10 W Low Line TRIAC Dimmable for BR30 PFC Non-Isolated Buck LED Driver

By | March 22, 2017

This design example report describes a 10 W, 88% efficiency low line TRIAC dimmable for BR30 power factor corrected, non-isolated buck LED driver employing LYTSwitch™-7 LYT7503D. It operates at an input voltage ranging from 90 VAC to 132 VAC and delivers an output of 60 V at 170 mA. This design features a single-stage power factor correction, accurate constant LED current (CC) regulation, high efficiency, low cost and low component count for compact PCB solution, TRIAC dimmable, and integrated protection.

The LYTSwitch-7 device is an SO-8 package LED driver controller IC designed for non-isolated buck topology applications. The LYTSwitch-7 ICs provide high efficiency, high power factor, accurate LED current regulation, and inherent dimming capability. LYTSwitch-7 ICs incorporate a high-voltage power MOSFET and variable frequency/variable on-time, critical conduction mode control engine for accurate current regualtion, high power factor and proprietary power MOSFET utilization for high efficiency. The controller also integrates protection features such as input and output overvoltage protection.

DER-586 is a single stage 10 W TRIAC dimmable LED driver with constant current output. The key design goals were design simplicity, high efficiency, low component count, accurate constant current regulation, compact PCB and acceptable dimming compatibility. The design is intended for BR-30 dimmable LED bulb applications.

This document contains the power supply specification, schematic diagram, billl of materials, printed circuit layout, and performance data.

Figure:1 Populated Circuit Board

Figure 1  Populated Circuit Board

Figure 2  Schematic Diagram

Circuit Description

The LYTSwitch-7 (U1-LYT7503D) combines a high-voltage power MOSFET switch with a variable frequency/variable on-time, critical conduction mode controller in a single SO-8 package. LYT7503D is configured to drive a 60 V LED string, TRIAC dimmable, non-isolated buck LED driver with 170 mA constant current output. The LYT7503D device was selected from the graph selection guidance located in the data sheet. The graph is a function of output power and output voltage requirement.

Input Stage

The fusible resistor FR1 provides safety protection against component failures that would lead to very high input current. Varistor RV1 provides clamping during differential line surge events to limit the maximum voltage spike across the primary. The maximum clamping voltage of RV1 must be lower than the Drain-to-Source breakdown voltage of the internal power MOSFET of LYT7503D (725 V) to ensure sufficient input overvoltage protection during line surge occurrence.

The AC input is full-wave rectified by BR1 to provide the pulsating DC input to the pi filter consisting of C1, C2 and L1. The values of C1, C2 and L1 were chosen to provide the best balance between high power factor, EMI performance, and dimming compatibility.

EMI Filter

The inductor L1 and capacitors, C1 and C2, form an EMI pi filter which works to filter differential mode noise. The LYTSwitch-7 ICs’ variable frequency/on-time states and critical conduction mode control engine limits RFI emission to significant level which enables design to use simple EMI pi filter even for high power bulb and tube applications.

LYTSwitch-7 Control Circuit

The topology used for this LED driver is a low-side buck converter. During the on-time of the LYT7503D internal power MOSFET, current ramps through the buck inductor winding, charging the output capacitor, and provides current to the output load. The energy stored in the magnetic field of the inductor winding during on-time of the power MOSFET is then delivered to the load during off-time via output diode D1. The output capacitor C3 provides filtering to minimize LED ripple current while resistor R3 serves as a pre-load.

Capacitor C5 provides local decoupling for the BYPASS (BP) pin of U1, which provides power to the IC during the switch on-time. The IC internal regulator draws power from high-voltage DRAIN (D) pin and charges the bypass capacitor C5 during the power switch off time. The typical BP pin voltage is 5.22 V. To keep the IC operating normally especially during the dead zone, where VIN < VOUT, the value of capacitor should be large enough to keep the BP voltage above the VBP vaule of 4.5 V. Additional bias resistor R2 was employed to maintain the BP pin voltage for very fast AC on/off power cycling event and during low conduction angle operation. Recommended minimum value for the BP capacitor is 10 µF.

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