Application Note AN-300 INTEGRATED CIRCUITS DIVISION MXHV9910 Design Considerations AN-300-R02 www.ixysic.com 1 AN-300 INTEGRATED CIRCUITS DIVISION 1 Off-line LED Driver using MXHV9910 This application note provides general guidelines for designing an off-line LED driver using the MXHV9910. The MXHV9910 is a constant frequency buck converter specifically designed to provide a low cost, minimal external component solution for off-line LED applications. The converter operates in a continuousconduction, peak-current control mode with no slope compensation. When designing an LED driver with the MXHV9910, the duty cycle must be restricted to less than 50% in order to prevent subharmonic oscillations. The MXHV9910 has two current sense thresholds: one is internally set at 240mV, and the other can be Figure 1 externally set at the LD pin. The lower of these two thresholds determines the LED peak current in conjunction with the current sense resistor (RSENSE) at the CS pin. A linear dimming function can be accomplished by adjusting the current sense threshold voltage up to the internal current threshold range. When the linear dimming function is not used, it is recommended that the LD pin be connected to VDD. Figure 1 shows the functional block diagram of the MXHV9910 device. Figure 2 shows a schematic of a typical application circuit for the device, and is referred to in all the discussions that follow. MXHV9910 Block Diagram VDD VIN 6 1 Voltage Regulator Voltage Reference 250mV RT 8 OSC + LD 7 PWM Control 4 GATE 2 CS + 2 PWMD 5 GND 3 www.ixysic.com R02 AN-300 INTEGRATED CIRCUITS DIVISION Figure 2 Application Circuit Diagram D1 LEDs L1 VIN BR VIN VDD FUSE GATE PWMD CC LD CVDD CBULK ROSC CS PGND MXHV9910 NTC1 2 FET RSENSE RT Typical Design Parameters Parameter AC Input Voltage Minimum Voltage Symbol VAC-min 90 - - Maximum Voltage VAC-max - - 130 AC Input Frequency fac 50 - 60 LED String Voltage VLEDstring - 60 LED String Current ILEDmax - - Estimated Efficiency Oscillator Frequency fS - 0.90 64 - kHz Dmax_spec - - 0.5 - Duty Cycle Min Typ Max • DC Bulk Voltage at Low and High Line Units V DC_bulk_min = V DC_bulk_min = 127.3V Vrms 2 V AC-max Hz V DC_bulk_max = - V V DC_bulk_max = 183.8V 350 mA • Average Input Current P in 23.33W I in_avg = ------------------------------- = -----------------V DC_bulk_min 127.3V I in_avg = 0.183A • Output Power Calculation P OUT = V LEDstring I LEDmax • Peak Input Current P OUT = 60V 350mA I in_pk = 5 I in_avg P OUT = 21W I in_pk = 0.915A • Input Power Calculation P OUT P IN = ------------ 21W P IN = ----------0.90 2 V AC-min Note: During a surge, the current could be as much as 5 times higher, hence the multiplier. P IN = 23.33W R02 www.ixysic.com 3 AN-300 INTEGRATED CIRCUITS DIVISION 3 Switching Frequency and Resistor RT Selection It is recommended that the switching frequency range for off-line applications ranges from 30kHz to 120kHz. The MXHV9910 requires an external resistor, RT , that sets the internal RC oscillator frequency. For this Figure 3 design, RT is selected to be 402k, which sets the oscillator frequency to about 64kHz. Figure 3 below shows the typical oscillator frequency for a given RT resistor value. Oscillator Frequency vs. Resistor Value Oscillator Frequency, fS, vs. RT (TA=27ºC) 250 Frequency (kHz) 200 150 100 50 0 0 200 400 600 800 1000 1200 RT (kΩ) 4 Selecting Fuse and NTC1 Thermistor The fuse protects the circuit from input current surges during turn-on. Choose a fuse that is rated five times the peak input current. 5 Diode Bridge Rectifier The selection of the diode bridge rectifier is based on DC blocking voltage, forward current, and surge current. I fuse = 5 I in_pk V rb = V DC_bulk_max I fuse = 4.575A V rb = 183.8V The thermistor in series with the input bridge rectifier limits the inrush charging current into the input bulk capacitor during startup. The value is determined by: R th_cold 2 V AC_max = ---------------------------------I in_pk R th_cold = 200.87 The diode forward current rating should be set to 1.5 times the input average current. I fb = 1.5 I in_avg I fb = 0.2745A The diode bridge can be subjected to currents as high as 5 times the forward current, and the diode bridge should be rated accordingly. I fsb = 5 I fb I fsb = 1.3725A 4 www.ixysic.com R02 AN-300 INTEGRATED CIRCUITS DIVISION 6 Input Bulk Capacitor, CBULK, and CC The AC line voltage is filtered by the input bulk capacitor (CBULK), which is selected based on the minimum peak rectifier input line voltage and peak-topeak ripple voltage. Assuming a 20% ripple: r DC_bulk = 0.2 V in_min = 1 – r DC_bulk V DC_bulk_min = 1 – 0.2 127.3 7 The VDD pin is the internal regulator output pin and must be bypassed by a low-ESR capacitor (typically 0.1F or higher) to provide a low-impedance path for high-frequency switching noise. 8 Duty Cycle and ON-Time From the design requirements, the duty cycle and ON-time can be calculated as: V in_min = 101.8V P in C bulk = -----------------------------------------------------------------------------2 2 f AC V DC_bulk_min – V in_min C bulk Bypass Capacitor, CVDD V LEDstring 60V D max_buck = ------------------------------- = ----------------V DC_bulk_min 127.3V D max_buck = 0.471 23.33W = --------------------------------------------------------------------2 2 60Hz 127.3V – 101.8V D max_buck 0.471 t on.max_buck = ------------------------ = ---------------fS 64kHz C bulk = 66.70F t on.max_buck = 7.366s For this example, the voltage rating of the capacitor should be more than VDC_bulk_max with some safety margin factored in. An electrolytic capacitor with a 250V, 68F rating would be adequate. Note that electrolytic bulk capacitors contain parasitic elements that cause their performance to be less than ideal. One important parasitic is the capacitor’s Equivalent Series Resistance (ESR), which causes internal heating as the ripple current flows into and out of the capacitor. In order to select a proper capacitor, the designer should consider capacitors that are specifically designed to endure the ripple current at the maximum temperature, and that have an ESR that is guaranteed within a specific frequency range (usually provided by manufacturers in the 120Hz to 100kHz range). The Effective Series Inductance (ESL) is another parasitic that limits the effectiveness of the electrolytic capacitor at high frequencies. The combination of the variation of ESR over temperature and a high ESL may require adding a parallel film or tantalum capacitor (CC) to absorb the high-frequency ripple component. This keeps the combined ESR within the required limit over the full design temperature range. Dmax_buck is less than 50% and meets the subharmonic oscillation requirement. 9 Inductor Design The inductor (L1) value is determined based on desired LED ripple current and the switching frequency. 64 kHz was chosen as the optimum switching frequency to minimize switching losses and to reduce circuit power dissipation at the expense of larger inductor size. Assuming a 30% peak-to-peak ripple in LED current, one can calculate the inductor requirements: r iout = 0.3 V DC_bulk_min – V LEDstring t on.max_buck L min_buck = --------------------------------------------------------------------------------------------------r iout I LEDmax 127.3V – 60V 7.366s L min_buck = ---------------------------------------------------------------0.3 350mA L min_buck = 4.7mH Inductor peak current rating: I Lmax = I LEDmax 1 + 0.5 r iout I Lmax = 350mA 1 + 0.5 0.3 I Lmax = 0.403A In some cases, when the design requires a higher current rating and there is no standard inductor available, a custom-made inductor should be considered. R02 www.ixysic.com 5 AN-300 INTEGRATED CIRCUITS DIVISION 10 Power MOSFET and Diode Selection 11 Current Sense Resistor, RSENSE Peak voltage seen by the discrete power MOSFET (FET) and diode (D1) are equal to the maximum bulk voltage. For safety reasons assume an additional 50% margin by design. The current sense resistor (RSENSE) is selected based on the desired LED current. In this case, the maximum LED current is set at 350mA. Note that there is a difference between the peak current and the average current in the inductor. This ripple difference should be included in resistor calculations. The current sense threshold is given in the MXHV9910 data sheet. V FET_BVDSS_buck = 1.5 V DC_bulk_max V FET_BVDSS_buck = 1.5 183.8V V FET_BVDSS_buck = 275.771V Assuming 30% ripple: V Diode_r_buck = 1.5 V DC_bulk_max V cs(high) = 250mV V Diode_r_buck = 1.5 183.8V r iout = 0.3 V Diode_r_buck = 275.771V V cs(high) 250mV R sense = ------------------------------------------------------------------ = -------------------------------------------------------------- 1 + 0.5 r iout I LEDmax 1 + 0.5 0.3 350mA Maximum RMS current though the FET depends on the maximum duty cycle seen by the FET. In this buck converter, the maximum duty cycle is set slightly less than 50%. Choose a MOSFET with a rating of 3 times this current. I FET_rms_buck = 0.5 I LEDmax I FET_rating_buck = 3 I FET_rms_buck Note that since the current sense threshold voltage of the MXHV9910 (Vcsth) is specified between 200mV and 280mV, 250mV, the nominal value, is used in the formula above. Power dissipation across the sense resistor: I FET_rating_buck = 0.743A 2 Average current though the diode is one-half of the LED current. Choose a diode with a rating 3 times this current. P = I LEDmax R sense P = 0.076W In practice, select a resistor power rating that is at least twice the calculated value. I Diode_buck = 0.5 I LEDmax = 0.5 350mA = 0.175A I Diode_rating_buck = 3 I Diode_buck I Diode_rating_buck = 0.525A 12 Layout Considerations For this design, the IXTA8N50P external power FET, in the SMD D2-Pak package, was selected from IXYS’ family of Polar N-channel devices. The Polar process features 30% reduction of RDS(on) and substantial reduction of total gate charge, QG. This helps with improved LED driver efficiency by minimizing conduction and switching losses. In addition, the Polar power FET family has very low thermal resistance, RJC, which improves the device’s power dissipation. The IXA8N50P can be used with an external heat sink similar to Aavid Thermalloy’s part number 573100. The high frequency switching of the buck LED driver requires the use of a fast recovery diode. The BYV26_B series diode, in the SOD 57 package, was chosen for this design. 6 R sense = 0.621 In all switching converters, proper grounding and trace length are important considerations. The LED driver operates at a high frequency, and the designer must keep trace length from the MXHV9910 GATE pin to the external power MOSFET as short as possible. Doing this helps to avoid such undesired performance characteristics as ringing and spiking. In high-frequency switching, current tends to flow near the surface of a conductor, so ground traces on the PC board must be wide in order to avoid any problems due to parasitic trace inductance. If possible, one side of the PC board should be used as a ground plane. The current sense resistor, Rsense, should be kept close to the CS pin in order to prevent noise coupling to the internal high-speed voltage comparator, which would affect IC performance. In addition, RT should be placed away from the inductor and away from any PCB trace that is close to switching noise. www.ixysic.com R02 AN-300 INTEGRATED CIRCUITS DIVISION 13 Design Idea This design idea features an inexpensive, off-the-shelf Triac Dimmer Controller used with the MXHV9910 LED driver. The simple circuit is a voltage divider that feeds into the LD pin. The voltage divider can be adjusted for 110VAC or 220VAC operation simply by changing the value of resistor, R3. For a 220VAC application, decrease the value of R3 to 7.8k. VIN TRIAC DIMMER CONTROLLER 120VAC EXT. POWER VIN FUSE F2 2A R1 402k AC AC X1 + VDD MONITOR LD MONITOR C1 0.01μF 400V D1 BYV26B df04s NTC1 R3 17k R4 10M 1 2 R5 100k L1 4.7mH 1 Q1 IXTA8N50P C2 2.2μF 16V MXHV9910 8-PIN SOIC 2 3 4 RT VIN DUT1 CS LD PGND VDD GATE PWMD 3 8 7 6 5 C3* 10μF LED+ pwmd LED CONNECTION LED- 1 R2 0.56Ω X2 2 3 PWMD * C3 can be between 0.1μF and 10μF For additional information please visit our website at: www.ixysic.com IXYS Integrated Circuits Division makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication and reserves the right to make changes to specifications and product descriptions at any time without notice. Neither circuit patent licenses nor indemnity are expressed or implied. Except as set forth in IXYS Integrated Circuits Division’s Standard Terms and Conditions of Sale, IXYS Integrated Circuits Division assumes no liability whatsoever, and disclaims any express or implied warranty, relating to its products including, but not limited to, the implied warranty of merchantability, fitness for a particular purpose, or infringement of any intellectual property right. The products described in this document are not designed, intended, authorized or warranted for use as components in systems intended for surgical implant into the body, or in other applications intended to support or sustain life, or where malfunction of IXYS Integrated Circuits Division’s product may result in direct physical harm, injury, or death to a person or severe property or environmental damage. IXYS Integrated Circuits Division reserves the right to discontinue or make changes to its products at any time without notice. Specification: AN-300-R02 ©Copyright 2012, IXYS Integrated Circuits Division All rights reserved. Printed in USA. 12/13/2012 R02 www.ixysic.com 7

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