Efficient and durable power supply is the backbone of any IoT Product. For a wearable or self-powered wireless device, power consumption and battery life are a big concern. In the coming years, with billions of “things” about to be connected to the Internet, power conservation is going to be a critical factor in deciding the economic feasibility of any IoT deployment. Apart from battery-powered devices, this is equally applicable for IoT gateways which run on 24/7 mains supply. In this post, we will discuss the essential design features needed to meet the requirements of an efficient power supply. Based on my experience of developing a power supply unit for a portable BLE IoT endpoint, I have also presented a prototype reference design of a battery charging circuit for standalone IoT devices.
So you are a hardware engineer, and you want to design a power source for an IoT device. At first, you might be tempted to choose a long lasting battery. But that’s not all. While choosing a battery, we must also take into account the following essential considerations
There are a wide variety of batteries available in the market, ranging in different form factors and electric charge capabilities. Here are some of the options available to us.
If the product is a “use and throw” kind, then Li/MnO2 battery is your best choice. It is available as a compact cylindrical or button-shaped package. If the product requires long lasting power and rechargeable option, then Lithium-Ion and Lithium Ion Polymer batteries are better choices.
Li-Ion & LiPo batteries offer thin packaging, which makes them very popular with mobile phones
Selecting a battery in today’s electronic market is quite challenging. As a hardware designer, you have many options. But the first thing that you need to know is the mAh rating of your desired battery. This is directly proportional to power consumption. The more power the device consumes, the higher becomes the mAh rating that you need to look for. Apart from power consumption, you should have some idea about the form factor of the final product. If the product requires compact batteries, the best choice will be LiPo which is available in in various ultra-thin form factors.
Here is a general strategy for power conservation. Most of the time, devices consume maximum power while communicating with interface peripherals or remote devices via wireless. Often, this happens as part of background activity. You would have surely experienced it when your mobile phone drains out faster with mobile data setting enabled. So as a thumb rule, put the device in sleep mode and wake up only when communication with the peripheral is needed. Most SoC vendors provide low power features for reduced power consumption and idle/hibernation modes. These features allow the device to be programmed to bring down the hardware to sleep state. Apart from this, the device should regularly monitor the battery level. When the battery is about to drain completely, the power supply hardware should power off or pull the device into hibernate state to avoid further damaging the battery.
In the case of IoT, most devices need rechargeable batteries. The obvious choice is LiPo or Li-Ion battery. Now let’s design a battery charging circuit. But before we get into the nitty-gritty of design, let us look at some of the essential features that every battery charging cum power supply circuit must have.
Note: Rechargeable Lithium battery charger works on the principle of high-performance linear charge with 1% power dissipation with charge accuracy of 1.5%. The charger circuit has an over-current protection of up to 3 Amps.
As a thumb rule, for charging a 3.7v LiPo / 3.6v Li-ion battery, the charger circuit should drive 4.5 V to 5.5 V and 1A current.
The circuit contains three major components,
The TP4056 is a complete constant-current/constant-voltage linear charger for single cell Lithium-Ion batteries. It has thermal feedback mechanism to regulate the charge current to limit the die temperature during high power operation or high ambient temperature. The charge voltage is fixed at 4.2V, and the charge current can be programmed externally with a single resistor. The TP4056 automatically terminates the charge cycle when the charge current drops to 1/10th the programmed value once the battery attains maximum voltage.
The DW01-A battery protection IC with MOSFET 8205A is designed to protect Lithium-Ion/Polymer batteries from damage due to overcharge, over discharge, and/or overcurrent. This also protects against degradation of recharge lifecycle. The ultra-small package along with minimum external components makes it ideal for integrating the DW01-A into the limited space available within a battery pack. It provides an accuracy of ±50mV overcharging detection voltage which ensures safe and full utilization of charging. It also has a very low standby current which drains very little charge from the battery while in storage. The MOSFET 8205A makes it possible for users to monitor the status of the Lithium-Ion rechargeable battery. A built-in voltage detection circuit with high accuracy and a delay circuit aids in monitoring the battery.
The above circuit schematic is also available as off the shelf PCB unit, and most local electronics market dealers would stock it. Check the specification of the circuit you are buying to see if the components used are the same as described in the schematic here.
For the benefit of those curious minds, I am offering a bonus. If you would want to design your PCB and perhaps do some modifications or enhancements to the existing circuit, I have provided the Allegro OrCAD PCB design software files.
The entire OrCAD project folder is available in GitHub under this repository.
Thumbs up! We have designed a high-performance power supply with battery charging circuit for powering your IoT product. There is only one limitation. This circuit does not provide safety on reverse polarity which can damage the batteries. So make sure that the battery is hooked up to the charger with the right polarity.
If you are working on designing power supply systems for IoT then I would love to hear from you. Please share your experiences or any specific challenges that you might have overcome through your design. And in case you want to replicate this design in your work and have any technical doubts then you can post your comments below and I will get back to you.
Surya is a hardware buff and tinkerer who loves all the stuff around IoT. He is a specialist in building proof-of-concepts around electronic hardware and conceptualizing use cases and solutions for Internet of Things. When not working with hardware, Surya can still be found fiddling with some crazy hardware design, trying to bring out an idea out of a raw circuit.
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