AUTOMOTIVE ELECTRONICS the Long Run Unlike in the buck converter, the capacitors CIN3 and CIN2 filter ac between the VIN and CIN4 and –VOUT are not
optional in this solution; they function as the input filter. The capacitors CIN1
rails. The
following expressions can be used to estimate the stresses on the power train components, assuming CCM operation.
Converter functionality and testing
There’s plenty of literature covering the basic and even advanced functionality of these two types of converters. In the remainder of this article we’ll examine rarely discussed factors.
First, there is a fundamental difference in functionality of the output filters between the buck and buck-boost topologies. In the buck configuration, the inductor is hardwired to the output filter, providing continuous output current in CCM. Unlike the buck, the buck-boost topology does not connect the inductor only to the output. During the Q1/Q2 on-time, the inductor L1 is disconnected from the output filter and the output filter capacitance is the only source of energy to the load. Consequently, it’s important to have enough output capacitance to absorb the discontinuous output capacitor current and support the specified output voltage ripple. There is a drawback in negative buck-boost and, in fact, most inverting topologies. At startup there is a reverse voltage swing at the output filter with amplitude not more than one diode voltage drop, as shown in Figure 3. This brief reverse voltage is due to the flow of the controller’s operating current through the forward-biased diode to system ground. The existence of the reverse voltage on polarized capacitors appears unacceptable at first glance. Hence, some designers eliminate polarized capacitors from the output filter, resorting to ceramic-only capacitors. This approach creates other problems associated with the size, cost, and dc bias of the ceramic capacitors. Nevertheless, it is possible to use polarized capacitors in inverting buck-boost applications with some limitations.
Figure 3: An inverting buck-boost converter with start-up waveforms. Channel 2’s VIN
is 5 V/div, while channel 3’s VOUT
The converters shown in Figure 1 and Figure 2 were thoroughly tested and evaluated. Their efficiency is shown in Figure 4. To simplify the design with a low pin count and wide input voltage range, making it applicable to a wide variety of solutions, the LTC7803 advanced controller was used in both cases. The evaluation board DC2834A was used as a basis (with some modification) to verify both applications. To reduce EMI, the spread spectrum feature of this controller can be employed. Figure 5 shows a photo of the buck DC2834A converted to inverting buck-boost. This article presents a way to use the same controller and a number of identical components for positive step-down and negative buck-boost converters. In this way, costs for qualifying components can be reduced. Costs can be further reduced by using a controller that requires a minimal number of power train components and supports synchronous rectification, resulting in efficient, low EMI, wide input voltage range solutions.
is 0.5 V/div with a 2 ms/div timescale.
Figure 5: DC2834A converted to inverting buck-boost from the original, off-the-shelf step-down converter.
Figure 4: Efficiency of the converters in Figure 1 and Figure 2 (VIN 12V, natural convection cooling, no air flow).
Analog Devices
www.analog.com
JUNE 2021 | ELECTRONICS TODAY 17
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