LED power supply solution based on Mean Well switching power supply

1. How to choose Mean Well LED power supply?

a. Determine the appropriate wattage based on the customer's system requirements and application methods, including the power safety margin to be designed, and consider the system's drive method design. Use Mean Well power supplies to "directly drive" LED lamps. For selection points, please refer to questions (2) and (3). Use Mean Well power supplies with LED driver ICs that use constant current sources to achieve more precise constant current drive. For selection points, please refer to questions (2) and (3).

b. Confirm the working environment of the LED power supply to select the appropriate waterproof and dustproof (IP) level or the appropriate model structure (metal shell, plastic shell, PCB type).

c. Whether it is necessary to have a power factor correction (PFC) function: Models with a single-stage PFC architecture can only be used for LED loads, while models with a dual-stage PFC architecture can be used for general loads.

d. If the system design uses a power supply to directly drive the LED, is a model with adjustable output voltage/current or a model with dimming function required?

2. What are the configuration and application methods of LED lighting systems? What are their advantages and disadvantages?

Table LED drive mode and power supply selection 3. What should be paid attention to when selecting LED power supply?

a. The lighting system adopts direct drive design 

The total of the upper and lower limits of the LED operating voltage range must be within the output voltage range of the LED power supply. For example, if the LED specification is 3.4~3.6V, the series voltage of 6 strings is 20.4~21.6V. In this case, you should choose a model with an output of 24V (constant current range 18~24V). 

For models with power factor correction function, the PF value must be greater than 0.9 if required, and the load must be greater than the PFC definition range in the specification. The relationship between power factor and output load is shown in Figure (1). It will vary depending on the model definition, and the general design value is above 75% LOAD. 

If the product is used in a location with unstable input voltage, such as a generator or in a heavy industrial area, please select the "General Purpose" product in Table 1.

b. The lighting system adopts constant current IC design

The startup voltage design of the driver IC needs to be close to the output voltage of the LED power supply. The driver IC has high requirements for voltage stability, and it is recommended to use the "general-purpose" products in Table 1. 

For models with power factor correction function, the load must be greater than the PFC definition range in the data sheet. The relationship between power factor and output load is shown in Figure (1). It will vary depending on the model definition, and the general design value is above 75% LOAD. 

The use of driver ICs may cause EMI matching problems. After the lamp design is completed, the EMI needs to be confirmed. If there are EMI issues, please refer to LED power supply application issues (11).

Figure 1 Relationship between power factor and output load 4. In LED lighting design, if the LED light strip is designed with 12 LEDs in a string (VF is assumed to be 3.5V), 4 strings are connected in parallel, and the current of each string is 0.7A. When it is directly connected to the power output terminal without a constant current driver in series, how should the appropriate power supply be selected?

First, we need to make sure that the LED power supply can work in constant current mode :

Each LED string operating voltage = 3.5VX 12 = 42V

Total current requirement of LED lamp = 0.7AX 4 series in parallel = 2.8A

LED power requirement wattage = 42V x 2.8A = 117.6W

The wattage/voltage of the LED power supply should be greater than and closest to the required wattage/voltage. First select the LED power supply based on the 150W/48V requirement, and then confirm whether the selected voltage/wattage power supply meets the definition requirements of the constant current range and PF>0.9. (117.6W/150W = 78.4% > 75%), for example, in this question, you can choose CLG-150A-48V to adjust the output current to 2.8A.

Note: Generally, the VF voltage of the same batch of LEDs may be in a range (such as 3.4~3.6V). Each LED is different, and this difference must be taken into consideration when selecting a power supply.

5. Same as the previous question, if the lighting design intends to use LED light strips connected in series with a constant current driver, how should we choose a suitable power supply?

First, we need to confirm the total forward voltage of each LED string, plus the cross voltage of the constant current driver, which is about 2V:

Each LED string operating voltage = 3.5VX 12 = 42V

Driving circuit voltage = 42V + 2V = 44V LED

Current = 0.7AX 4 series in parallel = 2.8A

Driving circuit wattage = 44V x 2.8A = 123.2W

The wattage/voltage of the LED power supply should be greater than and closest to the required wattage/voltage. First select the LED power supply based on the 150W/48V requirement, and then confirm whether the actual wattage of the drive circuit meets the definition requirement of PF>0.9 (123.2W/150W = 82.13% >75%). For example, in this question, you can choose the CLG-150A-48 model and adjust the output voltage to 44V or use it directly. 6. What are CV, CC, and CV+CC mentioned in the LED power supply specifications?

7. Why does the LED driver IC sometimes cause the LED power supply to fail to start up smoothly when the LED string light design has it?

(The voltage is clamped by the LED and cannot rise to the rated voltage value). Depending on the circuit used, there will be different operating problems as follows:

Boost Mode Current Driver IC

Since the starting voltage of the driver IC is much lower than the forward working voltage of the LED light string, the driver IC starts to start at a low voltage. Often, the IC starting voltage is even far less than half of the power supply voltage. At this time, to achieve the rated power output, the driving current must be at least twice the rated output current of the power supply. Therefore, the power supply cannot provide such a large current and cannot drive the LED constant current module.

Buck Mode Current Driver IC

When the power supply output voltage is much higher than the forward voltage of the LED light string (for example, the power supply output is 48V, while the LED light string is only 24V and the power between the two is equivalent), when the power supply is started and the LED starts to conduct, the LED power supply immediately enters the constant current operation mode. Since the power consumption of the LED is set much greater than the driving capability of the power supply constant current at this time, the driving circuit cannot work normally, and the power supply voltage is clamped at the forward voltage of the LED light string. Therefore, it is recommended to increase the starting voltage of the driver IC to close to the power supply output voltage or add a slow start function (as shown in Figure 3) when designing the LED driver, and start the driver module after the power supply output voltage is established. Or when selecting a power supply (used in a step-down module), a power supply with a power supply output voltage close to the LED light string voltage and a power supply with a sufficient driving power (LED power consumption/0.85) must be selected.

Figure 3 Schematic diagram of simple slow start circuit

DIM PIN is the PWM dimming control PIN and is also marked as EN (Enable). When DIM (or Enable) is kept at 0V, the internal SW is turned off; when the DIM voltage reaches 1.5V (Typ), it turns ON. Therefore, the driver start voltage Vstart is set, Vstart = (VDIM/RB) x (RA+RB). The Vstart setting is 5~10% higher than the LED working voltage. 8. Why does the LED have slight brightness changes or flickering when using the Mean Well LED power supply?

Mean Well has launched a series of products for LED power supplies . Some of them use a single-stage PFC circuit architecture for cost considerations. This architecture has the following limitations:

Poor resistance to AC changes

This architecture does not use large input capacitors. If used in areas with unstable input power quality, the output voltage and current will also be unstable, causing the LED to vary in brightness. This problem will not occur if the input power is stable.

Output voltage ripple

Because the input is not filtered by a large capacitor, the output ripple voltage will be larger than the general two-stage PFC architecture, as shown in Figure (4). Sometimes, the ripple voltage valley is too low, causing the LED to flicker due to insufficient drive voltage when the series constant current IC is used . At this time, the output voltage of the power supply can be increased to make the output voltage valley higher than the required drive voltage or choose a power supply with a higher voltage output.

Harmonic current

When a single-stage PFC power supply is used under constant voltage conditions (generally referring to the use of series constant current ICs), the harmonic current may deteriorate. It is recommended that when the application is in an area with unstable input power or a series constant current IC application, please select the "general-purpose" model in Table (1) (non-single-stage PFC circuit architecture) or contact Mean Well sales/engineering personnel.

Figure 4 Schematic diagram of output voltage ripple of single-stage PFC architecture 9. Can LED power supplies be connected in parallel?

Mean Well LED  power supply does not have parallel connection function and cannot be used in parallel. If you need a high wattage, it is recommended to choose a power supply with sufficient wattage or divide the LED into several blocks and supply power separately. The LED layout should also be as shown in Figure (5), and the output ground loops of each power supply should be separated. On the contrary, low-wattage LED light strings can be connected in parallel and then driven by a larger rated LED power supply (as shown in Figure 5), but the current sharing must be considered.

Figure 5 Dual power supply configuration for high-wattage LED loads

10. When LED lamps are used for lighting, what are the requirements for harmonic current and THD (total harmonic distortion)?

The harmonic requirements for lighting products are still based on EN61000-3-2, but the limit value will vary depending on the input power. Input power greater than 25W must meet Class C requirements, and less than 25W only needs to meet Class D. Currently, EN61347 does not have requirements for total harmonic distortion/THD (Total Harmonic Distortion), but Taiwan's national standard CNS15233 requires THD to be less than 33%; the US Energy Star standard for solid-state lighting (SSL) equipment is based on the ANSI_C82-77-2002 standard. General products require THD to be less than 32%, and some special products are 20%. Mean Well LED power supply equipment bipolar PFC models are generally less than 20%, and the test input and output conditions are 277VAC/60Hz/80% load. 11. Will there be EMI problems when using your company's LED power products? How to solve it?

Figure 6 PCB type and U type product EMI test configuration

LED modules have LED driver ICs:

The above EMC application countermeasures are likely to make EMI countermeasures more complicated if the customer uses LED driver IC as CC constant current driver. Since LED driver IC itself is a high-frequency switching (hundreds of kilohertz to several MHz) circuit structure, its own noise suppression is more important. The PCB layout configuration needs to pay attention to the IC grounding size and input and output capacitor and inductor configuration. It is generally recommended to add a common choke and a high-frequency X capacitor between the LED power supply output line and the LED driver PCB as shown in Figure (7).

Figure 7 EMI countermeasures for systems including LED driver ICs

Figure 8 Surge and EMI countermeasures built-in PCB/U housing type:

For example, the PLP/ULP series adopts the same metal structure as the LED module. Therefore, the EMI test is performed as shown in Figure (6). An iron plate is used to simulate a metal plane. However, due to the differences in LED systems, the input and output wiring has a significant impact. Therefore, the wiring treatment will also affect the EMI test results. It is recommended to twist the wires and add a CORE clamp at the end to suppress noise.

12.What is the maximum lightning surge that Mean Well LED drivers can withstand?

Among the Mean Well LED power supply product lines, the CEN/CLG and HBG, HLG, HLN, HLP, HVG, HSG series products have the highest surge tolerance, which can reach the heavy industrial grade (4kV). If a higher surge tolerance is required, an external ZNR (470V) or Gas Tube (500V) countermeasure should be used as shown in Figure (8), but the overall safety requirements must be considered. When multiple lamps are used, a surge protection device (SPD: Surge Protection Device) can be installed to meet regulatory requirements.

13.Can the output voltage and current of Mean Well LED power supplies be fine-tuned by the user?

Customers who wish to modify the output voltage and current can refer to the V/I adjustment field in Table 1 "Mean Well LED Power Supply Product Comparison Table" and select an LED power supply according to the voltage and current adjustment requirements. For the adjustment range, please refer to the product specification. The output voltage and current fine-tuning of Mean Well LED power supplies is adjusted via internal variable resistors. Except for the PLN and ELN series models, which require the top cover to be removed as shown in Figure (9) and adjusted by SVR1/SVR2, the other models can be adjusted directly by the Io ADJ and Vo ADJ holes on the outer shell as shown in Figure (10). In addition, after adjusting the voltage and current, it is necessary to confirm that the total output power cannot be greater than the output rated value, and it is also necessary to confirm that the top cover and waterproof plug must be assembled properly after adjustment.

Figure 9 V/I adjustment position for models with the cover removed Figure 10 Adjusting the V/I position