5 Isolated Gate Drivers

TI gate driver isolation—up to 5.7kVRMS—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 protection gate driver UCC2182xx-Q1
• The programmable gate driver UCC5881-Q1

The UCC2182xx-Q1 is an isolated gate driver with active protection and advanced safety features, such as multiple UVLO options (12V, 15V, and 17V) with reinforced isolation, desaturation (DESAT), and overcurrent (OC) protection, as well as overtemperature protection. Sink and source current is increased to 15A from legacy devices, while the 20-DFP package enables a 45 % smaller footprint. The device features a soft shutdown pin for fine-tuned current in shutdown, primary and secondary active short circuit (ASC) protection, and a built-in self-test during start-up to check comparators on the diagnostic features.

The programmable gate driver UCC5881-Q1 is named for the programmable registers, which can be set to modify different parameters, such as UVLO or DESAT per the data sheet for the device. These registers can be addressed using SPI. In the latest generation of TI’s programmable gate drivers, the user can also vary the gate drive strength. When driving the gate of a power stage, a strong drive signal leads to lower switching losses as a strong signal reduces the time the switch spends conducting current at a voltage drop. At the same time, strong drive signals create larger overshoot, which can come close to the voltage limit of the drain-source and damage the switch, especially at fully charged battery packs, where the headroom is the smallest. Weak drive signals lead to a lower overshoot and safer operation, but cause higher switching losses due to the lower slew rate, see Figure 5-1.

Figure 5-1 Weak Versus Strong Gate Drive and Effect on Losses and Overshoot

Programmable gate drivers, such as the UCC5881-Q1, can utilize both weak and strong drive signals to create a combined drive. The current of the drive is inversely proportional to the voltage rise time of the switch node, so strong drive currents increase the slope of the switch node voltage. As the current reaches the DC link (target) voltage, the switch node voltage continues to increase for a short amount of time before falling back to the DC link (target) voltage. This additional increase is called overshoot. The stronger the current of the drive, the higher the slope, so the voltage levels can easily reach the voltage limit of the drain-source.

The strength of the programmable gate drive (the exact operation mode can be set on-the-fly through SPI or GPIOs) works on a strong drive, up until a certain output current where the drive switches into weak gate drive mode. This combined drive reduces the risk of overshoot damaging the switch (compared to only a strong gate drive) while enabling considerable savings in power losses (compared to only a weak gate drive), see Figure 5-2.

Figure 5-2 Combined Drive Output of a Programmable Gate Driver

Based on the CLTC drive cycle, the savings in power loss using a variable strength drive results in:

• More than a 2% improvement in inverter efficiency
• –140 dollars (U.S. currency) in battery cost
• +15.5km in operating range
• –9kg in weight
• –7.5l in battery volume (physical size)

Figure 5-3 shows an implementation of the strength feature of the programmable gate drive in real application. As the turn-off transient (yellow curve) approaches the 1080V VDS overshoot limit at ILOAD = 300A using the strong gate drive (20A), a change in the signal strength of the GPIO drive (green curve) turns on the weak drive mode (5A) and the transient voltage reduces to 991V in the following switching cycle.

Figure 5-3 Programmable Drive Strength Application Example

The UCC5881-Q1 driver family includes the following additional features:

• Primary and secondary side ASC protection, enabling increased design flexibility in the system
• Split output driver with dual output for an adjustable gate drive (±15A and ±5A drive current) with an integrated 4A active Miller clamp or optional external drive for a Miller clamp transistor
• Interlock and shoot-through protection and programmable minimum pulse rejection
• Internal and external supply undervoltage and overvoltage protection
• Integrated diagnostics supporting ISO26262 functional safety requirements up to ASIL D, such as health monitoring (performing threshold voltage measurements over the lifetime of the system) and failure anticipation (provide power switch data to MCU to predict failures)
• Shunt resistor based overcurrent protection
• Overtemperature (PTC, NTC, or diode) and DESAT detection
• Extended temperature range from –40°C to +125°C

The UCC5880-Q1 inverter evaluation module (EVM) can be used standalone to test the gate driver with a 100nF capacitor load soldered on the board or to drive half-bridge power modules directly based on a Wolfspeed® XM3 SiC MOSFET for high power tests, see Figure 5-4.

Two UCC14241-Q1 isolated bias supplies are included on the board. The EVM is flexible to configure different SPI communication methods, including regular SPI, daisy-chain, and TI address-based. The EVM can interface with a Sitara™ and C2000™ real-time microcontroller control card for 3-phase inverter testing up to 300kW.

Figure 5-4 UCC5880-Q1 Evaluation Module for a Variable Isolated Gate Drive in Traction Inverters