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AppNote180328 - Parallel Configuration of E21As on VirEnce MCPCB

Without current balancing or current mirroring, driving LED strings in parallel is not a good options. This is because of the thermal runaway phenomenon inherent to LED's characteristic. LED is a diode and will only pass current at certain voltage (hence the terms forward voltage or Vf). Thus, current will flow more towards LED (or LED string) with lower voltage. As the LED's die heats up things worsen because the Vf gets lower - current imbalance gets higher.

E21A combine with VirEnce board lessen this current imbalance greatly due to the extremely low thermal resistance of both the E21A and the MCPCB. So any thermal imbalance minimized making it's possible to be configured in parallel without the needs of special current balancing system. No matter how good they are, the thermal runaway will still be there and some good measures have to be taken to maintain long term reliability.

Use series configuration whenever possible. It's always best to use current balancing circuitry or multi output driver whenever parallel configuration is needed. It's a proven facts that VR16SP4 can be configured to work with E21A in parallel successfully. But several considerations must be taken:
- Use only the same LED from the same reel
- Minimize LED variations per parallel string(s).
- Distribute them to get the most even total Vf per string.

Example:
- Driving 2000K, 4000K, 5000K, and 6500K E21As in parallel is not a good idea because those LEDs will certainly have different Vf. Usually the warmer the LED the lower the Vf, even when the dies are of the same Vf. This is because thicker phosphor of warm white LED makes the LED runs hotter compared to the cool white LED. The same also true for high CRI vs low CRI, the latter has thinner phosphor layer and run cooler. So, in this case the best option is to run them in series.
- If you must run different LEDs in parallel make sure at least there are even numbers between different LEDs used. 2x 3000K and 2x 4000K arranged in diagonal pattern to each other is better. This is to make an even heat spreading over the MCPCB surface (geometrically)
- Make the thinnest possible solder joints to reduce further thermal deviations (either from solder joint thermal and electrical resistance).
- If connecting wires are needed to connect between strings, use from the same diameter and
lenght.

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