New isolated DC/DC Topology maximizes reliability while mitigating Cost & Supply-Chain Disruption

2024/03/25
New isolated DC/DC Topology maximizes reliability while mitigating Cost & Supply-Chain Disruption Image
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.

Table of contents

  1. Abstract
  2. Introduction
  3. Minimum cost barrier
  4. Next generation DC/DC converters
  5. Summary (Download/Login)

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1. Abstract

Many different DC/DC converter topologies have been developed, and each is unique. However, it is rare that a novel construction breaks down the barriers between the traditional, tried-and-tested topologies and a modern solution when implementing low power designs.

In this whitepaper, we will review how a legacy workhorse of the industry has been improved to drive a more optimal balance between size, weight, power, and cost metrics while concurrently enabling flexible product roadmaps by supporting a variety of input voltage ranges and output isolation and regulation options. Employing a topology that is more amenable to automated manufacturing processes is beneficial in that it helps mitigate human error in the process.

2. Introduction

The heart of any isolated DC/DC converter is its transformer. Transformers function with AC only, so all DC/DC converters are actually DC-to-AC : AC-to-DC converters (Figure 1).

The simplest DC/AC converter is a free-running oscillator that generates a square wave across the isolation transformer primary winding. The simplest AC/DC converter is simply a diode and capacitor circuit which rectifies and smooths the transformer secondary output into a DC output voltage.


Fig. 1: Isolated DC/DC converter block diagram
The simplest DC/DC converter circuit is the self-oscillating Royer topology - an unregulated push-pull converter (Figure 2):
This circuit has only eight low-cost components besides 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 the 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 has many advantages: low BoM, compact size (down to <0.5cm3) and high isolation (up to 4kVDC/1s). It is also very easy to create a dual +/- output by adding an extra capacitor and reversing D2, making it ideal for powering dual rail op-amps, A-to-D converters or bipolar sensor circuits. The main disadvantage is that the output is unregulated, but with a stable supply voltage and 10-100% load range, the output voltage is typically within ±10%, an acceptable figure for cost effective designs.

For applications that need a low power, yet galvanically isolated supply, it is the most popular DC/DC converter solution on the market.


Fig. 2: Royer Topology

3. Minimum cost barrier

Despite the success of Royer converters, there is a constant push from the market to drive down costs further. The main fixed cost is the transformer construction (figure 3).

These are typically hand-wound on toroidal ferrite cores because their very small size (6mm outside diameter and 3mm inside diameter) means that they are too small for traditional transformer winding machines.


Fig. 3: Typical hand-wound miniature transformer
The material cost of the toroidal transformers (core, transformer wire) will go down with increased production volumes, but the assembly time per transformer remains fixed. This creates a minimum manufacturing cost — even with production volumes in the millions (Figure 4):
With such a low BoM count, there are limited opportunities to reduce component costs further. As much of the assembly work is manual (winding the transformers, soldering the transformer wire ends to the PCB), costs could be reduced further by moving assembly to low wage countries, but as RECOM is a responsible employer and we value the experience and skills of our operators very highly, we insist on paying a fair wage.

The solution is to make a paradigm shift and move to a different topology and transformer design that is more suitable to fully automated manufacturing.


Fig. 4: Manufacturing cost vs. quantity

4. 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 needed, depending on the input and output voltage, as well as split centre-tap primaries and secondaries. For the Royer topology, two additional feedback windings are needed. This requirement for the six different windings is what makes the Royer topology transformer so labour intensive.

One alternative used by a competitor is to make the windings around the ferrite core by using a multilayer PCB with vias, making the …

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