12
LT1737
1737fa
APPLICATIO S I FOR ATIO
WUUU
TRANSFORMER DESIGN CONSIDERATIONS
Transformer specification and design is perhaps the most
critical part of applying the LT1737 successfully. In addi-
tion to the usual list of caveats dealing with high frequency
isolated power supply transformer design, the following
information should prove useful.
Turns Ratios
Note that due to the use of the external feedback resistor
divider ratio to set output voltage, the user has relative
freedom in selecting transformer turns ratio to suit a given
application. In other words, “screwball” turns ratios like
“1.736:1.0” can scrupulously be avoided! In contrast,
simpler ratios of small integers, e.g., 1:1, 2:1, 3:2, etc. can
be employed which yield more freedom in setting total
turns and mutual inductance. Turns ratio can then be
chosen on the basis of desired duty cycle. However,
remember that the input supply voltage plus the second-
ary-to-primary referred version of the flyback pulse (in-
cluding leakage spike) must not exceed the allowed external
MOSFET breakdown rating.
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a spike after output switch turnoff. This
is increasingly prominent at higher load currents, where
more stored energy must be dissipated. In many cases a
“snubber” circuit will be required to avoid overvoltage
breakdown at the output switch node. Application Note
AN19 is a good reference on snubber design.
In situations where the flyback pulse extends beyond the
enable delay time, the output voltage regulation will be
affected to some degree. It is important to realize that the
feedback system has a deliberately limited input range,
roughly ±50mV referred to the FB node, and this works to
the user’s advantage in rejecting large, i.e., higher voltage,
leakage spikes. In other words, once a leakage spike is
several volts in amplitude, a further increase in amplitude
has little effect on the feedback system. So the user is
generally advised to arrange the snubber circuit to clamp
at as high a voltage as comfortably possible, observing
MOSFET breakdown, such that leakage spike duration is
as short as possible.
As a rough guide, total leakage inductances of several
percent (of mutual inductance) or less may require a
snubber, but exhibit little to no regulation error due to
leakage spike behavior. Inductances from several percent
up to perhaps ten percent cause increasing regulation
error.
Severe leakage inductances in the double digit percentage
range should be avoided if at all possible as there is a
potential for abrupt loss of control at high load current.
This curious condition potentially occurs when the leak-
age spike becomes such a large portion of the flyback
waveform that the processing circuitry is fooled into
thinking that the leakage spike itself is the real flyback
signal! It then reverts to a potentially stable state whereby
the top of the leakage spike is the control point, and the
trailing edge of the leakage spike triggers the collapse
detect circuitry. This will typically reduce the output volt-
age abruptly to a fraction, perhaps between one-third to
two-thirds of its correct value. If load current is reduced
sufficiently, the system will snap back to normal opera-
tion. When using transformers with considerable leakage
inductance, it is important to exercise this worst-case
check for potential bistability:
1. Operate the prototype supply at maximum expected
load current.
2. Temporarily short circuit the output.
3. Observe that normal operation is restored.
If the output voltage is found to hang up at an abnormally
low value, the system has a problem. This will usually be
evident by simultaneously monitoring the V
SW
waveform
on an oscilloscope to observe leakage spike behavior
firsthand. A final note—the susceptibility of the system to
bistable behavior is somewhat a function of the load I/V
characteristics. A load with resistive, i.e., I = V/R behavior
is the most susceptible to bistability. Loads which exhibit
“CMOSsy”, i.e., I = V
2
/R behavior are less susceptible.
Secondary Leakage Inductance
In addition to the previously described effects of leakage
inductance in general, leakage inductance on the second-
ary in particular exhibits an additional phenomenon. It
forms an inductive divider on the transformer secondary,
