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Battery powered lawn and garden tools are steadily replacing their noisy gasoline-powered counterparts, thanks to the high performance of lithium-based batteries. To satisfy the demand for higher power and longer runtime, battery pack voltage and capacity are steadily increasing. However, high energy density and small form factors create significant challenges for equipment designers, the foremost of which is product safety.
Modern homeowners and professionals demand high power output from outdoor tools, and battery powered equipment manufacturers have delivered. Power output is surprisingly high, and the tools are extremely convenient, eliminating the need for homeowners to fill and store containers of volatile gasoline. Shorter runtime is easily overcome with a quick spare battery swap and recharge. All is not roses however as this rapidly growing industry struggles with battery durability and high certification cost.
Battery Pack Construction
The preferred battery chemistry for power tools is lithium manganese oxide (LMO) or lithium nickel manganese cobalt (NMC) because of high cell voltage. Cylindrically shaped battery cells are series-connected to achieve the desired battery pack voltage, and fewer cells yield a lighter, more compact battery pack. Meeting the demand for high power requires high series cell count, designated with suffix S. Common battery pack configurations range from 10S to 20S corresponding to nominal voltages of 32 to 80 V depending on chemistry.
With higher cell count comes increasing risk of cell voltage mismatch, corresponding to charge mismatch. Cells with lower-than-average voltage not only degrade battery pack performance, but cells with higher voltage could overcharge, which is extremely hazardous for lithium-based batteries. Preventing charge imbalance requires intelligence within a high cell count battery pack.
One approach is for the charger to check for cell voltage imbalance, and if out of range, the charger simply refuses to charge the battery pack. This maintains product safety but leads to frustration as expensive batteries quickly become useful only as garden shed doorstops while the tool sits idle. However, the service life of battery packs can be greatly extended.
Simply Reliable
Clever schemes exist to shuttle energy from higher to lower voltage cells through a matrix of power interconnections and one or more DC-DC converters. These are called battery management systems, and they include power conversion. A much simpler solution is to alternately partially discharge energy in higher voltage cells, with heat dissipated in power transistors and resistors. This is the approach taken in Qorvo’s Power Application Controller (PAC) series of battery monitoring systems (BMS). Each Qorvo BMS does the following:
- Precisely measure all cell voltages and current
- Measure battery and internal temperatures
- Automatically interrupt battery current in emergency conditions
- Function as a battery disconnect when not in use
- Actively balance cells with transistors and resistors
Figure 1. PAC2xxxx BMS cell balancing with external transistors
Each PAC2xxxx BMS can balance cell voltages with internal 25 Ω FETs that support up to 50 mA, limited by internal heating. However, outdoor tools usually require higher balance current, so the internal FETs control external transistors capable of hundreds of milliamps. One implementation is shown in Figure 1 where the internal FET biases a PNP transistor on with a voltage drop across resistors Rf.
Much higher current flows through the PNP and load resistor to partially discharge a higher voltage cell, which could be anywhere in the battery pack. This balancing process happens while the tool operates with sufficiently high current, or while the tool is idle. Balanced cell charge yields long runtime and service life.
Balancing cell voltages requires highly precise voltage measurements, and for this reason, each PAC2xxx BMS includes a 16-bit analog-to-digital converter (ADC) dedicated to cell voltage measurement and safety checks. Battery current measurement also requires high precision to enable Coulomb counting for battery gauge and service life calculations. A second 16-bit ADC serves this purpose.
Figure 2. PAC2xxxx BMS battery disconnect and circuit breaker
Integrated within each Qorvo BMS are protection circuitry and drivers for external MOSFETs, working together to function as battery disconnect and circuit breaker. Figure 2 shows a simplified schematic with two common-drain MOSFETs that can separately block charge or discharge current, automatically interrupt any fault current, and disconnect the battery from the power terminals when not in use. Both MOSFETs are on during normal charge or discharge operation to minimize conduction loss.
Discharge in low power/precharge mode shuts down the charge pump and switches on an internal source-follower mode MOSFET. A third MOSFET drives an optional self control fuse (SCF) and/or an LED or relay. The integrated charge pump provides power for the high-side gate drivers, minimizing component count on the circuit board.
Figure 3. PAC intelligent BMS and motor controller functions
Firmware development is a huge investment for outdoor power equipment manufacturers and therefore a critical factor when selecting both BMS and motor control solutions. Safety and performance requirements are also increasingly stringent. The motor controller must work seamlessly with the BMS and allow code reuse as the product roadmap evolves. Qorvo intelligent motor controllers address each of these challenges and complete the PAC product lineup.
All PACs share most functions as shown in Figure 3. The PAC22xxx BMS and PAC52xx motor controllers contain a 50 MHz Arm Cortex-M0 32-bit microcontroller core with 32 KB of embedded flash and 8 KB of SRAM. The PAC25xxx BMS and PAC55xxx motor controllers contain a 150 MHz Arm Cortex-M4F 32-bit microcontroller core with floating point unit (FPU), 128 KB of flash, and 32 KB of SRAM.
They all contain peripherals, including pulse width modulation (PWM) and other timers, ADCs, various serial communications, and security. Each PAC has industry leading hibernate mode that keeps a battery pack ready for use with full charge even after months of storage.
The Configurable Analog Front End (CAFE) inside each PAC contains one or more precision differential and single-ended programmable-gain amplifiers for current and other sensing, and multiple function-specific protection comparators with dedicated digital-to-analog converters (DAC). The DAC outputs and internal parameters are multiplexed to an ADC for periodic safety checks.
Acting Independently of Firmware
All safety features of both BMS and motor controllers are configurable to act independently of firmware. For example, if the battery pack voltage surpasses a safe threshold and for any reason the firmware fails to react, the BMS safety circuitry automatically switches off the breaker MOSFETs, which shuts down the tool or charger and protects the battery. The same is true of internal temperature.
Likewise, if current in either the BMS or motor controller exceeds a safe level, the PAC automatically shuts down the current. Some motor controllers feature configurable cycle-by-cycle overcurrent protection that allows the motor to keep running while being protected from overcurrent. VDS sense (DESAT detection) is also available. Some PAC motor controllers also feature brake and drive disable modes, activated and controlled by a redundant safety microcontroller.
To further ensure safe, reliable operation, each PAC includes a watchdog timer, and PAC2xxxx and PAC55xx feature a windowed watchdog timer with a separate clock. All safety features are compliant with UL / IEC60730 Class B safety standards. To further reduce risk and accelerate product development and certification, Qorvo provide Class B pre-certified firmware.
Simple by Design
With the main theme of Qorvo PACs being safety, the harmony is high integration, enabled by a high voltage fabrication process. Integrated power management is designed for certain operating voltage ranges spanning from 20 V to 160 V. Internal Multi-Mode Power Manager features include:
- Voltage regulators to generate various internal voltages
- High-side gate driver for an external power transistor
- Switch-mode controller with an internally compensated (stabilized) feedback loop, supporting buck and SEPIC topologies
- Bootstrap supply clamp
Figure 4 shows a typical buck implementation for a BMS or motor controller. Its function is to produce a regulated power supply for internal use. Recommended component values are listed in datasheets and user guides. This integrated approach to power management eliminates numerous components from the circuit board, including DC-DC controller IC, feedback compensation components, an isolated gate driver, and a voltage supervisor. Importantly, the design and test time for the converter are greatly reduced as a result.
Figure 4. Buck power supply implementation
Application Specific Power Drivers bring three-phase bridge gate drivers inside the motor controller PACs. Besides the external drive MOSFETs, the usual gate resistors and bootstrap resistor-capacitor-diode are all that remain on the circuit board. Figure 5 shows a typical drive configuration for one of three half-bridges that drive a brushless DC (BLDC) or permanent magnet synchronous machine (PMSM).
Figure 5. Typical drive configuration showing one half-bridge
A buck converter like the one shown in Figure 4 powers three low-side gate drivers, and a bootstrap powers three high-side drivers. Note that there are three separate bootstraps, one per phase, but the node labelled VP_BST is common (connected) to all three phases. Low resistance totem-pole FETs minimize power dissipation within the drivers, and an internal temperature sensor protects them from overheating.
Pulse width modulation (PWM) peripheral features include:
- Multiple 16-bit timers
- Up to 300 MHz input clock for high resolution in PAC55xx
- Flexible latch modes and interrupts
- 12-bit deadtime & interlock generators
High PWM resolution facilitates very fine motion control, not so much needed for outdoor power tools but certainly for robotics and similar applications.
Qorvo provides firmware libraries for both trapezoidal and sinusoidal motor control, greatly reducing development time and cost.
High Integration with a Different MCU
Migrating a large code base written for a certain series of microcontrollers might not be a viable option. However, the advantages of simplified design and cost and space reduction are still attainable by using a motor control driver with analog front end. For example, ACT72350 derives from the PAC5532 but with the MCU removed. It is a 3-phase, 2 A source/sink motor driver with integrated power management, CAFE, and protection comparators with DACs. It works with any microcontroller to allow retention of the code base and familiar toolset while providing features and benefits of other PAC motor controllers.
ACT72350 includes a buck converter with integrated control to supply the internal gate drivers and linear regulators. A second buck supplies 5 V, which can power the external microcontroller. ACT72350 features 25 to 160 V input voltage range, cycle-by-cycle current limit, voltage and temperature lockout, SPI for configuration and status, and hibernate, brake, and driver disable modes.
Safe Operation and Long Service Life
PAC intelligent battery monitors and motor controllers ensure safe operation and long service life of both the battery and tool. Precision measurements enable fuel gauge calculations and battery cell charge balancing. Flexible PWM and pre-certified firmware libraries simplify code development.
Precision amplifiers, comparators, bias circuitry, isolated gate drivers, driver power supplies, a PWM controller with associated feedback and compensation components, voltage regulators, and voltage supervisors were transplanted from the circuit board into two types of ICs. And those ICs work seamlessly together to greatly simplify, de-risk, and accelerate battery powered tool development while simultaneously reducing cost and space.
Visit Qorvo’s website for datasheets and user guides, firmware libraries, evaluation kits, thermal and reliability data, and detailed design examples.
All images used courtesy of Qorvo.
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