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EET426 Power Electronics II
Thermal Management Prepared by : Mohd Azrik Roslan EET 426 – Power Electronis II
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What you should know after this lecture
Thermal basic Dissipated power vs junction temperature Thermal management Mosfet Schottky EET 426 – Power Electronis II
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EET 426 – Power Electronis II
CONVERTER OBJECTIVE: DELIVER POWER Converter objective is to deliver power However due to the switching action and parasitic components within the converter, it will produce lots of heat. So we need to manage this heat so that it will not affect the operation or even damage the converter. CONVERSION PROCESS: GENERATES HEAT EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Thermal Basics EET 426 – Power Electronis II
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temperature difference current I PD power dissipated power P Q heat
Electrical ï ANALOGY ð THERMAL voltage V T temperature potential difference DV DT temperature difference current I PD power dissipated power P Q heat conductivity thermal conductivity resistivity thermal resistivity Conductivity (Sigma) Conductivity is the inverse of resistivity Resistivity (RHO) The electrical resistivity ρ is defined as the ratio of the electric field to the density of the current it creates: EET 426 – Power Electronis II
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EET 426 – Power Electronis II
PD,max IDEAL WORLD TJ,max PD REAL WORLD Tambient T Junction temperature is the highest operating temperature of the actual semiconductor in an electronic device In ideal world, we want the temperature a device to maintain at ambient temperature even though the dissipated power increases. However this will not happen. In real world, the temperature will normally increase linearly with the power dissipated. PD = dissipated power RTH = Thermal Resistance EET 426 – Power Electronis II
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EET 426 – Power Electronis II
PD,max TJ,max PD T Tambient EET 426 – Power Electronis II
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EET 426 – Power Electronis II
PD,max TJ,max PD T Tambient EET 426 – Power Electronis II
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EET 426 – Power Electronis II
PD,max TJ,max PD Total heat loss of the device is the combination of the external system heat and heat generated by the device package T Tambient EET 426 – Power Electronis II
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NOMINAL OPERATING POINT
PD DEVICE PD / T NOMINAL OPERATING POINT SYSTEM HEAT REMOVAL = DEVICE DISSIPATION Tambient T At nominal operating point The rate of the device generating heat is the same as the rate of device dissipating heat. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
JUNCTION COOLS SYSTEM HEAT REMOVAL SYSTEM HEAT REMOVAL > PDEVICE In the case that system heat removal slope is greater than the device heat slope, we can assure that the device will be maintained cool PD DEVICE PD / T TJN,op T EET 426 – Power Electronis II
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EET 426 – Power Electronis II
JUNCTION HEATS UP DEVICE PD / T SYSTEM HEAT REMOVAL < PDEVICE PD However if the other way round, the device will heat up and this will cause a problem to the device. SYSTEM HEAT REMOVAL TJN,op T EET 426 – Power Electronis II
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Non-linear characteristic
DEVICE PD / T PD Tambient T EET 426 – Power Electronis II
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EET 426 – Power Electronis II
DEVICE PD / T PD Please observe and compare the slope of the graph. Tambient T EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Thermal instability boundary unstable stable TJN,op2 TJN,op1 DEVICE PD / T PD thermal runaway T Tambient EET 426 – Power Electronis II
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non-linear power dissipation vs device junction temperature
curve has a boundary operational junction temperature above which power dissipation > the removal capability resulting in thermal instability EET 426 – Power Electronis II
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Schottky off-state and mosfet on-state
exhibit non-linear power dissipation curves versus device junction temperature and have a boundary operational junction temperature above which power dissipation exceeds the removal capability resulting in thermal instability EET 426 – Power Electronis II
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Thermal Management Mosfets
EET 426 – Power Electronis II
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temperature difference current I PD power dissipated power P Q heat
Tj Rth,j-c Tc Rth,c-s(case) Ts Rth,s-a(heatsink) Ta Electrical ï ANALOGY ð THERMAL voltage V T temperature potential difference DV DT temperature difference current I PD power dissipated power P Q heat conductivity thermal conductivity resistivity thermal resistivity EET 426 – Power Electronis II
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HIGHER VOLTAGE RATED MOSFETS LARGER Pconduction loss
LARGER RDSON LARGER Pconduction loss HIGHER VGS DRIVE LOWER RDSON EET 426 – Power Electronis II
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TENDS to be INDEPENDENT of IDS
RDSON TENDS to be INDEPENDENT of IDS if HIGH VGS DRIVE EET 426 – Power Electronis II
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RDSON INCREASES at HIGHER TJN op LARGER Pconduction loss
RDSON value RDSON INCREASES at HIGHER TJN op LARGER Pconduction loss EET 426 – Power Electronis II
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EET 426 – Power Electronis II
RDSON NORMALISED value EET 426 – Power Electronis II
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EET 426 – Power Electronis II
+ gate charge loss EET 426 – Power Electronis II
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ON-STATE LOSS versus JUNCTION TEMPERATURE
same shape as RDSon versus TJN ON-STATE LOSS versus JUNCTION TEMPERATURE Irms 2 RDS on fn (T) EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Thermal Runaway Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
TJN TRANSCENDENTAL fn (TJN) fn (PD) fn (RDSon) TJN RDSon Power MOSFETs typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop. However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel, so current hogging does not occur. If a MOSFET transistor produces more heat than the heatsink can dissipate, then thermal runaway can still destroy the transistors. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. PD THERMAL RUNAWAY EET 426 – Power Electronis II
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transcendental equations resulting from the non-linear characteristics
requires graphical or computer solutions to avoid laborious iteration Mosfet on-state loss Irms2 RDS,on is the problem due to the non-linear RDS,on versus temperature characteristic EET 426 – Power Electronis II
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Solving Transcendentals Iterative Process
EET 426 – Power Electronis II
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EET 426 – Power Electronis II
ASSUME TJN op < TJN max RDSon @ TJN op data sheet calculate PD @ TJN op SELECT Tambient RTH J-A data sheet calculate D TJ-A calculate TJN conservative re SELECT HIGHER TJN op re SELECT LOWER TJN op Y N TJN << TJN op This is called the iterative process Y OK N TJN > TJN op EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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Mosfet conduction loss
THERMAL INSTABILITY THERMAL STABILITY DETERMINE THERMAL STABILITY BOUNDARY Usually ambient temperature will be given T (oC) Tamb TJN,op < TJN,boundary TJN,boundary EET 426 – Power Electronis II
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Mosfet conduction loss
Pop slope of any line represents the thermal conductivity TJN,op T (oC) Tamb EET 426 – Power Electronis II
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LINE PLOTTING : JUNCTION-CASE
Rth,j-c Rth,s-a(heatsink) Tjunction Tcase Tsink Tamb Rth,c-s(contact) T (oC) Tcase TJN,op EET 426 – Power Electronis II
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LINE PLOTTING: CASE-AMBIENT
Rth,j-c Rth,s-a(heatsink) Tjunction Tcase Tsink Tamb Rth,c-s(contact) PD,op T (oC) Tamb Tcase TJN,op EET 426 – Power Electronis II
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Thermal Management Schottky Rectifiers
The Schottky diode (named after German physicist Walter H. Schottky); also known as hot carrier diode is a semiconductor diode with a low forward voltage drop and a very fast switching action. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Schottky Rectifiers EET 426 – Power Electronis II
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EET 426 – Power Electronis II
SCHOTTKY RECTIFIER IF VF VF T TJmax EET 426 – Power Electronis II
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SCHOTTKY on-state loss no problem provided Tjn,op < Tjn,max
SCHOTTKY RECTIFIER IF VF VF T TJmax typical rectifier VF as Tjn SCHOTTKY on-state loss no problem provided Tjn,op < Tjn,max EET 426 – Power Electronis II
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same shape as VF versus Tjn
SCHOTTKY RECTIFIER VF T TJmax same shape as VF versus Tjn EET 426 – Power Electronis II
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EET 426 – Power Electronis II
SCHOTTKY RECTIFIER IR mA VR T IR TJmax EET 426 – Power Electronis II
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same shape as IR versus Tjn
SCHOTTKY RECTIFIER IR T TJmax same shape as IR versus Tjn EET 426 – Power Electronis II
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possible THERMAL RUNAWAY
SCHOTTKY RECTIFIER OFF-STATE predominant ON-STATE predominant possible THERMAL RUNAWAY EET 426 – Power Electronis II
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EET 426 – Power Electronis II
TJN TRANSCENDENTAL fn (TJN) fn (Prev) fn (Irev) TJN IREV Prev THERMAL RUNAWAY EET 426 – Power Electronis II
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transcendental equations resulting from the non-linear characteristics
requires graphical or computer solutions to avoid laborious iteration Schottky reverse leakage current versus temperature has an exponentially rising characteristic that creates the problem EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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EET 426 – Power Electronis II
SCHOTTKY PROCEDURE same as MOSFET EET 426 – Power Electronis II
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DETERMINE THERMAL STABILITY BOUNDARY
SCHOTTKY TOTAL LOSS THERMAL INSTABILITY THERMAL STABILITY DETERMINE THERMAL STABILITY BOUNDARY T (oC) Tamb TJN,boundary TJN,op < TJN,boundary EET 426 – Power Electronis II
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“Thermal stability requires a heat removal capability
20 oC margin is a rule of thumb “Thermal stability requires a heat removal capability that is greater than the heat dissipation” SCHOTTKY operational junction temperature should be lower than the temperature at the tangent from Tambient to the PSCH1 curve EET 426 – Power Electronis II
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EET 426 – Power Electronis II
NR NP NS Vout D1 D2 DR L R C Ein Schottky rectifier forward loss curves reduce with junction temperature increase due to the reduction in forward voltage drop SCH2 conduction loss SCH1conduction loss P T EET 426 – Power Electronis II
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increase exponentially with junction temperature
NR NP NS Vout D1 D2 DR L R C Ein Schottky rectifier reverse loss curves increase exponentially with junction temperature SCH2 reverse loss SCH1 reverse loss T P EET 426 – Power Electronis II
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EET 426 – Power Electronis II
T (oC) P (W) SCH2 SCH1 17.5 (W) 3.5 (W) Tamb Tbdy SCH1 Power dissipation starts the upward rise at lower temperature and has a much greater increase with temperature hence this curve determines the designers operational boundary. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Pmos PSCH1 PSCH2 Ptotal efficiency Teff,max below Teff,msax the Schottky losses are predominantly on-state losses and the combined ‘on’ and ‘off’ losses exhibit a dropping loss curve as temperature increases due to the reduction in Schottky forward voltage drop with rise in junction temperature combined forward and reverse losses usually exhibit a dropping loss curve at lower junction temperatures where the conduction losses are predominant. As the junction temperature rises and reverse loss starts to increase faster than the conduction loss falls the combined curve then starts an upward path EET 426 – Power Electronis II
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EET 426 – Power Electronis II
T (oC) P (W) SCH2 SCH1 17.5 (W) 3.5 (W) Tjn1,op 3.5 W P1tot Tamb P2tot 15.8 W 19.3 W P1tot + P2tot EET 426 – Power Electronis II
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EET 426 – Power Electronis II
THERMAL IMPEDANCE THERMAL RESISTANCE POWER PULSE DURATION EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Determine a single heat sink thermal management design for the mosfet and Schottky rectifiers operating at 50oC ambient temperature and with a minimum 20oC Schottky junction temperature boundary margin. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Finding Tsink Rth,j-c Rth,s-a(heatsink) Tjunction Tcase Tsink Tamb Rth,c-s(contact) EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Finding junction temperature for mosfet Rth,j-c Rth,s-a(heatsink) Tjunction Tcase Tsink Tamb Rth,c-s(contact) Based on Rth,ju-sink =0.42 oC/W Difficult to draw line too small What if we try to find how much power change if temperature increase by 5oC EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Find Ptotal EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Rth,j-c Rth,s-a(heatsink) Tjunction Tcase Tsink Tamb Rth,c-s(contact) EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Q2) Thermal Management Rth,junction-case 1.5 oC / W Rth,case-sink 0.5 oC / W Determine the common heat sink design requirement for thermal management of the Schottky rectifiers at an ambient temperature of 50 oC if the operating junction temperature of SCH1 is 125 oC. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Finding Tsink Rth,junction-case 1.5 oC / W Rth,case-sink 0.5 oC / W Rth,j-c Rth,s-a(heatsink) Tjunction Tcase Tsink Tamb Rth,c-s(contact) We want to make sure that EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Verify Rth,j-c Rth,s-a(heatsink) Tjunction Tcase Tsink Tamb Rth,c-s(contact) EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Find Ptotal EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Rth,j-c Rth,s-a(heatsink) Tjunction Tcase Tsink Tamb Rth,c-s(contact) EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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