[047] Forward Converter Design - Part XIII Current Transformer Design

Desiging current transformers for fast sensing on the forward converter. 

Introduction

This article continues the series in which Dr. Ridley documents the processes involved in taking a power supply from the initial design to the full-power prototype. At this point in the design, the second layout of the PC board is complete, with several significant changes to the power specifications being implemented. The new power supply layout is an opportunity to redesign magnetics components, starting with the current transformer.

Current Sensing Using a Current Transformer

When working with high power supplies, above 100 W or so, the best way to sense the current in the power stage is usually with a current transformer. This approach has the following advantages over resistive sensing of the current:

1.Sense voltage can be large (>1 V).

2.Dissipation can be arbitrarily low.

3.Galvanic isolation eliminates grounding problems. (very important for bridge current-balancing.)

A current transformer consists of a multi-turn secondary wound on an ungapped core, and a single turn primary which usually consists of just a wire jumper on the board, or a copper strap.

Figure 1 shows the schematic of the forward converter with a current transformer in the high side of the input rail, above the drain of the upper FET. The location of the current transformer is important – it should be placed in a part of the circuit where the common-mode ac voltage is small, and where only the desired current is sensed. If the current transformer in Figure 1 were placed on the return of the input rail, it would also sense the gate drive current pulse in addition to the drain current.

Figure 1: Forward Converter with Current Transformer Placement and Waveforms

The first consideration in selecting a current transformer is the turns ratio. The primary side of the transformer is driven by the current in the power stage, and is set by the load and power stage operating conditions. If you want a given signal size for the control circuit to use, the turns ratio of the current transformer will determine how much voltage drop, and hence how much dissipation, will be seen on the primary. The lower the dissipation that you want, the higher the turns count of the current transformer.