Analysis of the safety circuit design of automotive LED lighting driver attributes

In some applications, such as automotive LED lighting, due to the location of the LED is usually far away from the driver, which need to increase the short-circuit protection, JOHN RICE explained.

Non synchronous step-up power conversion topology is commonly used for the LED driver, in these cases, the input voltage is not enough for a group of series / parallel LED lamp string of forward bias. The inductive switch topology generates a constant current output voltage regulation system to achieve the LED current, and is often used for LCD backlight applications. In the LED matrix applications, such as automotive interior and exterior lighting, as far away from the driver, output short circuit to ground the possibility is real, even could be disastrous. The protection circuit can limit the current as the electronic circuit breaker, can prevent catastrophic failure.

As shown in Figure 1, boost converter input through the boost inductor boost diode (L1) and (D1) the physical connection of the output. Therefore, the short circuit output will boost inductor saturation caused by current spikes, enough to damage the boost diode. What is worse, the short circuit can also destroy all connected with the input, including pulse width modulation (PWM) controller. Obviously, in the use of this topology for remote LED power supply, the circuit needs some kind of protection. We need to consider a general circuit and low cost, can optimize the protection of the boost converter and to prevent short circuit load case input. In addition, we will introduce a circuit simulation, verification of the desired response.

Figure 1. non isolated LED driving circuit based on boost topology

Current limiter and electronic circuit breaker

Current Shunt Monitor (CSM) is a kind of high precision, high gain differential current sense amplifier, often used to monitor the input and output current. Figure 2 shows the typical configuration. This particular device incorporates an open drain comparator, which can be programmed in a preset line current on the trip, latch and reset.

Figure 2. current shunt monitor module increased protection

The output of the comparator can be used to control the external MOSFET switch, can load short-circuit interruption in a few microseconds. In addition to interrupt input current at the output fault condition, the analog output can be used to solve the switching regulator on the negative input impedance of the so-called "". When the input voltage decreases the negative input impedance will lead to increased input current.

The input current and output current to the logical configuration with control input. Its purpose is to generate a composite feedback signal driving PWM controller, as shown in figure 3. Subsequently, CSM covers the output current feedback, and forcing the LED current decreased with the input voltage to a preset level, thereby limiting the input current.

Figure 3. input limiter depends on the input and output current sensing

Circuit operation

Figure 4 shows the driver circuit of LED boost converter with output short circuit protection function realization way. The circuit shown in OSRAM Opto Semiconductors Ostar LED is aimed at the automotive headlamp application device, the insulation is actually a complete five LED chip on a metal substrate. Surge current rating of the device is 2A, less than 10 s, and 1A 18V of the typical forward voltage. LED current detection resistor (RSNS) by setting the internal bandgap reference with PWM converter is proportional to the value of it (RSNS = VREF/ILED). The use of boost converter with low voltage reference is helpful to achieve higher conversion efficiency, and reduce the thermal stress components.

Figure 4. with short circuit fault protection LED boost driver circuit.

Although the LED itself can reach more than 50000 hours of life, but LED on temperature and electric stress is very sensitive, but the design challenge LED dynamic impedance characteristics tend to select and control loop regulator elements. Therefore, the development of the simulation circuit 4, to analyze the LED driver / protection circuit complexity, and to predict the circuit characteristics under the different operating conditions.

This paper selected the PWM controller with feedback reference voltage 0.26V. Therefore, in the current LED 1A, only 0.26W of power dissipation in LED resistance detection. The gain of CSM is 50, so the use of a smaller value detection resistor to sense current output. When the current through the CSM shunt resistor exceeds the detection resistor limits set by the CSM, CSM gain and comparator threshold (R, R), PMOS channel transistor load current is interrupted, as electronic circuit breaker.

The RESET pin switch to a low level, can be reset latch output. However, for the purposes of this article, RESET has been disabled, in order to check the response speed. The response speed and the peak current depends on many variables, including component selection, CSM bandwidth, noise filter, output capacitance, FET selection and output inductor. Together, these factors will affect the output impedance converter. In order to accurately assess the operation, we simulate the maximum time step is set to 50 ns DC, the relative tolerance is set to 0.001%. In the analysis of TINA-TI, a free Berkeley SPICE 3f5 compatible simulator. Boost converter to 300 kHz operation of the 5 ms simulation run in just 30 seconds to start to steady state.

Current Shunt Monitor (CSM) placed in where?

The input or output of CSM can be placed in the boost converter. In the simulation, CSM is placed on the output side, sensing current flowing through the shunt 10-m. The shunt and the output PMOS channel element (T5) series. According to the layout of CSM, the circuit can prevent the internal or external short circuit. However, CSM must be designed with enough common mode range in all operating conditions (CMR).

If