LED system design based on FAN100

Introduction
All LEDs, regardless of their color, size, or power, can fully perform as long as the driving current is constant. LED manufacturers will list the specifications of their products. For example, the data sheet will list the lumens, beam waveform, and color of the product when driven by a specified forward current (IF) rather than a forward voltage (VF). The brightness of LEDs varies with the current, and the voltage and current curves of manufactured LEDs are slightly different. Therefore, the brightness of LED lighting is often unstable with changes in the power supply voltage. In order to maintain a stable and consistent brightness, a constant current driver for LEDs is required. Constant current drivers can make LEDs work in a fixed current mode, so the brightness is highly stable. Constant current drivers also allow LEDs to work at a certain current for a long time, allowing them to maintain a longer life. The advantages of LED lighting are energy saving and safety, but due to constant current operation, energy consumption is also relatively increased, so the lighting system design aims at low energy consumption. As mentioned above, the voltage drop of the constant current driver is within 2 V, which means that the design takes low energy consumption into consideration. If the voltage drop between the power supply end of the system and the voltage drop of the series-connected LED exceeds 2 V, it is necessary to consider using a voltage converter to achieve the low energy consumption goal, but still maintain the constant current working mode. The low-energy voltage converter works in a switching mode, and controls the switching cycle according to the feedback circuit to achieve a stable output voltage. However, in order to maintain the constant current working state of the LED, the feedback circuit controls the converter switching cycle with the output current.

There are currently two types of power input systems on the market: one is the front-end AC power input system plus the back-end current control module. Such products include freezer light strips, indoor lamps, street lamps, table lamps, MRl6, ARlll, etc. The other is the AC power direct input system integrating AC/DC converters and constant current circuits. Such products include E27 and GUl0 bulb-type LED lamps, PAR lamps, T5 and T8 LED tubes.

This article uses a constant current source to drive the diode to emit light. When the current of the light-emitting diode decreases, the constant current source circuit collects the changing (decreasing) current value, amplifies it, and transmits it to the control circuit. The control circuit inverts the sampling signal, and the output pulse width increases. The output pulse with increased width drives the power tube of the power conversion stage, so that the secondary output voltage increases. In this way, the voltage across the series LED also increases, so the current flowing through the light-emitting diode also increases, which maintains the current of the light-emitting diode constant. Similarly, if for some reason, the current of the light-emitting diode increases, the control process is opposite. The constant current source drive method can overcome the shortcomings of inconsistent voltage drop of high-power light-emitting diodes and changes in current and luminous efficiency of the tube due to poor temperature characteristics.

1 Hardware Circuit
1.1 Introduction to FAN100
FAN100 is a primary-side regulated PWM controller that meets the key needs of the high brightness (HB) light-emitting diode (LED) market. Built-in proprietary TRUECURRENTTM technology and a strict constant voltage (CV) range are used to achieve the most accurate constant current (CC) control without the need for secondary-side feedback circuits. By accurately constant current over a wide voltage range, the same circuit can be used for different numbers of LED strings, thereby improving design flexibility, shortening time to market, and extending the service life of HBLEDs. Due to the high integration of these PSR PWM controllers, board space can be saved to match the trend of shrinking bulb form factors.
FAN100 has a proprietary energy-saving mode that provides off-time modulation to linearly reduce the PWM frequency under light load conditions. In addition, they also minimize power consumption (standby power consumption <0.15 W under no load) by reducing secondary-side feedback circuits and components. All of these energy-saving features are essential for power supplies to comply with stringent energy efficiency regulations. In addition, these PSRPWM controllers provide robust protection functions such as undervoltage lockout (UV-LO), overvoltage protection (OVP) and overtemperature protection (OTP).
1.2 Overall Circuit
The overall circuit is shown in Figure 1.

FANl00 Description: Pin 1 is analog input, current detection, connected to the current detection resistor for peak current mode control constant voltage mode, for the current detection signal, also provides output current regulation in CC mode. Pins 2 and 6 are ground terminals. Pin 3 is analog output. Pin 4 is analog output, voltage compensation. Pin 5 is analog input, voltage terminal. Pin 7 is voltage reference. Pin 8 is the output of the drive power device.
Working process: frequency hopping PWM working mode, for EMI problem method uses minimized filtering components. VDD terminal (pin 7) is equipped with overvoltage protection and undervoltage lockout, pulse current limit and CC control to ensure overcurrent protection. The gate output is clamped at 15 V to protect the external MOSFET from overvoltage damage, and the internal overtemperature protection function shuts down and the controller is locked when overheating. The startup current is 10μA, and the low startup current allows the startup power supply of the power supply controller with high resistance and low startup resistance. A 1.5 MΩ, 0.25 W start-up resistor, 10 μF/25 V is an AC to DC power adapter with a wide input range (100VAC to 240VAC). FANl00 built-in temperature compensation provides better constant voltage regulation at different ambient temperatures. This internal compensation current is a positive temperature coefficient (PTC) current that compensates for the temperature change of the forward voltage drop diode, which causes the increase in output voltage temperature.
The voltage across the current sense resistor is sensed for current mode control and pulses are used by pulse current limiting. The slope compensation is built-in to improve stability and prevent subharmonic oscillations due to peak current mode control. The FANl00 has synchronous, positive, ramped ramps built-in on each switching cycle.
The BiCMOS process of the FANl00 output stage is a fast gate driver. Minimize heat dissipation, improve efficiency, and enhance reliability. The output driver is clamped by an internal 15 V Zener diode to protect the power MOSFET transistor against unwanted overvoltage gate signals.
Overvoltage protection prevents damage caused by overvoltage conditions. When the voltage exceeds 28 V, due to abnormal conditions, the PWM pulse drops below the UVLO voltage and is disabled until it drops below 28 V and then restarts. Overvoltage conditions are usually caused by an open feedback loop.
1.3 Experimental
simulation The relationship between the experimental simulation voltage input and output is shown in Figure 2. The vertical axis is the output and the horizontal axis is the temperature change. It can be seen from Figure 2 that when the input voltage is between 15 V and 17 V, it can be seen that when the temperature rises, the voltage output is decreasing, so the circuit has a wide temperature fluctuation range.

Figure 3 shows the relationship between current input and temperature. When the circuit input current is between 75% and 95%, the output current fluctuation is relatively small. Under such circumstances, the service life of the LED can be extended.

2 Summary
This system can extend the life of the LED and maintain the stability of the output voltage and current within a relatively large temperature fluctuation range. In this way, the DC-DC converter from the battery to the LED can both gradually increase the power supply voltage to the standard LED forward voltage and gradually decrease the power supply voltage to the forward voltage, and keep the LED current unchanged (for constant brightness). At the same time, when the overall input current is higher, a larger inductor is required, and a current with smaller ripple is also required to limit the peak switch current to below the maximum rated current of the IC.