Power dissipation calculation for Linear and Switching regulators

For any hardware designer, power consumption control is a big challenge. Lower the power consumption of your product higher the success rate. For a battery operated devices like mobile, tablet and other smart devices power consumption is a critical specification. One critical component in this power consumption/wastage across the board is regulator. Power from the main source like battery, usb power is distributed across the board using regulators. Regulators can either step-up or step-down the input a DC voltage. Based on our requirement we choose a regulator type.

Regulators used in embedded hardware boards can be mainly classified as linear and switching regulators. There are several sub classifications in linear and switching regulators which we will discuss later. For now let us calculate the power dissipation calculation across Linear and switching regulator. For this we will take an example of devices from Linear technologies.

Linear Regulator: LT3014B

Switching Regulator: LTM8020

Power Dissipation calculation for LT3014B

I/P voltage range: 3V-80V

O/P voltage range - 1.22V-60V
Dropout voltage - 350mV

For a linear regulator, as the output current from the regulator increases, drop out voltage increases which is shown in the graph below from datasheet of LT3014B

Let us assume i want a output voltage of 5V @20mA from LT3014B. To achieve this output, minimum input required is 5.35V. Let us assume, we are providing a input of 7.4V which is Li-ion standard battery pack voltage.

The power dissipated across the linear regulator is (7.4-5)*20m = 48mW

If the input is 50V and output is 5V, then power dissipated is (50-5)*20m = 0.9W

So, we can clearly see that as the difference between input and output is increasing in a linear regulator, power dissipation across regulator is going high. Also, imagine a case where input to output voltage difference is low but current is in amperes, in this case also, power dissipation across linear regulator is very high.

Important note: If the input to output voltage difference is high, or if the current output from regulator is high, power dissipation across linear regulator is very high in which case heat dissipation is high. We may need a huge heat sinks if we use linear regulators in such cases. Also, power is unnecessarily wasted.so, we have to go to switching regulator in these cases.

Power Dissipation calculation for LTM8020

I/P voltage range: 4-36V

O/P voltage range - 1.25-5V

The main factor to consider in switching regulators is efficiency. For a given load current, the efficiency can be calculated from the graph below as from LTM8020 datasheet.

To calculate power dissipation across switching regulator LTM8020, we have to take above graph into consideration. Let us assume my load is drawing 100mA for a input of 24V at output of 5V. In this case, from the graph efficiency is ~82%.

For above requirements, output power = 5v *100mA = 0.5W
Input required power = output/effeceincy = 0.5/0.82 = ~0.6W

So, power dissipated across LTM8020 is, 0.6-0,.5 = 0.1W

Important note: If either the input to output voltage difference is high, or if the current output from regulator is high, power dissipation across switching regulator is less in which case heat dissipation is less. We may not need a huge heat sinks if we use switching regulators in such cases. Also, power is not unnecessarily wasted.So, we have to go to switching regulator in these cases.

Diodes - Part 1 (Button diode, Cell diode, Pressfit diode)

Engineers outside automotive domain might never have thought how a automotive battery gets charged. A hardware engineer in automotive is very much familiar of those technical details. The whole mechanism happens with the help of alternators which convert mechanical energy to electrical energy. The electrical energy generated at this point is alternating one. So, either to charge the battery or to provide the power to the ECU (Engine control unit). So, rectification has to happen. The process of rectification converts alternating current to direct current. For these purpose we use rectifier diodes.

Button diode, Cell diode and Press fit diode are types of rectifier diodes used in rectification process. These are especially designed for automotive applications.

Advantages of rectifier diodes (Button type, Press fit, Cell diode) used in automotive applications:

• Low leakage
• High reverse voltage rating
• Low cost
• High surge current handling capability
• High operating temperature
• Plastic Casing on these diodes meet flammability standards

Thermal paste/compound - A small note

Thermal paste is a heat sink compound used on top of the processors (whether it be graphical or central processor), regulators for heat to follow smoothly to external medium. The mechanical designers suggest using a heat sink based on the thermal calculations. The principal behind using a heat sink in that the operating temperature of the device should not exceed the maximum specified ratings in the datasheet.

Operating temperature of a specific heat generating device is calculated by summation of junction temeparature and heat generated (Heat generated is the product of thermal resistance and power of operation). When this sum exceeds operating temperature specification that effect is catstrophic and can lead to device failure.

So, we have to use heat sink on top to enable the heat to escape from the device. Keep in mind that thermal paste is filling the imprefections of heat sink. The heat sink will be mounted on top of the device. Even though the heat sink is placed on top of device touching it there will be a air gap in between. Air is always a poor conductor of heat in which case another media must enable smooth heat flow. For this purpose we use a thermal paste. Thermal paste is a highly heat conductive paste.You mave have a large heat sink with exception fin structure for smooth heat flow, but, without a thermal paste the entire cost you have put on heat sink is a mere waste.

The thermal paste applied on top can be of different types. These can be electrically conductive as well as non-conductive. In a electronic board, it is always prefferable to use a non-conductive type. Cermaic based paste is one type of thermal paste. Arctic silver is the most preffered thermal paste.

What is Integrated Heat Spreader?

Keep in mind that the modern processors have a metal kind of enclosure for heat flow assistance. This we call a integrated heat spreader (IHS).

I already have a heat sink compound on the device. How do i clean it?

Usin Isopropyl Alcohol. Use a isopropyl dipped smooth cloth to rub off the paste.

Conformal coating for PCB

Engineers who worked on hadware boards which have to operate very low temperatures might not have come across conformal coating requirement. But who work on marine, defence, military, aerospace domains and hardware board designers who worked on board designs which have to work up to high temperatures and humidity might have used conformal coating. Now a days we can see conformal coating being used very extensively across other domains also (Telecomm, industrial, automotive, instrumentation). We can say that where there is a need for reliability and high protection conformal coating of PCB is a must.

Conformal coating is a chemical coating on top of the assmebled PCB which helps electronics to get shielded against harsh environments. If you have seen some relays, transformers available in the market, they have a plastic enclosure for internal electronics with a gel compound filled for protection against shock, vibration, stress, thermal, corrosion, moisture. This is what we call "plotting". You can see the huge black ics in the below figure.

Plotting is an expensive technique and conformal coating helps to achieve the protection simpler. The also disadvantage is that specific temperatures are used to form the protective layer which may cause the electronics to loose bonding. Conformal coating also adds dielectric strength to the PCB. Also, conformal coating ensures board reliability. Conformal coating can protect electroincs boards against the following problems:

• Temperature (which we call thermal cycling in environmental tests)
• Humidity
• Moisture
• Mechanical stress
• Dust accumulation
• Fungus formation
• Corrosion because of salt spray
• Shock & Vibration
• Chemical contamination

There are specifications available forconformal coating guidelines especially in military and aerospace. For example, MIL-1-46058 is one such sytndard. Silicone is the most commonly used conformal coating material used for protection against high temperatures.

What are the various other conformal coating materials available?

Silicone, Epoxy, Acrylic, Urethane, Parylene. Urethane is the most widely used.

Once conformal coated, can we remove it?

Yes, it can be removed and there are several techniques available. Approach the manufacturer to get to know the type of material used and technique to remove coating. Acrylics are the easiest to remove.

What care need to be taken for resoldering a component on already conformally ciated PCB?

Once conformally coated, very high temperature must be applied by soldering machine to remove that coating. This may cause SMT pads to get lifted. In susch cases, careful attention must be provided while aplying temperature beyond certain range. Also, once rework is done take care that conformal coating residue is not present on the board which can give a problem in the long term. Take care of ESD which performing such procedures for removing conformal coating.

What can be the thickness of conformal coating?

Have seen a conformal coating of thickness up to 50um.

What are the advantages of conformal coating?

• Eliminates need for costly mechanical enclosures
• Less weight
• Easy removal

What are the techniques used for conformal coating?

• Brush
• Spray
• Dip
• Automated

What care need to be taken for conformal coating?

Eliminate any surace defects. This means you have to equally spray without any gaps

Chose the conforming material as per the application and requirement

Choose the conformal coating technique as per expertise available

The drying process after coating must be simple. Can be air dired, normal dried or UV dried.
Coating material complies to industrial standards and is environmental friendly

How can suraface defects be detected in conformal coating?

Using UV light inspection

Film capacitors in various applications - A note

The following are the advantages of film capacitors that make them suitable for various applications:

• No polarity
• Reliable
• Stability
• Available from pF to uF
• High surge current capability
• Low inductance
• Low cost
• Low ESR
• High precision capacitor value
• Can withstand high power 
• Value retention for longer time, long life
• High voltage withstanding capability
• High frequency handling capability
• Customization

What is a film capacitor?

A film capacitor uses a thin layer of plastic as dielectric. Later, this is treated to come up with various film capacitor types like metallic film capacitor, polyester film capacitor, PTFE film capacitor, etc.

Various applications:

There are several advantages of metal film capacitors that make them useful in various applications. Have you seen the latest LED bulb applications? The most preferred capacitor is metal film type. These are relatively stable and cheaper which makes them suitable for low margin and stability desired LED market.

We can see PTFE film capacitors (polytetraflouroethylene) used in aerospace and military applications. PTFE film capacitors are heat resistant, so, this is preferred.

Snubber circuits use polypropylene film capacitors. This is because of their higher current carrying capability and low inductance.

Other applications:

• Filters
• Lighting ballasts
• decoupling capacitors
• RFI suppression

What are the disadvantages of film capacitors?

• Bulky compared to other capacitors
• Difficult to find SMT parts for these capacitors for all values
• Little costly