7.
6 THE PUSH-PULL CONVERTER
Another dc-dc converter that has transformer isolation is the push-pull
converter shown in Fig. 7-8a.
The transformer magnetizing inductance is not a design parameter.
The transformer is assumed to be ideal for this analysis.
Power Electronics by D. W. Hart Chapter 07 1
7.6 THE PUSH-PULL CONVERTER
Switches Sw1 and Sw2
turn on and off with the
switching sequence
shown in Fig. 7-8b.
Analysis proceeds by
analyzing the circuit with
either switch closed and
then with both switches
open.
Power Electronics by D. W. Hart Chapter 07 2
Switch Sw1 Closed: Closing Sw1 establishes the voltage across primary
winding P1 at
Diode D1 is forward-biased
Power Electronics by D. W. Hart Chapter 07 3
Switch Sw2 Closed Closing Sw2 establishes the voltage across primary
winding P2 at
The voltage across P2 is transformed to the three other windings,
resulting in
- -
+ +
- -
Diode D2 is forward-biased
+ +
Same as Sw1 Closed
Power Electronics by D. W. Hart Chapter 07 4
Both Switches Open With both switches open, the current in each of the
primary windings is zero.
-
Note that the result is similar to
that for the buck converter,
Power Electronics by D. W. Hart Chapter 07 5
EXAMPLE 7-6 Push-Pull Converter
A push-pull converter has the following parameters:
Power Electronics by D. W. Hart Chapter 07 6
7.7 FULL-BRIDGE AND HALF-BRIDGE DC-DC ONVERTERS
Output of the full-bridge converter is analyzed as for
the push-pull converter, resulting in
Correction
Note that the maximum voltage across
an open switch for the full-bridge
converter is Vs, rather than 2Vs as for
the push-pull and single-ended forward
converters.
Reduced voltage stress across an open
switch is important when the input
voltage is high, giving the full-bridge
converter an advantage.
Power Electronics by D. W. Hart Chapter 07 7
Power Electronics by D. W. Hart Chapter 07 8
The half-bridge converter of Fig. 7-10a has capacitors C1 and C2
which are large and equal in value. The input voltage is equally divided
between the capacitors. Voltage v is the same form as for the push-pull and
x
the full-bridge converters, but the amplitude is one-
half the value.
The relationship between the input
and output voltages for the half-
bridge converter is
Full Half
Power Electronics by D. W. Hart Chapter 07 9
Power Electronics by D. W. Hart Chapter 07 10
7.9 MULTIPLE OUTPUTS
Multiple outputs are useful when different output voltages are necessary.
The duty ratio of the switch and the turns ratio of the primary to the specific
secondary winding determine the output/input voltage ratio.
An example is a single converter with three windings on the output producing
voltages of 12, 5 and -5 V with respect to a common ground on the output
side.
Multiple outputs are possible with all the dc power supply topologies
discussed in this chapter.
Note, however, that only one of the outputs can be regulated with a feedback
control loop.
Other outputs will follow according to the duty ratio and the load.
Power Electronics by D. W. Hart Chapter 07 11
Fly-back Converter with two outputs
Power Electronics by D. W. Hart Chapter 07 12
Forward converters with two outputs
Power Electronics by D. W. Hart Chapter 07 13
7.10 CONVERTER SELECTION
In theory, any power supply circuit can be designed for any application,
depending on how much the designer is willing to spend for components and
control circuitry.
In practice, some circuits are much more suited to particular applications than
others.
See the book for power handling ranges
Power Electronics by D. W. Hart Chapter 07 14
7.11 POWER FACTOR CORRECTION
Full-wave rectifier
The source current is highly
non-sinusoidal because the
diodes conduct for a short time
interval.
The result is a large total
harmonic distortion (THD)
of current coming from the
ac source.
Power Electronics by D. W. Hart Chapter 07 15
A rectifier circuit used to produce a high power factor and low THD;
Current in the inductor for continuous
current mode (CCM) operation
Current
from the ac
source. Fig. 7-14a
A way to improve the power factor (and reduce the THD) is with a power
factor correction circuit, as shown in Fig. 7-14a.
A boost converter is used to make the current in the inductor approximate a
sinusoid.
When the switch is closed, the inductor current increases.
When the switch is open, the inductor current decreases.
By using appropriate switching intervals, the inductor current can be made
to follow the sinusoidal shape of the full-wave rectified input voltage.
Power Electronics by D. W. Hart Chapter 07 16
This type of switching scheme is called continuous-current mode
(CCM) power factor correction (PFC).
In an actual implementation, the switching frequency would be much
greater than is shown in the figure.
Power Electronics by D. W. Hart Chapter 07 17
Another type of switching scheme produces a current like that shown
in Fig. 7-15.
In this scheme, the inductor current varies between zero and a peak that
follows a sinusoidal shape.
This type of switching scheme is called discontinuous-current mode (DCM)
power factor correction.
DCM is used with low-power circuits, while CCM is more suitable for high-
power applications.
In both the CCM and DCM schemes,
the output of the power factor
correction (PFC) stage is a large dc
voltage, usually on the order of 400 V.
The output of the PFC stage will go to
a dc-dc converter.
For example, a forward converter can be used to step down the 400-V output
of the PFC stage to 5 V.
Other converter topologies can be used for power factor correction.
The SEPIC and ´Cuk converters are well suited for this purpose.
Power Electronics by D. W. Hart Chapter 07 18
