Optimizing Circuits For Low Power
Originally written for Maxlab
Power consumption is the main limitation when it comes to portability. Therefore, it is necessary to pay attention to the power consumption of the entire system. Minimizing the overall power consumption of such a device is essential as it favors the lowest possible battery weight, battery life, and size requirements to take advantage of runtime. Therefore, in portable devices, low power design is an important design consideration.
DC/DC component selection
System power requirements
As with most power supplies, the primary consideration in selecting a DC/DC converter is the output load power requirement of the system. DC/DC output requirements include the output voltage and current supplied by the converter. Output voltage may have tolerance specifications that depend on environmental conditions such as input voltage, output load current, and ambient temperature. The load current requirement specification should include minimum, maximum, and typical values.
DC/DC converter application requirement specifications differ from AC/DC power supplies in that the input voltage is not standardized like AC/DC power supplies. When choosing a DC/DC converter, it is necessary to specify the range of input voltage that will be applied to the converter.
Regulated/unregulated output voltage
Most DC/DC converters, like AC/DC power supplies, produce a tightly regulated output voltage. Applications that can tolerate an unregulated output voltage can use smaller or less expensive converters. A choice of regulated or unregulated output voltage is often only available in small power converters.
Isolated/non-isolated configurations
An isolated converter is designed so that the input and output circuits are galvanically isolated and there is no DC current path between the input and output. These converters are often used to isolate input and output circuits for electrical noise or hazardous voltage isolation.
An isolated converter can have multiple output voltages from a single converter. The non-isolated converter has a DC connection between the input and output through a common ground between the input and output circuits.
This class of converters can be smaller and cheaper than isolated converters. Some non-isolated converters can produce a negative output voltage from a positive input voltage.
Package type and mounting
DC/DC converters are available in a variety of housings and mounting styles. Surface mount (SMT) or through hole mount (THM) as well as single in-line pin (SIP) or dual in-line pin (DIP) configurations for applications where the converter is mounted directly on a printed circuit board.
Chassis mount converters are available for applications requiring this type of mounting. Many converters are available in DIN rail mount configurations for industrial applications. Both open and sealed converters are found in most housing and mounting configurations.
EMI, EMC and safety concerns
Most electronic products sold must meet regulatory requirements for EMI and EMC (electromagnetic interference and compatibility). Regulatory requirements are designed to ensure that products do not interfere with the operation of other products and that external electrical noise does not interfere with the proper functioning of certified products.
DC/DC converters can be certified to meet regulatory requirements, but in most applications the complete system is certified and the internal subcircuits do not require certification.
As with EMI and EMC regulations, most electronic products sold must comply with government safety regulations.
Products are given end-product safety certifications as well as EMI and EMC regulatory certifications. Certification is often not required for internal subcomponents, but can be obtained if required for current flowing through a protective conductor to earth.
Current that can flow from the surface of a conducting or non-conducting part to earth if there is no earth connection or if there is a conducting path (such as the human body). External currents always flow through protective conductors.
Typical current leakage issues
There are two types of leakage current: AC leakage current and DC leakage current. DC leakage current generally applies only to end-product devices, not power supplies.
AC leakage current is caused by the parallel combination of capacitance and DC resistance between the voltage source (AC line) and the grounded conductive part of the equipment.
Leakage currents caused by DC resistance are usually insignificant compared to AC impedances of different capacitances connected in parallel. Capacitance can be intentional (like an EMI filter capacitor) or unintentional.
Examples of unintentional capacitance include printed circuit board gaps, isolation between semiconductors and grounded heat sinks, and primary-to-secondary capacitance of isolation transformers in power supplies.
Power switch design
The power switch provides the electrical connection from the voltage source or ground to the load. Multiple voltage rails save power and protect subsystems from damage. It also improves component protection, inrush current protection, and minimizes PCB size.
There are several circuit breaker topologies with different features for different applications. Load switches form the basis of circuit breakers by providing safe and reliable power distribution.
Applications that commonly use load switches include power sharing, power sequencing, inrush current control, and leakage current reduction. Integrated power MUX devices are similar to load switches, but allow multiple input sources.
This set of electronic switches is used to select two or more input power paths to switch to one output while providing input power protection. The eFuse and Hot Swap Controllers provide additional input power path protection features such as current sense monitoring, current limit, undervoltage and overvoltage protection, and thermal shutdown.
This makes these devices ideal for hot plugging and transient events that can damage system components. These benefits help reduce system maintenance costs and maximize equipment uptime. The ideal diode-OR controller protects against reverse-battery conditions by monitoring an external FET to significantly reduce power consumption and block reverse current. Whenever a transient event occurs, the controller monitors and adjusts the external FETs to prevent damage to upstream components.
An intelligent high-side switch is used for off-board load protection. They monitor output load current and provide additional diagnostic telemetry to detect short and open circuit events.
The smart high-side switch has an adjustable current limit, allowing for more reliable integration into start profile applications with large inrush currents or low peak currents. Adding an intelligent high-side switch to your design gives you a smarter, more robust solution for driving capacitive, inductive, and LED loads.
Power profiling tools
The purpose of a power profiler is to plot the current (and associated charge) drawn in real time, over a certain period of time.
Nordic Power Profiler Kit
- Instantaneous and average current measurement on all Nordic DKs in addition to custom boards
- Variable supply voltage from 0.8V to 5.0V (software configurable)
- Maximum current measurement from 1A
- Up to approx. 200 nA accurate measurements
- Up to 0.2 µA resolution
- 100 kS/s sampling rate
- Automatic switching between 5 current measurement ranges for optimum resolution
- Greater than ±20% measurement accuracy (average current measurement)
- USB communication for easy portability to other devices
- Desktop application for meter analysis
- Real-time power meter display
- 8 digital inputs to support low-end logic analyzers
- For Power Profiler app on Desktop Supported by nRF Connect
STM32CubeMonitor-Power
- Compatible with STM32 Nucleo development boards
- Built-in display that allows current to be read directly and eliminates the need to use a multimeter
- Measurement range - 100nA to 50mA