Control device of a switching converter and relative switching converter

A control device for a switching converter having an input terminal and an output terminal, a half-bridge of a first and a second transistor coupled between the input terminal and a reference voltage the control device including a first circuit structured to detect signal on the output terminal of the converter and to integrate the detected signal and regulate on the average value of the detected signal by comparison with a further reference signal, and then drive the first and second transistor as a function of the regulation. The control device further includes a switching circuit for turning off the first circuit so that the control device carries out a regulation on the detected signal by comparison with a further reference signal and drives the first and second transistors when current passing between the output terminal of the converter and the half-bridge crosses zero.

BACKGROUND

1. Technical Field

The present disclosure pertains to a control device for a switching converter and relative switching converter, preferably a converter from direct voltage to direct voltage.

2. Description of the Related Art

In the state of the art converters from direct voltage to direct voltage or DC-DC converters are generally known; a switching converter with controller in pulse frequency mode and with a constant turn-on time is shown inFIG. 1. The converter includes a first MOS transistor HS having a non-drivable terminal connected to an input voltage Vin and another non-drivable terminal connected to a terminal of an inductance L and to a non-drivable terminal of another MOS transistor LS that has its other non-drivable terminal connected to ground GND. The inductance L has its other terminal connected to a sense resistance Rsense that in turn is connected to a filter constituted by a resistance ESR in series to a capacitor Cout where the resistance ESR is the parasitic resistance of the capacitor Cout; the filter is placed in parallel to the load LOAD. The converter comprises a control circuit2having in input on the terminals CSENSEPLUS and CSENSEMINUS the current detected at the terminals of the resistance Rsense, the output voltage Vout at the terminals of the load LOAD on the input terminal VFB, a reference voltage VREF and the clock pulses MIN_FREQ coming from a timer3. The control circuit2is suitable for driving the transistors HS and LS by means of the drive signals HSIDE and LSIDE.

InFIG. 2the control circuit2is shown in more detail to include a comparator21suitable for comparing the voltage Vout, present on the terminal VFB, with the voltage VREF, a comparator22having the input terminals coinciding with the terminals CSENSEPLUS and CSENSEMINUS and suitable for detecting the zero crossing of the current that flows through the inductance L and three set-reset latches or flip-flops23-25in which the flip-flop23has the input set S coupled with the output of the comparator21, the flip-flop24has the input reset R coupled with the output of the comparator22and the flip-flop25has the input set S connected with the output of the oscillator3. The outputs of the flip-flops23and24are respectively the drive signals HSIDE and LSIDE for the transistors HS and LS. The circuit2also includes a timer26which when the input is at a low logic level has a low output. Initially the set reset flip-flops23and25are reset while the flip-flop24is set. When the signal Vout falls below the value VREF the comparator21sets the flip-flop23; in this manner the signal HSIDE is raised while the signal LSIDE is lowered and the voltage Vout rises above the value of the voltage VREF. After a period given by the turn-on time Ton of the transistor HS the timer26changes the output signal taking it to a high logic level; and the signal resets the flip-flop23, which in turn lowers the signal HSIDE and raises the signal LSIDE. In these operating conditions, that is for loads exceeding half the ripple on the induction current IL in pulse width modulation, the period Tp of repetition of the charge transfer cycles in output in the converters is equal to Ton*Vin/Vout.FIG. 3shows the time diagrams of the voltages Vout and VREF, of the current IL on the inductance L and of the signals HSIDE and LSIDE.

When the load LOAD absorbs low value currents, for example on the order of milliamperes, it can happen that the inductor current IL becomes negative during the turn-off period Toff of the transistor HS. In this case the comparator22resets the flip-flop24so as to lower the signal LSIDE. In this manner the half-bridge constituted by the transistors LS and HS is left at high impedance to prevent the inversion of the sign of the current, and the output voltage Vout is discharged on the load LOAD. When the voltage Vout falls below the value of VREF the flip-flop24is set and the previous cycle is repeated with the turn-on of the transistor HS; the control circuit2works in pulse frequency mode. The control circuit2can also comprise a timer27suitable for establishing the minimum turn-off time Toff of the transistor HS; in this manner the stability is ensured in regard to the noise induced by the switching of the transistors HS and LS.

In the case of low load and in the presence of pulse frequency modulation a charge

Q=12⁢Vin-VoutL⁢Ton⁡(Ton+Toff)=12⁢Vin-VoutL⁢VinVout⁢Ton2
is transferred at every cycle. The frequency fp of repetition of the charge transfer cycles in output in the converters is directly proportional to the current on the load Iload because fp=Q/Iload; if the current becomes low, the frequency fp can return within the range of frequencies audible by man. For this reason the converter has a device for limiting the minimum frequency; the device in this embodiment is implemented by the timer3. When in the conditions of detection of negative current IL and consequent lowering of the signals HSIDE and LSIDE, the timer3prevents the pulse period Tp, inverse of the frequency fp, from exceeding a predefined value Tpmax by sending a pulse to the set input of the flip-flop25which, in turn, sends a signal on the set input of the flip-flop24to raise the value of the signal LSIDE. When the voltage Vout falls below the value VREF, the flip-flop25is reset. InFIG. 4the course of the voltage Vout, of the current IL and of the signals HSIDE, LSIDE and MIN-FREQ if the flip-flop25is activated can be seen.

A converter of this type suffers from an error in direct current given by half of the ripple on the output signal Vout; this comes about because the regulation is carried out on the minimum value of the voltage Vout. An integrator can be inserted whose object is to correct the error, as shown inFIG. 5. The integrator4comprises a transconductance amplifier41having the inverting input connected to the reference voltage VREF and the non-inverting input connected to the voltage Vout. The integrator consists of a capacitor Cint connected between the voltage Vout and the output terminal of the amplifier41connected to the control circuit2so that the voltage VFB is

VFB=GmsC⁢int⁢(Vout-Vref)+Vout
where Gm is the transconductance gain of the amplifier41. In this case the comparator21compares the voltage VREF with the voltage VFB. To reach the stationary state the average of the voltage Vout within a cycle must be constant. Given that the comparator PWM compares the voltage VFB with the voltage VREF, the time average of the voltage VFB must also be constant, and therefore Vout=VREF must be direct. The regulation that is operated on the signal VFB is on the minimum values of the signal or valley of the signal VFB.

If the load LOAD absorbs low value currents the regulation on the signal Vout is made on the average value. After a cycle of turn-on time Ton and turn-off time Toff in which the current IL goes to zero, the output voltage Vout is overloaded in relation to the value VREF. While the output voltage remains above the regulated value VREF the integrator4raises the voltage VFB. When the load LOAD brings the output voltage below the voltage VREF, the voltage VFB decreases until it reaches the voltage VREF and the comparator21is triggered, as can be seen in the time diagrams ofFIG. 6.

Nevertheless it is possible that, when a charge transient is applied at the output of the converter by starting from a current of zero value, it has an undershoot at the output of the converter because the output of the integrator is overloaded and it is necessary to wait that the output of the integrator is brought at the regime situation before the control device reacts to the transient, as shown inFIG. 7wherein Vout is the output voltage of the converter inFIG. 5and IL is the current passing through the inductance.

BRIEF SUMMARY

The present disclosure provides a control device of a switching converter that overcomes the above-mentioned drawbacks.

In accordance with one embodiment, a control device for a switching device is provided that includes an input terminal and an output terminal, the converter including a half-bridge of a first and a second transistor coupled between the input terminal and a reference voltage. The control device further includes a circuit capable of detecting a signal on the output terminal of the converter and first means suitable for integrating the detected signal, the control device adapted to carry out a regulation on the average value of the detected signal by comparison with a further reference signal and being suitable for driving the first and second transistors as a function of the regulation. The control device also includes a circuit suitable for turning off the first circuit so that the control device carries out a regulation on the detected signal by comparison with a further reference signal and drives the first and second transistors as a function of the regulation when the current passing between the output terminal of the converter and the half-bridge crosses the zero.

In accordance with one embodiment of the present disclosure, a circuit is provided that includes first and second transistors coupled as a half-bridge between an input and an output of the circuit; a controller coupled to the first and second transistors; an integrator circuit coupled to the output and to the controller, the integrator circuit structured to integrate a signal on the output; and a switching circuit coupled to the integrator circuit and the controller and structured to turn off the integrator circuit and enable regulation of the signal on the output by comparison with a reference signal.

In accordance with another aspect of the foregoing embodiment, the integrator circuit includes a capacitor coupled to the output and to the controller and an amplifier having a first input coupled to the controller and to a reference voltage, a second input coupled to the output of the circuit, and an output coupled to the switching circuit. Ideally a voltage divider is coupled between the output of the circuit and both the second input of the amplifier and the switching circuit.

In accordance with another aspect of the present disclosure, the integrator circuit includes a capacitor coupled between the output and the controller and an amplifier having a first input coupled to the switching circuit, a second input coupled to the output of the circuit, and an output coupled to the capacitor and the controller, and further wherein the switching circuit is coupled to an output of the controller and to a reference voltage.

DETAILED DESCRIPTION

InFIG. 8a switching converter according to a first embodiment of the present disclosure is shown. The switching converter has an input terminal IN on which the voltage Vin is present and an output terminal OUT to which the load LOAD is connected. The converter has a half-bridge of a first transistor HS and a second transistor LS coupled between the input terminal IN and a reference voltage, preferably ground GND, an inductance L coupled to the half-bridge and to the output terminal OUT, a control device100including a circuit capable of detecting a signal Vout on the output terminal OUT of the converter.

The control device includes an integrator201suitable for integrating the signal detected Vout and a device102suitable for imposing a preset minimum frequency to the signal detected Vout. The control device is suitable for carrying out a regulation on the average value of the signal detected Vout and for driving the first HS and second LS transistor as a function of the regulation. The control device100also includes a switching circuit205suitable for turning off the integrator201so that the control device carries out a regulation on the signal Vout when the current passing through the inductance L crosses the zero. The control device100has a logic circuit300having in input the signal VFB, which can be the output signal from the integrator201or a signal proportional to the output signal Vout, the signal VREF and the signal MIN_FREQ in output from the device102, and it generates and sends the signals LSIDE and HSIDE for driving the transistors HS and LS.

The converter includes a series of two resistances Rfbh and Rfbl arranged between the terminal OUT and ground GND, and a series of resistances Rfbh and Rfbl are placed between the terminal OUT and ground GND.

The integrator201preferably has a transconductance amplifier41having the inverting input terminal connected with the reference voltage VREF and the non-inverting input terminal connected with the voltage Vout. The integrator further includes a capacitor Cint connected between the voltage Vout and the output terminal of the amplifier41connected to the control circuit300so that the voltage VFB, when the switching circuit205are not active, is

VFB=GmsC⁢int⁢(Vout-Vref)+Vout
where Gm is the transconductance gain of the amplifier41.

The switching circuit205has a switch adapted to connect a terminal of the capacitance Cint with the output terminal of the transconductance amplifier41or with the common terminal of the resistances Rfbh and Rfbl. The switch205is suitable for disconnecting the capacitance Cint from the output terminal of the amplifier41and for connecting it with the common terminal of the resistances Rfbh and Rfbl when the current IL passing through the inductance L crosses the zero. In this case the voltage across the capacitance Cint is maintained at a value substantially equal to the regulation value. The switch is commanded by the signal ZCLATCH coming from the logical circuit300. In this manner the transconductance amplifier41acts, together with the capacitance Cint, as an integrator to correct the error given by half ripple on the output voltage Vout only when the capacitance Cint is connected with the output terminal of the amplifier41.

The switching circuit205allows to reduce the undershoot at the output of the converter when a load transient is applied at the output by starting from a current value equal to zero. The undershoot is due to the fact that the output of the integrator is overloaded and it is necessary to wait until the output is brought to the regime condition before the control device reacts to the transient. The regulation is controlled by the offset voltage of the integrator when the amplifier41acts as an integrator and it is controlled by the ripple voltage and by the offset voltage of the comparator inside the control circuit300when the amplifier41does not act as an integrator.

The control circuit300of the control device100can be seen better inFIG. 9to include a comparator21suitable for comparing the voltage VFB, present on the terminal VFB, with the voltage VREF; a comparator22having the input terminals coinciding with the terminals CSENSEPLUS and CSENSEMINUS, that is the terminals of a resistance Rsense placed between the inductance L and the terminal OUT, and suitable for detecting the zero crossing of the current that flows through the inductance L; and three set-reset flip-flops23-25in which the flip-flop23has the set input S coupled with the output of the comparator21, the flip-flop24has the reset input R coupled with the output of the comparator22and the flip-flop25has the set input S connected with the output of the timer3. The outputs of the flip-flops23and24are respectively the drive signals HSIDE and LSIDE for the transistors HS and LS. The circuit300also includes a timer26which when the input is at a low logic level has a low output.

Initially the set-reset flip-flops23and25are reset while the flip-flop24is set. When the signal VFB falls below the value VREF, the comparator21sets the flip-flop23; in this manner the signal HSIDE is raised while the signal LSIDE is lowered and the voltage Vout rises above the value of the voltage VREF. After a period given by the turn-on time Ton of the transistor HS, the timer26changes the signal in output taking it to a high logic level. The signal resets the flip-flop23, which in turn lowers the signal HSIDE and raises the signal LSIDE. The circuit300also includes a timer27suitable for establishing the minimum turn-off time Toff of the transistor HS. In this manner the stability is assured in relation to the noise induced by the switching of the transistors HS and LS. The signal ZCLATCH is the signal Q at the output of the flip-flop24.

InFIG. 10a switching converter according to a variant of the second embodiment of the present disclosure is shown. Differently from the case inFIG. 8, the switch205is suitable for disconnecting the inverting terminal of the amplifier41from the reference voltage Vref when the current passing through the inductance L crosses the zero and it is suitable for connecting the inverting terminal of the amplifier41with the output of the amplifier41so that the amplifier41is in a buffer configuration. The switch205is controlled by the signal ZCLATCH coming from the circuit300. In this manner the transconductance amplifier41acts, together with the capacitance Cint, as an integrator to correct the error given by half ripple on the output voltage Vout and acts as a buffer when the inverting input terminal of the amplifier is connected with the output of the same amplifier41.

With the circuit configuration inFIG. 10the feedback divider is uncoupled by the presence of the buffer in the case wherein the current passing through the inductance L crosses the zero. The undershoot at the output of the converter is minimized. The regulation is controlled by the offset voltage of the integrator when the amplifier41acts as an integrator, and it is controlled by the offset voltage of the amplifier41, by the ripple voltage, and by the offset voltage of the comparator of the part300when the amplifier acts as a buffer.

The factor due to the offset voltage of the amplifier41is present both when the amplifier acts as integrator and when it acts as buffer; this factor is balanced. Also the feedback divider is uncoupled from the control circuit300.

FIG. 11shows timing diagrams of the inductor current IL, the output voltage of the integrator Vint, the voltage Vp across the resistance Rfbl and the output voltage Vout. From the above-mentioned time diagrams it is observed that the control device responds immediately to a variation of the current IL without undershoot at the voltage output Vout.

InFIG. 12a switching converter according to another variant of the second embodiment of the present disclosure is shown. Differently from the converter shown inFIG. 8the capacitor Cint is connected between the output of the amplifier41and ground GND, the non-inverting input terminal of the amplifier41is connected with the reference voltage Vref and the inverting input can be connected with the output of the amplifier41(when the amplifier acts as a buffer) or with the common terminal of the resistance Rfbh and Rfbl (when the amplifier41acts as an integrator) by means of the switch205controlled by the signal ZCLATCH. Differently from the converter inFIG. 8, the output of the amplifier is connected with the terminal Vref of the control circuit300and the common terminal of the resistance Rfbh and Rfbl is connected with the terminal Vfb of the control circuit300. In this case wherein the amplifier41is in buffer configuration, the reference voltage Vref occurs at the terminal Vref of the control circuit300while the terminal Vfb of the control circuit300is always connected with the common terminal of the resistance Rfbh and Rfbl. In this case across the capacitance Cint is stored a voltage equal to Vref.

In the case of the converter inFIGS. 8 and 10, when the amplifier41acts as integrator, it has:

where Vpwm is the voltage difference between the terminals of the comparator21of the part300and

When the amplifier41is in buffer configuration it has:

Vpwm=Vout⁢α+sC⁢int⁢⁢α⁢⁢Rfbh1+sC⁢int⁢⁢α⁢⁢Rfbh-Vref
in the case of the converter inFIG. 8and:

Vpwm=Vout⁢α+sC⁢intgm1+sC⁢intgm-Vref
in the case of the converter inFIG. 10.

In the case of the converter inFIG. 12, when the amplifier41acts as an integrator, it has:

while with the amplifier41in buffer configuration it has:
Vpwm=αVout−Vref.