UM9989B

Overview

The UM9989B is a silicon PIN diode designed for MRI receiver protection. It operates as a passive limiter, self-biased by RF transmitter pulses without requiring external forward bias voltage. The device features a hermetic glass axial package and exhibits low insertion loss in both on and off states to preserve receiver sensitivity. Key electrical parameters include a peak repetitive reverse voltage of 75 V, a non-repetitive peak forward surge current of 2.5 A, and an operating temperature range from -65°C to +150°C. At 0 V bias, it maintains a capacitance of 1.2 pF typical at 1 MHz and conductance of 40 µS at 64 MHz.

Feature Summary

• Passive self-biasing operation activated by RF pulse power
• Hermetic glass axial construction with metallurgical bond
• Ultra-low magnetic design with planar passivated chip
• Compatible with automatic insertion equipment

Applications

• MRI receiver protection
• Body coil isolation

Procurement Notes

Verify the specific ordering suffix against the manufacturer's official documentation to confirm package style and compliance status. Confirm RoHS compliance requirements, as matte tin finish versions are designated with different suffixes such as UMX9989B. Ensure the procurement request matches the exact part number to avoid receiving surface mount variants instead of the axial leaded configuration.

Selection Notes

Select this component when designing circuits that require robust protection against high RF energy fields and spike pulses in MRI environments. The hermetic glass package provides environmental stability suitable for demanding medical imaging applications. Consider the thermal resistance and surge current capabilities when evaluating the diode's ability to handle long RF pulses without failure.

Part Context

The UM9989B belongs to the UM9989 series of fast MRI protection diodes manufactured by Microsemi (now Microchip Technology). This series includes variations such as the UMX9989B and UMX9989SM, which offer alternative packaging options like surface mount configurations. The UM9989B specifically denotes the hermetic glass axial variant within this product family.

Replacement Considerations

UMX9989B