Reinforced isolated gate drivers are key components in 3-phase inverters for industrial motor drives, and DESAT is a popular approach for overcurrent protection (OCP) or short circuit protection (SCP) in these applications. This application note presents a small form factor, cost-optimized design based on the 6-pin opto-compatible reinforced isolated gate driver UCC23513 with a discrete DESAT implementation using the isolated comparator AMC23C11. This combination achieves a smaller PCB size and lower cost, compared to the 16-pin package smart gate drivers with integrated DESAT protection, and helps to enhance flexibility in applications of compact motor drives. The design also keeps the flexibility to configure the application parameters of the DESAT function.
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In 3-phase inverters for motor drives, OCP and SCP are critical to protect the system from damage caused by abnormal operating conditions. Shunt-based system level OCP or SCP are often implemented by sensing the current through the negative DC bus or the three low-side switches; especially in many lower power, compact models, where form factor and system cost are critical. These protections are effective for the commonly seen fault patterns of arm shoot-through and phase-to-phase short. However, neither of them can detect an earth ground short when the fault current flows through a high-side switch, as shown in Figure 1-1. A DESAT function on the gate driver can help to protect the power switch against this fault. In fact, device level DESAT protection is effective to all these fault modes in a 3-phase inverter, thus has been widely used in many high power, high performance models.
Many industrial motor drives also have a regeneration brake switch to shunt the current to the negative VDC- bus and discharge the bulk capacitor when the voltage goes too high during a regeneration brake operation. Often this brake resistor needs to be installed externally and then connected to the system by a specific terminal on the drive. If a user makes an error in connecting this resistor, or mistakenly used one with a very low resistance, an overcurrent fault can occur once a brake operation is started by the system controller, as shown in Figure 1-2. In this case, a DESAT function on the gate driver can detect the problem and protect the power switch in time.
A typical approach to protect the system against these faults uses an isolated smart gate driver with DESAT function, like the UCC21750 reinforced isolated gate driver with CMOS input. As seen in Figure 1-3, a DESAT pin monitors the voltage drop of VCE when the IGBT is turned ON. Once this voltage drop goes up and reaches the set threshold, which means an over current or short circuit condition is happening, the output of the gate driver will be pulled to low at once and a fault output wii be activated to inform the system controller on the fault.
Reinforced isolated smart gate drivers with integrated DESAT function are typically offered in a 16-pin SOIC package, which is physically much larger than a compact gate driver without DESAT function in a stretched SO-6 package, as is shown in Figure 2-1. Consider placing six pieces of such devices of a 3-phase inverter on a power inverter PCB, the package length will stack up accordingly. A design using a shorter length compact device can offer an advantage on the PCB size. Even for a regeneration brake power switch, a smaller size gate driver can help to reduce the application layout area significantly. However, such gate drivers sacrifice the overcurrent protection function for application circuit simplicity and cost reasons.
An alternative approach is to use the smaller footprint compact gate driver without DESAT and implement the DESAT function discretely using an isolated comparator.
For circuit configurations that only require DESAT function on either the three low-side switches or the three high-side ones, this discrete DESAT design allows all six switches to use a same 6-pin reinforced isolated gate driver; thus avoids mixing simple gate drivers with smart gate drivers in one application system. The external DESAT function can be added to the low-side or high-side gate drivers, respectively. This discrete DESAT implementation adds flexibility to the application design to configure parameters of DESAT voltage, DESAT bias current, DESAT detection blanking time, and DESAT output deglitch filter, thus helps to increase immunity against the PWM switching noise.