TMC603A Datasheet, PDF (40/44 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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8.1.1 Calculating the MOSFET power dissipation
The power dissipation in the MOSFETs has three major components: Static losses (P_STAT) due to
voltage drop, switching losses (P_DYN) due to signal rise and fall times, losses due to diode conduction
(P_DIODE). 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).
where
I_MOTOR is the motor current, e.g. 10A
R_DSON is the on-resistance of the MOSFETs at a gate voltage of about 10V, e.g. 20mΩ
t_DUTY is the actual duty cycle of the chopper, e.g. 80% = 0.8
V_VM is the motor supply voltage, e.g. 24V or 48V
f_CHOP is the chopper frequency, e.g. 20kHz
t_SLOPE is the slope (transition) time, e.g. 300ns
Example:
With the example data for a 10A motor at 24V, we get the following power dissipation:
P_STAT = 3.2W
P_DYN24 = 2.88W
For comparison: The motor output power is 10A*24V*0.8=192W
The dynamic and static dissipation here are in a good ratio, thus the choice of a 20mΩ
MOSFET is good.
At 48V, the dynamic power dissipation doubles:
P_DYN48=5.76W
Here, the dynamic losses are higher than the static losses. Thus, we should reduce the slope time.
Given that the gate capacity would not allow for faster slopes than 300ns, we could go for a 30mΩ
MOSFET, which has a lower gate charge and thus allows faster slopes, e.g. 200ns. With these
modifications we get a static loss of 4.8W and a dynamic loss of 3.84W. This in sum is 8.64W, which is
slightly less than the 8.96W before. At the same time, system cost has decreased due to lower cost
MOSFETs. The loss is still low when compared to a motor power of 384W.
Copyright © 2009 TRINAMIC Motion Control GmbH & Co. KG