AC-DC power supplies

One of the hardest to analyze and fix are probably the chain reaction problems. For example, in an AC-DC power supply, we know all too well that the switch can blow... 

For example, in Figure 3-7, we have a typical AC-DC power supply (Flyback or Forward). The threshold voltage of such a high-voltage Fet is typically high (around 8V). It is common to try to achieve fast turnoff but relatively slower turn-on. The concern is that if... 

In this particular chapter, we will focus a great deal on ceramic capacitors since these have become extremely popular today. However, in commercial AC-DC power supplies, the aluminum electrolytic (or elko) is still king, so we will talk about that component too. Unfortunately, we will have to pretend none of the others even exist. We just don t have the space for all of them here. 

When making commercial AC-DC power supplies, you will find that large electrolytic capacitors and also transformers and inductors can easily tear off the board in any standard... 

On a typical one-sided board (still very common in commercial AC-DC power supplies), as the number of capacitors you try to parallel goes up, so does the intervening PCB trace impedance. Take, for example, Figure 5-1, where we have the simple case of one output capacitor. A small advisory here—if you try to reduce the impedance further by making the current loop smaller and smaller, the capacitor would eventually start comparing notes with the heatsink on the topic of temperature, and that can t be good for its life expectancy. 

Let us take the popular 384x family for the purpose of illustrating some key routing principles in AC-DC power supplies. 

In general, most converters are tested on the bench with the electronic load set to constant current (CC mode). True, that s not benign, nor as malignant as it gets. But the implied expectation is that converters should at least work in CC mode. They should, in particular, have no startup issues with this type of load profile. But even that may not be the end of the story Some loads can also vary with time. For example, an incandescent bulb has a resistive profile, but its cold resistance is much lower than its hot resistance. That s why most bulbs fail towards the end of their natural lifetime just when you throw the wall switch to its ON position. And if the converter is powering a system board characterized by sudden variations in its instantaneous supply current demand, that can cause severe problems to the converter, too. The best known example of this is an AC-DC power supply inside a computer. The 12V rail goes to the hard disk, which can suddenly demand very high currents as it spins up, and then lapse back equally suddenly into a lower current mode. 

Once you have a stable clock, you might like to check the jitter. So how much jitter is acceptable In an AC-DC power supply with no PFC correction, especially at low line, the input voltage ripple is quite high. As this instantaneous input voltage moves up and down, a... 

Now, where could the problem lie Is it within the power supply stage itself, or is it external If you have peeled the onion as I suggested previously, you are hopefully sure the efficiency is not being lost within the EMI filter of your AC-DC power supply. Bypass or disconnect everything external and confirm you have a problem. 

Figure 11-1 One Possible Way of Conducting a Noise and Ripple Measurement for an AC-DC Power Supply... 

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