ISL6551EVAL1 Intersil, ISL6551EVAL1 Datasheet - Page 7

• Manufacturer Part Number: ISL6551EVAL1
• Description: EVALUATION BOARD ISL6551
• Manufacturer: Intersil

Specifications of ISL6551EVAL1

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Isolated
• Voltage - Output: 3.3V
• Current - Output: 60A
• Voltage - Input: 36 ~ 75V
• Regulator Topology: Buck
• Frequency - Switching: 470kHz
• Board Type: Fully Populated
• Utilized Ic / Part: ISL6551
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

discussion (Figures 10 to 13) is just the repetition of another
half cycle.
T4 --> T5, QC-to-QB Power Transfer Period
[Figure 10]
When QB is turned on, QC has been already turned on in
the previous period, the resonant delay. In this transfer
(active) period, the full input voltage (V
primary of the transformer, and V
secondary of the transformer once the primary current
catches the reflected output active current. The primary
current first flows from QB to QC due to the prior resonant
current and then reverses in direction until the current
reaches zero and starts ramping up at a rate determined by
V
Simultaneously, Q1 should stay off for eliminating shoot-
through currents, and Q2 is turned on to reduce conduction
losses; the current through the Lo1 is a positive ramp, and
the current through Lo2 is a negative ramp. The ON-time of
QB is a function of V
the output load Io. QB is turned off when the peak of the
modified current ramp signal hits the error voltage, and
another freewheeling period then begins.
T5 -->T6, QC-to-QA Clamped Freewheeling Period
(Passive) [Figure 11]
Once QB is turned off, the primary current continues flowing
into the output capacitance (Coss) CB of QB, which will be
charged up from the switch Rds(on) Drop to V
Drop. Simultaneously, the primary capacitance (Cp) of the
transformer and the output capacitance CA of QA are
+
QA
Vin
QB
-
IN
SYNC DRIVE
INV_LOW DRIVE
INV_SYNC DRIVE
SYNCHRONOUS FETS
, the magnetizing inductance, and the output inductance.
FIGURE 10. QC-TO-QB POWER TRANSFER PERIOD
-
DA
CA
DB
CB
QB = QC = ON, QA = QD = OFF
Lk
-
IN
, Vo, the transformer turns ratio N, and
Vp
Vs
Q1
Cp
Q2
T
+
7
QC
QD
OFF
OFF
OFF
Q1
IN
+
D1
D2
/N is across the
IN
) is across the
Lo1
Lo2
DC
CC
DD
CD
IN
+ Diode
Application Note 1002
ON
ON
ON
Q2
Vo
Co
discharged from V
transition is accomplished using the energy stored in the
leakage inductance of the transformer, the magnetizing
inductance, the reflected output inductance, and any
external commutating inductance. After the transition, the
primary current flows in the same direction and the real
freewheeling period begins. One end of the transformer is
shorted to V
clamped to V
path that the primary current can go through. Refer to the
T1-->T2 period for more detailed discussion.
T6 --> T7, Q2-to-Q1 Dead Time Period
[Figure 12]
The dead time is used to prevent simultaneous conduction of
QA and QB, which would cause shoot-through currents. The
dead time period is still part of the freewheeling period, the
drive control signals for the power switches therefore do not
change states while the drive signals of the synchronous
FETs change levels. In the SYNC DRIVE scheme, both Q1
and Q2 now are turned off, the load current free wheels
through the body diodes of both FETs, which introduces high
conduction losses in high output current applications.
Shunting both synchronous FETs with schottky diodes can
reduce the losses. In the INV_SYNC DRIVE scheme, both
Q1 and Q2 are turned on, therefore, schottky diodes are not
required, so are not in the INV_LOW DRIVE scheme.
+
QA
Vin
QB
-
SYNC DRIVE
INV_LOW DRIVE
INV_SYNC DRIVE
FIGURE 11. QC-TO-QA CLAMPED FREEWHEELING PERIOD
SYNCHRONOUS FETS
-
IN
DA
CA
DB
CB
IN
by the channel of QC, and the other end is
by the body diode of QA, which is the only
QB = OFF, QC = ON, QA = QD = OFF
Lk
-
IN
to zero voltage (~diode drop). This
Vp
Vs
Q1
Cp
Q2
T
+
QC
QD
OFF
OFF
ON
Q1
+
D1
D2
Lo1
Lo2
DC
CC
DD
CD
ON
ON
ON
Q2
Vo
Co