Control device for a flyback converter

A control device for regulating the constant output current of a flyback converter, the control device adapted to control the on time period and the off time period of a primary winding switch and including a first circuit adapted to multiply a first signal representative of current flowing through the primary winding and a second signal representative of an input voltage and outputting a signal representative of the multiplication, a second circuit adapted to compare the output signal of the first circuit and a third signal representative of the direct output voltage, the control device determining, on the basis of the output signal of the second circuit, the on time period and the off time period of the switch so that the output signal of the first circuit is equal to the signal representative of the direct output signal to have the output current of the flyback converter constant.

BACKGROUND

1. Description of the Related Art

The present disclosure refers to a control device for a flyback converter.

2. Description of the Related Art

Flyback converters are known in the state of the art as DC to DC converters with a galvanic isolation between the input and the output. More precisely, the flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. The transformer includes a primary winding connected with the input of the converter and a secondary winding connected with the output of the converter.

A flyback converter can be used to supply LED diode chains.

Recently high brightness LEDs are becoming a prominent source of light and often have better efficiency and reliability when compared to that of conventional light sources. While LEDs can operate from an energy source as simple as a battery and resistor, most applications require an efficient energy source not only for the reduction of losses, but also for the lumen maintenance of the LED itself. For these reasons driving circuits for high brightness LEDs have been developed; these driving circuits assure a constant current to the LEDs.

Two of these circuits are the VIPer12A and VIPer22A developed by STMicroelectronics; they are integrated switching regulators capable of providing a constant current to the LEDs. VIPer12A and VIPer22A use typically secondary regulation for keeping the output current constant, that is the current of the LEDs, because it is necessary for proper LED driving. In fact the controller of the power switch of the flyback converter in VIPer12A and VIPer22A needs a feedback signal derived from the secondary side (the circuit part of the flyback converter that is coupled to the secondary winding of the transformer) to control the switch and to produce a constant output current. Secondary regulation also needs an optocoupler, several passive components, and an active component. If the output voltage is too high, for example higher than 36 V, it is even necessary to use additional secondary windings on the transformer to generate a proper supply voltage for secondary part as it is shown in the application notes AN2067 and AN1916 (documents AN2067 and AN1916 are available on the STMicroelectronics web site).

The main drawback for the circuits VIPer12A and VIPer22A is due to the use of numerous components with secondary regulation, which make them complex and increase the cost and size, as the use of the optocoupler to separate the feedback signal from secondary to primary side. Also, an additional secondary winding on the transformer is needed for high output voltage, higher than 36 V, to generate proper supply voltage for the secondary side.

In view of the state of the art, the present disclosure provides a control device for a flyback converter that has fewer components than the known circuits.

BRIEF SUMMARY

According to the present disclosure, a control device for regulating the constant output current of a flyback converter is provided. The flyback converter includes an input voltage and outputs a direct output voltage with a constant output current to supply a load. The flyback converter has a transformer with a primary winding coupled with the direct input voltage and a secondary winding coupled with the direct output voltage, and further including a switch coupled to the primary winding to regulate the current flowing through the primary winding and to regulate the output direct voltage. The control device is adapted to control the switch determining the on time period and the off time period of the switch. The control device further includes a first circuit adapted to multiply a first signal representative of the current flowing through the primary winding and a second signal representative of the input voltage and outputting a signal representative of the multiplication, a second circuit adapted to compare the output signal of the first circuit, and a third signal representative of the direct output voltage, the control device adapted to determine, on the basis of the output signal of the second circuit, the on time period and the off time period of the switch so that the output signal of the first circuit is made equal to the third signal representative of the direct output voltage and to have the output current of the flyback converter maintain a constant value.

In accordance with one aspect of the present disclosure, a circuit is provided that includes a current supply circuit coupled to a load to supply a current to the load; a control circuit coupled to the current supply circuit to maintain a constant current level of the current supplied to the load, the control circuit including an auxiliary primary winding in a transformer in the current supply circuit adapted to sense a direct output voltage from a secondary winding in the transformer and to output a representative signal of the direct output voltage; a first circuit structured to multiply a signal representative of current flowing in the primary winding and a second signal representative of an input voltage to the primary winding in the transformer and to output a multiplied signal that is representative of the product of the multiplication; a second circuit coupled to the auxiliary primary winding and structured to compare the multiplied signal with the representative signal of the direct output voltage and to output a comparison signal; and a driving circuit structured to receive the comparison signal and to generate a driving signal to a switch in the current source to maintain the current to the load at a constant level.

In accordance with another aspect of the present disclosure, the control circuit includes an auxiliary primary winding in the transformer adapted to sense the direct output voltage and to output the representative signal of the direct output voltage.

In accordance with the present disclosure, the representative signal of the current in the primary winding represents an average primary current flowing through the primary winding.

In accordance with another aspect of the present disclosure, the driving circuit referenced above is formed of a pulse width modulation circuit that is ideally structured to determine a turn on time period and a turn off time period of the switch. Preferably, the driving circuit controls the switch so that a value of the representative signal of the direct output voltage is equal to a value of the multiplied signal.

In accordance with another aspect of the present disclosure, a control circuit for a flyback circuit that supplies a load with an output current is provided. The flyback circuit has a transformer with a primary winding that receives an input voltage and a secondary winding that outputs a direct output voltage with the output current, and a switch coupled to the primary winding to regulate current through the primary winding and, thus, the output current of the transformer, the control circuit including a first circuit structured to multiply a first signal representative of the current through the primary winding and a second signal representative of the input voltage to the primary winding and to output a multiplied signal that is a product of the multiplication; a second circuit structured to compare the multiplied signal with a signal representative of the direct output voltage and to output a comparison signal; and a driving circuit structured to receive the comparison signal and to generate a driving signal to the switch to maintain the output current constant.

In accordance with another aspect of the present disclosure, a method is provided for regulating the output current of a flyback converter. The flyback converter has an input voltage and outputs a direct output voltage with an output current to supply a load, the flyback converter having a transformer with a primary winding receiving an input voltage and a secondary winding that outputs the direct output voltage, and a switch coupled to the primary winding to regulate the current flowing through the primary winding and thereby regulate the direct output voltage, the method including determining the on time period and the off time period of the switch by multiplying a first signal representative of current flowing through the primary winding and a second signal representative of the input voltage, comparing a signal generated by the multiplication and a third signal representative of the direct output signal, determining the on time period and the off time period of the switch as a function of the signal generated from the comparison so to make equal the signal representative of the multiplication and the third signal representative of the direct output signal in order to have the output current of the flyback converter maintain a constant value.

In accordance with another aspect of the method of the present disclosure, a control method for regulating the output current of a flyback converter is provided, said flyback converter having an input voltage and outputting a direct output voltage with a the output current to supply a load, the flyback converter having a transformer with a primary winding receiving an input voltage and a secondary winding that outputs the direct output voltage, and a switch coupled to the primary winding to regulate the current flowing through the primary winding and regulate the output direct voltage, the method including determining the on time period and the off time period of the switch by multiplying a first signal representative of the current flowing through the primary winding and a second signal representative of the input voltage, comparing a signal generated by the multiplication and a third signal representative of the direct output signal, determining the on time period and the off time period of the switch as a function of the signal generated from the comparison so to make equal the signal representative of the multiplication and the third signal representative of the direct output signal to thereby have the output current of the flyback converter maintain a constant value.

In accordance with yet a further aspect of the method of the present disclosure, the method includes obtaining the third signal from an auxiliary primary winding in the transformer, which is adapted to sense the direct output voltage and to generate the third signal as a representative signal of the direct output voltage.

In accordance with yet a further aspect of the method of the present disclosure, comparing a signal generated by the multiplication and a third signal representative of the direct output signal results in the generation of a comparison signal, and determining the on time period and the off time period of the switch further includes generating a driving signal with a pulse width modulation circuit adapted to determine the on time period and the off time period of the switch by comparing the comparison signal and a triangular waveform generated by an oscillator in the driving circuit.

DETAILED DESCRIPTION

A schematic of a flyback converter having a control device according to the present disclosure is shown inFIG. 1. The flyback converter includes a rectifier block1adapted to rectify an input voltage Vin for obtaining a voltage Vinr, a transformer10having a primary winding L1and a secondary winding L2, a switch M connected with the primary winding L1and coupled to ground GND, preferably an NMOS transistor having the drain terminal connected with a terminal of the primary winding L1and the source terminal coupled to ground GND by means of a resistance Rs; the primary winding L1having one terminal connected with the switch M and the other terminal coupled to the input voltage Vin, particularly with the input voltage Vinr. A current Ip flows though the primary winding L1while a constant current Iout flows through the load6; and an output voltage Vout is present across the secondary winding L2.

The transistor M is driven by a control device100comprising an auxiliary winding L3of the transformer10adapted to sense the reflected output voltage Vout and to generate a supply voltage for a driving circuit3adapted to drive the NMOS transistor M. The auxiliary winding L3has one terminal connected with ground GND and the other terminal connected with a rectifier block2that provides the voltage Vrout to supply the driving circuit and the voltage V1at the input terminal of a comparator4adapted to compare the voltage V1with another voltage V2.

The voltage V2is the output voltage of a multiplier5that is adapted to multiply a voltage V3representative of the rectified input voltage Vinr from a voltage divider13and a signal14representative of the average primary current flowing through the primary winding of the flyback converter. Particularly the signal14is the current flowing through a circuit comprising a capacitor Ca having one terminal connected to ground GND and the other terminal connected with the input of the multiplier5and with a terminal of a resistance Ra connected with the sensing resistance Rs and the source terminal of the transistor M.

The output signal E of the comparator is at the input of the driving circuit3adapted to regulate the output power of the flyback converter. Preferably the driving circuit3is a PWM (Pulse width Modulation) circuit adapted to determine the time period Ton for the turning on of the transistor M and the time period Toff for the turning off of the transistor M by comparing the signal E and a triangular waveform generated by an oscillator preferably internal to the driving circuit3.

The driving circuit3on the basis of the signal E is adapted to control the switch M so that the signal V1is made equal to the signal V2to have the constant output current Iout of the flyback converter.

In fact V1=K1*Vout, V3=K3*Vinr and I4=K4*Ip wherein K1, K3and K4are constants. In the multiplier5the signal V3is multiplied by the signal I4obtaining V2=V3*I4=K4*K3*Vinr*Ip=K4*K3*Vinr*Pin/Vinr=K4*K3*Pin because the input power at the primary side of the flyback converter is Pin=Vinr*Ip. Also since the input power Pin is proportional to the output power of the flyback converter Pin=Pout*K2=Vout*Iout*K2, wherein K2is a constant, it has V2=K4*K3*Iout*Vout*K2=K5* Iout*Vout where K5is a constant.

The signals V1and V2at the input terminals of the comparator4must be equal; in fact the comparator includes, as better shown inFIG. 2, an operational amplifier41having the inverting input terminal coupled with the output terminal by means of a series of a resistor R5and a capacitor C5; the voltage V1is present at the inverting input terminal of the operational amplifier while the voltage V2is present at a non inverting input terminal. If the signals V1and V2are not equal, the control device according to the present disclosure reacts, driving the switch M so as to change the value of the current Ip and the value of the output voltage so that V1=V2. The driving circuit3is adapted to change the on time period Ton and the off time period Toff of the switch M and to consequently modify the current Ip flowing through the primary winding L1.

Also V1=V2, that is K1*Vout=K5* Iout*Vout and Iout=K1*K5=K, where K is a constant.

As shown inFIG. 2, the flyback converter has a filter20to filter the input voltage Vin before the rectifier block1. The voltage V3is produced by means of a resistor divider formed by the series of the resistances R3and R4arranged across the voltage Vinr, that is V3=R4*Vinr/(R3+R4).

The voltage V1is produced by means of a resistor divider formed by the series of the resistances R1and R2coupled with the terminal of the auxiliary winding L3, that is V1=R2*Vrout/(R1+R2).

The rectifier block2is a diode and a circuit block is provided between the secondary winding and the load6, which is formed by a diode having the anode connected with a terminal of the secondary winding L2and the cathode connected with a terminal of a capacitor having the other terminal connected with the other terminal of the capacitor; across the capacitor is arranged the load6.

The load6preferably includes one chain61of LED diodes wherein the LED diodes DL1, DL2. . . DLn are arranged in series; the load6can include a plurality of chains of LED diodes wherein each chain of the plurality is made as the chain61. Even preferably the LED diodes are high brightness LED diodes. The circuit described herein maintains a constant, uniform visible brightness of these diodes, which can be used for illuminating an area, providing visual indication of a condition, and other uses known in the art.

As shown inFIG. 2the driving circuit3and the switch M, in particular the NMOS transistor M, are integrated together by forming an integrated circuit30. The circuit30has at the input terminals the voltage Vrout to supply the integrated circuit30, one terminal of the primary winding L1, one terminal of the resistance Rs and the signal E deriving from the comparator4.

The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent application, foreign patents, foreign patent application and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, application and publications to provide yet further embodiments.