Engineering Guide
Parallel LED Array Design: Current Sharing, Ballast, Thermal Balance, and Faults
A parallel LED array is reliable only when every branch has a defined current-control element and the design is checked across forward-voltage spread, temperature, supply tolerance, distribution loss, and faults.
Parallel Array Design Workflow
| Step | Action | Engineering Check |
|---|---|---|
| Define each branch | Record LEDs in series, target current, VF limits, temperature range, wiring, and control method. | |
| Choose current control | Use one ballast resistor or one regulated current channel per branch; do not rely on LED matching alone. | |
| Solve electrical corners | Evaluate maximum supply with minimum VF for high current and minimum supply with maximum VF for low current or dropout. | |
| Check power | Calculate resistor, LED, driver, wiring, and total supply power at consistent corners. | |
| Check thermal feedback | Use hot VF and junction-temperature limits; verify that neighboring branches have comparable cooling. | |
| Analyze faults | Review open LED, shorted LED, open resistor, wiring faults, connector faults, and driver-channel failure. | |
| Size distribution | Include cable and PCB drop, connector current, supply margin, startup, dimming, and injection points. | |
| Validate branches | Measure individual currents, temperatures, brightness, startup, PWM waveforms, and fault response over production and environment corners. |
Electrical Model
Formula reference
Branch and array relationships
Vstring = NseriesVFRbranch = (VS − Vstring) / IbranchIbranch = (VS − Vstring) / RbranchItotal = ΣIbranchPR,branch = Ibranch²RbranchPLED = VF Ibranch NseriesVariable definitions
- Evaluate supply and VF limits at the same operating corner
- Use one resistor or regulator per parallel branch
- Include wiring and connector drop in available headroom
- Sum actual branch currents when branches are not identical
Use the LED Parallel Array Calculator for branch count, current, resistor, power, and supply estimates after defining credible electrical limits.
Choose a Current-Control Method
| Method | Strength | Boundary |
|---|---|---|
| One resistor per branch | Simple and low cost | Current varies with supply, VF, resistor tolerance, and temperature |
| Linear constant-current sink | Good branch regulation and diagnostics | Requires compliance headroom and dissipates dropout power |
| Switching current channel | Higher efficiency for power LEDs | Adds switching ripple, EMI, control-loop, and layout requirements |
| One driver with series strings | Same current flows through all LEDs in the series path | Open-string handling and compliance voltage limit string count |
| One source feeding bare parallel LEDs | Few components | Not a controlled design; small VF differences can cause large current imbalance |
Thermal Balance and Fault Behavior
Forward voltage generally decreases as junction temperature rises. A weakly controlled hot branch can therefore draw more current. Use branch ballast or regulation, comparable cooling, and junction-temperature checks. For faults, recalculate headroom and dissipation rather than assuming an open or short affects only brightness.
Worked Examples
Four equal branches
Four 20 mA branches require 80 mA nominal supply current before design margin.
Per-branch resistor
At 5 V, VF = 2.0 V, and 20 mA, each branch needs 150 Ω and dissipates 60 mW in its resistor.
High-current corner
At VS,max = 5.25 V and VF,min = 1.8 V with 150 Ω, branch current rises to 23 mA.
Low-current corner
At VS,min = 4.75 V and VF,max = 2.2 V with 150 Ω, branch current falls to 17 mA.
Shared-resistor failure
Two bare parallel LEDs behind one resistor do not necessarily split current in half because their exponential I-V curves and temperatures differ.
Current sink compliance
A sink requiring 0.5 V dropout cannot regulate when supply minus string VF and wiring drop falls below 0.5 V.
Open branch
An open branch removes its current; whether other branches change depends on supply regulation and whether they have independent current control.
Shorted LED
A short in a resistor-fed series branch increases resistor headroom and current; recalculate current and resistor power at that fault corner.
Thermal mismatch
A hotter LED generally has lower VF, which can increase current in weakly ballasted parallel devices and reinforce heating.
Wiring drop
Remote branches with unequal cable resistance receive different voltage; star distribution or local regulation can reduce imbalance.
PWM dimming
Average branch current is approximately peak current times duty only when the driver reaches the intended peak during each pulse.
Measurement
Use one shunt or calibrated current probe per branch and measure after thermal equilibrium as well as during startup and PWM edges.
Common Mistakes
Related Calculators
Support reference
FAQ
Can LEDs be connected in parallel?
Yes, but each parallel branch should normally have its own ballast resistor or regulated current channel because LED forward voltage and temperature are not identical.
Why should each LED branch have a resistor?
Per-branch resistance converts forward-voltage differences into smaller current differences and prevents one low-VF branch from being limited only by the other LEDs and wiring.
Why do parallel LEDs share current poorly?
Their nonlinear I-V curves, forward-voltage spread, temperature coefficient, wiring resistance, and cooling differences produce unequal branch current.
How is total array current calculated?
Add the actual current of every branch. For N identical controlled branches, the nominal result is Itotal = N times Ibranch.
How is a branch resistor calculated?
Use R = (VS - NseriesVF) / Ibranch, then select a practical value and recalculate current at supply, VF, resistor, and temperature corners.
When should I use constant-current drivers?
Use them when brightness matching, high LED power, wide supply range, efficiency, diagnostics, or controlled dimming justify the additional circuitry.
What is LED driver compliance voltage?
It is the voltage headroom the current regulator needs in addition to the LED string and distribution drops to maintain the commanded current.
Can matched LEDs share one resistor?
Matching reduces spread but does not control temperature, aging, wiring, and residual VF variation. Independent branch limiting remains the robust approach.
What happens if one parallel branch opens?
Its current becomes zero. Other branches remain controlled if they have independent limiting and a stable supply; shared regulation can redistribute current.
What happens if one LED shorts?
The affected string voltage falls. A resistor-fed branch can draw more current, while a regulated branch may maintain current but needs a fault and power review.
How does temperature affect sharing?
LED VF usually falls as junction temperature rises. Without adequate ballast or regulation, a hotter branch can take more current and create destabilizing feedback.
How should current sharing be verified?
Measure every branch across supply, ambient, warm-up, PWM, startup, and fault conditions using low-burden sensing and representative production devices.
Summary
Give every branch its own current-limiting or regulating element, solve supply and VF corners, include thermal and distribution effects, analyze open and short faults, and verify individual branch current rather than relying on total-array measurements.