SLOA044A March   2000  – June 2021 TL2842 , TL2842B , TL2843 , TL2844 , TL2845 , TL3842 , TL3842B , TL3843 , TL3844 , TL3845 , TLC081 , TLC2201 , TLV2231 , TLV2361 , TLV2362 , TLV2470 , TLV2471 , TLV2770 , TLV2771 , TLV341 , TLV342 , UC1842 , UC1843 , UC1844 , UC1845 , UC2842 , UC2842A , UC2843 , UC2843A , UC2844 , UC2844A , UC2845 , UC2845A , UC3841 , UC3842 , UC3842A , UC3842M , UC3843 , UC3843A , UC3844 , UC3844A , UC3845 , UC3845A , UC3845AM , UCC1800 , UCC1801 , UCC1802 , UCC1803 , UCC1804 , UCC1805 , UCC2800 , UCC2801 , UCC2802 , UCC2802M , UCC2803 , UCC2804 , UCC2805 , UCC2813-0 , UCC2813-1 , UCC2813-2 , UCC2813-3 , UCC2813-4 , UCC2813-5 , UCC28C40 , UCC28C41 , UCC28C42 , UCC28C43 , UCC28C44 , UCC28C45 , UCC3801 , UCC3802 , UCC3803 , UCC3804 , UCC3805 , UCC3813-0 , UCC3813-1 , UCC3813-2 , UCC3813-3 , UCC3813-4 , UCC3813-5 , UCC38C40 , UCC38C41 , UCC38C42 , UCC38C43 , UCC38C44 , UCC38C45

 

  1. 1Introduction
  2. 2Circuit Design
  3. 3Advantages
  4. 4Choosing the Op Amp
  5. 5Conclusion
  6. 6Revision History

2 Circuit Design

A switch-mode power supply often switches current on the primary side of a transformer through a MOSFET and measures the primary current with a sense resistor (RSense) as shown in Figure 1-1 (a). The pulse width modulator IC (PWM) usually requires a current-sense signal (VS) in order to provide short-circuit protection or for use in current mode control, or for both protection and control. The peak value of VS depends on the PWM IC used, but it is typically 1 volt.

The value of the sense resistor RSense in Figure 1-1 (a) is chosen based on the peak value of the primary-side current (IPeak) and the required value of VS. Therefore, RSense is determined by:

R s e n s e = V s I p e a k

The power dissipation in RSense is based on the RMS value of the primary-side current (Irms), which depends on the peak value as well as on the waveshape and the duty cycle. The power dissipated is:

P s e n s e   =   I r m s   2 ×   R s e n s e

As an example, let:

IPeak = 6.67 A

Irms = 4 A

VS = 1 V

These values result in an RSense of 0.15 Ω, and a power dissipation in RSense equal to 2.4 W. Typically, a 5-W-rated resistor would be used in this application.

The circuit of Figure 1-1 (b) can be used to significantly reduce the cost and power dissipation of RSense. First, let us review how the circuit of Figure 1-1 (b) operates. This op-amp circuit is configured as a typical differential amplifier. The circuit operates by multiplying the differential sense signal (VSense) by the differential gain of the op-amp circuit. If Rf = R3 and Ri = R2, this gain is:

G a i n   =   R f R i   a n d   V s   =   V s e n s e   x   R f R i  

Using the previous example, assume that the design goal is to use a lower-power sense resistor with a standard value, such as a 0.01-Ω resistor rated at 0.5 W, and to limit the dissipation of this resistor to no more than 0.25 W. From this information, the gain of the circuit can be calculated.

P s e n s e   =   I r m s   2 ×   R s e n s e   =   4   A r m s 2 ×   0.01   Ω   =   0.16   W
V s e n s e   =   I p e a k   ×   R s e n s e   =   6.67   A p k   ×   0.01   Ω   =   66.7   m V
G a i n = V s V s e n s e = 1 V 66.7   m V = 15

Based on these results, let Rf = R3 = 15 kΩ and Ri = R2 = 1 kΩ in Figure 1-1 (b).