PCB/Electronics: Thermal Management, Cooling and Derating

In this lesson you will learn the basic concepts and definitions, among which, thermal resistance, junction temperature, the electrical analogy, ambient temperature, airflow and power dissipation. 

In this section we will see how to calculate the dissipated power in simple components like transistors and resistors, and in more complex digital devices,
where we have to consider factors like the I/O switching power and ODT
power factor.

In this lesson, we will apply what we have learned form the previous lesson to a SRAM digital chip Cypress CY7C1381D, whose dissipated power will be calculated as an example, using the device datasheet.

I this lesson we will see the most used temperature ranges, and we will have a quick glance at the JEDEC standard and its use, in order to be able to perform reliable comparison between thermal performance of different components.

In this lesson we will see the peculiarities of one of the most used and more affected by the power dissipation performance devices, the linear regulators. We will see hot to calculate the dissipated power in the worst case scenario, using the data available in the datasheet.  

As an example, we will see the Low Drop Out Linear Regulator Microchip TC1264.

In this lesson we will talk about heat sinks, how to calculate when hey are necessary, which performance is required and how to choose them for a specific device, once given the ambient temperature, the Junction to Case thermal resistance, the dissipated power and the maximum Junction Temperature allowed by the chip manufacturer.

We will provide a numerical Example, at the end of which an AAVID Heatsink will be chosen after having checked its datasheet.

In this lesson we will introduce thermal interface materials (TIM), which are the materials used between the heat sink and the device. We will explain the different types of TIM, with strengths and weaknesses, and we will provide an example of thermal resistance calculation with the Laird Technologies TPCM585 phase change TIM.

In this lesson we will see the most used techniques to apply an heat sink onto a chip, with strength and weaknesses for each siltation. We will also provide some tips to avoid short circuits between heat sinks and nearby capacitors.

In this lesson you will learn what the Spreading Resistance is, under which circumstances it occurs and how we take into account of these aspects by augmenting the rated heat sink thermal resistance with a safety multiplication factor.

In this section we will introduce the most important fan parameters (CFM, LFM) and how to calculate it. We will use as an example the Datasheet of the fan RS Pro DC Axial Fan.

We will also see a simple rule to derated the fan CFM with respect to the back-pressure.

In this section we will explain how the use of a fan impact the thermal resistance of heat sinks. We will refer to the impact of the fan used in the previous lesson example, the RS Pro DC Axial Fan, on a heat sink with 9.5 °C/W thermal resistance. 

In this lesson we will talk about the thermal resistance variation with airflow predicted by the manufacturers and reported under the form of graphs. 

We will see the two most common graphs.

In this lesson, we will see how altitude impact a heat sink performance, with and without forced air flow/fans, showing a table with derating factors.

In this lesson, we will give you some tips on fan and heat sinks positioning and some other advice.

In this lesson we will talk about when and why the PCB design represent a critical factor in the thermal management.

In this section we will explore how manufacturers provide information on a component PCB soldering and placement in order to provide a Thermal Resistance through the PCB of a specified value.

In this lesson, we will provide some tips on the PCB design and component placement in order to improve the thermal performance.

In this lesson we will talk about the most influent stress factors and what derating is. We provide an example on a resistor.

In this lesson we talk about the difference between mechanical and chemical failure, and we introduce two example of direct stress factors derating, which can be applied to every stress factor but power dissipated. The latter indeed, requires one more step before derating, which is called re-rating and it is shown in the next lesson.

In this lesson we will talk about the power dissipation Re-Rating and De-Rating for electronic components. 

We will see how manufacturers provide re-rating information for resistors and semiconductor devices, showing specific datasheets of component from Vishay and ON Semiconductor.

Furthermore, as an example, we will derate a 200W 2N6338 transistor using the graph method with the derating graph and mathematical method with the derating coefficient, both provided by the manufacturer in the device datasheet.

In this lesson, mostly theoretical, we will introduce a formula that describe the relation between failure rate and electrical stress, and we will show how a variation of 15% of the stress voltage, can double the failure rate.

This lesson is quite theoretical, and shows a formula that relates the thermal variations to the failure rate. It shows how a variation a 10°C may double the expected life of a device.

In this lesson, we will provided the recommended derating factors for the most common electronic components, to be used when the component manufacturer does not provide information on this matter.

Final lecture! Thank you and good luck!