The performance of light-emitting diodes (LEDs) is highly dependent on its working parts and circuit configuration. In order to save space and reduce production costs, some manufacturers are incorporating non-isolated designs without isolating transformers. Below shines light on the differences between isolated and non-isolated power supplies for LEDs.
Isolated Power Supply
LEDs with isolated power supplies provide electrical protection via an isolating transformer. The LED driver input and output ends are connected, and the isolating components help protect the fixture from surges and transient currents. Electrical efficiency for units with an isolated power supply is roughly 88 percent, and output voltages can reach 30-42V DC. When it comes to safety, this configuration is more reliable, compared to non-isolated power supplies (more on this later). However, costs for producing lights with isolated drivers are higher, because of the additional, numerous parts involved. This option is ideal for LEDs that use high power circuits above 15-20 watts, such as outdoor lighting.
Non-Isolated Power Supply
A fixture that is missing a transformer is an indicator that the LED driver is non-isolated. Such fixtures apply configurations that do not rely on an isolating transformer. Usually found in low power LED applications below 15 watts, non-isolated drivers offer superior efficiency rates at 92 percent, due to no loss of transformer electricity during consumption. It also boasts simple circuitry, as well as wide input and output voltage ranges. This suggests that non-isolated power supplies are ideal for high voltage, low current circuits.
Additionally, costs for producing LEDs with non-isolated drivers are lower, compared to lights with isolated drivers. Traditional incandescent bulb designs support non-isolated configurations- if the glass of the fixture was broken during operation, the operator would have direct access to the AC line power.
Advantages of non-isolated drivers:
• High efficiency – up to 92 percent
• Lower heat dissipation
• Lower temperature of the MOS devices and power inductors
A drawback of non-isolated power supplies is instability. Such systems are more prone to electrical surges. The circuit is not protected from the AC source, which increases the risk of electrocution. As a solution to this dilemma, manufacturers typically apply fusible resistors in the design to interrupt over-currents and provide protection. Fusible resistors work by responding to fluctuations in electrical current. When the voltage increases, the resistance of the piece decreases. In the event a voltage spikes outside of the resistor’s threshold, it will open, causing the circuit to break. A fusible resistor also limits the current surge, making it very efficient in mitigating over-currents. The piece’s resistance wires have an average melting point of 1,300-1,400 degrees Celsius. This unusually high threshold makes fusible resistors prone to fire hazards.
UL Compliance for Non-Isolated LEDs
A primary risk with non-isolated power supplies is the possibility of damage caused by short circuits. Without adequate protection, high input current could cause heat generation, smoke and fire. Furthermore, various components in the fixture, such as bridge diodes, capacitors and MOSFETs can fail during such occurrences. To ensure the safety of LEDs that apply this type of electrical configuration, the fixtures must meet guidelines set forth in UL 8750 on safety standards for LED Equipment for Use in Lighting Products.
Recommendations from the agency includes the following:
• 8.5.2 Component failure test
• 184.108.40.206 A unit shall not exhibit a risk of fire or electric shock when a simulated short circuit is imposed on electrolytic capacitors or semiconductor devices.
• 220.127.116.11 Each electrolytic capacitor and semiconductor device is to be short circuited, one at a time (one fault per test). Each test shall continue until either the unit is no longer operable, or until conditions are obviously stable (as determined by no visual changes or detectable thermal increase) for at least 30 minutes.