An LED driver is a constant-current source wrapped around a magnetic component, and the magnetics decide three things at once: efficiency, flicker, and whether the product passes EN 55015 and IEC 61347 without a redesign. This guide covers the practical design choices for drivers from 5 W retrofit lamps to 300 W high-bay and street-lighting units.
1. Topology decides the magnetic
| Power range | Typical topology | Magnetic component |
|---|---|---|
| 5–30 W | Non-isolated buck / buck-boost | Single inductor, often on a ferrite drum or molded core |
| 30–100 W | Isolated flyback (with or without PFC) | Gapped flyback transformer acting as energy-storage inductor |
| 100–300 W | Boost PFC + LLC or flyback | PFC inductor plus resonant transformer |
| Any, output side | Ripple filter | Output choke, sometimes coupled |
2. The flyback transformer stores energy — treat it as an inductor
In a flyback the transformer is not a transformer in the classical sense: it stores energy during the on-time. That changes the design method. The core needs a defined air gap, the inductance is set by the gap not the AL value, and saturation at maximum current is the hard limit. Typical design sequence:
- From input range, output current and switching frequency, compute the primary inductance that keeps peak current inside the chosen device rating at minimum input voltage.
- Choose a core whose window fits the primary and secondary turns at the required creepage.
- Gap for the target inductance and verify Bsat at worst-case peak current plus 20% margin.
- Check the hot spot at maximum ambient inside the luminaire — this is always hotter than the bench.
- Measure leakage inductance; too high a value moves energy into the snubber instead of the LED string.
3. Isolation and creepage for SELV output
Most LED drivers must provide reinforced isolation between mains and the SELV output. In a small EE16 or EE25 bobbin, reinforced insulation typically means a triple-insulated wire or a margin tape plus a defined creepage path. The constraint to watch is that the insulation consumes winding window, which pushes the core size up — decide this before tooling, not after the first hipot failure. IEC 61347-1 and IEC 60598 set the clearance and creepage distances by pollution degree and working voltage.
4. Flicker, audible noise and dimming
Flicker. Low-frequency output ripple shows up as flicker at 100 or 120 Hz. A larger output choke and enough output capacitance reduce it. Percent flicker below 5% at full load is a reasonable target for architectural lighting.
Audible noise. Magnetically induced noise comes from magnetostriction and from loose winding layers. Potting or varnish and a lower flux swing both help. Dimming to 1% often drops the switching frequency into the audible band — check the controller’s behaviour before blaming the core.
Dimming range. Deep dimming means very small duty cycles, and at that point the transformer’s interwinding capacitance and the leakage inductance dominate the waveform. A design that is clean at 100% can be unstable at 1%.
5. Efficiency and thermal in a sealed luminaire
A street-light housing reaches 70–80 °C internally on a summer afternoon with no airflow. Design to that ambient, not to 25 °C. Ferrite with a 100 °C Curie margin is not the constraint; the winding insulation class and the core loss at the actual hot-spot temperature are. Class F insulation with a measured 30 K margin is usually the correct answer, not Class H everywhere.
6. EMI: the choke is part of the driver, not an accessory
EN 55015 conducted emissions at 150 kHz–30 MHz are commonly met with a common mode choke plus an X-capacitor. The choke’s impedance at the switching frequency and its first harmonics matters more than its 100 kHz inductance. A nanocrystalline or high-permeability ferrite core gives more impedance per turn in the small space available.
Shaanxi SiRui Technology produces Magnetic Core series ferrite cores, powder cores and nanocrystalline cores for LED driver magnetics, together with wound flyback transformers and common mode chokes built to the customer’s creepage and insulation drawing.
Send the driver specification and enclosure ambient to sales@sirui-tech.com.