TMC603 DATA SHEET (V. 1.06 / 26. Mar. 2009)
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8 Designing the application
8.1 Choosing the best fitting power MOSFET
There is a huge choice of power MOSFETs available. MOSFET technology has been improved
dramatically in the last 20 years, and gate drive requirements have shifted from generation to
generation. The first generations of MOSFETs have a comparatively high gate capacity at a moderate
RDSon. Their gate-source capacity is two to five times as high as the capacity of the gate-drain
junction. These MOSFETs have a high gate charge and thus require high current gate drive, but they
are easy to use, because internal feedback is low. In the early 2000s new MOSFETs have emerged,
where RDSon is much lower, and gate-source capacity has been improved by minimizing structural
overlap. Thus, the capacitance ratio has shifted, and feedback has become quite high. These
MOSFETs thus are much more critical, and power drives have to actively force the gate off to prevent
the bridges from cross-conduction due to feedback from the drain to gate. Latest generation
MOSFETs, like the Vishay W-Fet technology, further reduce RDSon, while reducing the capacity
between the channel and the drain. Thus, these MOSFETs have lowest gate charge, and again, are
easier to control than the previous generation of MOSFETs.
When choosing the MOSFET, the following points shall be considered:
Maximum voltage:
Choose at least a few volts above your maximum supply voltage, taking into account that the
motor can feed back energy when slowing down, and thus the supply voltage can rise. On the
other hand, a transistor rated for a higher voltage is more expensive and has a higher gate charge
(see next chapter).
RDSon:
A low RDSon gives low static dissipation, but gate charge and cost increases. Take into account,
that a good part of the power dissipation results from the switching events in a chopped drive
system. Further, to allow a current measurement, the RDSon should be in a range, that the
voltage drop can be used for measurement. A voltage drop of 50mV or higher at nominal motor
current is a good target.
Gate charge and switching speed:
The switching speed should be compared to the required chopper frequency. Choose the chopper
frequency low to reduce dynamic losses. When the application does not require slow, EMV
optimized switching slopes, choose a low gate charge transistor to reduce dynamic losses.
Gate threshold voltage:
Most MOSFETs have a specified on-resistance at a gate drive voltage of 10V. Some MOSFETs
are optimized for direct control from logic ICs with 5 or even 3.3V. They provide a low gate
threshold voltage of 1V to 2V. MOSFETs with higher gate threshold voltage should be preferred,
because they are less sensible to effects of the drain gate capacity and cross conduction.
Package, size and cooling requirements
Cost and availability
8.1.1 Calculating the MOSFET power dissipation
The power dissipation in the MOSFETs has three major components: Static losses (PSTAT) due to
voltage drop, switching losses (PDYN) due to signal rise and fall times, losses due to diode conduction
(PDIODE). The diode power dissipation depends on many factors (back EMF of the motor, inductivity
and motor velocity), and thus is hard to calculate from motor data. Normally, it contributes for a few
percent to some ten percent of overall power dissipation. Other sources for power dissipation are the
reverse recovery time of the transistors and the gate drive energy. Reverse recovery also causes
current spikes on the bridges. If desired, you can add Schottky diodes over the (chopper) transistors to
reduce the diode losses and to eliminate current spikes caused by reverse recovery.
The following sample calculation assumes a three phase BLDC motor operated in block commutation
mode with dual sided chopper. At each time, two coils conduct the full motor current (chopped).
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