Some notes about LED modules design. Breakdown voltage

This post addresses some aspects of  LED modules design guide relating to electric strength of LED modules: choice of material of PCB and  PCB topology.

LED modules must have sufficient electrical strength. Electrical strength measures MCPCB material’s ability to resist electrical breakdown. This is regulated by the standards. Requirements of the standards (may vary slightly depending on the country):

LED module. MCPCB. Aluminum, dielectric, copper.
Figure 1. LED module. MCPCB. Dielectric layer.
  • < 42V DC circuitry needs 500 V AC for 1 minute test,
  • 42 to 1000 V DC circuitry needs 1000 + 2 U  (U – maximum working voltage) for 1 minute test. 

Usually, PCB for LED module is made of Aluminum Based Copper-clad Laminate (ACCL). And one of the factors that affect the electrical strength of LED modules is the properties of the dielectric in ACCL, Figure 1.

Recipe of dielectric ACCL/MCCL (MCPCB MCPCB material)
Figure 2. The dielectric layer of MCPCB. The recipe.

The typical electrical strength value for ACCL dielectric is more than 750 V/mil [1]. That is the typical breakdown voltage of 100 µm dielectric is > 3000 V AC. Which more than exceeds the requirements of the standards. Pay special attention to the choice of a dielectric of MCPCB.  Manufactures very serious about keeping the recipe a secret, Figure 2. We don’t know why we associate the recipe of dielectric with Salt Bae.

Breakdown voltage depends on the properties of the dielectric of ACCL. But don’t forget the rules of LED module PCB design. It is necessary to ensure the distance between the copper pads/traces and metal parts (any grounded surface) of MCPCB, Figure 3:

LED modules design. Breakdown voltage.
Figure 3. LED module design. Breakdown voltage.
  • head of a screw,
  • in the grooves, in the holes,
  • edges of PCB along the contour. 

This figure is almost a symbol of the Olympic Games. The minimum suggested distance is 3 mm, at least 2 mm [2] or 5-7 mm [3] when feasible in accordance with the standards.

There is a temptation to reduce this distance due to the insulating properties of the solder mask (typical value 500 V DC). But first, 500 V DC does not correspond to a large distance, and secondly, there may always be defects in the form of open areas of the solder mask along the copper traces (although this is not always allowed).

Brief and interesting information about LED modules design is presented in our Instagram account.

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PREMALSYS Team

REFERENCES

[1] CS-AL-88 AD2. Specification of Aluminum Based Copper-clad Laminate. Csem (Available for download at:  http://www.csem.com.tw/en/products.php?id=3).
[2] LUXEON Illumination LEDs. Circuit Design and Layout Practices to Minimize Electrical Stress. Lumileds (Available for download at: https://www.lumileds.com/wp-content/uploads/files/AB06.pdf).
[3] «Electrical Over Stress – How to Prevent an LED Failing Earlier than Expected», LED Professional Review, No.66 August, 2018.