7004 Datasheet, PDF (7/12 Pages) Bourns Electronic Solutions – Network Interface Device

Double-Ended Topologies
There are several commonly employed variants of double-ended converters. The terminology
implies that the magnetic flux swing in the transformer primary is bi-directional. In other words,
the transformer is being actively driven in two directions, as opposed to the forward and fly-
back topologies where the transformer is driven in one direction and allowed to reset its core
flux naturally. The double-ended approach has the advantage of utilizing the core volume more
efficiently, and therefore allowing the use of a physically smaller transformer core. The price
paid for this better utilization is more power switches and more complex transformer windings.
There are two main topologies to discuss, push-pull and bridge.
Push-Pull Designs
Figure 8 shows a basic push-pull design. The main advantage of this design is grounded-
source switches. This allows for a simple gate-drive design, requiring only two switches. The
main disadvantage, like the single transistor forward and flyback designs, is the drain voltage
of the two switches is inherently unconstrained. Therefore, there will typically be some form of
snubbing and/or clamping, usually passive, on the FET drains. Operation is quite simple. Each
FET gets turned on for some fraction of one half of an entire cycle. If each FET were to be
operated for exactly 50% of a full period, the effective duty factor at the output of the second-
ary rectifiers would be 100%. This cannot be accomplished in practice, but a close approxima-
tion can be achieved. Also, the effective secondary side ripple frequency is doubled. This
helps to minimize the size of the secondary inductor. Figure 9 shows idealized gate drive, FET
drain, and transformer secondary waveforms. It should be noted that the theoretical minimum
drain voltage seen by a primary FET in this configuration is 2-times the DC input voltage. As
one FET is driven on, a voltage of Vin is developed across one half of the transformer primary.
An equal voltage will be induced in the other half of the winding. This voltage is added to the
input voltage, and impressed on the drain of the off FET. Since there is also unclamped leak-
age inductance associated with the transformer primary, there will be spikes substantially
higher than twice the input. Consequently, only the 200V FDS2670 is suitable for applications
where inputs as high as 72V could occur. About twice the power level of a single transistor
forward converter can be obtained.
+
V_IN ^{CIN CONTROL}
–
Q1 _T
Q2
D
L
+
D
C_OUT ⁺ V_O
–
Figure 8. A basic push-pull design.
©2001 Fairchild Semiconductor Corporation
7
Rev. A, June 2001