Applying a motor-controller IC for buck-boost conversion

There are two fundamental methods to control the bridge MOSFETs in a non-inverting, buck-boost converter that uses a fixed-frequency PWM. The first method stores energy in the inductor by switching on both Q1 and Q4 simultaneously (Fig. 2). The stored energy is then diverted to the output capacitor by turning on both Q2 and Q3, after Q1 and Q4 have been turned off. This is analogous to the operation of traditional inverting buck-boost and flyback converters. The input-to-output DC transfer function is given by:

With this control method, the inductor must be able to store all of the energy transferred from the input to the output.

The second control method facilitates true buck conversion when the input voltage is higher than the output voltage, and true boost conversion when the input voltage is lower than the output voltage (Fig. 3).

This method is used most by newer non-inverting, buck-boost controllers. In the buck mode design, the duty cycle through Q1 and Q2 is modulated to control the output voltage; Q3 and Q4 are off. As the input voltage decreases and approaches the output voltage, the duty cycle of Q1 approaches 100 percent. Below this point, the converter operates in the boost mode, where Q1 remains on and Q2 remains off, while Q3 and Q4 are modulated. The input-to-output DC transfer function for this control method is given by:

At the boundary condition where the input voltage equals the output voltage, Q1 and Q3 are both on, while Q2 and Q4 are both off. Along with lowering the inductor's energy storage demands, this approach also reduces the ripple current on both the input and output capacitors.

Chips such as the UC3638, on the other hand, were designed for PWM motor drive and amplifier applications. However, the UC3638's programmable oscillator and the structure of its two PWM comparators are well-suited for controlling the full-bridge, buck-boost converter in either of the two previously described operation modes. The oscillator's functional block diagram and PWM comparators are shown in Fig. 4.

Its frequency, as well as average and peak-to-peak magnitudes can be programmed using external components. In operation, the oscillator's triangle-wave output is presented to two separate PWM comparators. One comparator offsets the error amplifier's output with a positive dead- band voltage, while the other uses a negative offset. This dead-band offset is externally adjustable also.