Back to Basics: LEDs: Part 1

Posted under Back to Basics, Educational on September 17th, 2023 by
LED Basics

I thought I should probably do a series on the ubiquitous LEDs in electronics design. You might be thinking what’s there to learn about that? TBH there are a lot more things than you think. Most times when I start embedded hardware interviews, I start with basic LED circuits and how to use them in circuits. Lots of fundamentals to probe there. Surprisingly, even seasoned experts trip up on this area.

At the heart of every LED is a semiconductor chip composed of layers of materials, typically gallium arsenide/phosphide/nitride. When a voltage is applied across these layers, it excites electrons, causing them to jump from the conduction band to the valence band. As they return to their stable state, they release energy in the form of photons, creating visible light. It’s called electroluminescence.

Every LED has forward voltage(Vf) and it changes with color. Everyone knows the exponential IV curve from your first electronics class. LEDs don’t pass current till Vf is reached and then increases drastically. Approx Vf values for diff colours are as follows

Red-1.8- 2.2V

Green/Yellow- 2 – 2.2V

Blue/White- 3 – 3.4V

What you also need to remember is Vf changes(by upto even 20%) even between the same type of LEDs, from the same manufacturer, even from the same batch. Because of this, LEDs are driven with a constant current rather than a constant voltage. Usually more current, more light an LED produces till a point. A very basic LED drive design is there in the pics. It’s our job as circuit designers to ensure what’s the minimum current that needs to pass, to produce the required light output. We will see all about that and more in the upcoming posts.

Feel free to follow or share with a colleague if you believe this series could be valuable, or simply as a helpful refresher.

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Back to Basics: Planar Transformers

Posted under Back to Basics, DIY, Educational, Technology on September 10th, 2023 by
Planar Transformers

Anyone remotely associated with electrical systems would have seen a transformer. These tend to stick out as big rectangular-shaped units in your circuit and are called wire wound transformers. Most of you would have seen this in use in step or step-down voltage use cases for low frequencies(50Hz) AC power signals. They are built by wrapping enameled wire over a plastic part(bobbin) for both primary and secondary wires. Whenever there is a current change in the primary, there will be an induced current in the secondary. There are different types of ways by which you wind a transformer, but all of it is just to ensure that coupling is high between 2 sides and leakage is low.

Now coming to high-frequency transformers, which are used in SMPSs, where the frequency of switching is several times larger. As freq is higher, you can use thinner wires(Skin effect, refer to older posts) This will help reduce the size of transformers. Now an even better way to reduce spacing(and increase coupling) is to ditch circular wires altogether and get them printed in PCB tracks. That’s exactly what planar transformers are. It’s a stack of PCBs with multiple layers with both primary and secondary current coils printed on them and connected in a certain way. Insulation is provided by the PCB dielectric and you can have more areas for better coupling. This ensures that the height of the unit is drastically reduced. Since these are PCBs, it has better heat dissipation ensuring that the transformers remain cooler. Manufacturability also goes up as it’s repeatable to make PCBs that are SMD mountable with machines than the older wire wounding semi-manual process. You can even create slots on the PCBs to mount your ferrite cores directly in it.

This is one of the key reasons for the size reduction in your phone chargers(refer to older posts). To learn more about the design and state of art of this tech, I would suggest watching a few videos on YouTube on the international workshop conducted by IISc this year. Search IWPMT 2023 for a series of talks on the same.

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