Thermal Positive Feedback in SiC Schottky Modules: Why Current Crowds Into One Die as Temperature Rises
Why “hotter draws more” Occurs in SiC Schottky
A Schottky diode is a unipolar device. Forward voltage is governed by the drift-region resistance and the Schottky barrier. At low current density, the barrier term dominates; the effect of barrier lowering with temperature exceeds the effect of mobility degradation, and VF decreases with rising temperature—a negative temperature coefficient. As current density increases, the drift-resistance term becomes dominant, lattice scattering intensifies, and VF turns to increase with temperature—a positive temperature coefficient. The crossover is the ZTC point.
This means the temperature coefficient of a SiC Schottky is not a fixed label but a curve that varies with current. In light-load, low-duty-cycle, or standby conditions, the device may operate below the ZTC point. In a paralleled array where each die carries only a fraction of the total current, the per-die operating point is even more likely to fall into the negative-coefficient region. The sequence “higher Tj → lower VF → more current → higher loss → higher Tj” then forms a positive feedback loop, and current sharing is not achieved by the device but actively undermined.
Silicon PiN diodes have long been recognized as having a similar issue in paralleling, and engineering practice typically addresses it through derating or external current-balancing measures. The lower ZTC point of SiC Schottky means the negative-coefficient region extends further along the current axis, leaving a narrower “safe zone” than intuition suggests. Applying silicon-device paralleling experience directly to SiC Schottky, or selecting on nominal VF without checking its temperature slope at the target current, leaves this risk in the field.
What to Check: Replace “Low VF” with “Where Is the ZTC Point?”
For SiC Schottky paralleling or thermal design, a more reliable check sequence does not start with nominal forward voltage. It starts with locking the per-die operating current range under the target profile, then determining which side of the ZTC point that range occupies.
In Hypersemi’s SiC Schottky portfolio, CR2065SX100 is a 650 V / 20 A SiC SBD in a TO-263 package with a nominal VF of 1.38 V. That VF figure is competitive in single-device evaluation, but the question that matters for paralleling is whether its VF temperature slope is positive or negative at the target load current. The 1700 V SiC Schottky modules in the same portfolio target high-voltage DC-DC and PCS DC-side applications, where multi-die paralleling within a single module is more typical and ZTC verification carries greater weight. For each specific model, the ZTC location should be taken from the VF–Tj curve or ZTC parameter in the corresponding datasheet, not inferred from nominal VF.
Check item | Intuitive approach | More reliable engineering approach |
Parallel current-sharing judgment | Low nominal VF means natural sharing | Lock per-die operating current; verify whether it is above the ZTC point |
Thermal design input | Use typical VF only | Use VF temperature coefficient at target current in the electro-thermal model |
Derating basis | Apply a uniform derating from rated current | Set parallel derating from the gap between ZTC current and operating current |
It should be noted that SiC Schottky does exhibit a positive temperature coefficient above the ZTC point, which favors current sharing in parallel operation. The point is not that “SiC Schottky cannot be paralleled,” but that paralleling must specify the current range and the basis for judging which side of the ZTC point the design sits on. Treating the ZTC point as an explicit selection parameter is closer to engineering fact than a general belief that “SiC is easy to parallel.”
Conclusion: Current Sharing Is Earned by Design, Not Given by the Device
The low VF and low reverse-recovery charge of SiC Schottky are genuine advantages, but they do not automatically translate into paralleling safety. Below the ZTC point, rising temperature causes one die to draw more current, and the direction of the positive feedback is opposite to the intuition of “automatic current sharing.” For multi-die SiC Schottky modules or discrete paralleled arrays, selection should at minimum complete three steps: define the per-die operating current range, locate the ZTC point in the datasheet, and set parallel derating based on the gap between operating current and ZTC. A fuller range of SiC Schottky models and datasheets can be located on the SiC Schottky diode selection page by voltage, current, and package.
FAQ
Q1: Is a SiC Schottky positive-temperature-coefficient or negative-temperature-coefficient?
Both are possible, depending on operating current. Below the ZTC point, VF decreases as temperature rises, giving a negative coefficient; above the ZTC point, VF increases with temperature, giving a positive coefficient. Selection should not assign a fixed label to the device; it should examine the temperature slope at the target current.
Q2: If several SiC Schottky dice are paralleled from the same batch with close VF, is current crowding ruled out?
Same batch and close VF reduce initial mismatch, but they do not change the direction of the temperature coefficient. If the operating current falls below the ZTC point, small initial VF differences are amplified by the positive feedback loop, one die runs persistently hotter, and current crowding intensifies over time. Pairing consistency is necessary but not sufficient; ZTC verification is the basis for judging whether the sharing direction is stable.
Q3: What should a design that already uses silicon PiN diode paralleling experience watch for when switching to SiC Schottky?Silicon PiN has a relatively higher ZTC point, while SiC Schottky has a lower one, extending the negative-coefficient region toward higher currents. A current level that was “high enough” for silicon may still fall below the ZTC point for SiC Schottky. Replacement should re-confirm the operating current relative to the ZTC point rather than carrying over the silicon derating factor.