Buck/boost

Buck/Boost Converter Buck/Boost Converter... 

The operation of a discontinuous-mode, flyback converter is quite different from that of a forward-mode converter, and likewise their control-to-output characteristics are very different. The topologies that fall into this category of control-to-output characteristics are the boost, buck/boost, and the flyback. The forward and flyback-mode converters operating under current-mode control also fall into this category. Only their dc value is determined differently. Their representative circuit diagram is given in Figure B-12. 

Active power factor correction circuits can take the form of nontransformer isolated switching power supply topologies, such as buck, boost, and buck/boost. The buck topology in Figure C-3 produces an output dc voltage lower than found at its input, whenever the PFC stage is operating (F > Fom). In other... 

We all know that the losses in the output capacitor of any Flyback are high, due to the choppy waveform of the current they encounter coming through the diode. It is obvious that reducing the ESR to zero would totally knock off a major chunk of losses, boost the published efficiency curves, and allow a much higher maximum achievable power for the device. However, OmQ would have been much too obvious, wouldn t it So it was a case of Buck the ESR and Boost the efficiency. In other words, a perfect Buck-Boost. 

Note that in Figure 6-3, we have a negative to positive Buck-Boost. But in fact the very same arguments and conclusions apply to a positive to negative Buck-Boost. There is no difference in layout principles, except, of course, for the fact that Ground is different. 

Figure 6-3 Buck-Boost 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 the Boost and the Buck-Boost, we see that the output capacitor is in the critical path. So this capacitor should be close to the control IC, along with the diode. A paralleled ceramic capacitor can also help, provided it does not cause loop instability issues (especially in voltage mode control). 

Note that in the Buck and the Buck-Boost, the input capacitor is included in the critical path. That implies we need very good input decoupling in these topologies (for the power section). So, besides the necessary bulk capacitor for the power stage (typically a tantalum or aluminum electrolytic of large capacitance), we should also place a small ceramic capacitor (about 0.1 to IpF) directly between the quiet end of the switch (i.e., at the supply side) and the ground—and also as close as possible to the switch. 

But in a Boost or Buck-Boost, the average inductor current is equal to 70/(l - D). Further, the peak current in all cases is typically 20 to 30% higher... 

In other words, the transformer-based Flyback behaves just like an inductor-based Buck-Boost, with the difference that the output voltage is V0R, not V0. So decreasing V0R calls for a decrease in D. However, the same input power still has to be drawn from the switch. So if the width of its waveform decreases, the height of the waveform must increase. Which means that the inductor current must also increase. So, decreasing V0R could also end up decreasing efficiency. That is why for most universal-input Flybacks, the best Vqr compromise is about 90V to 105V. 

There is something puzzling about the statements above in case you haven t noticed How are we concluding that a decrease in D causes an increase in the inductor current So far we have been led to believe that in a Buck-Boost or Boost topology, the inductor current equals 70/(l - D), which implies that the inductor current goes up as D increases, not decreases ... 

But, if we keep the turns ratio fixed, the Flyback certainly follows the known behavior of the Buck-Boost and Boost with respect to changes in D. 

What happens if Vsw < VD In fact that is the situation in most commercial Flybacks. But note that to do a proper comparison, you have to reflect the diode drop to the primary side. And for that we have to multiply the diode drop by the turns ratio (see the equivalent Buck-Boost models of a Flyback section in my book, Switching Power Supply Design Optimization). So, for example, if the turns ratio is 20 and the diode drop is 0.6V, the effective VD we need to compare with Vsw for our time-sharing analysis is 0.6 x 20 = 12V. And that is usually greater than the (average) drop across the switch. Therefore, we tend to say that in a Flyback, decreasing D (increasing input) will worsen the total conduction loss and decrease the efficiency. But of course that never happens, because as we increase the... 

Blog Entry 3 A more simple solution to implement is to use the 3478 Low-side N-channel controller in a SEPIC configuration. This application utilizes two inductors (instead of a Flyback transformer) to attain the Buck-Boost function. The 3478 requires an external user-selectable Fet switch, so you can choose the one that suits your load current requirement. The datasheet provides an application rationale for SEPIC configuration on page 19, Figure 13. The output voltage can be set to 12V by changing the value of the feedback resistor. 

Figure 12-6 A Simple 4-switch Noninverting Buck-Boost...

What follows is what you should really know about the Boost and Buck-Boost topologies themselves. 

More sophisticated systems are able to remove charges from cells which have a higher voltage than the others and to bring them to cells which have a lower voltage in the series connection [58], Among the main converter topologies, the bidirectional nondissipative buck-boost converter [59,60] and the flyback converter [61,62] are the most studied. Active systems appear to be sophisticated circuits and thus may be expensive in relation to the cost of the cell they are intended to protect. 

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