Skip to content
RFrftools.io
Power

Flyback Converter Calculator

Calculate flyback converter turns ratio, peak primary and secondary currents, and power levels for isolated DC-DC converter design.

Loading calculator...

Formula

N=Vin⋅DVout(1−D),Ip,peak=2IinDN = \frac{V_{in} \cdot D}{V_{out}(1-D)},\quad I_{p,peak} = \frac{2 I_{in}}{D}

Reference: Unitrode Power Supply Design Seminar SEM600

N— Turns ratio Np/Ns
D— Duty cycle
Vin— Input voltage (V)
Vout— Output voltage (V)

How It Works

The flyback converter calculator computes turns ratio, peak currents, and magnetizing inductance for isolated DC-DC conversion — essential for AC-DC adapters, PoE systems, and multi-output power supplies. Power electronics engineers, charger designers, and industrial equipment developers use this tool to achieve galvanic isolation while maintaining 85-92% efficiency. According to Erickson & Maksimovic's 'Fundamentals of Power Electronics', flyback converters dominate the <75 W isolated supply market due to their single magnetic component and low parts count. The coupled inductor stores energy during the switch on-time (Epri = ½Lm×Ipk²) and transfers it to the secondary during off-time. The turns ratio N = Vin×D / ((Vout+Vf)×(1-D)) determines voltage transformation, while boundary conduction mode (BCM) at 50% duty cycle maximizes power transfer capability. TI application note SLVA057 specifies magnetizing inductance Lm = Vin×D/(fsw×ΔIL) for continuous conduction mode. Critical design parameter: the leakage inductance (typically 1-3% of Lm) causes voltage spikes exceeding 2×Vin on the primary switch, requiring RCD snubber or active clamp circuits per Power Integrations AN-19.

Worked Example

Design a 12 V to 5 V/2 A isolated flyback converter for industrial sensor interface. Requirements: 3.75 kV isolation, 88% minimum efficiency, <100 kHz switching frequency. Step 1: Calculate turns ratio — At D = 0.4, Vf = 0.5 V (Schottky): N = 12×0.4 / ((5+0.5)×0.6) = 1.45. Use N = 1.5 for standard winding ratio. Step 2: Calculate primary peak current — Pout = 10 W, η = 0.88. Pin = 11.4 W. At D = 0.4, Ipk = 2×Pin/(Vin×D) = 2×11.4/(12×0.4) = 4.75 A. Step 3: Select magnetizing inductance — For 30% ripple in CCM at 100 kHz: Lm = 12×0.4/(100k×0.3×4.75) = 33.7 µH. Use 33 µH. Step 4: Calculate output capacitor — For 50 mV ripple: Cout = 2×0.4/(100k×0.05) = 160 µF. Use 2×100 µF low-ESR electrolytics. Step 5: Snubber design — Leakage inductance ≈ 1 µH (3% of Lm). Peak voltage without snubber: Vin + N×(Vout+Vf) + Lleak×dI/dt = 12 + 8.25 + 1µ×4.75A/100ns = 67.5 V. Use 100 V MOSFET with RCD snubber (R=10k, C=1nF, D=UF4007).

Practical Tips

  • ✓Per Fairchild AN-4137, use quasi-resonant (QR) switching to achieve valley switching, reducing turn-on losses by 50% and EMI by 10 dB compared to fixed-frequency PWM
  • ✓Design the RCD snubber to clamp voltage spike at 80% of MOSFET Vds(max) — for 100 V MOSFET, clamp at 80 V; dissipated power = ½×Lleak×Ipk²×fsw = ½×1µH×25×100k = 1.25 W
  • ✓Use planar transformers for power densities >10 W/cm³ — PCB-integrated windings achieve 1% leakage inductance versus 3-5% for bobbin-wound transformers

Common Mistakes

  • ✗Ignoring transformer leakage inductance — 2% leakage (1 µH) with 5 A turn-off in 50 ns generates 100 V spike; without snubber, this destroys 60 V MOSFETs within microseconds
  • ✗Undersizing the transformer core — flyback transformers must not saturate at peak current; EE16 cores handle only 15-20 W at 100 kHz; use RM8 or EE25 for 50 W designs
  • ✗Using standard diodes for secondary rectification — PN diodes exhibit 100 ns reverse recovery, causing 5-8% efficiency loss; Schottky diodes (SS34, 40 V/3 A) essential for <24 V outputs

Frequently Asked Questions

Per TI's 'Designing Flyback Converters' guide, flyback offers: (1) galvanic isolation with single magnetic component versus 2-3 for forward/bridge topologies, (2) inherent multiple output capability with single transformer, (3) wide input range (85-265 VAC) compatibility, (4) low component cost at <75 W. Disadvantages: higher output ripple (5-10% vs 1-2% for forward), limited to 150-200 W due to discontinuous secondary current.
Per Unitrode (now TI) design seminar: Flyback stores energy in transformer air gap during on-time, transfers during off-time — no output inductor needed but discontinuous output current causes higher ripple. Forward converter transfers energy continuously — requires output inductor and transformer reset mechanism (third winding or active clamp). Forward preferred above 100 W for lower ripple; flyback preferred below 75 W for simplicity and cost.
Per Power Integrations design guide: 65-132 kHz for universal AC input (avoiding AM radio band), 200-400 kHz for DC-DC applications where EMI filtering is easier. Higher frequency enables smaller magnetics (core size ∝ 1/fsw^0.5) but increases switching losses. Quasi-resonant designs vary frequency with load, typically 30-130 kHz.
Saturation occurs when magnetic flux density B = Lm×Ipk/(N×Ae) exceeds core material limit (typically 300-400 mT for ferrite). Solutions: increase turns count (N), select larger core area (Ae), add air gap (reduces effective permeability, increases saturation margin). Per Ferroxcube design guide, 0.5 mm air gap in E25 core allows 3× higher DC current before saturation.
Per CISPR 32 Class B compliance guidelines: (1) use Y-capacitors (4.7 nF/250 VAC) between primary and secondary ground for common-mode filtering, (2) add pi-filter on AC input (2×4.7 µF + 1 mH common-mode choke), (3) implement spread-spectrum frequency modulation (±5%) to reduce peak emissions by 10 dB, (4) shield transformer with copper foil between primary and secondary windings.

Shop Components

As an Amazon Associate we earn from qualifying purchases.

DC-DC Buck Converter Modules

Adjustable step-down converter modules for bench and prototype use

Amazon →

LDO Voltage Regulator Kit

Assorted low-dropout linear regulators for prototyping

Amazon →

Electrolytic Capacitor Kit

Aluminum electrolytic capacitor kit for power supply filtering

Amazon →

Power Inductor Kit

Assorted shielded power inductors for switching supply designs

Amazon →

Related Calculators

Power

Transformer Turns Ratio

Calculate transformer turns ratio, secondary current, apparent power, and real power delivered. Accounts for transformer efficiency.

Power

Buck Converter

Design a synchronous buck (step-down) converter: calculate duty cycle, inductor value, output capacitor, and input capacitor.

Power

MOSFET Power Loss

Calculate MOSFET conduction loss, switching loss, total power dissipation, junction temperature, and efficiency for power electronics design

Power

Voltage Divider

Calculate voltage divider output voltage, current, Thévenin impedance, and power dissipation from Vin, R1, and R2. Ideal for bias networks and level shifting.