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Introduction:

This document provide’s the RF amplifier design Engineer with a useful reference to aid in thermal considerations and calculations, applied to the VIMOS portfolio of RF power transistors. Rather than focusing on junction temperature measurement and modeling techniques, the application notewill focus on the basic practical application of device specific thermal characterization data, to calculate the junction temperature given the variables: pulse condition, flange temperature, and power dissipation .

It is useful to review terms and symbols commonly used for thermal analysis of characterization ofRF Transistors:

REV B

Thermal Characteristics & ConsiderationsVIMOS Product Portfolio

The thermal resistance, from the semiconductor junction to the back side of the flange or case, is identified on RF transistor data sheets in various ways: R(th)

The RF power transistor is comprised of a die of semiconductor material mounted on a metal flange, diamond or ceramic substrate, wire bonds, leads, and a plastic or ceramic lid. The heat is generatedin the semiconductor junction and is conducted away via the flange/substrate to the backside of theflange (also known as the “case”). The flange is typically mounted to a heat sink in the amplifierapplication. Although heat is being dissipated through numerous paths, such as convection from the lid or top surface of the device or via conduction from the wire bonds to the leads of the device package, the dominate heat transfer path is from the junction of the semiconductor through any substrates involved to the flange or case of the device, and finally to the heat sink. All other heat transfer paths can be ignored as they are insignificant.

TM

Measurement of resistance of heat flow through a specific material. The reciprocal of heat conductivity.

Measurement of the conductivity of heat flow through a specific material. The reciprocal of thermal resistance.

Thermal Resistance Junction to Case

C Heat Capacity / Capacitance

K Thermal Conductivity

A measure of a materials capacity to store heat, analogous to a capacitors ability to store charge in electrical circuits.

R

R(th) Thermal Resistance

Symbol / Abbreviation Term Description

P Power Dissipation The amount of power being dissipated in the RF Transistor.

DISS

R

Thermal resistance between the bottom side of the flange/case (c = case) and the semiconductor junction (J-Junction).

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The practical application of thermal data provided in data sheets, with regards to the RF amplifier designer, is to obtain the junction temperature given specific flange temperatures. This is easilycalculated once the thermal resistance, from the back side of the device flange to the actual semiconductor junction (referred to as ) is known.

Factors that impact the junction temperature include:

- thermal resistance of the materials in the heat path

- temperature of the materials in the heat path (As temperature rises, the ability for materials to conduct heat generally degrades. For example, with regard to Si the thermal conductivity decreases by about 0.3% per °C increase in temperature.)

- pulse width and duty cycle (High power pulsed RF transistors contain various materials in the heat path from the semiconductor junction to the back side of the flange have sufficient heat capacitance to absorb and dissipate heat between pulses. As the pulse conditions become more demanding the heat storage of the materials approaches saturation and the effective thermal resistance increases.)

- power being dissipated by the device

The values for the various VIMOS products in the ASI portfolio, at the time of this writing,at specific pulse conditions are shown in Table 1.

R

R JC

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Part Number R(th) PULSE CONDITIONS

HVV0405-175 0.400 300uS 10%

HVV0405-300 0.360 300uS 10%

HVV0405-1000 TBD 300uS 10%

HVV0912-150 0.130 10uS 10%

HVV0912-450 0.050 10uS 10%

HVV0912-800 TBD 10uS 10%

HVV1011-035 1.500 50uS 5%

HVV1011-075L 0.280 32uS on 10uS off x 48

repeat every 24ms

HVV1011-180L 0.140 32uS on 10uS off x 48

repeat every 24ms

HVV1011-300 0.140 50uS 5%

HVV1011-500L 0.180 32uS on 10uS off x 48

repeat every 24ms

HVV1011-600 0.075 50uS 5%

HVV1011-1000L 0.090 32uS on 10uS off x 48

repeat every 24ms

HVV1012-060 0.280 10uS 1%

HVV1012-250 0.033 10uS 1%

HVV1012-550 0.016 10uS 1%

HVV1214-025 1.500 200uS 10%

HVV1214-140 0.540 200uS 10%

Table 1. Thermal resistance from the semiconductor junction to the back side of

the device flange.

(Flange Temp = 25 Deg C)

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With the thermal resistance data provided on table 1, the junction temperature can be calculated for each of the devices listed, as follows:

The HVV1011-300 is a 300 Watt RF transistor characterized for a 50uS Pulse Width, 5% Duty Cycle, pulse condition. The thermal resistance from the junction to the back side of the flange is

of the back side of the flange, for every watt dissipated. Thermal resistance increases as the material temperature increases, the thermal resistance is only valid for a back side flange

of the junction temperature for higher case temperatures.

delivered to the load and 300 watts of power will be dissipated in the device itself. In this case the junction temperature can be calculated as follows:

(300W x .14W/˚C) + 25 ˚C

( TCASEPD R JC=

(

TJ +

TJ

Calculating the thermal resistance and junction temperature for various pulse conditions is simplified by using an electrical equivalent circuit loaded into a circuit analysis tool. Thermal resistance, heat capacity and temperature have their electrical counter parts; electrical resistance,capacitance and voltage. D. Rice, J. Crowder, and B. Battaglia authored a paper titled “Dynamic Models for Predicting the Thermal Behavior of Vertical MOSFET Transistors under Pulsed Conditions.” detailing their thermal transient electrical equivalent model for the HVV1011-300. The model is shown in Figure 1.

Figure 2 shows various pulse conditions, drawn to scale, to provide a visual representation of the periods of heating and cooling at various pulse conditions. Following figure 2 are the results of simulations of the maximum junction temperature using an electrical equivalentthermal model. The calculated thermal resistance is shown, which can then be used to calculate the junction temperature under those specific pulse conditions, for various flange temperatures, and dissipation levels.

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Pulse Period = 3000µS (3mS)

Duty Cyle = Pulse Period

Pulse Width 300µS

3000µS = = 10%

On

Off

Ground Based Radar 1214-1400MHz 200uS Pulse Width,10% Duty Cycle

Ground & Air DME, TCAS and IFF 960-1215MHz10uS Pulse Width, 10% Duty Cycle

TCAS, IFF, Mode-S Applications 50uS Pulse Width, 5% Duty Cycle

Airborne DME 1025-1150MHz 10uS Pulse Width, 1% Duty Cycle

UHF Band Weather & Long Range Radar 300uS Pulse Width,10% Duty Cycle

Mode S-ELM Interrogator 1030-1090MHz 32uS on/18uS off x 48, repeated every 24mS

300uS 2700uSPulse Width

Figure 2. Common pulse conditions for various high power pulsed applications drawn to scale.

R4.5 Ohms

R3.4 Ohms

R2.26 Ohms

R1.2 Ohms

C1.15F

C2.0005F

C3.007F

C4.033F

Voltage at this nodeis equivalent to thejunction temperatureV = ˚C

+

-

Current source establishesdissipated power(300 amps = 300 Watts Dissipation)

Voltage source establishescase temperature(25 volts = 25 ˚C)

Voltage at this nodeis equivalent to thecase temperatureV = ˚C

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Figure 1. Electrical Equivalent Thermal Transient Model.

The maximum junction temperature for each of the specific pulse conditions shown in figure 2 have been simulated using the equivalent electrical circuit in figure 1. by pulsing the current source,and measuring the voltage at the node shown above. The voltage source in the model was set to 25 volts, equivalent to having a 25˚C flange temperature. The simulation applies to the HVV1011-300, HVV0405-300, and HVV1012-250. Once the junction temperature is obtained, the thermal resistance from the junction to the case is calculated and then the junction temperature can be calculated forvarious power dissipation levels. The maximum voltage at the node shown in the model above is equivalent to the maximum junction temperature in degrees Celsius.

The HVV1011-600 and HVV1012-550 are simply two die, rather than one, packaged in a larger package.Each die has the same thermal impedance and capacitance characteristics as the model shown above,but the dissipation is roughly twice as high. Therefore an electrical equivalent transient thermal modelis simply two of the models (shown in figure 1 above) in parallel. Future planned updates to this document will expound on that model. Devices operating at longer pulse conditions, such as the HVV0912-450, HVV0912-800, HVV1011-500L, and HVV1011-1000L feature an internal construction scheme that has significantly lower thermal resistance than that of the shorter pulse devices such as the HVV1011-300. At the time of this writing the thermal resistance has been measured, modeled, and verified via the internal body diode method. The electrical equivalent thermal transient model has not yet been created.

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UHF Band Weather & Long Range Radar 300uS Pulse Width,10% Duty Cycle

10 20 30 40 50 60 70 80 90 100 1100 120

40

60

80

100

120

20

140

me, ms ec

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Simulation Results of Equivalent Electrical Circuit

time, msec

Volta

ge /

Tem

pera

ture

˚C

(TJ TCASEPD

R JC =

(

(136-25)300

R JC =

R JC = 0.370

40

60

80

100

120

140

160

180

0 50 100 150 200 250 300 350 400 450

Power Dissipation (watts)

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10 20 30 40 50 60 70 80 90 100 1100 120

30

40

50

60

70

20

80

me, ms ec

V1,V

Ground & Air DME, TCAS and IFF 960-1215MHz10uS Pulse Width, 10% Duty Cycle

Simulation Results of Equivalent Electrical Circuit

Time (msec)

Volta

ge /

Tem

pera

ture

˚C

(TJ TCASEPD

R JC =

(

(74-25)

300R JC =

R JC = 0.163

30

40

50

60

70

80

90

100

0 100 200 300 400 500

Power Dissipation (watts)

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TCAS, IFF, Mode-S Applications 50uS Pulse Width, 5% Duty Cycle

10 20 30 40 50 60 70 80 90 100 1100 120

30

40

50

60

20

70

me, ms ec

V1,V

Simulation Results of Equivalent Electrical Circuit

Time (msec)

Volta

ge /

Tem

pera

ture

˚C

(TJ TCASEPD

R JC =

(

(68-25)

300R JC =

R JC = 0.143

30

40

50

60

70

80

90

0 50 100 150 200 250 300 350 400 450

Power Dissipation (watts)

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10 20 30 40 50 60 70 80 90 100 1100 120

26

28

30

32

34

24

36

me, ms ec

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Airborne DME 1025-1150MHz 10uS Pulse Width, 1% Duty Cycle

Simulation Results of Equivalent Electrical Circuit

Time, msec

Volta

ge /

Tem

pera

ture

˚C

(TJ TCASEPD

R JC =

(

(35-25)

300R JC =

R JC = 0.033

30

40

50

60

70

80

90

100

0 100 200 300 400 500

Power Dissipation (watts)

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Ground Based Radar 1214-1400MHz 200uS Pulse Width,10% Duty Cycle

10 20 30 40 50 60 70 80 90 100 1100 120

40

60

80

100

120

20

140

me, ms ec

V1,V

Simulation Results of Equivalent Electrical Circuit

Time (msec)

Volta

ge /

Tem

pera

ture

˚C

(TJ TCASEPD

R JC =

(

(123-25)

300R JC =

R JC = 0.326

Power Dissipation (watts)

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C

40

60

80

100

120

140

160

180

0 50 100 150 200 250 300 350 400 450

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References:D. Rice, J. Crowder, B. Battaglia, Dynamic Models for Predicting the Thermal Behavior of Vertical MOSFET Transistors under Pulsed Conditions

Acrian, Inc. Thermal Time Constant for High Piower Pulsed Transistors - Power Flow Calculations

Acknowledgements:Special thanks to Srdjan Pajic for circuit simulations and graphs.