ISL6551EVAL1 Intersil, ISL6551EVAL1 Datasheet - Page 5

• 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: -

T0 -->T1, QA-to-QD Power Transfer (Active) Period
[Figure 6]
When QD is turned on, QA 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 current. The primary current first
flows from QD to QA 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
magnetizing inductance, and the output inductance.
Simultaneously, Q2 should stay off for eliminating shoot-
through currents, and Q1 is turned on to reduce conduction
losses; the current through the Lo2 is positive ramp, and the
current through the Lo1 is negative ramp. The ON-time of
QD is a function of V
the output load Io. QD is turned off when the peak of the
modified current ramp signal hits the error voltage, and the
freewheeling period then begins.
T1 --> T2, QA-to-QC Clamped Freewheeling
(Passive) Period [Figure 7]
Once QD is turned off by trailing edge pulse width
modulation, the primary current continues flowing into the
output capacitance (Coss) CD of QD, which will be charged
up from the switch Rds(on) Drop to V
Simultaneously, the primary capacitance (Cp) of the
+
QA
Vin
QB
-
SYNC DRIVE
INV_LOW DRIVE
INV_SYNC DRIVE
SYNCHRONOUS FETS
FIGURE 6. QA-TO-QD POWER TRANSFER PERIOD
-
DA
CA
DB
CB
QA = QD = ON, QB = QC= OFF
Lk
-
IN
, Vo, the transformer turns ratio N, and
Vp
Vs
Q1
Cp
Q2
T
5
+
ON
ON
ON
Q1
IN
QC
QD
/N is across the
+
D1
D2
IN
IN
- Diode Drop.
) is across the
IN
Lo1
Lo2
, the
DC
CC
DD
CD
Application Note 1002
OFF
OFF
OFF
Q2
Vo
Co
transformer and the output capacitance CC of QC are
discharged to from V
This 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
the channel of QA, and the other end is clamped to V
body diode of QC, which is the only path that the primary
current can go through. The losses due to the body diode
conduction at the freewheeling period could be significant if
the primary current (the lumped sum of the magnetizing
current and the reflected secondary winding freewheeling
current), is relatively high. These conduction losses can be
minimized by employing the maximum allowable turns ratio of
the main transformer, i.e, the maximum allowable duty cycle in
the design. In some applications, shunting upper switches
with Schottky diodes might be another possible way to reduce
the conduction losses. For a wide range input application, if a
pre-regulator is implemented, then a fixed, high duty cycle
(~100%) post full-bridge regulator can be achieved and the
freewheeling time is minimized. The power dissipation of the
upper FETs can be therefore reduced significantly.
Three different synchronous rectification drive schemes can
be implemented with the ISL6551 as shown in Figures 4 and
5. The INV_LOW DRIVE scheme is the one that would
provide an additional path for the secondary freewheeling
+
QA
Vin
QB
-
SYNC DRIVE
INV_LOW DRIVE
INV_SYNC DRIVE
SYNCHRONOUS FETS
FIGURE 7. QA-TO-QC CLAMPED FREEWHEELING PERIOD
-
DA
CA
DB
CB
QA= ON, QD = OFF, QB = QC = OFF
Lk
-
IN
to zero voltage (~diode drop).
Vp
Vs
Q1
Cp
Q2
T
+
ON
ON
ON
Q1
QC
QD
+
D1
D2
Lo1
Lo2
DC
CC
DD
CD
OFF
OFF
ON
Q2
IN
by the
IN
Vo
by
Co