HP HP-Bench-Briefs-1986-07-10 free download

File name HP-Bench-Briefs-1986-07-10.pdf

HEWLETT PACKARD Barry Halm, HP New Jersey Division Flyback Converters - Figure 1. Flyback converter We previously discussed the half- bridge converter and its use in the transfer this stored energy to the Now we will define the term auto- off-line switching power supply. Re- output filter and load. Both the dy- ranging, as it is related to dc power member, off-line is defined as a power namic voltage range and energy supplies. supply whose input rectifier circuits transferred are controlled by the time operate directly from the ac power that Q1 and Q2 are on and Ipri is Figure 2 shows the output character- line without transformer isolation. flowing. Note that the primary and istics of three different power sup- This article discusses the flyback secondary current do not flow at the plies. The typical CV/CC power converter and its use in off-line au- same time. Thus the input and output supply, Figure 2A, delivers maximum toranging power supplies. We have are not coupled at the same time so output power at only one combination two new terms, flyback converter and that faults that occur on the second- of output voltage and current. autoranging, to define and relate to ary are not directly reflected to the each other. primary. Diodes D1 and D2 protect The dual-range power supply delivers the transistors from inductive surges maximum output power at two points First, let's look at the flyback con- that occur when the transistors turn by using switch-selectable voltage/ verter in Figure 1 and describe its off. We can describe this circuit by current ranges. operation. saying that "The flyback switching converter works by cyclically storing The autoranging power supply deliv- Transistors Q1 and Q2 are power energy in a magnetic field and then ers maximum output power not only field-effect transistors (FET) that are transferring this stored energy to a at the points described above, but in series and are switched simulta- load." also over a wide and continuous range neously. Both Q1 and Q2 are turned on, initiating current flow in the primary of T1. The primary current MAX (Ipri) is a linear ramp whose rise time is controlled by the magnetizing inductance of the power transformer T1. The output rectifier D3 is reverse biased during this period so that the energy is stored in the magnetic field