How to choose regulators for PCB design
Unless you are working with a purely AC system, your PCB will need to receive stable DC voltage to operate properly. A voltage regulator circuit provides the necessary DC voltage with fixed magnitude, even if the input voltage (line) or the output current (load) changes. Some circuits are more sensitive to voltage fluctuations than others, and some power lines are noisier than others. Any designer should understand how to choose the right voltage regulator for their board. Let’s take a look at the different types of DC regulators and go through some factors to consider when selecting a voltage regulator.
Important parameters for choosing regulators
One question I often see on forums is how to choose a voltage regulator for different applications. There is no single answer to this question. You should consider the following characteristics when looking for a voltage regulator; note that these aspects apply to both linear and switching regulators:
If you need a step-up regulator, then you will have to use a switching regulator in boost configuration. If you are using multiple supplies in a system, typically one regulator is used for each supply. Regulators usually supply a fixed output voltage, although there are some regulators available on the market which you can use with some adjustable settings.
Efficiency vs. Noise
Switching regulators provide higher efficiency than linear regulators, but they generate more noise. Unnecessary electronic noise can interfere with other circuits when the regulator outputs a high current. If you want your product to hit the market, it can be more difficult to receive EMC certification. If other circuits on your board are sensitive (e.g., purely analog components), then it’s better to choose a linear regulator.
Selecting a regulator with high power losses can make it almost impossible to meet efficiency goals. Linear regulators are less noisy, but they are very inefficient (except LDO), meaning some power gets converted to heat. If your regulator will run at a high current, you should include heat sinks on the board. If you don’t have room on the board for a heat sink or if power dissipation is a concern, then a switching regulator might be a better option.
When output current changes quickly, there is a small spike in the output. The voltage regulator takes some time before it switches back to the same voltage. This is called transient response. The transient response is usually a function of the output capacitance and loads current. A fast transient response ensures that the regulator can deliver the required power. Check your component data sheets and look for the recommended bypass capacitor you should include on the regulator’s output.
Laying Out Each Circuit Block
Now that we can see the overall system architecture, we can get a sense of how to layout each circuit block in a switching power supply and the overall system to ensure low EMI and safety. Think about the entire block diagram when creating your PCB layout:
Layout in sections: Much like other boards with multiple functional blocks try to layout the power supply board in sections. It’s okay to do this in a linear fashion progressing from the input to the output in the block diagram.
Plan to layout with feedback: Sometimes, such as in precision high current regulators, you’ll have some feedback between sections. Use optocouplers to bridge the ground gap between each section.
Follow ground return paths: If there is any guideline that is universal in PCB design, it is probably “follow your ground return path.” For power supplies, this is critical for identifying locations where common-mode currents can develop and for ensuring low loop inductance in each supply section.
Pay attention to high current and high voltage rails: Designing for high voltage and high current are sometimes intermixed. The maximum potential difference between two conductors will determine their minimum spacing (see IPC-2221), and the current a conductor carries will determine its required width to ensure low temperature (see IPC-2152 for internal layers or external layers).