How to Select a Silicon Carbide Heating Power Supply? From Start-up Control to Ageing Compensation-Technical Articles-KULUN Electric

Bottom line first: the difficulty of selecting a power supply for silicon carbide (SiC) heating is not "being able to adjust voltage", but covering two characteristics of SiC elements — their resistance changes nonlinearly with temperature and increases gradually over service life. Selection therefore has to confirm four things at once: how the start-up current is controlled according to the element data, whether constant power can be maintained during operation, whether the voltage ceiling leaves enough headroom for ageing compensation, and whether broken elements and current imbalance can be detected when several elements are wired in series and parallel. Once these four points are settled, capacity and model selection become meaningful.

Silicon carbide heating elements are widely used in glass, ceramics, metallurgy and laboratory high-temperature furnaces. Their resistance–temperature relationship differs clearly from that of ordinary resistance wire, and many site problems — "the breaker trips at start-up", "the power keeps dropping", "the current is too high after fitting new elements" — originate in a mismatch between the power supply and the element characteristics. The sections below follow the selection order.

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Why does the resistance of silicon carbide elements change with temperature and service time?

The resistance–temperature relationship of SiC elements is nonlinear: the difference between the cold section and the working-temperature section is significant, and the shape and magnitude of the curve vary considerably between manufacturers and element types. There is no rule of thumb that can be applied directly. The correct engineering approach is to obtain the resistance–temperature curve and the cold and hot resistance data of the specific element type from the element manufacturer, and to calculate the start-up and operating currents from that data — instead of assuming in advance that "resistance rises with temperature" or "resistance falls with temperature".

The second change comes from ageing. SiC elements oxidise slowly during long-term high-temperature operation, and their resistance increases gradually with service time. For a given power supply, if the voltage ceiling is designed exactly around new-element parameters, the system will eventually reach the state of "voltage at the limit, power still insufficient".

Taken together, the power supply must satisfy two requirements at the same time: the start-up current is calculated from the element data with control means reserved, power is held steady during operation, and the voltage ceiling leaves headroom.

Why is constant-power control usually preferred over simple voltage adjustment?

A supply that only performs open-loop voltage adjustment lets the output power follow the load resistance: as resistance rises, power falls and the furnace temperature drifts. Constant-power control samples voltage and current, calculates the output power in real time and closes the loop, keeping the power delivered to the furnace at the setpoint and automatically compensating part of the resistance drift caused by element ageing.

"Constant-current start, then constant-power operation" is a common engineering practice, but not every furnace type must copy it — whether to adopt it depends on the SiC element type, the transformer tap arrangement, the voltage adjustment range and the control method. When adopted, the typical process is: limit the current during start-up to establish the current, then switch to constant-power or constant-temperature closed-loop control once the current reaches the permitted value, which suppresses abnormal start-up current and stabilises the operating power. The current-limit value and the switching point should be determined from the element manufacturer's data together with site commissioning.

Control method	Characteristics	Typical use
Open-loop voltage adjustment	Simple structure; power drifts with load resistance	Heating duties with low temperature-accuracy requirements
Constant-current control	Limits the start-up current; current under control	Duties where the element data shows that current limiting at start-up is required
Constant-power control	Stable power; partially compensates ageing drift	Loads whose resistance changes with temperature, such as SiC elements
Constant-temperature closed loop	Furnace temperature as the controlled variable, with PID	Processes with a defined temperature profile

Which parameters should be confirmed during selection?

The parameters of a SiC heating power supply are not just "how many kilowatts". Confirm the following items one by one:

Parameter	Why it matters
Element type and quantity	Determines total resistance, wiring method and required current
Per-element operating voltage/current	Determines the series/parallel combination and the transformer output voltage
Cold and hot resistance	Used to calculate the start-up and operating currents
Maximum working temperature	Affects the element ageing speed and the voltage headroom
Voltage adjustment range	Must cover the resistance change from new elements to late-life ageing
Wiring method	Series, parallel or grouped; affects current sharing and broken-element detection
Control method	Constant-current start, constant power or constant-temperature closed loop
Cooling and installation environment	Determines the cabinet cooling design and protection rating

Among these items, the voltage adjustment range is the one most easily overlooked. SiC elements need a higher output voltage to maintain the same power in the later stage of their life, so the output ceiling of the supply (or the matched transformer) should normally leave headroom instead of being sized tightly around new-element conditions.

What should be noted when elements are wired in series and parallel?

A real furnace rarely uses a single element; elements are usually combined in series, in parallel or in groups. Three points deserve attention:

Impedance matching: the resistance of elements in the same group should be as close as possible, so that no single element carries an excessive current;
Broken-element effect: when one branch of a parallel group opens, the current of the remaining elements redistributes and rises, so current-imbalance or broken-element detection is needed;
Grouping and control: when groups are controlled independently, the synchronisation and power-distribution strategy between groups must be considered.

For furnaces that need independent temperature control by zone, a multi-channel power control unit or a zone-by-zone control scheme is easier to match.

What matters for start-up and protection?

For the start-up characteristics of SiC elements, the protection scheme should cover: overcurrent protection, open-circuit and short-circuit protection of the load, current-imbalance (broken-element) detection, over-temperature protection, phase-loss protection and interlocking with the cooling system. How complete the protection scheme is determines whether an element or supply abnormality ends in a protective shutdown or in wider damage.

FAQ

Q: Can an ordinary voltage regulator replace a dedicated SiC heating power supply?
A: It depends on the process requirement. An ordinary regulator changes the output voltage but generally has no constant-power closed loop, no start-up current limiting and no broken-element detection. For glass, ceramic or laboratory furnaces with demanding temperature profiles, a power control unit with constant-power control is normally required; whether additional current limiting is needed at start-up should be determined from the element characteristics.

Q: After replacing old SiC elements with new ones, why is the current too high at the same voltage?
A: SiC elements age in the direction of increasing resistance, so old elements that have been in service for a long time have a higher resistance than new ones, and the current naturally rises at the same voltage after the change. In addition, when the furnace is first loaded the temperature is low and the elements are cold, where their resistance differs from the working-temperature value. The reasonable approach is to re-check the voltage and current settings against the new elements' resistance data and to confirm that the current-limiting and constant-power functions are actually active.

Q: How much voltage headroom should be reserved for a SiC power supply?
A: The headroom depends on the ageing speed of the elements and the maximum working temperature, and differs considerably between element types. It should be determined from the resistance–temperature and ageing data provided by the element manufacturer together with the process temperature. In engineering practice the voltage ceiling is normally not designed right at the critical value of the new-element condition.

Q: How to choose between phase-angle control and zero-cross (burst) control?
A: Phase-angle control gives a continuous, fine-grained output but a relatively higher harmonic content; zero-cross control produces fewer harmonics but a discontinuous on/off output. For high-power installations the grid conditions and the process tolerance for temperature fluctuation must be evaluated together.

About the timeliness of this article

This article reflects general engineering practice as of October 2026 and discusses the selection logic and key checkpoints for SiC heating power supplies. It does not constitute a selection conclusion for any specific model or capacity. Parameters differ considerably between furnace types, element types and process temperatures; actual selection should be based on the element manufacturer's datasheet and the site conditions.