STMicroelectronics ACST435-8FP

Overview

ACST435-8FP from STMicroelectronics is a 4 A, 800 V insulated AC switch with integrated overvoltage clamping, designed as a self-protected triac replacement for mains-connected inductive load control. It features 35 mA gate trigger current (IGT), 1000 V/μs static dV/dt immunity at 125 °C, and TO-220FPAB package with 2000 VRMS insulation - used in compressor and universal motor drive circuits in refrigerators and washing machines.

For engineers reviewing the ACST435-8FP datasheet, ACST435-8FP pinout, ACST435-8FP application, or ACST435-8FP equivalent, key selection criteria include gate drive compatibility with MCU outputs, IEC 61000-4-4/4-5 transient robustness without external snubbers, thermal derating in TO-220FPAB at Tc = 102 °C, and clamping voltage (VCL ≥ 850 V) under inductive turn-off stress.

Technical Context

The ACST435-8FP implements a planar triac structure with monolithically integrated overvoltage crowbar protection, enabling safe conduction during IEC 61000-4-5 line surges up to 2 kV. Its high-noise-immunity gate design supports direct MCU interfacing via a single series resistor (68 Ω recommended).

It operates in all four quadrants with guaranteed latching (IL ≤ 60 mA) and holding (IH ≤ 25 mA) currents, and sustains repetitive 30 A surge (60 Hz, 16.7 ms) without degradation. The device recovers full blocking capability after transient clamping at the next zero-crossing.

Key Specifications

Pinout & Package

ACST435-8FP uses the TO-220FPAB package: insulated plastic body with metal tab (electrically isolated), UL-certified (E81734), 2000 VRMS isolation, 0.4–0.6 N·m mounting torque, and footprint per Figure 19 (DS2762 Rev 12 p.11).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar AC mains switching applications.

Compared with ACST410-8FP, ACST435-8FP trades lower gate sensitivity for superior noise immunity - critical in compressor applications with high di/dt. Versus BTB16-800CW, it delivers drop-in surge robustness and eliminates three external components, reducing total solution size by 38%.

Availability

ACST435-8FP is available at Aetrix Electronics and suitable for refrigerator compressor control, washing machine motor drive, and HVAC damper actuation requiring stable component supply, long-term lifecycle support, and certified insulation performance.

Supply support for ACST435-8FP includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, designing and manufacturing analog, microcontroller, power, and sensor solutions for automotive, industrial, and consumer markets.

The ACST4 series belongs to ST's ACS/ACST family of protected AC switches - engineered specifically for relay replacement in home appliance motor control, emphasizing intrinsic surge immunity, gate simplicity, and thermal resilience in insulated packages.

FAQ

What is the maximum allowable gate resistor value for reliable triggering with a 3.3 V MCU output?

The datasheet specifies 68 Ω for ACST435-8FP in IEC 61000-4-5 test conditions. With a 3.3 V GPIO sourcing 35 mA max, Ohm's Law gives R = 3.3 V / 0.035 A ≈ 94 Ω theoretical upper limit. However, ST recommends 68 Ω to ensure margin against voltage drop, temperature drift, and gate capacitance effects - verified across -40 °C to 125 °C operation.

Does ACST435-8FP require a heat sink in a 4 A continuous load application?

Yes - at 4 A RMS, on-state power dissipation is ~2.7 W (VTM × IT). With Rth(j-c) = 4.6 °C/W and max Tj = 125 °C, case temperature must stay ≤110.3 °C. In free-air convection, TO-220FPAB's Rth(j-a) is 60 °C/W, resulting in >160 °C junction rise - exceeding rating. A minimum 10 cm² aluminum heatsink is required to maintain safe thermal margin.

Can ACST435-8FP replace a standard triac like BT139 in an existing design?

Yes, with minor layout adjustments: the TO-220FPAB pinout matches BT139 (MT1-COM, MT2-OUT, G-GATE), but COM is electrically isolated from the tab. You must insulate the mounting screw and verify gate drive strength - BT139's IGT is typically 50 mA, while ACST435-8FP requires ≤35 mA. No changes to snubber or protection circuitry are needed, as ACST435-8FP provides built-in clamping.

How does the VCL parameter function during an IEC 61000-4-5 surge event?

VCL (clamping voltage) defines the threshold at which the device transitions into low-impedance conduction to shunt surge energy. When line voltage exceeds ~850 V, ACST435-8FP enters crowbar mode, conducting for <10 ms until the next zero-crossing. This limits peak voltage across the load to ≤VCL and safely dissipates surge energy through the load path - confirmed in Figure 18 (DS2762 p.9) with 90 A peak current and 130 A/μs di/dt.