7004 Datasheet, PDF (8/12 Pages) Bourns Electronic Solutions – Network Interface Device

V_{SW 0}
I_{PRI 0}
2_VIN
V_IN
SW₂
SW₁
V_SAT
I_PK
TIME
TIME
I_MIN
V_GS(Q1)
PWM'ED EDGE
V_GS(Q2)
V_DS(Q1)
-2V_IN
-V_IN
V_DS(Q2)
D1(ANODE)
D2(ANODE)
I₀
I_L1
Figure 9. Voltage and current waveforms for the basic push-pull design.
Full-Bridge Designs
Full-bridge designs, sometimes called four transistor bridges, are generally used at very high
power levels, mainly due to the cost of these designs. Two of the four switches have floating
sources, so they each require an isolated gate drive, usually transformer coupled. The power
transformers primary winding design is a simple single winding. The secondary, is generally
center-tapped, at least for very low output voltage designs. The main advantage of the bridge
topology is that the drain voltages of all four switches are constrained to Vin. Energy in the
transformer’s leakage inductance gets returned to the input and can help force commutate the
switches. The switches get turned on across a diagonal of the bridge. Figure 10 shows the
basic circuit design, and Figure 11 shows the idealized waveforms. Since the drain voltage is
well clamped to Vin without snubbers, there is no concern for passive energy losses. This also
allows the use of the 80V, FDS3570 in all four locations. With these devices, output power
©2001 Fairchild Semiconductor Corporation
8
Rev. A, June 2001