Parallel Current Sharing in Data Center PSUs: The ZTC Boundary of SiC Diodes
Under what conditions can thermal positive feedback occur?
When data center PSUs are paralleled to increase current, the diode's positive temperature coefficient is often used as the basis for "natural current sharing." However, the temperature coefficient of SiC Schottky diodes varies with current; below the zero temperature coefficient (ZTC) point, thermal positive feedback can still occur. The key to device selection is not how low VF is, but the relative position of the target operating current and the ZTC point.
As power density in data center PSUs continues to rise, paralleled redundancy has become standard when a single module’s output cannot cover the peak power of AI servers or high-density racks. The N+M architecture imposes a more complex requirement at the current-sharing level than “each device meets specification”: parallel branches must maintain balanced current distribution in both steady state and transients, or effective output must be derated and reliability compromised.
The diode’s position in a paralleled PSU—particularly in DC-DC secondary rectification and ORing paths—makes its temperature characteristics a hidden variable in current-sharing design. SiC Schottky diodes enter this scenario with zero reverse recovery and a positive VF temperature coefficient, often summarized as “natural current sharing.” But this label needs a qualifier: the positive temperature coefficient holds only above the zero-temperature-coefficient (ZTC) point.
The ZTC Point: The Dividing Line for Current-Sharing Direction
The VF of a Schottky diode is governed by both the barrier term and the drift-region resistance term. At low current, the barrier term dominates; barrier lowering with temperature outweighs mobility degradation, and VF decreases as temperature rises—a negative temperature coefficient. As current increases, the drift-resistance term takes over, lattice scattering intensifies, and VF turns to increase with temperature—a positive coefficient. The crossover is the ZTC point.
For a parallel system, the ZTC point’s location determines the direction of current sharing. Above the ZTC point, the hotter device has higher VF, current shifts toward the cooler device, and self-balancing occurs. Below the ZTC point, the hotter device has lower VF, current crowds toward the hot die, and thermal positive feedback closes.
An IEEE comparative electrothermal study of paralleled 600 V SiC Schottky and silicon PiN diodes offers a notable conclusion: SiC Schottky has a lower ZTC point than silicon PiN, giving it better temperature convergence and more equal current sharing under mismatched initial junction temperatures and thermal boundary conditions. This supports SiC’s electrothermal stability advantage in paralleling, but “lower ZTC point” is a double-edged property: it extends the positive-coefficient region toward higher currents, making light-load or low-current conditions more likely to fall on the negative-coefficient side.
The Data Center PSU Load Profile: Light Load Is Not a Rare Condition
Data center PSUs rarely operate at constant full load. IT load fluctuations, standby rotation of redundant modules, and low-utilization nighttime periods cause individual modules in a paralleled PSU to operate frequently at 30%–50% of rated current or lower.
In this range, whether a SiC Schottky remains on the positive-coefficient side cannot be inferred from the general statement that “VF has a positive temperature coefficient.” The VF–Tj curve in the datasheet must be consulted to confirm where the target operating current sits relative to the ZTC point. If the ZTC point is above the light-load operating current, the premise of “natural current sharing” does not hold in that profile, and thermal positive feedback risk remains.
This shifts the focus of current-sharing design from “selecting a device with good-looking VF” to “confirming whether the loss structure of the target profile falls on the safe side.” For paralleled secondary rectification or ORing diodes in a PSU, the variables to lock at selection include the target operating current range, the ZTC point location, and the weight of light-load periods in the overall mission profile.
Model Entry Points and ZTC Verification
In Hypersemi’s 650 V SiC SBD portfolio, CR1565PEX100 (650 V / 15 A / VF 1.5 V / PDFN8×8) and CR1065TCX100 (650 V / 10 A / VF 1.39 V / TO-220AC) cover high-density surface-mount and through-hole thermal layouts respectively. In data center PSU DC-DC secondary rectification or ORing paths, the ZTC location of these devices should be verified against the VF–Tj data in each datasheet, not inferred from nominal VF or the generic “positive temperature coefficient” description.
Check Dimension | What to Confirm | Selection Implication |
Target operating current range | Per-device current in parallel branches under typical and light-load profiles | Whether it is above the ZTC point |
ZTC point location | The crossover of positive and negative slopes in the datasheet VF–Tj curve | Whether the current-sharing direction is stable in that profile |
Light-load share | Time weight of 30%–50% load in the mission profile | Whether thermal positive feedback risk is acceptable |
Conclusion: Current-Sharing Direction Must Be Verified, Not Assumed
The electrothermal stability of SiC Schottky in paralleling is real in the operating range above the ZTC point, and the IEEE comparative study supports this judgment. But data center PSU load profiles contain substantial light-load periods, and the negative-coefficient region below the ZTC point cannot be glossed over with a “positive temperature coefficient” label.
The goal of current-sharing design is not to prove that a device “will share current in theory,” but to confirm that at every current level in the target profile, the sharing direction points toward stability. This requires extracting the ZTC point from the background of the datasheet and placing it in the selection checklist. A fuller range of SiC SBD voltage, current, and package combinations can be located on the SiC diode selection page according to actual topology conditions.