Two basic IC topologies are employed in dc power sources. If the output is lower than the input voltage, the IC is said to be a step-down, or buck converter. If the output is higher than the input voltage, the IC is said to be a step-up, or boost converter.
Buck, or step-down topology is a non-isolated power management configuration shown in Fig. 1.Its advantages are simplicity and low cost. However, it has a limited power range and its direct dc path from input to output can pose a problem if there is a shorted power switch.
Fig. 1. Non-isolated step-down converter.
In the simplified circuit (Fig. 1), the regulator IC accepts a dc input, converts it to a PWM (pulse width modulator) switching frequency that controls the output of the power MOSFET (Q1). An external Schottky rectifier, inductor and output capacitors produce the regulated dc output. The regulator IC compares a portion of the rectified dc output with a voltage reference (VREF) and varies the PWM duty cycle to maintain a constant dc output voltage. If the output voltage tends to increase, the PWM reduces its duty cycle causing the output to reduce and keeping the regulated output at its proper voltage. Conversely, if the output voltage tends to go down, the feedback causes the PWM duty cycle to increase and maintain the regulated output.
Fig. 2. Non-isolated inductive-boost dc/dc converter.
As shown in the simplified inductive-boost dc-dc converter circuit (Fig. 2), turning on the power MOSFET causes current to build up through the inductor. Turning off the power MOSFET forces current through the diode to the output capacitor. Multiple switching cycles build the output capacitor voltage due to charge it stores from the inductor current. The result is an output voltage higher than the input.
In Fig. 2, the PWM control turns the MOSFET on and off. Without feedback, the PWM duty cycle determines the output voltage, which is twice the input for a 50% duty cycle. Stepping up the voltage by a factor of two causes the input current to be twice the output current. In a real circuit with losses the input current is slightly higher.
Its advantages are simplicity, low cost and the ability to step-up the output without a transformer. Disadvantages are a limited power range and a relatively high output ripple due to the off-time energy coming from the output capacitor.
Inductor selection is a critical part of this boost circuit design, because the inductance value affects input and output ripple voltages and currents. An inductor with low series resistance provides optimal power conversion efficiency. Choose the inductor’s saturation current rating so that it is above the steady-state peak inductor current of the application.
To ensure stability for duty cycles above 50%, the inductor requires a minimum value determined by the minimum input voltage and maximum output voltage. This depends on the switching frequency, duty cycle, and on-resistance of the power MOSFET.
Fig. 3. Simplified forward converter can operate as a step-up or step-down converter. Theoretically it should use an” ideal” transformer with no leakage fluxes, zero magnetizing current and no losses.
Forward converter topology (Fig. 3) is essentially an isolated version of the buck converter. Use of a transformer allows the forward converter to be either a step-up or step-down converter, although the most common application is step-down. The main advantages of the forward topology are its simplicity and flexibility.
Fig. 4. Flyback converter’s transformer has an air gap, enabling it to store energy during the on-time and deliver the energy to the diode during to off-time.
Another transformer-isolated topology, the simplified flyback converter (Fig. 4) operates in the indirect conversion mode. Flyback topology is one of the most common and cost-effective means for generating moderate levels of isolated power in ac-dc converters. It has greater flexibility because it can easily generate multiple output voltages by adding additional secondary transformer windings. A disadvantage is that regulation and output ripple are not as tightly controlled as in some of the other topologies and the stresses on the power switch are higher.