Have you ever hosted a dinner party, only to realize with horror that you didn’t cook enough food? Well, I have and the looks of confusion and disappointment on my guests’ faces still haunt me to this day. This is what happens when you do not adequately plan for everyone that needs to be fed. Similarly, not planning for all the loads on your boards will leave you with an inadequate power distribution network (PDN). That’s why it’s essential to properly design your boards power distribution network (PDN) so that no IC goes hungry. To be clear here, I’m talking about power supply and distribution rather than power integrity or quality. We’re concerned with getting the right currents and voltages to the correct places, not filtering noise from supplies or eliminating ground loops. The first things to consider when designing your PDN is what voltages your components need and how much current they draw. After you’ve nailed down those requirements, you’ll need to investigate what kind of power supply to use and how to route power to each part of your PCB.
Some of my friends are gluten-free or have other dietary restrictions, so when I have them over, I have to make sure they can actually eat the food. Similarly, different chips or components on your board may require varying voltages. Your required voltages can have an affect on the supply scheme you choose, so make sure you know them all. They will also each draw different amounts of current, so you need to be sure your supply can provide enough to ensure operation.
The most common voltage you’ll probably find on a PCB is +3.3 V, which is used for most ICs. Other components or circuits may need different voltages; for example, I have used an input protection chip that required +/- 5 V, so I had to procure a DC/DC converter to provide those voltages. Make sure you list out all the voltages you need and whether they’re AC or DC before you select your power supply so you can be sure you’re making an informed choice. For instance, if your circuit includes AC motors, you may want to bring AC power into your board and route it directly to the motors, then convert some to DC to power the rest of your PCB. You could also bring in DC and use an inverter to convert it into AC.
Sometimes, your application can also inform your initial supply voltage level and type. If you have a sensor operating outside in a bright place, you may want to use a solar panel to power your board. Solar panels output DC voltage that can be at varying levels, depending on how many cells are wired in series. If you’re building a PCB for a distributed Internet of Things (IoT) sensor, you’ll probably want to use a battery to supply power so that the device can be small and portable. However, you need to consider where your board will end up before you choose a supply.
It’s important to take current as well as voltage into account. Sometimes, through-hole components are more power hungry than their SMT counterparts and can draw more current, so even the form of your components can make a difference. Be sure to look at the power ratings for your ICs and calculate how much current they’ll need. This matters because every power supply has a limit as to how much current it can supply. If you’re using 120 VAC into a DC converter, you may think you can source an endless amount of current; however, that converter will have a max amperage that it can provide. Even different batteries have different discharge rates, so you’ll need to choose the correct type in order to supply the right amount of current for your board. You’ll also need to understand the peak power requirements for each component and ensure you can source the current for above nominal operation.
In many cases, though, you can use something as simple as a capacitor to provide the right amount of peak amps. Certain ICs have very short power spikes during operation. It’s often imprudent to oversize your power supply just so one chip can draw extra power for a few milliseconds. In this case, you can simply use a bypass capacitor to make up the difference. The capacitor will charge during the times that the component is operating at its normal level, then discharge when the extra current is needed. Just make sure the capacitor is large enough to cover the overdrive and you can lower costs as well as complexity.
Routing Power to Components
It doesn’t matter if your power supply can source enough current for a component if your traces can’t get it there. Conductor size has an effect on power transmission and board temperature.
Real estate on a PCB comes at a premium, which is why designers are often attempting to reduce trace width in various ways. Take care when doing this because if you don’t have enough copper to conduct the required current, your traces can fail. Just like in power grid distribution, you need to be sure that your conductors are large enough to handle the current passing through them.
Even if your traces are within spec for amperage, they may bring it out for temperature. Smaller traces have greater current densities, which adds more heat to the system as a whole. You may want to check for potential hotspots during the design or prototyping phase to ensure that your board doesn’t fail due to high temperatures. Adding more copper to the trace will lower the current densities and thus cool your board off too.
If you’re using low power ICs, you will also need to be aware of voltage drops. Supply voltage will drop as you go down long traces with high current densities. If you’re using a low power chip, it may be significant enough that the chip will operate intermittently, or not at all. Make sure you calculate voltage drop across your PDN and adjust trace width to ensure all your chips get the voltages they need.
There’s no greater social faux pas than running out of food when hosting a dinner party. At best, your friends will leave hungry and sad, and at worst, they may never come back again. The stakes are a little higher in the electronics world. If you don’t supply your board with the right voltages or currents, your PCB may work intermittently or not at all. Before choosing your supply, it is important to know each voltage you need on your board, and understand how much peak and nominal current everything will draw. After you’ve selected the right power source, you’ll need to make sure that the rest of your distribution network can channel the electricity where it needs to go.
|Tempo‘s Custom PCB Manufacturing Service|
Here at Tempo Automation, we may not be much good at planning parties, but we certainly know a lot about PCB design and manufacturing. Follow these tips and your customers are sure to come back for seconds.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.