Abstract: In today's era of energy shortages, improving energy efficiency is a primary goal of scientific research. It is well known that LEDs are highly energy-efficient compared to other lighting technologies. With the continuous development of LED lighting technology, LEDs have gradually evolved from ordinary LEDs to high-power LEDs. This will promote the integration of LED lighting technology into people's daily lighting and accelerate the process of LEDs replacing other lighting products. However, the widespread adoption of high-power LEDs in daily lighting is currently limited by LED driver technology.
This paper conducts an in-depth study on high-power LED driving technology. It elucidates the necessity and purpose of researching high-power LED drivers. The principles and performance of LED driving methods are introduced in detail, and a design scheme is proposed: a high-power LED driver control design based on the HV9910B chip manufactured by Supertex Corporation (USA). The circuit design of this scheme mainly includes two parts: the design of the power supply pre-stage rectification and filtering circuit and the design of the chip's peripheral driving circuit. The principles of these circuits are analyzed in detail, and the parameters of each key component involved in the driver design are configured. Furthermore, the performance of the high-power LED driver designed in this paper is analyzed, ultimately completing the design of the high-power LED driver.
1 Introduction
With the development of science and technology, human lighting has evolved significantly, from traditional fire lighting and incandescent lamps to fluorescent lamps and now, with the advent of semiconductor solid-state lighting. In fact, the development of lighting technology is essentially a process of continuously improving lighting efficiency. Because the spectrum of firelight is mostly outside the visible light spectrum, and approximately 90% of the energy produced by combustion is converted into heat, the efficiency of firelight lighting is very low. Furthermore, the combustion of chemical fuels often produces polluting gases. Therefore, people began to search for and develop lighting devices with higher luminous efficiency. As the latest emerging fourth-generation lighting technology, semiconductor solid-state lighting technology is considered another historic leap forward in the history of human lighting, following fluorescent lamps. The gradual improvement of semiconductor technology has also taken a considerable amount of time.
With the development of LED technology, LEDs have expanded from their initial application in digital watches, indicator lights, and calculator displays—areas with low brightness requirements—to high-power LED everyday lighting applications with ultra-high brightness. As LED efficiency continues to improve and processing costs decrease, the emergence of high-power LEDs has expanded LEDs into the general lighting field. High-power LEDs, i.e., LEDs with a power greater than 1W, have the following advantages compared to traditional incandescent and fluorescent lamps: (1) energy saving; (2) pure light color; (3) long lifespan; (4) reliability and durability; (5) flexible application; (6) fast response; (7) flexible control.
As an emerging technological field, high-power LEDs offer many advantages over traditional lighting sources. However, several technical challenges remain to be overcome before they can truly replace traditional light sources. Due to the voltage-current and temperature characteristics of LEDs, high-power LEDs require low-voltage DC drive. Therefore, traditional drivers for tungsten bulbs, fluorescent lamps, energy-saving lamps, and sodium lamps are unsuitable for directly driving high-power LEDs, necessitating the design of specialized drivers. While the brightness of a high-power LED is typically proportional to the current flowing through it, it saturates under high current, leading to a significant reduction in luminous efficiency and even failure. Therefore, the current used with high-power LEDs must not exceed their rated value. Furthermore, LED brightness output is inversely proportional to temperature, so minimizing heat generation and designing a robust cooling system are crucial. Moreover, the cost of commonly used incandescent lamps, fluorescent lamps, and high-intensity discharge lamps is relatively low. High-power LEDs can only replace these traditional light sources if their production costs are significantly reduced, and the production cost of high-power LEDs primarily depends on the value of the chip itself and the driver power supply.
In conclusion, as high-power LEDs gradually become the next generation of light sources, the quality of LED drivers will directly constrain the reliability of high-power LED products and determine whether high-power LEDs can achieve long lifespan and high efficiency in practical applications. To realize true semiconductor lighting, designing high-performance high-power LED driver circuits based on market demand has significant economic and practical value.
Analysis of 2 High-Power LED Drivers
With the gradual improvement of high-power LED technology, the application of LEDs has become increasingly widespread. However, the application of high-power LEDs has also placed high demands on LED driving technology. This chapter provides a comparative analysis of the mainstream high-power LED driving solutions currently on the market. The main driving solutions include linear driving, switching driving, and constant current driving.
The advantages of linear driving are simple circuit design, low cost, and fewer required components. However, its energy conversion efficiency is low, failing to achieve energy saving effectively. It also generates significant heat and is prone to voltage fluctuations, leading to unstable LED brightness. Therefore, this linear driving method is unsuitable for everyday lighting applications.
Switching drives control the output voltage by utilizing the duty cycle of a switching transistor, resulting in extremely high energy conversion efficiency, exceeding 80% in practical applications. Furthermore, compared to linear drives, switching drives offer significant improvements in voltage stability, efficiency, and circuit size, while also being relatively low-cost. They also offer three structural options: buck, boost, and buck-boost, providing flexibility in application. However, because the working principle of switching drives for LEDs is the same as that of linear drives, they do not meet the requirements of the nonlinear voltage-current characteristics of LEDs. Therefore, the problems of inconsistent and unstable brightness in high-power LEDs connected in series and parallel still exist.
Constant current source drive circuits, due to the use of constant current control chips, have advantages such as high current control accuracy, relatively simple structure, and high reliability. However, they also have some shortcomings: First, constant current source drives require strict matching between the power supply voltage and the number and operating voltage of the high-power LED loads in the circuit, allowing no large-scale changes. Therefore, constant current source drives have poor adaptability to changes in power supply voltage and high-power LED loads. Furthermore, since the integrated constant current control chips used in the circuit consume a lot of power during operation, their heat generation is relatively high, especially when the chip's operating voltage varies significantly. This leads to severe heat generation, requiring additional heat sinks, which increases power loss and reduces efficiency.
Based on the above comparative analysis, this paper adopts commercially available integrated constant current control chips. This makes it clear that high-power LED constant current source driving solutions often employ a switching power supply for voltage regulation at the front end of the driver, and then use a constant current control chip for constant current design at the back end. Using a linear constant current chip driving circuit is the simplest and most direct driving method among constant current source drivers.
The core of a linear constant current chip is to utilize the power transistor integrated in the chip, which operates in the linear region and can act as a variable resistor to control the load, so that the current flowing through the high-power LED is maintained at a relatively stable value.
The linear constant current source driver selected in this paper is the NU511 integrated driver chip manufactured by Digital Energy Technology Co., Ltd. The NU511 is a single-channel open-drain LED linear constant current driver chip with a maximum drive current of 1.2A and supports PWM dimming control up to 1MHz. The NU511 can be used to drive high-power LEDs from 0.5W to 3W. In high-power LED lighting applications, the NU511's wide voltage range allows it to operate stably in uncertain power supply systems. When the power supply voltage fluctuates, the NU511's output current remains constant, thus keeping the current of the entire LED lighting circuit constant, achieving the purpose of constant current.
As shown in Figure 1, the peripheral circuit driven by the NU511 linear constant current source only requires three components: a series resistor Rs, a current regulating resistor Rf, and an anti-interference and vibration damping capacitor C. Based on the relationship between the NU511 current setting and the external resistor, the required constant current value I in the lighting system can be determined according to the value of the current regulating resistor RF, as shown in equation (1):
(1)
Where K is a constant.
Figure 1 NU511 constant current drive circuit
With a fixed current value, the NU511's output voltage increases with the power supply voltage, thus increasing the NU511 chip's power consumption and heat generation. To reduce the NU511's power consumption and increase the efficiency of the drive circuit, the output voltage needs to be reduced. To address this, we added a power resistor Rs to the circuit to share the NU511 chip's power consumption and reduce its heat generation. Additionally, to minimize voltage fluctuations across the load, we selected an appropriate capacitor C to improve system efficiency.
In addition, for AC-powered drives, a switching power supply circuit needs to be added in the front stage to provide power to the constant current control chip.
From the above analysis, we can see that the driving circuit structure based on the linear constant current chip is relatively simple, uses fewer components, and has a lower cost. In addition, due to the use of a constant current control chip, this constant current source driving method has the advantages of high control accuracy and high reliability.
3 High-Power LED Driver Control Design Schemes
This paper synthesizes the analysis of various aspects of high-power LED driving in previous chapters and designs an LED control circuit based on a buck converter. To reduce the impact of high-power LED voltage and input voltage fluctuations on the high-power LED current, feedback control is adopted for the buck converter with peak current control, and a suitable buck converter is selected to achieve an efficiency of 90%.
3.1 Introduction to the HV9910B Drive Controller
To better promote the application of LEDs, various companies have designed various LED driver products. Supertex, an American company, is one of the manufacturers that has launched the most high-voltage LED constant current direct drive ICs. In 2005, they first launched the HV9921/22/23 series, which could drive low-power LEDs powered by AC. The drive current of the HV9921/22/23 series was in the range of 20-50mA. With the emergence of high-power LEDs, the HV9921/22/23 series could no longer meet the driving needs. Therefore, Supertex developed the high-power LED driver controller HV9910, which could be powered by AC and could output a constant drive current of 1A. Then, in 2007, they launched the HV9910B driver controller, an improved version of the HV9910. Its development played a positive role in promoting the application of high-power LEDs. Below, we will introduce the HV9910B chip in detail.
The Supertex HV9910B chip is a general-purpose LED driver controller. It can be used in high-power LED drivers (DC/DC or AC/DC), constant current sources, RGB backlight LED drivers, flat panel display backlight drivers, chargers, and LED decorative lights or signal lights.
The HV9910B chip can be powered by AC mains or by 12-24V or 48V solar cells or DC batteries. Its input DC voltage range is extremely wide, from 8 to 450V. Its output constant current drive current can range from tens of milliamps to over one amp, also with a very wide range. This allows it to drive both high-power and low-power LEDs, up to hundreds of LEDs, with output power ranging from a few to tens of watts. Furthermore, its application is very flexible; it can be configured in buck or boost/buck structures to meet various needs.
The HV9910B chip can be programmed in either off-time mode or constant power mode, and is an integrated LED open-loop current control driver. It includes a linear regulator that can adjust the input voltage to 7.5V for internal power supply, while its input voltage tolerance ranges from 8V to 450V, allowing it to operate without an external low-voltage power supply. Furthermore, the HV9910B can directly drive external MOSFETs without any additional circuitry.
Furthermore, the HV9910B easily implements PWM dimming, allowing a duty cycle from 0 to 100% and a frequency of several kilohertz. It can also achieve linear dimming from 0 to 250 millivolts. Its internal structure is shown in Figure 2. (The diagram shows the internal structure of the HV9910B.)
Figure 2 shows the circuit schematic of HV9910B.
VIN is the input terminal for the operating voltage;
GND is the ground terminal;
VDD is the output terminal of the internal 7.5V regulated power supply. A capacitor with low equivalent impedance needs to be connected between it and the ground terminal. This capacitor should usually be greater than 0.1 microfarads.
LD is the dimming input, used to adjust the brightness of the LED;
GATE is connected to the gate of the MOSFET switch;
PWM_D is the enable terminal and input terminal for PWM dimming. When a low level is applied to PWM_D, the MOSFET is turned off and there is no output. When a high level is applied to PWM_D, the GATE outputs a high level and the MOSFET is turned on.
CS is the detection terminal, connected to an external current sampling resistor RCS;
Rosc is used to set the oscillation frequency. The external working frequency setting resistor Rosc is connected to the ground when working in the mode with constant off time.[37]
As shown in Figure 2, the HV9910B has two current sampling threshold voltages: one is the external voltage of the LD pin, and the other is an internal voltage of 250mV. In practical applications, we usually choose the lower of these two voltages because a lower sampling voltage means a smaller resistance for current detection, thus increasing efficiency. Analysis shows that Supertex's HV9910B chip is an integrated chip specifically designed for LED driver design, allowing for the creation of more efficient and simpler high-power LED drivers.
3.2 Hardware Circuit Design Scheme
This paper designs a high-power LED driver control system based on the HV9910B chip. It mainly consists of EMI filtering, rectifier filtering circuits, the HV9910B chip, and its external circuits. The system block diagram is shown in Figure 3. In the figure, the power supply is rectified and filtered to become a stable DC voltage, which is then supplied to the high-power LED. This, combined with the HV9910B chip, enables the entire driving process.
Figure 3 Basic circuit block diagram
The input stage circuit consists of a full-bridge rectifier, a thermistor, and a filter capacitor. Its main function is to rectify and filter the 220V AC power. The control circuit is based on the HV9910B chip. The input voltage of the circuit is provided to the chip by the voltage filtered by the input stage circuit, as shown in Figure 4. To ensure the stability of the VDD, PWM_D, and LD pin voltages, each pin must be grounded through a capacitor. The switching signal for the control switch is provided by a square wave pulse signal of a certain frequency generated by the GATE pin. The pulse width is determined by the high-power LED current signal fed back by the sampling resistor Rcs connected to the CS pin, and the resistor connected to the Rosc pin determines the pulse frequency. The inductor L in the circuit provides energy storage, filtering, and continuous power supply to the drive circuit, keeping the current in the load balanced. The inductor L plays a crucial role in the circuit.
Figure 4 Circuit schematic
The power supply process is basically as follows: during the half-cycle when the switch signal is in the off state, the circuit is powered by a loop consisting of the input diode, the charged inductor L, and the high-power LED; during the half-cycle when the switch signal is in the on state, the circuit is directly powered by the pre-stage filter circuit to the high-power LED load. In this way, we can achieve continuous driving of the high-power LED within one cycle.
3.3 Rectifier and Filter Circuit Design
The rectifier circuit described in this paper consists of a filter and a bridge rectifier circuit. The filter mainly comprises a thermistor and two capacitors. The thermistor is primarily used to limit inrush current, while the electrolytic capacitor is used for voltage regulation. The other capacitor, a metal-plated polypropylene capacitor, assists the electrolytic capacitor in absorbing high-frequency ripple current.
This paper uses a bridge rectifier circuit as the core component of the rectifier circuit, utilizing the unidirectional conductivity of diodes to convert alternating current (AC) into direct current (DC). This bridge rectifier circuit is derived from the full-wave rectifier circuit. While the full-wave rectifier circuit has high rectification efficiency, it also has drawbacks. The full-wave rectifier circuit requires each rectifier diode to withstand a very high maximum reverse voltage, which places high demands on the performance of the diodes themselves.
The bridge rectifier circuit, by adding two diodes, not only has high rectification efficiency, but also reduces the requirements for rectifier diodes compared to the full-wave rectifier circuit. The specific circuit diagram of the bridge rectifier is shown in Figure 5.
Figure 5 Bridge rectifier circuit
The bridge rectifier circuit uses four diodes connected in pairs. When the input is the positive half-cycle of a sine wave, rectifier diodes VD2 and VD4 are subjected to a reverse voltage, so they are cut off. Meanwhile, VD1 and VD3 are subjected to a forward voltage, so they are turned on, resulting in a positive output. Similarly, when the input is the negative half-cycle of a sine wave, rectifier diodes VD1 and VD3 are subjected to a reverse voltage, so they are cut off. Meanwhile, rectifier diodes VD2 and VD4 are subjected to a forward voltage, so they are turned on, resulting in a forward voltage at the output, opposite to the negative half-cycle of the sine wave.
From the above analysis, we can see that repeating these two processes yields the positive portion of a sine wave. Based on the above process, we can conclude that the voltage across the rectifier diodes in a bridge rectifier circuit is half that of a full-wave rectifier circuit.
4 Circuit Parameter Design
After determining the overall circuit design scheme, the parameters of different components in the circuit need to be set, which plays a decisive role in whether the circuit design can achieve the intended requirements.
First, we need to determine the switching frequency fs of the circuit according to the HV9910B chip datasheet, because the switching frequency determines the size of the inductor L. A higher switching frequency results in a smaller inductor, and vice versa. A larger inductor also increases switching losses in the circuit. Faced with this contradiction, this paper adopts a compromise, choosing a switching frequency fs of 80kHz. According to the chip datasheet, at a switching frequency of 80kHz, the value of the resistor Rosc should be 470kΩ.
Next, we need to select the input diode D. The selection of the diode mainly considers its rated voltage and rated current values. The rated voltage of the diode is determined by the maximum voltage input to the diode. Typically, a 50% safety margin is considered when selecting a diode, multiplied by 1.5, therefore:
(2)
The rated current of the diode is primarily determined by the maximum average output current of the HV9910B, referring to the current at the minimum input voltage and maximum output power. The minimum input voltage Vdcmin must be greater than half the voltage across the high-power LED to ensure system stability. Therefore,
(3)
(4)
As can be seen from the above formula, a diode with a rated voltage of 600V and a rated current of 1A needs to be selected.
The thermistor in the circuit diagram is used to limit surge current. Therefore, when the AC current is at its peak, the current that the thermistor needs to limit must be higher than 5 times the steady-state current. We can calculate the value of the thermistor at room temperature (25℃) using the following formula.
(5)
In practical applications, a 120Ω thermistor with a rated current of 1A can meet the requirements.
In the circuit diagram, the capacitor C1 connected to the output terminal of the full-bridge rectifier circuit should be calculated according to the following formula:
(6)
That is, since the rated voltage of the capacitor must be greater than the peak value of the input voltage,
(7)
Therefore, capacitor C1 can be a 33μF electrolytic capacitor with a rated voltage of 450V. Electrolytic capacitors have good voltage regulation, but their equivalent series resistance is relatively large. Therefore, a metal-plated polypropylene capacitor C2 is connected in parallel across electrolytic capacitor C1 to better absorb high-frequency current. Its capacitance value can be calculated using the following formula:
(8)
The metal-plated polypropylene capacitor C2 has a capacitance of 0.33μF and a rated voltage of 400V, which can reduce the range of high-frequency circulating currents on the circuit board.
When the gate of the MOSFET is charged with a high-frequency pulse, capacitor C3 plays a role in maintaining a stable internal power supply voltage. This paper selects a 2.2μF capacitor with a rated voltage of 16V as C3. The gate current changes with the switching frequency. During switching, to prevent undervoltage shutdown due to the internal voltage drop of the chip, the voltage at the input power supply pin needs to be set higher.
The value of the sampling resistor R should be calculated according to the following formula:
(9)
U is the internal threshold voltage, which is equal to 0.25V, or it can be calculated using the voltage value of the LD pin, resulting in a value of 0.625Ω.
The inductor L provides energy to the high-power LED during the switch-off period. According to the formula, , where dt is the off-time, and di is the ripple current. Let the total ripple be 30%, then di equals I<sub> omax </sub> x 0.3, which means:
(10)
Therefore, an inductor of 4.7mH should be selected for L. As mentioned above, increasing the inductance will increase the switching losses.
The selection of the switching transistor in this paper is based on its peak voltage and maximum effective current. The rated voltage of the switching transistor is determined by the maximum input voltage. A safety margin of 50% is typically considered, so multiplying by 1.5 yields a VFET of 562V. The maximum effective current of the switching transistor is determined by the maximum duty cycle, which is 50% in this paper. Typically, the rated current of a switching transistor is three times its rated current, which is 0.247A. Therefore, the selected switching transistor is the STD2NM60 MOSFET with a rated current of 1A and a rated voltage of 600V. It produces very low power loss at a 50% duty cycle, approximately 171mW.
5 Circuit Performance Analysis
In the high-power LED driver control system designed in this paper, an external switch and inductor are used to reduce the power loss of the LED driver. The specific power loss in the circuit is mainly generated by the sampling resistor, the switching transistor, the diode, the inductor L connected to the high-power LED, and the HV9910B chip itself. According to the component parameters, their power consumption in the circuit is relatively small. This makes the overall circuit efficiency relatively high, reaching approximately 85% or more.
Furthermore, in terms of driving principle, the circuit design in this paper adopts peak current mode PWM control technology. That is, based on the principle of PWM switching circuits, a PWM switching circuit was designed with the HV9910B integrated chip manufactured by Supertex Corporation of the United States as its core. This method can effectively drive high-power LEDs. In-depth analysis reveals that this circuit has a wide operating voltage range, high conversion efficiency, and can guarantee constant current output. By filtering the circuit, the output current becomes more stable, meaning the current ripple is reduced, which is more conducive to the LED emitting light normally.
Circuit analysis reveals that the high-power LED driver control scheme designed in this paper conforms to the current-voltage characteristic curve of high-power LEDs, i.e., nonlinear characteristics. This reduces the impact of voltage fluctuations on current, thus solving the problem of unstable brightness in series-connected high-power LEDs during illumination. Furthermore, since the system requires only a relatively small number of components to achieve the driving purpose, it reduces the overall cost and design complexity of the driving system, thereby improving its reliability in use.
In summary, this high-power LED driver design scheme based on the integrated chip HV9910 has significantly improved energy loss, design cost, design complexity, and current accuracy control.
6 Conclusions
High-power LEDs, as a novel energy-saving lighting technology, have played a positive role in promoting the future development of the lighting field. However, in the process of promoting high-power LEDs, the driving technology of LEDs has limited their application in everyday lighting with AC power input. This paper designs a high-power LED driver with AC power input. The designed driver is mainly based on the HV9910B chip produced by Supertex Corporation (USA). The front-end power input design adopts a full-bridge rectifier circuit, which, in conjunction with the HV9910B, provides a stable power drive for the high-power LED. To achieve high-performance driving, this project configures the parameters of each key component involved in the driver design and analyzes the performance of the designed high-power LED driver, thus realizing the design of the high-power LED driver.
A high-performance, high-power LED driver was designed using the HV9910B chip manufactured by Supertex, providing strong technical support for the gradual replacement of existing lighting technologies by high-power LEDs.
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