Bypass capacitors ensure a device has a stable and clean power supply. In most cases capacitors are chosen out of habit, such as the typical 0.1uF ceramic capacitor we use.
This app note describes how calculate, model, and use different types of bypass capacitors. Included is a table with all the relevant information on different types of capacitors, and a few examples of different circuits that need different bypass capacitors.
App note: Choosing and using bypass capacitors - [Link]
Electrolytic capacitors in an SMT package are not as often used as leaded radial or axial capacitors are. However they offer many advantages, which make the assembly easier and save a PCB space.
Electrolytic capacitors in an SMT package offer the same properties as their leaded familes (THT version), but they are much easier to assemble on a PCB. If you have a device, where most of components are in an SMT version, you probably proceed the way that SMT components are machine-assembled to PCB with a following reflow soldering. Leaded THT components (through hole technology) are then soldered in the 2-nd technological step – by a solder wave or manually. In a device containing many various THT components this 2-nd technological step is inavoidable. But in devices where only few THT components are, it is often possible to minimize their count or totally eliminate. This makes a production of the device significantly cheaper and quicker. Moreover in comparison to manual soldering, reflow soldering is much more consistent, ensuring a stable quality of soldering in various production batches.
Saving of a space is the second essential advantage. SMT electrolytic capacitors are available even with a very low profile. Thanks to the fact, that pads of SMT capacitors don´t require holes drilling into a PCB, like it is at THT technology, in many applications they can significantly simplify design of multilayer PCBs. Another advantage at HF circuits can be the fact, that integrity of the ground layer on the PCB won´t be corrupted by their usage. Read the rest of this entry »
If you’ve ever wondered how decoupling and bypass capacitors work, or why you should use them on every digital circuit, you need to check out Bertho’s excellent tutorial on the subject. He writes:
While enjoying the 7400 contest, it occurred to me that many of the submitted logic designs lacked some of the most elementary safeties to ensure a working result. One of the most disregarded aspects of the designs was the lack of bypass capacitors. Then, with an article about Murphy’s law linking a Maxim application note, it was decided to write a bit about decoupling and bypass capacitors.
As a person, who can be considered “old” in this line of work, I have experienced the problems of missing decoupling first hand. My first high-speed build was in the mid-eighties as an apprentice at a large electronics firm. The design I was building, a digital frequency measurement, used 74Fxx logic at a speed of 11MHz (which was very fast for the time). It was wire-wrapped on a double euro-card size board and used about 40 logic chips. When the time came to turn it on, I noticed that it didn’t work as expected and all kinds of stuff happened all over the place. After checking the build several times I talked to my supervisor about the problem and he just looked at it and said: “There are no bypass capacitors; mount them on all chips over the power supply and we’ll talk then.”. Completely bewildered I did what he said and, as a miracle, everything just worked. Why would seemingly inert capacitance on the power supply make things work?
My supervisor then told me all about switch/surge currents and inductance of the wiring and went on to tell the tale of decoupling. I admit that it took several years before I really understood what he was talking about, but the lesson was learned: always put capacitors on the power supply of logic chips.
Decoupling Capacitors Explained - [Link]
rsdio tipped us to an app note on isolated power-supply circuits: [via]
Wow! It’s quite interesting to see how to substitute capacitors for a transformer. Without digging deeper, though, it’s not clear whether voltage boost can be achieved.
An integrated H-bridge driver for isolated power-supply circuits (MAX256) usually drives the primary of a transformer, but it can also drive a pair of capacitors that substitute for the transformer in providing isolation and power transfer.
Isolated power using capacitors - [Link]
The fine people over at FaradNet have put together an appreciable set of notes on the electrolytic capacitors that appear in almost all consumer electronics devices. Although this is a good read for those who are interested using the devices in a safe manner (and getting the most performance out of them), there is a lot of text, so I will try to summarize the two features of electrolytics that seem to be most important: polarization and frequency response.
Some notes on electrolytic capacitors - [Link]