The structure and working principle of LED open circuit protector SMD602

The internal structure of SMD602 is shown in Figure 1. Its basic structure is the same as NUD4700, but with an additional reverse-connected diode, which can provide a current path when the polarity of the LED string is reversed. Since the forward voltage drop of the diode is 1.1-1.5V, it can protect the LED from reverse voltage breakdown (the general LED reverse voltage breakdown voltage is 5V).

Figure 1 Internal structure of SMD602 The working principle of SMD602 is exactly the same as that of NUD4700. Its positive electrode is connected to the anode of LED, and its negative electrode is connected to the cathode of LED, as shown in Figure 2. When the LED is not open, SMD602 is in the off state, and its leakage current is 100μA (typical value). The orange line in the figure is the LED current ILED.

Figure 2 SMD602 works in the LED conduction state. If there is an open circuit in the series LED, as shown in Figure 3, the voltage between the positive and negative electrodes of the parallel SMD602 is greater than its turn-on voltage of 4.65~5.25V, and the device is converted from the off state to the on state, and the voltage is reduced to 1~1.2V. The LED current flows to the next LED after passing through the thyristor inside the SMD602, ensuring the normal operation of other LEDs that are not open. The red line X in the figure indicates that this LED is open.

Figure 3: When the MD602 is working in the LED open circuit state, if the polarity of the LED string is reversed when connected to the driver, the LED may be damaged by excessive reverse voltage. Since there is a reverse diode inside the SMD602, when the polarity of the LED string is reversed, the polarity of the internal diode is correct, and the LED current forms a loop through the diode inside the SMD, and the LED is protected, as shown in Figure 4.

Figure 4 SMD602 works under LED reverse connection state. Main parameters and characteristics of SMD602. Main parameters of SMD602: maximum input voltage VDC is 38V; maximum current IBP=500mA in on state; maximum reverse current IR is 500mA; starting voltage V(BR) is 4.65~5.25V, voltage drop VT is 1~1.2V in on state; when LED polarity is reversed, voltage difference is 1.1~1.5V; leakage current is 100~150μA in off state; maximum holding current IH is 20mA, operating temperature range is -40~+85℃.

Figure 5 Characteristics of SMD602 SMD602 has two packages: 2mm×2mmFBP package and 3-pin SOT-89 package, and its pin arrangement is shown in Figure 6.

Figure 6 Pinout of SMD602 The two LED open circuit protectors introduced above are suitable for 1W high-power LEDs with an ILED current of 350mA. They can be mounted on a hexagonal aluminum substrate together with the LEDs.