Switching power supplies PWM

The Building-block Approach to PWM Switching Power Supply Design... 

Figure 3-6 Design flow for a PWM switching power supply.

The power switch and rectifier ac-current loops contain very high trapezoidal current waveforms typical in PWM switching power supplies. These waveforms are rich in harmonics which extend far above the basic switching frequency. These ac currents can have peak amplitudes two to five times that of the... 

The following examples, though tedious, should provide a good insight into the design process of a PWM switching power supply. 

Major Losses within the PWM Switching Power Supply... 

Section 3.4, Table 3-3, provides some insight as to where the major losses occur and to what degree. The losses highlighted in that table are for the basic PWM switching power supply without much effort placed into making them more efficient. These efficiency numbers, therefore, can be seen as the baseline... 

A PWM switching power supply that is designed with no extraordinary loss-control methods will exliibit efficiencies as seen in Table 3-3. For switching power supplies that have no problem in getting rid of the heat, such as some off-line applications, the aforementioned efficiencies may be satisfactory. For portable applications and equipment that must be small in size much better efficiencies must be sought. To improve the overall efficiency of a power supply, several techniques can be used. 

Switching losses occur at two equivalent nodes within every switching power supply the drain (or collector) of the power switch(es), and the anode of the output rectifier(s). These are the only ac nodes within each type of PWM switching power supply. Within the nontransformer isolated topologies, these nodes are physically one node where the collector (or drain) of the power switch is directly connected to the anode of the output rectifier. Within transformer-isolated topologies, these two nodes are separated by the transformer and the two nodes are treated slightly differently. 

As with PWM switching power supplies, there are comparable topologies within the zero-current switching (ZCS) and zero-voltage switching (ZVS) quasi-resonant families. You ll immediately recognize the family members upon seeing them. 

Selecting the core material is the first issue to be addressed. All core materials are alloys based on ferrite. The major factor in a material s worthiness is its loss at the frequency of operation and the flux density of the application. A good place to start is with the materials the core manufacturer s themselves recommend for PWM switching power supplies and those that are commonly used by the designers in the field (see Table D-f). 

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