New Isolated DC/DC Topology Maximizes Reliability while Mitigating Cost & Supply Chain Disruption
Mar 25, 2024
Transforming Legacy Industry Essential: Optimal Balance, Flexibility, and Automation. Explore how a traditional industry staple has evolved for improved efficiency, reduced errors, and adaptable product roadmaps.
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Abstract
Many different DC/DC converter topologies have been developed, each with unique characteristics. However, it is uncommon for a new design to transcend the limitations of established, proven architectures while introducing modern benefits — particularly in low-power applications.
In this whitepaper, we examine how a longstanding workhorse of the industry has been enhanced to deliver a more optimal balance of size, weight, power efficiency, and cost. At the same time, it enables flexible product roadmaps by supporting a broad range of input voltage options along with both isolated and regulated output capabilities. Adopting a topology compatible with automated transformer manufacturing also helps reduce manual intervention, thereby minimizing the risk of human error.
In this whitepaper, we examine how a longstanding workhorse of the industry has been enhanced to deliver a more optimal balance of size, weight, power efficiency, and cost. At the same time, it enables flexible product roadmaps by supporting a broad range of input voltage options along with both isolated and regulated output capabilities. Adopting a topology compatible with automated transformer manufacturing also helps reduce manual intervention, thereby minimizing the risk of human error.
Introduction
The core of any isolated DC/DC converter is its transformer. Since transformers only operate with alternating current (AC), every isolated DC/DC converter fundamentally performs a DC-to-AC and then an AC-to-DC conversion process (Figure 1).
The most basic form of DC/AC conversion involves a free-running oscillator that produces a square wave across the primary winding of the isolation transformer. On the secondary side, the simplest AC/DC converter consists of a diode-capacitor network that rectifies and filters the waveform to produce a DC output voltage.
The most basic form of DC/AC conversion involves a free-running oscillator that produces a square wave across the primary winding of the isolation transformer. On the secondary side, the simplest AC/DC converter consists of a diode-capacitor network that rectifies and filters the waveform to produce a DC output voltage.
Fig. 1: Isolated DC/DC converter block diagram
The simplest complete DC/DC converter design is the self-oscillating Royer topology-an unregulated push-pull converter (Figure 2):
This circuit includes only eight low-cost components in addition to the transformer: two transistors, two resistors, two diodes, and two capacitors. The transistors TR1 and TR2 are alternately switched on and off in antiphase by the two feedback windings T1af and T1bf, and the secondary winding output is rectified by diodes D1 and D2 before being smoothed by the output capacitor C2. For a full analysis of the Royer topology, refer to the DC/DC Book of Knowledge.
A Royer topology DC/DC converter offers several advantages: a low bill of materials (BoM), compact size (as small as <0.5 cm³), and high isolation (up to 4kVDC for 1 second). It is also easy to configure for dual ± output by adding an extra capacitor and reversing D2, making it ideal for powering dual-rail op-amps, analog-to-digital converters, or bipolar sensor circuits. The main drawback is that the output is unregulated, but with a stable supply voltage and a 10–100% load range, the output voltage typically remains within ±10%—an acceptable range for low-cost applications.
For low-power applications requiring galvanic isolation, it remains the most widely used DC/DC converter solution on the market.
A Royer topology DC/DC converter offers several advantages: a low bill of materials (BoM), compact size (as small as <0.5 cm³), and high isolation (up to 4kVDC for 1 second). It is also easy to configure for dual ± output by adding an extra capacitor and reversing D2, making it ideal for powering dual-rail op-amps, analog-to-digital converters, or bipolar sensor circuits. The main drawback is that the output is unregulated, but with a stable supply voltage and a 10–100% load range, the output voltage typically remains within ±10%—an acceptable range for low-cost applications.
For low-power applications requiring galvanic isolation, it remains the most widely used DC/DC converter solution on the market.
Fig. 2: Royer Topology
Minimum cost barrier
Trotz des Erfolgs der Royer-Wandler besteht weiterhin ein starker Marktdruck, die Kosten weiter zu senken. Der größte Fixkostenfaktor ist der Aufbau des Transformators (Abbildung 3).
Diese Transformatoren werden üblicherweise von Hand auf Ferrit-Ringkerne gewickelt, da sie aufgrund ihrer sehr geringen Größe (6mm Außendurchmesser und 3mm Innendurchmesser) für herkömmliche Transformatorwickelmaschinen zu klein sind.
Diese Transformatoren werden üblicherweise von Hand auf Ferrit-Ringkerne gewickelt, da sie aufgrund ihrer sehr geringen Größe (6mm Außendurchmesser und 3mm Innendurchmesser) für herkömmliche Transformatorwickelmaschinen zu klein sind.
Fig. 3: Typical hand-wound miniature transformer
The material cost of the toroidal transformers (core, transformer wire) decreases with increased production volumes, but the assembly time per transformer remains fixed. This results in a minimum manufacturing cost—even at production volumes in the millions (Figure 4):
With such a low BoM count, there are limited opportunities to reduce component costs further. Since much of the assembly work is manual (winding the transformers, soldering the transformer wire ends to the PCB), costs could be lowered by outsourcing to low-wage countries. However, as RECOM is a responsible employer and values the experience and skills of its operators, we insist on paying a fair wage.
The solution is to make a paradigm shift—adopting a different topology and transformer design that supports fully automated manufacturing.
The solution is to make a paradigm shift—adopting a different topology and transformer design that supports fully automated manufacturing.
Fig. 4: Manufacturing cost vs. quantity
Next Generation DC/DC Converters
For a transformer to work, it needs at least one turn on the primary and one on the secondary. In practice, many more turns are required, depending on the input and output voltage, as well as split center-tap primaries and secondaries. For the Royer topology, two additional feedback windings are also necessary. This requirement for six separate windings is what makes the Royer topology transformer so labor-intensive.
One alternative used by a competitor is to create the windings around the ferrite core using a multilayer PCB with vias, forming electrical connections from top to bottom to create ...
One alternative used by a competitor is to create the windings around the ferrite core using a multilayer PCB with vias, forming electrical connections from top to bottom to create ...
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