9014 查看數據表（PDF） - Fairchild Semiconductor

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9014

Fairchild QFET for Synchronous Rectification DC to DC Converters

Fairchild Semiconductor 

Equations (4-5), (4-6) and (4-7) show that the MOSFET used in the DC-DC converter needs

low RDS(on), low Qg, and fast switching speed characteristics in order to increase the system

efficiency.

Now let's calculate the losses using the measured data, after having applied FQP60N03L to

Q1 and Q2, as shown in Figure 4-1, and where the test conditions are input voltage = 5V, out-

put voltage = 2V, and load current = 7A.

(1) Q1

Pc = 4.972[A] × 19.03[mΩ] ≅ 470.06[mW]

Pg – c = 6.30[V] × 18.04[nC] × 311[kHz] ≅ 35.35[mW]

Pt(on) = 0.7[V] × 2.8[A] × 130[ns] × 311[kHz] ≅ 79.24[mW]

Pt(off) = -5---.--5----0----[--V-----]----×-----8---.--0----4----[--A------]---×---2--2----4---1----[---n----s---]----×-----3---1----1----[---k----H-----z----] ≅ 1657.17[mW]

Pt = Pt(on) + Pt(off)

≅ 79.24[mW] + 1657.17[mW] = 1736.41[mW]

PQ1 = Pc + Pg – c + Pt

≅ 470.06[mW] + 35.35[mW] + 1736.41[mW]

= 2241.82[mW]

(2) Q2

Pc = 2.892[A] × 11.55[mΩ] ≅ 96.47[mW]

Pg – c = 10.32[V] × 27.22[nC] × 311[kHz] ≅ 87.36[mW]

PQ2 = Pc + Pg – c

≅ 96.47[mW] + 87.36[mW]

= 183.83[mW]

*Ptotal = PQ1 + PQ2

≅ 2241.82[mW] + 183.83[mW] = 2425.65[mW]

The RDS(on) value used in the above calculation was taken from the Figure 2-3 graph, and it

corresponds to the RDS(on) value when the MOSFET temperature has reached saturation after

1 hour of set operation. Q2 transition loss is almost zero because there is almost no current

flowing between Q2's drain and source during the transition period, but rather most of the cur-

rent flows through D2, which forms a current pass during dead time. The waveforms in Figure

4-3 are of currents flowing between the drain and source of Q1 and Q2 MOSFETs.

13

Rev. B, July 2000

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