AUTOMOTIVE ELECTRONICS Reduce Costs in
Victor Khasiev, senior applications engineer atAnalog Devices investigates why you should use the same qualified parts for both positive and negative output DC-to-DC converters
ne relatively easy way for manufacturers to reduce the cost of electronic components is by using the same designs or components for disparate applications. The cost savings manifest not only in the obvious bulk procurement of identical parts, but also in minimising the number of required qualification processes. Qualification is especially critical for the transportation industry - particularly for automobile manufacturers. Usually it is an expensive process involving testing for ruggedness, reliability, and longevity of units.
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This article shows how to use the same components - an IC controller and power train - in two very different topologies: a common buck converter and in an inverting buck-boost converter. The component requirements are explored for the inverting buck- boost converter by specifically examining the reverse voltage fluctuation on the output of the inverting buck-boost, and ways to use the least expensive polarized capacitors in this topology. As a result, a simple and cost-effective solution for designing positive buck and negative buck-boost converters using the same IC is presented.
Positive output, buck converter
The electrical schematic of the positive output, buck converter is presented in Figure 1. The converter generates a stable VOUT 15A from the input voltage VIN
and CIN2 of 5V at range of 5V to 38V. The power train
includes modulating (high-side) MOSFETs Q1 and Q2, rectifying (low- side) MOSFETs Q3 and Q4, inductor L1, a combination of the electrolytic and ceramic input filter capacitors CIN1
, and a
similar combination of capacitors for the output filter and the controller.
Negative output, negative buck-boost converter The schematic of the negative buck-boost converter presented in Figure 2 is similar to the buck converter schematic in Figure 1. Notably, both use the same components for the power train, interconnections, and controller. Differences arise in the grounding of the controller, switching MOSFETs, and input/output filter. The ground of these inverting converter components is –VOUT terminated to the system (input) ground.
. The inductor L1 is
The resistor RS can be used as a current-sense element if the peak current mode controller is employed, or as part of the short-circuit protection circuitry in voltage mode control. The input capacitors CIN1 and CIN2
are terminated to GND; however, the optional CIN3 and CIN4
are terminated to the output and are employed in the negative buck-boost solution.
The functionality of buck converters is widely studied and easily obtainable. In this article, we just briefly note voltage and current stress on the power train components. It is relevant to the preliminary selection of components in new designs and rough evaluation of the existing solutions. Assuming continuous conduction mode (CCM) operation, the following expressions can be used.
Figure 1: Electrical schematic of a step-down, buck converter with VIN 38V, and VOUT
5V at 15A. 16 JUNE 2021 | ELECTRONICS TODAY
5V to
Figure 2: Electrical schematic of an inverting buck-boost converter with VIN to 33V, VOUT
–5 V at 15A, and start-up input voltage +5V.
2V
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