DESIGN IDEAS L High Power 2-Phase Synchronous Boost Replaces Hot Diodes with Cool FETs—No Heat Sinks Required by Narayan Raja, Tuan Nguyen and Theo Phillips Introduction For low power designs, non-synchronous boost converters offer a simple solution. However, as power levels increase, the heat dissipated in the boost diode becomes a significant design problem. In such cases, a synchro- nous boost converter, with the diode replaced with a lower forward voltage drop switch, significantly improves efficiency and relieves many issues with thermal layout. Although the topology is more complicated, Linear Technol5V ogy offers controller ICs that simplify the design of high power synchronous boost applications. The LT3782A boost controller, for instance, includes predrive outputs for external synchronous switch drivers. It also integrates strong PD3S160 Q1 VCC 1µF GND LTC4440-5 IN 4.7µF VIN 10V TO 14V + 680µF 1µF TG DFLS160 TS SGATE1 10µF s4 BST L1 8.3µH 475k 1% 825k BG1 VCC1 10µF s4 BG1 Q2 VOUT 24V AT 8A + 680µF 53.6k 1% Q3 + SENSE1 SGATE2 SGATE2 BGATE1 BG1 SGATE1 SGATE1 BGATE2 BG2 SENSE1+ 10Ω SYNC LT3782A L2 8.3µH GBIAS GBIAS1 DCL GBIAS2 4.7µF PD3S160 VCC 1µF VEE1 GND SLOPE VEE2 IN RSET GND LTC4440-5 BST DFLS160 1µF TG TS SGATE2 SENSE2– SENSE2+ FB SS 0.1µF 60.4k BG2 0.008Ω 0.01µF 220pF Q5 BG2 Q6 SENSE2+ VC 0.008Ω SENSE2– 15k 6.8nF 51.1k 5V Q4 SENSE1– SENSE2– 2.2nF 30.9k RUN DELAY 0.008Ω SENSE1+ 2.2nF SENSE1– 22pF 0.008Ω SENSE1– 316k 10Ω SENSE2+ Q1–Q6 = HAT2266 Figure 1. Compact high power boost application efficiently produces a 24V/8A output from a 10V–15V input. Linear Technology Magazine • January 2009 35 L DESIGN IDEAS 97 100 EFFICIENCY (%) 94 VIN = 24V 92 POWER LOSS 91 = 24V Figure 2. Layout of the circuit in Figure 1. Note that no heat sinks are needed, even at the high power levels produced by this relatively compact circuit. bottom switch drivers for high gate charge high voltage MOSFETs and uses a constant frequency, peak current mode architecture to produce high output voltages from 6V to 40V inputs. Its 2-phase architecture keeps external components small and low profile. Synchronous Operation At high current levels, a boost diode dissipates a significant amount of power, while a synchronous switch can burn far less. It all comes down to the forward voltage drop. The power dissipated in the boost diode is IIN • VD, while the power dissipated by the synchronous switch is I2IN • RDS(ON). (or IIN • VDS(ON)). A typical sub-10mΩ MOSFET running 10A dissipates 1W, while the 0.5V drop of a typical a. Thermal image of the board in Figure 2 built up with synchronous FETs 1 2 5 3 4 LOAD CURRENT (A) 6 7 1 Figure 3. Efficiency and power loss of the circuit in Figure 1 compared to the efficiency of the circuit when the synchronous FETs are replaced with non-synchronous boost diodes. Schottky diode burns a whopping 5W. Because the forward drop of a synchronous MOSFET is proportional to the current flowing through it, FETs can be paralleled to share current and drastically reduce power dissipation. On the other hand, paralleling boost diodes does little to reduce power dissipation as the forward drop through the diodes holds fairly constant. The non-synchronous boost diode topology is more than just inefficient relative to a synchronous solution—the extra heat generated in a boost diode must go somewhere, necessitating a larger package footprint and heat sinking. At high power levels, a non-synchronous boost application becomes larger in size and higher in cost over a synchronous solution. COOL FETs 90 10 VIN = 12V 93 POWER LOSS (W) VIN = 12V 95 = 24V VIN = 24V EFFICIENCY 96 Multiphase Operation Reduces Application Size There are a number of good reasons to choose a multiphase/multi-channel DC/DC converter over an equivalent single-phase solution, including reduced EMI and improved thermal performance, but the biggest advantage can be a significant reduction in application size. Although a 2-phase solution requires more components, two inductors and two MOSFETs instead of one, it offers a net reduction in space and cost. This is because the inductors and MOSFETs are more than proportionally smaller than those required in the single-phase solution. Moreover, because the switching signals are mutually anti-phase, their output ripples tend to cancel each continued on page 39 HOT DIODES HEAT UP THE WHOLE BOARD b. Thermal image of the board in Figure 2 built up with boost diodes Figure 4. The board in Figure 2 runs fairly cool (a), but when the synchronous FETs are replaced with boost diodes, the entire board heats up considerably with the diodes running significantly hotter than the FETs (b). (VIN = 12V, ILOAD = 6A, two minutes after power up.) 36 Linear Technology Magazine • January 2009 DESIGN IDEAS L 10V–50V L1 22µH VIN 9V TO 36V (6V UVLO) LED+ ILED 400mA D1 2.2µF 50V s2 110k 4.7k 499k VIN SHDN/UVLO 1M VIN GATE SENSE FB VLED = 10V 80 0.025Ω 3906 VLED = 50V 90 0.25Ω COUT 2.2µF 100V s2 M1 100 2.49k 130k EFFICIENCY (%) LED– 70 60 50 40 30 20 10 0 10 15 LT3756 PWMOUT CTRL 140k 4.7µF VREF 100k SS ISN VC 0.1µF 10k GND 5.1k RT 28.7k 400kHz M1: VISHAY SILICONIX Si7454DP D1: DIODES INC. PDS3100 L1: SUMIDA CDRH127-220 4700pF Figure 4. A buck-boost mode LED driver with wide-ranging VIN and VLED current; the peak inductor current is also equal to the peak switching current—higher than either a buck mode or boost topology LED driver with similar specs due to the nature of the hookup. The 4A peak switch current and inductor rating reflects the worst-case 9V input to 50V LED string at 400mA. Below 9V input, the CTRL analog dimming input pin is used to scale back the LED current to keep the inductor current under control if the battery voltage drops too low. The LEDs turn off below 6V input due to undervoltage lockout and will not turn back on until the input rises above 7V, to prevent flickering. In buck-boost mode, the output voltage is the sum of the input voltage and the LED string voltage. The output capacitor, the catch diode, and LT3782A, continued from page 36 to level shift the SGATE signals and drive the synchronous MOSFETs. The 250kHz switching frequency optimizes efficiency and component size/board area. Figure 2 shows the layout. Proper routing and filtering of the sense pins, placement of the power components and isolation using ground and supply planes ensure an almost jitter free operation, even at 50% duty cycle. Figure 3 shows the efficiency of the circuit in Figure 1 with synchronous MOSFETs (measured to 8A) and the efficiency of an equivalent non-synchronous circuit using boost diodes (measured to 6A). The 1% improvement in peak efficiency may not seem significant, but take a look at the difference other out, thus reducing the total output ripple by 50%, which in turn reduces output capacitance requirements. The input current ripple is also halved, which reduces the required input capacitance and reduces EMI. Finally, the power dissipated as heat is spread out over two phases, reducing the size of heat sinks or eliminating them altogether. 24V at 8A from a 10V–15V Input Figure 1 shows a high power boost application that efficiently produces a 24V/8A output from a 10V–15V input. The LTC4440 high side driver is used Linear Technology Magazine • January 2009 30 the power MOSFET can see voltages as high as 90V for this design. ISP OPENLED 25 Figure 5. Efficiency for the buck-boost mode converter in Figure 4 PWM INTVCC 20 VIN (V) Conclusion The 100V LT3756 controller is ostensibly a high power LED driver, but its architecture is so versatile, it can be used in any number of high voltage input applications. Of course, it has all the features required for large (and small) strings of high power LEDs. It can be used in boost, buck-boost mode, buck mode, SEPIC and flyback topologies. Its high voltage rating, optimized LED driver architecture, high performance PWM dimming, host of protection features and accurate high side current sensing make the LT3756 a single-IC choice for a variety of high voltage input and high power lighting systems. L in heat dissipation shown in Figure 4, which shows thermal images of both circuits under equivalent operating conditions. The thermal advantages of using synchronous switches are clear. Conclusion The 2-phase synchronous boost topology possible with the LT3782A offers several advantages over a nonsynchronous or a single-phase boost topology. Its combination of high efficiency, small footprint, heat sink-free thermal characteristics and low input/output capacitance requirements make it an easy fit in automotive and industrial applications. L 39