An Analysis of Three Major Packaging Technologies for Fast-Recovery Diodes

The ultra-fast recovery diode in the TO-252 package is a power semiconductor device manufactured using surface-mount technology, characterized by a short reverse recovery time and low switching losses, making it well suited for high-frequency circuits.

Short reverse recovery time: Typically on the order of tens of nanoseconds, this brief reverse recovery time enables rapid transitions from conduction to cutoff in high-frequency switching circuits, thereby reducing switching losses and enhancing circuit efficiency.

Low reverse leakage: Manufactured using silicon epitaxial technology, it features a reduced reverse leakage current, thereby enhancing device reliability.

Good thermal performance: The TO-252 package features a back-side thermal pad that can be soldered to a large copper area on the PCB, effectively dissipating heat and reducing chip temperature, making it suitable for medium-power applications.

The TO‑252 is a three‑lead surface‑mount package whose defining feature is a large metal thermal pad on the back, serving as the device’s primary heat‑dissipation path. Its typical dimensions are 6.45–6.70 mm in length, 6.00–6.20 mm in width, and 2.19–2.39 mm in maximum height, making it well suited for thin‑profile PCB designs. The standard lead center‑to‑center spacing is 2.286 mm, with pin assignments typically designated as: Pin 1 – Anode, Pin 2 – Cathode, and Pin 3 – Anode; Pin 3 is electrically tied to the rear thermal pad. For recommended PCB pad layouts, the main thermal pad should measure at least 5 × 5 mm, and it is advisable to include copper‑exposed window areas to enhance soldering and heat‑dissipation performance. Lead pads are approximately 1 mm wide and 2.5 mm long, and 2–4 vias may be added at the main pad location to connect to internal copper layers, further reducing thermal resistance.

The SOD-123FL is a surface-mount ultra-fast recovery diode featuring a compact footprint and rapid switching speeds, making it ideal for space-constrained high-frequency circuits. Its ultra-thin surface-mount package is well-suited for small‑power ultra-fast recovery diodes in the 1 A range, offering a low profile that accommodates high‑density PCBs and thin‑profile designs. It is widely used in applications such as low‑voltage high‑frequency switching power supplies, DC‑DC converters, and reverse‑polarity protection circuits.

The SOD-123FL is a two‑lead ultra‑thin surface‑mount package, featuring a flat, low‑profile structure that facilitates automated mounting and enables slim‑line designs.

Package dimensions: The package measures approximately 2.8 ± 0.1 mm in length, 1.8 ± 0.1 mm in width, and a maximum height of only 0.9–1.2 mm, making it thinner than the standard SOD‑123. The lead center-to-center spacing is about 1.27 mm, with polarity indicated by a color band on the cathode end; the cathode and anode leads are arranged symmetrically.

Pad and Thermal Design: It is recommended that the PCB pad be approximately 0.8 mm wide and 2 mm long. The primary thermal dissipation area can be implemented as a 2 × 2 mm exposed copper region with cutouts. Given the limited thermal dissipation capability of the package, 1–2 vias can be added adjacent to the pad to connect to the inner-layer copper plane, thereby reducing thermal resistance and enhancing power-handling capability.

Key Considerations for Selection

 . Extreme parameter

VRRM: Select based on 1.2–1.5 times the circuit’s reverse peak voltage, covering the range of 50 V to 600 V.

IF (AV): Rated at 1 A; the actual current shall not exceed 0.7 times the rated value, taking into account both the duty cycle and thermal dissipation calculations.

IFSM: Must exceed the inrush/short-circuit transient current; typical withstand value is 30 A–50 A.

Both the junction temperature and the storage temperature range from −55°C to +150°C, ensuring ample margin for high-temperature applications.

 . Dynamic performance

trr: Typical 25–50 ns; for high-frequency applications above 100 kHz, prioritize models with ≤35 ns to reduce switching losses and EMI.

Qrr: Select a smaller value to reduce reverse recovery and drive‑induced switching losses.

di/dt and dv/dt: must meet the transient stress requirements of fast switching in the circuit.

 . Static and Thermal Characteristics

VF: 1.3 V–1.7 V at the rated current of 1 A; select the lower value to reduce conduction losses.

IR: At the rated reverse voltage, it is at the μA level; the leakage current increases slightly at elevated temperatures, which can help reduce standby power consumption.

RθJA: Typically 80–120 °C/W, strongly dependent on the copper foil area of the PCB; junction temperature must be calculated in conjunction with power dissipation.