Switching power supplies topologies

Figure 3-59 The major current loops within the major switching power supply topology types (a) the nonisolated buck converter (h) the nonisolated boost converter (c) the transformer-isolated converter.

Quasi-resonant Switching Power Supply Topologies... 

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... 

There is one node within each switching power supply that has the highest ac voltage compared to the others. This node is the ac node found at the drain (or collector) of the power switch. In nonisolated dc/dc converters, this node is also connected to the inductor and catch (or output) rectifier. In transformer-isolated topologies, there are as many ac nodes as there are windings on the transformer. Electrically, they still represent a common node, only reflected through the transformer. Special attention must be paid to each ac node separately. 

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. 

The zero current switching (ZCS) quasi-resonant (QR) switching power supply forces the current through the power switch to be sinusoidal. The transistor is always switched when the current through the power switch is zero. To understand the operation of a ZCS QR switching power supply, it is best to study in detail the operation of its most elementary topology—the ZCS QR buck converter (and its waveforms) as seen in Figure 4-10. 

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. 

Topology The overall design technique, or layout, by which an electronic device, such as a switching power supply regulator, operates. 

As seen in Section 4.1, the major types of losses are the conduction and switching losses. Conduction losses are addressed by selecting a better power switch or rectifier with a lower conduction voltage. The synchronous rectifier can be used to reduce the conduction loss of a rectifier, but it can only be used for forward-mode topologies, and excludes the discontinuous boost-mode converters. The synchronous rectifier will improve the efficiency of a power supply about one to six percent depending upon the average operating duty cycle of the supply. For further improvements, other techniques must be pursued. 

Answer To reduce stresses at various points inside a power supply, and also to generally reduce the overall size of its components, a current ripple ratio (V) of about 0.4 is considered to be a good compromise for any topology, at any switching frequency. 

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. 

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