Semi-regulated power supply using forward converter leakage energy

A forward converter circuit transfers a reset current to a capacitor each switching cycle that is independent of the value of input voltage. The reset current stored in a capacitor is transferred to an inductor and, in turn, transferred to a filter capacitor each cycle, thereby providing a semi-regulated voltage to a constant resistive load.

DESCRIPTION 
1. Technical Field 
This invention relates to power supplies, and more particularly, to a DC/DC 
forward converter in which a regulated voltage is needed for either the 
forward converter or another device. 
2. Background of the Invention 
Power supplies are commonly used to provide DC power for electronic devices 
from either AC power or DC power at a different voltage. DC power supplies 
that receive DC power at a different voltage are sometimes known as power 
converters. 
There are a wide variety of power converters in existence. Power converters 
generally apply the DC input power to a switch circuit which is controlled 
to maintain the output voltage or current at a predetermined value. One of 
the advantages of using switches to convert the DC input voltage to a DC 
output voltage is that switches dissipate relatively little power because 
they are either closed or open. Many power converters also use a 
transformer having a primary winding connected in series with a switch. 
A commonly used power converter circuit periodically switches a voltage 
across the primary winding of a transformer using a switch connected in 
series with the primary winding. The periodically applied voltage causes 
an AC voltage to be generated across the secondary winding that is 
rectified, filtered and applied to the output of the power converter. A 
power transformer used in this type of converter must be "reset" each 
cycle because the voltage applied to the primary of the transformer is not 
symmetrical. When the switch is closed, the current in the primary of the 
transformer linearly increases as a result of the inductance of the 
primary winding. When the switch is opened, the magnetic field that has 
been generated in the transformer dissipates thereby generating a 
substantial reverse voltage across the primary winding of the transformer. 
Power can be dissipated in the transformer because of the resistance of 
the transformer primary and secondary as well as leakage inductances in 
the transformer. It is important to minimize the power dissipated in a 
power converter in order to maximize the efficiency of the converter and 
minimize the amount of heat that must be removed from the converter. This 
period during which the magnetic field in the transformer dissipates is 
known as a transformer reset. The voltage generated across the primary of 
the transformer during transformer reset is sometimes dissipated in the 
transformer and switching circuitry connected to the transformer. However, 
dissipating the reset energy in this manner wastes power, thereby reducing 
the efficiency of the power converter. As a result, circuits known as 
"reset snubbers" have been developed for forward converters to couple this 
reset voltage back to the input of the power converter so that it is not 
wasted. Reset snubbers have greatly improved the efficiency of forward 
converters. 
At least some of the circuitry used in conventional forward converters must 
receive a regulated DC voltage in order to operate. In the past, this 
regulated DC voltage was often supplied by a power supply circuit 
contained in either the forward converter itself or in a separate unit. 
The expense of a regulated power supply to provide a regulated voltage for 
forward converters and other circuitry can constitute a significant 
portion of the expense of such forward converters and other circuitry. 
SUMMARY OF THE INVENTION 
The inventive forward converter uses the reset energy that is normally 
either dissipated or returned to the input of the forward converter to 
instead provide a semi-regulated voltage. The forward converter includes a 
transformer having a primary winding connected in series with a 
selectively closing first switch between a pair of input terminals to 
which an input voltage is applied. The series combination of a capacitor 
and a second switch is connected between the junction of primary winding 
and the first switch and a voltage that varies according to the magnitude 
of the input voltage. When the first switch periodically opens, the 
primary winding generates a reset current that is coupled to the capacitor 
through the second switch. The second switch closes when the voltage on a 
lead of the capacitor has a predetermined magnitude with respect to the 
input voltage so that a charge is transferred to the capacitor each 
operating cycle, and the magnitude of the charge is independent of the 
magnitude of the input voltage. An energy transferring circuit coupled to 
the capacitor receives a portion of the energy stored in the capacitor 
during each operating cycle and transfers the energy to a pair of output 
terminals. 
The energy transferring circuit preferably includes an inductor and a third 
switch connected in series with the inductor between a fixed voltage and a 
lead of the capacitor. The third switch closes during at least a portion 
of the period that the first switch is closed so that the energy stored in 
the capacitor is transferred to the inductor. A second energy transferring 
circuit coupled to the inductor receives a portion of the energy 
transferred to the inductor during each operating cycle and transfers the 
energy to the output terminals.

DETAILED DESCRIPTION OF THE INVENTION 
A conventional forward converter 10 with a transformer reset snubber is 
illustrated in FIG. 1. The forward converter 10 includes a transformer 12 
having a primary winding 14 and a secondary winding 16. The primary 
winding 14 is connected between a first input terminal 20 and the drain of 
a MOSFET transistor 22. The source of the MOSFET transistor 22 is 
connected to ground, as is the other input terminal 24. The gate of the 
MOSFET transistor 22 is connected to a conventional switch control circuit 
26. The switch control circuit 26 periodically applies a positive pulse to 
the gate of the MOSFET transistor 22 to cause the MOSFET transistor 22 to 
periodically connect the input terminals 22, 24 across the primary winding 
14 of the transformer 12. The MOSFET transistor 22 thus essentially 
functions as a periodically closing switch. 
When the MOSFET transistor 22 conducts, current is drawn from the input 
terminal 20 through the primary winding 14 of the transformer 14 to 
ground, and back to input terminal 24. During this time, the input voltage 
V.sub.IN is applied across the primary winding 14 of the transformer 12. 
As a result, a voltage is generated across the secondary winding 16 of the 
transformer 12 that is substantially equal to the product of N and 
V.sub.IN, where N is the turns ratio of the transformer 12. The voltage 
generated across the secondary winding 16 is coupled through a first 
rectifying diode 30 to an inductor 32 which, with capacitor 34, acts as a 
filter. A relatively steady state DC voltage is then generated across the 
output terminals 40, 42. A second diode 36 provides a path from the 
inductor 32 to the capacitor 34 and output terminal 40 for the output 
current during the period when the MOSFET transistor 22 opens, as 
explained below. 
When the switch control 26 causes the MOSFET transistor 22 to open, the 
transformer 12 resets during which time the magnetic field in the 
transformer 12 dissipates and a reverse polarity voltage is generated 
across a secondary winding 46 of the transformer 12. One end of the 
secondary winding 46 is connected to ground through a diode 47 and the 
other end of the secondary winding is feedback to the first input terminal 
20. As a result, as the magnetic field in the secondary winding 46 
dissipates, current is coupled back to the input terminal 20. 
A preferred embodiment of the inventive forward converter circuit is 
illustrated in FIG. 2. The forward converter 100 primarily functions to 
convert an input voltage V.sub.IN to a first output voltage V.sub.01 at a 
different voltage, and it also uses reset energy to generate a second, 
semi-regulated output voltage V.sub.02. As in the forward converter 10 of 
FIG. 1, the preferred embodiment oft he inventive forward converter 100 
includes a transformer 112 having a primary winding 114 and a secondary 
winding 116. The primary winding 114 is connected in series with an input 
terminal 120, a MOSFET transistor 122, and ground. Another input terminal 
124 is also connected to ground. The MOSFET transistor 122 is periodically 
turned on by a conventional switch control circuit 126. 
The secondary winding 116 of the transformer 112 is connected to output 
terminals 140, 142 through a diode 130 and inductor 132. A filter 
capacitor 134 is connected across the output terminals 140, 142. Finally, 
a diode 144 provides a path for current flowing in the output inductor 
L132 during the MOSFET 122 OFF time. The components 130-146 connected to 
the secondary winding 116 of the transformer 112 operate in the same 
manner as the components 30-44 connected to the secondary winding 16 of 
the transformer 12 of FIG. 1. In the interest of brevity, the operation of 
these components will not be described further. 
The forward converter 100 also includes a number of components connected to 
the primary winding 114 that function to convert the reset energy of the 
transformer 112 to a semi-regulated voltage delivered to output terminals 
146, 148. The operation of the circuit containing these components will be 
described in connection with each of its operating states. 
The forward converter 100 illustrated in FIG. 2 undergoes several distinct 
states during its operation. The first state, which will be referred to 
herein as "State 1," starts when the MOSFET transistor 122 becomes 
nonconductive, thus terminating the flow of current through the primary 
winding 114 of the transformer 112. A second state, referred to herein as 
"State 2," starts when the MOSFET transistor 122 becomes conductive, thus 
starting the flow of current through the primary winding 114. A final 
state, referred to herein as "State 3," occurs during the time that the 
MOSFET transistor is conductive some time after the start of State 2. 
Prior to the start of State I1 the forward converter 100 assumes the 
operating configuration shown in FIG. 3. (The secondary winding 116 and 
the components connected thereto have been eliminated for purposes of 
clarity.) The voltage V.sub.A is at ground since the MOSFET switch 122 is 
closed, and the voltage V.sub.b will be assumed to be slightly less than 
V.sub.02 so that the diode 160 is back biased. Also, because V.sub.02 is 
assumed to be less than V.sub.IN, and V.sub.B is slightly below V.sub.02, 
the diode 152 is back biased. Since the MOSFET switch 122 has been closed 
for some time, the current flowing through the primary winding 114 has 
built up to a substantial level. 
At the start of State 1, the MOSFET switch 122 opens, and the reset of the 
transformer 112 causes a voltage to be generated across the primary 
winding 114. The forward converter 100 then assumes the operating 
configuration shown in FIG. 4, and the voltages V.sub.A and V.sub.B are 
shown as a function of time in FIG. 5. State 1 starts at t.sub.1 when the 
MOSFET switch 122 is opened. The reset current flowing from the lower end 
of the primary winding 114 quickly increases the voltage V.sub.A, thereby 
causing a corresponding increase in the voltage V.sub.B, since the diode 
152 is initially back biased, thereby leaving the node on which V.sub.B is 
measured floating. However, when V.sub.B rises to V.sub.IN, the diode 152 
becomes forward biased, so that the forward converter 100 assumes the 
configuration shown in FIG. 6. 
As shown in FIG. 6, V.sub.B is clamped at V.sub.IN as illustrated in FIG. 5 
while V.sub.A continues to rise as the capacitor 150 is charged by the 
reset current from the primary winding 114 of the transformer 112. Note 
that the diode 152, is represented as a short circuit in its forward 
biased condition, thus ignoring the forward biased diode drop. Also, since 
V.sub.b is now greater than V.sub.02 (as shown in FIG. 5), the diode 160 
is back-biased, as illustrated in FIG. 6. At time t.sub.2, as illustrated 
in FIG. 5, the voltage V.sub.A has reached a peak voltage V.sub.PK. The 
voltage V.sub.A reaches the peak voltage V.sub.PK when the magnetic field 
in the transformer 112 has dissipated and the flow of reset current into 
the capacitor 150 terminates. The magnitude of V.sub.PK is proportional to 
the magnitude of V.sub.IN as well as the duration that the MOSFET switch 
122 was closed prior to State 1. The input voltage V.sub.IN may change 
appreciably but the duty cycle of the MOSFET switch 122 varies inversely 
with the magnitude of V.sub.IN to maintain the energy stored in the 
primary magnetizing inductance of the transformer 112 fairly constant. 
However, the magnitude of V.sub.PK varies directly with the magnitude of 
V.sub.IN and does not vary with either the duty cycle or the magnitude of 
the load connected to the forward converter 100. After t.sub.2, V.sub.A 
will return to V.sub.IN at a rate determined by the transformer inductance 
and parasitic capacitance of the converter. The decrease in voltage 
V.sub.A is coupled through the capacitor 150 to V.sub.B, thereby causing 
V.sub.B to drop below the voltage V.sub.IN to which it had been clamped by 
the diode 152. Since the voltage V.sub.B is now less than V.sub.IN, the 
diode 152 opens, thereby once again isolating the node on which V.sub.B is 
measured and returning the effective configuration of the forward 
converter to that illustrated in FIG. 4. As a result, V.sub.B follows the 
decrease of V.sub.A, and the voltage across the capacitor 150 remains 
constant at V.sub.RESET, where V.sub.RESET is equal to the difference 
between V.sub.PK and V.sub.IN. At time t.sub.3, the voltage V.sub.A has 
fallen to V.sub.IN, and the voltages V.sub.A and V.sub.B remain stable for 
the remainder of State 1. 
It is significant that, at the end of State 1, the charge on capacitor 150 
is constant regardless of the magnitude of V.sub.IN. The charge on the 
capacitor 150 is directly proportional to the energy stored in the 
magnetizing inductance L of the primary winding 114. The energy stored in 
each cycle in the magnetizing inductance is 
##EQU1## 
Since the switch control insures that V.sub.IN is inversely proportional to 
V.sub.1 -V.sub.0, the energy stored is constant, thus the energy stored in 
capacitor 150 will be constant as the magnitude of V.sub.IN varies. It is 
this aspect of the preferred embodiment of the forward converter 100 that 
provides a semi-regulated output voltage V.sub.02, as explained in greater 
detail below. 
The MOSFET switch 122 closes at time h which constitutes the start of State 
2. In State 2, the forward converter 100 assumes the configuration shown 
in FIG. 7. As illustrated in FIG. 5, the voltage V.sub.A on capacitor 150 
is switched from V.sub.IN to 0 volts, thereby dropping V.sub.B by 
V.sub.IN. Since V.sub.B just prior to t.sub.4 was at 2 V.sub.IN -V.sub.PK, 
the voltage V.sub.B just after t.sub.4 drops to V.sub.IN -V.sub.PK. 
Thereafter, the capacitor 150 forms a series resonant circuit with the 
inductor 164 since the diode 160 is forward biased by the negative voltage 
V.sub.B. As a result, the voltage V.sub.B increases after t.sub.4 as shown 
in FIG. 5. 
When the voltage V.sub.A reaches V.sub.02, the diode 168 becomes forward 
biased so that the forward converter 100 assumes the configuration shown 
in FIG. 8. Thereafter, capacitor 150 is essentially placed in parallel 
with capacitor 170. However, since the capacitance of capacitor 170 is 
preferably significantly greater than the capacitance of capacitor 150, 
most of the current flowing from the inductor 164 as the magnetic field in 
the inductor 164 collapses flows into the capacitor 170. Capacitor 170 is 
sufficiently large that the voltage across capacitor 170 remains 
substantially constant at V.sub.02 as current from the inductor 164 flows 
into the capacitor 170. Thus, the voltage across the capacitor 150 changes 
from V.sub.PK -V.sub.IN at time t.sub.4 to -V.sub.02 at a time prior to 
t.sub.1 of the next cycle. The change in energy stored in the capacitor 
150 thus changes from 1/2C(V.sub.PK -V.sub.IN).sup.2 at T.sub.4 to 
-1/2CV.sub.02.sup.2 just prior to t.sub.1 of the next cycle, where C is 
the capacitance of capacitor 150. The difference in energy stored in the 
capacitor from t.sub.4 to t.sub.1, i.e., 1/2C(V.sub.PK -V.sub.IN).sup.2 
-V.sub.02.sup.2 ! is the amount of energy transferred to the capacitor 170 
each energy transfer cycle. As explained above, the amount of charge 
transferred to the capacitor 170 cycle remains constant. For a constant 
power load, the voltage V.sub.02 will therefore remain constant. 
In summary, by placing a charge on capacitor 150 each cycle that is 
independent of the voltage V.sub.IN, the voltage resulting from that 
energy transfer is constant for a constant power load. Although the 
preferred embodiment of the forward converter 100 illustrated in FIG. 2 
utilizes an inductor 164 and capacitor 170 as a vehicle for transferring 
energy from the capacitor 150, other configurations may be used. For 
example, instead of using an inductor 164, capacitor 170 and diode 168, 
the primary of a transformer may be used in place of the inductor 164 so 
that energy from the capacitor 150 is transferred by transformer coupling. 
Also, the capacitor 150 may be connected to the transformer 112 in other 
configurations as long as the energy transferred to the capacitor 150 is 
independent of the magnitude of the input voltage V.sub.IN. Other 
alternatives for transferring energy from the capacitor 150 each cycle 
will be apparent to one skilled in the art. Thus, it will also be evident 
that, although specific embodiments of the invention have been described 
herein for purposes of illustration, various modifications may be made 
without deviating from the spirit and scope of the invention.