LED driver is a voltage converter that converts the power supply into a specific voltage and current to drive the LED light. Usually: the input of LED driver includes high-voltage industrial frequency AC (i.e., utility), low-voltage DC, high-voltage DC, low-voltage high-frequency AC (such as the output of electronic transformers), etc. The output of the LED driver is mostly a constant current source that can change the voltage with the change of LED forward voltage drop value—input filter parts, output filter parts, etc. According to the requirements of different occasions, there are also input overvoltage protection circuits, input under-voltage protection circuits ；LED open circuit protection, overcurrent protection, and other circuits.
Classification of LED driver
The drive mode is divided into constant current type, constant voltage type, pulse drive, an AC drive.
The circuit structure is divided into resistor and capacitor step-down mode. Resistor step-down mode. Conventional transformer step-down mode. Electronic transformer step-down mode.RCC step-down mode switching power supply.PWM control mode switching power supply.
The installation location can be divided into an external power supply and a built-in power supply.
Driving method classification
Constant current type
A constant current driver circuit output current is constant. In contrast, the output DC voltage varies with the size of the load resistance within a specific range; the smaller the load resistance, the lower the output voltage, the larger the load resistance, the higher the output voltage.
B, constant current circuit is not afraid of the load short circuit, but the load is strictly prohibited entirely open circuit.
C,constant-current driver circuit to drive LED is quite ideal but relatively high price.
D should pay attention to the maximum withstand current and voltage values used, which limits the number of LEDs used
Constant voltage type
- When the voltage regulator circuit parameters are determined, the output voltage is fixed, while the output current changes with the increase or decrease of the load.
- The regulated circuit is not afraid of an open circuit of the load, but it is strictly forbidden to short circuit the load entirely.
- the voltage regulator drives the LED; each string needs to add a suitable resistor to make each LED display brightness average.
- the brightness will be affected by the voltage changes from rectification.
Pulse Drive
Many LED applications require a dimming function, such as LED backlighting or architectural lighting dimming. Dimming can be achieved by adjusting the brightness and contrast of the LED. Simply reducing the current of the device may be able to tune the LED luminance, but allowing the LED to operate at less than the rated current can have many undesirable consequences, such as chromatic aberration problems. Instead, the current adjustment method is integrated with the LED driver pulse width modulation (PWM) controller. PWM signal is not directly used to control the LED, but to control a switch, such as a MOSFET, to provide the LED with the required current. PWM controller usually operates at a fixed frequency and adjusts the pulse width to match the desired duty cycle. The main advantage of PWM control is that the dimming current through PWM is more accurate and minimizes the color difference when the LED is illuminated.
AC drives
AC drives can also be divided into three types of buck, boost, and converter, depending on the application. The difference between AC drive and DC drive is the need to rectify and filter the input AC. From the safety point of view ：isolation and non-isolation issues.
AC input drivers are mainly used for retrofit lamps: ten PAR (Parabolic Aluminum Reflector, an ordinary luminaire on professional stages) lamps, standard lamps, etc., they operate at 100V, 120V, or 230V AC input; while for MR16 lights, they need to work at 12V AC input operation. Due to certain complications, such as the dimming capability of standard triacs or leading and trailing edge dimmers and compatibility with electronic transformers (i.e., flicker-free operation). Therefore, the area covered by AC input drivers is more complex than that of DC input drivers.
AC power supply ( city power drive ) applied to LED drive, generally through the step-down, rectification, filtering, voltage stabilization ( or current stabilization ), and other processes. Then the AC power supply is converted to DC power, and finally, the LED is provided with the proper operating current through a suitable driver circuit. There should also be high-efficiency conversion, smaller size, and lower cost while solving the safety isolation problem. Consider the impact on the power grid. We need to solve its electromagnetic interference and power factor issues. For small and medium power LEDs, the best circuit structure is an isolated single-ended flyback converter circuit; for high-power applications, a bridge converter circuit should be used.
Circuit structure classification
According to the circuit, the structure is divided into a resistor, capacitor step-down mode; resistor step-down mode; conventional transformer step-down mode; electronic transformer step-down mode; RCC step-down mode switching power supply; PWM control mode switching power supply
Resistor and capacitor step-down method.
Through the capacitor buck, in the flashing use, due to charging and discharging, the instantaneous current through the LED is big, easy to damage the chip. Vulnerable to fluctuations in grid voltage, low power supply efficiency, low reliability.
Resistor step-down method
Through the role of capacitor buck charging and discharging, the instantaneous current through the LED is enormous and easily damages the chip. Vulnerable to grid voltage fluctuations, low power supply efficiency, and low reliability.
Conventional transformer step-down method.
Power supply small size, heavyweight, power supply efficiency low, generally only 45% to 60%, so commonly rarely used, reliability is not high.
Electronic transformer step-down mode.
Power supply efficiency is low, and the voltage range is not comprehensive, generally 180 ~ 240V, ripple interference is considerable.
RCC step-down mode
Switching power supply voltage range is relatively wide, and power efficiency is relatively high, generally can do 70% to 80%, the application is also more widely. Because the oscillation frequency of this control method is discontinuous, the switching frequency is not easy to control, the load voltage ripple coefficient is also relatively large, abnormal load adaptability is poor.
PWM control mode switching power supply
They are composed of four parts, input rectifier filtering amount, output rectifier filtering part, PWM regulator control part, switching energy conversion. The basic working principle of the PWM switching regulator is that in the case of input voltage, internal parameters, and external load changes, the control circuit, through the difference between the controlled signal and the reference signal for closed-loop feedback, adjust the pulse width of the primary circuit switching device conduction, so that Switching power supply output voltage or current stability (i.e., the corresponding regulated power supply or constant current power supply). Power supply efficiency is very high, generally can achieve 80% to 90%, the output voltage and current stability. Naturally, this circuit has perfect protection measures, is a high-reliability power supply.
Installation position classification
According to the installation location, the drive power supply can be divided into an external power supply and an internal power supply.
External power supply
As the name implies, an external power supply is the power supply installed outside. Generally higher voltage, there is a safety risk to people, you need an external power supply. The difference with the built-in power supply is the power supply plus a shell, standard street lights.
Built-in power supply
If the power supply installed in the lamps and lanterns generally lowers voltage, 12v to 24v, there is no safety hazard to people. This standard bulb lamp power supply
The characteristics of LED driving power supply
LED driver’s features. High reliability, high efficiency, high power factor, drive mode surge, protection function
High reliability
High reliability especially likes the LED street light driver, installed at high altitude. A waterproof aluminum case driver, good quality than not easy to break, reduces maintenance.
High efficiency
LED driver output current is constant, and the ideal circuit is that no matter how the LED characteristic curve changes, the driver’s current remains unchanged. But limited to the precision of the components, there constantly a tiny difference. This change is also an important parameter to determine whether the driver circuit is excellent; the LED on and voltage function is a non-linear “three-section” relationship, so it is very important to maintain constant current.
High efficiency LED is an energy-saving product, the efficiency of the driver power supply to be increased. The power supply is installed in the structure of the lamp, especially important. Because the luminous efficiency of the LED decreases as the temperature of the LED rises, the heat dissipation of the LED is very important. The high efficiency of the power supply, its power dissipation is small, the heat generated within the lamps and lanterns is small, it also reduces the temperature rise of the lamps and lanterns. It is conducive to delaying the light decay of LEDs.
High power factor
The start-up of the LED driver is soft-start due to the very poor consistency of LEDs and the instantaneous change in the activity of the internal PN junction during conduction. So the LED driver is generally designed for soft-start to avoid this defect.
A high power factor is a requirement of the grid for the load. Generally, there is no mandatory indicator for appliances under 70 watts. Although the power of a small single appliance power factor is a little lower on the grid, at night, everyone’s lights, the similar load is too concentrated, will produce more severe pollution of the grid. For 30 watts to 40 watts of LED drive power, that perhaps there will be specific indicators of power factor requirements soon.
Drive mode
Because many times, the circuit needs to be installed in a small space, with the convenience of LED lighting, the circuit should be as simple as possible, saving costs and reducing energy consumption.
LED Driver way prevails two kinds: one is a constant-voltage source for multiple constant-current sources, each constant-current source separately to each road LED power supply. This way, the combination of flexible, a group of an LED failure, does not affect the work of other groups of LEDs, but the cost will be slightly higher. The other is a direct constant-current supply, LED series, or parallel operation. It has the advantage of a little lower price, but the flexibility is poor. It also has to solve an inevitable LED failure, which does not affect the operation of other LEDs. These two forms, over time, coexist. Multiple constant current output power supply methods, in terms of cost and performance, will be better. Perhaps the mainstream direction in the future.
Surge protection
It generally does not require isolation because many products are similar to the same structure as ordinary lighting, safety aspects can be identical to illumination.
Surge protection LED anti-surge ability is relatively poor, especially the ability to resist reverse voltage. It is also essential to strengthen this aspect of safety. Some LED lights are installed in the outdoors, such as LED street lights. Due to the grid load start dumping, and lightning induction from the grid system will invade various surges, some will damage the LED. Therefore, the LED driver power supply should have the ability to suppress the intrusion of waves and protect the LED from damage.
Protection function
Power supply and the conventional protection functions, it is best to increase the LED temperature negative feedback in the constant current output to prevent the LED temperature is too high; to meet the requirements of safety regulations and electromagnetic compatibility.
Role of an LED driver.
LED driver power supply is the key to LED luminaire. It is like the heart of a person. The constant current source drive is the best way to drive LEDs. The current flowing through the LED is not affected by external supply voltage changes, ambient temperature changes, and LED parameter dispersion, thus maintaining a constant current and giving full play to the excellent characteristics of the LED.
With LED constant current power supply to LED lamps, the power supply will automatically detect and control the current flowing through the LED during operation. So do not worry about the moment of power on too high a current flow through the LED, and do not have to worry about the load short circuit burned power supply.
The constant current drive method can avoid the LED forward voltage changes caused by recent changes. In contrast, the constant current makes the LED brightness stable and facilitates the mass production of LED lighting factories to ensure product consistency. Therefore, many manufacturers have been fully aware of the importance of the drive power supply. Many LED lighting manufacturers have abandoned the constant voltage method and slightly higher cost stable current way to drive the LED lamps.
LED driver applications
LED applications are found in almost every field of electronic applications. Its variations in luminous intensity, light color, and on/off control are practically unpredictable. So LED drivers also become almost one-to-one servo devices, making the family of this device diverse by setting up different support circuits at the periphery of the LED driver IC. Build solutions for different LED applications. Small to cell phone display backlight and button light driver, large to high-power LED street lights, large outdoor LED display, etc.
How to choose a good LED driver
Not enough knowledge about LED power supplies
Production of LED lighting and related products of the company’s technical staff do not know enough about the switching power supply to make the power supply work properly. Still, some critical evaluation and electromagnetic compatibility considerations are not enough. There are still certain hidden dangers.
Most LED power supply manufacturers are transformed from ordinary switching power supplies to do LED power. The characteristics of LED and the use of knowledge are not enough.
Standards are not uniform
At present, there are almost no standards on the LED. Most of them refer to the means of switching power supplies and electronic rectifiers.
Now, most LED power supplies are not unified, so the volume is mostly relatively minor. Small procurement volume, the price is on the high side, and component suppliers are not very cooperative.
Stability of the power supply
The stability of LED power supplies: wide voltage input, high and low-temperature work, over-temperature, and over-voltage protection are not solved one by one.
The overall life of the driver circuit
First of all, the overall life of the driver circuit, especially crucial devices such as capacitors at high temperatures, directly affects the life of the power supply.
Higher conversion efficiency
LED drivers should be challenged to higher conversion efficiency, especially when driving high-power LEDs is more so. Because all the power is not dissipated as light output as heat, the power supply conversion efficiency is too low, affecting the energy-saving effect of LEDs.
Cost
At present, in the application of small power (1-5W), the proportion of constant current drive power supply cost has been close to 1/3 of the cost of the light source.
More LED driver knowledge
Calculate the number of LEDs by the known power supply power
Example: rated output power of 10W power supply, using the rated forward current of 20mA, dissipation power of 70mW under the conditions of how many LEDs can be configured? According to the above formula (that is, take the integer of the data obtained)
For constant voltage drive mode:
From the known output power supply voltage to calculate the number of series LEDs per branch and the number of parallel branches
(1) Calculate the number of LEDs per branch
(2) Calculate the number of parallel branches
Note: VLED values vary depending on the color of light, with a regulated power supply to drive LEDs, to control the current, usually need to be connected in series resistors.
Example: A-rated output voltage of DC 24V, power for 10W power supply, using a rated forward current of 20mA, dissipation of energy for 70mW rated forward voltage of 1.8V. How many LEDs can be configured? (Take the integer of the data obtained)
It can take ten groups of branches, each branch of 14 LED series circuits, a total of 140 LEDs.
2: For constant current drive mode
From the known power supply output current and LED current value to calculate the number of parallel branches and the number of each branch
(1) Calculate the number of branches in parallel
(2) Calculate the number of LEDs connected in series:
Example: A-rated output current of DC 0.35A, rated power of 10W power supply, drive dissipation power of 70mW, forward current of 0.02A LED, how can it be configured?
The number of parallel branches:
(Take the integer of the obtained data)
Number of series connections per branch: Number
That can take 17 groups, each group of 8 LED series, a total of 136 LEDs.
Line loss and line voltage drop calculation
P wire = I R V wire = IR
R wire = σ (Note: L is the length of the wire; S is the cross-sectional area of the wire; σ is the electrical conductivity of the wire) can also check the electrician’s manual.
Example: with a length of 10 meters (positive and negative wire 5 meters each), 24AWG copper-core wire, through the current of 2A, the power loss and line voltage drop is how much?
Check the electrician’s manual can be found: R wire = 0.737W
V wire = 2 × 0.737 = 1.474V
P wire = 2 × 0.737 = 2.948W
From the above calculations can be seen, the line current is larger. We must pay attention to the selection of the appropriate wire cross-section. Otherwise, the line loss and line voltage drop are pretty significant.
Only when we fully understand the essential characteristics of LEDs and LED power supplies can we design and use LED light sources correctly.
Examples of selection
If the LED light beads constitute the strip, you need to calculate according to the specific model.
Conventional products using the circuit structure are 3LED +1R mode, that is, 3 LEDs plus a resistor R together to form a group and then with other groups of the same connection together to form a parallel structure. When a group of LEDs has problems, this design will not affect the regular use of other groups. According to the current circuit structure, the current loaded on all LEDs in each group is 20mA. You can calculate the current of each unit of the LED strip; under the law of parallel shunt, the total current of the LED strip is the sum of the draft of each group.
For example, 1210 light bar, a meter of 60 lights, according to the 3LED + 1R circuit structure, then 60 lights / 3 equal to 20 groups, each current is 20mA (0.02A) 1210 light bar per meter of current calculation method is.
60/3 * 0.02 = 0.4 A. So, if you want to know the total number of meters of LED strip current, use 0.4 A x the number of meters can be the answer.
Another example is the 5050 light bar, and the structure is the same; the difference is that the 5050 has three outputs. So, according to the regular specification of 60 lamps per meter to calculate, then the total current of 5050 light bar is calculated as follows
60/3*0.02*3=1.2A/m.
The total number of meters of 5050 light strip, its total current is 1.2A X the number of meters, but, in engineering, generally, 1 meter of 60 beads of 5050 light strip with 1A power supply will do, 2 meters with 2A, and so on.
If it is an ordinary LED light board or bulb, choose according to the actual power of LED lights.
P=UI I=P/U, divide the lamp’s power by the 12V voltage (assuming that the LED light is 12V working voltage), and the result is the working current. Note that the choice of power supply is directly related to LED lights’ brightness, life, and safety. So must not use a shoddy power supply. Now on the market, LED lamps and lanterns are saving money on the power supply, which we should use the driver to meet the national safety standards. Such a power supply is expensive, should be careful about purchasing a cheap one
