MXPA99002363A - A switched-mode power supply control circuit - Google Patents

Abstract

A controller for a switch mode power supply includes an undervoltage protection circuit (106a) responsive to an input supply voltage indicative signal (VZ1). The input supply voltage indicative signal is also coupled to a foldback point correction circuit (106b). The correction circuit causes a decrease in a maximum duty cycle of a control signal (VOUT) when the input supply voltage increases and is still smaller than a predetermined magnitude. A zener diode (Z1) limits the input supply voltage indicative signal in a manner to prevent a further decrease in the duty cycle when the input supply voltage exceeds the predetermined magnitude.

Description

AN INTERRUPTED MODE POWER SUPPLY CONTROL CIRCUIT The invention relates to an interrupted mode power supply control circuit. Interrupted mode power supplies efficiently generate a variety of regulated voltages from a single line voltage level (eg, 110 volts AC). One important use of these power supplies is in a television signal receiver, where they are used to produce a regulated B + voltage for the horizontal deflection circuit, as well as other regulated voltages to energize various analog and digital circuits. Commonly, an interrupted mode power supply contains a full wave rectifier, a power supply controller, a switch, and an output transformer. The switch is commonly a high-energy transistor such as a MOSFET. To regulate the output voltages, the controller activates and deactivates (ie, pulse width modulations) the transistor gate in response to the power supply load and other control parameters. The interrupted voltage of the transistor activates a primary winding of the transformer, while several loads of the power supply are connected to one or more secondary windings. As such, the power supply converts an alternating current input voltage to one or more direct current voltages.

A particular controller is an integrated circuit available from Siemens as Model TDA 4605. This power supply controller is commonly used to activate the MOSFET transistor, which in turn activates the primary coil of the transformer. This specific integrated circuit as well as others used in the art, commonly contain a control mechanism that deactivates the power supply when the input voltage drops below a previously defined voltage level. Such protection is necessary because, in order to produce regulated output voltages, the interrupted mode power supply increases the duty cycle of the control signal which activates the transistor as the input voltage decreases. At some point, the input voltage decreases to a level where the output of the power supply is not regulated (for example, the maximum pulse length is used to activate the transistor). Such unregulated operation can damage the electronic circuits of the power supply, but it is more likely to damage electronic load circuits. For integrated circuit TDA 4605, as defined in the technical manual of the TDA 4605 available from Siemens AG, dated July 27, 1989, leg 3 of the integrated circuit is used to detect or monitor the primary input voltage (vp ) for the power supply (for example, rectified alternating current voltage). The minimum voltage to disable or disable the integrated circuit, and also the power supply, is previously established by the controller at one volt. As such, the primary input voltage (vp) is reduced by using a voltage divider at the input of pin 3. When selecting the appropriate values of the resistor in the voltage divider, a nominal value of the monitoring voltage is applied to the leg 3. Commonly, this voltage is approximately 2.0 volts for a primary input voltage of 120 volts. When the primary input voltage falls to a level that causes the monitoring voltage in pin 3 to fall below 1 volt, the power supply is deactivated to prevent unregulated operation. As mentioned above, this mode of interrupted mode power supply has found its use in television signal receivers. However, television receivers, in particular, have charging characteristics peculiar to an energy supply. Specifically, a television receiver power supply is used to produce a regulated voltage B +, commonly of approximately 140 volts, as well as a low direct voltage voltage level of 16 volts to energize all analog and digital circuits within the receiver. When the television receiver switches from the reserve mode to the operating mode, a heavy load is produced by the input of current to the filter capacitors connected to the regulated voltage B +. This heavy load causes the power supply to operate temporarily in an unregulated mode (maximum pulse width) and can cause the primary input voltage to drop to a low level. In addition, when the neutralization circuit is activated to neutralize the cathode ray tube (CRT), the voltage of the main AC supply is reduced due to the substantial load presented by the neutralization circuit. Consequently, the drop in line voltage could commonly cause the monitoring voltage to fall below 1 volt, the first minimum level, and as such, deactivate the power supply. Therefore, it is desirable to produce a monitoring voltage indicative of the primary input voltage, but to ensure that the power supply is not disabled by the expected heavy loads found in a television receiver. The TDA 4605 integrated circuit includes a retraction point correction circuit that reduces the maximum duty cycle of the MOSFET control signal, when the monitoring voltage exceeds a second minimum level of approximately 1.7 volts. The monitoring voltage is applied to the correction circuit also via the leg 3. In a circuit that includes an aspect of the invention, a resistive voltage divider that makes the monitoring input or sensor signal of the primary input voltage is designing such so that the first minimum level is not achieved during the expected temporary loading of the primary input voltage. However, said voltage divider causes a higher voltage to be applied to the monitoring voltage input of the controller during normal operation of the power supply. As such, an increase in the primary input voltage to a higher level, which is still within an acceptable tolerance range of the AC line voltage, can cause the monitoring voltage to rise to a level exceeding the second level. minimum at which the integrated circuit begins to limit the maximum duty cycle of the control signal that controls the MOSFET, that is, the controller applies a fallback correction technique. When the second minimum level is exceeded, the power supply automatically limits the power output of the power supply for an increase in the primary voltage. As a result of the design of the voltage divider that provides enough free space to overcome the voltage dips in the output generated by the load on the primary input voltage, the maximum power supply output could be, undesirably, significantly reduced to a high primary input voltage. In carrying out an aspect of the invention, to ensure that such inconsistent increase in the primary input voltage does not cause the power supply to significantly reduce the maximum duty cycle of the control signal, and therefore, the output of the Energy from the power supply, a zener diode is coupled to the voltage divider. The zener diode limits the magnitude of the monitoring voltage to a level that avoids an additional limitation to the maximum duty cycle when the primary input voltage increases additionally. Consequently, when the power supply is used in a television signal receiver, the voltage divider provides sufficient free space for the primary voltage to drop substantially due to activation of the neutralization circuit or other load conditions, while the diode zener ensures that the primary voltage can be increased above its nominal voltage without producing a significant energy limitation of the power supply output. An interrupted mode power supply, which incorporates an aspect of the invention, includes a source of an input supply voltage. A switch responds to a first control signal having a controllable work cycle and coupled to the source of the input voltage supply to generate a voltage. of output supply, according to the duty cycle of the first control signal. A duty cycle modulator responds to a second control signal to generate the first control signal and to control the duty cycle of the first control signal according to the present one in a way to control the current pulses. An increase in the duty cycle produces an increase in magnitude of the current pulses The modulator responds to a signal that is indicative of the input supply voltage to decrease the duty cycle when the input supply voltage increases. A limiter is coupled to the modulator for limit the decrease of the duty cycle for a given increase in said input supply voltage, when the input supply voltage exceeds a first quantity Figure 1 shows a schematic diagram of an interrupted mode power supply incorporating the teachings of the present invention. Figure 1 describes a schematic diagram of an interrupted mode power supply 100 embodying the present invention. The embodiment shown is designed to be used as a power supply for a television signal receiver wherein the power supply generates a regulated voltage B + (for example 140 volts) and a low voltage (for example 16 volts). The regulated voltage B + is used to energize a horizontal deflection circuit and the regulated low voltage is used to energize the digital and analog electronic circuits (continuous load 118). Other applications for power supply may require a small variation in the components described and their interconnections; however, such variations are within the scope of the present invention. The power supply contains a number of main components, including a full wave rectifier 102, the power supply controller 106, a MOSFET transistor Q1, a monitor voltage generator 110, an output transformer 112, and a plurality of circuit components to complete the electronic devices of the power supply. Illustratively, the input to the power supply is a voltage of 110 volts alternating current, 60 hertz. Rectifier 102 is a conventional full-wave bridge rectifier coupled to an AC input voltage source 101. The output of bridge rectifier 102 is coupled to capacitor C1 approximately 680 μF. An INPUT B + voltage forms unprocessed (unregulated) voltage B + (also referred to herein as the primary input voltage vp) having a nominal value of approximately 150 volts. The capacitor C1, connected from the rectifier output to ground, reduces the voltage of the bridge rectifier so that a direct current voltage, i.e., the primary input voltage is available in the upper terminal of the primary winding of the transformer W1. The primary input voltage forms an input to the monitor voltage generator 110 which produces a monitor voltage VZ1 for the controller 106. The monitor voltage generator is described in detail below. The controller is, illustratively, a TDA 4605 power supply controller available from Siemens AG of Munich, Germany. The eight legs of the controller are connected to signals and voltages that allow the controller to produce a pulse width or a work cycle modulated signal in leg five to control the duty cycle of transistor Q1. Specifically, pin four of controller 106 is connected to ground. Foot three is coupled to the monitor voltage. The leg two is information supplied concerning the primary current. An increase in primary current in the primary winding W1 is simulated as a periodic voltage increase, ramp voltage VC2 in the leg two using an external RC element formed by the resistor R3, the capacitor C2 and resistor R4 (where R3 is approximately 360 kW, and C2 is approximately 6,800 pF, and R4 is approximately 220 W). These elements are connected in series from the primary input voltage to ground. The leg 2 of the controller 106 is coupled to the junction of R3 and C2. A pulse width modulator 106c of the controller 106 controls the duration of the direct phase, and in addition, the primary current peak, using the ramp voltage VC2 which is proportional to the drain current of the transistor Q1. As indicated above, the ramp voltage is derived from the primary input voltage by using the RC elements connected to the leg two, that is, the ramp voltage simulates the primary current. Leg 1 of the controller is information supplied from the secondary voltage which internally compares the sampled control voltage of the regulating winding W3 of the transformer 112 and compares that sample voltage with an internal reference voltage. The fifth leg generates a modulated control signal of duty cycle or voltage VSALIDA by means of a push-pull output actuator for fast loading and unloading of the input capacitance of a MOSFET power transistor Q1 (model IRF740). Foot six is coupled to the voltage supply for the controller. Leg seven forms a soft entry start terminal. Capacitor C5 (0.1 μF) is connected from leg seven to ground to reduce the duration of the pulse during start. Finally, leg eight is the input leg for oscillator feedback. In operation, transistor Q1 is used as an energy switch controlled by controller 106. A damping circuit is connected to the drain of transistor Q1. The damping circuit contains a combination of diode D3, resistor R16 and capacitor C12, which together limit the overmodulation of voltage when the transistor is turned off. D3 is a MUR450 diode, C12 is a 1000 pF capacitor, and R16 is a 2-Watt and 30 kW resistor. Together with the dispersion capacitance of the transformer, the capacitor C7 (470 pF connected from the drain terminal to ground) determines the frequency of no load and consequently, the maximum speed of rotation of the drain voltage for a transistor Q1. The transistor Q1 is driven with the pulse width modulated signal VSALIDA produced in the leg 5 of the controller 106 and coupled to the gate terminal of the transistor via the resistor R11 (35W). In addition, a C6 capacitor (4700 pF) is coupled from the source terminal to the drain terminal. The source terminal is coupled to ground by means of resistor R13 (0.27 KW). The resistor R12 (10 kW) is optionally connected between the source terminal and the gate terminal to ensure that the transistor is not activated if power is applied to the power supply without the controller 106 being installed. The drain terminal is coupled to a terminal of the primary winding W1 of the transformer 112. Consequently, the transistor Q1 controls the current flow from the primary input voltage to the primary winding. The secondary circuit of the transformer 112 consists of several windings, each of which has a different number of turns, polarity and load capacity. Specifically, the winding W2 forms the regulated output voltage for B +, while the winding W4 forms the output winding for the regulated low voltage output of 16 volts, and the winding W3 generates the feedback voltage for the controller 106. load circuits include, connected to the winding W2, an output diode D4 and a capacitor C13 which couple power to the horizontal deflection circuit 116. Additionally, the center tap of the secondary output winding is connected to ground, and the winding W4 is connected to diode D5 and capacitor C14. This output is the 16 volts that energize the continuous load 118 of the television receiver ie all electronic devices and integrated circuits. This circuit 118 also controls the time when the neutralization circuit 114 is activated using the neutralization control line 120. The control line for continuous charging is the operation / reserve control signal that essentially turns the receiver on and off. TV. The continuous load circuits 118 are coupled to the horizontal deflection circuit 116 to provide control signals thereto. Controller 106 is activated using resistor R17 (100KW) as a start resistor. As such, the capacitor C11 (100 μF) is charged with half wave currents to the voltage supply leg of the controller 106, i.e., the pin 6. These half-wave currents are supplied from the primary input voltage across the resistor R17 (100 KW) to earth through the resistor connected in series R14 (202 W), diode D2 (148 W) and the regulating winding W3. When the voltage at C11 reaches the minimum ignition level, the interrupted mode power supply begins to operate and supplies the feedback voltage via the winding W3, the resistor R14 and the diode D2. This feedback voltage, when rectified by diode D2 and reduced by capacitor C11, forms the supply voltage (vcc) for controller 106 via pin six. A control signal or voltage VCT for leg one is generated in a circuit parallel to the supply voltage circuit of the controller. The control voltage is produced by the diode D1 (ERB43) the charge capacitor C3 (1.5 μF) through the resistor R8 (10W). The RC element, consisting of R15 connected in series (30 W) and C10 (0.01 μF), prevents the rectification of the peak value of high frequency components of the feedback signal. More specifically, the regulating winding W3 is coupled to a terminal of the resistor R15. The other terminal of resistor R15 is coupled to capacitor C10 to ground. The diode D1 is connected to the junction of the resistor R15 and the capacitor C10. Capacitor C9 (1000pF) is connected in parallel with diode D1. The diode D1 has an output voltage that is coupled to R8 connected in series and C3 which couple the output of the diode to ground. The output of the diode is also coupled through the network of resistive dividers R6 and R7 which are respectively connected in series to ground. The voltage at the junction of R6 and R7 forms the control voltage VCT and is coupled to the pin 1 of the controller 106. These resistors define the frequency of non-load oscillation of the controller 106. Therefore, they are commonly accurate resistors to 0.1 % being R6 which is 5.49 KW, and R7 which is 174 W. The control voltage VCT is coupled to a pulse width modulator 106c within the controller 106, which controls the modulation of the VSALIDA voltage duty cycle to regulate , for example, the regulated voltage B +. During the start of the power supply, capacitor C5 in the soft start leg (ie, leg 7) influences the duration of the direct phase by controlling the error voltage of the wavelength modulator. The controller detects the end of the discharge phase of the transformer via the resistor R10 (20KW) which is coupled at one end to the eight leg of the controller and at the other end to the resistor R14 and ultimately to the regulating winding W3. Additionally, capacitor C8 (0.022 μF) is coupled from the junction of R10 and R14 to ground. At this point, the voltage changes polarity from positive to negative, that is, the voltage represents zero crossings.

A voltage VZ1, which incorporates one aspect of the invention, is generated by the monitor voltage generator 110 and is coupled to the three leg of the controller 106. The voltage VZ1 is used to both determine the minimum line voltage that will allow the power supply operate, such as to control a retraction point correction circuit 106b within the controller 106. The monitor voltage generator 110 contains the resistor R1 (270 kW) coupled in series with the resistor R2 (5100 W) to form a divider network of resistive voltage with respect to the primary input voltage INPUT B +. The junction of the two resistors is coupled to the three leg of the controller 106. In addition, a zener diode Z1 (B2X55 / C3V0) incorporating an aspect of the invention, is connected in parallel with the resistor R2 from the point of attachment to ground . The zener diode Z1 forms a limiter to limit the maximum voltage across R2 to the breaking voltage of the zener diode Z1. Consequently, the voltage at the output of the monitor voltage generator 110 tracks the input primary input voltage B + to the minimum point where the zener diode starts to conduct. The controller 106 includes an undervoltage detector 106a that uses a fixed minimum internal voltage level that causes the controller to turn off the power supply when the monitor voltage VZ1 drops below the first minimum voltage. For the TDA 4605 integrated circuit, this first minimum voltage is one volt. As such. The divider network of R1 and R2 defines a voltage at the output that during a typical operation would not cause the controller to deactivate the power supply. In a particular application, for example, a television signal receiver is commonly connected to a neutralization circuit 114 directly along the AC power input. Consequently, when the neutralization circuit is activated, it will commonly produce a drop in the alternating current voltage which is applied to the input of the voltage rectifier 102. Consequently, the primary input voltage INPUT B + will drop significantly during the neutralization period. Since this is normal behavior of a conventional television receiver circuit, it is desirable that the monitor voltage generator 110 be designed such that the controller 106 does not deactivate the power supply during the neutralization period. For a primary input voltage of 120 volts and using a resistive divider of 270 KW for R1 and 5100 W for R2, the nominal voltage VZ1 on the input leg of the voltage monitor is 2 volts. Such a value for the voltage monitor voltage will prevent deactivation of the power supply during the neutralization period or other heavy load period. When the duty cycle of the VSALIDA voltage is at its maximum as a result of an overload condition, an increase in the INPUT B + voltage, produced by an increase in the AC line voltage, increases the voltage across the primary winding W1 As the primary input voltage B + IN increases, the available input power to the power supply increases, which could damage the power supply when the power supply is overloaded. During a period of overload, unregulated output, the modulator 106c generates that VSALIDA voltage having a maximum duty cycle for the conductive transistor Q1. As a result, an IP primary current in the winding W1 of the transformer 112 also has a maximum duty cycle. Therefore, undesirably, an increase in the INPUT B + voltage can produce a large voltage across the transistor that could damage the transistor or other circuits. To maintain the power supply operating within a safe operating range, the controller 106 includes what is known as a retraction or correction circuit. overload point 106b. This retraction point correction circuit reduces the maximum voltage duty cycle VSALID when the primary input voltage exceeds a predetermined amount. An increase above the predetermined magnitude causes the retraction point correction circuit 106b to decrease the maximum duty cycle of the VSALIDA signal as the INPUT B + voltage increases. The decrease is made by generating an ICOR current correction which is coupled to the capacitor C2 which produces an increase in the rate of change of the voltage VC2 in the leg 2 of the controller 106 when the voltage VZ1 exceeds one second of minimum voltage.

When the INPUT B + voltage increases and causes the VZ1 voltage to rise above the second minimum voltage, an increase in the ICOR current produces a decrease in the maximum working cycle of the VSALIDA signal, in a well known way The second minimum voltage occurs when voltage VZ1 is above the voltage level of approximately 1 7 Volts The result is that when the voltage of INPUT B + increases more, the maximum duty cycle decreases proportionally The decrease in the maximum duty cycle tends to stabilize the energy maximum produced in the power supply against an increase in voltage B + INPUT On the other hand, an increase in voltage VZ1 when the voltage VZ1 is below the level of 1 7 volts, does not affect the ICOR current and the duty cycle of the VSALIDA voltage Because the divider network (R1 and R2) establishes a sufficiently large monitor voltage VZ1 that produces enough free space to avoid Upon deactivation of the power supply when the neutralization circuit is activated, the primary input voltage INPUT B + may be at a level which causes the voltage VZ1 to exceed the second minimum voltage of the circuit 106b by an excessive amount even when the INPUT voltage B + is within the normal range of tolerance Therefore, disadvantageously, the maximum duty cycle can be further decreased by a significant amount in a way to reduce the maximum energy that can be derived. That significant reduction in energy capacity can occur even through of the primary input voltage is not really at such a high level that the power supply could. According to one aspect of the invention, to prevent the ICOR current from further reducing the maximum duty cycle of the VSALIDA voltage when the INPUT B + voltage increases above the minimum magnitude corresponding to the voltage VZ1 being equal to 3V, the generator monitor voltage 110 contains the zener diode Z1 operating as a limiter which limits the voltage indicative of the primary input voltage VZ1 to 3V. Consequently, the monitor voltage VZ1 can never rise above the predefined level (i.e., 3 volts) which could otherwise cause the retraction point correction circuit 106b within the controller 106 to further increase the maximum duty cycle. In this way, advantageously, the decrease in the maximum duty cycle as a function of an increase in the INPUT B + voltage is limited. The decrease in the duty cycle of the VSALIDA voltage, produced by the ICOR current, for a given increase in the INPUT B + voltage, is limited when the INPUT B + voltage is greater than a minimum quantity corresponding to the VZ1 voltage equal to 3 V. In contrast, the decrease in the duty cycle produced by the ICOR current is not limited but varies proportionally to the INPUT B + voltage when the VZ1 voltage is between 1.7V and 3V. In addition, the zener diode Z1 operates as a limiter to limit the decrease in the duty cycle when the input B + voltage exceeds the relative minimum magnitude when the input voltage B + does not exceed the minimum magnitude. An increase in the INPUT B + voltage that produces voltage VZ1 below the second minimum voltage of 1.7 V, does not affect the ICOR current. Specifically, for the control of the integrated circuit TDA 4605 the zener diode has a value of 3 volts. Consequently, the input signal to the monitor voltage generator can not be raised above the level of three volts before the zener diode begins to conduct current to ground. As such, the monitor voltage generator establishes a range of voltages that pre-define a range of primary input voltages to which the controller 106 operates in a normal manner that avoids an undervoltage power supply deactivation and an additional increase in the maximum work cycle. In this way the dynamic range of input voltage is extended.

Claims (10)

1. CLAIMS 1. An energy supply, comprising: a source (101) of an input supply voltage; a switch (Q1) that responds to a first control signal (VSALIDA) having a controllable work cycle and coupled to said input supply voltage source to generate an output supply voltage (B + REGULATED), in accordance with said work cycle of said first control signal; a work-cycle modulator (106b, 106c) that responds to a second control signal (VCT) to generate such a first control signal and to control said duty cycle of said first control signal in accordance therewith, said modulator responds to a signal (VZ1) which is indicative of such an input supply voltage to decrease said duty cycle when said input supply voltage is increased; and a limiter (Z1, R1) coupled to said modulator to limit the decrease in the duty cycle, for a given increase in said input supply voltage, when such an input supply voltage exceeds a first quantity.
2. 2. An energy supply in accordance with the claim 1, wherein said duty cycle of said control signal (VCT) varies within a control range, in accordance with said second control signal, and wherein said limiter (Z1, R1) limits a decrease of said cycle of work when such work cycle is at one end of said control range 3 A voltage supply according to claim 1, wherein said limiter (Z1, R1) comprises a fastener (Z1) coupled in a signal path of said signal indicative of the input supply voltage (VZ1) to clamp said signal indicative of the input voltage, when said input supply voltage exceeds said first quantity, and to disable the clamping thereof, said input supply voltage does not exceeds said first quantity 4 An energy supply according to claim 3, wherein said voltage clamp (Z1) comprises a diode An energy supply according to claim 3, further comprising a disabling circuit (106a) responsive to said input supply voltage signal (VZ1) for disabling said output supply voltage (B + REGULATED), when the said input supply voltage is smaller than a second magnitude and wherein said voltage clamp (Z1) is coupled in a common signal path of said signal indicative of input supply voltage with respect to each of an input of said disabling circuit and an input (cathode of Z1) of said limiter 6 A power supply according to claim 1, wherein said modulator (106b, 106c) comprises a retraction point corrector (106b) to decrease said cycle of work when such input supply voltage is increased (INPUT B +) and where said limiter (Z1, R1) is coupled to said correct or. 7. An energy supply according to claim 1, wherein said second control signal (VCT) is produced in a feedback path to regulate such an output supply voltage (B + REGULATED). An energy supply according to claim 1, wherein said supply voltage indicative signal (VZ1) is coupled to said modulator (106b, 106c) from said input supply voltage source (101) via a path of signal that deflects said switch (Q1). 9. An energy supply according to claim 8, wherein said limiter (Z1, R1) comprises a fastener (Z1) coupled to said signal path to hold said signal indicative of input supply voltage (VZ1), when said input supply voltage exceeds said first quantity, and to disable the clamping operation, when said input supply voltage does not exceed said first magnitude. 10. A power supply, comprising: an input supply voltage (B + INPUT), a transformer (112) and a switch (Q1) coupled for the interrupted mode generation of a regulated output supply voltage, said switch responds to a first control signal (VSALIDA) that has a controllable work cycle; a work cycle modulator (106b, 106c) for generating said first control signal that responds to a second control signal (VZ1) to limit a duty cycle of such a switch, said modulator operates in a first mode (VZ1 NOT LIMITED ) when said second control signal is in a predetermined range of voltage levels and operates in a second mode (VZ1 LIMITED) when said second control signal is outside such range, and a voltage monitor circuit (Z1, R1) , R2) to generate said second control signal, said second control signal represents a first proportion of said input supply voltage in a first range of input supply voltage values and a second proportion of such supply voltage of input into a second range of input supply voltage values 11 A power supply according to claim 10, wherein said voltage monitor circuit (Z1, R1 , R2) comprises a fastener (Z1) coupled in a signal path of said second control signal (VZ1) 12 A power supply according to claim 11 wherein said second control signal (VZ1) is coupled to said modulator of such input supply voltage (B + INPUT) via a signal path that deflects said switch (Q1) 13 A power supply, comprising an input supply voltage (B + INPUT), a transformer (112) and a switch (Q1) coupled for interrupted mode generation of a regulated output supply voltage, said switch responds to a first control signal (VSALIDA) having a controllable work cycle, a duty cycle modulator (106b, 106c) to generate said first control signal that responds to a second control signal (VZ1), such modulator operates in a first mode (VZ1 NOT LIMITED) when said second control signal is turned on is in a predetermined range of voltage levels and operates in a second mode (VZ1 LIMITED) when said second control signal is outside said range, and a voltage monitor circuit (Z1, R1, R2) to generate said second signal of control, said second control signal represents a first proportion of said input supply voltage in a first range of input supply voltage values and a second portion of said input supply voltage in a second range of voltage values of input supply, as long as said input supply voltage is in said first range of input supply voltage values, said second control signal varies when such input supply voltage varies and said modulator operates in said first operating mode 14 A power supply, comprising an input supply voltage (INPUT B +), a transformer ( 112) and a switch (Q1) coupled for interrupted mode generation of a regulated output supply voltage, said switch responds to a first control signal (VSALIDA) having a controllable work cycle; a work-cycle modulator (106b, 106c) for generating said first control signal responsive to a second control signal (VZ1) said modulator operates in a first mode (VZ1 NOT LIMITED) when said second control signal is in a predetermined range of voltage levels and operates in a second mode (VZ1 LIMITED) when said second control signal is outside said range; and a non-linear voltage divider circuit (Z1, R1, R2) coupled to said input supply voltage to generate said second control signal, said second control signal represents a first proportion of such input supply voltage in a first range of input supply voltage values and a second proportion of such input supply voltage in a second range of input supply voltage values.