Ordinary ceiling lights typically use electronic ballasts, or high-frequency transformers. However, the quality of commercially available electronic ballasts is relatively poor. For example, the author tested an ordinary ceiling light equipped with a Philips 32W ring tube. The measured results are as follows: Pin = 16W, PF = 0.62. Therefore, it only uses 50% of the ring tube's power, and the power factor is also poor. The author also tested the parameters of a 9W energy-saving lamp and measured Pin = 8.4W and PF = 0.563.
Both of these fail to meet the power factor requirement of > 0.7, yet they are still widely available on the market.
LED ceiling light power supplies can be categorized into two types: non-isolated and isolated.
Non-isolated constant current sources. Because LED ceiling lights are not easily touched by users like bulbs, and because they do not require contact heat conduction, their internal structure easily insulates the aluminum base plate or printed circuit board from the metal base plate. Therefore, using non-isolated power supplies can easily pass safety certifications such as CE and UL. Furthermore, its installation is usually performed by a professional electrician, which reduces the risk of electric shock to users.
LEDs must be driven by a constant current source. Otherwise, due to their negative temperature coefficient, the current will increase sharply, leading to elevated junction temperature and shortened lifespan. Constant current sources come in two types: linear and switching. The advantages of linear constant current sources are that they do not generate electromagnetic interference (EMI), are simple, and low-cost. Their disadvantage is relatively low efficiency.
1. Non-isolated Constant Current Source Using a Constant Current Diode
A constant current diode is a linear constant current source whose constant current function can be used to drive LEDs. The simplest method is to connect the constant current diode directly in series with the LED. However, when using it for LED driving, care must be taken to select the appropriate current and withstand voltage.
Minimum Voltage
Because a constant current diode requires a certain voltage (Vk) to maintain constant current, it will not operate with a power supply voltage that is too low. This Vk is typically around 5-10V.
Maximum Voltage
Because a constant current diode must be able to absorb variations in the power supply voltage, the variation range of a 220V diode is twice that of a 110V diode for the same percentage. For example, a +10% to -20% voltage fluctuation for 220V means a 22+44=66V fluctuation. After bridge rectification, this fluctuation is multiplied by 1.2 times, becoming 79.2V. For a 110V power supply, the same fluctuation only translates to a 39.6V fluctuation. Lower voltage means lower power consumption and higher efficiency. Therefore, constant current diodes are more suitable for countries with 110V mains power.
Maximum Current
Because constant current diodes have limited power consumption, excessively high currents are unsuitable. For example, a 1W LED typically requires 350mA, which a constant current diode struggles to provide. Even if it could, the power consumption would be excessive, significantly reducing overall efficiency.
Constant current diodes are most suitable for AC mains-powered LED fixtures that use many low-power LEDs in series, ideally suited for high voltage, low current applications.
The figure below shows a constant current diode driver for a ceiling light. The load is 80 3022 diodes in series, with a total power of 16W. The constant current diode used also has a constant current of 60mA. If you only have a 30mA constant current diode, you'll need to connect two in series and parallel.
Using a constant current diode as an LED driver
Here, the constant current diode's function is to maintain a constant output current when the input mains voltage fluctuates. It also eliminates the current increase caused by the LED's negative temperature coefficient. However, due to its voltage resistance, the constant current diode's ability to absorb voltage fluctuations is also limited. For example, a 100V CRD has a limited operating voltage range, minus its minimum operating voltage of 10V, resulting in a usable voltage range of only 90V. If used with a 220V mains power supply, a voltage fluctuation of +10% to -20% translates to a rectified voltage of 290 to 211V, a voltage change of 79V, which is within its voltage resistance range. If 80 LEDs are used and their forward voltage is 3.3V, the total voltage is 264V, which is exactly the value of 220V after bridge rectification. At this point, there's no voltage drop across the constant-current diode, but it won't work. A voltage drop of at least 10V is required, requiring a rectified voltage of 274V and a mains voltage of 228VAC. At that point, the constant-current diode's voltage drop is minimal, and so is its power consumption, at only 0.03A x 10V = 0.3W. Overall efficiency can reach up to 96% (of course, the rectifier's efficiency must also be considered, so it's actually lower). If the mains voltage increases to 242VAC, the constant-current diode's voltage increases to 26.4V, and its power consumption increases to 0.79W, bringing its efficiency to 91%.
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