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參數(shù)資料
| 型號(hào)： | LTC3766MPUFD#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 316 kHz SWITCHING FREQ-MAX, PQCC28 |
| 封裝： | 4 X 5 MM, LEAD FREE, PLASTIC, MO-220, QFN-28 |
| 文件頁數(shù)： | 17/60頁 |
| 文件大小： | 607K |
| 代理商： | LTC3766MPUFD#PBF |
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LTC3766
24
3766f
APPLICATIONS INFORMATION
choose a capacitor whose voltage rating is greater than
this maximum VCL voltage by 50% or more. Typically, a
good quality (X7R) ceramic capacitor is a good choice for
CCL. Also, be sure to account for the voltage coefficient of
the capacitor. Many ceramic capacitors will lose as much
as 50% of their value at their rated voltage.
The value of the clamp capacitor should be high enough
to minimize the capacitor ripple voltage, thereby reducing
the voltage stress seen by the MOSFETs. However, a larger
clamp capacitor will ultimately result a slower transient
response to avoid transformer saturation during load
transients. While Direct Flux Limit will automatically limit
the PWM on-time only as needed to prevent saturation, a
larger clamp capacitor will require a longer time to charge
ordischargeinresponsetoaloadtransient.Consequently,
the value of the clamp capacitor represents a compromise
between transient response and MOSFET voltage stress. A
reasonable value for the clamp capacitor can be calculated
using the following:
CCL =
1
2LM
4
2
πfSW
2
An additional design constraint on CCL occurs because of
the resonance between the magnetizing inductance LM of
the main transformer and the clamp capacitor CCL. If the
Q of this resonance is too high, it will result in increased
voltage stress on the primary-side MOSFET during load
transients. Also, a high Q resonance between LM and CCL
complicatesthecompensationofthevoltageloop,andcan
cause oscillations under certain conditions. To avoid the
problems associated with this resonance, always use
an RC snubber in parallel with the clamp capacitor as
shown in Figure 9. The values for this RC snubber can
then be calculated using:
RCS =
1
1–
VO
VIN(MIN)
NP
NS
LM
CCL
and
CCS = 6CCL
Figure 9 shows a typical arrangement of the active clamp
capacitor with an RC snubber. Be careful to account for
the effect of voltage coefficient for both CCS and CCL to
ensure that the above relationship between CCS and CCL
is maintained.
DUTY CYCLE (%)
20
ACTIVE
CLAMP
VOL
TAGE
NORMALIZED
TO
50%
DUT
YCYCLE
1.3
1.4
1.5
1.2
1.1
40
60
30
50
70
80
1.0
0.9
1.6
3766 F08
Figure 8. VCL Voltage vs Duty Cycle
VIN
LM
SWP
SNUBBER
CCL
ACTIVE
CLAMP
PMOS
CCS
3766 F09
RCS
Figure 9. Active Clamp Capacitor and Snubber
Direct Flux Limit
In active clamp forward converters, it is essential to es-
tablish an accurate limit to the transformer flux density
in order to avoid core saturation during load transients or
when starting up into a pre-biased output. The LTC3765
and LTC3766 implement a new unique system for moni-
toring and directly limiting the flux accumulation in the
transformer core. Unlike previous methods, the direct flux
limitdirectlymeasuresandmonitorsfluxaccumulationand
guarantees that the transformer will not saturate in either
direction, even when starting into a pre-biased output.
This technique also provides the best possible transient
response,asitwilltemporarilyallowveryhighdutycycles,
only limiting the duty cycle when absolutely necessary.
Moreover, this technique prevents overcurrent damage to
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