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The measure of a man is what he does with power
Let’s face it, without power the world would be dark and electrical engineers would be out of work. The power supply of a new project is usually the last thing to be designed, and rightly so, you need to know the circuit requirements of a board before you can do a good job designing the power supply.
Having said that, many supplies share similar characteristics and if the boards you design are similar in nature, then your supplies will start to look alike. All the leprechauns at Celtic Engineering Solutions have been very busy lately. We are in the design phase of a power supply that we will likely be releasing for sale as one of our fist products later this year.
The board is called the Hummingbird Power Supply, see featured image. It takes input power from a 5V 1A barrel connector or USB port. It has a battery recharge circuit designed for an 1100mAh LI-ion battery. Then it has three regulators, a 3V, a 5V and a 15V, with current outputs of 50, 240 and 100mA respectively.
Good resources lead to happiness
The point of the board, and todays newsletter is to provide a resource. The board will be open source to those who purchase it. All of the design files will be provided. This is because getting the supply right is a big part of a successful project.
The board has one chip by Linear Technology (battery recharge circuit) and 3 chips by Texas Instruments (voltage regulators). So why are all the regulators from the same company??? Truthfully, the linear regulator might have been any company, but the two switching supplies are from TI because of a wonderful resource they are providing for switching power supply design called WEBENCH Design Center.
Switching power supplies
In the good-old-day (last decade), putting a switching power supply together was a hair-raising experience. Let’s face it, some datasheets are better than others. I don’t want to point any fingers, but some company’s datasheets are so convoluted that they can inspire a nervous breakdown. It is almost as though they want to hide data from you rather than make it easy to find it, or they express it in such an ambiguous way that you wonder if they really want you to use their products.
TI has taken the guess work out of switching supplies. Not only do they provide excellent datasheets, but the WEBENCH almost holds your hand as it leads you through the design. We used it to design both switching supplies on the Hummingbird board.
It all starts with an easy to use input screen, see Figure 1. The entire “program” is web based, so there is no software to load onto your machine. You enter the range of input voltages that your system will have (really important with a battery-operated device where the input voltage varies over the charge of the battery), the output voltage you want and the output current. The next window is a list of designs that meet your criteria listed by the switching regulator chip. They give you the part count, the entire regulator footprint size, the regulator efficiency and even the BOM cost for each design.
Figure 1 New design input screen.
It is no wonder that TI has 17% of the market share. We like to work with people that make our lives easier, more enjoyable and of course, in the end, more profitable.
It’s not all Plug-and-play
Don’t get the wrong idea, you still need to read the datasheet and make good engineering decisions. We have found a few landmines along the way. If you use every part that WORKBENCH gives you in the BOM, you are pretty safe, but what if your company wants you to use parts already in their library. And what if you go to buy the recommended part and find it has a 20-week lead-time? These things happen. By not carefully selecting a replacement, can lead to you having a smoky time.
Capacitors are not just about capacitance!
A few of the landmines involve capacitors. A switching supply has a fundamental frequency. It is just how they work. Some of the capacitors, large ceramic, have resonant frequencies in the same range as the switching frequency of the supplies, it’s an inductor thing. What does that mean? As you approach resonance the total capacitance seen by the circuit will change, it will likely decrease. Many of the switching supplies have a minimum output capacitance to remain stable. If your output capacitance drops below that minimum, well bad things happen, some involving noise, other involving smoke. That is why you may see a design with two 4.7uF caps in parallel rather than a single 10uF in its place.
Another problem is the ESR, also know as Equivalent Series Resistance. When you buy a capacitor, you get inductance and resistance thrown in for free. ESR is the resistance that looks like it is in series with the capacitor. At low current draw, this is not a problem, but if you try to pull lots of little electrons off the cap at once you will see a drop in output voltage. Most of the time you will not see DC effects, ESR is an AC parameter eating up your energy at higher frequencies. These frequencies can be anywhere from 100Hz to 100Khz depending on circuit parameters. At 22uF a standard aluminum cap can have an ESR between 7 and 30 ohms while a ceramic of the same value will be less than 0.015 ohms.
Don’t fear the Regulator (switching regulator that is)
While not as straight forward as a linear regulator, switching regulators have gotten much easier to work with. Their parts count has dropped and the chips, datasheets and tools have become much easier to use. We will let you know more about the Hummingbird Power Supply later this year. In the meantime, check out the TI WEBENCH Design Center on their website.
This newsletter is sponsored by Celtic Engineering Solutions LLC, a design engineering firm based out of West Jordan, Utah, which can be found on the web at: www.celticengineeringsolutions.com. You can find the newsletter on the company blog, LinkedIn or in your inbox by subscribing. Send your emails to The Celtic Engineer at: [email protected], with the subject line SUBSCRIBE.