Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-01-09
2003-10-21
Berhane, Adolf D. (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C363S021010
Reexamination Certificate
active
06636025
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to control circuitry for power supplies and, in particular, to control circuitry for use on the load side of a switch-mode power supply.
2. Description of Related Art
Switching mode power supplies are frequently used to power small appliances such as portable computers and the like where electrical isolation from an AC power source is needed. One advantage of switching mode supplies is that they can be compact in size and yet deliver considerable power.
FIG. 1
is a simplified block diagram of a typical prior art switching mode power supply. A rectifier/filter circuit
10
is connected to an AC power source that is typically 110 V 60 Hz for domestic application and 85-265 V, 50/60 Hz for international applications. The output of the rectifier/filter
10
is connected to the primary side of an isolation transformer
12
. A Line-Side Controller
14
includes an active switch, such a power transistor
16
, which operates to periodically interrupt the connection between the transformer primary and the output of the rectifier/filter circuit
10
so as to introduce an AC component. The switching frequency is relatively high so that transformer
12
can be made relatively small. Details of an exemplary Line-Side Controller
14
are disclosed in U.S. Pat. No. 6,233,165, the contents of which are fully incorporated herein by reference.
The output of the secondary winding of transformer
12
is rectified by diode
18
and filtered by capacitors
20
and
24
and choke
22
to produce a DC output voltage Vout
+
. A Load-Side Controller
26
is included to provide various functions. Among other things, Load-Side Controller
26
provides a control input to the Line-Side Controller
14
by way of optical coupler
28
so as regulate the magnitude of Vout
+
using pulse width modulation. Load-side Controller
26
also typically provides output short circuit protection and output current limiting functions. Preferably, the Load-Side Controller
26
is powered by the output Vout of the power supply or output V
+
and does not require the use of an auxiliary power supply.
FIG. 2
is a schematic diagram of an exemplary prior art Load-Side Controller
26
for use in the
FIG. 1
switch mode power supply. The
FIG. 2
controller is fabricated from discrete components including Zener diode Z
1
which is connected to V+ by way of a resistor Rz. The output voltage Vout+, which is related to voltage V+, is equal to the sum of the forward voltage dropped across optical coupler diode
28
A, diode D
1
and the Zener voltage of Z
1
. The Zener voltage may be, for example, +4.7 volts. Thus, voltage V+ is approximately +6 volts. If voltage V+ should drop below the target voltage, the current through optical coupler diode
28
A will drop, with the Line-Side Controller
14
(
FIG. 1
) being implemented so as to respond by increasing the output voltage using pulse width modulation. The magnitude of the output voltage can be changed somewhat by changing the value of resistor Rz thereby changing the operating point of the relatively soft knew of Zener diode Z
1
.
The
FIG. 2
load-side controller
26
further includes an NPN transistor Q
1
that performs a current limit function. The load current flows through resistors RB and RC, with a fraction of the voltage developed across RB being applied to the base-emitter junction of transistor Q
1
. At normal output currents, the voltage across RB is not great enough to turn on Q
1
. However, at greater currents, transistor Q
1
will turn on thereby drawing current through resistor RE and the optical coupler diode
28
A. The Line-Side Controller
14
(
FIG. 1
) will respond by reducing the output voltage V+ thereby limiting the output current.
The base-emitter voltage needed to turn transistor Q
1
on will vary from transistor to transistor. Thus, it will typically be necessary to manually adjust the value of RB to achieve the desired current limit point. In addition, the base-emitter voltage has a fairly strong negative temperature coefficient. Resistor RT is thermistor device having a positive temperature coefficient resistance that tends to offset the negative temperature coefficient of the base-emitter voltage of transistor Q
1
. In the event that the output of the supply becomes shorted, that is V+ (or Vout+) is shorted to Vout−, the current limit circuit will continue to operate. No auxiliary power supply is needed to keep the current limit circuit functioning under these conditions.
Resistor RB is selected to produce a voltage greater than that necessary to turn transistor Q
1
on, typically the voltage across RB being around 800 millivolts. Resistor RA, RT and RD function to apply only a fraction of this voltage to the base-emitter junction of Q
1
. Resistor RC is added to produce another approximately 400 millivolts at current limit. Accordingly, when V+ and Vout− are shorted together, a total voltage of 1280 millivolts is dropped across resistors RB and RC for powering the current limit circuitry. That is sufficient to power the current limit circuitry which needs about 1300 millivolts to operate, with that being the sum of the voltage drop across resistor RE, coupler diode
28
A and the collector-emitter saturation voltage of transistor Q
1
.
In order to improve the operation of the
FIG. 2
controller, some prior art load-side controllers utilize a commercial integrated circuit programmable shunt regulator in lieu of Zener diode Z
1
. These shunt regulators, such as regulator sold by Fairchild Semiconductor under the designation TL431 include a band-gap circuit that produces a reference voltage and an error amplifier which compares the reference voltage with the output voltage Vout+, or some fraction of the output voltage set by a resistor divider, so as to provide an adjustable regulated output voltage.
The
FIG. 2
controller is inherently imprecise in terms of both voltage regulation and current limit set point. Further, a discrete implementation significantly increases the cost of manufacturing a power supply using the
FIG. 2
controller, with low cost being a important factor in this type of power supply.
FIG. 3
shows another prior art Load-Side Controller
26
, the primary components of which are implemented in integrated circuit form. Rectification is carried out by a Schottky diode D
3
connected to the V− output of transformer
12
(
FIG. 1
) which replaces diode
18
. The
FIG. 3
controller
26
includes an auxiliary power supply AS having an input terminal connected to V−. The auxiliary supply AS, which typically includes at least a rectifier diode and filter capacitor (not depicted), has an output connected to power input terminal VCC of the integrated circuit I
1
of the
FIG. 3
controller.
The integrated circuit I
1
includes a band-gap regulator circuit BR that produces a band-gap reference voltage of +2.5 volts. Circuit BR can be trimmed to vary the magnitude of the reference voltage. The reference voltage output of the circuit BR is buffered by a unity gain configured amplifier A
1
, with the output of A
1
being coupled to an inverting input of another amplifier A
3
by way of resistor RH. A frequency compensation capacitor Cd is connected between terminal Comp, connected to the inverting input of amplifier A
3
, and terminal Opto.
The non-inverting input of amplifier A
3
is connected a terminal V Sense of the integrated circuit I
1
. A selected fraction of the output voltage V+ is supplied to terminal V Sense by way of a resistive divider network comprising discrete resistors RI and RJ connected between V+ and Vout−. The output of amplifier A
3
is connected to terminal Opto by way of a diode D
1
. The anode of an optical coupler diode
28
A is connected to terminal Opto so that current is supplied to diode
28
A by way of diode D
1
when the amplifier A
3
output is positive.
An external discrete resistor RE is connected be
ASIC Advantage Inc.
Berhane Adolf D.
Girard & Equitz LLP
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