Meet Green Standards in 24VAC and 12VAC Lighting Systems: Replace Halogen Bulbs with LEDs Driven by High Power Factor, High Efficiency Converter Keith Szolusha LEDs are increasingly used in 24VAC and 12VAC lighting systems as a robust, energy efficient and high performance alternative to halogen lamps. Power converters that drive the LEDs should have a high power factor (above 90% in order to meet generally accepted green standards), should be efficient, use a minimal number of components and should run cool. They do not need isolation. One solution that meets these requirements combines a rectifier bridge and a current-controlled synchronous step-up/ step-down converter. Specifically, a synchronous 4-switch buck-boost converter can be paired with a 4-switch ideal diode rectifier bridge for high power LEDs; lower power solutions can use a standard diode bridge. Both solutions are shown here. The LT3791 60V 4-switch synchronous buck-boost controller IC can drive constant current (either DC or pulsating) into PVIN 24VRMS PULSATING 120Hz M5 RIN 0.003Ω M6 CIN 1µF 50V 51Ω 0.1µF TG2 24VAC 60Hz TG1 IN1 LT4320 IN2 1µF 50V 100k D3 OUTN BG2 470nF 1M OUTP IVINP CTRL IVINN VIN INTVCC TG1 22.6k M1 SWI BG1 INTVCC 200k LT3791 M2 IVINMON L1 7.8µH M4 1M M3 44.2k RSENSE 0.008Ω SHORTLED OPENLED 0.1µF OPENLED SNSN IVINMON PGND ISMON ISMON BG2 CLKOUT CLKOUT SW2 OVLO TG2 SYNC FB SGND ISP 0.1µF SS RT VC CSS 22nF CC 22nF 45.3k 500kHz D1, D2: NXP BAT46WJ D3: SMAJ60A L1: WÜRTH 744325780 7.8µH 8A M1, M2: RENESAS RJK0651DPB 60VDS M3, M4: RENESAS RJK0451DPB 40VDS M5–M8: VISHAY Si7414DN 60VDS Figure 1. 24V AC to 60W LED driver (600W halogen equivalent) features high power factor and high efficiency 28 | October 2013 : LT Journal of Analog Innovation RLED 0.022Ω PULSATING LEDs 120Hz 15V–25V 0A–4.4A ISN PWMOUT VREF COUT 4.7µF 50V ×4 SNSP 200k SHORTLED CVCC 4.7µF 10V 0.1µF BST1 EN/UVLO M8 D1 D2 PWM BST2 37.4k BG1 M7 a string of high power LEDs. It features an output current feedback loop used to drive constant current through a string of LEDs, and a CTRL dimming input pin that can be tied to the 120Hz half-sine wave design ideas This eco-friendly 60W LED lighting solution is roughly equivalent to 600W of halogen lighting without using lead, mercury, argon, xenon or krypton gases. IAC 2A/DIV PVIN 10V/DIV VLED 5V/DIV IL1 2A/DIV VAC 20V/DIV ILED 2A/DIV ILED 2A/DIV 5ms/DIV 5ms/DIV 5ms/DIV Figure 2. 60Hz 24VAC input waveforms Figure 3. 120Hz pulsating LED driver waveforms Figure 4. 120Hz pulsating PVIN output of a rectifier bridge to create a high power factor pulsating LED current output. output. When currents reach 5A and higher, the diodes in a standard rectifier bridge dissipate significant power and heat up. The LT4320 helps high power AC applications run efficient and cool by driving low resistance external N-channel FETs. 98.1% POWER FACTOR The LT4320 is an ideal diode rectifier bridge that drives four MOSFETs in place of four typical rectifier diodes for highest efficiency conversion of the 60Hz 24VAC input to 24VRMS 120Hz pulsating Figure 1 shows an LED driver that operates with 98.1% power factor directly from 24VAC. It can drive up to 25V of LEDs with 120Hz pulsating power with LED current peaking at 4.4A. At 120Hz, the pulsing of the light is not detectable by the human Figure 5. Components remain cool in the high efficiency LED driver shown in Figure 1. Note that the The LT4320 ideal driver remains cool at full LED current. The LT3791 high power buck-boost converter and supporting components rise less than 24°C while delivering 60W of LED power. The four ideal diode bridge MOSFETs on the back of the board (inset) temperature rise less than 13°C (23°C ambient). October 2013 : LT Journal of Analog Innovation | 29 PVIN 24VRMS PULSATING 120Hz D3 D4 D5 CIN 1µF 50V 51Ω 0.1µF 1µF 50V D7 24VAC 60Hz RIN 0.003Ω 470nF 1M D6 100k IVINP CTRL IVINN VIN INTVCC D1 D2 PWM BST2 37.4k TG1 22.6k M1 SWI BG1 INTVCC 200k LT3791 M2 IVINMON L1 15µH M4 1M M3 44.2k RSENSE 0.015Ω SHORTLED OPENLED 0.1µF SNSP 200k SHORTLED COUT 4.7µF 50V ×4 0.1µF BST1 EN/UVLO CVCC 4.7µF 10V OPENLED SNSN IVINMON PGND ISMON ISMON BG2 CLKOUT CLKOUT SW2 OVLO TG2 SYNC FB SGND ISP RLED 0.05Ω PULSATING LEDs 120Hz 15V–25V 0A–2A ISN 0.1µF PWMOUT VREF SS RT VC CSS 22nF CC 22nF 45.3k 500kHz D1, D2: NXP BAT46WJ D3–D6: PDS360 D7: SMAJ60A L1: WÜRTH 744071150 15µH M1, M2: RENESAS RJK0651DPB 60VDS M3, M4: RENESAS RJK0451DPB 40VDS Figure 6. Alternate, 24W solution uses a standard diode rectifier for simplicity eye and is seen as constant brightness. The high power factor 24VAC input voltage and current waveforms are shown in Figure 2. The 120Hz pulsating LED current waveforms are shown in Figure 3. start-up is not harsh and inrush currents do not affect the high power factor. LED current foldback with the CTRL pin voltage is used to achieve the high power factor. The maximum LED current is set by RLED at 4.5A, but the CTRL pin monitors the post-rectifier 120Hz PVIN input voltage (see Figure 4) and shapes the LED current waveform to match the input. When the input drops below the shutdown pin threshold, the IC goes into shutdown and switching stops. The LED current trails off as the output capacitors are discharged and soon enough, the input rises above the shutdown pin threshold and the LT3791 starts back up. With the CTRL pin folding back the LED current at low input, The 24VAC pulsating LED driver converter in Figure 1 delivers approximately 60W of LED lighting at 94% efficiency. This eco-friendly solution is roughly equivalent to 600W of halogen lighting replacement without using lead, mercury, argon, xenon or krypton gases. The four synchronous switches of the LT3791 buck-boost converter and those of the LT4320 ideal diode bridge are responsible for the high efficiency. Figure 5 shows the circuit components remaining cool despite the 60W conversion. The components have less than 24°C temperature rise, showing that there is plenty of room to spare for even higher power applications. 30 | October 2013 : LT Journal of Analog Innovation HIGH EFFICIENCY & HIGH POWER FACTOR 60W PULSATING LED DRIVER A standard rectifier bridge would produce about a 50°C temperature rise and run several efficiency points lower. Total efficiency is calculated by measuring the input power, the power factor, and the delivered output power separately. The values of 63.0W real input power, 64.4W apparent input power and 98.1% power factor are measured with an HP 6812A AC power source. Measurement of the output power is a bit more complex. A current probe and oscilloscope are used to capture the pulsing current and voltage waveforms at the output of the converter. From these waveforms, the converter output RMS current and voltage is calculated for the on-time (tON) of the LED. The on-time output power is POUT(ON) = VRMS(ON) • IRMS(ON). Output power is zero during LED off-time, where design ideas The principals of the 24W circuit are the same as the 60W circuit and the two operate in the same manner. Efficiency of the 24W circuit is 90%, lower than the 94% achieved by the 60W circuit. Nevertheless, this loss is acceptable due to the overall lower power. Figure 7. Thermal performance of 24W solution the current is zero. The output power of 60W is calculated via a simple duty cycle equation: POUT = POUT(ON) • tON • 120Hz. Overall efficiency = output power divided by real input power. HIGH EFFICIENCY & HIGH POWER FACTOR 24W PULSATING LED DRIVER The circuit in Figure 6 is a high efficiency and high power factor 24W pulsating LED driver that operates from 24VAC input. Because the power level here is less than half of the 60W LED driver in Figure 1, the rectifier bridge shown in Figure 8 is made from four discrete Schottky diodes, instead of ideal diodes. The trade-offs for simplicity are slightly lower efficiency and additional heat dissipation. The principals of the 24W circuit are the same as the 60W circuit and the two operate in the same manner. Efficiency of the 24W circuit is 90%, lower than the 94% achieved by the 60W circuit. Nevertheless, this loss is acceptable due to the overall lower power, making the temperature rise in the discrete rectifier bridge components comparable between the two. With the discrete diode rectifier bridge, the components only heat up to 49°C as shown in Figure 7, well within the requirements of most high power LED drivers. CONCLUSION The LT4320 and LT3791 synchronous buck-boost pulsating LED driver combine to deliver 60W of LED power at 120Hz with 98.1% power factor and 94% efficiency. This circuit can be used to easily replace high power 24VAC halogen lighting with more robust and ecofriendly LEDs. At lower power levels, the LT3791 can be used with a simple discrete diode rectifier bridge—such as in a 24W LED driver with 90% efficiency and similarly high power factor. n For higher efficiency, simply replace the discrete rectifier with a LT4320based rectifier. In general, as power levels and temperatures rise, the need for synchronous rectification in both the converter and rectifier goes up. October 2013 : LT Journal of Analog Innovation | 31