Topdiode manufactures Silicon Controlled Rectifiers (SCRs) and Triacs for AC power control, controlled rectification, motor control, industrial power conversion, appliances and protection circuits. The thyristor portfolio includes sensitive-gate devices, general-purpose power devices and high-current SCRs in industry-standard SMD and through-hole packages.

For engineers and purchasing teams qualifying an alternative semiconductor source, Topdiode also provides SCR and Triac cross-reference support for selected devices from STMicroelectronics, WeEn, Vishay, Littelfuse, onsemi, IXYS and other established semiconductor brands.

SCR & Triac Quick Answer

An SCR is a unidirectional thyristor that is triggered by a gate signal and remains conducting until its current falls below the holding current. SCRs are commonly used for controlled rectification, high-current power control, crowbar protection and industrial power conversion.

A Triac is a bidirectional thyristor capable of controlling both half-cycles of an AC waveform. Triacs are widely used in heater control, lamp dimming, fan speed control, universal motors, appliances and other AC load-control applications.

Silicon Controlled Rectifiers (SCRs)

Silicon Controlled Rectifiers are unidirectional gate-controlled power semiconductors. Once triggered, the SCR remains latched in the conducting state until the anode current falls below its holding-current threshold.

Topdiode SCRs are used in applications including controlled rectifiers,industrial motor control, welding equipment, battery chargers, power supplies, soft starters and overvoltage crowbar circuits.

Available SCR characteristics include:

• Low-current and high-current thyristor options
• Repetitive blocking-voltage classes from 200V to 2000V
• Sensitive-gate and standard-gate devices
• High surge-current capability for industrial loads
• Glass-passivated device structures
• SMD and through-hole power packages

Representative high-power products include S5560-12N 55A 1200V SCR, S7560-12N 75A 1200V SCR, S7560-16N 75A 1600V SCR, S10060-12P 100A 1200V SCR and S10060-16P 100A 1600V SCR.

A thyristor is a type of semiconductor device that acts as a switch, controlling the flow of electrical current in a circuit. Thyristors are typically used for switching and controlling high voltage or current in applications where the power must be turned on or off. They are made of four layers of semiconductor material (P-N-P-N) and typically have three terminals: anode, cathode, and gate.

Difference Between SCR & TRIAC

SCR (Silicon Controlled Rectifier):

1. Structure: An SCR is a type of thyristor that has a single direction of current flow. It has four layers (P-N-P-N) and three terminals: anode, cathode, and gate.
2. Control: It is controlled by a gate signal. Once triggered, it remains in the "on" state (conducting) until the current flowing through it drops below a certain threshold (called the "holding current").

TRIAC (Triode for Alternating Current):

1. Structure: A TRIAC is similar to an SCR but can conduct current in both directions, making it suitable for AC applications. It has a similar structure but with an additional set of layers.
2. Control: Like an SCR, a TRIAC is controlled by a gate signal, but it can conduct during both halves of an AC cycle, allowing bidirectional control.

How to Select an SCR or Triac

Selecting a thyristor requires more than matching voltage and current. The following parameters determine whether an SCR or Triac can operate reliably in the actual circuit.

1. Repetitive Blocking Voltage: VDRM and VRRM

VDRM is the repetitive peak off-state voltage. For SCRs, VRRM specifies repetitive peak reverse voltage. The selected voltage class must provide sufficient margin above the normal working voltage and expected transient conditions.

2. Current Rating: IT(RMS) and IT(AV)

Triacs are commonly specified using RMS on-state current, IT(RMS), because they conduct during both AC half-cycles. SCR specifications may also use average on-state current, IT(AV), depending on the application and conduction angle.

The current rating must always be considered together with case temperature, thermal resistance and heatsink conditions.

3. Gate Trigger Current: IGT

IGT is the gate current required to trigger the thyristor under specified test conditions.

Sensitive-gate devices can simplify control circuits driven from microcontrollers or optocouplers, while higher-current industrial devices often use stronger gate-drive circuits to improve noise immunity.

4. Surge Current: ITSM and I²t

ITSM specifies the non-repetitive surge on-state current. This is important in applications with inrush current, motor starting, capacitor charging or abnormal fault conditions.

The I²t value helps engineers coordinate semiconductor surge capability with fuses and other protection components.

5. dv/dt and di/dt

A high rate of voltage change can unintentionally trigger a thyristor, while excessive current rise after turn-on can create localized current stress before conduction spreads uniformly through the silicon.

For electrically noisy AC systems, inductive loads and hard-commutated circuits, dv/dt and di/dt ratings should be reviewed carefully.

6. 3-Quadrant vs 4-Quadrant Triacs

A conventional four-quadrant Triac can be triggered under a wider set of gate-polarity conditions. Three-quadrant Triacs are optimized to avoid one of the less favorable triggering quadrants and can provide improved commutation behavior in demanding AC-load applications.

The correct choice depends on the gate-drive circuit, load type and commutation conditions.

7. Package and Thermal Design

Topdiode SCRs and Triacs are available in packages including TO-92, SOT-223, TO-252/DPAK, TO-220, TO-263/D2PAK, TO-3P, TO-247 and TO-247PLUS.

For high-current applications, package thermal resistance, mounting method, insulation requirements, heatsink interface and terminal geometry must be evaluated together with the semiconductor electrical ratings.

A useful second source should provide more than a nominally similar part number. Topdiode supports engineers and procurement teams with a broad thyristor portfolio and cross-reference information designed to simplify initial alternative-source evaluation.

In the IC and Semiconductors market, many customers have faced significant challenges with lead times when dealing with certain US and Japanese IC brands. However, with China’s increasing investment in IC CHIPS, Topdiode is able to offer an extensive range of IGBTs, driver ICs, audio ICs at more competitive prices and shorter lead times.

Topdiode services are designed to meet the needs of customers seeking cost savings with faster project timelines, please contact info@topdiode.com for more information.

• Broad SCR and Triac portfolio: low-current through high-power devices across multiple voltage classes.
• Cross-reference support: candidate alternatives for selected ST, WeEn, Vishay, Littelfuse, onsemi and IXYS devices.
• Industry-standard packaging: SMD and through-hole options for existing PCB and mechanical platforms.
• Engineering-oriented selection: part selection based on voltage, current, IGT, surge capability, thermal performance and package requirements.
• Qualification support: datasheets and samples can be used by customers to evaluate a proposed cross reference before production approval.

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