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Rev. A, June 2001

©2001 Fairchild Semiconductor Corporation

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.

Figure 8. A basic push-pull design.

D

D

T

Q1

CONTROL

L

–

+

+

–

+

Q2