Converters

The full family of power converters: AC–DC, DC–DC (isolated and non-isolated), DC–AC and AC–AC. Learn how flyback, forward, LLC, dual-active-bridge, cycloconverter and matrix topologies work and how to choose one.

Every conversion — mains to 5 V USB, battery to motor, solar to grid, grid to EV battery — is built from a small set of topologies chained together. Knowing the families and their trade-offs is how engineers select an architecture before designing any circuit.

Isolation (a transformer between input and output) is required for safety whenever mains is involved and a user can touch the output, and it also enables large voltage ratios.

An analogy

Power converters are like a transport network: rectifiers, choppers, inverters and transformers are trains, trucks, ferries and pipelines. You combine them to get energy from where it is, in the form it's in, to where it's needed, in the form it's needed.

Four conversion families

Every converter falls into one of four families.

• AC–DC: rectifiers, PFC stages, SMPS.
• DC–DC: buck/boost (non-isolated); flyback, forward, push-pull, half/full bridge, LLC, DAB (isolated).
• DC–AC: inverters.
• AC–AC: AC voltage controllers, cycloconverters, matrix converters, or AC–DC–AC (VFDs).

Isolated DC–DC topologies

Each topology suits a different power range.

• Flyback: coupled inductor stores energy then releases it — fewest parts; ≤ ~150 W (chargers, auxiliary supplies).
• Forward: true transformer plus output inductor; 100–500 W.
• Half/full bridge and phase-shifted full bridge: 500 W – several kW.
• LLC resonant: soft switching, very high efficiency; server and TV supplies, EV chargers.
• Dual active bridge (DAB): bidirectional power flow; EV and battery storage.

Architecture thinking

A typical 1 kW supply is PFC boost (AC → ~400 V DC) → LLC (isolation + step-down to 48 V or 12 V) → point-of-load bucks. Each stage is chosen for its strengths, and overall efficiency is the product of stage efficiencies.

AC–AC conversion

Cycloconverters synthesise low-frequency AC from segments of the supply using thyristors (large, slow mill drives, ship propulsion). Matrix converters connect each input phase directly to each output phase with bidirectional switches, giving AC–AC with no DC-link capacitor but complex commutation.

Selection criteria

Choose by: power level, voltage ratio, isolation need, bidirectionality, efficiency target, power density, EMI, cost and component stress. Wide-bandgap devices shift the trade-offs toward higher frequency, making soft-switched resonant topologies attractive even at modest power.

Formulas

• Flyback (CCM): V_o = V_in · n · D / (1 − D)
• Forward: V_o = V_in · n · D
• LLC resonant frequency: f_r = 1 / (2π √(L_r C_r))
• Cascaded efficiency: η_total = η₁ · η₂ · η₃ …

Worked example: Flyback turns ratio

Given: V_in = 325 V DC (rectified 230 V); V_o = 12 V; D = 0.4 at nominal input

Find: Turns ratio N_p : N_s.

• 12 = 325 × n × 0.4/0.6 → n = 12/(325 × 0.667) = 0.0554
• N_p : N_s = 1/0.0554 ≈ 18 : 1

Answer: ≈ 18:1

Worked example: Chain efficiency

Given: PFC 97 %; LLC 97.5 %; POL buck 92 %

Find: Overall efficiency and loss for 500 W out.

• η = 0.97 × 0.975 × 0.92 = 0.870
• P_in = 500/0.870 = 575 W → loss = 75 W

Answer: 87 %, 75 W of heat

Common mistakes

• Wrong: Using a flyback for kilowatt power. — Right: Flyback stress and transformer size grow fast above ~150 W; use forward, bridge or resonant topologies.
• Wrong: Forgetting the leakage-inductance spike in flybacks. — Right: Clamp it with an RCD or active clamp, and rate the switch for V_in + reflected voltage + spike.

Applications

• Phone & laptop chargers: Quasi-resonant flybacks with GaN switches pack 65–140 W into pocket-size chargers.
• EV onboard chargers: Bidirectional PFC + DAB/CLLC stages charge the battery and can feed power back (V2G).
• Data centres: Titanium-rated (96 %+) PSUs use bridgeless PFC and LLC stages.

Key takeaways

• Four families: AC–DC, DC–DC, DC–AC, AC–AC.
• Flyback for low power, forward/bridge/LLC for higher, DAB for bidirectional.
• Real products chain stages; efficiencies multiply.
• Choose by power, isolation, ratio, direction, efficiency and cost.

Going further

• Soft-switching families: LLC, phase-shifted full bridge and DAB achieve ZVS using transformer leakage and magnetising inductance, eliminating turn-on loss.
• Magnetics design: Transformer design balances core loss (Steinmetz: P = k·f^α·B^β) against copper loss, including proximity effect; planar magnetics suit high-frequency, low-profile designs.
• Digital control: DSP/MCU controllers run PFC and LLC loops digitally, adding adaptive dead time, burst mode for light load and PMBus telemetry.