Forward converter

Within multiple-output forward converters, it is possible to easily combine the filter chokes of complementary outputs (i.e., +/- 5V, etc.) together on the same core (see Figure 3-2f). This offers several advantages it saves space, vastly improves cross-regulation of those outputs, and exliibits superior ripple voltage levels on both outputs. 

For those applications where high efficiency is important, synchronous rectification may be used on the higher current (power) outputs. Synchronous rectifier circuits are much more complicated than the passive 2-leaded rectifier circuits. These are power MOSFE B, which are utilized in the reverse conduction direction where the anti-parallel intrinsic diode conducts. The MOSFET is turned on whenever the rectifier is required to conduct, thus reducing the forward voltage drop to less than O.f V. Synchronous rectifiers can be used only when the diode current flows in the forward direction, that is in continuousmode forward converters. 

Determining the core size. TDK rates its cores by the amount of power that can be handled by the core in a one-transistor forward converter. Its volume requirements are very similar to a flyback converter. The EPC core that rated at 15 W or greater is the EPC 17 core size. The part numbers for this assembly are core, PC40EPC17-Z bobbin,BER17-llllCPH and clamp, FEPC17-A. 

The resulting control-to-output Bode plots for the voltage-mode controlled forward converter are given in Figure B-11. 

The current-mode controlled forward-mode converter exliibits the same dc gain as the voltage-mode controlled forward converter, as shown in Equation B.6. 

The current-mode controlled forward converter has one additional consideration there is a double pole at one-half the operating switching frequency. The compensation bandwidth normally does not go this high, but it may cause problems if the closed-loop gain is not sufficiently low enough to attenuate its effects. Its influence on the control-to-output characteristic can be seen in Figure B-14. 

The next task is to determine the plaeement of the eompensating zero and pole within the error amplifier. The zero is plaeed at the lowest frequency manifestation of the filter pole. Since for the voltage-mode controlled flyback converter, and the current-mode controlled flyback and forward converters, this pole s frequency changes in response to the equivalent load resistance. The lightest expected load results in the lowest output filter pole frequency. The error amplifier s high frequency compensating pole is placed at the lowest anticipated zero frequency in the control-to-output curve cause by the ESR of the capacitor. In short ... 

Figure B-23 An example of 2-pole-2-zero compensation used with a voltage-controlled forward converter.

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So in Figure 6-4, we do trace section analysis for a Forward converter, and find that there are two separate current loops we need to minimize here. The differences between Figure 6-4 and Figure 6-1 are subtle but important. The latter is in effect only one current loop, even though it spans both the input and output sections. 

Figure 6-4 Forward Converter from Top to Bottom—Switch ON, Switch OFF, Critical Traces...

In Figure 6-5 we carry out analysis for a Flyback PCB, and realize that unlike a Forward Converter, even the output capacitor has to have very short interconnecting leads and trace lengths. That situation is similar to the Buck-Boost in Figure 6-3, from which the transformer-coupled Flyback is essentially derived. 

In a Forward converter, the high-frequency current loop encloses the transformer, the output diode, and the freewheeling diode. This loop must be minimized. Note that the output choke and output capacitor see relatively smooth (low-frequency) current, so their positioning is not critical. 

Another inverter my colleague was making years ago looked a lot like Figure 8-5. He had been having some success, and was feeling optimistic, until I asked him where the output choke wasl You don t make a Forward converter without an output choke He had apparently been lured astray by similar looking schematics of traditional AC inverters made from iron laminations. But this was a high-frequency switcher, man ... 

Any Buck-derived topology (e.g., the Forward converter, the Half-Bridge, the Push-Pull, the Full-Bridge, etc.) needs an output choke. Otherwise it is akin to running a Buck without its inductor—you can thereby create a dead short cross the input supply rails. 

Note The off-line forward converter transformer is probably the only known exception to the above logic. We will learn that if we for example double the duty cycle (i.e double toN), then almost coincidentally, Von halves, and therefore the voltseconds does not change (and nor does AI). In effect, AI is then independent of duty cycle. 

Off-line converters are derivatives of standard dc-dc converter topologies. For example, the flyback topology, popular for low-power applications (typically <100 W), is really a buck-boost, with its usual single-winding inductor replaced by an inductor with multiple windings. Similarly, the forward converter, popular for medium to high powers, is a buck-derived topology, with the usual inductor ( choke ) supplemented by a transformer. 

The flyback inductor actually behaves both as an inductor and a transformer. It stores magnetic energy as any inductor would, but it also provides mains isolation (mandated for safety reasons), just like any transformer would. In the forward converter, the energy storage function is fulfilled by the choke, whereas its transformer provides the necessary mains isolation. 

The procedure presented in this section applies explicitly to the single-switch forward converter. However, the general procedure remains unchanged for the two-switch forward converter as well. 

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