How to reconfigure a buck converter for multiple outputs

-November 28, 2017


Power supply circuits come in the form of voltage step-up (boost) or the more common step-down (buck) DC/DC converter. Many of today’s applications require multiple voltage rails to drive a variety of ICs. These rails can be inverting or non-inverting, with or without isolation. While designers typically use multiple buck converters with single filter inductors, they add cost, footprint and height. A simpler alternative is to use a single buck converter with coupled inductors or transformers configured in isolated converter topologies. Designers can use the buck converter for inverting or non-inverting voltage rails, and they can configure it for use as an inverting buck-boost converter. Coupled inductors or transformers can also be used with a buck-boost converter to generate multiple inverting or non-inverting outputs with voltage step up/down function.

This article will highlight various isolated/non-isolated DC/DC converter topologies and demonstrate how they can be implemented using a single synchronous buck converter. We’ll also look at other topologies and show how they are suitable for various applications.


A look at three DC/DC converter topologies

The beauty of generating various converter topologies based on a single buck converter is that an opto-coupler and its related circuitry are not required. This provides the benefit of a smaller footprint, lower component count, reduced complexity and cost savings. Besides generating multiple outputs, the buck converter is configurable to operate as an inverting buck-boost converter, essentially providing a voltage step-up function. In addition, designers can create an isolated buck-boost converter using a similar concept.

1. Isolated buck topology

A. +/- Step-down output: circuit operation1

An inverting and non-inverting step-down output can be generated with an isolated buck topology. Figure 1 shows how it delivers a +/- output rail to any application that requires a positive and a negative supply.


Figure 1
Synchronous buck regulator uses isolated buck topology to generate ± Vout rail1

With reference to Figure 1, the primary and secondary outputs are given by the following equations, assuming the leakage inductance of the coupled inductor or transformer and the DC resistance of the windings is negligible:

where VIN is the input voltage, VO1 and VO2 are the primary and secondary outputs, respectively, D is the duty cycle, N is the turns ratio of the transformer, and Vdiode is the forward voltage drop across the diode.

During the cycle when the high side switch is on (current flow indicated by the green arrow in Figure 1), the primary current ramps up and stores the energy in the magnetizing inductance of the transformer and the primary output capacitor. The diode on the secondary side is reverse biased and the load current on the secondary side is supplied by the output capacitor.

During the cycle when the low side switch is on (current flow indicated by the red arrow in Figure 1), the primary current ramps down and releases the stored energy in the magnetizing inductance of the transformer, and the load current on the primary side is supplied by the output capacitor. The diode on the secondary side is forward biased and the current flows from the transformer to supply current to the load, and charges up the secondary output capacitor. At steady state, the voltage at the secondary output is proportionally inverted compared to the voltage at the primary output, assuming the diode voltage drop, transformer winding resistance, and leakage inductances are negligible. Figure 2 shows the operating waveforms for this architecture.


Figure 2
Operating waveforms for a +/- step-down design1


B. +/+ step-down output2

Employing the same concept of generating secondary outputs using a coupled inductor or transformer, the secondary side can be configured differently to generate positive or negative secondary voltages. To generate a positive secondary output, the polarities of the transformer/coupled inductor as well as the secondary side diode are reversed. Figure 3 shows an isolated buck topology to generate a dual +VOUT rail.


Figure 3
Isolated buck topology to generate a dual + VOUT rail2 C. +/+/- step-down output3

Figure 4 shows an isolated buck topology to generate three outputs (dual +VOUT and single –VOUT rail). For a multiple output configuration, the total current of the various outputs reflected to the primary side must accounted for to make sure the IC is able to handle the resultant current.


Figure 4
Isolated buck topology to generate three outputs, dual +VOUT and single –VOUT rail3

The equations for the above circuit are as given below:


Where VO1 is the primary output and VO2 and VO3 are the positive and negative secondary outputs, respectively, D is the duty cycle, N1 and N2 are the turns ratio of the transformer for VO2 and VO3, respectively. Vdiode is the forward voltage drop across the diode. IOUT1, IOUT2 and IOUT3 are the output current drawn from VO1, VO2 and VO3, respectively, IDS_pk is the peak current through the top switch and Δi is the triangular portion of the primary inductor ripple current.

2. Inverting buck-boost (step-up and step-down) topology4

An inverting buck-boost converter can be derived from the synchronous buck converter by connecting its GND terminal as the negative output of the buck-boost converter and the VOUT terminal of the buck converter as the GND of the buck-boost converter. Figure 5 shows the circuit diagram of configuring the ISL85415 buck switcher as an inverting buck-boost converter.


Figure 5
Configuring a buck converter into an inverting buck-boost converter4

The equation for output voltage and output current are as follows:

where VIN is the input voltage, VO1 is the output voltage, D is the duty cycle, IOUT is the output current, and IL is the inductor current.

During the cycle when the high side switch is on (current flow indicated by the green arrow in Figure 5), the inductor current ramps up and stores energy in the inductor and the output capacitor provides current to the load. During the cycle when the low side switch is on (current flow indicated by the red arrow in Figure 5), the inductor current ramps down and provides current to the load as well as charges the output capacitor. Operating waveforms for the inverting buck-boost design are shown in Figure 6.


Figure 6
Operating waveforms for an inverting buck-boost design4


3. Isolated buck-boost topology: +/- output5

A ± step up/down output voltage can be realized using the isolated buck-boost topology. The filter inductor can be replaced with a transformer (or coupled inductor) to obtain a positive secondary output. Figure 7 shows an isolated buck-boost topology to generate a ± step up/down VOUT rail. Figure 8 shows the operating waveforms for the isolated buck-boost design.


Figure 7
Isolated buck-boost topology to generate a ± VOUT rail5.

The voltage and current equations for the above circuit are given below:


where VIN is the input voltage, VO2 is the secondary output voltage, Vdiode is the forward voltage drop across the diode, D is the duty cycle, N is the turns ratio of the transformer, IDS_pk is the peak current through the top switch, Δi is the triangular portion of the primary inductor ripple current, and IOUT1 and IOUT2 are the output current drawn from VO1 and VO2, respectively.


Figure 8
Operating waveforms for an Isolated buck-boost Topology: +/- output5




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