What are the types of boost converter topologies?
Primarily, boost converter ICs are used in battery-operated or portable electronic devices to "boost" or "step-up" the battery voltage to a higher level. These non-isolated converters employ an inductor. Also, there is a galvanic path between the input and output. A charge pump (switched capacitor) converter would be another transformer-less approach. Here, we will only consider the switch-mode boost converter. We'll cover the charge pump boost converter in a future edition of Power Design FAQs.
Do isolated boost converter topologies exist?
Isolated step-up converters employ a flyback (buck-boost) or forward converter (buck) topology. Isolation requires a transformer, so any desired step-up or step-down ratio depends on the transformer turns ratio.
How does the basic transformer-less switch-mode boost converter IC work?
In the basic boost IC circuit, closing the switch allows current to build up in the inductor (Fig. 1). When the switch opens, the stored inductor energy freewheels into the output through the boost diode. After several switching cycles, the steady dc output voltage developed across the output capacitor is higher than the input voltage.
What determines the output voltage of the boost dc-dc converter?
In the more detailed circuit in Figure 2, a power MOSFET replaces the mechanical switch. A pulse-width modulator (PWM) control circuit drives the MOSFET to ensure a constantly regulated output. The duty cycle determines the output voltage, which is the ratio of the switch on-time to the entire switching cycle time. For a 50% duty cycle, the calculated output voltage is approximately twice the input voltage.
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