3 Advantages

Power Dissipation and Efficiency:

The power dissipation of the sense resistor (R_Sense) used in the second example results in an improvement of 2.4 W – 0.16 W = 2.24 W, which can result in a significant increase in efficiency as well.

Board Area:

The reduction in power rating results in a much smaller sense resistor. Consider for example the following sense resistors from Vishay Dale (www.vishay.com): WSR-3 3-W surface-mount and the WSL 1/2-watt surface-mount resistors. The 3-W device requires six times the board area of the 1/2 watt device and also needs significant copper area to dissipate the heat. The 1/2-W resistor combined with a SOT-23 op amp and sense resistors result in approximately the same overall board area. Therefore, there is no significant advantage or disadvantage in board area.

Cost:

The cost tradeoff in this design is not just the cost of a high-power resistor versus the cost of the lower-power resistor plus the op-amp circuit. The following considerations can also be made regarding cost:

• Reduced bill of materials and reduced-size inventory: custom or semicustom power-supply designs often choose sense resistors for the particular peak current and sense voltage required for each design. This can result in each individual power supply design requiring a unique value and power rating, resulting in many sense resistors in the company’s inventory. By utilizing one standard component, such as a 0.01-Ω 0.5-W resistor, each individual power supply design can be customized by changing the gain of the resistors around the op amp circuit. This also helps cost by allowing volume buying of one resistor.
• Through-hole construction versus surface mount: The cost of manufacturing large through-hole power resistors should be compared to that of manufacturing standard surface-mount components such as lower-power sense resistors, ICs, and standard SMD resistors.
• Thermal: The cost of additional cooling due to higher power dissipation should be considered.

Programmability:

The peak current of the switch-mode power supply can vary due to minor design or output specification changes, or for various other reasons. The ability to easily adjust the gain of the op amp circuit offers power-supply designers easy programmability by allowing changes to the circuit performance without having to change the sense resistor. These changes can be due to control-loop changes, or to power supply specification changes such as input-voltage range or output-voltage and current changes, all of which affect the primary-side peak current.

Noise/performance:

Several factors of this op amp design will improve performance with regard to noise immunity of the sensed current signal:

• Lower-inductance resistors: lower-power-rated resistors have smaller bodies and are almost always surface-mount devices. This results in significantly-less series inductance, producing less ringing and fewer noise spikes on the current signal.
• Differential sensing: differential sensing of the current signal results in an accurate measurement of the primary-side current without the ground variation (ground bounce) becoming a factor. This is not the case in the circuit of Figure 1-1 (a), where the current sensing is single-ended.
• Separating the grounds: the sense voltage (V_S) is often used by the PWM IC for both control and current limit protection. It is important to isolate the analog ground of the PWM IC from the noisy power ground of the sense resistor. The circuit of Figure 1-1 (b) allows the designer to use grounds that vary slightly. It also allows the power-supply designer to place the sense resistor away from the PWM IC using the op amp to buffer the signal. The op amp should be ground-referenced to the same analog ground as the PWM IC.