Boost Converter Duty Cycle Calculator – Step-Up D and Iin

The way a boost converter works is that it have an inductor which stores energy and releases that energy. In order for this to work, there’s a parameter called a duty cycle. This determine the length of time the switch is closed. If you get this wrong your components could heat up or your output voltage might decrease.

Duty cycle D represent the fraction of time main transistor stays closed; it’s equal to one minus the input voltage divided by the output voltage. That equation demonstrate how far voltage needs to be bumped up. To go from 3.7 volts to 5 volts, for instance, you’d need to use a 26-ish-percent duty cycle. That means the switch is engaged for approximately a quarter of every cycle and not switched for remainder. There’s an online calculator that will do all the math for you. When designing under time crunch, it saves you from possibility of making calculation error.

How Boost Converters Work

Output current are not equal to input current. Remember that energy must be conserved: Input power = Output power, Losses. Since the boost converter raise the voltage, it must draw more current from the source than it delivers to the load. You can’t get away with it. To get a 5-volt output at one ampere while boosting the input to 3.7 volts, you must draw about 1.35 amperes from your battery. That added current gets routed through your switch and inductor, resulting in increased heating.

With increasing duty cycle (above 80 percent), denominator in the equation decreases, meaning input current will rise quickly. For reliability, it’s best to maintain some safety margin. Unless your application has plenty of thermal headroom, don’t push that boost converter above an 85 percent duty cycle. The input current explode as the duty cycle climbs higher because denominator in that equation shrinks rapidly; see the reference table for the multiplier. For 90 percent duty, the input current will be ten times then output current. Your inductor has to handle that surge without saturating. And the switch cannot remains turned-on too long or it’ll short out the supply. All of these affect the long-term stability of the circuit.

How big do components have to be? That depends off how often you switch them. The higher your switching frequency, the smaller inductor you can use (because on-time per cycle is shorter). At a 500 kilohertz design, it switch on and off in microseconds instead of milliseconds. But as switching frequency goes up, so do the switching losses. It is a choice between how efficient you want to be and how small you want to go. Because of this, most compact modules runs somewhere in the 250-1 megahertz range. It is a trade-off between heat dissipation and device size.

And then there’s efficiency, which changes the necessary duty cycle a bit. In the real world, things aren’t ideal: your diode drops some voltage; your inductor has resistance; your switch gets hot. To account for inefficiency, you can tweak the duty equation in the calculator. Because some of the input energy will be lost, a 90 percent efficient system would of need a larger duty cycle than the idealized equation predicts. Otherwise, it will produce less than desired output voltage when loaded.

The boost converter can only step up voltage. It will only do that if your input voltage is lower then the desired output voltage. In that case, you should use a buck or a buck-boost converter. This is a condition flagged by the tool automatically. That saves you from making an error in calculations.

When designing a power stage, you are working with tradeoffs among efficiency, heat, and size. Getting the duty cycle correct first help make it easier to select other furnitures.