LED Driver Efficiency Calculator

This tool calculates overall driver efficiency (η), power loss (P_loss), junction temperature rise (ΔT_j), and thermal resistance requirement (R_θJA) based on electrical operating conditions. It also estimates RMS and peak currents for MOSFET and diode selection, supporting both constant-current and PWM-dimming designs.

Use it during early-stage driver selection and thermal design—especially when sizing heatsinks, selecting switching devices, or validating compliance with efficiency standards (e.g., ENERGY STAR, DLC). It’s particularly valuable for battery-powered, automotive, and architectural lighting where thermal headroom and efficiency directly impact lifetime and reliability.

PWM dimming reduces average output power but not switching losses proportionally—driver efficiency typically drops at lower duty cycles due to fixed control circuitry losses and increased relative conduction losses. The calculator adjusts power loss and thermal rise accordingly, helping you assess worst-case thermal stress (often at 100% duty) versus real-world dimmed operation.

Input voltage: 5–48 V (common for 12 V/24 V systems or wide-range AC-DC front-ends); Output current: 100–2000 mA; Output voltage: 2–42 V (matching series LED string V_f); Switching frequency: 100–2000 kHz (higher f_sw improves transient response but increases switching losses); Dimming duty: 1–100%.

Higher f_sw increases switching losses (proportional to f_sw × V × I), reducing efficiency and raising junction temperature—especially in MOSFETs and gate drivers. The calculator models this trade-off, helping you balance EMI, component size, and thermal performance. Optimal f_sw is often 200–600 kHz for mid-power drivers.

Verify units: output current must be in mA (not A), and all voltages in volts. Ensure V_out reflects the *actual* forward voltage of your LED string at I_out, not a datasheet maximum. Also confirm input voltage isn’t too close to V_out—low headroom increases conduction losses dramatically, especially in linear or low-dropout buck designs.

While it doesn’t auto-select topology, the ratio of V_in/V_out and resulting efficiency/loss distribution provide strong guidance: V_in > V_out favors buck; V_in < V_out requires boost; wide V_in ranges or variable LED strings need buck-boost or SEPIC. Low efficiency at extreme V_in/V_out ratios signals topology mismatch.

No—it computes *required* R_θJA based on assumed ambient temperature and max allowable junction temperature (typically 125°C). Real-world R_θJA depends heavily on copper area, layer count, thermal vias, and heatsink mounting. Use the result as a target to guide layout optimization and thermal validation—not as a final system prediction.

R_θJA (°C/W) is the *maximum allowable* total thermal resistance from junction to ambient needed to keep T_j ≤ your design limit (e.g., 125°C) under worst-case load. For example, if P_loss = 1.2 W and ΔT_j = 60°C, R_θJA must be ≤ 50°C/W. Compare this to your board + heatsink R_θJA estimate to verify margin.

The base calculation assumes nominal component parameters at 25°C ambient. No automatic derating is applied—but the junction temperature rise output lets you apply semiconductor derating curves (e.g., MOSFET R_DS(on) increase with T_j). For long-life designs (>50,000 hrs), consider adding 10–20% margin to P_loss to accommodate electrolytic capacitor aging and thermal interface degradation.