LINEAR LT3748 User Guide and Manual

Isolated Power Supplies Made Easy John D. Morris

The LT3748 is a switching regulator controller specifically designed to simplify the design of isolated power supplies

SW VOLTAGE

using a flyback topology. No third winding or opto-isolator

SW CURRENT

is required as the LT3748 senses the isolated output

DIODE CURRENT

voltage directly from the primary-side flyback waveform.

Figure 1. Idealized waveforms for an LT3748-based

One challenge in designing a flyback By contrast, the LT3748 infers the

flyback converter operating in boundary mode

converter is that information relating to isolated output voltage by examin- the output voltage on the secondary side ing the primary-side flyback pulse

simple and compact solution, as shown

of the transformer must be fed back to the waveform. In this manner, neither an

by the 12V, 30W demo board in Figure 2.

regulator on the primary side in order to opto-isolator nor an extra transformer maintain regulation. Historically, feedback winding is required to maintain regula- OUTPUT POWER across the isolation barrier is achieved tion, and the output voltage is easily Because the MOSFET power switch is using opto-isolators or extra transformer programmed with two resistors. located outside the LT3748, the maximum windings, though both methods present a output power is limited primarily by exter-

The LT3748 features a boundary mode

number of design problems. Opto-isolator nal components—not the LT3748. Output

control method (also called critical con-

feedback circuits add components, increas- power limitations can be separated into

duction mode), where the part operates at

ing converter size and cost. They also draw three categories: voltage limitations, cur-

the boundary between continuous conduc-

power, degrading efficiency and complicat- rent limitations and thermal limitations.

tion mode and discontinuous conduction

ing thermal design. Opto-isolators also The voltage limitations in a flyback design

mode, as illustrated in Figure 1. Due to

make it difficult to accurately regulate are primarily the MOSFET switch maximum

the boundary control mode operation, the

the output due to their limited dynamic drain to source voltage and the output

output voltage can be calculated from the

response, inherent nonlinearities, typical diode reverse-bias rating. The current

transformer primary voltage when the sec-

variation from unit-to-unit and varia- limitation on output power delivery is gen-

ondary current is approximately zero. This

tion with age. The usual alternative is erally constrained by transformer satura-

method improves load regulation with-

to add an extra transformer winding, tion current in higher power applications,

out external resistors and capacitors and

but this may introduce other problems, although the MOSFET switch and output

results in typical line and load regulation

including bigger, more expensive mag- diode may need to be rated for the desired

of better than ±5% while allowing for a

netics or limited dynamic response.

Figure 2. A no-opto-coupler Figure 3. The LT3748 is available in an MSOP-16 30W design with an 18V-to-90V package with four pins removed for high voltage input range (actual size) operation.

38 | January 2011 : LT Journal of Analog Innovation