# Boost Switching Converter Design Equations

Posted on Feb 6, 2014

Where VD is the voltage drop across the diode, and VTrans is the voltage drop across the transistor. Note that the continuous/discontinuous boundary occurs when io is zero. The main question when designing a converter is what sort of inductor should be used. In most designs the input voltage, output voltage and load current are all dictated by

the requirements of the design, whereas, the Inductance and ripple current are the only free parameters. It can be seen form Equation 1, that the inductance is inversely proportional to the ripple current. In other words, if you want to reduce the ripple, then use a larger inductor. Thus, in practice a ripple current is decided upon which will give a reasonable inductance. There are tradeoffs with low and high ripple current. Large ripple current means that the peak current is ipk greater, and the greater likelihood of saturation of the inductor, and more stress on the transistor. So when choosing an inductor make sure that the saturation current of the inductor is greater than ipk. Likewise, the transistor should be able to handle peak current greater than ipk. The inductor should also be chosen such that the it can handle the appropriate rms current. It should be noted that when there is a light load the circuit can slip into discontinuous mode, where the inductor becomes fully discharged of it`s current each cycle. When a load is reapplied the inductor needs to recharge, and so the transistor`s duty cycle increases pulling the inductor towards ground, and because of the increased duty cycle Vout decreases when we really want it to increase. This causes an instability, which is well known for boost converters, and not a problem with buck converters. One way to combat this instability is to choose a large enough inductor...

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