Telecom power is a hostile environment for magnetic components: −40 °C to +75 °C ambient, 4,000 m altitude derating, lightning surges on the input, salt fog at coastal sites, and a service life measured in decades. A rectifier that meets its datasheet on a lab bench can still fail in the field if the magnetics were specified for commercial-grade conditions.
1. The power chain and its magnetics
A −48 VDC telecom rectifier typically runs an input EMI filter, a PFC stage, an isolated DC-DC (phase-shifted full bridge or LLC), and an output filter. 5G remote radio units add a further stage: a wide-input DC-DC inside the AAU, often 48 V to 30–50 V at 1–2 kW in a sealed enclosure with no fan.
| Block | Magnetic | Telecom-specific requirement |
|---|---|---|
| Input filter | Common mode + differential choke | Surge withstand, high impedance 150 kHz–30 MHz, coastal corrosion resistance |
| PFC | Boost inductor | Stable inductance to 75 °C, low loss at high line and light load |
| DC-DC | Transformer + resonant inductor | Isolation for 3 kV impulse, low leakage, wide temperature swing |
| Output | Output choke | High DC bias, low DCR, thermal margin at 75 °C |
| AAU DC-DC | Planar transformer or integrated module magnetics | Very low profile, sealed enclosure thermal path, high power density |
2. Temperature is a material question
Ferrite permeability drifts with temperature, and a core chosen for 25 °C can lose 20–30% of its inductance at 75 °C. In a PFC inductor that shifts the ripple current; in a resonant converter it shifts the gain curve. Two practical responses: select a material with a flat permeability-versus-temperature curve over the operating range, or design the control loop to tolerate the drift and specify the inductance at the hot condition rather than at 25 °C.
Powder cores behave differently — their permeability is far more stable with temperature, which is one reason distributed-gap cores dominate telecom PFC inductors.
3. Altitude, humidity and corrosion
Above 2,000 m, air density reduces convection cooling; a part designed for 40 K rise at sea level can exceed its insulation limit at 4,000 m. Derate the loss budget accordingly, and require the supplier to state the altitude assumption in the thermal report.
Coastal sites need salt-fog-resistant finishes: a varnish or conformal coating that covers the core face as well as the winding, and terminal plating that does not corrode. Uncoated iron powder cores exposed to humid air can rust at the exposed grain boundaries over years.
4. Surge and inrush
Telecom inputs see 1.2/50 µs and 8/20 µs surge waveforms at kV levels. The common mode choke must not saturate during the surge, or the protection network is bypassed. Specify the choke’s saturation current against the surge current, not against the operating current, and confirm the winding insulation withstands the same event.
5. Reliability targets
Field life of 10–20 years with a FIT rate that supports the rectifier’s MTBF claim. That translates into derating rules: hot-spot temperature at least 20 K below the insulation class limit, flux swing well under saturation, and no electrolytic-dependent assumptions in the magnetic design. Thermal cycling between −40 °C and +75 °C is the qualification test that most often finds a marginal winding or a potting void.
6. Why the core should come from the same plant as the winding
Telecom programs change material grades during cost reduction more often than any other segment. When the core and the winding come from one manufacturing site, that change is a controlled engineering decision with a new qualification report. When they come from two, it becomes a documentation exercise. Shaanxi SiRui Technology produces the Magnetic Core series — including nanocrystalline and amorphous cores well suited to compact high-impedance chokes — and winds telecom-grade inductors and transformers in the same facility.
Send your environmental and surge specification to sales@sirui-tech.com for a derated proposal.