LTC3722-2 Linear Technology, LTC3722-2 Datasheet - Page 22

• Manufacturer Part Number: LTC3722-2
• Description: Synchronous Dual Mode Phase Modulated Full Bridge Controllers
• Manufacturer: Linear Technology

LTC3722-1/LTC3722-2
OPERATIO
Switching losses in the MOSFETs are dominated by the
power required to charge their gates, and turn-on and
turn-off losses. At higher power levels, gate charge power
is seldom a significant contributor to efficiency loss. ZVS
operation virtually eliminates turn-on losses. Turn-off
losses are reduced by the use of an external drain to source
snubber capacitor and/or a very low resistance turn-off
driver. If synchronous rectifier MOSFETs are used on the
secondary, the same general guidelines apply. Keep in
mind, however, that the BV
be greater than V
secondary is snubbed. Without snubbing, the secondary
voltage can ring to levels far beyond what is expected due
to the resonant tank circuit formed between the secondary
leakage inductance and the C
the synchronous rectifier MOSFETs.
Switching Frequency Selection
Unless constrained by other system requirements, the
power converter’s switching frequency is usually set as
high as possible while staying within the desired efficiency
target. The benefits of higher switching frequencies are
many including smaller size, weight and reduced bulk
capacitance. In the full-bridge phase shift converter, these
principles are generally the same with the added compli-
cation of maintaining zero voltage transitions, and there-
fore, higher efficiency. ZVS is achieved in a finite time
during the switching cycle. During the ZVS time, power is
not delivered to the output; the act of ZVS reduces the
maximum available duty cycle. This reduction is propor-
tional to maximum output power since the parasitic ca-
pacitive element (MOSFETs) that increase ZVS time get
larger as power levels increase. This implies an inverse
relationship between output power level and switching
frequency. Table 1 displays recommended maximum
switching frequency vs power level for a 30V/75V in to
3.3V/5V out converter. Higher switching frequencies can
be used if the input voltage range is limited, the output
voltage is lower and/or lower efficiency can be tolerated.
22
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IN(MAX)
/N, depending on how well the
DSS
OSS
rating needed for these can
(output capacitance) of
Table 1.Switching Frequency vs Power Level
Closing the Feedback Loop
Closing the feedback loop with the full-bridge converter
involves identifying where the power stage and other
system poles/zeroes are located and then designing a
compensation network around the converters error ampli-
fier to shape the frequency response to insure adequate
phase margin and transient response. Additional modifi-
cations will sometimes be required in order to deal with
parasitic elements within the converter that can alter the
feedback response. The compensation network will vary
depending on the load current range and the type of output
capacitors used. In isolated applications, the compensa-
tion network is generally located on the secondary side of
the power supply, around the error amplifier of the
optocoupler driver, usually an LT1431 or equivalent. In
nonisolated systems, the compensation network is lo-
cated around the LTC3722-1/LTC3722-2’s error amplifier.
In current mode control, the dominant system pole is
determined by the load resistance (V
capacitor 1/(2 • R
1/(2 • ESR • C
load regulation can be obtained if there is high loop gain at
DC. This requires an integrator type of compensator
around the error amplifier. A procedure is provided for
deriving the required compensation components. More
complex types of compensation networks can be used to
obtain higher bandwidth if necessary.
Step 1. Calculate location of minimum and maximum
output pole:
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<200W
<500W
<50W
<1kW
<2kW
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600kHz
450kHz
300kHz
200kHz
150kHz
100kHz
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