Embedded System Power Supply Guidelines for Power Integrity
This system will require its own power supply and regulation
Embedded systems that require a significant amount of processing power and that need to interface with analog components require important embedded system power supply guidelines in order to ensure power and signal integrity. Whether you design your own external power supply, or embed a power supply directly alongside your board, you’ll need to follow some important power integrity practices when laying out your power supply.
Your Power Supply Design Strategy
Smaller embedded systems, such as a small UAV, single board computer, or similar systems do not normally require huge battery packs or dedicated power supplies. With larger systems, such as the mass of electronics in automobiles, you’ll have a large power source that supplies voltage and current to a number of electromechanical and electronic systems simultaneously. You’ll need to have a rough idea of your power budget in all areas of the system before designing a power delivery network for the components in your embedded electronics.
Any power supply design and power conditioning system should meet the following requirements:
Stable voltage/current output: Your output should have as little noise as possible. This involves suppressing ripple (for AC->DC->DC conversion), suppressing or filtering switching noise (DC->DC conversion), and designing the rest of your board to meet EMC standards.
Maximum output current: You can generally get a decent estimate from assuming the current maximum values from datasheets for your components that will appear in the load.
Transient immunity: Many embedded systems carry standards that dictate the level of required immunity a system should have to the transient response from a power supply. This is related to the slew rate of a component receiving a transient power signal and is typically measured in V/μs.
EMC standards: Active components in switching regulators or buck-boost converters can create noise that interferes with nearby components. Designers should familiarize themselves with design guidelines to ensure EMC when designing a power supply for an embedded system.
Embedded System Power Supply Layout Guidelines
Choosing the right power supply topology can aid noise suppression in the output power signal. Noise output from the primary power source can propagate into the load electronics as EMI if the output is not put through power conditioning. As an example, the output from an alternator in an automobile or aircraft contains some residual ripple from a full wave rectifier circuit. Feeding this DC signal into a linear regulator with low dropout voltage can suppress ripple while stepping down the output voltage to the appropriate level for your embedded system.
Another option with a DC power source is the use of a buck-boost converter, as this allows the voltage to be adjusted by changing the PWM signal sent to the FET in the converter. A linear regulator can also be used with a switching regulator to reject more noise. The downside with any switching power source is that the active switching electronics generate their own noise, which can cause power integrity problems in downstream components when the converter outputs high current.
Be careful when embedding a customized switching regulator onto the same board as your embedded system as these regulators can produce significant EMI. Nowadays, power management ICs are available that integrate regulation and conditioning into a single component, and these are specifically designed to be compatible with embedded systems. They typically require their own continuous ground plane and proper bypassing to ensure power integrity.
Whether you included a regulator on your embedded board or on a separate board, you might also require some shielding techniques to prevent radiated EMI from interfering with components. In the case of high current switching regulators, it may be better to place this on its own board and add some modest shielding and filtering to prevent EMI problems.
Some of the components in a switching power supply
Other Design Aspects: Stability and Transient Immunity
When the primary power source is battery power, the output from a battery is not constant over time and may slowly drop off during operation if the batteries are not being charged. If you are using a buck-boost converter, you may need to include a circuit that can actively adjust the PWM signal sent to the switching FET in order to maintain steady DC power to your components. This can typically be implemented in an MCU, FPGA, CPLD, or other programmable device that supports the primary functions of your system.
At the PCB level, you’ll want to make proper use of bypass/decoupling capacitors on critical components to ensure further noise rejection from a switching power supply. Sufficient noise rejection in any components that process data is critical. Your capacitor network needs to be able to provide sufficient current upon discharging to counter any transients that interfere with DC power reaching your active components. It also should be placed with minimal loop area.
Finally, the loop inductance and impedance of any bypass/decoupling capacitor network should be as low as possible. This means placing your bypass/decoupling capacitor network as close as possible to the ground/power pins on an active component. This ultimately improves transient immunity of a component and can help ensure your board meets important design standards.
The Final Step: Power Integrity Simulations
One important aspect of ensuring power integrity throughout your device lies in simulations. It is important to examine power integrity throughout your power delivery network before sending your embedded device off for production. This allows you to identify the formation of hot spots in your board and examine the transient response in different signal nets.
Determining the immunity of different components to transient switching is important in any embedded system. You’ll need to ensure that your power supply can maintain steady DC output when many ICs switch simultaneously. Any power/ground potential change produced by simultaneous switching, called ground bounce, can lead to involuntary switching in upstream digital circuits in severe cases.
Current density (left) and voltage (right) in signal nets returning to a ground plane
Identifying power integrity problems is extremely important in embedded systems, and you’ll need design tools that can help you prevent these problems when designing an embedded system power supply. Altium Designer contains all these features and many more in a single program. Altium Designer includes a powerful PDN analyzer as an add-on that is conveniently available within the program.