Hot garments like jackets, vests, and gloves, and even insoles, rely on safe, soul-portable low-voltage battery platforms 2S Li-ion (7.4V) and 3S Li-ion (12V). The voltage choice in the first phase of product design plausibly impacts all of the characteristics of the printed circuit board assembly (PCBA), such as the current managing, trace scheme, component rating, power conversion plan, protection circuit, and thermal control.
A volume of a voltage platform which is not correctly matched can lead to either oversized battery packs, overboard heating, lower runtime, undermined safety margins or unwarranted complexity and cost.
System Voltage and PCBA Design Logic
Direct operating voltage influences the current directly, thermal behaviour during the PCB, and size of power-critical components.
Current Load and PCB Trace Width Requirements
At a given heating power (P = V I ), the value of V is lower and the value of current diminishes.
- One 30W heat region at 7.4V is loaded with about 4.05 A.
- Only a little over 2.5 A is required in the same zone at 12 V.
The reduced current of 12V designs enables smaller traces, reduced copper coverage, decreased vias, and less rigid board designs thinness – all these are very welcome in space-sensitive wearable designs. On the other hand, 7.4V define the existence of a trace width calculation, wider power planes, and in some cases use more copper pours, in order to control I 2 R heating where the board itself is concerned.
MOSFET and Power Switching Component Selection
Losses in conduction (I 2 R DS(on)) are increased in the lower voltages as the current is increased.
- The typical design requirements of 7.4 V are low R DS on-MPOSFETs (typically less than 10 m) with VDS ranging from 20-30 V and logic-level gate drive.
- Systems operating at 12V allow slightly high values of R_DS(on) with comparable efficiency.
Requirements on the type of gate driver can also vary: Gate-driven systems operating on 7.4V typically require actual logic-level MOS location-based gate-drives, or specially designed gate-drive ICs, whereas 12V systems are less restrictive in their the choice of a gate driver.
DC-DC Conversion and Auxiliary Power Rails
Secondary low-voltage rails (3.3 V or 5 V) of microcontrollers, Bluetooth modules, sensors, and user interfaces are common on most of the heated wearables.
- The smaller buck converters with wide input voltage spans can be comfortably made to operate at 12V systems.
- Systems 7.4V (nominal 6.084 V) work nearer to the minimum input voltage of numerous buck ICs, and sometimes so near the lowest possible voltage of many important batteries, that SEPIC, flyback, or buck-boost topologies are needed to ensure constant output.
The paper gives a complete overview of the considerations at the system level,see: low-voltage PCBA architecture for heated wearables.
7.4V vs 12V — Electrical Design Trade-Offs
Both voltage platforms come with its own pros and cons on major performance measurements.
Power Efficiency and System Losses
The typical 12V architectures have higher end-to-end efficiency because there are lower I 2 R losses in:
- PCB traces
- Internal wiring
- Connectors
- Heating elements
This benefit is greater in high-power designs (>50 total -60 W) where the layout discipline has to be strictly followed and high-quality components.
Heating Response Time
Greater voltage can give quicker current increase by heating circuit (and especially carbon-fiber ribbons and thin-wire heaters), so initial warm-up can be achieved much more quickly – a significant consideration with regard to user satisfaction in cold conditions.
Battery Pack Complexity, Weight, and Cost
- 7.4V (2S) are more lightweight, and less complex and costly to manufacture and defend.
- 12V (3S packs) must factor in balancing cells, more advanced cell protection, and marginally higher size which is fine; however one can probably utilize more total power with similar mass when long capacity cells are used.
To have a detailed information about the runtime, charging and user considerations in each platform, refer to: 7.4V vs 12V battery systems for heated clothing.
Safety, Protection & Compliance at Low Voltage
In spite of the fact that the operation of both of them uses the SELV (Safety Extra-Low Voltage), body-worn Li-ion systems still need good protection against electrical and thermal malfunctions.
Over-Current, Short-Circuit, and Thermal Runaway Protection
Both voltages demand:
- Quick response over-current protection (through sensitive resistor or in-built fuel-gauge IC)
- Hardware-level short- Circuit protection.
- Multi zone thermostat with automatic shut off.
NTC thermistors are used on the PCBA either on heating components as is, or adjacent to power components and directly on the heating elements.
Cell Balancing and Battery Management Complexity
- Passive and active balancing is also required from packs of practically all types (3S (12V) ) to keep every cell in step, which can be costly and consume board space.
- This protection layer is important to the conformity with standards like IEC 62133-2, UL 2054, and new wearable specific safety standards.
More guidance on these kinds of safeguards can be given in practice engineering by: see: battery protection circuit design for heated apparel.
Temperature Control & User Comfort
Wearable heating is best suited and demanded to perform one function, which is to provide stable and predictable warmth.
PWM vs Linear Control Methods
Pulse-Width Modulation (PWM) aside with power MOSFETs is the utilization in industry because of:
- Near-zero switching losses
- Low heat production in control electronics.
- Good linkage with microcontroller timers.
The efficiency losses associated with linear regulation are unacceptable, and it is hardly ever used in modern designs.
Sensor Placement and Voltage Influence on Temperature Stability
Other designs often use multiple NTC thermistor types, one connected to each heating zone providing real time feedback on temperature to a microcontroller indicating the PWM duty cycle to apply.
Since V R varies directly with V 2, a constant change in duty cycle induces a greater absolute power change at 12V than at 7.4 V. It is possible to cause slightly coarser temperature granularity in 7.4V systems without fine tuning of PID parameters.
The developed features are ambient temperature compensation, motion sense and a variety of user-choosable warmth settings.
To learn more about how these circuits are applied to provide measures of comfort to real-world applications, read: temperature control circuits in heated wearables.
Conclusion — Choosing the Right Voltage Platform
7.4V systems are generally preferred when the priorities include:
- Minimum weight and volume
- All simplest battery architecture.
- Lowest component cost
- Plausible power needs (light vests, gloves, insoles, etc.)
12V systems tend to be more suitable for:
- Of the systems that are more appropriate are:
- Full-body garments that have high power.
- Applications that require shortest warm-up time and maximum run time.
- Designs that are of optimal efficiency and thermal margin.
Depending on the platform, the PCBA needs to be designed in more of a holistic manner, i.e. both connector choice, power management/allocation, protection, temperature, etc. and not just scaling a generic board to different voltage families.
To realize the desired consistency across performance, regulatory compliance, and user satisfaction and reliability of the product designed in harsh environment such as body-worn heated apparel, it is crucial to make the voltage choice early and design the whole power delivery chain as a complete system.