Power supply designers should be aware that all switchmode supplies are made up of buck or boost regulators (or combinations of buck/boost regulators) with or without dc transformers. An excellent book by Severns and Bloom presents a convincing case.
Fig. 1 shows typical buck and boost regulators with their respective input and output current waveforms. When combined with a dc transformer, isolation may be provided and voltage transformation may be obtained. However, the waveforms and transfer functions of the basic buck and/or boost regulator are retained in any combination.
The buck regulator is characterized by a discontinuous input current and a continuous output current (with some ripple current, depending on the size of L1), while the boost regulator has a continuous input current and a discontinuous output current.
In low-voltage, high-current applications the discontinuous current can present a major problem for the smoothing capacitors C1 or C2. Assume we choose to use a buck regulator to deliver 100 A at 12 V from a 24-V supply. The output current would have a mean value of 100 A, and typically the triangular ripple current would be 10% (10 A peak-to-peak). This can be easily smoothed with a reasonable-size electrolytic capacitor bank. However, the 100-A square wave input current would require a huge bank of smoothing capacitors to adequately decouple the large discontinuous input current.
Fig. 2 shows how two buck regulators may be combined to virtually eliminate the smoothing requirement. The first advantage is that each regulator provides only 50 A, reducing the total I2R loss by 50%. But more important, for the example given above, the input for each regulator would be a true square wave with a conduction period of 50% of the total cycle. By phase shifting the drive to the lower section Q1(b) by 180°, the current at the input summing point “A” will be a near-continuous dc current of 50 A at 24 V. On the output side at summing point “B”, the ripple current also sums to a steady dc current of 100 A at 12 V output. (The current in L1(a) increases as the current in L1(b) decreases and inversely.)
Although this ideal condition will clearly degrade if the input and output voltages differ from the ideal 2:1 ratio (causing some gap or overlap in current pulses), generally the ripple current will still be much reduced and much smaller smoothing capacitors may be used. Where the ratio is not ideal, the duty cycle may be adjusted to the ideal 50% duty at the actual nominal working voltage, by tapping the flywheel diode D1 into the inductor, as shown in Fig. 3a. Further, if the input is near three times the output, then three regulators can be used phase shifted by 0 degrees, 120 degrees and 240 degrees.
It should be remembered that the buck regulator has very low output impedance, and current mode control or forced current sharing will be required to ensure each half shares the load current equally. For a 180-degree phase shift, a standard dual-output control IC can be used. For 120 degrees and other more complex phase shifting, a ring counter can normally be configured to give the required spacing to the drive pulses.
The same approach can be applied to the boost regulator. But in this case, the major advantage would be the summing of the output current pulses. Once again, for non-ideal ratios, the inductor may be tapped in a similar way to the buck regulator to give near-square output pulses as shown in Fig 3b.
Adding a DC-DC Transformer
Fig. 4a shows the basic elements of a dc-dc transformer. DC is applied to the input, and Q1 and Q2 are driven with a square wave drive to switch alternately into the push-pull primary. Full-wave rectification provides a dc output. Notice the input current and output current are continuous.
Fig. 4b shows that by adding an output inductor L1 and diode D1 and pulse width modulating Q1 and Q2, the action of the dc transformer and a buck regulator are combined in a single unit. Notice the input is now discontinuous and the output continuous, as found with the normal straightforward buck regulator. Also notice D1 can be eliminated, as D2 and D3 will provide the flywheel action. The advantage of the transformer is to provide isolation and the opportunity to ratio the voltages. By choosing the transformer ratio correctly, the input current can be made into a square wave, allowing two such units to be combined in anti-phase to minimize the ripple currents. Many such combinations are possible- using buck, boost, buck-boost and boost-buck combinations, with or without dc transformers, to form all known pulse-width modulated switch-mode power supplies.
References:
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Modern DC-DC Switchmode Power Converter Circuits by Severns and Bloom, published by Van Nostrand Reinhold Company.
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