What do you need to know about the forward transformer?

The energy storage device is located on the rear BUCK inductor, which makes the forward transformer less complex compared to a Flyback transformer. Its main functions include voltage and current conversion, electrical isolation, and energy transfer. When calculating the forward transformer, we typically start by analyzing the BUCK inductor on the secondary side of the transformer. The input voltage for this inductor is the secondary output voltage of the forward transformer minus the forward voltage drop of the rectifier diode. Therefore, we often refer to the forward power supply as an isolated version of the BUCK converter.

Q1

Primary and Secondary Turns Selection

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, provided the transformer doesn’t saturate under full load. However, this is a misunderstanding. The number of turns determines the primary inductance (without or with an air gap), and the inductance value affects the excitation current. This current does not participate in energy transfer but still consumes energy. A smaller excitation current leads to higher efficiency. If the number of turns is too small, it can increase the ΔB (change in magnetic flux density), potentially causing saturation if the air gap isn’t properly balanced.

Q2

Magnetic Reset in Single-Tube and Double-Tube Forward Topologies

Magnetic reset is crucial in both single-tube and double-tube forward converters. It's considered passive, and the reset current plays a vital role. If the reset current is too small, it may be affected by the transformer’s parasitic parameters, such as capacitance and leakage inductance. After the primary MOSFET turns off, the primary winding induces a reverse voltage, and the reset winding generates a current to demagnetize the core. Without an air gap, the primary inductance is large, so the reset current is naturally small. In high-power applications, adding a small air gap is often necessary to reduce leakage inductance and improve reliability.

Q3

Duty Cycle Determination

The positive duty cycle is primarily determined by the secondary freewheeling inductor. Since the secondary side operates as a BUCK circuit, the output voltage Vo = Vin × D, where D is the duty cycle. With known output voltage, we only need to determine the appropriate turns ratio to calculate the input voltage of the BUCK inductor. This means the output voltage of the transformer is essentially fixed. It's important to note that the duty cycle is closely related to the reset method, and it's generally recommended that D should not exceed 0.5.

Q4

Reset Winding Placement

Should the reset winding be placed close to the primary or between the primary and secondary? Ideally, it should couple best with the primary, though this poses challenges in terms of insulation. In practice, it's more common to place the reset winding on the innermost layer, especially from an EMC and manufacturing perspective. For commercial power or PFC-based inputs, the MOSFET must withstand at least twice the DC bus voltage, so 800V or 900V MOSFETs are often recommended.

Q5

High-Power Power Supply Reliability

In high-power designs, transformers usually have a large margin to avoid saturation. To do this, ΔB is often set to less than 0.2. Due to switching losses and EMI considerations, the operating frequency is typically kept below 40kHz. High-power supplies often use active PFC, resulting in a DC bus voltage around 400V. According to the transformer formula Np = Vin × Ton / (ΔB × Ae), increasing the number of turns increases leakage inductance and distributed capacitance, leading to higher AC and DC losses. Additionally, poor heat dissipation in high-power transformers can cause significant temperature rise. Adding a small air gap helps reduce remanence and prevents saturation under high-temperature and high-load conditions.

Q6

Why Do Some Transformers Reset Without a Reset Winding?

External reset methods like RCD, LCD, or active clamping can be used instead of a dedicated reset winding.

Q7

Transformer Winding Layout and Current Behavior

Some forward transformers use resonant reset through the magnetizing inductance and MOSFET junction capacitance, requiring detailed calculation of inductance and capacitance. A small air gap is usually needed. The reset current is typically low, so the heat generated is minimal. The reset winding is often placed inside to simplify the process. The layout of the secondary winding also affects performance—placing it inside reduces wire length and DC loss but may worsen heat dissipation, while placing it outside has the opposite effect.

Q8

Hard Switching Circuits

Increasing the switching frequency allows for fewer turns or smaller transformers, reducing size and improving power density. However, it also increases switching losses and EMI. Proper thermal design is essential to maintain reliability. The turns ratio is influenced by input voltage range and duty cycle. Unlike Flyback, the forward transformer’s inductance comes after the transformer. Under the same duty cycle, the secondary output voltages differ. The secondary side acts as a BUCK circuit, with its input voltage being the transformer’s secondary output minus the rectifier voltage drop.

Q9

CCM Operation in Forward Converters

Forward converters typically operate in CCM mode, which includes a large DC component. Using a larger ΔB requires a small air gap to balance the DC effect, but this increases excitation current and copper loss. Since the duty cycle is usually less than 0.5, the secondary freewheeling diode has a longer conduction time. The average current of the rectifier and freewheeling diodes should be similar. Forward converters are rarely used across the full voltage range due to the difficulty in designing the secondary inductor with a large duty cycle variation.

Design Considerations for Transformer Core and Windings

The first step in designing a forward transformer is selecting the core and bobbin. Factors such as core area product (AP), window area (AW), effective cross-sectional area (Ae), and operating frequency (fs) are crucial. The AP value is calculated using: AP = AW × Ae = (Ps × 10⁴) / (2ΔB × fs × J × Ku). While this formula provides a starting point, real-world designs require a safety margin of 1.5–2 times the calculated AP. ΔB is critical, as it determines the core’s operating range and iron loss. A higher ΔB reduces the number of turns but increases core loss. Selecting a core with high saturation flux density and low residual magnetism helps achieve compact, efficient designs.

After choosing the core, the number of turns is calculated based on the turns ratio, input voltage, and desired ΔB. The primary turns are calculated using Np = Vin(min) × Ton(max) / (ΔB × Ae), and the secondary turns are derived from the ratio. Careful rounding of fractional turns is necessary to ensure proper operation. The reset winding is often equal in turns to the primary to ensure reliable reset. Finally, the wire diameter is calculated based on RMS current, considering skin and proximity effects.

Designing a forward transformer involves careful consideration of many factors, including core selection, turns ratio, air gap, and winding layout. Each decision impacts performance, efficiency, and reliability. Through thorough analysis and practical adjustments, a well-designed forward converter can deliver excellent results in various applications.

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