August 2026 Volume 8

EQUIPMENT & TECHNOLOGY

Parameter

SCR

Power BJT

Power MOSFET

IGBT

400 to 500 Hz

Up to 10 kHz

100 kHz+

1 kHz to 50 kHz Moderate (~3V)

Operating Frequency

On-State Voltage Drop Very Low (< 2V)

Low (< 2V)

High (Load Dependent Losses)

Current Controlled Single pulse turn-on Critical (Unpolarized) 3000 V / 2000 A Minority Carrier Mandatory External

Current Controlled

Voltage Controlled

Voltage Controlled

Control Architecture Drive Requirements Snubber Circuitry

Continuous base drive Continuous gate drive Continuous gate drive

Critical (Polarized) 2 kV / 1000 A Minority Carrier

Often Optional 600 V / 200 A Majority Carrier

Often Optional 3300 V / 1500 A Minority Carrier

Max V/I Ratings

Carrier Type

None

None

None

Commutation Needs Temperature Coeff.

Negative

Negative High Risk

Positive

Flat / Positive Inherently Safe

Thermal Runaway Risk High Risk

Inherently Safe

Requires external equalization

Requires equalizing circuits

Exceptionally Easy

Exceptionally Easy

Parallel Scalability

Eliminating the Rivals: Why Other Devices Fall Short The SCR: Stuck in the Slow Lane Silicon Controlled Rectifiers (SCRs) are the giants of the power world, boasting massive voltage and current handling. However, SCRs are fundamentally limited by slow switching speeds, typically topping out below 1 kHz. Furthermore, they require complex, bulky external commutation circuits just to force the device to turn off. For medium-frequency induction heating, the SCR is simply too slow and architecturally inefficient. The Power BJT: A Thermal Liability The bipolar junction transistor (BJT) can stretch its operation up to 10 kHz, matching the lower boundary of medium-frequency heating. However, it is a current-controlled device that demands a massive, continuous base drive current to stay active. Worst of all, BJTs possess a negative temperature coefficient. As the device heats up, it draws more current, creating a dangerous feedback loop known as thermal runaway that can catastrophically destroy the inverter. The Power MOSFET: Fast but Fragile Power MOSFETs are high-speed champions, effortlessly operating well beyond 100 kHz, making them perfect for high-frequency surface hardening of small components. However, they struggle at medium frequencies due to high conduction losses. While MOSFETs excel at extremely high frequencies, their conduction losses and power-handling limitations often make them less practical for the high-current medium-frequency applications commonly found in forging operations. The Verdict: Why the IGBT Dominates The IGBT is a brilliant hybrid device. It seamlessly blends the best attributes of two worlds: the simple, low-power voltage-gated drive of a MOSFET and the high-current, low-conduction-loss capability of a BJT. • The Sweet-Spot Frequency: The IGBT effortlessly spans the exact 1 kHz to 10 kHz window required for medium frequency applications, with headroom to scale higher if needed.

• Simplified System Footprint: Because it turns off cleanly without forced commutation, engineers can eliminate bulky external circuitry. Snubber networks can also be minimized. • Rock-Solid Thermal Stability: Featuring a flat-to positive temperature coefficient, the IGBT maintains stable performance even as ambient temperatures rise inside a forge shop, thus helping to reduce the risk of thermal runaway. • Modular Scalability: When an induction furnace needs a power boost, multiple IGBT modules can be configured in parallel with minimal balancing effort, allowing for flexible, modular inverter designs. Through continuous R&D, we have perfected this integration. While Ajax TOCCO still designs and manufactures power supplies utilizing legacy architectures where specialized applications demand them, we heavily favor advanced-generation IGBTs for modern forging. For heavy-duty, deep-mass medium frequency industrial heating, IGBT devices dominate as the most efficient, dependable, and cost-effective choice on the market. At one automotive forging facility, an aging SCR-based induction system was replaced with a modern IGBT-driven power supply. The upgrade reduced maintenance requirements, improved heating consistency, decreased energy consumption by 10%, and decreased billet temperature variability to within ±10°C, significantly increasing production uptime. Looking Forward Ceramic high-power Insulated Gate Bipolar Transistors (IGBTs) could be used in the very near future. These are specialized semiconductor devices built on ceramic substrates (like Aluminum Nitride) rather than standard fiberglass. They are widely utilized in heavy-duty power electronics, industrial motor drives, and renewable energy inverters due to their superior thermal management. Advantages • Superior Thermal Conductivity: Ceramic substrates dissipate heat up to 100 times more effectively than standard, keeping high-power junctions stable and preventing thermal runaway.

FIA MAGAZINE | AUGUST 2026 15