How to Select a Boost Regulator/Controller IC and Select Peripheral Components – Part 2
April 19, 2021
In part 1 of this article series, we explored why switch current is important, what output current is required to satisfy expected loads, and the differences between selecting a buck and boost dc-to-dc converter.
In part 2, we’ll look at VSW, the transistor maximum allowable voltage and duty cycle limitation, how to understand peripheral stresses using LTspice, and the connection between VSW and duty cycle.
Usually, the VIN range of an IC is specified in the data sheet—both a recommended range and absolute maximum values. The highest output voltage possible from a boost regulator with an internal power switch is stated in the data sheet as its maximum VSW rating. If you use a boost controller with an external MOSFET as the power switch, the MOSFET’s data sheet-stated VDS rating is what limits the maximum output voltage.
For example, the LT8330 boost regulator has an input voltage range of 3 V to 40 V, an absolute maximum switch voltage of 60 V, and a fixed switching frequency of 2 MHz. Although the absolute maximum 60 V switch voltage rating makes it possible for the part to generate a boost output to 60 V, it is best practice to remain below this by at least 2 V.
The output voltage is also limited by the duty cycle. The maximum and minimum duty cycle might be in the data sheet, or it can be calculated. Using the LT8330 to convert 12 V to 48 V, for CCM omitting the diode voltage drop for a high conversion ratio, the duty cycle is computed from the input and output voltage:
- D = (VO – VIN)/VO = (48 V – 12 V)/48 V = 0.75 or 75%
- Check to see if the IC is able to work at the duty cycle required.
- The IC minimum duty cycle is given by:
- DMIN = minimum TON(MAX) × fSW(MAX)
- And the IC maximum duty cycle is given by:
- DMAX = 1 – (minimum TOFF(MAX) × fSW(MAX))
Minimum TON and Minimum TOFF can be found in the Electrical Characteristics table in the data sheet. Use the max values from the table’s min, typ, and max columns. Using the published values of the LT8330 and in the DMIN and DMAX equations result in DMIN = 0.225 and DMAX = 0.86. From the result, we can see that the LT8330 should be able to convert 12 V to 48 V, as the design calls for a duty cycle of 0.75.
Understanding Peripheral Stresses Using LTspice
The schematic shown in Figure 12 realizes the design concept previously introduced featuring the LT8330 in a 12 V input to 48 V output converter supporting 150 mA loads.
Figure 12. The LT8330 used in a 12 V to 48 V converter for 150 mA load current.
From the LTspice simulation we can plot and measure a number of parameters. Those that can help you choose an IC are described in Figure 13.
VSW and Duty Cycle
After running the simulation, you can view the SW node behavior as a waveform to understand what voltages are present on the power switch during a switching period. To do so, hover over the SW node so that the crosshair cursor changes to a red voltage probe. Click to plot the switch node behavior on the waveform viewer. The resulting graph corresponds to the drain of the internal power MOSFET.
As expected, when the MOSFET conducts, the voltage potential is close to ground, but more importantly, during TOFF, the MOSFET is off and the drain voltage is subject to the output voltage plus a diode drop. Now we know what stress is on the VDS of the MOSFET. If we chose a controller design that uses an external MOSFET as the power switch, we should have chosen a MOSFET that has a VDS rating of 60 V.
In the LTspice waveform viewer, one can use cursors to make horizontal and vertical measurements, similar to the cursors on an oscilloscope. To invoke the cursors, click the V(sw) label in the LTspice waveform viewer. This attaches the first cursor to the trace, and another click attaches a second cursor to the same trace. Alternatively, right-click the label and choose the desired cursors to a given probed trace. Using these cursors, you can measure TON and calculate the duty cycle, given by TON/Period.
TPERIOD = TON + TOFF = 1/fSW. Earlier, we calculated this to be 75% or 0.75. Using LTspice we get approximately 373 ns. The LT8330 uses a fixed switching frequency of 2 MHz, so TP = 1/2e6 = 500 ns, so the duty cycle is 373 ns/500 ns = 0.746.
Figure 13. Switch node plot on the graphical viewer in LTspice.
Figure 14. Measuring TON to verify the estimated duty cycle.
In part 3, we will go into detail about peak current through and voltage across the inductor, current and voltage through the diode, and diode power dissipation.
About the Author
Rani Feldman joined Analog Devices in 2017 as a senior field applications engineer. Previously, Rani worked for Linear Technology for three years. Rani has a bachelor’s degree in electronics engineering from Afeka College in Israel and holds a master’s degree in business and administration from Holon Institute of Technology in Israel. He can be reached at [email protected]