Electrolytic caps are often the life-limiting part of an LED lamp/driver. Here’s a practical, no-nonsense guide.
How to choose electrolytic capacitors for LED lights
1) Start from the topology (where the cap lives)
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Primary “bulk” cap (after bridge/PFC, before the flyback/buck):
Needs high voltage and long life. Typical: 400–450 V aluminum electrolytic. -
Secondary output cap (after the LED current-regulator):
Lower voltage, low ESR for ripple control. Typical: 16–63 V electrolytic or solid polymer (if voltage allows). -
Do NOT use electrolytics for line safety positions (X/Y). Those must be film safety capacitors (X2/Y2).
2) Voltage rating & derating
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Universal AC input (85–265 VAC): rectified peak ≈ 375 V worst-case → use 450 V bulk cap (gives margin for surges).
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120 VAC-only: peak ≈ 170 V → 200–250 V can work, but many designs still choose 400–450 V for global SKUs.
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Derate to ≤80% of rated voltage in normal operation when possible.
3) Temperature rating & lifetime
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Choose 105 °C (or 125 °C if available for critical spots).
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Endurance: prefer ≥5,000–10,000 h @ 105 °C for luminaires targeting 50k-hour service.
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Rough rule: capacitor life doubles for every 10 °C drop from the rated temperature (Arrhenius heuristic). Keep caps cool:
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Place away from hot LEDs/heat sinks.
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Give airflow and copper pours for heat spreading.
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4) Ripple current rating (heating = killer)
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Check the cap’s RMS ripple current rating at your switching frequency and ambient.
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Keep actual ripple ≤70–80% of the datasheet limit, or add margin per vendor curves (ripple capacity rises at higher freq, falls at high temp).
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Low ESR helps, but on the primary 450 V caps you’re limited—pick series designed for LED/PFC (high ripple endurance).
5) Capacitance sizing (what value?)
A) Bulk hold-up / mains ripple
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For a simple rectifier (no PFC), the DC bus sags at 2·f_line (100/120 Hz).
Use: C ≈ I_load / (2·f_line·ΔV_bus)
where ΔV_bus is the allowed ripple on the HV bus. -
With active PFC, the 100/120 Hz ripple is much smaller; you size C for hold-up time and PFC loop stability:
C ≈ I_bus · Δt_hold / ΔV_bus.
B) Secondary LED ripple (flicker & current quality)
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For output ripple target ΔV_out at switching ripple frequency f_sw:
C ≈ I_LED / (8·f_sw·ΔV_out) (triangle-ripple approximation; check your topology). -
If meeting flicker standards (IEEE 1789, Pst/SVM) is hard, either increase C, raise f_sw, add an LC, or use polymer caps on the secondary (lower ESR → smaller ripple for same C).
Quick reality check (why big bulk caps are common)
Example: 30 V/0.33 A LED string (≈10 W) fed from a rectified 60 Hz supply without PFC or high-frequency regulation. To limit 120 Hz ripple to just 1 V,
C ≈ 0.33 A / (120 Hz · 1 V) ≈ 2750 µF — impractically large at high voltage.
Hence LED drivers switch and regulate at kHz, so the secondary cap can be modest and effective.
6) ESR, impedance & noise
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Primary: pick “long-life, high-ripple” series intended for PFC/LED drivers.
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Secondary: choose low-ESR electrolytic or solid polymer (if ≤35–63 V). Polymer gives:
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Much lower ESR (less ripple, less audible buzz),
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Better low-temp performance,
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But lower voltage ratings and higher cost.
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7) Size, footprint, and safety margin
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Taller cans have better heat dissipation but watch mechanical limits and vibration.
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Check vent orientation and keep-out from hot resistors/transformers.
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Add inrush limiting (NTC or active) to protect the bridge and bulk cap.
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Verify surge and ripple multipliers in the datasheet (temperature/frequency correction tables).
8) Dimming & special cases
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Triac/leading-edge dimming can cause deep line notches → you often need more bulk C and careful EMI design.
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0–10 V/PWM dimming: ripple on the secondary can modulate light—keep ESR+ESL low (polymer + small MLCC in parallel).
9) What to use instead (when feasible)
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Film capacitors on the primary (e.g., DC-link film) give superb life and ripple, but they’re bigger/costlier; common in high-end drivers.
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MLCCs in parallel with the secondary electrolytic tame HF ripple, but mind DC-bias derating on ceramics.
10) A simple selection flow
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Identify position: primary bulk vs secondary output.
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Set V rating with margin**:** 450 V for universal input bulk; 1.5× to 2× V_out on secondary.
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Set C from ripple/hold-up targets (use formulas above).
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Check ripple current at operating temp/freq; add ≥20–30% headroom.
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Pick 105 °C long-life series (≥5k–10k h @105 °C).
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Verify ESR/impedance curves at your f_sw (and 100/120 Hz).
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Thermal/layout review: keep cool, short loops; add inrush limiting.
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Flicker compliance check (IEEE 1789 / PstLM/SVM) and EMI pass.
Quick “good/better/best” picks (rule-of-thumb)
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Primary bulk (universal input):
Good: 450 V, 105 °C, 2–3k h;
Better: 450 V, 105 °C, 5–10k h, high-ripple series;
Best: 450 V, 105/125 °C, 10–12k h, LED-specific long-life or film DC-link if space allows. -
Secondary output (≤60 V):
Good: low-ESR electrolytic 105 °C;
Better: low-ESR, high-ripple, 5–10k h;
Best: solid polymer (if voltage rating fits) + small MLCC in parallel.
Mini example (universal input, 30 W flyback, 60 V/0.5 A LED)
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Primary bulk: 450 V, C = 33–68 µF (PFC or optimized rectifier), ripple ≥ 0.6–0.9 A_rms rating, 105 °C, ≥10k h.
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Secondary: 63 V cap (≥1.5× V_out ripple peak), C = 220–470 µF low-ESR, ripple rating ≥ 0.7–1.0 A_rms; or 100–220 µF polymer + 1–4.7 µF MLCC.
