Output rectifiers

To estimate the loss within each output rectifier within a multiple output switching power supply, use the ratio of the desired output s power compared to the total output power as in Equation 3.14. 

Power Switch and Drive (%) Output Rectifier (%) Magnetics (%) Miscellaneous (%)... 

One last factor is the physical layout of the output stage when more than one output filter capacitor is used. The capacitors should be located radially symmetric from the output rectifier, and the printed circuit traces for the rectified voltage and the grounds should be of similar trace-widths and lengths. Any dissimilarity of these traces causes more series resistance and inductance to the... 

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. 

The output rectifier represents between 40 and 65 percent of the total losses within the typical nonsynchronous rectified switching power supply. So, it is very important to understand this section. The waveforms associated with the output rectifier can be seen in Figure 4-2. 

Analyzing the switching loss of an output rectifier is much more complicated. The inherent behavior of the rectifier itself causes problems within the local circuits. 

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. 

Continuous-mode converters, both forward and boost, suffer from one common problem. The output rectifiers have forward current flowing through them just... 

Since both ends of the primary winding have a single-ended loaded winding during their respective turn-off transitions, each of the MOSFE R accomplish ZVS turn-off. The output rectifiers gain some efficiency since their current transitions appear more zero-current switching in nature. 

Another major source of noise is the loop consisting of the output rectifiers, the output filter capacitor, and the transformer secondary windings. Once again, high-peak valued trapezoidal current waveforms flow between these components. The output Alter capacitor and rectifier also want to be located as physically close to the transformer as possible to minimize the radiated noise. This source also generates common-mode conducted noise mainly on the output lines of the power supply. 

One subtle, but major noise source is the output rectifier. The shape of the reverse recovery characteristic of the rectifiers has a direct affect on the noise generated within the supply. The abruptness or sharpness of the reverse recovery current waveform is often a major source of high-frequency noise. An abrupt recovery diode may need a snubber placed in parallel with it in order to lower its high-frequency spectral characteristics. A snubber will cost the designer in efficiency. Finding a soft recovery rectifier will definitely be an advantage in the design. 

Silicon diodes used as output rectifiers in SMPC circuits also contribute to power loss. As the diodes switch between the forward- and reverse-biased states, a... 

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