13
FN9181.3
October 31, 2008
capacitance. Each switch is designated by its position; upper
left (UL), upper right (UR), lower left (LL), and lower right
(LR). The beginning of the cycle, shown in Figure 10, is
arbitrarily set as having switches UL and LR on and UR and
LL off. The direction of the primary and secondary currents
are indicated by I
P
and I
S
, respectively.
The UL - LR power transfer period terminates when switch
LR turns off as determined by the PWM. The current flowing
in the primary cannot be interrupted instantaneously, so it
must find an alternate path. The current flows into the
parasitic switch capacitance of LR and UR, which charges
the node to VIN and then forward biases the body diode of
upper switch UR.
The primary leakage inductance, L
L
, maintains the current,
which now circulates around the path of switch UL, the
transformer primary, and switch UR. When switch LR opens,
the output inductor current free-wheels through both output
diodes, D1 and D2. During the switch transition, the output
inductor current assists the leakage inductance in charging
the upper and lower bridge FET capacitance.
The current flow from the previous power transfer cycle
tends to be maintained during the free-wheeling period
because the transformer primary winding is essentially
shorted. Diode D1 may conduct very little or none of the
free-wheeling current, depending on circuit parasitics. This
behavior is quite different than occurs in a conventional
hard-switched full-bridge topology where the free-wheeling
current splits nearly evenly between the output diodes, and
flows not at all in the primary.
This condition persists through the remainder of the half
cycle.
During the period when CT discharges (also referred to as
the deadtime), the upper switches toggle. Switch UL turns off
and switch UR turns on. The actual timing of the upper
switch toggle is dependent on RESDEL, which sets the
resonant delay. The voltage applied to RESDEL determines
how far in advance the toggle occurs prior to a lower switch
turning on. The ZVS transition occurs after the upper
switches toggle and before the diagonal lower switch turns
on. The required resonant delay is 1/4 of the period of the LC
resonant frequency of the circuit formed by the leakage
inductance and the parasitic capacitance. The resonant
transition may be estimated from Equation 25.
where τ is the resonant transition time, L
L
is the leakage
inductance, C
P
is the parasitic capacitance, and R is the
equivalent resistance in series with L
L
and C
P
.
The resonant delay is always less than or equal to the
deadtime and may be calculated using Equation 26.
where τ
resdel
is the desired resonant delay, V
resdel
is a
voltage between 0V and 2V applied to the RESDEL pin, and
DT is the deadtime (see Equations 1 through 5).
When the upper switches toggle, the primary current that was
flowing through UL must find an alternate path. It
charges/discharges the parasitic capacitance of switches UL
and LL until the body diode of LL is forward-biased. If
RESDEL is set properly, switch LL will be turned on at this
time. The output inductor does not assist this transition. It is
purely a resonant transition driven by the leakage inductance.
FIGURE 10. UL - LR POWER TRANSFER CYCLE
VIN+
VIN-
UL
LL
UR
LR
VOUT+
RTN
I
P
I
S
L
L
D2
D1
FIGURE 11. UL - UR FREE-WHEELING PERIOD
VIN+
VIN-
UL
LL
UR
LR
VOUT+
RTN
I
P
I
S
L
L
D2
D1
τ
π
2
---
1
1
L
L
C
P
-------------- -
R
2
4L
L
2
----------
–
-----------------------------------
=
(EQ. 25)
τ
resdel
V
resdel
2
--------------------
DT⋅= S
(EQ. 26)
FIGURE 12. UPPER SWITCH TOGGLE AND RESONANT
TRANSITION
VIN+
VIN-
UL
LL
UR
LR
VOUT+
RTN
L
L
D2
D1
I
S
I
P
ISL6752
