Power Dissipation And Cooling Methods; Power Dissipation - ST STM32 Application Note

4

Power dissipation and cooling methods

This section provides recommendations for efficient thermal analysis on STM32 applications, focusing on the
following points:
• Power dissipation and its variation with various factors named PVTA (process, voltage, (junction)
temperature and activity)
• How to minimize power consumption.
• How to optimize thermal dissipation.
4.1

Power dissipation

There are two types of current consumed by the device:
• Static current: also called leakage current, depends on the process, voltage and junction temperature but
does not depend on the activity.
• Dynamic current: depends on the process, voltage and activity but does not depend on the junction
temperature (at least in first approximation).
The static/leakage current grows exponentially with the junction temperature (whilst the dynamic current does not
significantly change).
Therefore, the total current can be written as follows:
The dissipated power varies with PVTA conditions and is calculated as the product of the voltage generated
across the device and the (average) current consumed.
The junction temperature increase above the ambient temperature is calculated as a product of the dissipated
power and the junction-to-room thermal resistance.
The junction temperature must be kept lower than the maximum target given by the following equation:
Important:
In this simplified formula used only for didactic purpose, Theta_j_room is a complex coefficient that is not a device
characteristic but a system one (device + other components + boards + casing).
The following methods to keep the junction temperature below the ambient temperature, are detailed in the next
sections:
• Limiting Pdiss
• Limiting Theta_j_room (cooling system, host board and casing design)
AN5036 - Rev 3
I Total P, V, T j , activity = I static P, V, T j + I Dynamic P, V, T j , activity
T j = T room + P diss × Tℎeta_ j_room < T j_max
AN5036
Power dissipation and cooling methods
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