FEATURE POWER ELECTRONICS
TOP 11 TIPS FOR SAVING POWER IN LOW POWER WIRELESS RADIO SYSTEMS
The IoT encompasses a plethora of low power radio devices connected to the Internet. These devices require careful design to prolong battery life, reduce environmental impact and to maximise manufacturer margins. Here Bytesnapexplores 11 considerations when reducing the power consumption of radio systems
1. SYSTEM CHOICE The type of radio to use depends on several factors, principally data rate, communication range and operating costs. The range of options is considerable from short-range ZigBee, Thread, Bluetooth and Wi-Fi, to long- range low power radio like LoRa and SigFox or cellular solutions.
2. WAVELENGTH Some radio wavelengths propagate much better than others – and this propagation may be correlated with increased power-efficiency. Typically, lower frequencies propagate better than higher frequencies, but the compromise is that the possible data rate is lower. Mesh networks conversely manage to stretch the transmission distance by hopping the data between nodes.
3. MICROCONTROLLER SLEEP POWER There’s a wealth of microcontrollers claiming to be low power. Some, however, have very low power sleep states only, whilst others are low power when running also. Depending on how often a device is asleep affects which the key parameter is. If your device is normally asleep, look to the deep sleep power consumption. If however it is always awake, for instance to listen to a network, then it is the operating power consumption that is key.
4. POWER SOURCE Power is a key consideration in restricted devices. Once you’ve decided if user- replaceable (AA, AAA, etc.) or rechargeable such as Li-Po is appropriate for the product, there’s plenty of optimisation to do. Environmental factors (intense cold or heat) will have a big impact on life, as will the battery discharge curve. Low power radios tend to use very little power whilst sleeping and then use large pulses of current when awake for receiving and transmitting, which some types of battery may not handle particularly well.
24 MAY 2018 | ELECTRONICS
8. ANTENNA TUNING If range is important, remember to tune your antenna – this maximises what you’ve got without adding to the power budget of your system.
9. TX POWER Don’t unnecessarily boost your output power beyond what you need. If your radio link only has to reach ten metres, for instance, 5dB output power is unlikely to be required.
5. PULL-UPS AND OTHER DESIGN TIPS Low power wireless radio system design requires attention to detail outside of just the radio circuit, such as pull-up resistors. These need to be optimised to maximise your battery life, for example by adding active components such as FET’s to turn devices and pull up resistors on and off. Choose FETs carefully. Even though you
may only be taking a few milliamps you’ll find that a higher power device rated for a few amps will have a lower VF (forward voltage), meaning less power is wasted in the FET when it is on.
6. SPREADSHEETS Using a spreadsheet can be incredibly useful to determine what compromises should be made to make the design workable.
A spreadsheet can be used to see how
compromises in transmit or receiver timing can affect the battery lifetime, how long the transmission should last, and also to determine efficiencies of regulators over the voltage range and current requirements.
7. POWER MEASUREMENT Standard multimeters have low sample rates that will miss the short RF bursts associated with transmission. Either use an oscilloscope maths function or a high frequency multimeter instead for power measurement. Alternatively, sum the power over a long period with a watt hour meter.
Figure 1:
ByteSnap - LoRa low power board
10. BATCH TESTING With battery operated low power devices you can be operating close to limits of component performance. With FETs and other active devices, where you rely on a low voltage drop for example, there will always be a variation in the characteristics between devices which may affect the performance. It’s worthwhile doing batch testing with a good sized batch to ensure any variation will not compromise operation of the device. Simulation of some simpler aspects of the design using a SPICE simulator, running tests across temperature and voltage extremes, can similarly save pain in mass production.
11. TRANSMITTER PULSES When the transmitter is on, a low power radio is at its maximum power state. It makes sense to minimise that ‘on’ time - minimising the amount of data that is transmitted - possibly by using binary, or compressing larger files using .zip format. When developing a smart home product, remote monitoring devices or a whole host of other IoT solutions, these 11 considerations are always taken into account by our embedded design consultants when low power consumption is a requirement, so Bytesnap hope that these tips are of use on your next low power project.
Bytesnap
www.bytesnap.co.uk T: 0121 222 5433
/ ELECTRONICS
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