SLUA560D June 2011 – March 2022 UCC28950 , UCC28950-Q1 , UCC28951 , UCC28951-Q1

11 Setting Up the Current Sense (CS) Network (CT, R_S, R_RE, D_A)

The current sense transforer (CT) chosen for this design had a turn’s ratio (a2) of 100:1. This transformer was selected to attenenuate the T1's primary current for current sensing to reduce power disipation in the current sense resistor (R_S) improving system efficency.

Equation 83.

a 2 = \frac{I_{P}}{I_{S}} = 100

Calculate nominal peak current (I_P1) at V_INMIN:

Peak primary current:

Equation 84.

I_{P 1} = (\frac{P O U T}{V O U T \times η} + \frac{Δ I L O U T}{2}) \frac{1}{a 1} + \frac{V_{I N M A X} \times D_{M A X}}{L_{M A G \times} 2 \times f_{s}} \approx 3.3 A

The voltage where peak current limit will trip.

Equation 85.

V_{P} = 2 V

Calculate current sense resistor (R_S) and leave 200 mV for slope compensation:

Equation 86.

R_{S} = \frac{V_{P} - 0.2 V}{\frac{I_{P E A K}}{a 2} \times 1.1} \approx 49.9 Ω

Select a standard resistor for R_S:

Equation 87.

R_{S} = 48.7 Ω

Estimate power loss (P_RS)for R_S:

Equation 88.

P_{R S} = {(\frac{I_{P R M S 1}}{a 2})}^{2} \times R_{S} \approx 0.03 W

Calculate maximum reverse voltage (V_DA) on D_A:

Equation 89.

V_{D A} = V_{P} \times \frac{D_{C L A M P}}{1 - D_{C L A M P}} \approx 29.8 V

Estimate D_A power loss (P_DA):

Equation 90.

P_{D A} = \frac{P_{O U T} \times 0.6 V}{V_{I N M I N} \times η \times a 2} \approx 0.01 W

Calculate R_S reset resistor R_RE:

Resistor R_RE is used to reset the current sense transformer CT.

Equation 91.

R_{R E} = 100 \times R_{S} = 4.87 k Ω

Resistor R_LF and capacitor C_LF form a low pass filter for the current sense signal (Pin 15). For this design we chose the following values. This filter has a low frequency pole (f_LFP) at 482 kHz. This should work for most applications but maybe adjusted to suit individual layouts and EMI present in the design.

Equation 92.

R_{L F} = 1 k Ω

Equation 93.

C_{L F} = 330 p F

Equation 94.

f_{L F P} = \frac{1}{2 π f \times R_{L F} \times C_{L F}} = 482 k H z

The current sense network dissipated roughly 0.04 W and had very little effect on the power budget.

Equation 95.

P_{B U D G E T} \approx P_{B U D G E T} - P_{R S} - P_{D A} \approx 5.96 W

11 Setting Up the Current Sense (CS) Network (CT, RS, RRE, DA)

11 Setting Up the Current Sense (CS) Network (CT, R_S, R_RE, D_A)