The energy storage device in a forward converter is typically located on the rear side of the BUCK inductor. This makes the forward transformer less complex compared to a Flyback transformer, as its main functions are voltage and current conversion, electrical isolation, and energy transfer. When calculating the forward transformer, engineers usually start by analyzing the BUCK inductor at the secondary side. The input voltage to this inductor is the secondary output voltage of the forward transformer minus the forward voltage drop of the rectifier diode. Hence, the forward power supply can be considered an isolated version of a BUCK converter.
**Q1: Choice of Primary and Secondary Turns**
Taking a three-winding reset forward transformer as an example, once the turns ratio is determined, the next step is to calculate the number of primary and secondary turns. Some engineers believe that the fewer the turns, the better, as long as the transformer doesn’t saturate under full load. However, this is a misconception. The number of turns determines the primary inductance (without air gap or with the same air gap), which directly affects the excitation current. A smaller excitation current leads to higher efficiency, but if the number of turns is too small, it can increase the change in magnetic flux density (ΔB). Without proper balancing of the air gap, the transformer may easily saturate.
**Q2: Magnetic Reset in Single-Tube and Double-Tube Forward Converters**
Whether it’s a single-tube or double-tube forward configuration, magnetic reset is essential. It’s a passive process, and the reset current plays a critical role. If the reset current is too small, the reset effect can be compromised due to the transformer's inherent parasitic parameters like capacitance and leakage inductance. After the MOSFET turns off, the primary winding generates a back EMF, and the reset winding has an opposite phase, allowing for a reset current. Proper demagnetization is crucial, especially when no air gap is used, as the primary inductance is large, leading to a naturally smaller reset current.
However, in high-power applications, adding a small air gap is often necessary to reduce leakage inductance and improve reliability. High currents and leakage inductance can cause significant changes in magnetic induction, so an air gap helps manage these effects.
**Q3: Determining the Positive Duty Cycle**
The positive duty cycle is primarily influenced by the input and output of the secondary freewheeling inductor. Since the secondary side operates as a BUCK circuit, the output voltage is given by Vo = Vin × D, where D is the duty cycle. Knowing the output voltage allows us to determine the appropriate duty cycle. It’s recommended not to exceed 0.5 to ensure stable operation. Once the transformer’s output voltage is known, the turns ratio can be calculated based on the input voltage, using the lowest DC input voltage to determine the maximum duty cycle.
**Q4: Reset Winding Placement**
The placement of the reset winding—whether close to the primary or sandwiched between the primary and secondary—depends on design considerations. From an EMC and manufacturing standpoint, placing the reset winding on the innermost layer is generally preferred. For commercial power supplies or those with PFC outputs, the MOSFET must withstand twice the DC bus voltage plus leakage inductance factors, so 800V or 900V MOSFETs are commonly used.
**Q5: Reliability in High-Power Supplies**
In high-power designs, the transformer’s margin is usually large to avoid saturation. A lower ΔB (typically below 0.2) is selected to prevent core saturation. Switching frequencies are often kept below 40kHz to minimize switching losses and EMI. Active PFC circuits are common, resulting in a DC bus voltage around 400V. With more turns, distribution parameters like leakage inductance and distributed capacitance increase, leading to higher AC and DC losses. Heat dissipation in high-power transformers is also a concern, as internal windings tend to get hot, increasing the risk of core saturation.
Adding a small air gap reduces remanence and improves reliability, helping the transformer withstand high temperatures and heavy loads without saturating.
**Q6: Why Some Transformers Reset Without a Reset Winding?**
Some transformers use external reset methods like RCD snubbers, LCD clamps, or active clamping to achieve reset. These techniques provide alternative ways to demagnetize the core without requiring an additional winding.
**Q7: Currents in Forward Transformers**
When the forward converter is turned on and off, the primary and secondary currents consist of the magnetizing current, the energy-transfer current, and the induced secondary current. During steady-state operation, the clamp diodes are on during reset, keeping the MOSFET voltage close to the DC bus voltage. Fast-recovery diodes like BYV26C are ideal for reset diodes.
**Q8: Hard-Switching Circuits**
Hard-switching circuits have trade-offs. Increasing frequency can reduce the size of the transformer and improve power density, but it also increases switching losses and EMI. Careful thermal management is crucial to maintain reliability. The number of turns and the duty cycle are interdependent, and the transformer’s performance depends on the correct balance between these parameters.
**Q9: Continuous Conduction Mode (CCM)**
Forward converters typically operate in CCM, which introduces a large DC component. A larger ΔB requires an air gap to reduce residual magnetism, but this increases the excitation current and copper loss. The secondary freewheeling diode has a longer on-time, and the average current of the rectifier and freewheeling diode should be balanced. Due to the wide variation in duty cycle, forward converters are rarely used across the full voltage range.
**Transformer Design Considerations**
Choosing the right core and bobbin is essential. The AP method (core area product) is used to estimate the core size based on apparent power, frequency, and magnetic flux density. The calculated AP value is usually multiplied by a factor of 1.5–2 to account for real-world variations. Core selection should aim for high saturation flux density and low residual magnetism to optimize performance. The number of turns is then calculated based on the required voltage transformation and duty cycle.
Wire diameter calculations consider RMS current, skin depth, and proximity effects. Both primary and secondary windings require careful design to minimize losses and ensure reliable operation.
Overall, designing a forward converter involves a balance between efficiency, reliability, and thermal management, making it a complex but rewarding task in power electronics.
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