Texas Instruments 28xxx, Reference Manual

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3.9 Controlling Zero Voltage Switched Full Bridge (ZVSFB) Converter
CTR=zero
CTR=CMPB
X
En
SyncOut
Phase reg
Ext SyncIn
(optional)
EPWM1A
EPWM1B
SyncOut
Phase reg
CTR=CMPB
CTR=zero
X
En
EPWM2B
EPWM2A
Slave
Master
V
out
EPWM1A
SyncIn
SyncIn
V
DC_bus
EPWM1B
EPWM2A
EPWM2B
Φ
=0
°
Φ
=Var
°
Var = Variable
Controlling Zero Voltage Switched Full Bridge (ZVSFB) Converter
The example given in Figure 3-15 assumes a static or constant phase relationship between legs
(modules). In such a case, control is achieved by modulating the duty cycle. It is also possible to
dynamically change the phase value on a cycle-by-cycle basis. This feature lends itself to controlling a
class of power topologies known as
phase-shifted full bridge , or zero voltage switched full bridge. Here the
controlled parameter is not duty cycle (this is kept constant at approximately 50 percent); instead it is the
phase relationship between legs. Such a system can be implemented by allocating the resources of two
PWM modules to control a single power stage, which in turn requires control of four switching elements.
Figure 3-16 shows a master/slave module combination synchronized together to control a full H-bridge. In
this case, both master and slave modules are required to switch at the same PWM frequency. The phase
is controlled by using the slave's phase register (TBPHS). The master's phase register is not used and
therefore can be initialized to zero.
Figure 3-15. Controlling a Full-H Bridge Stage (F
PWM2
= F
PWM1)
SPRU791D – November 2004 – Revised October 2007 Applications to Power Topologies 89
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