4pin Kelvin package boosts SiC FET performance

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By Nick Flaherty

By adding another lead to the source to take the majoirty of the current, the Kelvin package avoids gate ringing and false triggering which would otherwise require slowing of switching speeds to manage the large common source inductance of 3-leaded packages. The 4-leaded Kelvin connection packages offer maximum operating temperatures of 175°C with half the typical switching losses.

“We are seeing lots of new applications emerging fro 650V and 1200V and the big one we see is onboard chargers and that demands power density. One of the features we bring is being gate drive compatible with IGBTs and silicon MOSFEts which really helps people working on these problems right now,” said Anup Bhalla, VP Engineering at UnitedSiC. “Then we run into the problem that the older packages such as the 3 lead are very inductive and you have problems with ringing etc and to deal with these we take fast devices and slow them down.”

“Using the kelvin lead for the gate loop avoids false triggering. You have to work on layout and EMI but at least this way you can actually acheive the higher speeds,” he said. “It provides an additional connection to the top of  source metal – the 40 or 50A flowing has to flow through the source wires. If all of that is part of the gate driving loop there’s huge spikes. What we do with the additional lead is bypass this and make a direct connection to the source metal so all the high current is flowing the central lead so that the Kelvin lead only carries a small amount of the gate current so all the big voltage drops are not seen by the gate drive circuit.”

The parts are aimed at designers of Totem Pole PFC stages, LLC and Phase Shifted Full Bridge converters, used in EV chargers, telecom and server power applications for speeds up to 500kHz.

“This allows higher frequencies for smaller power supplies,” said Bhalla. “What I am seeing is 500kHz in actual designs. Previous to this, the 3 lead packages were used at 50 to 125kHz, so this is a big jump for say the 6kW systems which don’t usually run in the MHz range as the passive devices can’t keep up.”

Compared with other wide band-gap technologies, the SiC cascode devices offer standard 12V gate drive, and have assured avalanche ratings (100 percent production-tested). The 4-terminal packages, which form part of the UF3C FAST series, offer easy screw or clamp mounting with very low junction-to case-thermal resistance allowing lower temperature rise for a given power dissipation or higher power operation, taking advantage of the high junction temperature capabilities of SiC.

The range includes the following part numbers: UF3C120040K4S (1200 V / 40 mΩ), UF3C120080K4S (1200 V / 80 mΩ), UF3C065030K4S (650 V / 30 mΩ), UF3C065040K4S (650 V / 40 mΩ) and UF3C065080K4S (650 V 80 mΩ). Prices range from $11 for the UF3C065080K4S to $25 for the UF3C120040K4S at 1,000 pcs quantities.

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