Forward Converter - Duty Cycle and Output Inductor

A forward converter calculator solves the steady-state design equations for a transformer-isolated, single-switch step-down DC-DC converter. It switches current through a transformer during the on-time and uses a freewheeling diode and output inductor to keep load current flowing during the off-time. The page works in continuous conduction mode and returns duty cycle, turns ratio, output inductor, and output capacitor.

• • Designing an isolated 24 V to 5 V supply: Pick the turns ratio, output inductor, and capacitor for a 24 V bus feeding a 5 V sensor front end with isolation.
• • Verifying a textbook switching-converter design: Confirm hand-calculated duty cycle and inductor against Vout = Vin*(Ns/Np)*D and Lo = Vout*(1-D)/(f*dIL) before choosing a controller IC.
• • Teaching the magnetizing reset constraint: Show why a single-switch forward must keep D below Np/(Np+Nr) and how the reset winding demagnetizes the core each cycle.

Pick a forward converter when you need isolation together with output power above roughly 100 W. Below that range a flyback is usually cheaper because it stores energy in the transformer itself, and a non-isolated buck is the right call any time you can drop isolation. This page targets the single-switch forward with a reset winding, the most common 24 V to 5 V industrial supply.

For the non-isolated step-up sibling that shares the same PWM averaging approach, Boost Converter returns duty cycle and inductor from Vin, Vout, and switching frequency.

The forward converter calculator applies volt-second balance to the output inductor and charge balance to the output capacitor in continuous conduction mode. The turns ratio scales the input voltage, and the magnetizing-reset constraint limits D to Np / (Np + Nr) on a single-switch design.

• Vin: DC supply voltage feeding the primary.
• Vout: Average DC output voltage at the load.
• Ns/Np: Secondary-to-primary turns ratio; values below 1 step the voltage down.
• D: Switch duty cycle, fraction of each period the primary transistor is on.
• f: Switching frequency in hertz, set by the controller IC.
• Pout: Output power delivered to the load in watts.

Once D is set, Iout = Pout / Vout and the load resistance R = Vout / Iout follow. L = Vout * (1 - D) / (f * dIL) holds the inductor ripple, and C = dIL / (8 * f * dVout) holds the output ripple. The primary peak current adds the reflected load current and half the reflected ripple, setting the stress on the switching transistor.

According to MIT OpenCourseWare 6.622 Power Electronics, applying volt-second balance to the forward converter output inductor in continuous conduction mode gives the steady-state transfer function Vout/Vin = (Ns/Np) * D.

The output stage of a forward converter is essentially a buck, so Buck Converter returns the same inductor and capacitor values from the same ripple budgets.

Four concepts describe how the page treats a transformer-isolated switching converter. None require a derivation; each explains what it means inside a design.

If the load is so light that the output inductor current touches zero, the converter enters discontinuous conduction mode and Vout = Vin*(Ns/Np)*D no longer holds. To stay in CCM, raise the switching frequency or lower the inductor ripple fraction so the average inductor current stays above half of its peak-to-peak ripple.

Where a forward converter uses a separate output inductor after the transformer, Flyback Converter uses a coupled inductor that doubles as the energy-storage element.

Six steps walk through a typical design pass with this forward converter calculator. The example uses 24 V in, 5 V out, a 0.5 turns ratio, 100 kHz, and 5 W.

1. 1 Enter the input voltage: Type the DC supply feeding the primary. Typical values are 12 V, 24 V, or 48 V for industrial buses.
2. 2 Enter the target output voltage: Type the DC voltage the load needs; the page returns the duty cycle once Vin, Ns/Np, and Vout are in.
3. 3 Enter the transformer turns ratio: Type the secondary-to-primary turns ratio Ns/Np. Use 0.1 to 0.5 for most step-down designs.
4. 4 Enter the reset winding ratio: Type Nr/Np. Use 1.0 for a single-switch forward (D_max = 0.5); use 0 for a two-switch topology.
5. 5 Enter the switching frequency and load power: Type the switching frequency from the controller datasheet and the output power the load draws.
6. 6 Set ripple fractions and read filter values: Pick an inductor ripple fraction (0.2 to 0.4) and an output ripple fraction (0.01 to 0.05), then read Lo and Co as a starting point.

For a 24 V industrial bus feeding a 5 V, 5 W isolated sensor supply at 100 kHz with Ns/Np = 0.5 and a single-switch reset winding, the calculator returns D = 41.7 percent, Lo = 88 uH, Co = 4.16 uF, and a primary peak current of about 1.1 A.

Five reasons students and engineers prefer this tool over solving the forward-converter equations by hand each time.

• • Saves design time: Returns duty cycle, turns ratio, output inductor, and output capacitor in one pass, so you can move from spec to bill of materials quickly.
• • Catches invalid designs early: Flags duty cycles at or above 1, switching frequencies at zero, and single-switch designs that exceed the 0.5 magnetizing-reset limit before you spin a board.
• • Connects theory to numbers: The same Vout = Vin*(Ns/Np)*D transfer function taught in class becomes a concrete inductor and capacitor for a specific Vin, Vout, and load.
• • Makes ripple budgets visible: Entering an inductor ripple fraction and an output ripple fraction turns an abstract spec into concrete filter values.
• • Supports homework and lab prep: Students can confirm hand calculations before building a converter, which makes troubleshooting easier when measurements disagree with the prediction.

Treat the output as a starting point rather than a final answer. Real forward converters also need to budget for switch on-resistance, transformer leakage, snubber losses, and the controller current limit. Fine-tune the transformer and output filter on the bench to confirm ripple stays inside limits.

Five factors drive the design numbers. The first three are inputs the page already accepts; the last two are real-world caveats.

• • The page assumes continuous conduction mode. At light load the output inductor current may touch zero, and the Vout = Vin*(Ns/Np)*D relation no longer holds.
• • The model uses an ideal lossless converter. Switch on-resistance, transformer copper loss, core loss, snubber loss, and rectifier forward drop are not modelled, so reserve headroom on the duty cycle and on the input current limit of the chosen controller.

If the duty cycle returned is above about 0.45 on a single-switch design, the converter is close to the magnetizing-reset limit; lower the turns ratio, raise the switching frequency, or pick a two-switch topology. Below about 0.1 the step-down ratio is small and a non-isolated buck converter will usually be cheaper.

According to Texas Instruments application note SLVA477, the buck-class output inductor required to hold the peak-to-peak ripple current dIL is L = Vout * (1 - D) / (f * dIL).

According to Texas Instruments SLLA284 forward converter design brief, the single-switch forward converter must reset its magnetizing current each cycle, which caps the duty cycle at Np / (Np + Nr).

Once the calculator returns the equivalent load resistance, Ohm's Law Calculator covers the underlying V = IR relationship and any related circuit analysis.