Average input current

Id. For flyback-mode converters it is a good idea to select a power switch average current rating of about 1.5 times the maximum average input current of the supply. Another consideration is the loss. By overspecifying the current the /2i DS(on) loss (conduction loss) can be reduced with very little penalty on cost and input capacitance. 

The power winding of the inductor (transformer) not only must support the maximum average input current but the output current as well. So, the wire gauge of the winding should be... 

Note that in a Buck, the average input current is NOT the load current (as the engineer was rather intuiAvely visuaflzing), but the average switch current. The average switch current is of course related to the load current by 7SW = Io x D (for a Buck). 

Average input current 5 [inductor ripple current, peak-to-peak value Jin Avg [i-IPP 0.93 A (0.65 A... 

If Lpn, ton, and duty are all fixed, then the average input current is proportional to the input voltage, resulting in an ideal power factor of unity. An example of the flyback power factor corrector is shown in Fig. 4.82. 

Average Input Current equals Average Switch Current Average Output Current Is equal to average Diode Current... 

For all topologies the average input current (and therefore switch current) must increase as the input voltage decreases, so as to continue to satisfy the basic power requirement expressed by Pin = Iin x Vin = Po/q (where q is the efficiency, assumed fixed). Therefore, the switch RMS current is a maximum at Vinmin 0-c. Dmax) for all topologies. 

The average input current tells us what the actual duty cycle D is, because Iin/D is also the center of the primary-side current ramp, and must equal Ilr, that is,... 

As a corollary, the core loss in the transformer is independent of the input voltage. The copper loss, on the other hand, is always worse at low inputs (except for the dc-dc buck) — simply because the average input current has to increase so as to continue to satisfy the basic power requirement Pin = Vin x Iin = Po-... 

Question 33 What is the average input current equal to for the three topologies ... 

So the average input current is equal to the average inductor current — which we know is Io/(l — D) for the boost. Let us again do a check in terms of power... 

Coming to the buck-boost, the situation is not so clear at first sight. The input current flows into the inductor when the switch is ON, but when the switch turns OFF, though the inductor current continues to flow, its path does not include the input. So the only conclusion we can make here is that the average input current is equal to the average switch current. Since the center of the switch current ramp is Io/(l — D), its arithmetic average is D x Io/(l — D). And this is the average input current. Let us check this out ... 

Answer For the buck, we know that average inductor current is equal to the output current, that is, II = Io- For the boost we know it is equal to the input current, that is, II = Iin- But for the buck-boost it is equal to the sum of the (average) input current and the output current. Let us check this assertion out... 

Then by simple waveform analysis, the average input current (which is the average switch current for this topology) can be calculated from the rectangular waveform of height 2 A with a duty cycle of 0.25. This gives an average input current of 2 A x 0.25 = 0.5 A. So the input power is 20 V x 0.5 A = 10 W. We can see that that is exactly the same as the output power, and therefore we are at 100% efficiency. 

Peak Current Switch/Diode/ Inductor (A) Average Current in Switch (A) Average Current in Diode (A) Average Input Current (A)... 

Figure 4 shows both the motor current (inverter output) and the converter power (drive input kVA or kW). The converter provides current to the DC bus at unity power factor. Current is supplied at rated drive input voltage and power utility frequency. Because of the separation of the drive input and output sections provided by the DC bus, the average input current at rated input voltage will be less than the average motor current, which is supplied at a lower average output voltage. 

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