Detailed explanation of the design principle of high-power LED constant current drive

Photovoltaic power generation industry is an emerging industry with a rapid development trend. Photovoltaic lighting is a pillar industry in the photovoltaic industry. Since the electricity generated by photovoltaic cells is direct current if it is not converted once, LED light source, as a direct current light source, is particularly suitable for the photovoltaic lighting industry. However, if the advantages of LED's high efficiency and energy saving are to be guaranteed, its drive plays a particularly important role. This paper compares the suitable drives for high-power LED and low -power LED. And proposes a high-efficiency high-power LED constant current drive solution based on PT4115. This drive circuit is simple, efficient, and low-cost, suitable for the market development of today's solar products.

1 LED operating characteristics

LED is sensitive to voltage. When the voltage across the LED exceeds its conduction voltage, it can be approximately considered that its forward voltage VF and forward current IF are proportional. Therefore, changes in voltage will cause changes in current.

Figure 1 LED VF and IF characteristic curve

From Figure 1, we can see that a small change in voltage will cause a large change in current. Therefore, we can conclude that constant current drive should be used for LED to prevent large fluctuations in current flowing through the LED, which will affect the service life of the LED. Therefore, whether it is AC constant current drive or DC constant current drive, the peak-to-peak value of the voltage across the output LED is best controlled to be within tens of milliamperes.

2 Comparative study of common LED driving technologies

2.1 Resistor ballast drive

Figure 2 Schematic diagram of ballast resistor drive

As can be seen from Figure 2, the driving method using resistor ballast is to connect a ballast resistor in series to the LED light string. The ballast resistor reduces the voltage on the LED light string to prevent the LED from being over-voltage and breaking down. The driving method using the ballast resistor is actually what is commonly known as the constant voltage driving method. Although this driving method is simple, there will be losses in the ballast resistor, and the losses will increase with the increase of the input voltage. Therefore, as the earliest driving technology, this technology has been gradually replaced with the development of technology.

2. 2 PWM constant voltage drive mode

As we all know, the PWM driving method itself has the advantage of high driving efficiency. Therefore, the PWM constant voltage driving method has the advantages of high efficiency and simple driving circuit. However, the constant driving method of ballast resistor is no longer suitable for the simple and efficient trend of today's photovoltaic lighting. Therefore, PWM constant voltage IC appears.

Figure 3 Constant voltage drive principle

For constant voltage drive (see Figure 3), since the voltage drop across the LED load remains unchanged, if a short circuit occurs in one of the LEDs, the constant voltage drop of the output will be dropped across the other LEDs. The voltage borne by each remaining LED may exceed the voltage rating and burn it out.

LEDs cannot be made the same due to their VF value characteristics. Some VF values ​​will also change with temperature and current, so they are generally not suitable for parallel design. However, in some cases, multiple LEDs must be connected in parallel to solve the cost problem of driving multiple LEDs. For example, if a constant current source is used for each low-power LED, the driving cost will be greatly increased. Therefore, multiple sets of LED strings must be connected in parallel and driven by constant voltage.

Therefore, even if constant voltage driving is adopted, PWM constant voltage driving IC should be selected to improve driving efficiency.

In order for low-power LEDs to achieve the same illumination as high-power LEDs, a large number of them are required. If a constant current driver is used for each driver, the driver cost will be greatly increased. In view of this problem, low-power LEDs are suitable for constant voltage driving.

2.3 Constant current drive technology

In fact, constant current drive is largely based on the high efficiency characteristics of PWM constant voltage drive, which is modified to achieve constant current in the simplest way.

For PWM constant voltage IC, a reference voltage is built in. By sampling the voltage at the feedback terminal FB and comparing it with the built-in voltage, the PWM output duty cycle is controlled to achieve constant voltage drive.

To perform constant current control, a small resistor is connected in series at the chopper output end to sample its voltage to ground, then amplify it and feed it back to the constant voltage control end for constant current control. Since the sampling resistor is connected in series in the output loop, to reduce the power consumption on the resistor, the resistance value of the resistor should be minimized, usually 0.1 Ω.

When using constant current drive, each LED string must have a constant current source drive. When a single LED in the string fails to short-circuit, the output current remains unchanged, so it does not affect the light efficiency of other LEDs . Using constant current drive can greatly increase the service life of LEDs.

3 Constant current drive technology based on PT4115

3.1 PT4115 chip introduction

1) Very few external components

2) Wide input voltage range: from 8 V to 30 V

3) Maximum output current 1.2 A

4) Multiplexing DIM pin for LED switching , analog dimming, PWM dimming

5) 5% output current accuracy

6) LED open circuit protection

7) Up to 97% efficiency

8) Output adjustable constant current control method

9) It has frequency jittering feature inside, which greatly improves EMI

3. 2 Typical application circuit

Figure 4 PT4114 typical application circuit

PT4115 (see Figure 4) can be used for both 12~18 V AC and 8 V~30 V DC. Therefore, the application range is wider. In addition, the drive circuit is simple and the required components are all inexpensive. It is suitable for mass production and marketization.

3.3 PT4115 constant current principle

Keeping the input voltage value of the sampling terminal (CSN) at the internal setting value of the IC relative to the VIN voltage value unchanged can achieve constant current. Because:

Where:

ILED --- Current flowing through the LED;

VCSN ---voltage detection terminal voltage;

RS ---Current sampling resistor.

From equation (1), it can be seen that as long as the sampling terminal voltage is kept constant relative to the input terminal voltage, the current flowing through the LED can be kept constant.

3. 4 PT4115 dimming measures

PT4115 adopts PWM dimming measures. When the DIM pin voltage is lower than 0.3 V, the LED current is turned off, and when it is higher than 2.5 V, the LED current is turned on.

Compared with traditional linear dimming, PWM dimming does not affect the light effect of LED. The basic principle of PWM dimming is to keep the forward current of LED constant by controlling the time ratio of current on and off, that is, controlling the time of current on in each cycle. The advantage of PWM dimming is that the forward current of LED is always constant, and the color of LED will not change like analog dimming. PWM dimming can accurately control the brightness of LED while ensuring the color of LED light.

Linear dimming adjusts the lighting effect by changing the current flowing through the LED. The change of the current flowing through the LED will inevitably affect the chromaticity of the LED.

Therefore, PWM dimming is a great improvement over traditional linear dimming.

3.5 Principle of PT4115 frequency jitter to improve EMI

Frequency Jitter is a new method to solve EMI problems by dispersing harmonic interference energy. Frequency Jitter refers to a method to reduce EMI by periodically changing the operating frequency of the switching power supply from narrowband to wideband to reduce electromagnetic interference.

Frequency jittering reduces narrowband EMI by spreading the power supply noise spectrum. There are some limitations on how much the oscillator frequency (fs) can be jittered. Some of these limitations are switching losses and magnetic circuit design. To keep the boost inductor as small as possible and the switching losses manageable, the frequency jitter should not exceed 20% to 30% of the fundamental frequency.

3.6 Dynamic temperature regulation and over-temperature protection of PT4115

PT4115 has the function of dynamic temperature regulation, and over-temperature protection can be achieved based on this function.

3.6.1 Dynamic temperature regulation.

Figure 5 Schematic diagram of dynamic temperature regulation

As can be seen from Figure 5, the DIM terminal has a 1MΩ pull-up resistor connected to the internal 5V power supply. The DIM terminal voltage is determined by the internal pull-up resistor and the thermistor NTC voltage divider. From the characteristics of the thermistor, we can know that temperature changes will affect the resistance of the NTC, and then affect the DIM terminal voltage, so as to achieve dynamic temperature regulation of PT4115.

3.6.2 Implementation of over-temperature protection.

As can be seen from Figure 6, there is one more transistor compared to Figure 5. When the temperature rises, the resistance of the NTC resistor decreases, and the voltage divider on it also decreases. Then the voltage divider on the resistor below it increases accordingly. When it exceeds the turn-on voltage of the transistor, the transistor is turned on, the DIM terminal is grounded, and the LED current is turned off. When the temperature drops, the IC restarts. Therefore, the over-temperature protection of PT4115 is realized.

Figure 6 Schematic diagram of over-temperature protection

3.7 Experience in PT4115 Engineering Application

1) The larger the inductance, the lower the operating frequency and the better the constant current effect; 2) The larger the output current, the smaller the inductance value required, and the easier it is to choose the inductor; 3) Generally, the larger the inductance, the smaller the switching loss of the power switch, but the corresponding inductor loss will increase; 4) PT4115 has an internal over-temperature protection function, and the external over-temperature protection can be set to achieve double protection for the LED; 5) PCB wiring should increase the contact area between the copper foil and the Exposed PAD and GND of PT4115 as much as possible to facilitate heat dissipation; 6) The AC 12V rectifier and the freewheeling diode must use low-voltage drop Schottky diodes to reduce their own power consumption; 7) When selecting the inductor, its saturation current is required to be 1.5 times the output current.

4 Experimental Results

4.1 Efficiency determination

The output efficiency of PT4115 was measured using laboratory precision instruments . Taking the output of 3 LEDs in series as an example, the results are shown in Table 1.

Table 1 Output efficiency measurement at different input voltages

As can be seen from Table 1, the overall power supply efficiency of PT4115 is maintained at more than 91%, which is a high-efficiency product compared to the constant current source on the current market. In addition, a single channel can drive up to 7 1W LEDs in series. Due to the simple peripheral circuit and small IC package size, the constant current drive and LED load can be integrated on an aluminum substrate to achieve a module solution that integrates drive and heat dissipation.

4.2 Experimental waveform

The waveforms at both ends of the Schottky diode are sampled by an oscilloscope and the three LED loads are also used as an example for sampling.

Figure 7 Waveform when input voltage is 12 V and load is 3 LEDs connected in series

It can be seen from waveform 7 that the chopping waveform has no burrs, so the harmonic content is relatively low and the constant current drive loss is small.

5 Summary

This article compares various common driving technologies and concludes that high-power LEDs should be driven by constant current. In addition, the principle, advantages, and circuit implementation of high-power LED constant current driving based on PT4115 are introduced in detail. At the same time, the experience summary in the process of using PT4115 to achieve constant current driving is also described in detail. Finally, the experimental results are described, proving the rationality, high efficiency, simplicity and other outstanding advantages of the driving. The constant current driving has strong engineering practicality.