Duty cycle maximum

Since fhere cannof be fracfonal furns on an E-E core, round fhis up fo 5.0 furns. This will yield a maximum duty cycle at the minimum input voltage of... 

R8 charges the timing capacitor (C8) and R7 discharges the timing capacitor. First, one decides the maximum duty cycle of the converter. Since the output voltage is about 50 percent of the lowest input voltage, I will choose a 60 percent maximum duty cycle. From the data sheet ... 

Note however, that this is the operating DMAX.When we power down our converter for example, the duty cycle will actually increase further in an effort to maintain regulation (unless current limit and/or duty cycle limit is encountered along the way). Then depending upon the number of missing ac cycles for which we may need to ensure regulation (the holdup time specification), we will need to select a suitable input capacitance and also the maximum duty cycle limit, Dlim, of our controller. Typically, DLim is set around 70%, and the capacitance is selected on the basis of the 3 ptF/W rule-of-thumb. For example, for our 74 W supply with an estimated 70% efficiency at low line, we will draw an input power of 74/0.7= 106 W. Therefore we should use a 106 x 3 = 318 p,F (standard value 330 iF)... 

The second consideration is that even if, for simplicity, we assume zero forward voltage drops across both the switch and the diode, we still may not be able to deliver the required output voltage — because of maximum duty cycle limitations. So for example, in our case, what we need is a (theoretical) duty cycle of Vo/Vin = 14.5 V/15 V = 0.97, that is, 97%. However, most buck ICs in the market are not designed to guarantee such a high duty cycle. They usually come with an internally set maximum duty cycle limit ( Dmax X typically around 90 to 95%. And if that is so, D = 97% would be clearly out of their capability. 

In low-side current sensing, to save the expense of a separate low-resistance sense resistor, the Rds of the low-side mosfet (the one across the optional diode in Figure 4-3) is often used for sensing the current. The voltage drop across this mosfet is measured, and so if we know its Rds, the current through it is also known by Ohm s law. It becomes obvious that in fact for any low-side current sense technique, we need to turn the high-side mosfet OFF, and thereby force the inductor current into the freewheeling path, so we can measure the current therein. That means we need to set the maximum duty cycle to less than 100%. 

Here, we should also keep in mind that the n-channel mosfet is probably the most popular choice for switches, since it is more cost-effective as compared to p-channel mosfets with comparable drain-to-source on-resistance Rds- That is because n-channel devices require smaller die sizes (and packages). Since we also know that the ubiquitous positive buck topology requires a bootstrap circuit when using an n-channel mosfet switch, it becomes apparent why a good majority of buck ICs out there have maximum duty cycles of less than 100%. 

Another product is a fairly popular off-line switcher IC family meant for Flyback applications. In an off-line case, the value of Dmax has several more implications. Here we must recall that the earlier generation of this switcher family had a maximum duty cycle of about 67%. When the next generation was conceived, the one-man product-definition team heuristically assigned a Dmax of 78%. His idea was that by allowing a wider input current pulse, we would automatically get a lower current pedestal, and this would enhance the power capability of his device (since this figure was being based purely on current limit, not on dissipation). 

The forward converter (Fig. 10.82) is a variation of the buck converter. It is usually operated in the CCM to reduce the peak currents and does not have the stabihty problem of the flyback converter. The HF transformer transfers energy directly to the output with very small stored energy. The output capacitor size and peak current rating are smaller than they are for the flyback. A reset windingis required to remove the stored energy in the transformer. The maximum duty cycle is about 0.45, which limits the control range. This topology is used for power levels up to about 1 kW. 

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