Design Article

IMG1

Power Tips #5: Buck-boost design uses a buck controller

Robert Kollman, Texas Instruments

11/4/2008 9:19 PM EST

Electronic circuits typically operate from regulated positive output voltages, often provided by buck regulators. If a negative output voltage is also required, the same buck controller often can be configured in a buck-boost topology. A negative output voltage buck-boost, sometimes called a negative flyback, operating at 50% duty-cycle provides an output voltage equal to the input voltage, only opposite in polarity. It has the ability to "buck" or "boost" the output voltage to maintain regulation by adjusting the duty cycle as the input voltage fluctuates.

Figure 1 shows a simplified buck-boost circuit and the switching voltage present on the inductor. The circuit's similarity to a standard buck converter should be immediately apparent. In fact, it is identical to a buck converter, but with the output voltage and ground reversed. This arrangement also works for a synchronous buck converter. However, this is where the similarities to a buck or sync buck converter end, because the circuit operates differently than a buck converter.

The voltages present on the inductor during the FET switching intervals are different than that of a buck. Just as in a buck, it is necessary to balance the volt-microsecond (V-μs) product to prevent the inductor from saturating. While the FET is on, shown as the ton interval in Figure 1; the full input voltage is impressed across the inductor. This positive voltage on the "dot' side of the inductor causes the current to ramp upward. This causes an on-time V-μs product across the inductor. During the FET off-time (toff), the inductor's voltage polarity must reverse to maintain current flow, pulling the dot side negative. The inductor current ramps downward and circulates through the load and output capacitor, returning through the diode. The off-time V-μs product across the inductor must be equal to the on-time V-μs product. Since Vin and Vout are fixed, the duty cycle (D) expression: D=Vout/(Vout " Vin) is easily derived. The control circuit determines the correct duty cycle to maintain output voltage regulation. This expression and the waveform in Figure 1 assume continuous conduction mode of operation.


Figure 1: The buck-boost inductor requires balancing its volt-microSecond Product.
(Click this image to view a larger, more detailed version)

The buck-boost inductor must operate at a current level that is higher than the output load current. This is defined as IL = I/(1-D) or simply the input plus output current. For a negative output voltage that is equal in magnitude to the input voltage, operating at D = 0.5, the average inductor current is 2X that of the output.

Capacitor placement affects rms current
Interestingly, there are two choices of where to connect the return side of the input capacitor, which can affect the rms current in the output capacitor. Contrary to the typical capacitor placement between +Vin and Gnd, the input capacitor can be connected between +Vin and "V. Using this input capacitor configuration reduces the rms current in the output capacitor. However, with the input capacitor connected to "Vout, a capacitive voltage divider is formed at "Vout. This can create a positive spike on the output at turn on, before the controller starts functioning. To minimize this effect, it is usually best to use an input capacitor that is much smaller than the output capacitor. You can see this in the circuit in Figure 2. The current in the input capacitor alternates between sourcing the dc output current and sinking the average input current. The rms current level will be worse case at low input voltage where the input current is highest. Be careful when selecting the capacitors so that their ESR is not too high. Ceramic or polymer capacitors are often an appropriate choice for this topology.


Figure 2: A Buck Controller does Double Duty in a Buck-Boost.
(Click this image to view a larger, more detailed version)

A controller must be selected that can power up with the minimum input voltage minus a diode drop, but must also withstand Vin plus Vout while operating. The FET and diode must also be rated for this voltage range. The output voltage is regulated by connecting the feedback resistor at the output ground, since the controller is referenced to the negative output voltage. By carefully selecting just a few components' values and some minor circuit changes, a buck controller can work double-duty in a negative output buck-boost topology.

I'd like to express special thanks to John Betten of Texas Instruments for his contribution to this article. Next month, we will discuss properly measuring power supply ripple in Power Tips #6.

The Power Tips! series
#1, July: Picking the right operating frequency for your power supply
#2, August: Taming a noisy power supply
#3, September: Damping the input filter --- Part 1
#4, October: Damping the input filter --- Part 2



Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.



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