The FIS115NL is a compact 1:100 current sense transformer whose headline electrical figures set expectations for SMPS feedback and protection: approximately 1:100 turns ratio, ~18 mH magnetizing inductance, ~2 Ω secondary DC resistance, up to 500 kHz usable frequency, and a primary continuous rating near 25 A with a finite V·μs headroom. These specs determine measurement accuracy, bandwidth, and V·μs margin that protect the core from saturation and define usable dynamic range for control loops.
Accurate current sensing in switching supplies depends on three interacting envelopes: core magnetics (inductance and saturation), burden and secondary resistance (sets signal amplitude and thermal loss), and frequency response (sets amplitude error and phase). The following sections break down core specs, operational limits, bench validation steps, integration notes, and replacement guidance for robust SMPS design and trouble‑shooting.
| Parameter | Typical / Notes |
|---|---|
| Turns ratio | 1 : 100 (primary single turn, secondary ~100 turns) |
| Primary rating | ~25 A continuous (use derating for ambient/temperature) |
| Secondary current (calc) | 25 A → 0.25 A secondary at 1:100 |
| Secondary DC resistance | ~2 Ω (burden and tolerance affect voltage) |
| Magnetizing inductance | ~18 mH (low‑frequency headroom) |
| Leakage inductance | low — design dependent; impacts high‑frequency rolloff |
| Max frequency | ~500 kHz usable (amplitude/phase degrade toward limit) |
| V·μs limit | finite V·μs — calculate margin per waveform to avoid saturation |
| Insulation / HI‑POT | specified in datasheet — verify for system isolation needs |
| Thermal / derating | recommend derate above steady ambient; check datasheet curves |
Each row ties directly to measurement fidelity: turns ratio sets gain and required burden resistor; magnetizing inductance sets low‑frequency droop and V·μs headroom; DC resistance and winding losses determine burden heating and offset. Use the table values as design anchors and verify per‑lot variation on incoming inspection.
The transformer is a through‑hole PCB mount part with a small rectangular footprint and vertical lead exit. Recommended PCB keepouts include a minimal secondary loop area and clearance from high‑voltage switching nodes; lead spacing and standoff height determine routing and creepage. Mounting affects thermal dissipation and EMC: tight seating and short secondary traces preserve common‑mode immunity and reduce radiated emissions.
Point: DC offset or net flux from a nonzero average primary current will drive the core toward saturation. Evidence: finite magnetizing inductance (~18 mH) and specified V·μs headroom mean integrated voltage over a switching interval must remain below the core limit. Explanation: compute V·μs = Vprimary × ton; ensure the product divided by turns ratio and magnetics leaves margin. Derate for continuous DC or long transients.
Point: amplitude droop at low frequency and roll‑off at high frequency both reduce accuracy. Evidence: usable bandwidth near 500 kHz implies measurable amplitude error and phase shift as frequency approaches that limit. Explanation: low‑frequency droop follows magnetizing impedance; high frequency behavior follows leakage/winding capacitance. Characterize amplitude error and phase at 100 kHz, 250 kHz and 500 kHz to quantify closed‑loop impact.
Start with turns ratio, secondary DC resistance, and inductance using an LCR meter: verify ~1:100 ratio, ~2 Ω DC R, and ~18 mH inductance. Perform insulation/HI‑POT to the datasheet value and visual inspection for discoloration. Required gear: LCR meter, calibrated current source or power supply, oscilloscope with differential probe, and hipot tester. Compare measured values to table tolerances to flag defects.
Calculation box: 25 A primary at 1:100 → 0.25 A secondary. With a 2 Ω burden this produces 0.25 A × 2 Ω = 0.5 V peak; for RMS or pulse calculations, scale by waveform shape and duty cycle. Use this to size burden resistor and ADC input ranges.
Set up a test where a controlled triangular or rectangular primary current waveform is injected and secondary voltage observed. Use a burden chosen to produce a safe, measurable voltage (for 25 A primary, burdens between 0.5–2 Ω yield 0.125–0.5 V secondary typical). Apply worst‑case ramp to measure V·μs headroom and record onset of saturation. Pass criteria example:
Primary uses include primary current feedback for regulation, peak‑current monitoring in flyback or forward stages, and overcurrent protection. Match bandwidth to switching frequency and ensure V·μs margin in topologies with large magnetizing volt‑microsecond products (flyback primaries). Typical connection: single‑turn primary conductor through core, secondary to burden resistor, then to sense amplifier or ADC input with appropriate filtering.
Keep the secondary loop and burden resistor close and short to minimize loop area; use star grounding for the sense return and place the burden near the converter controller. Add small series ferrite or RC filtering on the secondary when necessary, and avoid routing secondary under noisy switching nodes to reduce induced error. Prevent stray DC by avoiding split primary paths or offset currents through adjacent copper.
Symptoms include DC offset (partial saturation), increased noise (layout or partial short), or open secondary. Diagnose by measuring DC resistance versus expected ~2 Ω, verifying turns ratio, performing hipot, and inspecting for thermal discoloration. Thermal drift or sudden offset usually points to localized core heating or partial shorting in windings; replacement is safest if specs deviate significantly.
Match turns ratio and V·μs headroom first, then magnetizing inductance, DC resistance, frequency rating and mechanical footprint. For burden equivalence, recalc expected secondary current (Isec = Ipri / ratio) and ensure the replacement burden produces the same voltage into the sensing ADC or amplifier while remaining within power limits.
The FIS115NL functions as a 1:100 current sense transformer whose usable envelope is defined by magnetizing inductance (~18 mH), secondary DC R (~2 Ω), frequency limit (~500 kHz) and V·μs headroom for avoiding saturation. Practical takeaways: always verify V·μs margin for your SMPS topology, bench‑test turns ratio and dynamic accuracy at switching frequency, and follow PCB and grounding best practices to preserve measurement fidelity for regulation and protection.
Use a known low‑frequency AC source or LCR meter: excite the primary with a small AC voltage and measure secondary voltage, or inject a calibrated DC current pulse and measure steady‑state secondary current. The ratio should be about 1:100; large deviations indicate winding damage or incorrect part.
Select the burden so the secondary voltage is within the amplifier/ADC range while keeping power dissipation acceptable. Example: at 25 A primary → 0.25 A secondary; a 2 Ω burden produces 0.5 V (peak) and 0.06 W dissipation—adjust burden to match sensing input and thermal budget.
Apply a worst‑case ramp current waveform and monitor secondary voltage over the switching interval. Compute V·μs = Vprimary × ton and ensure the integrated flux does not exceed the transformer's V·μs headroom. Record the point of waveform distortion to determine safe operational margin and apply derating for continuous DC offsets.