In applications such as automotive LED lighting, the driver is usually far away from the LED, so the short-circuit protection function needs to be added. John rice introduces how to prevent the output of LED driver from short circuit to ground in this paper.
Asynchronous, boost, and power conversion topologies are often used in applications such as LED drivers. In these applications, the input voltage (VIN) is not sufficient to forward bias a set of series / parallel LED lamp strings. This inductive switching topology generates the compliance voltage necessary to achieve LED current regulation and is usually used in LCD backlight applications. For example, in the application of LED matrix such as interior and exterior lighting far away from the driver, once the risk of output short circuit to ground occurs, it will have disastrous consequences. Limiting the current and operating the protection circuit as an electronic circuit breaker can prevent these catastrophic faults.
As shown in Fig. 1, the input of the boost converter is physically connected to its output through a boost inductor (L1) and a boost diode (D1). Therefore, a short circuit condition on the output will saturate the boost inductor, causing a current spike sufficient to damage the boost diode. Worse, this short circuit condition will also interfere with all devices connected to the input, including pulse width modulation (PWM) controller. Obviously, when using this topology, some type of circuit protection is required to power the remote led. Next, we will consider designing a multi-purpose and low-cost circuit, which can be optimized to protect the boost converter and prevent short-circuit load at the input. In addition, we will verify the required response through an analog circuit.
Figure 1 LED driver circuit based on non isolated boost topology
Current limiter and electronic circuit breaker
Shunt Monitor (CSM) is a high-precision, high gain differential current sensing amplifier, which is often used to monitor input and output currents. Figure 2 shows a typical configuration. This specific device integrates an open drain comparator; The comparator can be set to jump, latch and reset on the preset line current.
Figure 2 A shunt monitor component adds protection
The output of this comparator can be used to control an external MOSFET switch that can interrupt the load short circuit within a few milliseconds. In addition to interrupting the input current in case of fault on the output, the analog output can also solve the so-called “negative input impedance” problem of the switching regulator and prevent the input current from increasing with the decrease of the input voltage.
The input current can be clamped by connecting the input current with the output current in a logical “or” configuration. As shown in Fig. 3, the purpose is to generate a composite feedback signal driving the PWM controller. Then, the CSM invalidates the output current feedback and forces the LED current to decrease when the input voltage drops below a preset level, thereby limiting the input current.
Figure 3 The input current limiter relies on sensing input and output currents
Fig. 4 shows the circuit implementation of a boost converter LED Driver with output short-circuit protection function. The LED produced by OSRAM Opto Semiconductors OSTAR shown in the circuit is a device for automotive headlamp application. In fact, it is a single-chip LED located on an insulated metal substrate. This device has a surge current rating of 2A (less than 10 μ s) And a typical forward voltage of 18V at a current of 1a. The DC / DC boost converter senses the forward LED current on the feedback pin and fully adjusts the output voltage to adjust the LED current. The LED current is set by the sensing resistor (RSNs) whose value is proportional to the internal bandgap reference of the PWM converter (RSNs = VREF / iled). Using a boost converter with a low reference voltage can more easily achieve higher converter efficiency and reduce component thermal stress.
Figure 4 Led boost driver circuit with short circuit load fault protection function
Although the service life can be more than 50000 hours, LEDs are very sensitive to temperature and electrical over stress, and their dynamic impedance characteristics often pose difficulties for the selection of switching regulator components and the design of control loop. These selection and design challenges are described in this operating manual. According to this method, the circuit simulation shown in Figure 4 is developed to analyze the complexity of LED driver / protection circuit and predict the operation mode of the circuit under various working conditions.
The PWM controller selected for this analysis has a feedback reference voltage of 0.26v. Therefore, when the LED current is 1a, the power dissipation of the LED sensing resistor is only 0.26w. Since the CSM has a gain of 50, a much smaller sensing resistor is required to sense the output current. When the current flowing through the CSM shunt resistor exceeds the limit set by the CSM sensing resistor, the CSM gain and the comparator threshold (R, R), and the PMOS turns on the transistor to interrupt the load current – thus acting as an electronic circuit breaker.
The latch output can be reset by switching the reset pin to low level. However, for the purpose of this article, reset has been disabled to verify the response speed. The response speed and peak current depend on many variables. These variables include component selection, CSM bandwidth, noise filter, output capacitance, FET selection, and output boost inductor. These factors together will affect the output impedance of the converter. In order to accurately evaluate the operation mode, we have run the simulation with a maximum time step of 50ns and a DC relative tolerance set to 0.001%. This analysis runs in TINA-TI, a free Berkeley spice 3f5 compatible simulator. The 5ms simulation of the boost converter with an operating frequency of 300kHz can start to a stable state in only 30 seconds.