LED control

Part I: The Thermal of LEDs http://
4.1 Cold lumens and heat lumens

The luminous flux emitted by the LED is closely related to the magnitude of the forward current. The luminous flux listed in the general product manual refers to the rated luminous flux at the rated forward current If (for example, 350 mA). The input power required to reach the rated luminous flux can be calculated using P = If × Vf. Where Vf is the forward voltage of the LED at rated current If.

The luminous efficacy of LEDs (hereinafter referred to as "light effects") is expressed as the ratio of luminous flux to input power. However, these rated currents and voltages are all values ​​when the LED light-emitting layer Tj is 25 ° C, which is what we call cold lumens. In an actual luminaire, the junction temperature of the LED will rise significantly due to the heat consumption of the LED chip during energy conversion. The highest junction temperature Tj is typically between 130 ° C and 150 ° C depending on the type of LED. The higher the junction temperature, the greater the loss to the LED chip.

As shown in Figure 4.1.1, the luminous flux of the LED decreases as the junction temperature increases. Therefore, in practical applications, the lower the junction temperature of the LED, the better. The heat flux we refer to refers to the luminous flux emitted by the LED at a junction temperature (70-120 ° C) when it is stable.

Figure 4.1.1 Graph of relative luminous flux as a function of junction temperature

On the other hand, the higher the junction temperature, the smaller the internal resistance of the LED chip, and the higher the light efficiency. We generally use "temperature coefficient of voltage VT" to indicate such a decrease, ie mV / °C.

The most common type of LED luminaire is the passive cooling system. Several factors must be considered when designing such a system, such as the layout of the LED light source, the properties of the luminaire material, the shape and surface treatment of the heat sink, and other factors described below.

Figure 4.1.2 Features of the CREE XP-E HEW

Thermal Resistance, junction to solder point

Thermal resistance at solder joint

Viewing Angel(DWHM)- white

Illumination angle (white light)

Temperature coefficient of voltage

Temperature coefficient of voltage

ESD classification (HBM per Mil-Std-883D)

Electrostatic discharge rating (HBM per Mil-Std-883D)

DC Forward Current

Forward current

Reverse Voltage

Reverse voltage

Froward Voltage (for 350 mA)

Forward voltage (350 mA)

Froward Voltage (for 700 mA)

Forward voltage (700 mA)

Froward Voltage (for 1000 mA)

Forward voltage (1000 mA)

LED Junction Temperature

LED junction temperature

From Figure 4.1.1, we can see that when the junction temperature is 25 ° C, the LED emits 100% relative luminous flux; and when the junction temperature rises to 150 ° C, the relative luminous flux is reduced to 70%.

According to the equation:

Light efficiency = relative luminous flux corresponding to Tj / [(Vf + ΔV) × If ]

Where ΔV = (Tj — 25) × VT

(Note: Tj is the junction temperature, Vf is the rated voltage, ΔV is the voltage variable, If is the rated current, and VT is the temperature coefficient of the voltage)

We were able to calculate the efficacy of the LED Cree XP-E HEW at a junction temperature of 25 ° C for a relative luminous flux of 114 lm, with the following forward bias current of 350 mA (Figure 4.1.2):

ΔV = (25 - 25) × (-0.003) = 0 V

Then:

In the case of If = 350 mA, Tj = 25 °C,

Light efficiency = 114/[(3 + 0)×0.35]=108.57 lm/W

Now calculate the efficacy of the LED at a junction temperature of 150 ° C, and bias it by the same current (according to Figure 4.1.1, the junction temperature of 150 ° C corresponds to 79.8 lm of relative light):

ΔV = (150 - 25) × (-0.003) = -0.375 V

Then:

In the case of If = 350 mA, Tj = 150 °C,

Light efficiency = 79.8/[( 3-0.375)×0.35]=86.86 lm/W

By comparing the two, we can see that when the junction temperature rises, the power consumption will drop slightly, but the luminous flux will have a more significant drop. Under the combined effect of power and luminous flux, the light effect will decrease as the junction temperature rises.

In practical applications, the junction temperature is always higher than 25 °C, so the "cold lumen" data is good, but it does not represent the performance of the LED. The actual LED efficacy is always lower than the luminous efficiency calculated based on cold lumens, and will be affected by the ambient temperature, so we must consider the heat lumen when designing the luminaire.

4.2 LED dimming

First, forward current dimming

Since the amount of LED illumination depends on the magnitude of the forward current, the easiest way to dim LEDs is to change the forward current. Figure 4.2.1 reveals the change in luminous flux with forward current. Since the relationship between the two is close to a linear change, the dimming by changing the forward current makes the implementation of the control algorithm very simple.

Figure 4.2.1 Relative luminous flux as a function of forward current

However, as shown in Figure 4.2.2, the change in forward current causes the movement of the color coordinates (Cx, Cy) and directly affects the color quality parameters (correlated color temperature, color rendering index) of the LED.

Figure 4.2.2 Osram LCW W5PM's color coordinates move with (a) forward current and (b) junction temperature

When the forward current is 350 mA, the color coordinates are (0.440, 0.408), and the corresponding correlated color temperature is 2985 K. If the forward current is reduced to 100 mA, the color coordinate shift is (0.448, 0.406) and the corresponding correlated color temperature is 2838 K. The change in forward current causes the correlated color temperature to change by 147 K (Fig. 4.2.2a).

Similarly, changes in junction temperature can also cause changes in color coordinates. When the junction temperature is 20 ° C, the color coordinates are (0.436, 0.406), and the corresponding relative color temperature is 3036 K. When the junction temperature rises to 100 ° C, the color coordinate changes to (0.428, 0.399), and the corresponding relative color temperature is 3121 K. The color temperature of the two is different by 85K. This can result in an annoying relative color temperature difference that is substantially visible when several LED luminaires are simultaneously dimmed.

advantage:

Does not cause the LED to flicker.

Disadvantages:

When the lamp is dimmed, the relative color temperature changes.

Second, pulse width modulation (Pulse-width modulation, referred to as PWM) dimming

Another way to achieve LED dimming is pulse width modulation (PWM). The principle of this kind of dimming is to switch the LEDs at a high frequency using the rated current. The power supply and power-off time of the LED determine the dimming level of the LED. Since the switching frequency is already higher than the human eye can sense the frequency of flicker, the human eye will feel that the LED is always on, but the intensity of its illumination depends on the duty-cycle of the PWM.

Figure 4.2.3 shows an example of PWM dimming for each of the 50% and 70% duty cycles.

Figure 4.2.3 Pulse width modulation dimming example:

a) 50% duty cycle

b) 70% duty cycle

advantage:

The relative color temperature is stable within the full dimming range.

Disadvantages:

When the luminaire is dimmed, it may produce a flickering effect.

4.3 LED connection method

The luminous flux of a single LED is not enough, so we generally connect several LEDs together to achieve the desired luminous flux. LED connection methods are generally divided into four types:

LED particles connected in series

LED particles connected in parallel

LED particle series and parallel connection

LED particle matrix connection

First, the LED particles are connected in series

The series connected LEDs (Fig. 4.3.1) both emit the same luminous flux and are unaffected by the forward voltage, which can vary depending on the number of series connections. This type of connection uses a constant current drive power supply, which is stable even if one or several LED chips are short-circuited in one path. Connecting LEDs in series is ideal for both forward current and PWM dimming. Generally used in situations where stable CCT and CRI are required.

Figure 4.3.1 Series connection of LED particles

advantage:

The current of each LED is the same - stable CCT and CRI

High efficacy – no external balancing resistors required

LED does not affect when it is shorted

Ideal connection for forward current and PWM dimming

Disadvantages:

When an LED is broken, the whole string will go out.

LED constant current drive power is more expensive

Second, the LED particles are connected in parallel

One of the great advantages of parallel connection of LED particles (Fig. 4.3.2) is that even if one LED does not work properly, it will not affect the operation of other LEDs. A parallel drive power supply is required for parallel connection. This type of connection is generally used for situations where CCT and CRI are not demanding, mainly for decorative lighting.

Figure 4.3.2 Parallel connection of LED particles

advantage:

Constant voltage power supply is cheaper

Not affected by the open circuit failure of an LED

Disadvantages:

Low performance

An LED is unstable when it is shorted

Cannot be used for forward current dimming

Third, LED particles in series and parallel connection

Compared to parallel connection, the overall performance of the connection is better than the slight decrease in the fail-safe function, the light color quality parameters (CCT and CRI) are more stable, and can be driven with a lower-cost constant voltage power supply. The series-parallel connection (Fig. 4.3.3) is generally used in the case of a large number of LED particles, such as a long string of decorative LED strips.

Figure 4.3.3 Series and Parallel Connection of LED Particles

advantage:

Can drive a large number of LEDs simultaneously

LED open circuit and short circuit protection

Disadvantages:

Low performance

Cannot be used for forward current dimming

Fourth, LED particle matrix connection

When the LEDs are connected in a matrix (Figure 4.3.4), we can simultaneously control a large number of LEDs and maintain their associated color temperature stability (regardless of which LED is faulty). The matrix-connected LEDs can be driven using a constant current source and a constant voltage source.

Figure 4.3.4 Matrix Connection of LED Particles

advantage:

Can control a large number of LEDs simultaneously

High performance

Forward current and PWM (when using a constant current source) can be used for dimming

LED open circuit and short circuit protection

Disadvantages:

Printed circuit board design is quite complicated

4.4 LED driver

Figure 4.4.1 is a simple drive circuit for supplying current to the LED. The drive converts the AC power in the grid to DC and optimizes it. LED lamps with special functions (such as dimming, emergency lighting, personnel sensing, remote control) require more complex drive circuits.

Figure 4.4.1 Basic diagram of LED wiring

First, the type and important parameters of LED driver

According to the output signal of the driver, we divide the LED driver into three categories:

Constant current drive (CC): Ideal for driving LEDs in series, driving to provide stable current and ideal dimming.

Constant voltage drive (CV): ideal for driving LEDs in parallel, decorative LED strips, the number of LEDs that can be driven can be changed at will, generally not used for dimming;

Special drive (CC+CV): Both series and parallel LEDs can be used, and the more expensive drive mode.

Figure 4.4.2 OMS Elite Vega LED control device with multiple driving methods

Important parameters of LED driver:

Rated current/voltage: a predetermined output current or voltage;

Rated power: the output power of the drive;

Efficacy: the ratio of output power to input power (%), the higher the efficiency, the better the drive.

Second, other features of LED driver

Analog signal communication - the easiest way to dim LEDs

Analog signal communication is only used in the lighting industry to achieve dimming, which is the most common dimming system used in commercial lighting, such as spotlights in stores. The disadvantage is that the analog signal dimming does not completely turn off the LED.

There are two main ways to dim the analog signal:

Front cut/post cut thyristor dimming: only dimming of a single luminaire can be achieved;

0-10V, 1-10V dimming: Simultaneous dimming of multiple lamps (as shown in Figure 4.5.1)

Figure 4.5.1 1-10V dimming diagram

Digital signal communication – complex communication with luminaires

Digital signal communication can control multiple LED drivers through digital signals, and each drive can be individually controlled, and the system can get feedback signals for each fixture status. Digital signal communication can realize dimming, motion and static detection, remote control, white light adjustment, scene preset, etc. It is an ideal communication method for implementing a wide variety of lighting projects.

Commonly used digital signal communication:

DALI (digital addressable lighting interface), digital addressable lighting communication, the most common digital signal communication (Figure 5.4.2)

DSI (digital signal interface) digital signal communication

DMX (digital multiplex) multi-channel digital communication

KNX (worldwide standard for all applications in home and building control) applies to all global standards for home and public building control

Figure 4.5.2 Schematic diagram of DALI control

White light adjustment

If we use two sets of circuits in the LED driver, we can connect two different CCT white LEDs simultaneously on one drive for control, one for warm white light and one for cool white light. (Figure 5.4.3) This way, white light can be adjusted, which is suitable for various application scenarios with different needs.

Figure 4.5.3 Schematic diagram of white light adjustment

Temperature feedback

We can also adjust the drive current through the instant feedback of the LED operating temperature to avoid the short life caused by overheating of the LED when the ambient temperature is too high. In this way, the life of the LED can be easily maintained.

Figure 4.5.4 Schematic diagram of temperature feedback

remote control

The LED driver can be very simple to achieve wireless remote control, as shown in Figure 4.5.5.

Figure 4.5.5 Remote Control Schematic

Emergency lighting

An LED driver with emergency lighting monitors the input drive current in real time. When the input current is cut, the LED driver automatically switches to powering the LEDs from the backup Battery. A typical backup battery can provide 1 to 3 hours of emergency lighting. This is an important function required by laws and regulations in public building lighting.

Figure 4.5.6 Schematic diagram of emergency lighting Next: The top ten advantages of LED http://