Power system is in it's basic form contains, power generation , power transmission, power conversion , and lastly power usage. The large scale power system that are familiar to everyone is consisting of power plant for generation, high voltage transmission lines, substation and utility pole transformers, and lastly end customers. The lighting system, electrical equipment and electronic products are the end node on the edge of this massive cloud of power networks.
Likewise, power system architecture and its component of an electronic product is more of less mirror the same basic building blocks.
The goal is to equip electrical design engineers to strength their understanding of the basic principle of power system design through examples, design trade-offs, and best practices.
The main task in power system design is how to transfer energy from source to sink properly. power source design is based on sink requirements. So to start with any power system design, we must go propagate backwards from sink and trace the way back to source. The sink in power system is can be thought as a resistor with various power consumption characteristics, such as a constant power load (screen panel), constant current load ( battery), and highly complex load (processors)
AC output from US runs at 110Vrms/60Hz.
Typical power supply unit/charger rating
Laptop USB charger: 60W ~20V/3A
Phone USB charger: 18W ~9V/3A
Set top Box power supply: 15W ~12V/1.25A
4k Streaming Stick power supply: 5W ~ 5V/1A
A power system of a electrical product is consisting of AC adapter, cable, power management unit, voltage regulators, and point of load regulation (e.g. dedicated regulator for processor cores, memory, I/O pads, etc.), and lastly power distribution network that distributes power and return current from source to sink to ensure digital and analog circuit performance.
System Block Diagram
As seen from above block diagram, we bring AC power to each subsystem via power conversion, voltage regulation, and lastly complex power distribution network.
AC adapter: it is the source of electricity. It coverts from AC main voltage (120V) down to DC voltage needed for electronics. It's job to provide fixed voltage regardless of loading conditions.
Charger: for battery powered device, the charger takes power source from the ac adapter , regulate its charging current for the battery as well providing a regulated system voltage going to rest of the power network. Modern chargers also include other functions such as over voltage protection, short circuit protection, etc to protect internal circuitry.
Power management unit: It generally handles power control and power conversion for the product. Specifically, battery charging, voltage conversion and regulation, power-on/off sequencing, and supervising (over current projection, over voltage protection, under voltage lockout ,etc.). In the above example, we used two PMICs to illustrate that due to complexity of the syste, two different PMICs might needed to meet the PDN requirement for the network.
Voltage regulator: it's a general term describing discrete power chips that does voltage conversion and regulation. Regulation means maintaining a fixed output voltage given that input and output load condition can be dynamic.
Point of load regulation: it's a general term describing a dedicated regulator that is needed for specific sink power supply such as core or memory supply pins of a processor. In practice, these regulators are placed in close proximity to the load enhancing it's power delivery effectiveness.
LDO; LDO is a linear regulator that uses a simple FET and a feedback network to regulate it's voltage output. It's ideal for low current and high noise immunity load.
DCDC SMPS: it's a switch mode power supply that is very efficient and is widely used for high current loads without stringent noise requirements such as digital cores of a processor.
Note: For more information about LDO and DCDC design selections. Please navigate to fundamental-> power regulators.
Power distribution network: it's power transmission path of the PCB board including power plane, bulk and decoupling capacitors, and return current ground planes in order to meet the system power supply distribution requirements. Validation of PDN requires looking at DC as well as AC current paths to make sure each path impedance is under the certain frequency vs impedance limit specified by the chip vendor.
What is AC adapter rating depended on?
It's depended on type of battery and maximum charging rate, plus worst case system loading. The BC1.2 and USB PD specification dedicated the type of chargers and its rating. As an engineer, we need to choose the right standards with right sets of capabilities to meet the system load requirement. In practice, the adapter design can be reused from generation to generation to save cost. So the power rating is limited so as the complexity of the product involves, the designer has to optimize power efficiency and subsystem inseparability to reduce the total peak current load to be under the max current rating of the supply,
How does charger simultaneously charge the battery and provide system power?
Battery charging is mostly a constant current process; hence the charger has a built-in current source that charges the battery while also maintaining a system voltage around 3.6V. When battery voltage rises from low voltage 3.0V to around 3.6v=V, the system voltage also increase to track the battery voltage closely. The reason for the battery voltage to track system voltage to minimize the reduce power loss across the current source due to minimal voltage difference.
How to choose between a DCDC POL vs a LDO POL?
LDO has the advantage being low noise and rejects power supply noise in order to achieve low noise output. This low noise and high power supply noise rejection is needed for sensitive circuits such as ADC inside camera and sensors. Furthermore, LDO has roughly the same power efficiency as that of DCDC when current is under 100mA as rule of thumb. However, if a load consumes high current and does not have stringent output regulation requirements, DC to DC regulator used to improve power efficient and elongates battery usage.
Note: LDO efficiency does not dependent on current. LDO should be used for loads in 100 mA range just for purpose of cost saving. In the ampere range, DC to DC switching mode power supply must to be used to improve efficiency of power conversion.
Summary and Conclusion
In this article, we went over the principle of power delivery from a source to a sink and the stages involved in between. We have learned that power conversion is not just a passive behavior, we need to turn on or off based on processor or other digital circuits. Lastly, we learned that design cost constraint is very real, and an good system engineer needs to also consider power optimization by interleaving peak task loads so the overall peak draw does not exceed the power supply rating, hence not unexpected system power brown causing a device reset.