In the design of lamps and lanterns, how to estimate the voltage of LEDs is a seemingly simple subject that requires attention to many details. First of all, even if the LEDs produced by each factory are tested under the same conditions, the forward bias voltage will be more or less different. Secondly, LED voltage also changes with temperature. In order to ensure the usability and reliability of the lamp, we must understand several factors that will affect the LED voltage and properly evaluate the error value in order to correctly select the output voltage specification of the driver. This article introduces the three biggest factors that affect LED voltage, and briefly explains the considerations and estimates. In addition, in response to the influence of temperature, the new LED power supply has the function of environmental adaptation. The benefits of this to the design of lamps will be explained below. At the end of the article, an example of complete design considerations will be provided for reference.
Generally, the design of lighting fixtures involves optics, heat and electricity. The main purpose of the lamp is of course to meet the light requirements of the application environment, such as illumination or color temperature, etc. To meet these requirements, the model, type, quantity and driving current of LEDs must first be defined. When the above conditions are determined, the driving conditions of each single LED have basically been set, and the overall power of the lamp is also fixed. However, the overall voltage and current of the lamp depend on how the designer arranges and combines the LEDs, whether all in series, or in series or in parallel. This consideration is usually related to the maximum voltage required by safety regulations or the modular design of the LED. The overall approximate voltage of the lamp can be obtained by the following formula
Vforward_total = Vforward x Num/String
The above formula is relatively simple and can provide the designer with a general direction, but it is insufficient for the final design of the lamp. Complete design considerations need to pay attention to at least the following three points:
1) V-I characteristics of LED
2) LED production differences
3) Temperature coefficient of LED
The following is explained in three paragraphs.
V-I characteristics of LED
The ideal LED V-I curve is shown in Figure 1. The LED voltage is not affected by the flowing current. However, in fact, the driving current of the LED will affect the voltage on the LED. Taking Figure 2 as an example, when the LED operates at 350mA, the voltage is about 3.2V. When the LED operates at 1A, the voltage will rise to 3.8V. The small voltage difference of a single LED will become more obvious when multiple LEDs are connected in series. Designers need to pay attention to whether the current conditions of the nominal voltage test in the LED specification sheet are consistent with the lamp design. If the lamp uses another current, or the peak value of the ripple current of the power supply changes greatly, the voltage value should be re-estimated based on the V-I characteristic curve.
Table 1 Specification Vf parameters
Figure 1 Ideal V-I curve Figure 2 Actual V-I curve
LED production differences
There will be a certain degree of production variation in the forward bias voltage of LEDs. A mature LED production line usually has small differences and a normal distribution, as shown in Figure 3. The impact of production differences on LED voltage is usually less than 10%, which can be seen from the ratio of the maximum value to the standard value of the reference voltage in the LED specification sheet. For exact values or distribution curves, you must ask the LED manufacturer.
Although the most extreme situation may be +/- 10%, in actual use, the more LEDs are in the same string, the more likely the probability is to be close to the middle value. If objective conditions permit, it is usually recommended that the power supply retain a 10% margin. Firstly, it can ensure that the driving current is not affected by LED production differences. Secondly, even if this range is not used, slightly reducing the load of the power supply will help extend the life of the power supply. .
Figure 3 LED Vf production normal distribution
LED temperature coefficient
LED voltage has a negative temperature coefficient, which means that the voltage will decrease when the temperature increases. On the contrary, the voltage will be relatively high when the temperature is low. The heat flow design of the lamp usually balances the self-heating of the LED, so in the steady state, the temperature and voltage of the LED are relatively fixed. For voltage, the most severe conditions occur when the luminaire is cold started. The voltage in the LED specification table is usually set at the normal operating temperature. If you need to estimate the cold-crank voltage, you need to refer to the V-T curve or use the tool software provided by the LED manufacturer to enter the parameter (Tj).
The biggest difference between the temperature coefficient and the previous two factors (current, production) is that the effect of temperature on voltage is temporary. When the LED is turned on, the voltage will gradually drop back to the normal level due to its own heat. In other words, the power supply does not need to unconditionally retain the voltage margin required for cold start. It only needs to temporarily increase the output voltage range for a short period of time after startup.
Some advanced models on the market provide environmental adaptive functions, allowing them to detect and automatically adjust the output voltage and current during driving, and enter the constant current region after the lamp reaches a stable operating state. The HLG-480H-C series has this function. The power supply can automatically reduce the current and increase the voltage up to 120% to ensure that the lamp can be turned on smoothly at low temperatures. When the lamp is turned on and warms up, the current automatically returns to the original design value. This design ensures that the lamps can be turned on at low temperatures without affecting the life of the power supply. Designers do not need to spend more money on selecting a higher-power power supply to ensure the short-term need to turn on the lamp. To give a practical figure, HLG-480H-C1400 can operate at 171~343V, and at low temperatures (such as - 40°C), it can provide an instantaneous voltage of up to 412V until the lamp enters stable operation.
Figure 4 V-T curve
HVGC's constant power series also has a similar function. Users can temporarily reduce the current and increase the maximum output voltage through dimming control. There are also many possibilities for other models. If you have any questions about LED cold start, please feel free to ask MEAN WELL power experts around the world.
Voltage calculation example
A certain lamp uses a total of 100 LEDs driven at 1.05A, with 2 and 50 strings on the lamp board. The lowest ambient temperature of the lamp device is 0°. How should the designer determine the appropriate LED power supply?
Method 1: Ask the LED manufacturer if there is any tool software that can be used, input the parameters, and get the answer directly.
Method 2: Get the answer from the LED specification book and refer to relevant information.
Step 1: Check the LED V-I curve and determine the median voltage.
According to Figure 2, the LED is 3.8V at 1.05A
Step 2: 50 strings, so the total voltage is 50 times that of a single cell.
3.8 (V) x 50 (pcs) = 190 V
Step 3: Consider production errors.
a. The middle value of LED in Table 1 is 3.2V, and the highest value is 3.48V.
b.3.48 (V) / 3.2 (V) = 108.75%
c.190 (V) x 108.75% = 206.6 (V)
Summary
The total LED steady state voltage has a median value of 190V.
The maximum total voltage of the LED in steady state is 207V* (assuming that the LED power supply ripple current is extremely small and can be ignored).
Step 4: Reference temperature coefficient.
As can be seen from Figure 4, the LED (350mA) is 3.2V at 85°C and 3.6V at 0°C.
3.6 (V, Tj=0) / 3.2 (V, Tj=85) = 1.125 < 1.2
cold start
The total LED voltage midpoint is 190V x 1.2 = 228 V
The maximum total LED voltage is 207V x 1.2 = 248.4 V
LED power supply selection:
The recommended model is HLG-480H-C2100 for the following reasons.
The lamp will generally operate at 190V / 2.1A (399W) in steady state, and the worst-case condition is 207V (435W). This condition can be met by HLG-480H-C2100. At the same time, its ripple current is low and can be ignored. When the machine is cold, the maximum voltage will reach 249V. Although this is not within its fixed output range, its environmental adaptive function can provide an instantaneous voltage of up to 275V, ensuring the stability of startup. Therefore HLG-480H-C2100 is the best model.
