Vout1 must be positive. Feedback is derived from this winding.
Enter either VT Product or Isat. If VT is entered, Isat is calculated from VT / Lpri. If VT is blank, VT is calculated from Isat × Lpri.
Please enter all required values. Enter either VT Product or Isat.
RESULTS
Input Parameters Specification
Vin and Vout1Primary input voltage and feedback output voltage used for flyback duty cycle and power transfer calculation.
Lpri and VT ProductPrimary magnetizing inductance and volt-second product define maximum on-time and saturation limit.
Isat and Turns RatioTransformer saturation current and primary-to-secondary ratio determine stored energy and output transfer.
Leakage, Diode and Transistor DropsThese are design inputs kept from the original flyback calculator source for complete SMPS parameter entry.
Practical Operational Examples
Flyback SMPS Design
Estimate switching frequency, duty cycle, on-time, transformer energy and output current for an isolated flyback converter.
Transformer Saturation Check
Use VT product or saturation current with primary inductance to verify whether the flyback transformer can safely store energy.
Power Supply Repair
Compare known transformer VT product, input voltage and turns ratio with expected power and current limits.
Converter Frequency Planning
Use calculated minimum frequency to check whether the design can operate without exceeding transformer saturation.
Diagrams & Theory
A flyback regulator stores energy in the primary magnetizing inductance when the switch is on, then transfers that energy to the secondary through the diode when the switch turns off.
Formulas & Mathematical Logic
Step 1: Convert primary inductance Lpri from µH to H, VT from V·µs to V·s, and use absolute values for Vin, Vout and diode drop.
Step 2: If VT product is entered, Isat = VT / Lpri. If VT is blank, VT = Isat × Lpri.
Step 3: Duty Cycle = Vout1 / (Vin × N).
Step 4: Tonmax = VT / Vin.
Step 5: Minimum switching frequency F = Duty Cycle / Tonmax.
Step 6: Energy per cycle En = Lpri × Isat² / 2.
Step 7: Maximum power P = F × En.
Step 8: Output current Io1 = P / Vout1 and input current Iin = P / Vin.
Step-by-Step Example
Example: Vin = 24 V, Vout1 = 12 V, Lpri = 100 µH, VT = 120 V·µs, turns ratio N = 2, diode drop = 0.7 V, transistor drop = 0.2 V.
Step 1: Convert Lpri = 100 µH = 0.0001 H and VT = 120 V·µs = 0.00012 V·s.
Step 2: Isat = VT / Lpri = 0.00012 / 0.0001 = 1.2 A.
Step 3: Duty = Vout1 / (Vin × N) = 12 / (24 × 2) = 0.25.
Step 4: Tonmax = VT / Vin = 0.00012 / 24 = 5 µs.
Step 5: Frequency = Duty / Tonmax = 0.25 / 5 µs = 50 kHz.
Step 6: Energy and power are calculated from Lpri, Isat and switching frequency.
How to Use This Calculator
Enter primary voltage Vin and output voltage Vout1.
Enter primary inductance in µH.
Enter either VT Product or Isat. One of these two fields is required.
Enter leakage inductance, turns ratio, diode drop and transistor drop.
Click Calculate to get Ton max, duty cycle, minimum frequency, energy, power and current limits.
About This Calculator
Estimate flyback regulator timing, power and transformer current limits.
The CalcBoy Flyback Switch Mode Regulator Calculator calculates duty cycle, on-time, minimum frequency, stored energy, maximum power and current from transformer and input parameters.
A flyback switch mode regulator stores energy in the primary magnetizing inductance during switch on-time. When the switch turns off, that stored energy is transferred to the output through the secondary winding and diode. This makes transformer VT product, saturation current, primary inductance and turns ratio critical design values.
This calculator is useful for isolated flyback SMPS design, small DC-DC converters, auxiliary supplies, transformer checking, repair comparison and early power stage estimation. It lets you enter either VT product or saturation current. If VT is known, the calculator derives Isat. If Isat is known instead, it calculates the matching VT product from Lpri and Isat.
Use the result as a starting point. Real flyback designs also depend on leakage inductance, snubber design, MOSFET drain voltage, diode recovery, output capacitor ripple, transformer air gap, core saturation, winding resistance, thermal rise, insulation and PCB layout. Always verify final hardware with waveform measurement and component datasheets.
Best UseFlyback SMPS timing and transformer energy estimate.
Supported InputsVin, Vout1, Lpri, VT, Isat, leakage inductance, turns ratio and voltage drops.
Helpful ForIsolated supplies, auxiliary SMPS, transformer checks and repair analysis.
Design ReminderCheck MOSFET drain spike and transformer saturation on real hardware.
Tip: In flyback designs, leakage inductance can create a large drain voltage spike. Always check the waveform and snubber/clamp rating, not only calculated power.
Frequently Asked Questions
What does this calculator find?
It calculates flyback duty cycle, maximum on-time, minimum frequency, energy per cycle, maximum power and input/output current estimates.
What is VT product?
VT product is volt-second capability. It helps define how long voltage can be applied to the primary before reaching saturation.
Can I enter Isat instead of VT?
Yes. Enter either VT product or saturation current. If one is missing, the calculator derives it from primary inductance.
What is Ton Max?
Ton Max is the maximum switch on-time calculated from VT product and input voltage.
Does this include leakage inductance effects?
Leakage inductance is an input, but the preserved source logic does not directly use it in the displayed output calculations.
Can this replace full flyback SMPS design?
No. It is an estimate. Final design must verify transformer construction, snubber, MOSFET stress, diode rating, ripple and thermal behavior.
Related Calculators
Flyback Transformer Power Supply Design CalculatorEstimate flyback winding turns, inductance and currents.
Snubber Circuit Design CalculatorCalculate RC snubber values for flyback voltage spikes.
Inductor Current and Maximum Power CalculatorCheck current ramp, saturation and energy storage.
SMPS Duty Cycle CalculatorEstimate duty cycle for switching regulator topologies.