DC-DC Converters: Buck, Boost and Buck-Boost For students.

A DC-DC converter changes one DC voltage into another — stepping 48 V down to 12 V, or boosting 12 V up to 400 V — using an inductor or transformer as temporary energy storage and a transistor switching tens to hundreds of kilohertz. Unlike a linear regulator that burns the excess as heat, a switching converter shuffles energy in packets and routinely exceeds 90% efficiency.

The three basic non-isolated topologies are the buck (step-down), the boost (step-up) and the buck-boost (either, with inverted polarity). Every laptop adapter, solar charge controller, electric-vehicle auxiliary supply and phone power-management chip contains one or more of these three circuits, making them the most-built power converters in the world.

Key Concepts

• Duty cycle D: the fraction of each switching period the transistor is on (0 to 1) — the single control knob setting the output voltage.
• Buck converter: transistor, diode, inductor and capacitor in a step-down arrangement; output is always below input.
• Boost converter: the inductor charges from the input, then dumps its energy into the output at higher voltage; output is always above input.
• Buck-boost converter: inverts polarity and can step up or down; used where the input may swing above and below the desired output.
• Continuous conduction mode (CCM): inductor current never hits zero — the mode in which the simple voltage formulas hold exactly.
• Volt-second balance: in steady state the inductor's volt-seconds must sum to zero over each cycle, which is what derives every converter's voltage ratio.

Formulas

• Buck (CCM): Vout = D × Vin.
• Boost (CCM): Vout = Vin / (1 − D).
• Buck-boost (CCM): Vout = −D × Vin / (1 − D); the minus sign denotes inverted polarity.
• Buck inductor ripple: ΔIL = (Vin − Vout) × D / (L × fsw), with L in henries and fsw in hertz, giving amperes peak-to-peak.
• Power balance: Pin ≈ Pout (ideal); Iin = Iout × Vout/Vin, so boosting voltage always raises input current.

Worked Example

A buck converter steps 48 V down for a 16.8 V battery-charging rail, switching at 50 kHz with a 100 µH inductor. The required duty cycle is D = Vout/Vin = 16.8/48 = 0.35. The inductor current ripple is ΔIL = (48 − 16.8) × 0.35/(100×10⁻⁶ × 50,000) = 10.92/5 ≈ 2.18 A peak-to-peak. For a 5 A load this ripple is about 44% of the DC current — acceptable, and it shrinks if a larger inductor or higher frequency is chosen. The same hardware reconfigured as a boost with D = 0.5 would instead deliver 48/(1 − 0.5) = 96 V.

Common Misconceptions

• "Duty cycle sets the voltage exactly, always." The simple ratios hold in continuous conduction with ideal parts; in discontinuous mode or with heavy losses the ratio shifts and feedback must compensate.
• "Boost converters create free energy." Power is conserved — doubling voltage halves the available current (minus losses), and the input current actually rises.
• "Higher switching frequency is always better." It shrinks the inductor and capacitor but increases switching losses and noise; designers pick the sweet spot, often 50–500 kHz.

Real-World Applications

• Buck converters power every CPU and GPU core from the 12 V rail on a motherboard (multiphase bucks delivering 100+ A at 1 V).
• Boost converters raise a car's 12 V battery to the hundreds of volts needed for LED headlights and audio amplifiers.
• Solar MPPT charge controllers use buck or buck-boost stages to hold panels at their maximum-power voltage while charging batteries.
• Electric vehicles use bidirectional buck-boost stages between the battery and the high-voltage DC link for driving and regenerative braking.

Buck steps down, boost steps up, buck-boost does either with a polarity flip — three circuits, one duty-cycle knob, and the volt-second balance tying them together. Size the inductor for acceptable ripple, close a feedback loop around the duty cycle, and you have the core of nearly every modern power supply.