LO2 - Question 1 - Thermal Resistances And Thermal Transfer Flashcards
(10 cards)
How to draw thermal resistances with a heatsink.
Heat Sink:
Start by drawing a small rectangle called the junction, then draw a larger rectangle called the metal plate exactly under it, then draw the PCB which is also a larger rectangle exactly under the metal plate. Finally, draw a large E with 5 sections horizontally on the junction named the heat sink.
Thermal resistances:
Draw a triangle with 3 resistors on the left side of the heat sink - 1 on each line and on the junction of the right hand side, draw another resistor connected to the junction (J) of the heat sink.
Draw the Thermal resistance without a heat sink.
Start by drawing the junction, the case, and the PCB, then draw 2 resistors in series. The first resistor is connected from the junction to the case, named RthJC and the next resistor in series is connected from the RthJC and Ambient (A) (draw somewhere on the left side - just a dot) named RthCA.
What are the equations for Thermal resistance without a heat sink?
TA Res TC Res TJ
A RthCA C RthJC J
<-----------------
P
TC = TA + RthCA . P
TJ = TC + RthJC . P
Therefore:
TJ = TA + (RthJC + RthCA) . P
What are some packages for power semiconductor devices?
TOP3 insulated
TOP3
TO-247
D-61-8
RD91 insulated
SOT-227
D-56
Most commonly used is TOP3 and TOP3 insulated.
What is thermal resistance?
Thermal resistance is a measurement of a material’s or a component’s resistance to heat flow.
It is the reciprocal of thermal conductance, which is the ability to conduct heat.
Why and how is thermal resistance used?
Thermal resistance is used in PCB circuit design to measure a package’s heat dissipation and avoid overheating.
What is the Unit of measurement used for thermal resistance?
^o C/W
What is the equation for material thermal conductivity (k)?
Q = ((kAc)/L) (T1 - T2)
Where
Q is heat (W)
k is the thermal conductivity (W/m . ^oC)
Ac is the plane area (m^2)
L is the plane thickness (m)
T is temperature (^oC)
How to draw conduction resistance?
Draw 2 rectangular boxes named Thermal and Electrical, both containing a triangle pointed downwards and in series to these triangles, draw a resistor. On the top end of the boxes, write T1 and V1 respectively. Also write Q and i next to the triangles respectively. Write tita and R next to the resistor symbols respectively and finally, write T2 and V2 at the bottom of the rectangle all inside it respectively.
What are the equations used to calculate the Thermal transfer, Electrical Transfer, and Conduction resistance?
Thermal Transfer:
Q = (T1 - T2)/tita
Electrical Transfer:
i = (V1 - V2)/R
Conduction Resistance:
Tita = L/kAc
For conduction resistance and thermal transfer, tita is drawn as an O with a - in the middle. This is done because the temperature decreases with distance.
