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Get negative auxiliary output from a buck regulator

Posted: 07 Aug 2007 ?? ?Print Version ?Bookmark and Share

Keywords:Buck switching-power converters? buck regulator?

By Bob Bell
National Semiconductor Corp.

Buck switching-power converters are the most common method used to efficiently step down positive input voltages. Some applications require an additional low power negative output voltage. Often, for cost and size constraints an additional regulator is impractical. There are two possibilities to derive a second negative output from the main buck regulator power stage.

Figure 1 shows a method to derive a second output by adding an auxiliary winding (N2) to the output inductor.

At each cycle, when the buck switch is closed, the 'dot end' of each winding on the inductor is positive. The diode associated with the auxiliary winding is reverse-biased. When the buck switch opens and the buck stage re-circulating diode conducts, the voltage across the main output winding is Vout + Vd, where Vd is the re-circulating diode voltage drop. The 'non-dot end' of the main output winding is positive.

Assuming the auxiliary winding and the main winding have the same number of turns, then the same voltage level will be present across the auxiliary winding, with a polarity that will forward-bias the auxiliary diode.

The auxiliary output capacitor will charge to the same potential as Vout. During the next cycle, while the buck switch is closed, the auxiliary output will be supported by the auxiliary output capacitor. This auxiliary output is floating and can be referenced to produce either a positive or negative output. The auxiliary output can be set by adjusting the number of turns on the auxiliary winding. An approximation to design it for a specific auxiliary voltage is:

where N2 is the number of turns on the auxiliary winding and N1 is the number of turns that on the buck inductor.

This method has the advantage of providing an auxiliary output voltage that is semi-regulated, since it is set as a ratio from the main regulated output. The disadvantage is difficulty and cost of finding an inductor with an auxiliary winding along with the desired number of turns.

Factors that affect the auxiliary voltage regulation are the lack of matching between the diodes and coupling effects of the windings. Moreover, if the load on the main output is very small and the inductor is operating in discontinuous mode, there may be insufficient energy in the inductor to charge the auxiliary output which will cause the auxiliary output to droop during this condition. In normal operation, a fixed preload is often necessary if the main output is lightly loaded.

Figure 1: Negative auxiliary output using an auxiliary inductor winding.

Figure 2 shows a method to derive an auxiliary negative output through a charge pump from the main output switching node.

Figure 2: Negative auxiliary output using a charge pump.

Each cycle, when the buck switch is closed, capacitor C1 (1,000pF) is charged to Vin. When the buck switch opens, the switching node and the top of capacitor C1 are at the ground potential. Charge from capacitor C1 is then transferred to the auxiliary output capacitor C2. The 5.6 resistor in series with C1 limits the peak current through C1 as to not disturb normal operation of buck regulator. An approximation to design for a specific auxiliary voltage is:

where Fs is the switching frequency and R is the load resistance on the auxiliary output.

This methods has the advantage that it can be implemented with only a few additional discrete components, and not require any modification to the main output inductor. The equation shows the disadvantage: the auxiliary output voltage is not regulated and will vary with changes to the input voltage as well as the load resistance on the auxiliary output.

A reasonable design approach is to select the coupling capacitor C1 to achieve the desired minimum auxiliary output voltage while operating with a minimum input voltage and a minimum load resistance. A Zener diode or other clamp will be necessary to limit the maximum auxiliary voltage while operating with a maximum input voltage and a maximum load resistance. The auxiliary output capacitor can be selected using:

The equation is an approximation, where I is the maximum load current on the auxiliary output and VRIPPLE is the output ripple voltage.

Both of the methods described are practical for designing a negative auxiliary output with a power level less than about one watt. By rearranging the components, the circuit will produce a positive auxiliary output.

About the author
Robert (Bob) Bell
is the applications engineering manager for National Semiconductor's design center in Phoenix, Arizona. Previously, he designed power converters for military and space applications. Bob has published 25 power-design articles, six conference papers and has three patents. In his spare time he enjoys hiking, camping, tennis and travel.

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