DC-DC converters — buck, boost, buck-boost

Reference notes

Use Next → to walk through the three classical non-isolated DC-DC topologies and the L / C sizing rules.

The building blocks

Every DC-DC converter has four core elements:

• Switch (MOSFET or IGBT) — turns ON and OFF at high frequency (50 kHz – 2 MHz).
• Inductor L — smooths current.
• Capacitor C — smooths output voltage.
• Diode D — provides a return path when the switch is OFF (replaced by a second MOSFET in synchronous designs).

Control variable is the duty cycle D — fraction of each period the switch is ON. 0 < D < 1.

The three classical non-isolated topologies (CCM steady state)

Buck — the workhorse

Switch on: V_in across L → current ramps up. Switch off: L's stored energy keeps current through the diode to the load. Average inductor voltage = 0 → V_out = D·V_in.

Used as point-of-load converters everywhere — laptop chargers, smartphone power management, GPU rails. Dominant topology by volume.

Boost — when you need MORE voltage

Switch on: L sits across V_in → current ramps up, energy stored. Switch off: L's voltage flips, adds to V_in, pushes current through diode to a higher V_out. V_out = V_in/(1-D).

Used: PV MPPT inverter front-end (steps panel V_oc up to DC bus), battery-to-bus converters, LED drivers, PFC pre-regulators.

Buck-boost — inverting

Switch on: V_in connects to L (stores energy). Switch off: L discharges into C and load with REVERSED polarity. |V_out| can be larger or smaller than V_in depending on D.

Non-inverting alternatives that achieve the same buck-boost capability: Cuk converter and SEPIC converter (single-ended primary-inductor converter).

CCM vs DCM

• CCM (Continuous Conduction Mode): inductor current never reaches zero. The clean V_out = f(D) equations hold.
• DCM (Discontinuous Conduction Mode): at light load, i_L ramps to zero during the OFF interval; control transfer function depends on D AND load.

Boundary: a buck converter stays in CCM as long as average load current > ΔI_L/2. Designers usually size L to maintain CCM down to ~10–20 % of full load.

Sizing the inductor

Rule of thumb: target ΔI_L ≈ 30 % of the average load current. Smaller ripple → bigger L (more copper, larger footprint, slower transient response). Larger ripple → smaller L but higher conduction losses and risk of DCM at low load.

Sizing the output capacitor

Real capacitors have non-zero series resistance (ESR), so the ESR-driven ripple component is usually larger:

Total ripple = max of these two terms. For low ripple, use low-ESR ceramic or polymer capacitors (~1–10 mΩ) at the output. Aluminum electrolytics typically run 50–500 mΩ ESR and contribute most of the visible ripple.

Modern variants briefly

• Synchronous: diode replaced by a second MOSFET. Eliminates the diode V_F drop (0.3-0.5 V) — huge efficiency gain at moderate-to-high currents.
• Multi-phase: parallel converters interleaved at 1/N period offsets. Reduces input/output ripple by √N, increases effective f_sw.
• Isolated: flyback, forward, push-pull, half-bridge, full-bridge — add a transformer for galvanic isolation. Required by safety standards in line-powered supplies.

Keyboard shortcuts

• → next  ·  ← previous  ·  on canvas: click +/− to change D, click topology toggle to switch between buck/boost/buck-boost