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POWER MOSFETs FOR POWER TOOLS
A complete choice of MOSFETs for hand-held tools

The power tools market, and in particular that of hand-held tools such as electronic drills, is expanding rapidly. Tool makers continually strive to increase the overall efficiency in terms of space, weight, and power dissipation of their instruments while at the same time keeping the final product at a very low cost.
A key design hint that permits the best trade-off between electrical performance of the power switch and cost competitiveness, is to use low voltage, low current devices that are suitably rugged to meet the stressful working conditions of this application.

  
POWER MOSFETs FOR POWER TOOLS
ST today offers a product portfolio which includes the well known ‘EHD2’ technology while offering a new innovative solution that represents the “protected approach”.

Brief Application Description
DC motor control in power tool applications, consists of a supplied battery voltage which ranges from 14.5 to 24V, plus a PWM controller that drives the MOSFET.
There are two different ways to run the application: with the motor switched ON or OFF.
Motor Switched off;
In this case there is no connection between the system and the battery.
Motor Switched on;
In this case the motor has to work at different speeds, with the battery connected to the system and the controller driving the MOSFET so that the rotation speed of the motor can be changed.
Two stressful modes of operation can be identified for the MOSFET:
Full rotation speed;
When no current is flowing through the transistor.
Blocked rotor;
When the current intensity through the motor and the transistor is very high, causing large scale power losses due to the conduction status.
Since these severe working conditions could damage the MOSFET the right device must have a rugged technology and a very low RDS(on) in order to minimize the conduction losses.

The Protected Approach For Harsh Conditions
One of the most severe tests for electronic drills is to evaluate the transistor’s performance with the rotor blocked. In this case the rotor remains fixed for a minimum time of 20 seconds, while the MOSFET is running at the maximum duty cycle with very high current and voltage (about 75A, 50V).

POWER MOSFETs FOR POWER TOOLS
Power tool application schematic
POWER MOSFETs FOR POWER TOOLS
  
The new STP62NS04Z with zener, is produced following the MESH overlay process. It combines a very low RDS(on) with extra ruggedness due to a higher level of gate oxide.

P/N
BVDSS
(V)
Id
(A)
RDS(on) max
10V (Ohm)
Qg typ
(nC)
33
62
0.015
34

The integrated zener both protects against undesired voltage spikes and actively clamps at turn-off.
The main features are:
RDS(on) typ: 0.0125 Ohm;
175°C max junction temperature;
Low gate charge.

Product Comparison
To compare the performance of the new protected STP62NS04Z with an older product like STP60NF06, a DC motor taken from a common electronic drill was powered at a voltage of 18V at room temperature.
The easiest way to evaluate their performance is to consider the energy losses of the MOSFET under working conditions. The stressful condition of a blocked rotor was simulated.

POWER MOSFETs FOR POWER TOOLS
  
POWER MOSFETs FOR POWER TOOLS
STP62NS04Z turn-off energy with active clamping
  
STP60NF06 turn-off energy without clamping

From the waveforms related to STP62NS04Z, it is clear that if during the turn-off the device reaches a VDS close to 40V, its active clamp leads to a new turn-on due to the integrated zener diodes.
From the energy measured it is easy to evaluate the power losses by consulting the table below. The total power dissipation of the STP62NS04Z, has been reduced by approximately 11%.


Comparison Between MOSFETs in Power Tools
P/N
Turn-off
Power
[W]
Turn-on
Power
[W]
Conduction Power
[W]
Total
Power
[W]
13.48
7.84
39.74
61.06
12.7
4.34
50.96
68

Best Cost-Performace Trade-Off
ST today offers not only a basic product kit for all hand held tools, but also a new protected approach in order to guarantee the best cost-performace trade-off.
In the hand-held tool application the new STP62NS04Z has many benefits:
Lower cost solution compared to the existing one;
Total Power losses reduction;
Lower RDS(on).

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