The SAFeFET approach for cordless power drills
30 May 2007
Power Electronics / Power Management
R. Gulino, technical marketing: Power MOSFET, STMicroelectronics
Universal, PMDC and brush DC motors are widely used in small household appliances (blenders, food processors, shavers etc) and in handheld power tools (drills, table saws and planers etc).
For unidirectional operation, the single-switch chopper configuration is the most cost-effective solution to control the motor's speed and torque.
For portable tools, STMicroelectronics' SAFeFET (simple added features FET) family improves the system's performance, while still maintaining the advantages of a cost-competitive solution. The built-in zener diode for ESD protection ensures exceptional robustness against voltage spikes, while the built-in active voltage clamp withstands repetitive inductive switching current, avoiding operation in breakdown mode.
Added zener diode networks
The new STP75NS04Z and the STP62NS04Z are SAFeFET devices with embedded gate-to-source and drain-to-gate zener diode networks (Figure 1). Key features of these devices include: 175°C max operating temperature; standard threshold drive; 100% avalanche rated; ESD protected; device is fully-clamped. Main characteristics can be seen in Table 1.
Figure 1. Fully-clamped SAFeFET internal schematic
Table 1. Main characteristics @ 25°C ambient
In the example of a power drill (see schematic in Figure 2), the controller is made up of a very simple PWM (pulse-width modulation), which is controlled through seven steps, in a discontinuous mode. The last step causes the motor to spin at full speed and the Power MOSFET is short-circuited by a mechanical switch. No feedback loop has been introduced to control speed and torque. The circuit was analysed under the most severe conditions for the system, which is at locked rotor. The circuit was powered with an input DC voltage of 18 V and all the measurements were performed at room temperature (25°C) with the device mounted on its original location.
Figure 2. Cordless power drill schematic
Waveforms
As shown in Figure 3, the STP75NS04Z has an active clamp that makes the device turn-on again when the drain voltage exceeds approximately 40 V during turn-off. Since the drain-to-gate zener diode has an intrinsic breakdown voltage (VZ) lower than the BVDSS, the SAFeFET will never go in breakdown mode and the VDS voltage will be limited at VZ. The Power MOSFETs may be used with breakdown voltages lower than those of standard devices, which at a certain voltage would work in avalanche. This in turn allows the use of on-resistance products and this advantage can be translated in terms of conduction losses during the operation.
Figure 3. Switching-off waveforms for STP75NS04Z
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