This article first briefly introduces the general requirements of LED drivers, which limit the current flowing through the LEDs under certain operating conditions, regardless of the input and output voltages. Then provide several ways to choose LED drivers. Finally, I shared 5 tips on LED driver design! After reading this article, you will think that LED driver design is not difficult.
LED driver general requirements
The arrangement of the LEDs and the specification of the LED source determine the basic driver requirements. The main function of the LED driver is to limit the current flowing through the LED under certain operating conditions, regardless of the input and output voltage changes. The basic working circuit diagram of the LED driver is shown in Figure 1. The so-called "isolation" means that there is no physical electrical connection between the AC line voltage and the LED (ie, input and output). The most common is to use a transformer to electrically isolate. "Non-isolated" does not use high-frequency transformers for electrical isolation.
It is worth mentioning that in the LED lighting design, the two parts of the AC-DC power conversion and constant current drive can be configured differently:
(1) Integral configuration, that is, the two are fused together, all located in the lighting fixture, the advantages of this configuration include optimizing energy efficiency and simplifying installation;
(2) Distributed configuration, that is, the two exist separately, this configuration simplifies security considerations and increases flexibility.
How to choose LED driving method?
Typical LED drivers on the market today include two types, linear drivers and switch drivers; the approximate range of application is shown in Figure 2. Switching regulators are used for high current applications with currents greater than 500 mA because linear actuators are limited to their own structural reasons. Provides such a large current; in low current applications with currents below 200mA, linear regulators or split regulators are typically used; in medium to 200mA to 500mA, linear regulators can be used. A switching regulator can be used.
Switching regulators are energy efficient and offer excellent brightness control. The linear regulator structure is simple, easy to design, provides steady current and overcurrent protection, and has no electromagnetic compatibility (EMC) problems.
In low-current LED applications, the resistive driver, although low in cost and simple in structure, has a low forward current under low voltage conditions, which causes LED brightness to be insufficient and under transient conditions such as load dump. The LED may be damaged; and the resistor is an energy consuming component, and the overall solution is less energy efficient, see Figure 3.
For example, in LED lighting applications using DC-DC power supplies, the LED driving methods that can be used are resistance type, linear regulators, and switching regulators. The basic application diagram is shown in Figure 4.
In the resistive drive mode, the current sense resistor connected in series with the LED can be used to control the forward current of the LED. This drive mode is easy to design, low in cost, and has no electromagnetic compatibility (EMC) problem. The disadvantage is that it depends on the voltage and needs to be filtered. (binning) LED, and low energy efficiency.
Linear regulators are also easy to design and have no EMC problems. They also support current regulation and over-current protection (foldback), and provide an external current set point. The power dissipation problem is insufficient, and the input voltage must always be higher than the forward voltage. And energy efficiency is not high. The switching regulator continuously controls the opening and closing of the switch (FET) through the PWM control module to control the flow of current.
Switching regulators have higher energy efficiency, are independent of voltage, and can control brightness. Insufficient is relatively high cost, high complexity, and electromagnetic interference (EMI) problems. Common topologies for LEDDC-DC switching regulators include Buck, Boost, Buck-Boost, or Single-Ended Primary Inductor Converter (SEPIC).
Wherein, the lowest input voltage under all working conditions is greater than the maximum voltage of the LED string, and the step-down structure is adopted, for example, 6 series LEDs are driven by 24Vdc; in contrast, the maximum input voltage is less than the minimum output voltage under all working conditions. The voltage structure, such as driving 6 series LEDs with 12Vdc; and the input voltage and output voltage range overlap, you can use buck-boost or SEPIC structure, such as driving 12 series LEDs with 12Vdc or 12Vac, but this kind of The cost and energy efficiency of the structure is the least desirable.
LED driver standard
The LED driver itself is also evolving, with a focus on further energy efficiency, increased functionality and power density. The US "Energy Star" solid-state lighting specification proposes energy efficiency limits at the lighting fixture level, involving specific product requirements including power factor. The requirements of the European Union's IEC61347-2-13 (5/2006) standard for LED modules powered by DC or AC include:
Maximum safety extra low voltage (SELV) working output voltage ≤ 25Vrms (35.3Vdc)
"appropriate" / safe work under different fault conditions
No smoke or flammable when it fails
In addition, the ANSIC82.xxx LED driver specification is still under development. In terms of safety, it is required to comply with UL, CSA and other standards, such as UL1310 (Class 2), UL60950, UL1012. In addition, LED lighting design also involves product life cycle and reliability issues.
LED driver design 5 tips and essentials
1, chip fever
This is primarily for high voltage driver chips with built-in power modulators. If the current consumed by the chip is 2 mA, the voltage of 300 V is applied to the chip, and the power consumption of the chip is 0.6 W, which of course causes the chip to generate heat. The maximum current of the driving chip comes from the consumption of the driving power mos tube. The simple calculation formula is I=cvf (considering the resistance benefit of charging, the actual I=2cvf, where c is the cgs capacitance of the power MOS tube, and v is the power tube conduction When the gate voltage, so in order to reduce the power consumption of the chip, we must find ways to reduce c, v and f. If c, v and f can not be changed, then please find a way to divide the power consumption of the chip into the off-chip device, be careful not to Introduce additional power consumption. A simpler one is to consider better heat dissipation.
2, power tube fever
The power consumption of the power tube is divided into two parts, switching loss and conduction loss. It should be noted that in most occasions, especially for LED mains drive applications, the switching damage is much greater than the conduction loss. The switching loss is related to the cgd and cgs of the power tube and the driving capability and operating frequency of the chip. Therefore, to solve the heat generation of the power tube, it can be solved from the following aspects:
A. The MOS power transistor cannot be selected according to the on-resistance of the one-sided surface, because the smaller the internal resistance, the larger the cgs and cgd capacitance. For example, the cgs of 1N60 is about 250pF, the cgs of 2N60 is about 350pF, and the cgs of 5N60 is about 1200pF. The difference is too big. When the power tube is selected, it is enough.
B. The rest is the frequency and chip drive capability. Here we only talk about the frequency effect. The frequency is also proportional to the conduction loss. Therefore, when the power tube is hot, the first thing to think about is whether the frequency selection is a bit high. Find ways to reduce the frequency! However, be aware that when the frequency is reduced, in order to get the same load capacity, the peak current must be larger or the inductance is also larger, which may cause the inductor to enter the saturation region. If the inductor saturation current is large enough, consider changing the CCM (continuous current mode) to DCM (discontinuous current mode), which requires an additional load capacitor.
3, working frequency down frequency
This is also a common phenomenon in the debugging process of users. The frequency reduction is mainly caused by two aspects. The ratio of input voltage to load voltage is small and system interference is large. For the former, be careful not to set the load voltage too high, although the load voltage is high, the efficiency will be high.
For the latter, you can try the following aspects:
a, the minimum point of the minimum current setting;
b, the wiring is clean, especially the critical path of sense;
c. Select the small point of the inductor or use the inductance of the closed magnetic circuit;
d, add RC low-pass filter, this effect is a bit bad, C's consistency is not good, the deviation is a bit large, but for lighting should be enough. No matter how bad the frequency is, there is no advantage, only the bad, so we must solve it.
4, the choice of inductor or transformer
Many users react, the same drive circuit, the inductance produced by a is no problem, the inductor current produced by b becomes smaller. In this case, look at the inductor current waveform. Some engineers did not notice this phenomenon, directly adjust the sense resistance or the operating frequency to reach the required current, which may seriously affect the service life of the LED. Therefore, before the design, reasonable calculation is necessary. If the theoretical calculation parameters and the debugging parameters are a little far away, consider whether the frequency reduction and the transformer are saturated. When the transformer is saturated, L will become smaller, causing the peak current increase caused by the transmission delay to rise sharply, and the peak current of the LED will also increase. Under the premise that the average current is constant, only the light fades.
5, LED current size
We all know that if the LEDripple is too large, the LED life will be affected. For the LEDripple too high impact, some LED factories said that it is acceptable within 30%, but has not been verified. It is recommended to try to control the small points. If the heat solution is not good, the LED must be derated. I also hope that some experts can give a specific indicator, or else affect the promotion of LED.
LED driver design is not difficult, as long as the calculation before debugging, measurement during debugging, aging after debugging, I believe we can easily engage in LED.
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