LTC3706 Datasheet, PDF (14/20 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller with PolyPhase Capability

LTC3706
APPLICATIONS INFORMATION
Bias Supply Generation
Figure 2 shows a commonly used method of developing
a V_CC bias supply for the LTC3706. During start-up, bias
winding 1 uses a peak detection method to rapidly develop
a V_IN voltage for the LTC3706, which in turn drives the
linear regulator that generates the V_CC voltage (7V). When
the main output of the converter is in regulation, winding
2 (configured as a forward-style output) is designed to
produce a regulated auxiliary voltage of approximately
7.5V to 8.5V. Since the auxiliary voltage is greater than that
of the linear regulator, the linear regulator will effectively
be shut down. Note that the output inductor L1 must be
adequately large so that its ripple current is continuous
given the amount of V_CC load current, thereby providing
a stable output voltage.
BAS21
•
WINDING 2
NB2
•
1 _• 4.7Ω
WINDING 1
NB1
1mH
BAS21
MBRO530
FMMT491A
1μF
4.7μF
50V
16V
MAIN
TRANSFORMER
V_IN
LTC3706
REGSD NDRV
C_REGSD R_REGSD
V_CC
3706 F02
The turns ratio (NB1) of the bias winding 1 should be cho-
sen to ensure that there is adequate voltage to operate the
LTC3706 over the entire range for the DC/DC converter’s
input bus voltage (V_BUS). This may be calculated using:
NB1= ^{VCC(MIN) + 1.5V}
V_BUS(MIN)
V_CC(MIN) can be as low as 5V (if this provides adequate
gate drive voltage to maintain acceptable efficiency) or as
high as 7V. For V_CC(MIN) = 6V and V_BUS = 36V to 72V, this
would mean a turns ratio of NB1 ≈ 0.21 and a V_IN voltage
range at the LTC3706 of 7.5V to 15V.
Using the bias circuit of Figure 2, the linear regulator
would normally operate only for a brief interval during the
initial soft-start ramp of the main output voltage. Under
some fault conditions (e.g., output overload), the auxiliary
voltage produced by bias winding 2 may decrease below
7V, causing the linear regulator to again supply the V_CC
bias current. Since the amount of power dissipation in the
linear regulator pass device may be quite high, it can take
considerable board area when the linear regulator pass
device is sized to handle this power continuously. As an
alternative, the REGSD pin may be used to effectively detect
an overtemperature condition on the linear regulator pass
device and generate a shut down (soft-start retry) before
overheating occurs. This allows for the use of a small (e.g.,
SOT-23) package for the linear regulator pass device.
Figure 2. Typical Bias Supply Configuration
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