Every EV traction inverter requires a
DC link active discharge as a safety-critical function. The discharge circuit is
required to discharge the energy in the DC link capacitor under the following
conditions and requirements:
In an emergency situation or
during repairs, the voltage in the system must be safe to touch in less than 2
s
At vehicle key-off, the DC
link capacitor must remain discharged
System-level safety requirement
ASIL D
Shall be able to operate
independently from the MCU, in case of MCU failure
TI has several active discharge
designs targeted for different system-level requirements:
Power transistor on, off control
using the TPSI3050-Q1. The TPSI3050-Q1 reinforced
isolated switch driver has an integrated 10-V gate supply that can drive the
discharge power switches with no need for a secondary bias supply.
Controlled PWM using the
AFE539F1-Q1 device. The AFE539F1-Q1 smart AFE has built-in nonvolatile memory
for PWM and custom waveform generators. The device has added programmability and
logic which eliminates the need for software filling the gap between DAC-based
circuits, MCU-based circuits, and entirely discrete circuits. #FIG_UKL_MCZ_N5B and #FIG_GCM_5CZ_N5B show a design block diagram and testing waveforms.
Figure 8-1 DC Link Active Discharge
Based on the Smart AFE
CH1: AFE539F1-Q1 output CH2: Gate driver (UCC27531-Q1) PWM output CH3: DC link voltage after resistive
divider CH4: SiC FET drain-to-source
current
Figure 8-2 Testing Waveforms
Discharge through the power stage
by linear biasing or PWM-based pulsed-linear switching on the power module to
constitute a short circuit. TI’s isolated gate driver with tri-state capability
enables active discharge through a power module using discrete analog circuits.
The discharge profile is mirrored to a current source reference across a
capacitor, where a 100-µA constant current sink is representing 1-A constant
discharge current. A gate voltage regulator regulates the gate-to-source voltage
and drives the power module into the linear region.
Energy discharge through the
motor winding. Dividing a winding-based discharge into multiple stages is
possible. These stages include a rapid discharge stage or a bus voltage
regulation stage. Generating large d-axis current quickly reduces the DC link
energy, while the q-axis current must be at zero. Fast loop control from TI’s
Sitara or C2000 MCU and safety isolated gate driver include Serial
Peripheral Interface (SPI) programmability, Six ADC channels provides a reliable
and smoothly controlled discharge.