TMC603A Datasheet, PDF (39/44 Pages) TRINAMIC Motion Control GmbH & Co. KG. – Three phase motor driver with BLDC back EMF commutation hallFX™ and current sensing

TMC603A DATA SHEET (V. 1.15 / 2009-Nov-19)
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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
R_DSON. 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 R_DSON 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 R_DSON, 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. Further enhancements of MOSFETs have
been done, to reduce the reverse recovery charge of the bulk diode. The bulk diode reverse recovery
charge otherwise is a source for high current spikes an oscillations in push-pull output stages driving
inductive loads like motor coils.
When choosing the MOSFET, the following points shall be considered:
Maximum voltage V_DSS:
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).
On-resistance R_DSON:
A low R_DSON 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 R_DSON 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 Q_G and switching speed:
The switching speed of the TMC603 application depends on the gate charge and the gate drive
current setting. 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 V_GS(TH):
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.
Reverse recovery charge Q_RR of bulk diode:
A lower reverse recovery charge Q_RR and lower reverse recovery time t_RR reduce peak currents in
the bridge and allow for faster switching. Snubber elements at the output are required for high
reverse recovery charge transistors. Otherwise, Schottky diodes should be used to bridge the bulk
diode.
Package, size and cooling requirements
Cost and availability
Copyright © 2009 TRINAMIC Motion Control GmbH & Co. KG