Diodes catch

Figure 6-11 Catch Diode Placed Very Close to the IC, Between SW and GND Pins...

In one case, I remember that a standard Non-Synchronous Buck switcher IC was not working right. Everything seemed OK, the PCB, the decoupling, and so on. We tried swapping the switcher IC at first, but the problem stayed with that board. Eventually we traced it to the Schottky catch diode. We then discovered that cheap... 

What are the reasons for clock instability High-frequency noise is always generated at turn-on and turnoff in any switcher. This noise can infiltrate into the IC via various pins. It can be very hard to filter out and control. You may need to ultimately simply avoid turning the Fet OFF too dramatically. In most switchers, the turn-on transition is traditionally delayed (or slowed) just a little, so as to allow the output/catch diodes to recover... 

Though the detailed functioning of the modern buck regulator of Figure 1-2 will be explained a little later, we note that besides the obvious replacement of R with an L, it looks very similar to the bucket regulator — except for a mysterious diode. The basic principles of power conversion will in fact become clear only when we realize the purpose of this diode. This component goes by several names — catch diode, freewheeling diode, commutation diode, and output diode, to name a few But its basic purpose is always the same — a purpose we will soon learn is intricately related to the behavior of the inductor itself. 

The essential difference from a conventional buck regulator is that the low-side mosfet in a synchronous regulator is designed to present a typical forward drop of only around 0.1 V or less to the freewheeling current, as compared to a Schottky catch diode which has a typical drop of around 0.5 V. This therefore reduces the conduction loss (in the freewheeling path) and enhances efficiency. 

To control EMI, ferrite beads (preferably of lossy nickel-zinc material) are sometimes placed in series with catch diodes (often slipped on to their leads), such as at the output diode of a typical off-line flyback. However, these beads must be very small, as they can have a significant effect on the efficiency of the power supply. 

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