LTC3722-1 LINER [Linear Technology], LTC3722-1 Datasheet - Page 12

• Manufacturer Part Number: LTC3722-1
• Description: Synchronous Dual Mode Phase Modulated Full Bridge Controllers
• Manufacturer: LINER [Linear Technology]

LTC3722-1/LTC3722-2
OPERATIO
on the active leg from the ground reference point to V
always occur, independent of load current as long as
energy in the transformer’s magnetizing and leakage in-
ductance is greater than the capacitive energy. That is,
1/2 • (L
occurs when the load current is zero. This condition is
usually easy to meet. The magnetizing current is virtually
constant during this transition because the magnetizing
inductance has positive voltage applied across it through-
out the low to high transition. Since the leg is actively
driven by this “current source,” it is called the active or
linear transition. When the voltage on the active leg has
risen to V
circuitry. The primary current now flows through the two
high side MOSFETs (MA and MC). The transformer’s
secondary windings are electrically shorted at this time
since both ME and MF are “ON”. As long as positive
current flows in LO1 and LO2, the transformer primary
(magnetizing) inductance is also shorted through normal
transformer action. MA and MF turn off at the end of
State 2.
State 3 (Passive Transition)
MA turns off when the oscillator timing period ends, i.e.,
the clock pulse toggles the internal flip-flop. At the instant
MA turns off, the voltage on the MA/MB junction begins to
decay towards the lower supply (GND). The energy avail-
able to drive this transition is limited to the primary leakage
inductance and added commutating inductance which
have (I
magnetizing and output inductors don’t contribute any
energy because they are effectively shorted as mentioned
previously, significantly reducing the available energy.
This is the major difference between the active and passive
transitions. If the energy stored in the leakage and com-
mutating inductance is greater than the capacitive energy,
the transition will be completed successfully. During the
transition, an increasing reverse voltage is applied to the
leakage and commutating inductances, helping the overall
primary current to decay. The inductive energy is thus
12
MAG
M
+ L
IN
+ I
I
, MOSFET MC is switched on by the ZVS
) • I
OUT
M
2
/2N) flowing through them initially. The
U
> 1/2 • 2 • C
OSS
• V
IN
2
— the worst case
IN
will
resonantly transferred to the capacitive elements, hence,
the term passive or resonant transition. Assuming there is
sufficient inductive energy to propel the bridge leg to
GND, the time required will be approximately equal to
MOSFET MB is commanded “ON” by the ZVS circuitry.
Current continues to increase in the leakage and external
series inductance which is opposite in polarity to the
reflected output inductor current. When this current is
equal in magnitude to the reflected output current, the
primary current reverses direction, the opposite second-
ary winding becomes forward biased and a new power
pulse is initiated. The time required for the current reversal
reduces the effective maximum duty cycle and must be
considered when computing the power transformer turns
ratio. If ZVS is required over the entire range of loads, a
small commutating inductor is added in series with the
primary to aid with the passive leg transition, since the
leakage inductance alone is usually not sufficient and
predictable enough to guarantee ZVS over the full load
range.
State 4 (Power Pulse 2)
During power pulse 2, current builds up in the primary
winding in the opposite direction as power pulse 1. The
primary current consists of reflected output inductor
current and current due to the primary magnetizing induc-
tance. At the end of State 4, MOSFET MC turns off and an
active transition, essentially similar to State 2, but oppo-
site in direction (high to low) takes place.
Zero Voltage Switching (ZVS)
A lossless switching transition requires that the respective
full-bridge MOSFETs be switched to the “ON” state at the
exact instant their drain to source voltage is zero. Delaying
the turn-on results in lower efficiency due to circulating
current flowing in the body diode of the primary side
MOSFET rather than its low resistance channel. Premature
turn-on produces hard switching of the MOSFETs, in-
creasing noise and power dissipation.
• LC/2. When the voltage on the passive leg nears GND,
372212i