High-voltage switcher

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

Once, while working as an Apps engineer at this maker of high-voltage Flyback monolithic switcher ICs, I happened to express concern about the way they were selecting the input bulk capacitors for their evaluation boards. To this day, in my opinion, they are making several misleading/erroneous recommendations to their customers via their eval boards. 

Surprisingly, device eccentricities may on occasion play a part in defining the limits of r too. For example in Figure 2-8 we have presented the current limit plot of an integrated high-voltage flyback switcher IC called the Topswitch . On it we have superimposed a typical switch current waveform, just to make things a little clearer. 

Now returning to another issue I promised to touch upon in last month s column. High-voltage off-line integrated flyback switcher ICs are available from several vendors, but they are restricted because they usually come only in a family of fixed current limits. So if for example we have a 5 A part, the next lower part being a 3 A part, the 5 A part is certainly optimum for peak currents slightly below 5 A. But what if the peak current in our particular application is 4 A For lack of a suitably matching part, we would now be forced to use a... 

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. 

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