Thermal Design
1. Tj max, Tc max, and θj-c
Tj max, Tc max, and θj-c are stipulated in the maximum ratings as required parameters for thermal design.
• Tj max (junction temperature)
Tj max is a parameter that is due to the physical structure of the internal devices. Since devices will be degraded or
destroyed if Tj exceeds this value, the design must not allow this value to be exceeded.
• Tc max (operating substrate temperature)
Tc max is a parameter that arises from the internal devices, the materials used, and the circuit design. It is
determined based on comprehensive considerations, including reliability. Devices are not guaranteed if they are
operated with Tc exceeding this value.
• θj-c (thermal resistance)
θj-c differs for each device in the design, and is stipulated in the maximum ratings since it is required to calculate
Tj for the major devices in the design.
Note that Tj and Tc are independent parameters that depend on the operating conditions, and the thermal design must
fulfill the maximum ratings of both these parameters.
2. Approaches to Thermal Design
Let Pd be the IC case internal operating power dissipation, and Pc be the power dissipation per power transistor. The
required heat sink thermal resistance (θc-a) for this case internal power dissipation (Pd) can be derived as follows:
Condition 1: Taking Ta to be the end product guaranteed ambient temperature, the IC case temperature Tc must not
exceed 125°C.
Pd × θc-a + Ta < 125°C (Tc max)................................①
Condition 2: The power transistor junction temperature must not exceed 150°C.
Pd × θc-a + Pc × θj-c + Ta < 150°C (Tj max)..............②
Where θj-c is the thermal resistance per power transistor.
The thermal design must fulfill these above conditions.
3. Design Procedure
The figure at right shows a model of the STK392-020 channel 1 circuit.
Taking the Pd’s of each channel to be Pd1 (channel 1), Pd2 (channel 2)
and Pd3 (channel 3), the total Pd max can be determined as:
Pd max = Pd1 max + Pd2 max + Pd3 max
That is, condition ① gives:
θc-a < ..............................................③
which allows the required heat sink thermal resistance to be derived.
(Here, Tc max is taken to be 125°C.)
Also, taking the power dissipation per power transistor for each channel
to be Pc, the transistor junction temperature Tj will be:
Tj = Pd max × θc-a + Ta + Pc + θj-c............................④
and this can be used to confirm that Tj does not exceed Tj max (150°C).
At this point, if Tj exceeds 150°C in the design, Tc must be lowered by lowering the heat sink thermal resistance
θc-a so that Tj does not exceed 150°C.
Tc max – Ta max
Pd max
No. 4688-5/8
STK392-020
STK392-020 Circuit Model (channel 1)