Switchers

In ae off-line applications the common-mode choke is typically used and its design procedure can be found in Appendix E. The Alters resemble pi Alters, but are actually bi-directional L-C Alters. The important function is to Alter the noise generated by the switcher before it exits via the input power lines. 

In addition to circuit breakers, there are other classes of automatic switches that can be controlled or operated remotely, but with current-interrupting capability. These include circuit switchers, reclosers, and sectionalizers. 

Unless a dual-excitation filter cube is used, in combination with a excitation switcher. 

The EMI characteristics will change appreciably only if the switcher IC used has soft-switching characteristics... 

Figure 2-7 A Small Ceramic Decoupling Capacitor Needs to be Mounted Very Close to the Switcher 1C...

However, as you will see a little later, it is not a bad idea to always include this 0.1 pF input decoupling capacitor. The reasons may be different on different occasions, and for different types of switchers, but this component is generally always nice to have. 

In general, we always need to put a scope probe on the input pin of a switcher IC and confirm that the ripple is within 1% of the input voltage. Otherwise chances are high the control sections will exhibit weird behavior. If not on one prototype, on another Without an actual measurement, you will never know if the problem is just waiting to happen. 

Ultimately, the power supply is only part of a larger system. Therefore, besides being concerned about the effect of noise and ripple on the converter itself, we need to worry about its effect on the rest of the system. The good news is that if the system were excessively noise sensitive, no one would have touched switchers with a ten-foot pole (or a lOdB zero) in the first place. They would have been using those low-noise, power-guzzling LDOs (linear regulators) instead ... 

Yes, we could simply place a high-frequency ceramic capacitor directly across the output to kill the noise appearing there. But remember, many switchers (like the BJT-based switcher family mentioned previously) are actually relying on some minimum ESR at the output to... 

In such cases, we could try to reduce the high-frequency output noise by suppressing it at the input. So that could be a valid reason to place a small ceramic capacitor at the input of an older-generation switcher IC (i.e., one with a BJT switch). Its primary purpose is then not to ensure that the control does not go into chaos because of switch transient noise, but to reduce the output noise in noise-sensitive applications. 

Most of the evaluation boards of such ESR-sensitive parts are shipped out to customers with only aluminum electrolytic or tantalum capacitors at their outputs. But what really happens is that the customer happily connects the eval board (rather expectantly) into his or her system, and completely forgets there are a bunch of ceramic capacitors all over the system board (for local decoupling at different points). In effect, the switcher can lose that valuable zero in its control loop and break into oscillations (see Figure 3-5). More so if the connecting leads are short. 

Figure 3-5 System Input Impedance Can Make the Switcher Unstable...

When designing a PCB for a switcher IC, try to plan ahead and leave the option of moving back from a fixed-voltage part to an adjustable-voltage part (leave room for two resistors). That is a likely retreat in the face of various problems you may encounter. 

Test your switcher s phase margins with something very close to the final load as early as possible. As mentioned previously, even simple startup problems of switchers do not usually show up with resistive loads. For these, you really need to test using an electronic load, placed at least in constant current (CC) mode. 

Figure 3-6 Ways to Stabilize Converters When the System Ends up Destabilizing the Switcher...

In DC-DC converters, we can somewhat slow the turn-on of Fets if we insert a small resistor (10 to 20Q. typically) in series with the decoupling capacitor of their respective driver stages. For example, a small resistor can be placed in series with the bootstrap capacitor of the third-generation switcher family I used to cover. That helped with almost 10 to 20% of customers, but somehow this trick didn t find its way into the applications information section of their datasheet. If the Fets are external, we can try a small resistor in series with the Gate, but this affects both the turn-on and turnoff (with such low threshold voltages, a diode in parallel to the resistor will not do anything). 

Here s how a slightly bad layout can cost you dearly—up to 10% in efficiency. In fact, as I write this, this very issue is bothering us in one of our synchronous Boost switcher ICs. 

Table 5-1 The Parameters Used to Generate the Published Efficiency and Maximum Load Curves of a High-voltage Switcher Family...

Integrated Switcher 1C Solutions Versus Controller 1C Solutions... 

In a Buck we see that it is very important to have good input decoupling, and also to minimize the trace lengths between the ceramic capacitor and the IC. We have discussed that issue previously. But we now see that it is very important to also minimize the length of the trace section connecting the SW pin to the common node of L and D. This is one of the first things I try to check out when I troubleshoot a Buck switcher board handed over to me. 

In a particular case I handled, a major Japanese customer was managing to blow up his simple Buck switcher under short-circuits on the output. I knew that these parts had not only the usual cycle-by-cycle current limiting (their first line of defense), but in fact, a hidden second level of current limit protection, which if ever encountered, caused protective... 

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