Risk Of Thermal Runaway; Figure 3. Hdc (Heat Dissipation Capability); Figure 4. Hdc And Junction-Temperature-Dependent Dissipated Power (Case 1) - ST STM32 Application Note

4.4

Risk of thermal runaway

The cooling system of a design is characterized by the junction-to-room temperature thermal resistance,
Theta_j_room.
This thermal resistance gives the capability to dissipate power in the design while limiting the junction
temperature.
The heat dissipation capability (HDC) is given by the following equation:
The figure below shows the operating point at the intersection of the curve of the power that can be dissipated
inside the design at a given T
Case 1
The figure below shows the intersection point that defines a clear operating point. No thermal runaway.
The cooling design is safe (no thermal runaway) if an intersection point exists for T
junction temperature < 125 °C.
AN5036 - Rev 3
HDC = T j − T room / Tℎeta_j_room
(HDC) and the curve of the power effectively dissipated inside the design (P
j
Figure 3.
Power
0
T
room
Figure 4. HDC and junction-temperature-dependent dissipated power (case 1)
Power
Operating point
0
T
room
HDC (heat dissipation capability)
Better
Worse
@ T
room
Pdiss
Effective T 125 °c
j
AN5036
Risk of thermal runaway
HDC
Slope = 1 / Theta_j_room
T
j
HDC
Slope = 1 / Theta_j_room
T
j
= T , with a
room room_max
).
diss
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