DAK-48A

Engineering Prototype Report for EP48 – 1.4 W Non-Isolated Buck Converter Using LNK304P (LinkSwitch®-TN) Title Specification 85–265 VAC Input, 12 V, 120 mA, 1.44 W Output Applications Room Air Conditioners, White Goods, LED Lighting, and Other Applications Requiring a NonIsolated Supply Author Power Integrations Applications Department Document Number EPR-48 Date 02-May-2005 Revision 1.1 Summary and Features • • • • • • • • Low cost, low component count solution (only 16 components) No optocoupler required Much higher output current than “reactive dropper” type power supplies High efficiency (>69% over full input voltage range) Less than 1 W input power with 0.5 W load Low no-load consumption ( 6 dB Margin 1.2/50 µs surge, IEC 1000-4-5, Series Impedance: Differential Mode 2 Ω Common Mode: 12 Ω Free convection, sea level. For operation at >70 °C substitute D1 for a diode with trr ≤35 ns Table 1 - EP48 Specifications Power Integrations Tel: +1 408 414 9200 Fax: +1 408 414 9201 www.powerint.com Page 4 of 20 02-May-2005 EP48 – 12 V, 1.44 W, Non-Isolated Buck Converter 3 Schematic Figure 2 – EP48 Schematic. 4 Circuit Description 4.1 Input Stage and EMI Filtering The input stage is comprised of fusible resistor RF1, diodes D3 and D4, capacitors C4 and C5, and inductor L2. Two diodes are used to both increase the surge withstand to 2 kV and provide EMI gating (noise current only flows when the diodes conduct). Placing D3 and D4 directly in series would meet the surge requirements, but without EMI gating, conducted EMI on the neutral line would be higher. Resistor RF1 is a flameproof, fusible, wire wound resistor. It accomplishes several functions: (a) limits inrush current to safe levels for rectifiers D3 and D4, (b) provides differential mode noise attenuation and (c) acts as an input fuse in the event any other component fails short circuit. As this component is used as a fuse, it should fail safely open circuit without emitting smoke, fire or incandescent material to meet typical safety requirements. To withstand the instantaneous inrush power dissipation, wire wound types are recommended. Metal film resistors are not recommended. 4.2 LinkSwitch-TN LinkSwitch-TN integrates a 700 V power MOSFET and control circuitry into a single low cost IC. The internal fixed switching frequency of 66 kHz was selected to allow up to 120 mA of output current using a standard 1 mH inductor. Lower frequencies require higher value, more costly inductors while higher frequencies increase EMI and cause undesirable high di/dt values as the inductor value reduces. The device is completely self-powered from the DRAIN pin with local supply decoupling provided by a small 100 nF capacitor connected to the BYPASS pin. Page 5 of 20 Power Integrations Tel: +1 408 414 9200 Fax: +1 408 414 9201 www.powerint.com EP48 – 12 V, 1.44 W, Non-Isolated Buck Converter 02-May-2005 Here, the device is configured in a buck converter. The supply is designed to operate in mostly discontinuous conduction mode (MDCM), with the peak L1 inductor current set by the LNK304P internal current limit. The control scheme used is similar to the ON/OFF control used in TinySwitch®. The on-time for each switching cycle is set by the inductance value of L1, LinkSwitch-TN current limit and the high voltage DC input bus across C5. Output regulation is accomplished by skipping switching cycles in response to an ON/OFF feedback signal applied to the FEEDBACK (FB) pin. This differs significantly from traditional PWM schemes that control the duty factor (duty cycle) of each switching cycle. Unlike TinySwitch, the logic of the FB pin has been inverted in LinkSwitch-TN. This allows a very simple feedback scheme to be used when the device is used in the buck converter configuration. Current into the FB pin greater than 49 µA will inhibit the switching of the internal MOSFET, while current below this allows switching cycles to occur. In the event of a fault condition such as output overload, output short circuit, or an open loop condition, LinkSwitch-TN enters into auto-restart operation. If no feedback is received for >50 ms, the internal MOSFET is disabled for 800 ms and auto-restart alternately enables and disables the switching of the power MOSFET until the fault condition is removed and feedback is received. 4.3 Output Rectification During the ON time of U1, current ramps in L1 and is simultaneously delivered to the load. During the OFF time the inductor current ramps down via free-wheeling diode D1 into C2 and is delivered to the load. Diode D1 should be selected as an ultra-fast diode (trr ≤50 ns) with a voltage rating greater than the maximum DC voltage across C5, 600 V in this case. In designs that operate in continuous conduction mode, trr of ≤35 ns is recommended. Capacitor C2 should be selected to have an adequate ripple current rating (low ESR type). 4.4 Output Feedback The voltage across L1 is rectified and smoothed by D1 and C2 during the off-time of U1. To a first order, the forward voltage drops of D1 and D2 are identical and therefore, the voltage across C3 tracks the output voltage. To provide a feedback signal, the voltage developed across C3 is divided by R1 and R3 and connected to U1’s FB pin. The values of R1 and R3 are selected such that at the nominal output voltage, the voltage on the FB pin is 1.65 V. This voltage is specified for U1 at an FB current of 49 µA with a tolerance of +/-7% over a temperature range of –40 to 125 oC. This allows this simple feedback to meet the required overall output tolerance of +/-10% at rated output current. Operation down to 0 mA output current can be accomplished while still meeting +/-10% by increasing the size of the preload from 3.5 mA to 5 mA. Power Integrations Tel: +1 408 414 9200 Fax: +1 408 414 9201 www.powerint.com Page 6 of 20 02-May-2005 EP48 – 12 V, 1.44 W, Non-Isolated Buck Converter 4.5 Operation Below Minimum Drain Voltage Specification In certain abnormal conditions, the drain voltage can drop below the minimum drain voltage specification of 50 V. If these conditions exist in combination with very light output loading (