LTC3726 Datasheet, PDF (8/16 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller

LTC3726
U
OPERATIO
Main Control Loop
The LTC3726 is designed to work in a constant frequency,
current mode, one or two transistor forward converter.
During normal operation, the primary-side MOSFET(s)
is (are) “clocked” on with the forward MOSFET on the sec-
ondary side. This applies the reflected input voltage across
the inductor on the secondary side. When the current in
the inductor has ramped up to the peak value as com-
manded by the voltage on the ITH pin, the current sense
comparator is tripped, turning off the primary-side and
forward MOSFETs. To avoid turning on the synchronous
MOSFET prematurely and causing shoot-through, the
voltage on the SW pin is monitored. This voltage will
usually fall below 0V soon after the primary-side MOSFETs
have turned completely off. When this condition is de-
tected, the synchronous MOSFET is quickly turned on,
causing the inductor current to ramp back downwards.
The error amplifier senses the output voltage, and adjusts
the ITH voltage to obtain the peak current needed to
maintain the desired main-loop output voltage. The
LTC3726 always operates in a continuous current, syn-
chronous switching mode. This ensures a rapid transient
response as well as a stable bias supply voltage at light
loads. A maximum duty cycle (either 50% or 75%) is
internally set via clock dividers to prevent saturation of the
main transformer. In the event of an overvoltage on the
output, the synchronous MOSFET is quickly turned on to
help protect critical loads from damage.
For most forward converter applications, the PT⁺ and PT^–
outputs will contain a pulse-encoded PWM signal. These
outputs are driven in a complementary fashion with an
essentially constant 50% duty cycle. This results in a
stable volt-second balance as well as an efficient transfer
of bias power across the pulse transformer. As shown in
Figure 1, the beginning of the positive half-cycle coincides
with the turn-on of the primary-side MOSFET(s). Likewise,
the beginning of the negative half-cycle coincides with
the maximum duty cycle (forced turn-off of primary
switch(es)). At the appropriate time during the positive
half-cycle, the end of the “on” time (PWM going LOW) is
signaled by briefly applying a zero volt differential across
the pulse transformer. Figure 1 illustrates the operation of
this multiplexing scheme.
The LTC3705 primary-side controller and gate driver will
decode this PWM information as well as extract the power
needed for primary-side gate drive.
DUTY CYCLE = 15%
150ns
7V
DUTY CYCLE = 0%
150ns
7V
V_PT1+ – V_PT1–
–7V
1 CLK PER
–7V
3726 F01
1 CLK PER
Figure 1: Gate Drive Encoding Scheme (V_MODE = GND)
Gate Drive Encoding
Since the LTC3726 controller resides on the secondary
side of an isolation barrier, communication to the primary-
side power MOSFETs is generally done through a trans-
former. Moreover, it is often necessary to generate a low
voltage bias supply for the primary-side gate drive cir-
cuitry. In order to reduce the number of isolated windings
present in the system, the LTC3726 uses a proprietary
scheme to encode the PWM gate drive information and
multiplex it together with bias power for the primary-side
drive and control, using a single pulse transformer. Note
that, unlike optoisolators and other modulation tech-
niques, this multiplexing scheme does not introduce a
significant time delay into the system.
Self-Starting Architecture
When the LTC3726 is used in conjunction with the LTC3705/
LTC3725 primary-side controller and gate driver, a com-
plete self-starting isolated supply is formed. When input
voltage is first applied in such an application, the LTC3705/
LTC3725 will begin switching in an “open-loop” fashion,
causing the main output to slowly ramp upwards. This is
the primary-side soft-start mode. On the secondary side,
the LTC3726 derives its operating bias voltage from a
peak-charged capacitor. This peak-charged voltage will
rise more rapidly than the main output of the converter, so
that the LTC3726 will become operational well before the
output voltage has reached its final value.
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