Lighting a string of LEDs at a constant brightness requires driving it with a regulated current. A boost converter is commonly used to step up the voltage to a level high enough to bias the LEDs on and conduct current.
Typically the current in the string is regulated by adding a sense resistor in series with the LEDs and using the voltage across it as the feedback to a PWM controller. If a fault occurs with any of the series LEDs or its wiring, an open load condition can be created.
In this situation, the voltage across the current sense resistor drops to zero. The control circuit responds by increasing the PWM on-time to boost the output voltage higher in a failed attempt to increase the LED current.
In most instances, the output voltage rapidly rises in an uncontrolled manner until the output capacitors, diode and/or the power FET are overstressed and destroyed. Using the simple over-voltage protection circuit shown in Figure 1 below prevents this situation.
|Figure 1. A Simple Over-Voltage Protection Circuit for LED Drivers|
This boost circuit implements current mode control and senses the LED current via resistor R14. The output voltage is boosted to just over 30V to drive 10 LEDs at a regulated current of 0.35A.
Often, designers add series resistor R9 as a tool to measure and verify stability of the feedback loop. In production, this may be replaced with a zero Ohm resistor. The open circuit protection circuit shown utilizes R9, causing it to serve an additional function, in conjunction with Zener diode D2.
In normal operation, the LED current is set by the PWM controller's internal reference voltage of 0.26V divided by the value of R14. Since the voltage drop across R14 will always be 0.26V in normal operation, there is no voltage drop across the series combination of R5 and R9.
The sum of these two values is used to set the loop gain and does not affect the output current set point. D2 does not conduct since it is purposely set to be 20 percent higher than the normal output voltage.
During an open LED fault, the load across the output is now D2, R9 and R14. The controller forces the output voltage to increase until it reaches approximately 36V. D2 begins to conduct current-to-ground through R9 and R14, increasing the sense voltage at TP1 to 0.26V.
This provides the necessary feedback voltage to the controller. The output regulates to approximately 36V and source current equal to 0.26V, divided by ~51 Ohms or only 5mA. Power in D2 will be minimal. If D2 were placed directly across the LED string, the full output current would flow through D2 during an open and immediately fail, if not sized to handle the large amount of power.
|Figure 2. An Open LED Test Provides a Controlled Output Voltage|
Figure 2 above shows the LED current and boost converter output voltage during an LED disconnect test. The LED current immediately decreases from 0.35A to 0A, followed by an increase in the output voltage.
Once the Zener diode clamp voltage of 36V is reached and Zener current flows, regulation is reasserted. The output voltage is held at 36V. A slight overshoot in the output voltage can be seen during the transition period due to the finite control loop response time.
John Betten is an Applications Engineer and Senior Member of Group Technical Staff at Texas Instruments , and has more than 23 years of AC/DC and DC/DC power conversion design experience. John received his BSEE from the University of Pittsburgh and is a member of IEEE.