Engineering Reference
LED Current Sharing, PWM and Dimming Terms Reference
Reference parallel LED branch current sharing, ballast resistance, PWM duty cycle, peak and average current, dimming frequency, driver timing, flicker, and camera boundaries.
Parallel LEDs do not inherently share current. PWM normally changes average current by switching a defined peak current, and no single PWM frequency guarantees freedom from visible flicker, camera artifacts, or compliance concerns in every application.
Parallel LED Current-Sharing Terms
| Term | Engineering meaning | Design boundary |
|---|---|---|
| Parallel branch | One LED or series LED string connected across a common supply | Branch current must be limited or regulated independently |
| Forward-voltage spread | Device-to-device Vf variation at a stated current and temperature | A lower-Vf branch can draw disproportionately more current |
| Ballast resistor | A resistor placed in series with each branch | Adds negative feedback but dissipates power and does not create precision regulation |
| Constant-current channel | An active regulator assigned to a branch or string | Preferred when matching, efficiency, thermal behavior, or LED power is critical |
| Current hogging | Unequal current concentrated in one branch | Can be reinforced by junction heating and a falling LED Vf |
| Thermal coupling | Branches influence one another through temperature | Layout and heatsinking affect sharing even with nominally matched parts |
PWM and Dimming Terms
| Term | Engineering meaning | Unit or boundary |
|---|---|---|
| PWM frequency | Number of complete on/off cycles per second | Hz; separate from MCU clock and addressable-LED refresh rate |
| Period | Duration of one PWM cycle | T = 1/f |
| Duty cycle | Commanded on-time divided by period | % or ratio from 0 to 1 |
| Peak LED current | Current during the commanded on interval | Must satisfy LED and driver pulse/current limits |
| Average LED current | Cycle-averaged current for ideal rectangular pulses | Iavg = Ipeak x D; it is not the peak current |
| Minimum pulse width | Shortest pulse the driver and current loop reproduce reliably | Limits very low or very high duty cycles |
| PWM resolution | Number of available duty-cycle steps | More timer counts improve resolution but can constrain frequency |
| Gamma mapping | Nonlinear mapping between requested brightness and duty code | Perceptual or system-specific; not an electrical power law |
Core Relationships
Itotal = Ibranch x Nbranches
Rbranch = (Vs - Nseries x Vf) / Ibranch
Presistor = Ibranch^2 x Rbranch
TPWM = 1 / fPWM
ton = D x TPWM
Iavg = Ipeak x D
Pavg,ideal = Vf x Ipeak x D
Driver Timing
| Term | Effect | Design implication |
|---|---|---|
| Turn-on delay | Reduces effective on-time | Most significant for short commanded pulses |
| Rise and fall time | Creates transition intervals rather than an ideal rectangular waveform | Can change optical output and switching loss |
| Minimum on-time | Sets a lower useful pulse width | May establish a minimum controllable brightness |
| Minimum off-time | Limits duty near 100% | The driver may fail to reset or regulate correctly |
| Switching frequency | Raises transition count per second | Higher frequency can increase gate-drive loss and EMI |
Application-Dependent Frequency Review
| Application | Relevant effect | Required review |
|---|---|---|
| Human-viewed indicator | Visible modulation, peripheral vision, motion | Select frequency with the actual observer and environment |
| Camera or machine vision | Exposure time, rolling shutter, frame rate, beat patterns | Test the exact camera and exposure settings |
| Display backlight | Refresh synchronization, low-duty behavior, temporal artifacts | Review panel and driver specifications |
| High-power lighting | Driver response, current-loop settling, thermal cycling, EMI | Check pulse current, minimum on/off time, and switching loss |
| Automotive or regulated lighting | Application-specific modulation and safety requirements | Use the current product and jurisdiction requirements |
Interpretation Boundaries
| Topic | Useful interpretation | Important limitation |
|---|---|---|
| Average current | Useful first-order electrical/thermal average | Does not waive instantaneous peak-current limits |
| Average optical output | Often follows average current over part of the operating range | LED efficiency, temperature, phosphor, and driver dynamics can make it nonlinear |
| Flicker | Temporal light modulation perceived or measured in an application | Frequency alone cannot establish universal flicker safety or compliance |
| Camera banding | Image artifact caused by modulation and sensor timing | A frequency acceptable to people may still interfere with cameras |
| Parallel current balance | Similarity of current among branches | Identical schematic values do not guarantee identical branch current |
Canonical Calculation Anchors
| Case | Calculated result | Interpretation |
|---|---|---|
| Four 20 mA parallel branches | 80.0 mA total; 4 branch resistors | Each resistor controls one branch; equal current remains a design assumption |
| 1 kHz, 25% duty | 1.000 ms period; 250 us on; 750 us off | Ideal commanded timing |
| 20 mA peak at 25% duty | 5.000 mA average | Peak current remains 20 mA during on-time |
| 50% duty, 10 us minimum on/off | 50.000 kHz maximum | Timing-only boundary; not a universal recommended PWM frequency |
Engineering Review Checklist
- Use one ballast resistor or regulated channel per parallel branch.
- Check LED Vf distribution across current and temperature.
- Verify resistor tolerance, power, and temperature coefficient.
- Separate peak current limits from average current and thermal estimates.
- Check driver minimum on-time and off-time.
- Confirm timer frequency, actual resolution, and duty quantization.
- Evaluate EMI and switching loss at the selected frequency.
- Test visible modulation in the actual viewing environment.
- Test rolling-shutter and exposure interactions with the actual camera.
- Apply current product, lighting, automotive, and safety requirements where relevant.
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FAQ
Do parallel LEDs share current equally?
No. Forward-voltage, temperature, resistor, layout, and device variations can produce unequal branch current even when the LEDs have the same part number.
Should each parallel LED branch have its own resistor?
For resistor-limited arrays, each branch should normally have its own ballast resistor. High-power or tightly matched systems often need independently regulated constant-current channels.
What causes current hogging in parallel LEDs?
A lower-forward-voltage branch can draw more current, heat more, and experience further forward-voltage reduction, reinforcing the imbalance.
How is PWM average LED current calculated?
For ideal rectangular current pulses, average current equals peak on-current multiplied by duty cycle as a decimal: Iavg = Ipeak x D.
Does PWM reduce LED peak current?
Not inherently. PWM normally changes on-time while the driver maintains its programmed on-current. Both peak and average ratings must be checked.
What PWM frequency should I use?
There is no universal value. Human perception, motion, camera exposure, rolling shutter, driver response, timer resolution, EMI, acoustic interaction, and applicable requirements all matter.
Is a high PWM frequency always flicker-free?
No. Frequency alone does not establish visual comfort, camera compatibility, or regulatory compliance; modulation depth and the viewing or capture conditions also matter.
Why does PWM resolution fall as frequency rises?
A fixed timer clock provides fewer counter steps in each shorter PWM period, reducing the number of available duty-cycle levels.
Why does minimum pulse width matter?
Driver delay and current-loop rise or fall time can consume a short pulse, so commanded duty may not equal effective optical output near the dimming limits.
Can PWM control any LED driver?
No. Use the driver's specified dimming or enable interface and observe its pulse-width, frequency, current, and logic requirements.