原创 Switching Power Supply (二)

中文版：   http://blog.ednchina.com/likee/98420/message.aspx

       Tandem Switching Power Supply

In the last capture，(http://blog.ednchina.com/likee/97144/message.aspx)We know that in the basic power_transfer circuit,with the input vlotage fluctuating or the load changed,the output voltage will increase or decrease correspondingly!Now in order to make the ripple of the output vlotage as low as possible and the output voltage as stable as possible,what measure should we take?

We know that:

            Uo = qU1.

So, if we decrease the dutyfactor when the Uo increases, and increase the dutyfactor when the Uo decrease.the output vlotage will much stabler. So as a IT member, It is not so hard to user negative feedback to solve this problem.We use the sampling vlotage of Uo via feedback to control the dutyfactor of the uB. In this case , Tandem Switching Power Supply appears.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Tandem Switching Power Supply includes compensating pipe,swhich drive circuit,sampling circuit,triangle generate circuit, reference voltage circuit,compare-amplifier circuit filtering circuit and so on.

The basic circuit is as follows:

In this circuit,bacause all of the switch and the filtering circuit have no consumption,So it also abey the formula:

Uo = qU1.

                 Parallel Switching Power Supply

In the Tandem Switching Power Supply,The compensating pipe and the load is in series,so the output vlotage is always less than the input vlotage,So it also named as Buck Switching Power Supply.But in application,we need to make the direct current supply output rise above the input.so it named boost switching power supply.In this part of circuit,the compensating pipe and the load is paralleled,So it also named Parallel Switching Power Supply.Because the inductance can store the power,and the circuit add this voltage and the input voltage together to the load,So the output vlotage is larger than the input vlotage.

The basic circuit is as follows:U1 is the  input vlotage, transistor T is the compensating pipe, uB is square wave. Inductance  L  and the capacitance  C  consiste of filtering circuit, D is the fly-wheel diode .

The state of T is controled by uB,when the uB is high, U1 can electricize the L ,and the charge current almost lineary increase. The direction of the current is as the arrow in the circuit.When the uB is low , T is cut off. L generate the induced electromotive force, whose direction prevent the change of the current.So it is the same with U1,the two voltage added up together to electricize capacitance C via D.the equivalent circuit is showed as (c), so inspite of the state of the T and D,the direction of the load current will not changed.

Note that only if the L is large enough,the output voltage is higher than the input voltage. And only if the capacitance C is large enough,the ripple of output voltage is low enough, when the period of the uB is fixed , the larger the dutyfactor is ,the higher the output voltage is .