MOSFET Thermal Characterization in the Application - Vishay

1 AN819 Vishay SiliconixDocument Number: Thermal Characterization in the ApplicationWharton McDanielINTRODUCTIONThe use of surface-mount packages for power mosfets hasprogressed dramatically over the past 10 years. Today, powerMOSFETs are widely available in packages that continue toget smaller. Now the question is how to choose best device inthe smallest package and make a space-critical design aseffective as possible. A major part of the problem isdetermining the Thermal performance of the device on theprinted circuit board (PCB) where it is mounted. Fortunately,a simple test method can be used to establish thermalperformance of a MOSFET in a particular Characterization DATAThe main piece of Thermal data provided by MOSFET suppliers is typically RQJA, or junction-to-ambient thermalresistance.

2 The basic Thermal circuit (Figure 1) for thisparameter consists of two components: the Thermal resistanceof the package and the Thermal resistance of the PCB it ismounted on. The problem with the PCB component in RQJA isthat the Characterization board used to create data sheetspecifications does not accurately represent the boards usedin actual applications . The PCB area is different, the copperpatterns are different, many applications use multilayer board,and actual applications have many different componentsmounted on the board along with the MOSFET . All these thingscreate Thermal performance that tends to be different, and inmost cases better than the performance of thecharacterization board. More recently, suppliers have addedRQJF to the Characterization data, which defines the thermalperformance of the package itself.

3 This is an excellentparameter to use to compare package performance and to usein determining actual device Thermal CircuitRQFARQJFTHERMAL PERFORMANCE IN THEAPPLICATIONFor any given Application , the ideal MOSFET is the smallestdevice that provides the electrical and Thermal performancethat is required. The Thermal performance is the most difficultto predict since, as previously described, the thermalcharacteristics of each PCB are different. Although there isdata published showing the relationship between thermalresistance and the copper area used to spread heat, it isaccurate only for the PCB where it is mounted. When trying tooptimize the Thermal performance, this is not good most practical method of optimizing Thermal performanceis to characterize the MOSFET on the PCB where it will beused, or on a board that is very similar.

4 This characterizationcan be performed using the same techniques used for thedatasheet Characterization , although the datasheetcharacterization is performed by a dedicated Thermal basis of this method is to dissipate a known amount ofpower in the MOSFET , and to measure the amount oftemperature rise this causes in the junction, giving the datarequired to calculate the junction to ambient thermalresistance in _C/W. The following procedure provides a simplemethod of determining the steady-state Thermal resistance ofa MOSFET on the PC board where it will be CHARACTERIZATIONPROCEDUREThe procedure has two main steps. First is thecharacterization of the body diode. Second is the temperaturerise measurements and calculation of the Thermal CharacterizationCharacterization of the body diode is important because thebody diode is used to measure the junction temperature of theMOSFET.

5 As an inherent part of the MOSFET structure, thebody diode makes the ideal sensor for this purpose. Theforward voltage, VF, of the diode varies with temperature,therefore the diode s temperature coefficient is needed to getan accurate representation of the junction temperature. Theforward voltage is measured with a low level current flowingthrough it to insure there is no self heating, which would makethe junction temperature measurement inaccurate. If it is notpossible to perform the measurements of VF, the generic VFtemperature coefficient of 2mV/_C for a diode can be used atthe cost of Characterization of the diode, the MOSFET can bemounted on a PCB or just connected with wires. Electrically,the gate should be connected to the source to insure theMOSFET cannot turn Characterization is performed as follows:DMeasure room temperature TroomDMeasure VF of body diode with IF of 10 mA at roomtemperatureDMeasure VF of body diode with IF of 10 mA at 100_CDCalculate the diode temperature coefficient usingTC+VF @ 100_C VF @ room100 Troom mV _C AN819 Vishay Number: 7161914-May-01 FIGURE DiagramPower SupplyunConstant Current ModeDeviceUnderTest9-V SupplyforSense CurrentDigitalScopeIF (Heating Current)10 mA820 WTemperature Rise Measurement and Calculation ofThermal ResistanceThe MOSFET being characterized is mounted on theapplication PCB and has the gate shorted to the source, withthe drain and source connected to two power supplies(Figure 2).

6 The first power supply is configured as a constantcurrent source, which forces current through the body diode toheat the junction when the switch is closed. The second powersupply provides the sensing current for measuring the junctiontemperature. This supply is connected to the drain and source,forcing 10 mA though the body diode, with the VF of the bodydiode indicating the junction temperature. Also, it staysconnected throughout the test in order that the sensing currentwill flow immediately upon removal of the heating measurement procedure is as follows:DClose the current through the body diode such thatapproximately 1 W is dissipated after reachingequilibrium. Equilibrium has been reached when is defined as PD = VF the switch, dropping IF to 10 mA. Measure VFimmediately using a digital scope triggered on thenegative slope of of RQJADC alculate DVF, result will be negativeDCalculate DTJ using DTJ = DVF/TCDRQJA = DTJ/PDVerification of the MethodThis method was verified using an Si4410DY mounted on thestandard Thermal Characterization board.

7 The forward voltageof the body diode, VF, was measured to be V at a roomtemperature of The device was placed in an oven, theoven temperature raised to 100_C, and VF measured to V. This yieldsTC+ V V100_C + the device connected according to Figure 2, W wasdissipated in the body diode. The starting value was measuredto be V. A digital oscilloscope was used to measure VFimmediately after the heating current was turned off. This gavea VF of mV. VF is 180 mV. Therefore,DTJ+DVFTC+ 180 mV +75_CandRQJA+DTJPD+ WRQJA+ value compares well with the measured value , measured using an AnaTech Phase 10 RQJA calculated, junction temperature can be predictedfor any given power dissipation level using the equation aspreviously +PD RQJA)TAIt should be noted that if other components on the PCBgenerate heat, the effective RQJA will be higher since thetemperature of the PCB will be raised by those those components can be made to dissipate their typicalpower while performing the Characterization above, the valueof RQJA will reflect this additional process of selecting a power MOSFET on the basis ofelectrical parameters must be done with care.

8 Close attentionmust be paid to the Thermal performance of the MOSFET aspart of the PCB assembly. Characterization of the MOSFET being used on the PCB being used can help insure that thecorrect device is chosen on the basis of both electrical andthermal performance.