1 Introduction

Switch-mode power supplies almost always require knowledge of the switching current, often sensed on the primary side of the power transformer. Current Sensing Solutions for Power Supply Designers⁽¹⁾ reviews many methods for sensing the switched current. In lower-power switch-mode power supplies, the method most often used employs a sense resistor. The type of sense resistor used is often a high power, low inductance resistor that can add significant cost and power dissipation to the power supply design. This circuit is shown in Figure 1-1 (a).

Figure 1-1 Typical R_Sense Method and Proposed Op-Amp Method for Sensing Primary Side Switch Current

To overcome the cost and power dissipation of such power resistors, the circuit of Figure 1-1 (b) is proposed. Using a differential amplifier made up of a low-power op amp and discrete resistors can result in several advantages including lower power dissipation (efficiency), noise immunity, cost, and programmability.