Engineering Guide
LED Power Delivery, Driver Efficiency, PWM Dimming, and Strip Design
LED systems need both current control and power delivery. A sound design connects strip load, driver loss, wiring drop, PWM timing, thermal limits, perceived brightness, and camera behavior.
LED System Workflow
| Step | Action | Engineering Check |
|---|---|---|
| Define optical operating point | Set required light output, color, dimming range, peak LED current, and acceptable ripple. | |
| Build electrical load | Determine strip length or channel count, forward voltage, on-state current, duty cycle, and simultaneous-channel factor. | |
| Select driver topology | Choose regulated current or voltage architecture with appropriate headroom, range, protection, and control input. | |
| Budget power | Calculate LED output power, driver input power, losses, standby power, supply margin, and startup demand. | |
| Budget distribution | Calculate round-trip wire resistance, voltage drop, connector loss, copper heating, and injection spacing. | |
| Choose PWM timing | Set frequency, period, duty resolution, minimum pulse width, dimming ratio, and control bandwidth. | |
| Check thermal limits | Translate driver and LED losses into temperature rise using the real thermal path. | |
| Validate | Measure current waveform, endpoint voltage, power, temperature, flicker, camera artifacts, and color over range. |
Peak, Average, Power, and Resolution
| Quantity | Meaning | Boundary |
|---|---|---|
| Peak LED current | Current during PWM on-time | Must remain within LED and driver pulse/continuous limits |
| Average LED current | Ipeak x duty for ideal rectangular PWM | Relates to average electrical loading, not every optical effect |
| LED power | Vf x Ipeak x duty | Approximation because Vf and light output vary with current and temperature |
| Driver loss | Pin - Pout | Becomes heat across driver components and PCB |
| Strip voltage drop | I x round-trip path resistance | Creates brightness and color nonuniformity |
| PWM resolution | Available timer codes | Limits low-end steps and interacts with PWM frequency |
| Perceived brightness | Often gamma-adjusted | Human perception and LED transfer are not perfectly linear |
Strip Power and Distribution
Formula reference
Strip and wiring model
Nominal power = length x power densityOperating power = nominal power x load factorTotal current = operating power/supply voltageCable drop = current x effective round-trip resistanceCable loss = current squared x resistanceVariable definitions
- Power density must match the strip and channel state
- Effective resistance includes supply and return paths
- Distributed injection changes current in each segment
Use the LED Strip Power Calculator for source and cable planning. Verify segment copper, connectors, injection topology, supply derating, and endpoint voltage.
Driver Efficiency and Thermal Loss
Formula reference
Driver power balance
DC input power = Vin x IinAC real input power = Vrms x Irms x PFLED output power = Vout x IoutEfficiency = Pout/PinDriver loss = Pin - PoutTemperature rise = loss x effective thermal resistanceVariable definitions
- Use real input watts for efficiency
- Standby power matters at low output
- Thermal resistance must represent the actual board, enclosure, and airflow path
The LED Driver Efficiency Calculator supports power and loss estimates. Efficiency varies with input voltage, output current, dimming state, switching frequency, and temperature.
PWM Dimming
Formula reference
Ideal rectangular PWM
Period = 1/fPWMOn time = period x dutyAverage current = peak current x dutyAverage LED power = Vf x peak current x dutyPWM code = round[duty x (2^bits - 1)]Gate-drive power = Qg x Vgate x fPWM x channelsVariable definitions
- Peak current remains the on-pulse current
- Timer topology determines the exact TOP equation
- Gamma mapping changes command-to-duty relationship
- Camera compatibility requires exposure and rolling-shutter validation
Use the LED PWM Dimming Calculator for timing and average quantities. Check driver minimum pulse width, turn-on delay, current settling, timer resolution, gate-drive loss, EMI, and optical behavior.
Fundamentals and Selection Boundary
Use Understanding LEDs for device behavior and How to Choose the Right LED for device selection. This guide owns system delivery, driver loss, and PWM implementation.
Worked Examples
Strip load
A 5 m strip rated 14.4 W/m uses 72 W nominal. At 80% load factor, operating power is 57.6 W.
Supply current
At 24 V, 57.6 W requires 2.4 A before supply and distribution margin.
Supply margin
Adding 25% margin gives a recommended source rating of at least 72 W, subject to startup and derating.
Cable drop
A 2.4 A load through 0.25 ohm effective round-trip resistance drops 0.6 V and dissipates 1.44 W.
Driver efficiency
A driver delivering 40 W from 44 W real input is 90.9% efficient and dissipates 4 W.
Driver temperature
At 4 W loss and 12 C/W effective thermal resistance, first-order rise is 48 C.
PWM timing
At 2 kHz, period is 500 us. A 20% duty cycle gives 100 us on and 400 us off.
Average current
A 700 mA on-pulse at 20% duty gives 140 mA ideal average current; the 700 mA peak limit still applies.
Timer resolution
An 8-bit timer has 256 nominal duty codes; minimum pulse constraints may make fewer levels usable.
Gamma mapping
For normalized brightness 0.5 and gamma 2.2, duty is about 0.218, not 0.5.
Common Mistakes
Related Calculators
Support reference
FAQ
How do I calculate LED strip power?
Multiply strip length by rated power per unit length, then apply the intended channel or load factor. Add justified supply and distribution margin.
How do I calculate LED strip current?
Divide operating electrical power by supply voltage for a first-pass estimate. Verify segment topology and manufacturer data.
Why does voltage injection matter on long LED strips?
Copper resistance causes voltage drop and heating. Multiple injection points shorten current paths and reduce brightness or color variation.
How do I calculate LED driver efficiency?
Divide useful LED output electrical power by real driver input power measured across the same operating condition.
Is LED driver loss converted to heat?
Nearly all electrical loss ultimately becomes heat, but its location and thermal path determine component and enclosure temperature.
How does PWM control LED brightness?
PWM switches regulated LED current on and off. For ideal rectangular pulses, average current is peak current times duty cycle.
Can I increase peak LED current when using low duty cycle?
Only when the LED and driver datasheets explicitly permit the pulse amplitude, width, repetition rate, and thermal condition. Average-current arithmetic alone is not enough.
What PWM frequency should I use for LEDs?
Choose a frequency high enough for the visual and camera requirement while preserving timer resolution, minimum pulse width, driver switching performance, and acceptable losses.
Why does PWM resolution decrease at higher frequency?
With a fixed timer clock, fewer counter counts fit into each shorter PWM period, reducing available duty codes.
What is gamma correction in LED dimming?
Gamma mapping converts a desired perceived-brightness command into a nonlinear duty command. The appropriate curve depends on the visual system and application.
Why can LEDs flicker on camera but not to the eye?
Camera exposure, frame rate, rolling shutter, and PWM phase can create banding or beat artifacts even when visible flicker is not obvious.
How should I validate an LED power and dimming design?
Measure far-end voltage, current waveform, input/output power, driver and LED temperature, low-end steps, flicker, camera artifacts, and color over supply and ambient corners.
Summary
Build LED load and distribution budgets from actual operating states, calculate driver heat from real power loss, and choose PWM frequency and resolution around the LED, driver, timer, human, and camera constraints. Validate current waveforms, endpoint voltage, temperature, light, and color on hardware.