Converters Excel In AC Applications
converters are rapidly dominating the next generation of high performance
military and aerospace applications due to their small size.
volume reduction of miniature AC input converters have lagged behind
their DC input counterparts. This is unfortunate, since many AC
applications also demand the small size, light weight and high performance.
By using a small rectifier and filter ahead of a suitable hybrid
DC/DC converter, a very small AC/DC power supply may be implemented.
DC/DC converters were not regulated at all or had relatively narrow
input voltage ranges. Second generation parts operate over a 2 1/2
:1 or 3:1 range. This is an impractically narrow range for running
on highly pulsating full wave rectified AC.
For wide operating
voltage range DC/DC converters, the choice of circuit topology is
critical. It has been shown that the flyback topology is well suited
for extreme operating voltage ranges. Reference #2 discusses a flyback
construction operating over a 37:1 range.
The most common
nominal voltage range for DC input units is 28 VDC followed by 270
VDC and 120 VDC. The 120 VDC range is used by the International
Space Station Alpha program as well as some newer satellites. Modular
Devices, Inc. is now manufacturing four families of hybrid 120 VDC
input converters using flyback topology, ranging in power from 6.5
to 80 watts. The operating range of these parts encompasses 200
VDC on the high end to 80 VDC on the low end. In addition, these
converters are current mode types and have a high degree of ripple
The wide operating
voltage range feature makes this series of parts ideal for operating
on pulsating DC derived from direct full wave rectification of nominal
115 VAC inputs.
A typical circuit
configuration is shown in Figure 18. AC voltage is fed to a bridge
rectifier, then to a capacitor in parallel with the DC/DC converter.
For low power outputs the capacitor is not necessary since the internal
capacitor of the DC/DC converter suffices
has excellent EMI characteristics. The high frequency emissions
are controlled within the DC/DC converter itself. The low frequency
conducted emissions are minimized by keeping the filter capacitance
as small as possible. Since the DC/DC converter is designed to operate
over a large input voltage range, it can cope with the high pulsating
voltage resultant from a small capacitor.
A typical input
waveform is shown in Figure 19. The small capacitor results in a
large conduction angle. The large conduction angle provides a high
power factor and allows a relatively fast fall off of low frequency
benefit of being able to operate with little or no external capacitance
is the small physical dimensions of the resulting power supply.
Tables 9 through
12 show how much minimum capacitance is needed. In practice, an
additional factor of safety in capacitance value may be desirable
to counter the capacitance change over temperature, increase low
line operating margins or reduce output ripple voltage.
family of hybrid DC/DC converters typically has 60 dB of ripple
rejection with 50-60 Hz inputs and 42 dB of ripple rejection with
a 400 Hz input.
With the capacitor
values shown, typical full wave rectified ripple component is 0.5%
p-p at 50/60 Hz., and 2.5% p-p at 400 Hz. This ripple is reduced
directly as capacitance is increased. For 400 Hz applications it
may be desirable to use a small inductor for filtering as this may
be volumetrically better than capacitors and will improve power
to operate from an AC power line with a battery backup is often
encountered in critical applications. A simple configuration for
battery backed up power supplies using two hybrid DC/DC converters
is shown in Figure 20. Highly compact uninterruptable or battery
backed up power supplies can be implemented with hybrid converters.
AC power is
full wave rectified and fed through a blocking
diode to the principal DC/DC Converter, #1. This converter produces
the desired regulated outputs.
A second diode
connected to the pulsating full wave rectified voltage feeds a trickle
charging resistor connected to a 20 cell Ni-Cad battery pack. The
blocking diode is used to prevent unintentional back feed from the
charge also supplies the no load current for DC/DC Converter # 2.
Converter # 2 operates from the nominal 24 VDC battery output and
delivers an output voltage in the 90 to 100 VDC range. This means
that so long as AC power is present, DC/DC Converter # 2 is reverse
biased and drawing only standby current. However, when the AC power
source is low or not present, dropping below the output set point
of DC/DC Converter #2, Converter #2 delivers the input voltage to
Converter #1, drawing its power from the Ni-Cad battery.
of the oscillator/counter/FET switch is to disconnect DC/DC converter
#2 from the battery after a preset time. This prevents the battery
from discharging below an unusable potential.
have a minimum voltage rating of 200 to 250 VDC for this application
or as derating criteria demands. The user can use a film, multilayer
ceramic, tantalum foil or a high performance aluminum electrolytic
capacitor. All of these part types are available in a height that
is consistent with the height of the converters.
factors of the capacitors should be reviewed since they are operating
with a high ripple voltage. A high dissipation factor can cause
power dissipation. At 400 Hz it may be desirable to use more capacitance
than indicated to reduce the converter's output ripple voltage.
R. Design Of Solid State Power Supplies, Third Edition. Van Nostrand
Reinhold, New York, 1989.
E. and Zuckerman, Leonard. "Wide Input Range Multiple Output Power
Supply." Proceedings of the Sixth International PCI Conference,