Easily Boost 12V to 140V with a Single Converter IC Design Note 1033 Victor Khasiev INTRODUCTION Generating a high voltage from a much lower voltage presents a number of challenges for the classical single stage boost topology. For instance, the maximum duty cycle limitation of a boost controller may not allow the required step-up ratio. Even if it does, there is often a sharp decrease in efficiency at high duty cycles. The duty cycle can be shortened by choosing discontinuous mode of operation, but this leads to high peak input current, higher losses and EMI challenges. An alternative to a single boost converter is a 2-stage boost converter, where the first stage produces an intermediate voltage and the second stage boosts to the final high voltage. A 2-stage converter can be produced with a single controller IC, such as the LTC ®3788, a high performance 2-phase dual output synchronous boost controller, which drives all N‑channel power MOSFETs. power losses and eases thermal requirements. The maximum output voltage of this controller is 60V, when using synchronous rectification. If greater than 60V is required, the second stage can be designed to run non-synchronously, as described below. 2-STAGE BOOST PRODUCES 140V FROM 12V The block diagram in Figure 1 shows the LTC3788 in a 2-stage boost configuration. This block diagram also reveals a few caveats that must be observed in this design: nn The output of the first stage (Q1, CINT) is connected to the input of second stage (RS2, L2). The output of the first stage should not exceed 40V, because maximum absolute rating of the SENSE pins is 40V. nn The gate drive voltage of 5V is suitable for logic level MOSFETs, but not for high voltage standard MOSFETs, with typical gate voltages of 7V to 12V. The external gate driver DR, controlled by the BG2 signal can be used as shown here to drive high voltage standard MOSFETs. nn To generate an output voltage above maximum limit of 60V, the synchronous rectification MOSFET is replaced by a single diode D1. The LTC3788 can be configured such that the first boost stage takes advantage of its synchronous rectification feature, which maximizes efficiency, reduces CINT 40V ABS MAX TG1 SW1 PGOOD1 SS1 ILIM FB1 ITH1 SENSE1+ Q1 RS1 Q2 SENSE1– VIN GND BOOST1 FREQ BG1 PHASMD VBIAS CLKOUT EXTVCC SGND INTVCC 5V TG2 SW2 PGOOD2 SS2 ITH2 BOOST2 FB2 BG2 RUN2 SENSE2+ RUN1 SENSE2– VOUT COUT PGND LTC3788 PLLIN/MODE GND L1 U1 D1 L2 DR Q3 GND 7V TO 10V MOSFET DRIVER DN1033 F01 Figure 1. Block Diagram of LTC3788-Based 2-Stage Boost Converter 09/15/dn1033f GND RS2 Figure 2 shows the complete solution. Transistors Q1, Q2, and inductor L1 compose the first stage, which generates an intermediate bus voltage of 38V. The first stage employs synchronous rectification for maximum efficiency. The output of the first stage is connected as input to the second stage, comprised of Q3, D1, L2. The output of second stage produces 140V at 1A. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 12.1k V_INT SENSE1– V_INT EXTVCC SGND INTVCC RUN1 BG2 12.1k SW2 PGOOD2 SS2 ITH2 FB2 SENSE2+ SENSE2– GND 309k 4.7μF SNS2+ SNS2– 100pF 2.7k 6.98k + VOUT 22μF/200V EEVEB2D220SQ GND D1 SBR10U200P5130 0.1μF 15nF GND 2x0.47μF/450V C4532X7T2W474M 8.2V VIN 82μF/50V 50HVH82M VOUT 140V AT 1.0A DRIVER BIAS CIRCUIT Q4 MMBTA42LT1G 1μF + Q2 BSC028N06LS3G V_INT 5.1k BOOST2 RUN2 VIN 3V TO 36V 4x4.7μF TG2 2.21k SENSE1– SENSE1+ L1, 6.8μF SER2915H682 RS1, 0.002Ω 0.1μF PGND LTC3788EUH PLLIN/MODE 82μF/50V 50HVH82M Q1 BSC067N06LS3G VBIAS VBIAS CLKOUT VBIAS BAS140W BG1 PHASMD + 4x4.7μF 0.1μF BOOST1 FREQ 42.2k TG1 SW1 PGOOD1 SS1 FB1 ITH1 SENSE1+ SNS1– U1 INTERMEDIATE BUS 15nF 0.1μF ILIM 8.66k SNS1+ 100pF 374k U2 L2, 100μH PCV210405L VBIAS LTC4440 BST VCC GND TG IN TS SENSE2– SENSE2+ RS2, 0.01Ω Q3 BSC320N20NS3 806k DN1033 F02 Figure 2. Full Schematic of 2-Stage 140V Output, 1A Boost Converter This solution features an input voltage range from 3V to 36V, nominal 12V. To decrease components’ thermal stress, the output current should be reduced when the input voltages falls below 10V. Figure 3 shows measured efficiency, and Figure 4 shows the start-up waveforms. A 93% efficiency is shown with VIN = 24V and with the 140V output loaded from 0.4A to 1A. This converter can operate at full load with no airflow. 94 92 EFFICIENCY (%) Transistor Q3 is standard level MOSFET, driven by the LTC4440. Here, an LDO, based on transistor Q4, biases the gate driver, but a switching regulator can be employed instead (such as one built around the LTC3536) to further increase overall efficiency. 90 88 86 84 24V 12V 8V 0 0.2 0.6 0.4 LOAD CURRENT (A) 1 0.8 DN1033 F03 Figure 3. Efficiency of the 2-Stage Converter in Figure 2 [VIN 8V to 24V, VOUT 140V] CONCLUSION LTC3788 is a high performance 2-phase dual output synchronous boost controller, suitable for high power, high voltage applications. Its dual outputs can be used in tandem to achieve extremely high step-up ratios to high voltages. DN1033 F04 Figure 4. Start-Up Waveforms [from VIN 12V to VOUT 140V at 1A] Data Sheet Download www.linear.com/LTC3788 Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 3161 dn1033f LT 0915 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2015