Active Clamp Synchronous Controllers for Forward Converters with 6.5V to 100V+ Inputs

April 2014
I N
T H I S
I S S U E
robust RS485/RS422
transceivers 9
high voltage surge stoppers
ease MIL-STD-1275D
compliance 15
Volume 24 Number 1
Active Clamp Synchronous
Controllers for Forward Converters
with 6.5V to 100V+ Inputs
Wei Gu, Randyco Prasetyo and Fei Guo
boost-then‑buck LED driver
for high PWM dimming
ratios 22
cost-effective high voltage
isoSPI™ coupling 26
ideal diode combines 200V
busses 30
The LT3752, LT3752‑1 and LT3753 are highly integrated, high
performance active clamp forward controllers that minimize external
component count, solution size and cost. Two of these controllers,
the LT3752 and LT3753, are designed for inputs up to 100V, while
the LT3752-1 is designed for applications with input voltages greater
than 100V—suitable for HV car battery and offline isolated power
supplies, industrial, automotive and military systems. All produce
compact, versatile and efficient solutions for single-IC output power
levels up to 400W. Higher power levels are
supported by stacking converter outputs
in series. See Table 1 (on page 4) for a
feature comparison of these devices.
NO-OPTO MODE OPERATION REGULATES WITH
ACCURATE PROGRAMMABLE VOLT-SECOND CLAMP
Figure 1 shows a complete 150W forward converter that
requires no opto-couplers thanks to the LT®3752’s accurate, programmable volt-second clamp. For a forward
converter operating in continuous conduction mode, the
output voltage is VOUT = VIN • N • D, where VIN is the input
voltage, N is the secondary to primary turns ratio and D
is the duty cycle. The duty cycle clamp on the OUT pin of
the LT3752, LT3752-1 and LT3753 inversely tracks VIN to
maintain constant VOUT over the input voltage range.
(continued on page 4)
The LT8614 Silent Switcher™ selected for EDN & EE Times ACE Award (page 3)
Caption
w w w. li n ea r.com
Table 1. Feature comparison of LT3752, LT3752-1 and LT3753
PART
INPUT RANGE
ACTIVE CLAMP DRIVER
HOUSEKEEPING FLYBACK CONTROLLER
LT3753
8.5V–100V
Lo-Side
No
LT3752
6.5V–100V
Lo-Side
Yes
LT3752-1
100V–400V+
Hi-Side
Yes
(LT375x) continued from page 1)
If the resistor that programs the duty
cycle clamp goes open circuit, the part
immediately stops switching, preventing the device from running without
the volt-second clamp in place.
In an active volt-second clamp scheme,
the accuracy of VOUT depends heavily on the accuracy of the volt-second
clamp. Competing volt-clamp solutions
use an external RC network connected
from the system input to trip an internal
comparator threshold. Accuracy of the
RC method suffers from external capacitor error, part-to-part mismatch between
the RC time constant and the IC’s switching period, the error of the internal
comparator threshold and the nonlinearity of charging at low input voltages.
The housekeeping supply can be used to
overdrive the INTVCC pin to take power
outside of the part, improve efficiency,
provide additional drive current and
optimize the INTVCC level. The housekeeping supply also allows bias to any
secondary side IC before the main forward converter starts switching. This
removes the need for external startup circuitry on the secondary side.
INTEGRATED HOUSEKEEPING
FLYBACK CONTROLLER
The LT3752/LT3752-1 includes an internal constant frequency flyback controller for generating a housekeeping
supply. The housekeeping supply can
efficiently provide bias for both primary and secondary ICs, eliminating
the need to generate bias supplies from
auxiliary windings in the main forward
transformer, significantly reducing
transformer complexity, size and cost.
To ensure accurate regulation part to part,
the LT3752, LT3752-1 and LT3753 feature
trimmed timing capacitor and comparator thresholds. Figure 2 shows VOUT versus
load current for various input voltages.
PRECISION UNDERVOLTAGE
LOCKOUT AND SOFT-START
The precision LT3752/LT3752-1 undervoltage lockout (UVLO) feature can be used for
supply sequencing or start-up overcurrent
protection—simply apply a resistor divider
to the UVLO pin from the VIN supply.
Figure 1. 150W forward converter in No-Opto mode
VIN
18V TO 72V
INTVCC
4.7µF
100V
×3
D2
•
T2
2.2µF
•
VAUX
D3
•
2.2µF
•
T1
4:4
L1
6.8µH
•
20k
D4
15nF
Si2325DS
100nF
M2
499Ω
0.15Ω
10k
M4
M3
LT3752
OVLO
1.82k
22.6k
34k
31.6k
49.9k
7.32k
60.4k
4 | April 2014 : LT Journal of Analog Innovation
22nF
0.33µF
•
560Ω
22nF
COMP
FB
2.8k
CSW
FB
VAUX
4.7µF
499k
4.7µF
1.1k
CSN
CSP
PGOOD
SYNC
T3
INTVCC
HFB
LT8311
GND
10k
HCOMP
SS2
SS1
RT
TBLNK
IVSEC
TAS
TOS
TAO
GND
•
220pF
SOUT
INTVCC
VIN
2.2nF
SS
ISENSEN
RSENSE
0.006Ω
2.2µF
100Ω
CG
5.9k
2k
VAUX
INTVCC
PMODE
TIMER
UVLO_VSEC
M1
FG
OC
ISENSEP
SYNC
FSW
OUT
OPTO
100k
AOUT
22µF
16V
×2
D1
100Ω
HOUT HISENSE
VIN
2.2nF
250V
+
COMP
M5
470µF
16V
VOUT
12V
12.5A
D1, D2, D3: BAS516
D4: CENTRAL SEMI CMMR1U-02
L1: CHAMPS PQI2050-6R8
M1, M4: INFINEON BSC077N12NS3
M2: VISHAY Si2325DS
M3: FAIRCHILD FDMS86101
M5: DIODES INC. ZVN4525E6
T1: CHAMPS G45R2_0404.04D
T2: BH ELECTRONICS L00-3250
T3: PULSE PE-68386NL
design features
In an active volt-second clamp scheme, the accuracy of VOUT depends
heavily on the accuracy of the volt-second clamp. Competing volt-clamp
solutions use an external RC network which suffers from a number of error
sources. To ensure accurate regulation part to part, the LT3752, LT3752-1 and
LT3753 feature trimmed timing capacitor and comparator thresholds.
The UVLO pin features adjustable input
hysteresis, allowing the IC to resist input
supply droop before engaging soft-stop.
During soft-stop the converter continues
to switch as it folds back the switching frequency, volt-second clamp and
COMP pin voltage. The LT3752, LT3752-1
and LT3753 have a micropower shutdown threshold of approximately
400mV at the UVLO pin—VIN quiescent
current drops to 40μA, or lower.
Adding capacitors to the soft-start pins,
(SS1 and SS2) implements the soft-start
feature, which reduces the peak input
current and prevents output voltage
overshoot during start-up or recovery
from a fault condition. The SS1/2 pins
reduce the inrush current by lowering the
current limit and reducing the switching
frequency, allowing the output capacitor
to gradually charge toward its final value.
Figure 2. Output voltage vs load current at various
input voltages
SHUTDOWN WITH SOFT-STOP
In a reversal of soft-start start-up, the
LT3752/LT3752-1 and LT3753 can gradually discharge the SS1 pin (soft-stop)
during shutdown. Figure 3 shows
shutdown waveforms of the converter
shown in Figure 5. Without soft-stop,
the self-driven synchronous rectifier
feedback transfers capacitor energy
to the primary, potentially causing
shutdown oscillation and damaging
components on the primary side.
Figure 4 shows shutdown waveforms
with soft-stop. The converter continues to
switch as it folds back switching frequency, volt-second clamp and COMP pin
voltage, resulting in clean shutdown.
CURRENT MODE CONTROL
The LT3752/LT3752-1 and LT3753 use a current mode control architecture to increase
supply bandwidth and response to line
and load transients over voltage mode
controllers. Current mode control requires
fewer compensation components than
voltage mode control architectures, making it much easier to compensate a broad
range of operating conditions. For operation in continuous mode and above 50%
duty cycle, required slope compensation
can be programmed by a single resistor.
PROGRAMMABLE FEATURES
SIMPLIFY OPTIMIZATION
The LT3752/LT3752-1 and LT3753 include
a number of programmable features that
allow the designer to optimize them for
a particular application. For instance,
programmable delays between various
gate signals can be used to prevent crossconduction and to optimize efficiency.
Each delay can be set with a single resistor.
Programmable turn-on current spike
blanking (adaptive leading edge blanking plus programmable extended blanking) of the main MOSFET greatly improves
the converter’s noise immunity. During
gate rise time, and sometime thereafter,
Figure 3. Shutdown waveforms of circuit in Figure 5
without soft-stop show oscillations.
Figure 4. Shutdown waveforms of circuit in Figure 5
showing soft-stop in action
PRIMARY
NFET
DRAIN
VOLTAGE
(50V/DIV)
PRIMARY
NFET
DRAIN
VOLTAGE
(50V/DIV)
VOUT
2V/DIV
VOUT
2V/DIV
14.0
13.5
13.0
VOUT (V)
12.5
12.0
11.5
VIN = 70V
VIN = 60V
VIN = 48V
VIN = 36V
VIN = 20V
11.0
10.5
10.0
0
2
6
8
4
LOAD CURRENT (A)
10
12
500µs/DIV
500µs/DIV
April 2014 : LT Journal of Analog Innovation | 5
The LT3752/LT3752-1 and LT3753 include a number of programmable
features that enable optimization for particular applications. For
instance, programmable delays between various gate signals can be
used to prevent cross-conduction and to optimize efficiency.
VIN
36V TO 72V
D1: BAS516
D2: CENTRAL SEMI. CMHZ5229B
L1: CHAMPS PQI2050-3R3
M1: INFINEON BSC190N15NS3
M2: IRF6217
M3, M4: INFINEON BSC0902NSI
T1: CHAMPS G45R2-0209
4.7µF
100V
×3
250V
0.22µF
M2
105k
5Ω
200Ω
+
5Ω
VOUT
5V
20A
560µF
10V
M4
47µF
10V
M3
OUT
M1
OC
SYNC
ISENSEP
UVLO_VSEC
1.96k
1k
RSENSE
0.012Ω
1.87k
4.7nF
SOUT
INTVCC
44.2k
30.1k
57.6k
6 | April 2014 : LT Journal of Analog Innovation
VOUT
D2
22µF
10V
1µF
10V
V+
LT1431
GND-F
GND-S
137k
REF
137k
COLL
100k
1k
2.2nF
250V
36V–72V INPUT, 5V/20A FORWARD
CONVERTER
Figure 5 shows a 5V, 20A output converter that takes a 36V–72V input. The
Figure 6. Efficiency of the converter in Figure 5
96
94
92
90
36VIN
48VIN
72VIN
88
86
1k
100k
1µF
EFFICIENCY (%)
The operating frequency can be programmed from 100kHz to 500kHz range
with a single resistor from the RT pin to
ground, or synchronized to an external
clock via the SYNC pin. The adjustable
operating frequency allows it to be set outside certain frequency bands to fit applications that are sensitive to spectral noise.
100Ω
PS2801-1
1µF
22nF
Figure 5. 5V at 20A forward converter that takes an input of 36V to 72V
noise can be generated in the current
sensing resistor connected to the source
of the MOSFET. This noise can false trip
the sensing comparators, resulting in
early switch turnoff. One solution to this
problem is to use an oversized RC filter
to prevent false trips, but programmable
turn-on spike blanking can eliminate
the need for additional RC filtering.
4.7µF
25V
COMP
FB
SS2
SS1
RT
TBLNK
IVSEC
TAS
TOS
GND
14.7k
100Ω
ISENSEN
LT3753
OVLO
TAO
•
D1
AOUT
VIN
L1
3.3µH
68nF
250V
100nF
10k
•
T1
9:2
0
2
4
6 8 10 12 14 16 18 20
LOAD CURRENT (A)
3.3nF
34.8k
active reset circuit consists of a small
P-channel MOSFET M2 and a reset capacitor. The MOSFET M2 is used to connect
the reset capacitor across the transformer T1 primary winding during the
reset period when M1 MOSFET is off.
The voltage across the reset capacitor automatically adjusts with the duty
cycle to provide complete transformer
reset under all operating conditions.
Also the active reset circuit shapes the
reset voltage into a square waveform that
is suitable for driving the secondary synchronous MOSFET rectifier M4. The MOSFETs
are on the secondary side and are driven
by the secondary winding voltage. Figure 6
shows the efficiency for this converter.
design features
The LT3752/LT3752-1 and LT3753 use a current mode control architecture
to increase supply bandwidth and response to line and load transients when
compared to voltage mode controllers. Current mode control requires fewer
compensation components than voltage mode control architectures, making
it easier to compensate a broad range of operating conditions.
INTVCC
D2
2.2µF
•
T2
•
VAUX
D3
•
4.7µF
100V
×3
•
2.2µF
T1
4:4
•
15nF
M5
0.15Ω
10k
M3
D1
1.82k
SOUT
INTVCC
22.6k
34k
31.6k
49.9k
7.32k
71.5k
22nF
0.33µF
COMP
FB
22nF
1.1k
100k
100k
560Ω
3.16k
100Ω
4.7µF
HFB
2.8k
CSN
CSP
CSW
FG
PGOOD
SYNC
T3
•
LT8311
FB
68pF
100k
GND
INTVCC
10k
HCOMP
SS2
SS1
RT
TBLNK
IVSEC
TAS
TOS
TAO
GND
•
220pF
100k
11.3k
VAUX
SS
ISENSEN
LT3752
RSENSE
0.006Ω
VIN
100Ω
CG
5.9k
2k
2.2µF
INTVCC
PMODE
TIMER
UVLO_VSEC
VAUX
FSW
OC
ISENSEP
OVLO
M1
OUT
SYNC
+
22µF
16V
×2
2.2nF
250V
M4
OPTO
100k
AOUT
VOUT
12V
12.5A
D4
100Ω
HOUT HISENSE
VIN
470µF
16V
20k
M1, M4: INFINEON BSC077N12NS3
M2: VISHAY Si2325DS
M3: FAIRCHILD FDMS86101
M5: DIODES INC. ZVN4525E6
100nF
M2
499Ω
L1
6.8µH
COMP
VIN
18V TO 72V
220nF
4.7µF
PS2801-1
499k
68pF
13.7k
4.7nF
1µF
1k
2.2nF
D1, D2, D3: BAS516
D4: CENTRAL SEMI CMMR1U-02
L1: CHAMPS PQI2050-6R8
T1: CHAMPS G45R2_0404.04D
T2: BH ELECTRONICS L00-3250
T3: PULSE PE-68386NL
Figure 7. 18V–72V input, 12V at 12.5A output forward converter
150V–400V INPUT, 12V/16.7A
FORWARD CONVERTER
Figure 7 shows an 18V–72V input,
12V/12.5A output forward converter. The
LT8311 is used on the secondary side of
forward converters to provide synchronous MOSFET control and output voltage
feedback through an opto-coupler. A
pulse transformer (see T3 in Figure 7) is
required to allow the LT8311 to receive
synchronization control signals from
the primary-side IC. These control signals are interpreted digitally (high or
low) by the LT8311 to turn on/off the
catch and forward MOSFETs. Figure 8
shows the efficiency for this converter.
Figure 9 shows a 150V–400V input,
12V/16.7A output isolated flyback converter. For high input voltage applications, the voltage rating of the available
P-channel MOSFETs may not be high
enough to be used as the active clamp
switch in the low side active clamp topology. An N-channel approach using the
high side active clamp topology should
be used. This topology requires a high
side gate driver or a gate transformer
to drive the N-channel MOSFET to switch
in the active clamp capacitor. Figure 10
shows the efficiency for this converter.
Figure 8. Efficiency for the converter in Figure 7
96
94
EFFICIENCY (%)
18V–72V INPUT, 12V/12.5A FORWARD
CONVERTER
92
90
24VIN
48VIN
72VIN
88
86
0
3
9
6
LOAD CURRENT (A)
12
15
April 2014 : LT Journal of Analog Innovation | 7
The LT3752/LT3752-1 includes an internal constant frequency flyback
controller for generating a housekeeping supply. The housekeeping supply
can efficiently provide bias for both primary and secondary ICs, eliminating
the need to generate bias supplies from auxiliary windings in the main forward
transformer, significantly reducing transformer complexity, size and cost.
D2
•
10µF
374k
•
VAUX
4.2Ω
INTVCC
C10
4.7µF
ACPL-W346
D3
•
D5
VOUT
ANODE
CATHODE
VEE
10µF
499Ω
0.15Ω
•
•
47nF
630V
D1
VCC
M5
402Ω
10nF
630V
0.22µF
0.002
M3
2k
UVLO_VSEC
5.76k
ISENSEN
LT3752-1
OVLO
2.94k
40.2k
100k
78.7k
95.3k
13k
124k
0.22µF
100Ω
4.7µF
HFB
806Ω
22k
560Ω
4.7µF
PS2801-1
8 | April 2014 : LT Journal of Analog Innovation
Figure 10. Efficiency for the converter in Figure 9
96
95
94
93
EFFICIENCY (%)
The LT3752, LT3752-1 and LT3753 simplify
the design and improve performance
to isolated power supplies with a voltsecond clamp architecture that produces
accurate regulation. An integrated flyback controller can be used to produce
a housekeeping supply, simplifying the
magnetics. Current mode control improves
bandwidth and allows compensation for
a broad range of operating conditions.
Soft-stop features protect the supply
and other components from potentially
damaging voltage and current spikes. n
92
91
90
89
88
VIN = 150V
VIN = 250V
VIN = 350V
VIN = 400V
87
86
85
0
2.5
68pF
11.3k
1µF
432k
D1: CENTRAL SEMI CMR1U-10
D2, D3, D5: BAS516
D4: CENTRAL SEMI CMMR1U-02
L1: COILCRAFT AGP2923-153
2.2nF
Figure 9. 150V–400V input, 12V/16.7A output isolated forward converter
CONCLUSION
VAUX
100k
FB
LT8311
PGOOD
SYNC
T3
•
CG
GND
100pF
5.11k
22nF
1µF
1.2k
22k
3.3nF
100k
VIN
3.16k
22k
0.47µF
•
2.2µF
INTVCC
10k
COMP
FB
HCOMP
SS2
SS1
RT
TBLNK
IVSEC
TAS
TOS
TAO
GND
RSENSE
0.022Ω
220pF
SOUT
INTVCC
VAUX
SS
ISENSEP
CSN
SYNC
CSW
OC
100Ω
120pF
INTVCC
PMODE
TIMER
M1
OUT
FG
AOUT
FSW
HISENSE
+
VOUT
12V
16.7A
33µF
16V
×4
M2
OPTO
499k
HOUT
VIN
330µF
16V
10nF
250V
M4
100Ω
499k
10k
D4
M1: RENESAS IPD65R25OC6
M2: RENESAS IPD60R1K4C6
M3: RJK0653DPB ×2
M4: FAIRCHILD FDMS86200 ×3
M5: INFINEON BSP300
CSP
INTVCC
374k
T2
COMP
VIN
150V 2.2µF
TO 630V
400V
L1
15µH
T1
31:5
7.5
10 12.5
5
LOAD CURRENT (A)
15
17.5
T1: CHAMPS LT80R2-12AC-3124005
T2: WÜRTH 750817020
T3: PULSE PE-68386NL
Similar pages