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Tutorial: Forward Converter

Design and simulate an isolated forward converter.

Difficulty: Intermediate (⅔) | Duration: 30 minutes

Theory

Topology

The forward converter is an isolated version of the buck converter:

              Np : Ns
    Vin ──[FET]──●══════●──|>|──[L]──●── Vout
                 ║      ║       │      │
                 ║  Lm  ║     [D2]   [C]  [R]
                 ║      ║       │      │    │
                GND─────●──────●──────●────┘

Unlike the flyback, energy transfers during the switch ON time.

Operating Principle

Switch ON: Power transfers through transformer to secondary. Output diode D1 conducts.

\[V_{sec} = n \cdot V_{in}, \quad V_L = n \cdot V_{in} - V_{out}\]

Switch OFF: Transformer resets. Freewheeling diode D2 conducts. Inductor supplies load.

\[V_L = -V_{out}\]

Voltage Conversion

\[V_{out} = n \cdot D \cdot V_{in}\]

Where n = Ns/Np.

Key Difference from Flyback

Feature Forward Flyback
Energy transfer During ON time During OFF time
Output filter LC filter needed Capacitor only
Output ripple Lower (LC filtered) Higher
Core utilization Unipolar (needs reset) Unipolar
Power level Medium (50-500W) Low-Medium (5-150W)

Core Reset

The forward converter needs a mechanism to reset the transformer core each cycle. With third winding:

\[D_{max} = \frac{N_p}{N_p + N_r}\]

For Np = Nr: D_max = 0.5

Design Example

Specifications

Parameter Value
Input voltage 48V DC
Output voltage 12V DC
Output power 100W
Switching frequency 200 kHz
Max duty cycle 0.45

Calculations

Turns ratio:

\[n = \frac{V_{out}}{D \cdot V_{in}} = \frac{12}{0.45 \times 48} = 0.556 \approx 1:1.8\]

Output inductor:

\[L = \frac{(n \cdot V_{in} - V_{out}) \cdot D}{f_{sw} \cdot \Delta i_L} = 16\ \mu H\]

Output capacitor:

\[C = \frac{\Delta i_L}{8 \cdot f_{sw} \cdot \Delta V_{out}} = 8.7\ \mu F\]

Exercises

Exercise 1: Duty Cycle Sweep

Vary D from 0.1 to 0.45. Plot V_out and compare with theory.

Exercise 2: Forward vs Flyback

Build both topologies for the same specs. Compare output ripple and switch stress.

Exercise 3: Core Reset

Observe the magnetizing current waveform. Verify the core resets before the next ON period.

Next Steps