Selecting self-clamping MOSFETs to improve power tool system reliability

Power tools have been widely used in various occasions due to their advantages such as light design, strong power and convenient use. Power tools are generally implemented with DC brushed motors and electronic stepless speed regulation circuits, which have functions such as sensitive starting and forward and reverse speed regulation, such as hand drills and electric screwdrivers. Stepless speed regulation circuits are generally implemented using PWM working mode. Since the internal resistance of the motor is small, generally only more than 100 milliohms, the peak current during the PWM on period is very large; when the PWM is turned off, the high induction voltage generated on the line lead inductance due to high di/dt puts high demands on the robustness of the MOSFET in the system. This article analyzes how to optimize the switching waveform and how to choose a suitable MOSFET.

Working principle of drive circuit

Figure 1 is a block diagram of an electric tool and a controller. The drive circuit in the figure is usually composed of a chip 555, and the operating frequency is generally within 10KHz. Its working process is described as follows.

Figure 1: Block diagram of power tool and controller

(a) Power tool; (b) Schematic diagram of power tool controller circuit

When the MOSFET is turned on, the current forms a loop through the positive electrode of the battery → line lead inductance → motor coil → MOSFET → negative electrode of the battery. The motor coil current is equal to the current in the MOSFET, and the freewheeling diode is cut off. When the MOSFET is turned off, the motor coil continues to flow through the diode D1, and the motor current remains basically unchanged. However, the current in the MOSFET and the lead inductance quickly becomes zero as the MOSFET is turned off, and a large induced potential LW di/dt is generated in the line lead inductance, and its direction is shown by the red arrow in Figure 1b. Such an induced potential superimposed on the battery voltage will produce a very high spike voltage, as shown in the red part in Figure 2. If these spike voltages exceed the breakdown voltage of the MOSFET, the system reliability will be greatly reduced. By adjusting the PWM duty cycle of the MOSFET, different average currents can be obtained in the motor coil, thereby achieving stepless speed regulation of the motor.

Figure 2: Switching waveforms of a MOSFET in a power tool

(a) MOSFET switching waveform; (b) turn-on waveform; (c) turn-off waveform

MOSFET power dissipation calculation

Figure 2 shows the switching waveform of MOSFET (AOT500) in power tools. The power loss of MOSFET consists of conduction loss and switching loss, which are as follows:

⑴ Conduction loss

Among them, (2) switching loss

Since there is a relatively large motor coil inductance when the MOSFET is turned on, the current flowing through the MOSFET from the off state to the fully turned on state is very small, so the turn-on loss is very small and can be ignored. The turn-off loss is as follows:

Where: VCLAMP is the clamping voltage when the MOSFET is turned off, E is the battery voltage, and LW is the line inductance.

The total loss of the MOSFET is:

The extreme temperature of MOSFET is:

Where: TJ is the junction temperature of MOSFET, TC is the surface temperature of MOSFET,

RTHJC is the thermal resistance of the MOSFET.

From the above formula, we can know that for a certain system, there are several ways to reduce the junction temperature of MOSFET: ⑴ Select MOSFET with lower RDS(ON) and lower RTHJC. ⑵ Set the appropriate MOSFET turn-off speed to minimize the switching loss. ⑶ Reduce the lead inductance of the main loop as much as possible. Because in each turn-off process, the energy in the lead inductance of the loop will be absorbed by the MOSFET.

Key points for selecting MOSFET

From the application point of view, the factors that affect the reliability of MOSFET are mainly the following: ⑴ The junction temperature of MOSFET. Too high a junction temperature will affect the reliability of MOSFET and cause the MOSFET to fail prematurely. ⑵ If the voltage spike on the drain of MOSFET exceeds its avalanche breakdown voltage, the MOSFET will also fail prematurely. Therefore, we must choose a suitable MOSFET to design the power tool drive circuit. For example, the self-clamping MOSFET - AOT500, which is designed and produced using an advanced trench process, has a maximum on-resistance of only 5.3 milliohms, and it has a VDS voltage self-clamping function, which is very suitable for power tool design applications.

Figure 3: Self-clamped MOSFET

(a) AOT500 appearance; (b) AOT500 internal structure

A Zener diode is integrated between the drain and gate of the self-clamped MOSFET, as shown in Figure 3. When the drain voltage is greater than the clamping voltage, a very small current will flow through the Zener diode between the drain and gate, generating a voltage drop through the gate resistor. When the voltage drop is greater than the MOSFET turn-on voltage VTH, the MOSFET will turn on and clamp the drain voltage to ensure that the MOSFET will not be in an avalanche state. Figures 4(a) and 4(b) are waveforms measured in a power tool system using an unclamped MOSFET and a clamped MOSFET, respectively. If an unclamped MOSFET is used, the highest voltage can reach 72V, and the MOSFET may be in an avalanche state. In this case, it is best to use a high-voltage MOSFET. When using AOT500, the voltage is clamped at 40V, and the spikes in the system are significantly reduced, greatly improving the reliability of the system.

Figure 4: VDS waveform of MOSFET

(a) Using unclamped MOSFET; (b) Using clamped MOSFET

Figure 5 is a comparison of the stall test time of AOT500 in power tools. It can be seen that the stall time that AOT500 can withstand is much longer than that of other MOSFETs commonly used in power tools, thus greatly improving the reliability of power tools.

Figure 5: Comparison of stall test results of different MOSFETs in power tools.

In summary, when using MOSFET to design power tools, the following points need to be noted: minimize the parasitic inductance of the line, especially the lead inductance, to minimize the energy absorbed by the MOSFET when it is turned off; increase the turn-off speed of the MOSFET to reduce the turn-off loss; select self-clamping MOSFET to improve the reliability of the system.