6
STEVAL-CTM005V1 bus link capacitor board
In EV inverter systems, bus link capacitors reduce ripple current and suppress voltage spikes caused by leakage
inductance and switching operations. These capacitors provide a low impedance path for the ripple currents
caused by output inductance load, the bus voltage and PWM frequency.
The bus link capacitors must sustain a ripple current given by the following formula:
ΔI0.5t = 0.25 ×
Vbus
f × L
Where:
•
ΔI
0.5t
is the maximum ripple current when duty cycle is 50%
•
V
bus
is the bus voltage
•
f is the switching frequency
•
L is the load inductance.
For a very low inductance motor (worst case scenario), ΔI
0.5t
is about 48 A
RMS
(V
bus
= 52 V, f = 8 kHz and
L = 12 μH). If we add 10% to ΔI
0.5t
and choose electrolytic capacitors with a ripple current of 2.4 A, 22 electrolytic
capacitors are required. The resulting capacitance is about 6 mF, leading to a negligible ripple voltage on the bus.
Figure 7.
STEVAL-CTM005V1 bus link capacitor board
6.1
STEVAL-CTM008V1 current sensing board
The STEVAL-CTM008V1 current sensing board is a general purpose board for motor control that can read up to
three phase motor currents and DC bus currents if four ICS are on-board. The board included in the kit hosts two
ICS to read two phase currents.
This sensing feature determines motor currents for digital control based on FOC algorithms. The sensors provide
high accuracy, with 4 mV/A over a temperature range of -40 °C to +105 °C and a nominal current of 200 A
RMS
.
The internal reference voltage of the ICSs (according to their V
CC
) is generally used, but the reference voltage
can be overdriven by providing an external reference voltage through the J1 connector. A female to female flat
cable is used to connect CON2 on the driver board with J1 on the current sensing board.
The signals from the sensors center around 1.65 V (average value at zero current).
UM2458
STEVAL-CTM005V1 bus link capacitor board
UM2458
-
Rev 1
page 10/40
