvantage of this method is the low accuracy due to the large measurement error of the pulse voltage due to the influence of transient thermal and electrical processes when the diode switches from one mode - heating mode to another - measurement mode [2]. Another significant disadvantage of this method is the long measurement time associated with the calibration operation.

Existing methods for measuring thermal resistance consist in preliminary calibration and subsequent measurement in a state of thermal equilibrium, which requires significant time (several hours). In this case, it is necessary to use a heat sink using heat-conducting paste, which significantly increases the measurement error, and for power semiconductor devices with a thermal resistance of hundredths of a degree per watt, the measurement error due to the use of heat-conducting paste can be about 40-100%, which makes it generally impossible to get the right result.

The closest in technical essence to the claimed method is the express method [3] for measuring the thermal resistance of the junction-case power semiconductor devices in a housing design, which consists in the fact that the semiconductor crystal is heated by passing a constant current I ₀ [A] of a given amplitude through it and during heating, the value of its temperature-sensitive parameter is measured, for which a direct voltage drop on the crystal (U _P ) [V] is used.

This method of measuring the thermal resistance of the junction-case is carried out according to two waveforms, the so-called "base" and "module" waveforms, of a temperature-sensitive parameter recorded and recorded using a TDS 340 oscilloscope in the computer's memory.

Obtaining waveforms "base" is produced by heating the crystal with direct current of small amplitude for the first hundred seconds of heating and the last time count in the steady state of thermal equilibrium. The waveform "base" is obtained once for the entire series of devices and stored in the computer's memory.

The waveform "module" is removed within a few seconds when the crystal is heated by direct current, providing a temperature difference between the transition Т _П and the base of the case Т _К not more than 30-50 ° С during the experiment, satisfying the condition Т _П <125 ° С.

Upon receipt of the oscillograms at the initial and final moments of measurement, the temperature of the base of the housing at the selected point is additionally measured and stored.

For the oscillogram of the temperature-sensitive parameter U _P at a constant heating current and a linear dependence of U _P on temperature, one can write [4]:

where Δ U _P (t) [V] is the change in the direct voltage drop across the crystal relative to the initial value at t = t ₀ ,

k [B / ° C] - temperature coefficient of the temperature-sensitive parameter,

I [A] - constant heating current,

t [s] - current time,

Z _Тп-к (t) [° С / W] - dynamic characteristic of transitional thermal resistance transition-case of a semiconductor device,

Z _Tk-a (t) [° C / W] - dynamic characteristic of the total transient thermal resistance of the housing-medium of a semiconductor device.

Each of the dynamic characteristics Z _Тп-к (t) and Z _Тк-а (t) can be reduced to the sum of exponential functions with parameters of an equivalent thermal model, in which thermal resistances and capacities are determined by the time constants of exponential functions.

The waveform Δ U _P (t) "base" determines the thermal components of the parameters of the transient thermal resistance of the housing-medium Z _Tk-a (t) of the semiconductor device.

From the dependence “module”, the curve constructed using the found thermal components of the parameters of the total transient thermal resistance of the housing-medium Z _Tk-a (t) is subtracted, and the change Δ U _P (t) characterizing only Z _Tn-k (t) is found by which the steady-state thermal resistance of the junction-case of the device is determined.

The most significant drawbacks of this method are the long measurement time upon receipt of the “base” waveform, the need to store it in a computer database, and the lengthy process of iteratively searching for a solution with least squares correlation according to the [5] method.

The technical result is a reduction in measurement time, a decrease in hardware costs during the implementation of the method and an increase in shelf life in the process.

The technical result is achieved by the fact that in the rapid method for measuring the thermal resistance of the junction-case power semiconductor devices in a housing design, namely, that the semiconductor crystal is heated by passing a constant current I ₀ [A] of a given amplitude through it, the value is measured during heating its temperature-setting, which is used as forward voltage drop U crystal _n, the interval heating temperature is additionally measured base Corp. ca T _K [° C] of the device at the selected point, storing these values, receiving them depending on the t time, stop heating of the semiconductor chip when the temperature T _{to a} predetermined value, and a cooling mode of the semiconductor device, when applied to the crystal of short measuring pulses a current amplitude I ₀ and the duty ratio does not affect the thermal equilibrium of the device is measured and stored temperature-parameter values U and the temperature t _{P K} U _P depending yield (t) and _K t (t) over the interval already oh azhdeniya, wherein the duration of the cooling interval is selected from the unconditional execution conditions t &γτ;&γτ; 3τ, where τ [s] is the largest thermal constant of the device design, the moment of dynamic equilibrium t ₁ [s] is determined on the heating interval and the transition-case thermal resistance at a given point t _{1 is} calculated from the obtained dependences.

Power semiconductor devices use crystals, including transistor and diode structures.

When the crystal is heated by direct current temperature T _P increases with respect to increasing T _K of the exponential dependence with τ until dynamic equilibrium constant time t ₁ after its occurrence at t ₁ ≥ 3τ excess _TP over T _K occurs a linear relationship, and their difference is constant value.

The signs that distinguish the claimed technical solution from the prototype are the following actions:

measure on the heating interval the temperature of the base of the device at a selected point;

get the time dependence of U _P (t) and T _To (t) on the heating interval;

stop heating the semiconductor crystal when the case temperature reaches a predetermined value,

in free cooling mode, a temperature-sensitive parameter and the temperature of the base of the case are measured when short measuring current pulses with an amplitude of I ₀ equal to the amplitude of the heating current and a duty cycle that do not change the thermal equilibrium of the device are applied to the crystal;

form and remember the dependencies U _P (t) and T _K (t) already on the cooling interval;

the duration of the cooling interval is selected from the condition that t &γτ; &γτ;3τ;

determine the moment of dynamic equilibrium t ₁ on the heating interval;

calculate the thermal resistance of the transition-case at the point of dynamic equilibrium;

In power semiconductor devices, crystals are used, including transistor and diode structures.

In the known technical solutions, no signs are found that are similar to those that distinguish the claimed solution from the prototype.

At time t _{1, the} preliminary value of the transition-case thermal resistance is calculated in the dynamics R _Tdin [° C / W], it is used to calculate the calibration characteristic for the heating interval and transition temperature in the cooling interval, which allows us to calculate the transitional thermal resistance of the transition-case ( _{Z-Ts k)} for cooling interval, then it is found by thermal components equivalent thermal model of the semiconductor device and is calculated by the thermal resistance of these components, the transition to the steady- rpus, which is usually determined statically _{K-T to} [° C / W].

A positive effect of the proposed technical solution consists in combining the process of measuring and calibrating the measured dependencies U _P (t) and T _K (t), which allows the entire measurement to be carried out in 2-3 minutes in the absence of standard measuring instruments.

The essence of the proposed solution is illustrated by drawings.

Figure 1 - curves illustrating the method of measuring thermal resistance transition-case:

and - machine waveform of the current with an amplitude of I ₀ in the interval t ₀ <t _i <t ₂ heating and in the interval t ₂ <t _j <t ₄ cooling,

b - machine waveform temperature-sensitive parameter U _P (t) on the intervals of heating and cooling,

in - a machine waveform of the measured temperature of the base of the housing T _K (t) and the calculated transition temperature T _P (t) on the heating and cooling intervals,

g - calculated diagram of the dynamics of the preliminary R _Tdin (t) values of the thermal resistance of the transition-case, explaining the determination of the moment of dynamic equilibrium t ₁ .

Figure 2 is a structural diagram of a measuring device.

When a semiconductor device is heated with direct current I _{0, the} heat flux through the semiconductor crystal, a solder layer, ceramic metallized on both sides, another solder layer and a copper base propagate to the base of the device body and further into the environment, creating a temperature field distribution in accordance with internal thermal resistances of structural elements. As a control selected point when measuring temperature T _To use a point under the center of the heated crystal as the hottest point on the base of the body, or in the center of the base, if the positions of the crystals are unknown.

The values of direct current I ₀ , power losses P and the temperature of the base of the housing Т _К satisfy the condition of limiting the transition temperature Т _П ≤ 125 ° С, which does not exceed the limit temperature with a margin of 20-30 ° С, where Т _П = Т _К + Р • 2 · R _Тп-кТУ , P = I ₀ • U _П [W], 2 · R _{Тп-кТУ -} assumed or known from the technical specifications (TU) or reference data value of thermal resistance with a guarantee against overheating of the transition in terms of power loss with coefficient " 2 ". Switching off the constant heating current is carried out at a temperature of the base of the housing Т _К0 ≈ 80-90 ° С.

The semiconductor crystal is heated with direct current I ₀ (figa) until the case temperature reaches the set value T _K0 at time t ₂ . On the time interval t ₀ <t <t ₂ measure the temperature-sensitive parameter U _P (t) (Fig.1B), which is used as a direct voltage drop on the crystal, and the temperature T _To (t) (Fig.1B) of the base of the body in the selected point. The values of the measured parameters are remembered. They stop heating the semiconductor crystal in the mode of natural cooling of the device when a short measuring current pulse of the same amplitude I ₀ with a duty cycle that does not affect the thermal equilibrium of the device is supplied to the crystal, the values of U _P (t) and T _K (t) are already measured the interval t ₂ <t <t ₄ cooling.

Analysis of the equivalent thermal model of a semiconductor device at constant current allows us to describe the establishment of the transition temperature T _P (t) by the expression

An approximate three-element thermal model of R _i C _i chains, where R _i is the thermal resistance [° C / W], C _i is the heat capacity [J / ° C], τ _i = R _i · C _i is the thermal time constant of the corresponding i -chains associated with the internal structure of the design of the device. It can be conditionally accepted that a chain with a thermal time constant τ ₀ = R ₀ · C ₀ describes the structure of the elements of the first soldered layer (crystal-solder-metallization), respectively, τ ₁ = R ₁ · C ₁ - the second soldered layer (metallized ceramic-solder -base) and τ ₂ = R ₂ · C ₂ are the heat transfer conditions at the base-environment interface. Under static conditions, when the temperature on the surface of the base remains unchanged with respect to the environment, R ₂ = 0.

The fastest thermal equilibrium is established in the elements closer to the crystal (the heat capacity C ₀ , then C ₁ will be used (charged) earlier than the others), and much longer the thermal equilibrium is established at the border of the housing base with the medium. This means that for the design of the device the condition

The total transitional thermal resistance from the equivalent thermal model in the interval t ₀ <t <t ₂ heating is determined by the expression

as t → ∞, the process of establishing thermal equilibrium will lead to a steady-state equilibrium in statics at which the temperature T _k at the boundary with the environment remains constant, the value of Z _Tp-k tends to the steady-state value of thermal resistance R _Tp-k = R ₀ + R ₁ ( in static R ₂ = 0). The temperature difference will be determined by a constant

For the interval t ₂ <t <t ₄ cooling, the dynamic characteristic Z _Tn-k has a simpler expression

as t → ∞ Z, _Tn-k tends to zero and T _P → T _K.

Since condition (4) is ensured by the design of the device, for exponential expressions Z _Tn-k it can be accepted that the process of establishing thermal equilibrium in accordance with the total transient thermal resistance practically ends at t≥ 3τ ₂ . In this case, the exponential terms of the expression become negligible. Further supply of power leads to a proportional increase in temperature relative to the ambient temperature in accordance with the internal R _i -elements of the structure. In this case, we can assume that the state of dynamic thermal equilibrium of the device occurs at the moment the temperature difference between any points of the device reaches a constant value (for example, T _P -T _K ≈ const).

From this it is obvious that one of the moments of reducing the time for measuring thermal resistance can be the application of a method in which the base temperature does not reach the state of thermal equilibrium with the environment, but the temperature difference becomes already constant (when T _{P П} const, T _K ≠ const, but T _P -T _K = const).

This means that in the heating interval t ₁ <t <t ₂ at t> 3τ _{2 the} temperature of the base T _K differs from the crystal temperature T _P by a constant

and on the cooling interval t ₃ <t <t ₄ for t> 3τ ₂ we obtain

therefore, at t> 3τ ₂ T _K on the cooling interval it performs the function of the transition temperature T _P in the calibration characteristic

Determine the time t ₁ on the heating interval to calculate the thermal resistance in the dynamics. Assuming that expression (9) is satisfied over the entire cooling interval t ₂ <t <t ₄ , and not only at t> 3τ ₂ , it is assumed that the calibration characteristic is valid over the entire cooling interval. Using it and the values of the temperature-sensitive parameter U _P (t) in the interval t ₀ <t <t _{2 of} heating, determine the preliminary values of the transition temperature T _P (t) _I (curve with index I in Fig.1c) according to the algorithm below, and calculate the thermal resistance R _Tdin (t) (Fig.1d) in dynamics over this interval by the expression

The maximum value of R _{Tmax of} this dependence R _Tdin (t) corresponds to the moment of dynamic equilibrium t ₁ .

The preliminary values of T _P (t) _{I are} found at time t = t _i in the interval t ₀ <t _i <t _{2 of} heating from the values of the transition temperature from the calibration characteristic (10), which correspond to the readings of the measurement t _j in the interval t ₂ <t _j <t _{4 of} cooling, with correspondingly equal values at these time points t _i and t _{j of the} values of the direct voltage drop, as T _P (t _i ) = T _K (t _j ) at U _P (t _i ) = U _P (t _j ).

The construction of preliminary values of T _P (t) _{I is} illustrated (fig.1b and 1c) for the point of dynamic equilibrium t ₁ . Assume that the time t ₃ (Fig.1c) satisfies the condition t ₃ > 3τ ₂ , then the transition temperature at this point is taken equal to the temperature on the basis of the housing T _P (t ₃ ) = T _K (t ₃ ) or T _P3 = T _K3 . The transition temperature T _P3 corresponds to the temperature-sensitive parameter U _P3 (figb). Find the time t ₁ in the heating interval in which U _P1 = U _P3 (figb), and determine T _P (t ₁ ) _I according to the temperature level T _P3 , i.e. T _P (t ₁ ) _I = T _P1 = T _P3 . Such constructions are performed for all time samples of the cooling interval, starting from the final measurement sample until the first after the cessation of heating.

Next, determine the transition temperature T _P (t) _II (curve with index II in Fig. 1c) already at the site of the heating interval t ₁ <t <t ₂ according to the expression

The calculated dependence T _P (t) _II makes up with the dependence U _P (t) the second calibration characteristic U _P (t) = f (T _P (t) _II ) on the time interval t ₁ <t <t ₂ . It is used to determine the transition temperature T _P (t) _III (curve with index III in Fig. 1c) in the cooling interval according to an algorithm similar to the construction of preliminary values of T _P (t) _I. Next, calculate the dependence of the transitional thermal resistance in the interval t ₂ <t <t ₄ cooling according to the expression

approximate it by the sum of the exponential dependencies according to expression (7) and determine by the method developed by the authors, the thermal component parameters R ₀ , R ₁ , R ₂ , τ ₀ , τ ₁ , τ ₂ and the steady-state transition-case thermal resistance as

Comparative characteristics of the methods for measuring the thermal resistance of the transition-housing according to the method of the prototype and the proposed express method are presented in tables 1-3.

Table 1 shows the measurement conditions and the results of the calculation of the experiment "base".

Table 2 summarizes the conditions and results of the “module” experiment.

Table 3 shows the initial data of the proposed express measurement method, the results of calculating the thermal resistance parameter in dynamics at the moment of dynamic equilibrium R _Тmax and the calculation results of thermal parameters, which determine the value of the established thermal resistance of the junction-case R _Тп-к .

The prototype method and the proposed express method use an equivalent thermal model of a semiconductor device, but they differ in the way it is analyzed. According to the prototype method, analysis is carried out only according to the heating procedure. In the proposed express method, the analysis is built according to the heating procedure and the subsequent cooling procedure, which allows you to combine the measurement process with the calibration procedure. Due to the different approach in the analysis of thermal processes in the device, it is impossible to compare the initial data of the measurement process. Comparison is permissible for the final result - the measured value of the thermal resistance parameter and the convenience of their operation - the duration of the measurement procedure to obtain the final result and the number of necessary measurement procedures.

The proposed express method does not require the use of a heat sink, excludes a lengthy calibration procedure and allows the measurement of steady-state thermal resistance for several minutes. This method can be used to assess the quality of semiconductor devices in the technological cycle, which allows to reduce the reject yield and increase the shelf life of devices in the production process.

The proposed method is implemented using the device, the structural diagram of which is shown in figure 2, and the machine waveforms of the measured parameters obtained with it, in figure 1.

The device comprises a clamping device 1, a power supply 2, a contactor 3, a programmable controller 4, a PC 5 with a Pentium IV processor.

The controlled device is connected to the clamping device in accordance with the requirements of the measured semiconductor structure - "transistor switch" 6 or diode 7. The structure of the semiconductor "key" refers to the structure of an IGBT or MOSFET transistor.

The temperature-sensitive parameter U _{P is} measured on the transistor structure at the collector-emitter terminals with short-circuited gate and collector outputs.

Elements of the flowchart perform the following functions.

The power source 2 is a constant current source I ₀ with an adjustable amplitude of 10 to 600 A. The current is set under the control of the controller 4 according to the type of the measured device.

The switch 3 in the initial state is closed, and when the power source 2 is turned on, current flows through it. When opening 3, the current from the power source flows through the measured device.

The measurement process takes place in an automated mode under the control of controller 4. The controller generates a signal for setting the current amplitude setting I ₀ and transfers it to a power source 2, which provides an output current of a given amplitude; controls the contactor 3, providing the mode of passage of short measuring current pulses that do not heat the device, in the initial state of the measurement system (at t <t ₀ ) and at a predetermined time interval (t ₂ <t <t ₄ ) cooling, or heating direct current ( t ₀ <t <t ₂ ); and also controls the temperature on the basis of the housing; reads and writes into the own memory the measured dependences — machine oscillograms I ₀ (t), U _P (t) and T _K (t) at the heating and cooling intervals and transfers the recorded information and control to the PC.

The device operates as follows.

Connect the power source 2 to the network, turn on the PC 5 and download the measurement program. Using the on-screen menu, the measurement conditions are set: indicate the type of device and the structure of the semiconductor element - "key" or diode.

The automated measurement process starts when you press the START button in the on-screen menu. The measurement parameters are automatically generated from the database for transferring them to the controller — constant current amplitude value I ₀ , temperature limit value on the case, measurement time on the cooling interval, measurement step. In this case, a corresponding command is received from the PC to the controller 4, which starts the controller measurement program.

The setting value for the current I ₀ from the controller enters the power source, the formation of current I ₀ begins, the contactor 3 operates in the mode of short measuring current pulses, until the amplitude current I ₀ is generated at time t = t ₀ . The initial values of I ₀ (t ₀ ), U _P (t ₀ ) and T _K (t ₀ ) are measured and stored in the controller memory. At time t = t _0, 3 is opened and a constant heating current I _{0 is} applied to the crystal. In the process of heating, the measured values of I ₀ (t), U _P (t), T _K (t) are measured and stored in the controller memory. When T _K reaches the specified value, the contactor switches to the mode of supplying to the crystal of short measuring current pulses I ₀ with a duty cycle that does not heat the crystal. Counting the measured values of I ₀ (t), U _P (t), T _K (t), which are also stored in the memory of the controller. After the set measurement time has elapsed in the cooling interval, the controller stops the measurement process. The information is read from it to the PC, where it is processed according to a special program until the value R _{Tp-k is} displayed on the monitor screen with the possibility of receiving on the screen and in the form of hard copies of the measurement process parameters, thermal parameters R ₀ , R ₁ , R ₂ , τ ₀ , τ ₁ , τ ₂ and machine waveforms U _П (t) and Т _К (t).

The advantage of the measuring device is the lack of standard equipment, difficult to operate, and the inertialess process of switching DC through a controlled device in the mode of measuring pulses.

Literature

1. RF patent No. 2178893 G 01 R 31/26, publ. January 27, 2002 Bull. No 3, 2002

2. Vikulin I.M., Stafeev V.I. Physics of semiconductor devices. - M .: Owls. Radio, 1980, p. 51.

3. Express method for determining the thermal resistance of power modules. Harzbein V.M., Ivanov S.V., Romanovskaya L.V., Florentsev S.N. ELECTRICAL ENGINEERING, No. 12, 2000, pp. 14-20 (prototype).

4. Siemens "Power IGBT modules". Materials for use. M .: "DODEKA", 1997, pp. 101-107.

5. William J. Orvis. "EXCEL for scientists, engineers and students." Kiev: "Junior", 1999, p.270-275.

Claims (2)

1. The express method of measuring the thermal resistance of the junction-case power semiconductor devices in the housing design, namely, that the semiconductor crystal is heated by passing through it a direct current I ₀ , A, of a given amplitude, the value of its temperature-sensitive parameter is measured during heating, in being used as the forward voltage drop across the crystal is U _n, V, characterized in that additionally to the heating interval measured temperature T of the base body _to, ° C, in the device The selected point is stored these values, receiving them depending on the t time, with, stop heating of the semiconductor chip when the temperature T _to a predetermined value and a cooling mode when applied to the crystal of short measuring current pulses with amplitude I ₀ and the duty ratio does not affect thermal equilibrium of the device is measured and stored parameter values and temperature-housing base temperature, yielding, depending U _n (t) and t _c (t) is already in the cooling interval, the duration of the inte Shaft cooling conditions selected from the unconditional execution t >> 3τ, where τ - maximum thermal time constant of the device structure to determine the time t ₁ of dynamic equilibrium, with, in the interval of heating and the obtained dependencies calculated thermal resistance junction housing at the point t _1. 2. The express method for measuring the thermal resistance of the junction-case semiconductor devices in a housing design according to claim 1, characterized in that crystals are used in power semiconductor devices, including transistor and diode structures.

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