Flyback Regulator Operation

The LM2585 is ideally suited for use in the flyback regulator

topology. The flyback regulator can produce a single output

voltage, such as the one shown in

Figure 4, or multiple out-

put voltages. In

Figure 4, the flyback regulator generates an

output voltage that is inside the range of the input voltage.

This feature is unique to flyback regulators and cannot be

duplicated with buck or boost regulators.

The operation of a flyback regulator is as follows (refer to

Figure 4): when the switch is on, current flows through the

primary winding of the transformer, T1, storing energy in the

magnetic field of the transformer. Note that the primary and

secondary windings are out of phase, so no current flows

through the secondary when current flows through the pri-

mary. When the switch turns off, the magnetic field col-

lapses, reversing the voltage polarity of the primary and sec-

ondary windings. Now rectifier D1 is forward biased and

current flows through it, releasing the energy stored in the

transformer. This produces voltage at the output.

The output voltage is controlled by modulating the peak

switch current. This is done by feeding back a portion of the

output voltage to the error amp, which amplifies the differ-

ence between the feedback voltage and a 1.230V reference.

The error amp output voltage is compared to a ramp voltage

proportional to the switch current (i.e., inductor current dur-

ing the switch on time). The comparator terminates the

switch on time when the two voltages are equal, thereby

controlling the peak switch current to maintain a constant

output voltage.

DS012515-21

As shown in

Figure 4, the LM2585 can be used as a flyback regulator by using a minimum number of external components. The switching waveforms of this

regulator are shown in

Figure 5. Typical Performance Characteristics observed during the operation of this circuit are shown in Figure 6.

FIGURE 4. 12V Flyback Regulator Design Example

DS012515-22

A: Switch Voltage, 20 V/div

B: Switch Current, 2 A/div

C: Output Rectifier Current, 2 A/div

D: Output Ripple Voltage, 50 mV/div

AC-Coupled

Horizontal: 2 µs/div

FIGURE 5. Switching Waveforms

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