Industry Article

# Modern Motor Drivers Enable a Common Platform for Power Tool Designs

In today’s power tool market, the wide range of battery voltages and torque requirements often result in designs that use different platforms that support a limited range of products, which can introduce various compatibility challenges. Overall, significant time and cost could be cut from research and development schedules if a single industry-wide platform were used to serve the range of planned products.

This article discusses the range of voltage and torque requirements for power tools and provides example calculations for low- and high-power gate drivers. Additionally, it aims to explain how to account for widely divergent requirements for heat sinks, the number of MOSFETs being used, and more.

### Accounting for the Wide Range of Power Tool Options

Some power tool lineups span the 12 V to 24 V range, while others span the 12 V to 60 V range (or even up to 80 V with some lawn tools). With these higher operational battery voltages come harsher environments that introduce larger negative transients on the driver output stages, necessitating multiple PCB designs that draw from a grab bag of low- and high-power motor driver chips.

A common brushless DC (BLDC) or half-bridge, an example power circuit shown in Figure 1, driver chip that spans the entire range of power tool designs available in the market would reduce the time and effort required to develop and produce new power tools, consolidating software development time and PCB design/test cycles.

##### Figure 1. An example power section of a half-bridge MOSFET.

Such a platform must allow for the full range of maximum voltages across those devices: many motor driver devices, for example, have a maximum supply voltage of only 40 V. This does not leave adequate margin in most 24 V or 36 V systems to deliver a robust design that can withstand the harsh voltage transients that may arise on the power-tool voltage supply during motor operation.

A standalone gate driver with a broader supply voltage range that can withstand these transients—such as the 50 V and higher gate drivers discussed below—would allow system designers to achieve the time and resource savings of a common design across a large span of power-tool batteries.

For higher-power 48 V, 60 V, or 80 V systems, fewer solutions exist for the integrated three-phase BLDCs. An example application diagram can be seen in Figure 2.

##### Figure 2. An example application diagram for an 80 V gate driver.

The necessary power tool design may be easier to achieve when power is distributed around the board with compact high-voltage half-bridges. A 100 V half-bridge in an ultra-small 3 mm × 3 mm DFN package could help toward this goal.

The wide supply voltage range allows for use in small 12 V drill motors or more-powerful 80 V string trimmers with a single PCB architecture, where—to save cost for low-power tools—various MOSFETS can be swapped in and out according to the power level needed.

### Calculations for High-power and Low-power Gate Drivers

High-end power tools often support extended operation times or operations with frequent, rapid, high-power bursts. They may also have peak torque ratings in excess of 1200 in-lbs or 130 Nm, commonly calculated at 2,000 RPM.

On the other end of the spectrum, battery-powered lawn mowers require less torque, yet still, demand high-speed operation for long durations. This dictates that the gate driver for a common platform needs to be able to drive both a 12 V, 30 kW peak power drill and an 80 V, 4.5 kW lawnmower.

When the common torque ratings of two tools are converted into power, the span that a driver needs to accommodate is shown by:

$Power [kw] = \frac {Torque [N \cdot m] \times Speed [rpm]}{9550}$

Take, for example, a high-power drill peak:

$Power = \frac {130 N \cdot m \times 2100 rpm}{9550} = 27.6 kW$

On the other hand, a low-power, long-duration lawnmower calculation yields the following result:

$Power = \frac {12 N \cdot m \times 3500 rpm}{9550} = 4.4 kW$

The above power levels are required for designers to determine what driver and MOSFET to use in any given system. Appropriately accounting for the wide range of outcomes here is a pivotal step of the design process when working with various power tools.

Most drivers supply gate drive voltages in the range of 7 V to 13 V. The total gate charge is the measure of how much current is needed to turn on a MOSFET, and it varies significantly among the most commonly used MOSFETs at their nominal 10 V. A low-profile 40 V rated DFN MOSFET may have a total gate charge of 65 nC, while a 100 V rated MOSFET may have a total gate charge of only 35 nC.

To ensure support for the full power spectrum of the tool lineup, designers must accommodate the average VREG current that the driver can supply to the gate of the MOSFET to hold the MOSFET in the on-state.

Additionally, designers must consider the maximum source and sink currents to ensure that the MOSFET advances quickly through the Miller region; however, the limiting factor of the pulse-width-modulated (PWM) drive frequency and MOSFET size will be the average VREG current the driver can supply the gate drive.

The equation to determine the necessary average VREG drive (Iavg) current needed to hold the MOSFETs in the on state at a given PWM frequency is:

$I_{avg}[mA] = Number \enspace of \enspace MOSFETs \enspace being \enspace driven \times f_{PWM}[kHz] \times Q_{G(tot)}[nC] \times 1000$

For example:

$I_{100V \enspace FET,avg} = 6 \times 20kHz \times 35nC \times 1000 = 4.2mA$

$I_{40V \enspace FET,avg} = 6 \times 20kHz \times 65nC \times 1000 = 8mA$

$I_{80V \enspace FET,avg} = 6 \times 20kHz \times 140nC \times 1000 = 17mA$

The number of MOSFETs being driven changes with the driver scheme:

• Six (6) are used for sinusoidal drivers
• Two (2) are used for trapezoidal drivers
• Four (4) are used for two-phase sinusoidal drivers

In these examples, a PWM frequency is set at 20 kHz to be above the audible range.

The graph below in Figure 3 shows an example of the relationship between the current supplied by the gate driver and the drive frequency.

##### Figure 3. Graph of common driver IREG capabilities and calculation of current needed to maintain the on-state for six 35 nC MOSFETs at various PWM frequencies.

Using the average current formulas above, we can see what frequencies can be supported with the selected number of MOSFETs and selected MOSFET gate charge value.  The dots on the IREG line show the current required to drive 6 MOSFETs in sinusoidal mode at different frequencies.  The horizontal lines show reference lines of the max current selected gate drivers can supply.

Many more MOSFET options exist and the values for total gate charges are infinite. The key point is that, in any system, the designer must address the interplay of components that affect the average VREG driver current before the driver is selected. Using a MOSFET with a 65 nC total gate charge at 10 V and a driver with an Ireg average current of 15 mA driven at 20 kHz would provide plenty of margin for a strong gate drive.

Using the same design for a lower-power tool, the MOSFET could be swapped with a lower ID-rated device with a higher total gate charge, a high-level example shown in Figure 4.

##### Figure 4. A high-level example showing how capable motor drivers can allow for flexible PCB designs.

The next consideration in the design of a power tool system is the robustness of the driver. How will it perform in harsh environments with large transients generated by high-torque motors?

When a driver switches the MOSFETs that control a motor with a peak power rating of 30 kW, large positive and negative transient pulses are bound to occur. A system designer can either place numerous capacitors on the MOSFET bridge supply or choose a driver that has built-in transient protection and save PCB space and BOM cost.

### Allegro MicroSystem's Gate Driver Portfolio

Systems that span the 12 V to 80 V range often require a driver with a higher supply rating that supports the high-power 18 V drill and the 80 V mower. Although the choice of suitable integrated three-phase BLDC drivers is limited, a set of capable 100 V half-bridges may meet the need.

One example product to consider for power tool designs is the Allegro A89500, a 100 V-rated half-bridge that can drive systems up to 30 kW. The peak sink and source currents are high enough to quickly switch the MOSFETs to the on state and can be easily set with external resistors for a highly flexible and robust electromagnetic compatiblility (EMC) design. The separate gate drive supply then supports all the current needed to keep the MOSFETs in the on state during high-current 100% duty cycle situations.

The Allegro portfolio of power tool gate drivers—like the 50 V-rated A4919 and the 100 V-rated A89500—offer negative transient protection built directly into the circuitry. A chart can be seen down below in Figure 5.

##### Phase connection transient robustness and maximum supply voltage rating of Allegro and other vendor gate drivers.

The high-side gate driver output of the A89500 can withstand voltages on the phase connections of short-duration transients at ~18 V up to 100 V.

Although a few other choices for this market are robust to ~8 V at the phase connection, many vendors can only support ~2 V below ground. These less-robust solutions would require separate PCB designs for the harsher high-power tools or significant protection circuitry that would be otherwise unneeded on the low-power end of the power tool market.

Whatever the system, devices are currently available that can enable a common platform for power tool designs. The A4919 is a small direct-drive gate driver with robust gate drive circuitry capable of supporting most systems below 40 V.

The A4915 is a similarly sized device for sub-40 V tools and features an integrated Hall-effect sensor supply and feedback along with the motor drive control logic. The A4915’s built-in control logic saves space with a simple interface that offloads motor-control algorithms.

For tool portfolios that span the range from 12 V to 80 V, the small and capable A89500 half-bridge is the best choice, easily driving high-power MOSFETs with high total gate charges or small, multipack low-power MOSFETs. All of these devices aim to allow system designers to condense a power tool lineup to a single PCB, which saves test time, offloads software resources, and enables faster development.

All images used courtesy of Allegro MicroSystems

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