Application of MOSFET in LED lamps

The design of LED lamps often use MOS tube, for LED constant current drive, the general use of NMOS.power MOSFET and bipolar transistors are different, its gate capacitance is relatively large, before the on to charge the capacitance, when the capacitive voltage exceeds the threshold voltage (VGS-TH) when MOSFET began to on. Therefore, it is important to note that the gate driver load capacity must be large enough to ensure that the equivalent gate capacitance (CEI) is charged within the time required by the system.

The switch speed of MOSFET has a lot to do with the charge and discharge of its input capacitor. Although the user can not reduce the value of Cin, but can reduce the gate drive circuit signal source resistance Rs value, thereby reducing the gate loop charge and discharge time constant, speed up the switching speed general IC drive capacity is mainly reflected here, we talk about the selection of MOSFET refers to the external MOSFET drive constant current IC. The IC with MOSFET built into it certainly does not need us to consider it again, generally greater than 1A current will consider external MOSFET. In order to achieve greater and more flexible LED power capabilities, external MOSFETs are the only option, the IC needs the right drive capability, and MOSFET input capacitors are key parameters.

Generally, IC's PWM OUT output is integrated with the current limit resistance, the specific numerical size is related to the IC's peak drive output capacity, it can be approximated that R-Vcc/Ipeak.general combined ic drive capacity Rg selection in about 10-20 s.

The drive of the IC in a typical application can drive THE MOSFET directly, but given that the driving line is not a straight line, the sensitivity may be greater and the Rg drive resistor is used to suppress it in order to prevent external interference. This resistance is as close to the gate of MOSFET as possible, taking into account the influence of the trace distribution capacitor.