LED lighting technology knowledge: realization of step-down structure

With the widespread application of LEDs, linear power supplies are increasingly being used in many applications. Linear Power Supplies

This simple structure can no longer meet the needs. In general, when the forward current required by the LED is set by a resistor, this simple driving method can continuously provide energy from the power supply to the load. Since the current of the LED is the same as that of the resistor, the power consumption of the resistor will increase with the increase of the input voltage. For example, an LED driven by a linear power supply has an efficiency of 70%. A 5V linear power supply provides 1A current to a typical white light InGaN LED (VF = 3.5V). Under the same working conditions, when the input voltage rises to 12V, its efficiency will drop to 30%. It cannot be used under such low efficiency. power supply

A power supply is a device that provides power to electronic devices. It is also called a power supply. It provides the electrical energy required by all components in a computer. [Full text]

Switching Power Supply Switching Power Supply

The switching power supply solves the problem of large efficiency changes due to input changes. This method controls the duty cycle to meet the output voltage or current required. Since the switching power supply will generate pulsed voltage and current, it is necessary to use some energy storage devices (inductors). inductance

The switching power supply can also achieve high efficiency over a wide input or output range. In the previous example, after replacing the linear power supply with a step-down switching power supply, the efficiency of the circuit changes from 95% to 98% when the input voltage changes from 5V to 12V.

The switching power supply has been greatly improved in efficiency and structural flexibility, but the periodic switching causes an increase in noise, and the complexity of the structure reduces the reliability of the circuit and increases the cost. The constant current LED circuit can be simply considered as a constant current source. Constant current source

The choice of topology should take into account the least external components and the best performance as the standard, which can improve the stability of the circuit and reduce the cost. In view of the good dynamic dimming characteristics of LED, it is necessary to consider making this characteristic convenient to apply during the design. Fortunately, the basic buck switching circuit performs very well in realizing these characteristics, so the LED driver generally chooses a buck switching power supply.

Constant current output stage

The most commonly used switching regulator is a voltage regulator. Figure 1a shows a basic constant voltage buck regulator. The buck controller can keep the output voltage constant by controlling the duty cycle or frequency when the input voltage changes. The output voltage required is calculated by the following formula (Eq. 1)

Figure 1a: Basic step-down voltage regulator

The inductor L is used to set the peak-to-peak value ΔIpp of the inductor current ripple, and the capacitor Co is used to set the output voltage ripple and the load transient response of the output voltage. Inverter

The average current in the inductor is equal to the load current, so we can control the load current by controlling the peak-to-peak value of the inductor current ripple. This can convert the voltage source control method into a current source control method. Figure 1b shows a basic current-type buck regulator. Similar to the constant voltage type, the constant current buck regulator can keep the output current IF constant by controlling the duty cycle or frequency when the input voltage changes. The required output current is calculated by the following formula (Eq. 2): Current Source

The current source is a basic element of the circuit. It is a two-terminal element. The internal resistance of the current source is large relative to the load impedance, and the fluctuation of the load impedance will not change the current. It is meaningless to connect a series resistor in the current source loop because it will not change the current of the load, nor the voltage on the load. Such resistors should be simplified in the schematic diagram. The load impedance is only meaningful when it is connected in parallel to the current source, and it has a shunt relationship with the internal resistance. Due to many reasons such as internal resistance, the ideal current source does not exist in the real world, but such a model is very valuable for circuit analysis. In fact, if the current fluctuation of a current source is not obvious when the voltage changes, we usually assume that it is an ideal current source. [full text]

Figure 1b: Basic current-mode buck regulator

After we set the LED current IF, we must accurately detect the current on the inductor. Theoretically, there are many ways to detect the inductor current, such as using the on-resistance Rdson of the MOSFET or using the DC resistance of the inductor. But in reality, these detection methods cannot meet the requirements of LED current setting in terms of accuracy (the accuracy of high-brightness LEDs is 5%-15%). If the resistor RFB is used directly to detect IF, the accuracy can meet the requirements, but additional power consumption will be generated in the resistor. Reducing the feedback voltage VFB can reduce the resistance value of the detection resistor at the same detection current IF (Fig. 2), so that the power consumption can be minimized. Most of the latest LED drivers provide a reference voltage (feedback voltage) between 50-200 mV.

The unique feature of constant current buck regulators is that they do not require output capacitors. Because there is a continuous output current and no load transients, the output capacitor in this regulator is limited to the current filter. filter

When we set it to a constant current buck regulator without capacitor, the output impedance will increase significantly. For the boost type, due to the increase in output impedance, in order to meet the constant output current, the output voltage will also increase significantly. As a result, the dimming speed and dimming range are significantly improved. In the application process, the dimming range is a very valuable feature from the perspective of backlighting and machine vision. Backlight

The backlight of the screen. Often used in LCD displays, the light source may be an incandescent bulb, electro-optical panel (ELP), light-emitting diode (LED), cold cathode fluorescent lamp (CCFL), etc. Electro-optical panels provide uniform light across the entire surface, while other backlight modules use diffusers to provide uniform light from uneven light sources. The backlight can be any color, and monochrome LCDs usually have yellow, green, blue, white, etc. backlight. Color displays use white light because it covers the most colors. [full text]

On the other hand, due to insufficient output capacitance, the AC current ripple circuit requires a relatively large inductor to meet the LED ripple requirements (forward current ΔIF = ±5 to 20%). At the same current ripple, a large inductor will increase the area and cost of the LED driver. Therefore, in the constant current buck circuit, the use of output capacitance must be weighed between cost, area, and dimming speed and range.

For example, to drive a 1A white LED (VF ≈ 3.5V) with a ripple current, ΔIF needs to be within ±5%, with an input voltage of 12V and a frequency of 500kHz, which only allows the use of a 50mH inductor at an inductor current amplitude of 1.1A. However, if the ripple current of the inductor is allowed to increase by ±30%, the inductor will be less than 10mH. If the 10mH and 50mH inductors are made of the same material and have the same rated current, the 10mH inductor is about half the cost and volume of the 50mH inductor. In order to achieve the required ΔIF (±5%) with a 10mH inductor, the output capacitor needs to be calculated based on the dynamic resistance rD of the LED and the sense resistor RFB and the impedance of the capacitor at this switching frequency, which can be calculated using the following expression (Eq. 3)

Loop Control Structure

Buck-based topology can be well matched with many loop control structures and does not consider stability limitations, such as the right half plane zero problem. In addition to being compatible with other dimming methods, this buck structure makes PWM dimming easy. LED drivers based on this structure can provide system designers with more options. Hysteretic control is very suitable for applications where the switching frequency changes quickly and the input range is small, such as white paper bulbs and traffic lights. Since hysteretic control does not consider stability limitations, loop compensation does not need to be considered. It is not limited by bandwidth like loop control. Using hysteretic control to drive a buck LED driver (Fig. 2a) simplifies the design and reduces the number of components and cost. This structure also allows a better PWM dimming range than other structures. LED drivers using hysteretic control are very suitable for applications where a very large dimming range is required and the dimming frequency is relatively high and the switching frequency changes are very large.

Figure 2a: Basic hysteretic controlled buck driver

A similar hysteretic buck LED driver can provide a good compromise between fixed frequency operation and hysteretic control without requiring switching frequency variation. The buck LED driver with controlled on-time (Figure 2b) uses a hysteretic comparator. Comparator

And the on-time controller. Let the on-time be inversely proportional to the input voltage, so that the change of the switching frequency can be minimized. Using this structure can also avoid the bandwidth limitation of the loop control. Using different dimming structures can make the dimming range very wide.

Figure 2b: On-time controlled buck LED driver

In some cases, such as many automatic control applications, it is required to reduce noise interference when synchronizing the LED driver with an external clock or with the driver. Hysteresis control and quasi-hysteresis control structures without clocks will cause difficulties in implementing synchronous frequency. In contrast, this problem is easier to achieve for a regulator controlled by a clock, such as the fixed-frequency buck LED driver in Figure 2c. Fixed-frequency control can solve this complex problem, but it also affects the dimming range due to its limited dynamic response.

Figure 2c: Basic fixed-frequency buck LED driver

In summary, many features of buck LED drivers make them very attractive. It can be easily set as a current source and can also achieve the minimum number of peripheral components. Fewer components can make the design simpler, improve the stability of the driver, and reduce costs. The buck structure LED is suitable for many control methods, making its application more flexible. Its output can omit the output capacitor and can also be well matched with other different dimming methods. These features allow it to be used in high-speed dimming and wide-range dimming. When the application allows, all of these features make the topology of the buck LED driver have many options.

What kind of application conditions do not allow the use of this structure? For example, home or commercial lighting requires thousands of lumens, and a method is designed to drive a string of LEDs. The total forward voltage drop on the LED string is equal to the sum of the forward voltage drops of each LED in it. In some cases, the input voltage range of the system may be lower than the forward voltage drop of a string of LEDs, or sometimes higher and sometimes lower. In these cases, a boost structure may be required, and a buck-boost switching regulator may also be required. In the next section, we will discuss LED drivers with boost and buck-boost structures.