This engineering report describes a low cost, non-isolated non-dimmable buck LED driver designed to drive a 120 V LED voltage string at 170 mA output current from an input voltage range of 190 VAC to 300 VAC. The LED driver utilizes the LYT1604D from the LYTSwitch-1 family of devices.
LYTSwitch-1 is a SO-8 package LED driver controller IC designed for non-isolated buck topology applications. The LYTSwitch-1 provides high efficiency, high power factor and accurate LED current regulation. LYTSwitch-1 incorporates a high-voltage power MOSFET and Variable Frequency / Variable On-Time, Critical Conduction Mode Control Engine for tight current regulation, high power factor and proprietary FET utilization for high efficiency. The controller also integrates protection features such as input and output overvoltage protection, thermal fold-back, over temperature shutdown, output shortcircuit and overcurrent protection.
This design example report offers a low cost compact size solution for 20 W LED drivers. The key design goals were high efficiency, accurate constant current regulation and low components count.
The LYTSwitch-1 device (U1-LYT1604D) combines a high-voltage power FET and variable frequency/ variable on-time, critical conduction mode controller in a single SO-8 package. LYT1604D is configured to drive a 120 V output non-isolated buck LED driver with 170 mA constant current output. The LYT1604D device was selected from the power table based on maximum output power of 22 W in the datasheet.
The input fuse F1 provides safety protection from component failures. Varistor RV1 acts as a voltage clamp that limits the voltage spike on the primary during line transient voltage surge events. A 275 VAC rated part was seleted being with a maximum clamping voltage specification lower than the device drain voltage (710 V). The AC input voltage is full wave rectified by BR1 to achieve good power factor and low THD. For higher surge capability such as for this design > 1 kV, C1 and L1 were placed before the bridge rectifier BR1, but a safety X-capacitor is required for C2, and the RV1 was placed after BR1.
Inductor L1 serves as differential choke. Inductor L1, C1 and C2 capacitors forn as an emi pi filter which works to filter differential and common mode noises. Resistor R1 damps the resonance of L1 to make it more effective in blocking high frequency noises. LYTSwitch-1's variable frequency/on-time states and critical conduction code control engine limit RFI emission to significant level which enables design to use simple emi pi filter even for high power bulb and tube applications.
LYTSwitch-1 Control Circuit
The LED driver circuit topology is a low side buck configuration, where the internal mosfet of U1 and the power inductor L2 are connected to the ground rail. During the FET on time, current ramps through the inductor winding storing energy in the form of magnetic field which is then delivered to the output load via flywheel diode D1 during the FET off-time.