Designing More Efficient HV Power Supplies Using Planar Flyback Transformers: Power supply efficiencies exceeding 90 percent achievable through proper transformer design.
Flyback topologies provide the simplest and lowest cost solutions all isolated power conversion architectures. It's no wonder that they are the most commonly used topology for AC/DC power supplies up to 100 W. Because of their low resistance and minimal number of turns in helical windings, planar magnetics are the high-frequency application converter of choice for engineers. Planar transformers also offer a slimmer mechanical profile that makes them useful in space-constrained designs.
Planar topologies also present a number of technical challenges, such as their high inductance values and the level of isolation the design requires for safety. A solution to these challenges is the availability of customized planar flyback transformers. Working with a knowledgeable component supplier gives designers the ability to offer peak power supply efficiencies of over 90 percent.
Strengths and Weaknesses
Compared to traditional wound transformers, planar transformers permit a smaller number of windings turns as their cores have wider surface areas than traditional E, EC or EP cores. In addition, these wider cores allow the DC resistance of the copper to be lower.
Another strength is that the rigid structure of the planar transformer's PCB removes the need for a plastic carrier or bobbin. This contributes to a slimmer, lower profile solution as opposed to the size of a wound transformer. The simplified manufacturability of a PCB, too, enables higher inductance, resistance and turns ratios tolerances in transformer specifications.
A potential weak spot for high voltage applications is that some planar transformers use PCB substrates like FR4, which do not always meet insulation safety requirements. There is also a potential for power supply damage as the result of differential surges that can jump between PCB vias and the transformer core. In addition, using multiple layers can be problematic and costly particularly if thick copper plating is required.
The Custom Flyback Transformer Design
Using a USB-based power delivery system as an example, a custom planar flyback transformer can be designed to deliver up to 100 W (20 V, 5 A) with the knowledge that a continuous conduction mode is necessary for powers greater than 10 W. A planar transformer design such as this prevents the high peak currents that can cause switching losses and overheating of the core. The losses in this custom transformer will primarily be from the copper; not the core. That's because the flyback transformer needs to have a primary inductance value of 530 pH to keep peak currents under 2.3 A, so as not to overstress the external 650 V MOSFET. By calculating the saturation current, inductance value and area of the core (EC26), this transformer requires 30 turns on the primary side.
The design example (Figure 1) has a 12-layer winding incorporating a primary and secondary winding produced using 2-ounce copper, FR4 PCB material and two identical substrates.
Cutting Core Losses and Leakage Inductance
Because the flux density is at its highest at the edge of the core, the flux needs to travel through the side wall of the core to complete its path. As the image of the analysis of the transformer in Figure 3 on page 25 illustrates, the flux density will increase at the side.
To keep the overall efficiency high, leakage inductance must be kept to a minimum as it is a source of energy waste. Calculating the leakage inductance value is dependent upon the magnetic field between the primary and secondary winding. Using Ampere's Law, the ampere turns across the interwinding region is the same as the ampere turns in either winding. Therefore, the flyback transformer's winding width was maximized in order to reduce the field strength in this region.
Also, the interwinding region volume needed to be as small as possible to reduce leakage. Leakage inductance was further reduced by increasing the winding width, which was accomplished by interleaving the layers. This design had four turns on the secondary implemented using a single layer insulated wire, so it can eliminate creepage and clearance space requirements between the primary and secondary windings. Doubling the winding breadth of the primary by interleaving it with secondary effectively cut the leakage inductance value in half.
The interwinding volume is better controlled if the secondary is also implemented in PCB material. However, on the downside, interwinding capacitance is adversely affected if the distances between layers are decreased, and is serious for high voltage applications as it worsens the coupling of AC power line noise through to the power supply output.
The leakage inductance test is carried out by shorting the secondary winding and measuring the primary inductance. The leakage inductance on this custom planar flyback transformer design was measured as 14 pH at 130 kHz.
AC Loss Control
In a flyback transformer, the AC resistance doesn't improve by interleaving the primary and secondary windings as these windings are out of phase. To control the AC resistance on the primary side, the copper must be kept to less than the skin depth at the switching frequency. The skin depth was maintained in this transformer at 0.2 mm, with a copper thickness of 0.07 mm.
At maximum load, the secondary AC current is 7.3 A, due to the pulsating nature of current on the secondary side that causes copper losses of 1.5 W due to the thin diameter (0.8 mm) of the wire. Using Finite Element Analysis (FEA) analysis revealed that the conductor closest to the gap experienced a hot spot as a result of the high circulating AC and eddy currents induced by fringing effects.
It was found that reducing copper losses at high frequencies required the insulated secondary wire to be replaced with a helical winding. The designers also realized that FR4-type PCB material is not considered safe at high frequencies, so a barrier, as Mylar or polyimide tape, needed to be bonded to the PCB substrate.
AC/DC Adaptor Application Example
The design of the custom planar flyback transformer is based on a Discontinuous Conduction Mode (DCM) Flyback Converter topology with valley switching and synchronous rectification. Both the valley switching and synchronous rectification reduce power losses in the external MOSFET and rectifier, respectively. Operating in DCM mode meant that there would be zero ampere turns in the transformer for a period every switching cycle.
It was tested on an AC/DC adaptor where the operating mode of the transformer is shown by the voltage across the drain of the MOSFET (Figure 2).
For purposes of this example, the AC/DC power supply operated at 115 V AC input with a 5V output. Bourns measured the efficiency at different output powers. The testing demonstrated that the biggest loss came from the snubber diode, which was the hottest component at 51,3[degrees]C. The solution helping to improve power loss was to optimize the snubber resistor value and reduce the leakage inductance in the transformer.
Maximizing Power Supply Efficiency
Optimizing conversion in high frequency, high current applications including high voltage AC/DC adaptors can be accomplished with planar magnetics. Bourns set out to demonstrate how a customized planar flyback transformer could maximize energy efficiencies, and tested its design in a variety of functions. By reinforcing the insulation, through splitting the primary into two PCBs and sandwiching a four-turn secondary using triple insulated wire, the transformer design provided the necessary barrier between primary and secondary windings. Furthermore, the leakage inductance was reduced via splitting the primary, which resulted in a 50 percent leakage decrease. Finally, Bourns tested its flyback transformer on an AC/DC adaptor in DCM mode with 5 V output, and it was shown to deliver a peak efficiency of 91.05 percent.
By Cathal Sheehan, Technical Market Manager, Bourns, Inc.
Caption: Figure 1: The PCB-based transformer design splits the primary winding enclosing the secondary made from triple insulated wire. The triple insulated wire on the secondary side provides the reinforced insulation between primary and secondary windings.
Caption: Figure 2: The planar transformer was tested in an AC/DC adaptor with the bulk voltage across the primary plus the reflected output voltage from the secondary.
Caption: Figure 3. The design team used a simulation tool to explore the transformer's magnetic behavior in the USB power supply circuit under various operating conditions. This simulated cross section of the transformer reveals that the flux density in the center leg was well below saturation.
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|Title Annotation:||Engineering Answers: TRANSFORMERS|
|Publication:||Product Design & Development|
|Date:||Oct 1, 2017|
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