LED lighting driver selection and design tips revealed

1. LED driver General requirements

The arrangement of LEDs and the specifications of LED light sources determine the basic driver requirements. The main function of LED drivers is to limit the current flowing through LEDs under a certain range of operating conditions, regardless of how the input and output voltages change. The basic working circuit diagram of LED drivers is shown in Figure 1, where the so-called "isolation" means that there is no physical electrical connection between the AC line voltage and the LED (i.e., input and output). The most common method is to use a transformer for electrical isolation, while "non-isolation" does not use a high-frequency transformer for electrical isolation.

It is worth mentioning that in LED lighting design, the two circuits of AC-DC power conversion and constant current drive can adopt different configurations:

(1) Integral configuration, where both are integrated and located inside 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.

2. How to choose LED driving mode

There are two types of typical LED drivers on the market today, namely linear drivers and switching drivers. The approximate application range is shown in Figure 2. For example, high-current applications with currents greater than 500mA use switching regulators, because linear drivers are limited by their own structure and cannot provide such a large current; in low-current applications with currents less than 200mA, linear regulators or separation regulators are usually used; and in medium-current applications between 200 and 500mA, both linear regulators and switching regulators can be used.

Switching regulators are energy efficient and provide excellent brightness control. Linear regulators are relatively simple in structure, easy to design, provide current regulation and overcurrent protection, and have no electromagnetic compatibility ( EMC ) issues.

In low-current LED applications, although the resistor-type driver has low cost and simple structure, the forward current of this driver is low under low voltage conditions, which will lead to insufficient LED brightness. In transient conditions such as load dump, the LED may be damaged. In addition, the resistor is an energy-consuming component , so the energy efficiency of the entire solution is low, as shown in Figure 3.

For example, in LED lighting applications using DC-DC power supplies , the LED drive methods that can be used include resistor type, linear regulator and switching regulator. The basic application diagram is shown in Figure 4.

In the resistive drive mode, the forward current of the LED can be controlled by adjusting the current detection resistor in series with the LED. This drive method is easy to design, low cost, and has no electromagnetic compatibility (EMC) issues. The disadvantages are that it is voltage-dependent, requires binning of LEDs, and has low energy efficiency.

Linear regulators are also easy to design and have no EMC issues. They also support current stabilization and overcurrent protection (fold back) and provide external current set points. Their disadvantages are power dissipation issues, the input voltage must always be higher than the forward voltage, and the energy efficiency is not high. Switching regulators use a PWM control module to continuously control the on and off of the switch (FET) to control the flow of current.

Switching regulators have higher energy efficiency, are voltage independent, and can control brightness, but their disadvantages are relatively high cost, higher complexity, and electromagnetic interference ( EMI ) issues. Common topologies for LED DC-DC switching regulators include buck, boost, buck-boost, or single-ended primary inductor converters (SEP ICs ).

Among them, a buck structure is used when the minimum input voltage under all working conditions is greater than the maximum voltage of the LED string, such as using 24 Vdc to drive 6 LEDs in series; on the contrary, a boost structure is used when the maximum input voltage under all working conditions is less than the minimum output voltage, such as using 12 Vdc to drive 6 LEDs in series; and a buck-boost or SEPIC structure can be used when the input voltage and output voltage range overlap, such as using 12 Vdc or 12 Vac to drive 4 LEDs in series, but the cost and energy efficiency of this structure are the least ideal.

3. LED driver standards

LED drivers themselves are also evolving, with a focus on further improving energy efficiency, increasing 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 EU IEC 61347-2-13 (5/2006) standard has requirements for LED modules powered by DC or AC, including:

Maximum safety extra-low voltage (SELV) operating output voltage ≤ 25 Vrms (35.3 Vdc)

"Proper"/safe operation under different fault conditions

No smoke or flammability when faulty

In addition, the ANSI C82.xxx LED driver specification is still being developed. In terms of safety, it is necessary to comply with UL, CSA and other standards, such as UL1310 (Class 2), UL 60950, and UL1012. In addition, LED lighting design also involves product life cycle and reliability issues.

4. Five key points of LED driver design

1. Chip heating

This is mainly for high-voltage driver chips with built-in power modulators. If the current consumed by the chip is 2mA, and a voltage of 300V is applied to the chip, the power consumption of the chip is 0.6W, which will of course cause the chip to heat up. The maximum current of the driver chip comes from the consumption of the driving power MOS tube. The simple calculation formula is I=cvf (considering the resistance effect of charging, the actual I=2cvf, where c is the cgs capacitance of the power MOS tube, and v is the gate voltage when the power tube is turned on. Therefore, in order to reduce the power consumption of the chip, you must find a way to reduce c, v and f. If c, v and f cannot be changed, then please find a way to distribute the power consumption of the chip to devices outside the chip, and be careful not to introduce additional power consumption. To put it simply, just consider better heat dissipation.

2. Power tube heating

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 in LED mains drive applications, the switching loss is much greater than the conduction loss. The switching loss is related to the cgd and cgs of the power tube, as well as the driving ability and operating frequency of the chip. Therefore, the heating of the power tube can be solved from the following aspects:

A. You cannot select MOS power tubes based on the on-resistance alone, because the smaller the internal resistance, the larger the cgs and cgd capacitances. 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 selecting a power tube, it is enough to use it.

B. The remaining is the frequency and chip driving capability. Here we only talk about the impact of frequency. Frequency is also proportional to conduction loss, so when the power tube is hot, the first thing to think about is whether the frequency is a bit high. Find a way to reduce the frequency! However, it should be noted that when the frequency is reduced, in order to obtain the same load capacity, the peak current must become larger or the inductance must also become larger, which may cause the inductor to enter the saturation region. If the inductor saturation current is large enough, you can consider changing CCM (continuous current mode) to DCM (discontinuous current mode), so you need to add a load capacitor.

3. Reduce the operating frequency

This is also a common phenomenon in the debugging process. Frequency reduction is mainly caused by two aspects: the ratio of input voltage to load voltage is small and the system interference is large. For the former, be careful not to set the load voltage too high, although the efficiency will be higher if the load voltage is high.

For the latter, you can try the following:

a. Set the minimum current to a smaller value;

b. Clean the routing, especially the key path of sense.

c. Select a smaller inductor or choose an inductor with a closed magnetic circuit;

d. Add RC low-pass filtering. This has a bad effect. The consistency of C is not good and the deviation is a bit large, but it should be enough for lighting. In any case, frequency reduction has no benefits, only disadvantages, so it must be solved.

4. Selection of inductor or transformer

Many users have reported that for the same drive circuit, there is no problem with the inductor produced by a, but the current of the inductor produced by b becomes smaller. In this case, you need to look at the inductor current waveform. Some engineers do not pay attention to this phenomenon and directly adjust the sense resistance or operating frequency to reach the required current. This may seriously affect the service life of the LED. Therefore, before designing, reasonable calculation is necessary. If the theoretical calculation parameters are a bit different from the debugging parameters, consider whether to reduce the frequency and whether the transformer is saturated. When the transformer is saturated, L will become smaller, causing the peak current increment caused by the transmission delay to rise sharply, and then the peak current of the LED will also increase. Under the premise that the average current remains unchanged, you can only watch the light decay.

5. LED current

We all know that if the LED ripple is too large, the LED life will be affected. Regarding the impact of excessive LED ripple, some LED manufacturers said that 30% is acceptable, but it has not been verified. It is recommended to control it as small as possible. If the heat dissipation is not well solved, the LED must be used at a reduced rating. I also hope that experts can give a specific indicator, otherwise it will affect the promotion of LED.

For this article, I believe you will find that LED driver design is not difficult. You must have a clear idea in mind. As long as you calculate before debugging, measure during debugging , and age after debugging, I believe we can easily make LED.