Currently, power input systems on the market fall into two categories: one type consists of an AC power input system at the front end plus a current control module at the back end. Products in this category include freezer light strips, indoor lighting fixtures, streetlights, table lamps, MR16, AR111, etc. The other type is a direct AC power input system integrating an AC/DC converter and a constant current circuit. Products in this category include bulb-type LEDs such as E27 and GU10, PAR lamps, and T5 and T8 LED tubes.
This paper uses a constant current source to drive the LED to emit light. When the LED current 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 sampled signal, increasing the output pulse width. This increased output pulse drives the power transistor in the power conversion stage, increasing the secondary output voltage. This, in turn, increases the voltage across the LED, thus increasing the current flowing through the LED. This maintains a constant LED current. Similarly, if the LED current increases for some reason, the control process is reversed. Using a constant current source overcomes the inconsistencies in voltage drop across high-power LEDs and the variations in current and luminous efficiency due to poor temperature characteristics.
1 Hardware Circuit
1.1 Introduction to FAN100
The FAN100 is a primary-side regulated PWM controller designed to meet the critical needs of the high-brightness (HB) LED market. Employing proprietary built-in TRUECURRENT™ technology and a tight constant voltage (CV) range, it achieves the most accurate constant current (CC) control without the need for secondary feedback circuitry. By providing precise constant current over a wide voltage range, the same circuitry can be used for strings of LEDs with varying numbers, increasing design flexibility, shortening time-to-market, and extending the lifespan of HBLEDs. The high integration of these PSR PWM controllers saves board space, keeping pace with the trend of shrinking bulb sizes.
The FAN100 features a proprietary energy-saving mode, providing off-time modulation to linearly reduce the PWM frequency under light load conditions. Furthermore, they minimize power consumption (standby power consumption under no-load) by reducing secondary feedback circuitry and components.
1.2 Overall Circuit
FAN100 Description: Pin 1 is an analog input for current sensing. Peak current mode connected to the current sensing resistor controls the constant voltage mode. A CC mode for output current regulation is also provided for the current sensing signal. Pins 2 and 6 are ground terminals. Pin 3 is an analog output. Pin 4 is the analog output with voltage compensation. Pin 5 is the analog input, voltage terminal. Pin 7 is the voltage reference. Pin 8 is the driver power output.
Operating Process: Frequency-hopping PWM operating mode, using minimal filtering components to address EMI issues. The VDD terminal (pin 7) is equipped with overvoltage protection and undervoltage lockout, pulse current limiting with pulse blocking, and CC control to ensure overcurrent protection. The gate output is clamped at 15V to protect the external MOSFET from overvoltage damage. Internal over-temperature protection is disabled when the controller is locked due to overheating. The startup current is 10μA, allowing for power supply startup with high and low resistance. A 1.5 MΩ, 0.25 W startup resistor and a 10μF/25V AC to DC power adapter with a wide input range (100VAC to 240VAC) are provided. The FAN100 features built-in temperature compensation for better temperature regulation in different environments. This internal compensation current is a positive temperature coefficient (PTC) current that compensates for temperature variations in the forward voltage drop diode, which are caused by the increase in output voltage due to temperature rise.
The entire current sensing resistor voltage is sensed for current-mode control and pulsed current limiting. Built-in slope compensation improves stability and prevents subharmonic oscillations due to peak current-mode control. The FAN100 has a synchronous, positive, and sloped ramp built-in in each switching cycle.
The FAN100 output stage uses a BiCMOS process for a fast gate driver. This minimizes heat dissipation, improves efficiency, and enhances reliability. The output driver is internally clamped by a 15V Zener diode to protect the power MOSFET transistor from unwanted overvoltage gate signals.
Overvoltage protection prevents damage caused by exceeding overvoltage conditions. When the voltage exceeds 28V, due to abnormal conditions, the PWM pulse drops below the UVLO voltage, disabling the circuit until it falls below 28V, then restarting. Overvoltage conditions are typically caused by an open feedback loop.
1.3 Experimental Simulation
Figure 2 shows the voltage input and output relationship in the experimental simulation. The vertical axis represents the output, and the horizontal axis represents temperature change. Figure 2 shows that within the input voltage range of 15 V to 17 V, the voltage output decreases as the temperature increases, indicating that 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 V and 95 V, the output current fluctuation is relatively small. Under these conditions, the lifespan of the LED can be extended.
2 Summary
This system can extend the lifespan of LEDs and maintain the stability of output voltage and current within a wide temperature fluctuation range. This allows the DC-DC converter from battery to LED to gradually increase and decrease the power supply voltage to the standard LED forward voltage while maintaining a constant LED current (for constant brightness). Simultaneously, higher overall input current requires a larger inductor and a current with lower ripple to limit the peak switching current below the IC's maximum rated current.
