Forward Converter¶

Overview¶

The forward converter is an isolated DC-DC topology derived from the buck converter. Unlike the flyback, it transfers energy to the output during the switch-on time and requires a transformer reset mechanism. Forward converters are commonly used in medium-power applications (50-500W) such as telecom power supplies, industrial equipment, and server power systems.

Difficulty: Intermediate

Estimated Time: 30-45 minutes

Learning Objectives¶

After completing this example, you will be able to: - Understand forward converter operation and energy transfer mechanism - Design transformer reset circuits (third winding, RCD clamp, active clamp) - Analyze the relationship between turns ratio and duty cycle limits - Compare forward and flyback converter characteristics

Prerequisites¶

• Complete Buck Converter Tutorial
• Understanding of transformer operation
• Familiarity with Flyback Converter for comparison

Circuit Description¶

Topology (Third-Winding Reset)¶

                     Transformer
    Reset             n1:n3:n2
    Winding  Dr        │  │
      ┌──────|<|───────●  ●
      │                ║  ║
      │     n1:n2      ║  ║
+Vin ─┼──┬──[S]──┬─────●  ●─────[D1]──┬──[L]──┬── +Vout
      │  │       │     ║  ║           │       │
      │ ┌┴┐     ┌┴┐    ║  ║          ┌┴┐     ┌┴┐
      │ │ │ Cin │ │Lm  ║  ║          │ │ D2  │ │ C
      │ │ │     │ │    ║  ║          │ │ FW  │ │
      │ └┬┘     └┬┘    ║  ║          └┬┘     └┬┘
      │  │       │     ●  ●           │       │
      └──┴───────┴─────────────────────┴───────┴── GND

    Primary Side                    Secondary Side

Key Components: - S: Primary switch (MOSFET) - n1:n2: Main turns ratio (primary:secondary) - n3: Reset winding (typically n3 = n1) - Dr: Reset diode - D1: Rectifier diode - D2: Freewheeling diode (continuous current through L) - L: Output inductor (energy storage) - C: Output capacitor

Operating Principle¶

Switch ON (Energy Transfer Phase): 1. Current flows through primary, transformer transfers energy to secondary 2. D1 conducts, current flows through L to output 3. Inductor current ramps up: diL/dt = (Vout/n - Vout)/L 4. Core magnetizes (flux increases)

Switch OFF (Reset + Freewheeling Phase): 1. Primary current stops, transformer must reset 2. Reset winding conducts through Dr, core demagnetizes 3. D1 blocks, D2 (freewheeling) conducts 4. Inductor current ramps down: diL/dt = -Vout/L 5. Reset time: Treset = Ton × (n1/n3)

Key Parameters¶

Voltage Conversion Ratio¶

Vout = (Vin × D) / n

Where: - D = duty cycle (limited by reset requirement) - n = turns ratio (n1/n2)

Maximum Duty Cycle¶

For third-winding reset with n1 = n3:

Dmax = n3 / (n1 + n3) = 0.5 (for equal windings)

For n3 < n1 (faster reset, higher Dmax):

Dmax = n3 / (n1 + n3)

Design Equations¶

Turns Ratio Selection¶

For Vin = 48V, Vout = 5V, D = 0.4:

n = (Vin × D) / Vout = (48 × 0.4) / 5 = 3.84 ≈ 4:1

Output Inductor¶

For CCM operation with 30% ripple at minimum load:

L = (Vin/n - Vout) × D / (ΔIL × fs)
L = (48/4 - 5) × 0.4 / (0.3 × Iout × 200k)

For Iout = 20A, ΔIL = 6A:

L = (12 - 5) × 0.4 / (6 × 200k) = 2.3 μH

Use 5-10 μH for margin.

Switch Voltage Stress¶

With third-winding reset (n1 = n3):

Vds = Vin + Vin × (n1/n3) = 2 × Vin

For Vin = 48V: Vds = 96V → Use 150V MOSFET

With RCD clamp, voltage can be higher due to leakage.

Diode Voltage Stress¶

Rectifier diode D1:

Vr,D1 = Vin/n + Vout × (n3/n2)  ≈ 2 × Vin/n (worst case)

For Vin = 48V, n = 4: Vr,D1 ≈ 24V → Use 40V Schottky

Reset Methods Comparison¶

Building in GeckoCIRCUITS¶

Step 1: Create Primary Circuit¶

1. Add DC voltage source (Vin = 48V)
2. Add input capacitor (Cin = 100μF)
3. Add MOSFET switch

Step 2: Add Transformer¶

1. Add ideal transformer with n1:n2 ratio
2. For reset winding: Add second transformer or coupled inductor
3. Connect reset diode (Dr) from reset winding to Vin+
4. Observe dot convention: reset winding dots opposite to primary

Step 3: Create Secondary Circuit¶

1. Add rectifier diode D1 (in series with secondary winding)
2. Add freewheeling diode D2 (cathode to D1 output)
3. Add output inductor L
4. Add output capacitor C
5. Add load resistor (R = Vout²/Pout = 0.25Ω)

Step 4: Add PWM Control¶

1. Add PWM signal generator (fs = 200kHz)
2. Set duty cycle D = 0.4 (below 0.5 limit)
3. Connect to switch gate

Step 5: Configure Simulation¶

• Simulation time: 1-5 ms
• Time step: 5-25 ns (1/200 of switching period)
• Solver: Trapezoidal

Expected Results¶

Steady-State Waveforms¶

Design Verification¶

For Vin=48V, D=0.4, n=4:

Vout = Vin × D / n = 48 × 0.4 / 4 = 4.8V

Efficiency Estimate¶

Exercises¶

Exercise 1: Duty Cycle Limit¶

1. Build forward converter with n1 = n3 (1:1 reset)
2. Start with D = 0.3, increase gradually
3. Question: What happens when D > 0.5? (flux doesn't reset!)

Exercise 2: Compare with Buck¶

1. Build equivalent non-isolated buck: Vin = 12V, Vout = 5V
2. Compare waveforms: inductor current, output ripple
3. Question: How do the topologies relate?

Exercise 3: Reset Winding Ratio¶

1. Test n1:n3 = 1:1 (Dmax = 0.5)
2. Test n1:n3 = 2:1 (Dmax = 0.33)
3. Test n1:n3 = 1:2 (Dmax = 0.67)
4. Question: What are the trade-offs?

Exercise 4: Current-Mode Control¶

1. Add current sense resistor in primary
2. Implement peak current-mode control
3. Apply load step (50% to 100%)
4. Compare transient response to voltage-mode

Forward vs Flyback Comparison¶

Common Issues¶

Related Examples¶

• Flyback Converter - Alternative isolated topology
• Buck Converter - Non-isolated equivalent
• Full Bridge - Higher power isolated

References¶

1. Pressman, A. "Switching Power Supply Design" - Forward Converter Chapter
2. Texas Instruments SLUP126: "Forward Converter Design"
3. Infineon AN-SMPS-ICE1PCS01: "Forward Converter Design Guide"
4. Erickson & Maksimovic: "Fundamentals of Power Electronics" - Chapter 6

Circuit Files¶

Note: Example circuits to be added: - forward_basic.ipes - Basic forward with third-winding reset - forward_rcd.ipes - With RCD clamp reset - forward_active_clamp.ipes - Active clamp topology - forward_two_switch.ipes - Two-switch forward (inherent reset)

Example Version: 1.0 Last updated: 2026-02 GeckoCIRCUITS v1.0