SLUAAY2 December 2024 ISO5451 , ISO5451-Q1 , ISO5452 , ISO5452-Q1 , ISO5851 , ISO5851-Q1 , ISO5852S , ISO5852S-EP , ISO5852S-Q1 , UCC21710 , UCC21710-Q1 , UCC21717-Q1 , UCC21732 , UCC21732-Q1 , UCC21736-Q1 , UCC21737-Q1 , UCC21738-Q1 , UCC21739-Q1 , UCC21750 , UCC21750-Q1 , UCC21755-Q1 , UCC21756-Q1 , UCC21759-Q1
Identifying and protecting short circuit (SC) and over current (OC) scenarios are critical for high power systems like HEV-EV traction inverters and EV charging and solar inverters system. In high-power systems, SiC FETs or IGBTs are generally used depending upon the power level and switching frequency. This application note discusses the key considerations and design approaches to implement the right protection circuit based on SiC FETs and IGBTs. It walks through the timings involved from detecting the SC/OC event to safe shut-down, the circuit implementation criteria and experiment data for both IGBT and SiC FETs. It summarizes the right protection driver for IGBTs and SiCs based on the released isolated gate drivers from TI.
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Isolated gate drivers are commonly used in automotive and industrial high-power systems such as HEV/EV traction inverters, solar inverters, and motor drives. In these applications, silicon carbide (SiC) MOSFETs and Si IGBTs are usually ideal candidates; their abilities to handle high voltage, high current are beneficial in high power systems in the hundreds of kW range. All the high-power applications use power modules with increased voltage and current capability.
This application note talks about some of the common failure modes of the SiC and IGBT power switches, characteristics, the best suitable protection approach based on the power module type and the protection circuit component design aspects.
Though both used in high-voltage, high-power systems, IGBT and SiC FETs are intrinsically different in their voltage-current characteristics, resulting in difference in their overvoltage and short-circuit protection timing and shutdown energy.
Both Si IGBT's and SiC FET's regions of operation are shown in Figure 2-1. For IGBTs, at low collector-emitter voltage (VCE), the device is in its linear region, and the collector current (IC) increases as VCE increases. IGBT has a saturation VCE voltage, and beyond the VCE saturation point, it operates in the active region, meaning the current is relatively flat as VCE increases. This saturation VCE voltage is usually used to determine when the short-circuit protection starts to engage, with the corresponding IC being the short circuit threshold current (ISC). Since only VCE increases and Ic stays stable during IGBT short circuit, the power dissipation increases relatively slowly, so IGBTs usually can tolerate longer duration of short-circuit event (around 10μs).
SiC, on the other hand, usually operates in the linear region. As a short-circuit event happens, the drain-source voltage (VDS) and the drain current (ID) increase simultaneously, resulting in faster-rising power dissipation. Because of this operating mode, the timing is more critical. SiC can usually only tolerate a short duration of short-circuit event (typically 2-3μs) before the power switch starts breaking down.
Thus, it is crucial to select the right short-circuit protection mechanism as well as suitable protection voltage (VCE/VDS) and load current (IC/ID) threshold to safely and efficiently turn off the device when a short-circuit event happens.