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LT3837EFE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P28
LT3837
13
3837fd
APPLICATIONS INFORMATION
Primary Winding Feedback
The previous work was developed using a separate wind-
ing for voltage feedback. It is possible to use the primary
winding as the feedback winding as well. This can simplify
the design of the transformer.
When using the primary winding the feedback voltage
will be added to the V
IN
voltage so:
V
FLYBK
=
V
OUT
+I
OUT
• ESR +R
DS(ON)
( )
N
SP
where N
SP
is the transformer effective secondary to
primary winding turns ratio. Use the circuit of Figure 2
to get more accurate output regulation. In this case the
regulation equations becomes:
R1=
R2
V
FB
•
V
OUT
+I
OUT
• ESR+R
DS(ON)
( )
N
SP
− V
BE
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
where V
BE
is the base emitter drop of the PNP (approxi-
mately 0.7V).
Likewise the load compensation equation needs to be
changed to use N
SP
instead of N
SF
so:
R
CMP
=K1•
R
SENSE
• 1–DC
( )
ESR+R
DS(ON)
•R1•N
SF
•
MS
MP
R2
R1
SECONDARY
PRIMARY
C
OUT
V
IN
V
FLYBK
3837 F02
•
LT3837
FB PG
Figure 2
Transformer Design
Transformer design/specification is the most critical part
of a successful application of the LT3837. The following
sections provide basic information about designing the
transformer and potential tradeoffs.
If you need help, the LTC Applications group is available to
assist in the choice and/or design of the transformer.
Turns Ratios
The design of the transformer starts with determining
duty cycle (DC). DC impacts the current and voltage stress
on the power switches, input and output capacitor RMS
currents and transformer utilization (size vs power).
The ideal turns ratio is:
N
IDEAL
=
V
OUT
V
IN
•
1–DC
DC
Avoid extreme duty cycles as they, in general, increase
current stresses. A reasonable target for duty cycle is
50% at nominal input voltage.
For instance, if we wanted a 9V to 3.3V converter at 50%
DC then:
N
IDEAL
=
3.3
9
•
1–0.5
0.5
=
1
2.72
In general, better performance is obtained with a lower
turns ratio. A DC of 52% yields a 1:3 ratio.
Note the use of the external feedback resistive divider
ratio to set output voltage provides the user additional
freedom in selecting a suitable transformer turns ratio.
Turns ratios that are the simple ratios of small integers;
e.g., 1:1, 2:1, 3:2 help facilitate transformer construction
and improve performance.
When building a supply with multiple outputs derived
through a multiple winding transformer, lower duty cycle
can improve cross regulation by keeping the synchronous
rectifier on longer, and thus, keep secondary windings
coupled longer.
For a multiple output transformer, the turns ratio between
output windings is critical and affects the accuracy of the
voltages. The ratio between two output voltages is set with
the formula V
OUT2
= V
OUT1
• N21 where N21 is the turns
LT3837
14
3837fd
APPLICATIONS INFORMATION
ratio of between the two windings. Also keep the secondary
MOSFET R
DS(ON)
small to improve cross regulation.
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a spike after the primary side switch
turn-off. This is increasingly prominent at higher load
currents, where more stored energy is dissipated. Higher
flyback voltage may break down the MOSFET switch if it
has too low a BV
DSS
rating.
One solution to reducing this spike is to use a snubber
circuit to suppress the voltage excursion. However, sup-
pressing the voltage extends the flyback pulse width. If
the flyback pulse extends beyond the enable delay time,
output voltage regulation is affected. The feedback system
has a deliberately limited input range, roughly ±50mV re-
ferred to the FB node. This rejects higher voltage leakage
spikes because once a leakage spike is several volts in
amplitude, a further increase in amplitude has little effect
on the feedback system.
So it is advisable to arrange the snubber circuit to clamp
at as high a voltage as possible, observing MOSFET
breakdown, such that leakage spike duration is as short
as possible. Application Note 19 provides a good reference
on snubber design.
As a rough guide, total leakage inductances of several per-
cent (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.
Avoid double digit percentage leakage inductances as there
is a potential for abrupt loss of control at high load current.
This curious condition potentially occurs when the leakage
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 typically reduces the output voltage
abruptly to a fraction, roughly one-third to two-thirds of
its correct value.
Once load current is reduced sufficiently, the system snaps
back to normal operation. When using transformers with
considerable leakage inductance, 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 is usually evident
by simultaneously viewing the primary side MOSFET drain
voltage to observe firsthand the leakage spike behavior.
A final note—the susceptibility of the system to bistable
behavior is somewhat a function of the load current/volt-
age characteristics. A load with resistive—i.e., I = V/R
behavior—is the most apt to be bistable. Capacitive loads
that exhibit I = V
2
/R behavior are less susceptible.
Secondary Leakage Inductance
Leakage inductance on the secondary forms an inductive
divider on the transformer secondary, reducing the size
of the feedback flyback pulse. This increases the output
voltage target by a similar percentage.
Note that unlike leakage spike behavior, this phenomenon
is independent of load. Since the secondary leakage in-
ductance is a constant percentage of mutual inductance
(within manufacturing variations), the solution is to adjust
the feedback resistive divider ratio to compensate.
Winding Resistance Effects
Primary or secondary winding resistance acts to reduce
overall efficiency (P
OUT
/P
IN
). Secondary winding resistance
increases effective output impedance degrading load regu-
lation. Load compensation can mitigate this to some extent
but a good design keeps parasitic resistances low.
Bifilar Winding
A bifilar or similar winding is a good way to minimize
troublesome leakage inductances. Bifilar windings also
improve coupling coefficients and thus improve cross
regulation in multiple winding transformers. However,
LT3837
15
3837fd
APPLICATIONS INFORMATION
tight coupling usually increases primary-to-secondary
capacitance and limits the primary-to-secondary break-
down voltage, so it isn’t always practical.
Primary Inductance
The transformer primary inductance, L
P
, is selected based
on the peak-to-peak ripple current ratio (X) in the trans-
former relative to its maximum value. As a general rule,
keep X in the range of 50% to 70% ripple current (i.e., X =
0.5 to 0.7). Higher values of ripple will increase conduction
losses, while lower values will require larger cores.
Ripple current and percentage ripple is largest at minimum
duty cycle; in other words, at the highest input voltage.
L
P
is calculated from:
L
P
=
V
IN(MAX)
•DC
MIN
( )
2
f
OSC
• X
MAX
•P
IN
=
V
IN(MAX)
•DC
MIN
( )
2
•Eff
f
OSC
• X
MAX
•P
OUT
where:
f
OSC
is the OSC frequency
DC
MIN
is the DC at maximum input voltage
X
MAX
is ripple current ratio at maximum input voltage
Continuing with the 9V to 3.3V example, let us assume a
10A output, 9V to 18V input power with 88% efficiency.
Using X = 0.7, and f
OSC
= 200kHz:
P
IN
=
3.3•10A
88%
= 37.5W
DC
MIN
=
1
1+
N• V
IN(MAX)
V
OUT
=
1
1+
1
3
•
18
3.3
= 35.5%
L
P
=
18V • 0.355
( )
2
200kHz • 0.7 • 37.5W
= 7.8µH
Optimization might show that a more efficient solution
is obtained at higher peak current but lower inductance
and the associated winding series resistance. A simple
spreadsheet program is useful for looking at tradeoffs.
Transformer Core Selection
Once L
P
is known, the type of transformer is selected.
High efficiency converters use ferrite cores to minimize
core loss. Actual core loss is independent of core size for
a fixed inductance, but decreases as inductance increases.
Since increased inductance is accomplished through
more turns of wire, copper losses increase. Thus trans-
former design balances core and copper losses. Remem-
ber that increased winding resistance will degrade cross
regulation and increase the amount of load compensa-
tion required.
The main design goals for core selection are reducing
copper losses and preventing saturation. Ferrite core mate-
rial saturates hard, rapidly reducing inductance when the
peak design current is exceeded. This results in an abrupt
increase in inductor ripple current and, consequently, out-
put voltage ripple. Do not allow the core to saturate! The
maximum peak primary current occurs at minimum V
IN
:
I
PK
=
P
IN
V
IN(MIN)
•DC
MAX
• 1+
X
MIN
2
now :
DC
MAX
=
1
1+
N• V
IN(MIN)
V
OUT
=
1
1+
1
3
•
9
3.3
= 52.4%
X
MIN
=
V
IN(MIN)
•DC
MAX
( )
2
f
OSC
•L
P
•P
IN
=
9 • 0.52
( )
2
200kHz • 7.8µH• 37.5W
= 0.380
Using the example numbers leads to:
I
PK
=
37.5W
9V • 0.524
• 1+
0.380
2
= 9.47A
Multiple Outputs
One advantage that the flyback topology offers is that ad-
ditional output voltages can be obtained simply by adding
windings. Designing a transformer for such a situation is
beyond the scope of this document. For multiple windings,
realize that the flyback winding signal is a combination of
activity on all the secondary windings. Thus load regulation
is affected by each windings load. Take care to minimize
cross regulation effects.
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LT3837EFE#TRPBF

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Switching Voltage Regulators Iso No-Opto Sync Fly Cntr
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