HP HP-Bench-Briefs-1984-03-04 free download

File name HP-Bench-Briefs-1984-03-04.pdf

HEWLETT PACKARD f-1 SERVICE INFORMATION FROM HEWLElT-PACK,, MARCH-APRIL 1984 egullated Linear Power Supplies Idc power supplies Preregulator Don Fielder, Hewlett-Packard control In the JanuarylFebruary issue of Bench Briefs we saw that one of the Series regulator major considerations of a linear supply is the internal power dis- sipated by the series regulator. o < = 1 We found that by adding a pre- regulator t o control the voltage input, we could reduce the power dissipated a n d t h e number of transistors in the series regulator. OUT 0-40V c What the Preregulator Does The preregulator, as shown in 3-43v Figure 1, monitors t h e voltage A across t h e series regulator and adjusts the input Vin to maintain the regulator voltage at Figure 1. Simple diagram of a series regulated power supply with preregulator control. approximately three volts. This regulator voltage is held relatively constant regardless of input o r output conditions. Therefore, when troubleshooting a unit that has no control (either locked up or down), in almost every case, if you have any output the preregulator circuit is probably operating properly. If you have no output but do have regulator voltage (three to six volts), again no problem with the pre- regulator. ONLY when you have BOTH no output AND no regulator voltage (<1volt) should you suspect the preregulator. How the Preregulator Works Figure 2 is a simplified diagram of Figure 2. Simplified block diagram of the preregulator. Letters within circles refer to f the preregulator and Figure 3 shows waveforms shown in Figure 3. the waveforms monitored at various voltage a t point A, rectifies i t , the summing capacitor. The series points in t h e circuit. The line divides it down to about two volts regulator voltage is als