Boost 12V to 140V with a Single Converter IC Victor Khasiev 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 LTC3788, a high performance 2-phase dual output synchronous boost controller, which drives all N‑channel power MOSFETs. The LTC3788 can be configured such that the first boost stage takes advantage of its synchronous rectification feature, which maximizes efficiency, reduces power losses Figure 1. Block diagram of LTC3788-based 2-stage boost converter CINT 40V ABS MAX TG1 SW1 PGOOD1 ILIM SS1 ITH1 FB1 SENSE1+ Q1 RS1 Q2 SENSE1– VIN GND BOOST1 FREQ BG1 PHASMD VBIAS CLKOUT PGND LTC3788 BG2 RUN2 BOOST2 SS2 ITH2 FB2 SENSE2 TG2 RUN1 SW2 INTVCC 5V PGOOD2 SGND + EXTVCC SENSE2– PLLIN/MODE GND L1 U1 30 | November 2015 : LT Journal of Analog Innovation VOUT COUT D1 L2 DR 7V TO 10V MOSFET DRIVER Q3 GND RS2 GND 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: •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 the maximum absolute rating of the SENSE pins is 40V. •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. •To generate an output voltage above maximum limit of 60V, the synchronous rectification MOSFET is replaced by a single diode D1. 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 design ideas 12.1k V_INT 2.21k V_INT PLLIN/MODE EXTVCC SGND INTVCC RUN1 BG2 SW2 SS2 PGOOD2 ITH2 FB2 SENSE2+ SENSE2– GND 12.1k DRIVER BIAS CIRCUIT 5.1k Q4 MMBTA42LT1G 8.2V SNS2+ SNS2– 1μF 100pF 2.7k 6.98k + Q2 BSC028N06LS3G VIN 82μF/50V 50HVH82M GND V_INT VOUT 140V AT 1.0A 2x0.47μF/450V C4532X7T2W474M + 22μF/200V EEVEB2D220SQ VOUT GND D1 SBR10U200P5130 0.1μF 15nF VIN 3V TO 36V 4x4.7μF 4.7μF BOOST2 RUN2 309k SENSE1– SENSE1+ L1, 6.8μF SER2915H682 RS1, 0.002Ω 0.1μF PGND LTC3788EUH 82μF/50V 50HVH82M Q1 BSC067N06LS3G VBIAS VBIAS CLKOUT VBIAS BAS140W BG1 PHASMD + 4x4.7μF 0.1μF BOOST1 FREQ 42.2k TG1 SW1 ILIM SS1 ITH1 SENSE1– U1 TG2 SNS1– FB1 SENSE1+ 0.1μF INTERMEDIATE BUS 15nF PGOOD1 8.66k SNS1+ 100pF 374k U2 VBIAS LTC4440 BST VCC GND TG IN TS L2, 100μH PCV210405L SENSE2– SENSE2+ RS2, 0.01Ω Q3 BSC320N20NS3 806k Figure 2. Full schematic of 2-stage 140V output, 1A boost converter of Q3, D1, L2. The output of the second stage produces 140V at 1A. Transistor Q3 is a standard level MOSFET, driven by the LTC4440. Here, an LDO, based on transistor Q4, biases the gate driver, but a switching 94 regulator can be employed instead (such as one built around the LTC3536) to further increase overall efficiency. This solution features an input voltage range from 3V to 36V, nominal 12V. To decrease components’ thermal stress, the VOUT = 140V EFFICIENCY (%) CONCLUSION VIN = 12V IOUT = 1A 92 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. n 90 VOUT 20V/DIV 88 86 84 output current should be reduced when the input voltages fall 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. VIN = 24V VIN = 12V VIN = 8V 0 0.2 0.6 0.4 LOAD CURRENT (A) 0.8 Figure 3. Efficiency of the 2-stage converter in Figure 2 1 5ms/DIV Figure 4. Start-up waveforms November 2015 : LT Journal of Analog Innovation | 31