Dual stage AC to DC switching power supply with high voltage, low current intermediate DC and low voltage, high current regulated DC output

AC power is converted to a low-voltage, high-current DC output by a power supply which contains two stages. The first stage is an AC to DC converter using a full wave bridge rectifier to convert the voltage from the AC source to an intermediate raw DC voltage. The second stage is a DC to DC forward converter using a transformer with its primary winding connected to the intermediate raw DC voltage through a switch which is cycled "on" and "off" at a predetermined frequency. During the switch "on" portions of the cycle a current flows in the primary winding powering the output and charging an inductor which is connected to the secondary winding. When the switch is turned "off", no current flows through the transformer and the load is powered by the energy stored in the inductor. A secondary coil is magnetically coupled to the inductor and has a induced voltage proportional to the output voltage. A controller is used to control the duty cycle of the switch based on the voltage in the secondary coil which determines the power supply output. The duty cycle of the switch is varied in response to variations in the load.

This invention relates to power supply circuits which convert AC inputs to 
DC outputs. More particularly, it relates to AC to DC power converters 
which provide a DC output isolated from the AC input and which compensate 
for load changes. 
BACKGROUND OF THE INVENTION 
Power conversion deals with converting electrical power from one form to 
another. In one form of power conversion, an alternating current (AC) 
input voltage is converted to a direct current (DC) output voltage. Such 
conversions are necessary to power DC devices (such as solid state 
devices) from conventional AC power sources such as standard wall outlets 
and the like. Modern power supplies are preferably small in size, 
lightweight, low cost and exhibit high power conversion efficiencies. In 
many applications it is also desirable that the power supply electrically 
isolate the load from the source to prevent transmission of noise or 
surges in the source to the load as well as to isolate the end user from 
the AC line for safety considerations. Such isolation is critical when the 
load contains sensitive electronics. Along with electrical isolation, the 
power supply should monitor the output conditions and compensate for 
changes in the load which result in changes in the output voltage and 
current of the power supply. The monitoring of the load to compensate for 
changes in the output voltage must be done while still maintaining the 
electrical isolation described above, and without significantly affecting 
power supply efficiency or output. 
SUMMARY OF THE INVENTION 
The present invention provides a power supply which converts power from an 
AC source to a DC voltage for powering a DC load. The output voltage, 
however, is electrically isolated from the source voltage and is 
continuously monitored by a coil magnetically coupled to the output 
voltage so that variations in load conditions which result in changes in 
output voltage are quickly compensated for by the power supply. 
The power supply is divided into two stages. The first stage is an AC to DC 
converter whose DC output is a raw high-voltage, low-current DC source. 
The second stage is a DC to DC converter which converts the raw 
high-voltage, low-current DC source into a low-voltage, high-current DC 
source across which a DC load is connected. A controller regulates the 
output voltage to ensure that it remains essentially constant despite 
variations in the DC load or AC input voltage. The controller regulates 
the output voltage by sampling the output voltage and varying the 
properties of the second stage DC to DC converter when the output voltage 
indicates a change in the DC load conditions. 
In the preferred embodiment the first stage is formed by a full wave bridge 
rectifier. The AC power source is connected to the full wave bridge 
rectifier through a fuse, a thermistor to limit in-rush current, and an 
EMI filtering capacitor. The output of the full wave bridge rectifier is a 
raw high-voltage, high ripple DC. For an AC input between 108V AC and 132V 
AC the output of full wave bridge rectifier is a nominal 170V DC. 
The DC to DC converter of the preferred embodiment includes a transformer 
with a primary and secondary winding. The primary winding of the 
transformer is connected across the raw DC voltage from the full wave 
bridge rectifier through a switching MOSFET transistor. The switching 
transistor is switched "on" and "off" at a nominal frequency of 200 kHz 
resulting in a 5 .mu.s cycle time. During the portion of the cycle in 
which the switching transistor is "on", voltage is imposed across the 
primary winding of the transformer and which causes a voltage to be 
induced in the secondary winding. The voltage on the secondary winding is 
coupled through a diode (or forward rectifier) which becomes forward 
biased as well as a coil acting as an inductor and the DC load before 
returning to the secondary coil. This transformer output voltage forces 
inductor current to increase with time. 
When the switch transistor is turned "off", current is stopped from flowing 
in the primary coil, resulting in no current in the secondary coil reverse 
biasing the forward rectifier. The inductor current then begins to 
decrease with time thus delivering its stored energy to the DC load and a 
diode (or freewheel rectifier) which is shunted across the secondary coil 
and forward rectifier. At some point during the discharge of the inductor 
energy the switch MOSFET transistor turns back "on", causing the inductor 
current to increase with time. A capacitor is shunted across the DC load 
to filter variations in the output current and provide a constant voltage 
to the DC load. A coil is magnetically coupled to the inductor to produce 
a voltage proportional to the output voltage. The load on the coil is kept 
very small such that substantially all the power produced by the power 
supply is delivered to the DC load. 
Switching of the transistor is controlled by an integrated circuit 
controller using a method termed "peak-current-control". The controller 
regulates the voltage at the output terminals by monitoring the voltage at 
the coil and controlling the duty cycle of the switch transistor. The 
longer the duty cycle the more power provided to the output. Variations in 
the output are accounted for by lengthening or shortening the duty cycle. 
As stated the controller monitors the output voltage using the coil 
electrically coupled to the inductor in the output which has an induced 
voltage across the coil is proportional to the output voltage and, 
therefore, indicates when variations in load conditions occur. The 
amplified difference between this voltage and an internal reference 
voltage, referred to as the error amplifier output, is sent to a pulse 
width modulator which compares the output voltage of the error amplifier 
with a voltage proportional to the current flowing through the switch 
transistor using an internal comparator in the controller. The output of 
the pulse width modulator is used to determine the duty cycle of the 
switch transistor. 
The controller is powered in steady state operation by the voltage 
developed across the secondary coil of the output inductor, thereby 
eliminating the need for an outside power source to power the controller.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A circuit diagram 10 of a preferred embodiment of the AC to AC power supply 
of the invention is illustrated in FIG. 1. Circuit 10 is provided with an 
AC input 12 which is applied to an AC to DC converter formed by full wave 
bridge rectifier 14. Fuse 16 connected at AC input 12 prevents power 
supply circuit 10 from drawing excessive input current in the event of 
circuit failure. Negative temperature coefficient thermistor 18, also 
connected at AC input 12, limits in-rush current during the initial power 
up of power supply circuit 10. Inductor 20 and capacitor 22 form a 
low-pass filter which is used to attenuate noise generated by the 
converter switching action. Because capacitor 22 is connected across the 
input terminals of AC input 12, resistor 24 is placed in parallel with 
capacitor 22 to bleed off charge when the power source is disconnected 
from AC input 12. This insures that AC input 12 will not have the voltage 
across capacitor 22 at the input terminals when AC input 12 is 
disconnected which would be a serious shock hazard. 
The raw DC output of full wave bridge rectifier 14 is applied to a DC to DC 
converter. The raw DC output is connected across primary winding 28 of 
transformer 26 in series with transistor 30 and resistor 32. When 
transistor 30 is "on" current flows through primary winding 28, transistor 
30, resistor 32 and then back to primary winding 28 through capacitor 34. 
Resistor 32 is used to provide a voltage proportional to the current 
through primary winding 28. 
Secondary winding 36 of transformer 26 is connected to DC output 46 of 
power supply circuit 10 through forward rectifier 38, freewheel rectifier 
40, output capacitor 44 and primary coil 42 acting as an inductor. Output 
capacitor 44 and primary coil 42 act as a filter to prevent AC voltage 
components which appear at the node between primary coil 42 and forward 
rectifier 38 from being imposed across capacitor 44, thereby providing a 
smooth output. Resistor 48 and capacitor 50 are connected across secondary 
winding 28 and act to damp parasitic ringing which could damage forward 
rectifier 38 and freewheel rectifier 40. Resistors 52 and 54 are connected 
across DC output 46 to provide a minimal preload to prevent excessive 
output voltage rise at no load conditions. 
Transformer 26 includes shield 60 between the windings to prevent the 
primary switching frequency and associated harmonic voltages from causing 
capacitive current flow between the primary winding 28 and secondary 
winding 36. Capacitors 56 and 58 are connected across primary winding 28 
and secondary winding 36 and provide a low impedance, high-frequency path 
which is used to attenuate common mode EMI developed across transformer 
26. Capacitors 56 and 58 are preferably Y-type capacitors connected in 
series to provide double insulation. 
Transformer 26 also includes clamp winding 62. Clamp winding 62 is coupled 
to primary winding 28 and acts to return energy stored in transformer 26 
back into capacitor 34. When transistor 30 is "off" a rectifier diode 64 
is caused to conduct by the action of transformer 26 through clamp winding 
62. 
Controller 66 is used to control power supply circuit 10. As stated above, 
resistor 32 is used to develop a voltage proportional to the current 
through primary winding 28. This voltage is used as an input to pin 66c of 
controller 66 through resistor 68. Capacitor 70 is used to attenuate the 
voltage spike produced at the turn "on" of transistor 30 which could 
otherwise prematurely trip controller 66. Output 66f of controller 66 is 
connected to the gate of transistor 30 through resistor 72 and controls 
the switching of transistor 30. Diode 74 is connected in parallel with 
resistor 72 and allows the rapid discharge of transistor 30 gate 
capacitance in order to achieve rapid turn "off" of transistor 30. 
Resistor 75, connected between the gate and the source of transistor 30, 
is used to guarantee that transistor 30 stays "off" between the 
application of power at AC input 12 and the activation of controller 66. 
Resistor 76 (connected between pin 66h and pin 66d of controller 66) and 
capacitor 78 (connected between pin 66d and ground pin 66e) are used as 
clock timing components. In the preferred embodiment, resistor 76 and 
capacitor 78 set the operational frequency of controller 66 to a nominal 
200 kHz. 
Secondary coil 80, inductively coupled to primary coil 42, provides an 
output feedback voltage to controller 66. Secondary coil 80 charges 
capacitor 82 during a portion of the time that transistor 30 is "off" 
while diode 88 prevents capacitor 82 from discharging back through 
secondary coil 80. While transistor 30 is off, current in primary coil 42 
flows in a loop consisting of primary coil 42, the output load, and 
freewheel rectifier 40. Since the voltage across freewheel rectifier 40 is 
small compared to the output voltage, the voltage across primary coil 42 
is substantially equal to the output voltage, resulting in the voltage 
induced in secondary coil 80 being substantially proportional to the 
output voltage. 
Resistors 84 and 86 form a voltage divider to provide a voltage 
proportional to the voltage across capacitor 82 to voltage feedback pin 
66b. The voltage at voltage feedback pin 66b is compared to an internal 
reference voltage in controller 66 and is used to stabilize the voltage at 
DC output 46. Capacitor 96 and resistor 98 are connected between feedback 
voltage pin 66b and pin 66a and serve as an error feedback amplifier 
components, the values of which are chosen to insure feedback loop 
stability. 
Secondary coil 80 is also used to charge capacitor 90 through diode 94. The 
voltage across capacitor 90 provides the necessary power to controller 66 
in steady state operation. Capacitor 92 acts as a high frequency bypass 
capacitor. At initial power "on", capacitor 90 is initially charged 
through resistor 68. When capacitor 90 reaches a sufficient voltage (about 
sixteen volts), controller 66 becomes active. Controller 66 draws current 
from capacitor 90 until DC output 46 reaches operating voltage and 
secondary coil 80 recharges capacitor 90. 
Power supply circuit 10 accepts AC input from 108V AC to 132V AC and 
provides a single DC output at 12.8 volts at 5.25 amps. Power supply 
circuit 10 provides output voltage stabilization against load variations; 
output voltage stabilization against line regulation; input/output 
isolation to satisfy safety agency requirements; and voltage conversion to 
deliver a low-voltage, high-current DC from the high-voltage, low-current 
AC source. 
Power supply circuit 10 operates in two stages. The first stage accepts the 
AC source input at AC input 12. Full wave bridge rectifier 14 converts the 
AC source into a raw DC voltage which is nominally 170V. This 170V raw DC 
serves as the input to the second stage DC to DC forward converter. 
The DC to DC converter functions by the switching of transistor 30. 
Controller 66 switches transistor 30 at a frequency of 200 kHz as set by 
resistor 76 and capacitor 78. Transistor 30 is "on" and conducting for a 
period which is less than half the 5 .mu.s period. For the remaining 
portion of the period transistor 30 is "off" and not conducting. 
When transistor 30 is "on", primary winding 28 is connected across the raw 
DC voltage from the full wave bridge rectifier 14. By action of 
transformer 26, the secondary winding is biased so as to forward bias 
forward rectifier 38 and reverse bias freewheel rectifier 40. Current then 
flows through secondary winding 36, forward rectifier 38 and primary coil 
42 to the load connected to DC output 46. The load on secondary coil 80 is 
scaled to conduct minimal current compared to primary coil 42 such that 
primary coil 42 acts as an inductor. 
When transistor 30 is "off", no current flows through secondary winding 36 
and forward rectifier 38 is reverse biased. Energy stored in primary coil 
42 is released causing current to flow through the load connected across 
DC output 46 and back to primary coil 42 through freewheel rectifier 40. 
This action results in the current in primary coil 42 ramping up during 
the period when transistor 30 is "on" and ramping down during the period 
when transistor 30 is "off". Primary coil 42 and capacitor 44 attenuate AC 
voltage components resulting in the voltage across the load being 
essentially constant. 
During transistor 30 "on" periods, magnetizing current and reflected 
secondary current appears in primary winding 28 of transformer 26. When 
transistor 30 is turned "off", magnetizing current appears in clamp 
winding 62 and flows through the rectifier diode 64 to capacitor 34. Thus 
substantially all energy stored in transformer 26 during transistor 30 
"on" periods is returned to capacitor 34. Clamp winding 62 also serves as 
a voltage clamp to limit the maximum voltage stress on transistor 30. 
Output voltage regulation at DC output 46 is achieved by controlling the 
"on" time (or duty cycle) of transistor 30. This duty cycle control is 
provided by controller 66 which contains an internal reference voltage, 
error amplifier, pulse width modulator comparator, and a current sense 
amplifier. Controller 66 uses a mode known as "peak-current-control" to 
control the converter. 
Peak-current-control functions using the current through transistor 30. A 
representation of this current during a cycle of transistor 30 is shown in 
FIG. 2. When transistor 30 is turned "on", its current jumps to an initial 
value determined by the current through primary coil 42 as reflected 
through transformer 26. The current in transistor 30 ramps up from the 
initial value (as shown in FIG. 2) due to increasing primary coil 42 and 
transformer 26 currents. As stated, the current through transistor 30 is 
converted to a voltage by resistor 32 and the voltage developed is sensed 
by controller 66. 
The voltage developed at resistor 32 (and seen by pin 66c of controller 66) 
serves as one input to the internal pulse width modulator comparator of 
controller 66. The other input to the internal pulse width modulator 
comparator is derived from the output of the error amplifier at pin 66a. 
When the voltage at pin 66c exceeds pin 66a voltage the "on" time of 
transistor 30 is terminated. The error amplifier output is the amplified 
difference between the internal reference of controller 66 and a voltage 
proportional to the voltage at DC output 46. 
In operation, the output of the internal error amplifier would increase if 
the voltage at DC output 46 were to decrease because of an increase in 
load. By action of the internal pulse width modulator comparator, the peak 
current through transistor 30 needed to cause the termination of the 
transistor 30 "on" time would increase, causing the average current 
through switch transistor to increase and resulting in a higher power 
supply output current. This higher level of supplied output current 
results in a constant output voltage despite variations in load. Output 
voltage sensing is accomplished using secondary coil 80 as discussed 
above. 
Output overload protection is provided by an internal clamp placed on the 
maximum internal error amplifier voltage. This maximum value forces switch 
transistor to turn "off" at a maximum switch transistor current despite 
greater demand from the load. This maximum current is set to represent an 
output current a bit greater than full load on the power supply. 
All components shown in FIG. 1 except transformer 26 and controller 66 are 
standard electrical components and are readily available. Transformer 26, 
in the preferred embodiment, has the following characteristics: winding 
ratios of 37:37:9 for primary winding 28, clamp winding 62, and secondary 
winding 36, respectively; a primary winding inductance of 3.78 mH (+/- 
20%); primary winding DC resistance of 230 m.OMEGA.; clamp winding DC 
resistance of 675 m.OMEGA.; and secondary winding DC resistance of 9.2 
m.OMEGA.. Controller 66 is preferably part number UC 3842 available from 
Universal Scientific Industrial Co., Ltd. 141 lane 351, Tai Ping Rd. Sec. 
1, Tsao Tuen Nan-Tou Hsien, Taiwan R.O.C. 
While the invention has been shown and described with particular reference 
to a preferred embodiment, it will be understood by those skilled in the 
art that various changes in form and detail may be made therein without 
departing from the spirit and scope of the invention as defined by the 
appended claims.