ISL6551EVAL1 Intersil, ISL6551EVAL1 Datasheet - Page 6

• 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: -

current since both Q1 and Q2 are turned on during the
freewheeling time, which could reduce the conduction losses
and the reflected output current in the primary. The amount
of the load current split into Q1 and Q2 depends on the
voltage drop across the secondary winding, the Rds(on) of
Q1 & Q2, and/or the body diode drop of Q1 & Q2. The
optimum performance of the converter happens when the
load current is split into both turned-on Q1 and Q2 evenly. In
reality, the body diode drop at one of upper FETs, the
leakage inductance, and the shorted primary winding force
one of the synchronous FETs to carry the majority of the
output current while the other conducts a minority of the
load.
T2 --> T3, Q1-to-Q2 Dead Time Period
[Figure 8]
The dead time is used to prevent simultaneous conduction of
QC and QD, which would cause shoot-through currents. The
dead time 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 and the load current freewheels
through the body diodes of both FETs. This 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
SYNCHRONOUS FETS
-
FIGURE 8. Q1-TO-Q2 DEAD TIME PERIOD
DA
CA
DB
CB
QA= ON, QD = OFF, QB = QC = OFF
Lk
-
Vp
Vs
Q1
Cp
Q2
T
6
+
QC
QD
OFF
ON
ON
Q1
+
D1
D2
Lo1
Lo2
DC
CC
DD
CD
Application Note 1002
OFF
ON
ON
Q2
Vo
Co
T3 --> T4, Lower Left-Leg (QB) Resonant Period
[Figure 9]
The dead time period is followed by the lower left-leg
resonant period. It begins with QA turned off and QC turned
on. At the beginning of this transition, the input voltage is
applied first across the commutating inductance (leakage
and any external inductances), i.e, the real primary stays
zero until the current through these inductors changes in
direction in the next time interval. This can be seen in the
voltage waveforms across the primary winding and the
secondary winding, discussed in the EXPERIMENTAL
RESULTS section on pages 24-25. The direction of the
current through the primary winding remains the same as
that in the previous time interval. The current flows into the
transformer primary capacitance (Cp) and the output
capacitance (Coss) CA of QA, which will be charged up from
zero voltage (~Rds(on) Drop) to V
output capacitance CB of QB is discharged to from V
Rds(on) Drop to zero voltage (~diode drop). This transition is
accomplished with the energy stored in the primary
inductance (including leakage inductance, magnetizing
inductance, and any external inductance). It takes a longer
time to complete this transition than the one reaching the
freewheeling period since the energy stored in the resonant
inductances decreases due to the conduction losses of the
power switches and the primary current is decaying in the
freewheeling period. Once QB is clamped to zero voltage by
its own body diode, QB is turned on at zero voltage (ZVS
transition). Another power transfer period is followed by the
other diagonal power switches (QC-to-QB). The rest of the
+
QA
Vin
QB
-
SYNC DRIVE
INV_LOW DRIVE
INV_SYNC DRIVE
SYNCHRONOUS FETS
FIGURE 9. LOWER LEFT-LEG RESONANT PERIOD
-
DA
CA
DB
CB
QA = OFF, QC = ON, QB = QD = OFF
Lk
-
Vp
Vs
Q1
Cp
Q2
T
+
OFF
OFF
OFF
Q1
QC
QD
IN
. Simultaneously, the
+
D1
D2
Lo1
Lo2
DC
CC
DD
CD
ON
ON
ON
Q2
IN
-
Vo
Co