Author: John Betten, Texas Instruments (TI) Applications Engineer
To ensure that the brightness of the LED string is constant, its drive current must be adjustable. A boost converter is typically used to boost the voltage level to a high enough level to bias and turn the LED on. A typical method of adjusting the LED string current is to add a sense resistor in series with the LED and use the voltage across it as a feedback input to the pulse width modulation (PWM) controller. If one of the LEDs in the series LED or a certain segment of the conductor fails, the circuit will be in an open load condition.
In this case, the voltage across the current sense resistor drops to zero. When the output voltage fails to increase by increasing the PWM on-time, the control circuit response will attempt to increase the LED current. In most cases, the output voltage will rise sharply until the output capacitor, diode, and/or power FET are overstressed and destroyed. This can be avoided by using the simple LED open-circuit overvoltage protection circuit shown in Figure 1.
Figure 1 A simple LED driver overvoltage protection circuit
The boost circuit measures the LED current through resistor R14 and implements current mode control. This circuit boosts the output voltage above 30V and drives 10 LEDs with a regulated current of 0.35A. Designers often add series resistor R9 and use it to measure and verify the stability of the feedback loop. In practical applications, this resistance may be replaced by a zero ohm resistor. The open circuit protection circuit shown in the figure uses R9, which provides more functionality along with Zener diode D2.
In normal operation, the LED current is dependent on the 0.26V PWM controller internal reference voltage divided by the value of the R14 resistor. Since the voltage drop across R14 will remain at 0.26V under normal operating conditions, there is no voltage drop across the R5 and R9 series resistors. The sum of R5 and R9 will be used to set the loop gain without affecting the output current regulation point. D2 is not turned on at this time because it is intentionally set to be 20% higher than the normal output voltage.
When the open LED fails, D2, R9, and R14 become the load across the output. The controller forces the output voltage to rise until the output voltage reaches approximately 36V. D2 begins to conduct, causing current to flow to ground through R9 and R14, thereby boosting the induced voltage on TP1 to 0.26V. This provides an essential feedback voltage to the controller. The output is adjusted to approximately 36V and the source current is equal to 0.26V divided by 51 ohms (approximately equal to 5mA). This minimizes power on D2. If D2 is directly connected to both ends of the LED string, the total output current during the open circuit will flow through D2, and if D2 is unable to withstand such large power, it will be burned immediately.
Figure 2 shows the LED current and the output voltage of the boost converter during the open circuit test. The LED current immediately drops from 0.35A to 0A, and the output voltage rises. Once the Zener diode reaches the clamp voltage of 36V, the Zener current is generated and the regulation process is re-established, and the output voltage will remain at 36V. Due to the response time of the control loop, there is a slight overshoot of the output voltage during the conversion. (Edit: Looking up at the stars)
Solar Panel Light,75W Solar Panel,Solar Cell Panel,Solar Poly Panel,20W Solar Panel,30W Solar Panel
Solar Garden Light Solar Wall Light Co., Ltd. , http://www.chsolarlight.com