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Advantages and Disadvantages of Driving Multi LED String Design

The parallel / series scheme for driving multiple LED strings has its own advantages and disadvantages

Advantages and Disadvantages of Driving Multi LED String Design 1

To drive multiple LED lamp strings, the simplest method is to use a power supply that can convert the power supply voltage into a direct current (DC) output voltage (e.g. 12 volts or 24 volts), then use this power supply to drive parallel LED lamp strings, and use resistors in each lamp string to adjust the current. The cost of this method is very low, but today's high brightness LEDs will consume more than 350 ma. Therefore, this method has great loss, low efficiency and poor current regulation effect, which makes the light difference between lamp strings very obvious.   

To improve this method, a linear regulator must be used instead of a resistor to improve the light output consistency of all lamp strings. However, this can only make the light output consistent, and the efficiency or power consumption has not been significantly improved. Reducing power consumption is very important to maximize the service life of LEDs. In both methods, using resistance or linear regulator as fixed heat source will greatly shorten the service life of LED.   Another equally simple method is to make a long single series lamp string and use a single power supply that can produce a high-voltage DC stable current source. The high voltage operation of this method will reach above the 60VDC or 42 volt root mean square (RMS) safety extreme low voltage (SELV) level, and the lighting equipment or accessories in it must be approved by the safety authority, which greatly reduces the flexibility of applying the same motor design to other applications.   

Another consideration of the single lamp string method is reliability. If only one LED is on, the light output of the whole lighting device will be released. Although many arc suppression circuits or devices can be installed to control each LED on, this will increase the cost and complexity of the lamp.  Light emitting diode (LED) has the characteristics of power saving and no frequent maintenance. It has become the mainstream of street lamps and other high-power lighting applications. Although the LED lighting driving methods are all inclusive, in order to meet the requirements of the market for higher efficiency and lower system cost, it is imperative to drive multiple LED light strings with a new simple topology circuit.Observing the development trend of street lamps or high patio lamps used in stadiums, as well as other high-power lighting applications, they are gradually turning to solid-state lighting using light emitting diodes (LEDs) as light sources, mainly because LEDs have higher energy efficiency and less frequent maintenance needs, and these two factors also prove the necessity of such a turn.

In such high-power lighting applications, various methods are being considered to drive these lamps. This paper will discuss a new topology that can drive multiple LED lamp strings with higher efficiency and lower system cost. In order to fully understand the advantages of this topology, we will first explore various methods that are currently considering solutions or have achieved good results in low-power LED applications.Using step-down converter to regulate current and reduce the cost of driving circuit is the keyIn fact, in high-power LED lighting applications, multi lamp string architecture with switching regulator to adjust current is most often used, in which a single main power supply will convert alternating current (AC) power supply to a single DC bus voltage generally below SELV level; This bus then supplies power to parallel LED strings, each of which has a step-down converter (the most common) or a step-up converter. For simplicity, the analysis of this paper is limited to the buck converter, because the buck converter is very similar to the boost converter in cost and number of components.   

Advantages and Disadvantages of Driving Multi LED String Design 2

For example, figure 1 shows a low-cost simple step-down regulator circuit, including a pulse width modulation (PWM) controller, an inductor, a metal oxide semiconductor field effect transistor (MOSFET), a diode, and a plurality of resistors and capacitors. If higher efficiency is needed, MOSFET can be used instead of diode, and PWM controller which can achieve synchronous buck operation can be used.Figure 1 simple step-down regulatorFigure 2 shows the high-power multiple lamp string lighting application subsystem block with current regulation by step-down regulator. When the AC power input is rectified, it is supplied to the power factor correction (PFC) boost circuit, where the PFC will generate a high voltage of 400 volts and provide input power to the downstream isolated DC-DC converter. Then, the converter output will be used to generate a low-voltage bus (generally 12 volts or 24 volts) to supply power to the step-down regulated LED lamp string.

Figure 2 general high power LED lighting system using step-down regulatorThis method has high efficiency and is an ideal choice for LED lighting with the minimum number of LED lamp strings. However, for high-power applications with more than four lamp strings, the number and cost of components will increase. For electronic component manufacturers and supply chains, product sales increase; However, for lighting equipment manufacturers and their users, such a high cost is not conducive to the wide use of products, because the stable development of solid-state lighting must rely on low-cost driving circuits in order to form and grow the market stably.Both low-cost / high-efficiency electrical insulation design scheme is eye-catching

Figure 3 shows the series input multiple parallel LED simple driver, which is a cost-effective multiple LED lamp string driving method. In addition to PFC, this is also a two-stage method, including reverse stable current step-down regulator and downstream DC-DC transformer circuit. This method has high efficiency and excellent lamp string current regulation function. Most importantly, it is a low-cost method.Figure 3 simple drive multi transformerIn addition, this method can also achieve backup effect for a single passive silicon controlled rectifier (SCR) arc suppression circuit added to each lamp string. If one LED or light string is on, the light output will not be higher than other light strings.   

Before further studying its operation, we must first discuss the problems arising from the use of simple drive multi transformer method. First of all, it should be noted that this is an electrical insulation design, in which the secondary side output voltage can be designed to be maintained below the SELV level, so it is not necessary to combine and interconnect the lighting equipment with the power supply, so as to obtain the permission of the safety authority. The reason is the same as all offline solutions discussed in this article. The power supply still needs a security license, but the lamps do not need it, so a process is omitted.  

 In addition, maintaining the output below these levels can also increase its elasticity, so that various lamps can meet the needs of many other lighting applications.   In addition, from the perspective of heat dissipation management, this insulation design is ideal because there are no restrictions on the proximity or contact of LEDs with metal accessories. The more remarkable feature is that this insulation design does not need the feedback of the output end, so it is not necessary to use photoelectric or other safety rated insulation feedback devices. Therefore, this paper will also discuss the simplicity of the secondary side, because there are only a few passive components on the secondary side, and there are no bias voltage sources, active components or control devices.   

In conclusion, in terms of operation, the simple driver has more than 1% excellent lamp string current matching, and has high-efficiency resonant operation, which can achieve higher cost-effectiveness with the increase of the number of lamp strings.PFC circuit output reduces switching lossThen, the output of PFC circuit is discussed. It is the input mode of reverse step-down circuit, which can be configured to produce stable current output, and the closed loop of the system is located near this current. Therefore, the generated current output will be supplied to the DC-DC transformer circuit downstream, which includes a half bridge controller, two MOSFETs, capacitor C1 and capacitor C2, And multiple transformers.

Then, the current will flow through the half bridge MOSFET switch to the primary side of the series transformer. Among them, capacitors C1 and C2 will play many functions. They can not only be used to establish a voltage divider for the half bridge, but also be a component of the resonant circuit and a DC blocking capacitor, which helps to avoid transformer saturation. The resonant operation allows the MOSFET switch to switch with zero voltage switching (ZVS), which can reduce the switching loss and force the output diode to achieve zero current switching (ZCS) to maximize efficiency.It must be noted that the DC current that has been converted to AC current will resonate back and forth through the primary side of all series transformers. The number of primary sides of transformers that can be connected in series is quite flexible, because the winding turns ratio can be selected to support many transformers or LED lamp strings. However, the number of lamp strings must be considered in calculating the turn ratio because it specifies the number of transformers and the forward voltage of each lamp string.Giving full play to the benefits of power conversion is indispensable for PFC design

In order to achieve the highest efficiency of power conversion, the minimum power must be handled as much as possible. In this way, it must be operated as close to the input voltage as possible. Since most high-power lighting applications support the use of active PFC, it can be regarded as a functional block for simplicity, and some typical value outputs represent the outputs therein.Since most active PFC circuits can play the role of boost converter, the PFC output voltage must be set higher than the peak of the highest AC line voltage. In the general input range of 85 265vac, it is about 375 volts. After increasing some dynamic ranges of tolerance and tolerance, 400 Volts becomes a typical setting.In addition, in order to ensure that the downstream Buck has more dynamic range of PFC output changes, more tolerance must be added to adapt to the ripple of about 40 volts, which makes the lower limit of the operation point of the reverse buck input about 360 volts. In addition, in order to ensure that the step-down output has a certain maximum pressure drop for normal operation, it is also necessary to provide a certain dynamic range and limit the output range to 280 volts.

It is easy to calculate the steady current value of step-down / transformer turns ratioAfter understanding each range limit, the next step is to understand how to calculate the stable current value of this design example through step-down and transformer turns ratio. In this design, two transformers are used to drive four LED light strings with a current of 1 ampere (a), each of which has ten high-power LEDs. Assuming that the led forward voltage is 3.5 volts and the lamp string voltage is 35 volts, since the output operation point of DC step-down is set at 280 volts, it becomes the input of DC-DC transformer circuit.This situation means that the voltage applied to the series primary side will be half of the voltage of the capacitor voltage divider (composed of C1 and C2), so that the voltage configured on the series primary side reaches 140 volts. As shown in Formula 1:

Through the equation of Formula 1, the primary side voltage (VP) = bridge voltage / number of transformers = 140 volts / 2 = 70 volts, and the calculation of turn ratio becomes quite easy, where NP = number of turns on the primary side; Ns = number of secondary side turns; Vs = secondary side or LED lamp string voltage; VP = voltage across each primary winding.On the other hand, if you want to calculate the current output setting value of reverse step-down when each transformer drives two LED lamp strings, you must first confirm that only one lamp string of each transformer is conductive in the alternating half cycle.However, to maintain the LED conductive state during sleep, the current provided by the pilot light string must be twice the LED current; That is, when the required LED current is 1 Amp, the current supplied to the LED and filter capacitor every half cycle is 2 amps. To calculate the step-down regulator, the current value (Iset) must be set as shown in formula 2:

As mentioned above, determine the transformer needs

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