5 Isolated Gate Drivers

TI gate driver isolation – up to 5.7 kV_RMS – helps protect against electric shock while offering higher working voltages, and wider creepage and clearance for improved system reliability. There are two major isolated gate driver families: the smart driver UCC21750-Q1 family and the safety driver UCC5870-Q1 family. The UCC21750-Q1 family includes protection features for the power modules in traction inverters such as fast overcurrent and short-circuit detection, shunt current-sensing support, fault reporting, active Miller clamp, input and output-side power supply undervoltage lockout detections. An isolated analog-to-PWM sensor facilitates easier temperature or voltage sensing.

The UCC5870-Q1 driver family includes the following features:

• Functional Safety-Compliant, isolated, single-channel gate driver, supporting up to 1-kV_RMS working voltage and longer than 40 years isolation barrier life, as well as providing low part-to-part skew, and >100 V/ns common-mode noise immunity (CMTI)
• A high 30-A peak drive strength for minimizing power switching losses and removes the buffer circuit on the drive circuit, thus reducing cost.
• A temperature sensor to monitor the temperature of the power module and allow operation up to a certain temperature limit, helping support a wide operating range
• Has a Miller clamp to prevent false turn on and enables switches to be switched as fast as needed to achieve efficiency targets

#FIG_GCM_5CZ_N5B and #FIG_IFD_DDZ_N5B show the 30-A drive strength of the UCC5870-Q1 and a competing device under the following test conditions:

• Vcc2 – Vee2 = 23 V
• R_gon = R_goff = 0 Ω
• Load capacitance = 1 µF

One way to improve the traction inverter efficiency and reduce EMI is to adjust the gate-drive output for controlling the slew rate, thereby changing switching speeds under varying conditions such as temperature, load, and voltage. For example, when depleting the battery voltage, the transient voltage (dv/dt) is naturally smaller, and the gate-drive output can be adjusted to push the switch to transition faster.

#FIG_QC5_GDZ_N5B and #FIG_SC5_GDZ_N5B illustrate an adjustable gate-drive implementation based on the UCC5870-Q1. #FIG_QC5_GDZ_N5B shows the design diagram, while #FIG_SC5_GDZ_N5B shows the design board, which is connected to the XM3 half-bridge power module family from Company WolfSpeed.

#FIG_OBX_GDZ_N5B and #FIG_QBX_GDZ_N5B show the double pulse testing waveforms. The average switching dv/dt speed of rising edge increased from 4.6 kV/µs to 21 kV/µs. The average switching dv/dt speed of the falling edge increased from 3.8 kV/µs to 13.5 kV/µs.

Both of the following images were collected with a double pulse testing waveform under an 800-V bus.

Table 5-1 shows the switching energy comparison between weak drive (5.5-Ω gate resistance) and strong drive current (0.5-Ω gate resistance), under a 400-V bus voltage.

Table 5-2 shows the switching energy comparison between weak drive and strong drive current, under 800-V bus voltage.