Maintaining high conversion efficiency across a wide load range for a planar transformer primarily involves optimizing the winding turns ratio to adapt to the voltage and current requirements under varying loads, balancing excitation losses and copper losses. The output current and input voltage for different loads vary. A fixed turns ratio can easily lead to excessive excitation current at light loads (resulting in higher excitation losses) and excessive secondary current density at heavy loads (increasing copper losses). Therefore, an adaptive turns ratio must be designed based on current variations across a wide load range. For example, for light loads, the turns ratio can be appropriately adjusted to reduce the excitation magnetomotive force and minimize hysteresis and eddy current losses in the core. For heavy loads, the turns ratio can be optimized to reduce the current amplitude in the secondary winding to avoid a significant increase in copper losses due to excessive current. Furthermore, the turns ratio must be adjusted in accordance with the planar transformer's PCB winding characteristics to ensure that the winding maintains a good coupling coefficient even after the turns ratio changes, avoiding additional losses caused by increased leakage inductance and laying the foundation for stable efficiency across varying loads.
The winding's trace width and number of layers must be dynamically adapted to changes in load current to minimize variations in copper losses under varying loads. The PCB winding width of a planar transformer directly affects its current-carrying capacity. Under heavy loads, where current is high, wider winding widths are required to reduce current density and minimize Joule heating losses. Under light loads, where current is lower, the width can be appropriately reduced or the number of winding layers adjusted to ensure current carrying capacity while reducing winding space and minimizing the impact of parasitic parameters. Furthermore, optimizing the number of winding layers requires a balance between coupling efficiency and heat dissipation. While multiple layers can improve the coupling coefficient, too many layers can lead to poor heat dissipation between layers, exacerbating losses particularly under heavy loads and high temperatures. Therefore, the appropriate number of layers should be determined based on current fluctuations within the load range. For example, fewer layers can be used to reduce parasitic capacitance under light loads, while multiple layers of wider windings can be used to balance current density and heat dissipation under heavy loads, ensuring consistently low copper losses across a wide range of loads.
The staggered winding arrangement optimizes leakage inductance and coupling coefficient, reducing the additional losses caused by leakage inductance under a wide range of loads. Improper PCB winding layout in a planar transformer can easily generate leakage inductance, which can cause voltage spikes and reactive power loss during load fluctuations. This loss can significantly impact efficiency, especially in high-frequency, wide-bandwidth load scenarios. By staggering the primary and secondary windings, alternating their placement on the PCB, the magnetic coupling between the windings can be significantly improved, reducing leakage inductance. Staggering also ensures a more uniform magnetic field distribution, reducing core losses caused by localized excess magnetic flux density. For a wide load range, it's crucial to ensure that leakage inductance remains consistently low across all loads. For example, at light loads, reactive power losses due to leakage inductance contribute more significantly, requiring further reduction through more precise staggering. At heavy loads, while leakage inductance contributes relatively little, it still needs to be kept within a reasonable range to avoid current waveform distortion caused by leakage inductance and ensure stable conversion efficiency.
A combination of parallel and series windings allows for flexible adaptation to current and voltage requirements across a wide load range, balancing the loss characteristics of varying loads. For scenarios that need to cover both high-current heavy loads and low-current light loads, the secondary winding can be designed with multiple parallel windings. Under heavy loads, multiple windings operate simultaneously, distributing the high current and reducing the current density and copper losses of a single winding. Under light loads, the control circuit can shut down some of the parallel windings, leaving only one or two windings active. This reduces the effective number of turns and wire length, thereby minimizing excitation losses and copper losses. For scenarios with a wide load voltage range, the primary winding can be designed with multiple taps in series. Taps can be switched based on the input voltage and load requirements, adjusting the effective turns ratio and avoiding abnormal excitation currents caused by voltage mismatch. Furthermore, the series tap design reduces winding turn redundancy, lowering parasitic parameters and ensuring stable efficiency across a wide voltage and load range.
Control of the winding's parasitic parameters requires optimization of wide-load characteristics to minimize parasitic losses at high frequencies. The PCB windings of a Planar transformer have parasitic capacitance (interlayer capacitance, winding-to-ground capacitance) and parasitic inductance. These parameters can cause resonant losses and conduction losses under high frequency and wide loads. In particular, under light load and high frequency scenarios, the capacitive reactance of the parasitic capacitance decreases, which can easily lead to increased reactive current and reduced efficiency. Parasitic capacitance can be controlled by adjusting the winding wire spacing, interlayer insulation thickness, and PCB material. For example, under light load and high frequency, increasing the wire spacing and interlayer insulation thickness can reduce parasitic capacitance and avoid resonant losses. Under heavy load and low frequency, the impact of parasitic capacitance is relatively small, so the wire spacing can be appropriately reduced to improve winding utilization. Furthermore, optimizing the position and length of the winding lead wires reduces parasitic inductance, avoids current lag caused by parasitic inductance, ensures voltage and current phase matching under wide loads, and reduces reactive losses.
The winding's heat dissipation design prevents efficiency degradation caused by temperature rise under wide loads, maintaining stable winding performance. Planar transformers experience significant copper losses in their windings under heavy loads, generating significant heat. If heat dissipation is poor, the resulting temperature increase will lead to increased winding resistance (conductor resistance increases with temperature), further exacerbating copper losses and creating a vicious cycle of heat and loss. While heat generation is minimal under light loads, prolonged low temperatures can degrade the winding insulation and potentially impact efficiency. Designing heat dissipation vias near the windings, using a high-thermal-conductivity PCB substrate, or applying thermally conductive material to the winding surface can enhance the winding's heat dissipation capacity. This allows for rapid dissipation of copper loss heat under heavy loads, controlling winding temperature rise, while maintaining a stable winding temperature under light loads to prevent insulation degradation. Furthermore, the winding's wire width and number of layers must be designed with heat dissipation in mind. For example, wide-wire windings offer a larger heat dissipation area, allowing for natural heat dissipation. Multi-layer windings require thermal dissipation channels to prevent heat accumulation between layers, ensuring that the winding temperature remains within a safe range, resistance remains stable, and losses are manageable under a wide load range.
Dynamic winding parameter adjustment strategies must be coordinated with control circuits to achieve real-time efficiency optimization under a wide load range. If a planar transformer relies solely on fixed winding parameters, it will be difficult to fully adapt to the dynamic changes of a wide load. It requires a controller to dynamically adjust the winding parameters. For example, a current sensor monitors the load current in real time. If a heavy load is detected, the controller automatically adjusts the number of parallel winding groups or the line width (using switches to control the switching of multiple winding groups) to reduce current density. If a light load is detected, the controller switches the series taps of the windings or shuts down some windings to reduce excitation losses. At the same time, the controller can also adjust the switching frequency based on load changes, coordinating optimization with the winding parameters. For example, under light loads, the switching frequency can be reduced to reduce switching losses and parasitic parameter losses, in conjunction with the low leakage inductance design of the windings. Under heavy loads, the switching frequency can be appropriately increased to shorten the on-time, in conjunction with the wide line design of the windings to reduce copper losses. Through the synergistic combination of "winding parameters + control strategy", closed-loop efficiency optimization can be achieved across a wide load range, ensuring that the conversion efficiency at each load point remains high.
