Pulse width modulator for switching power supply

An improved pulse width modulation type switching power supply uses an opto-isolator as an output voltage error amplifier which feeds back an error signal to control pulse width modulation as well as to provide isolation between the primary and secondary voltages of the power supply.

TECHNICAL FIELD 
This invention relates to switching power supplies that utilize pulse width 
modulators. 
BACKGROUND OF THE INVENTION 
Switching power supplies utilize a pulse width modulator to change the 
pulse width of the switching voltage to provide appropriate power to the 
output. Usually a pulse width modulator (PWM) integrated circuit (IC) is 
used for this purpose. However, additional discrete components are needed 
to make the PWM IC work properly. 
FIG. 1 shows a typical switching power supply implementation (see Billings, 
K. H., Switch Mode Power Supply Handbook, McGraw-Hill, 1989, in Chapter 1, 
pages 2.3-2.15.) FIG. 1 shows the switching power supply being used in an 
off-line AC to DC switching power supply arrangement. To isolate the high 
voltages at the primary from the lower DC voltages at the secondary, a 
transformer (T10) and an opto-isolator (U10) are used. Opto-isolator U10 
also feeds an error signal to the PWM (IC10). This error signal adjusts 
the pulse width of PWM IC10 to ensure that the switching power supply 
provides adequate power to the output terminals. Error amplifier IC11 
generates the error signal by comparing the output voltage with the 
voltage reference provided by zener diode CR11. 
Undesirably, the switching power supply circuit of FIG. 1 uses too many 
components to accomplish pulse width modulation. These extra components 
increase both the cost and power usage of the circuit. Additionally, extra 
components increase the area utilized on the printed wiring board needed 
to implement the circuit. 
SUMMARY OF THE INVENTION 
The invention provides an improved pulse width modulation (PWM) type 
switching power supply which uses an opto-isolator as the error amplifier 
to control pulse width modulation as well as to provide isolation between 
the input and output voltages of the power supply. More particularly, the 
switching power supply is connected as an off-line AC to DC power supply 
having its output terminals DC isolated from its input terminals. An 
oscillator and drive circuit connects to a rectified AC voltage source and 
the opto-isolator controls the pulse width of a switching voltage. The 
opto-isolator device also acts as an error amplifier. The opto-isolator 
device includes an input diode optically coupled to the base of an output 
transistor. The input diode is connected in series with a zener diode 
across the power supply output terminals. The emitter and collector of the 
output transistor are connected to control the pulse width of the 
oscillator circuit in response to changes in current through the input 
diode. A resistor connects between the base of the output transistor of 
the opto-isolator device and the source terminal of the switching 
field-effect transistor (FET) so that the resulting base current biases 
the output transistor in a linear operating region.

DETAILED DESCRIPTION 
With reference to FIG. 1, we describe the operation of the previously 
referenced prior art switching power supplies. A rectified AC voltage 
V.sub.in is received at the input terminals (V.sub.i+) and (V.sub.i-). 
This input voltage is filtered by capacitor C10. Pulse width modulator and 
drive circuit IC10 provides a switching voltage to field effect transistor 
F10. When transistor F10 turns ON, current flow through the primary of 
transformer T10 couples power to the secondary of transformer T10. The 
output voltage at the secondary of transformer T10 is rectified by diode 
CR10 and filtered by capacitor C11 to form the output DC voltage V.sub.out 
(V.sub.o+ and V.sub.o-). This output DC voltage is provided to load 
impedance RL. A feedback circuit is provided by zener diode CR11, error 
amplifier IC11, and opto-isolator U10. This feedback circuitry controls 
the pulse width and frequency of pulse width modulator IC10. 
In switching circuits, especially in switching power supplies, the pulse 
width modulator (PWM) IC10 is used to change both the operating frequency 
and duty cycle of the switching voltage to provide adequate output power 
to load impedance RL. When the switching power supply is used in an 
off-line AC to DC power supply arrangement, opto-isolator U10 is used for 
isolating the high primary input rectified AC voltage from the secondary 
DC output voltage. A feedback signal, generated by error amplifier IC11, 
is sent to PWM IC10 via opto-isolator U10. The IC10 is used to control the 
pulse width modulation and U20 is used to isolate the primary and 
secondary voltages. 
The error amplifier IC11, present at the output of the switching power 
supply, compares the voltage appearing at its two input terminals. When 
the output voltage V.sub.out exceeds the voltage of zener diode CR11 plus 
the turn-on voltage of error amplifier IC11, current flows in the 
light-emitting diode D1 of opto-isolator U10. When the output voltage 
V.sub.out is not sufficient to turn on the error amplifier IC11, no 
current flows through diode D1 of opto-isolator U10. 
When the current flow through diode D1 generates sufficient photon flow, it 
will provide the base current needed to operate transistor T1 of 
opto-isolator U10. When this happens, transistor T1 turns ON and sends a 
feedback signal to PWM IC10 to change the pulse width and frequency of the 
switching wave form. 
With reference to FIG. 2, we describe in detail the operation of the 
present invention. In accordance with the invention, the opto-isolator U20 
is used to provide the function of the error amplifier IC11 of FIG. 1. 
Additionally, an oscillator IC20 is used to replace the pulse width 
modulator IC10 of FIG. 1. The invention saves cost since the PWM IC10 and 
error amplifier IC11 of FIG. 1, along with the discrete components used to 
make these integrated circuits operational, are no longer needed. This 
also eliminates the area on the printed wiring board used to mount these 
ICs and components. Additionally, it saves the power utilized by error 
amplifier IC11 and PWM IC10. 
The operation of the invention shown in FIG. 2 is described as follows: 
Zener diode CR21 turns ON once the output voltage V.sub.out has exceeded 
the sum of the zener voltage and the turn-on voltage of diode D2 of 
opto-isolator U20. This induces current flow through transistor T2 of 
opto-isolator U20 which reduces the operating frequency and pulse width of 
oscillator IC20, thereby providing less power at the output terminals. If 
transistor T2 is not ON, the oscillator IC20 operates at its maximum 
frequency and pulse width to provide maximum power to the output load RL. 
When oscillator IC20 turns ON, field effect transistor (FET) F20 turns ON, 
causing current through FET F20 to start increasing, which increases the 
voltage across resistor R20. This voltage increase across resistor R20 
adds to the voltage across R21 and slowly turns ON transistor T2 of U20, 
which in turn shortens the pulse width and thus slows the operating 
frequency of oscillator IC20. Thus, the maximum current flowing through 
FET F20 is limited by the transistor T2 of U20. The sum of the voltage 
across R21 and the voltage across R20 together control transistor T2 of 
U20 to change the frequency and pulse width of oscillator IC20. 
For example, when the output load RL is drawing maximum current, the output 
voltage V.sub.out decreases and zener CR21 turns OFF. Hence, diode D2 no 
longer provides a photonic bias current to transistor T2 of U20. At this 
point, the frequency and pulse width of oscillator IC20 is increased since 
transistor T2 of U20 only turns ON in response to the current through FET 
F20. This is because transistor T2 of U20 is controlled only by the 
current flow through resistor R20. Consequently, the output voltage 
V.sub.out increases. 
Conversely, when the output load decreases (RL increases), the output 
voltage V.sub.out increases, causing zener CR21 to turn ON. At this point, 
both the photonic feedback current from diode D2 and the current through 
resistor R20 act to turn ON transistor T2 earlier in each cycle, thereby 
causing the frequency and pulse width of oscillator IC20 to be reduced. 
Consequently, the output voltage V.sub.out decreases. Thus, the switching 
power supply is constantly adjusting the pulse width and frequency of 
oscillator IC20, and hence the output voltage V.sub.out, to accommodate 
for changes in the output load RL. 
Shown in FIG. 3 is an illustrative circuit implementation of FIG. 2. A 
Schmitt trigger chip IC30 includes a first Schmitt trigger circuit A30, 
which is connected as an oscillator, and a second Schmitt trigger circuit 
A31, which is connected as an inverter circuit. A driver circuit IC31 
consists of two transistors Q30 and Q31 which connect to the output of 
circuit A31 and are used to drive transistor F30. The supply voltage V cc 
provides power to IC30 and IC31. Resistor R32 and capacitor C32 connect to 
Schmitt trigger circuit A30 to form an oscillator circuit A32. Resistor 
R32 and capacitor C32 determine the free-running frequency of oscillator 
circuit A32. The free-running frequency is the maximum frequency at which 
transistor F30 can be switched. The oscillator A32 has its maximum pulse 
width when it is operating at its free-running frequency. When the 
oscillator A32 is free-running, transistor T3 of opto-isolator U30 is OFF. 
When oscillator A32 is free-running, the increased frequency and pulse 
width enables the switching power supply to handle its maximum load RL. 
Shown in FIG. 4 are illustrative voltage wave forms appearing across 
capacitor C32 (V.sub.C32) and at the output of buffer circuit A31 
(V.sub.oA31) for a predetermined output load RL. At time t.sub.1, the 
voltage V.sub.C32 shows that capacitor C32 starts charging through 
resistor R32. At time 12, the Schmitt trigger circuit A30 turns OFF and 
starts to discharge capacitor C32. At time t.sub.3, the voltage V.sub.C32 
has decreased to the turn-ON voltage of Schmitt trigger circuit A30 and 
capacitor C32 again starts to charge. The longer the pulse width t.sub.3 
-t.sub.2, the greater the power provided to output load RL. The frequency 
of operation is the reciprocal of the time period t.sub.3 -t.sub.1. 
As the load current increases (i.e., resistor RL decreases), zener CR31 
begins to turn OFF, which means less current flows through R31 and thus 
more voltage is required from R30 to turn ON the transistor T3 of U30. 
This means transistor T3 remains OFF longer in the cycle and thus less 
current is pulled from C32 by transistor T3. Thus, the pulse width and the 
frequency increase as shown in FIG. 4A. 
When the output current is at a maximum level, the output voltage drops and 
does not exceed the sum of the zener CR31 voltage and the diode D3 
voltage. Consequently, transistor T3 of U30 is in an OFF condition for its 
longest time. At this point, the pulse width and frequency of the wave 
forms of FIG. 4A would be at their maximum rate (not shown). 
When the load decreases (i.e., resistor RL increases), the output voltage 
will start to increase. When the output voltage exceeds the sum of the 
zener CR31 voltage and diode D3 voltage and transistor T3 turns ON, it 
draws current away from capacitor C32, thereby slowing the frequency of 
oscillator circuit A30 and decreasing the pulse width of the output wave 
form. The reduced frequency and pulse width reduces the power provided to 
the output load RL. Shown in FIG. 4B are the wave forms for a minimum 
output current (i.e., maximum RL) where, consequently, both the frequency 
and pulse width are less than those shown in FIG. 4A. 
It should be understood that, when primary to secondary voltage isolation 
is not required, the transformer (T20 of FIG. 2 and T30 of FIG. 3) can be 
replaced with an inductor, and the rectifier diode (CR20 of FIG. 2 and 
CR30 of FIG. 3) connects between the inductor and the drain terminal of 
the FET (F20 of FIG. 2 and F30 of FIG. 3 ). Additionally, it should be 
understood that a current limiting resistor is typically placed in series 
with reference diode (CR21 of FIG. 2 and CR31 of FIG. 3) to limit the 
current flow into the opto-isolator. 
What has been described is merely illustrative of the application of the 
principles of the present invention. Other arrangements and methods can be 
implemented by those skilled in the an without departing from the spirit 
and scope of the present invention.