Pre-bias controller for switching power converters

A pre-bias control circuit for a switching power converter detects the slope of the output voltage over time and outputs an OPEN command when the slope detected is more NEGATIVE than a pre-defined threshold and a pre-charge current that flows back through the switching power converter has reached a maximum value. In response, the synchronous rectifier switch OPENs overriding the typical control waveform to control the energy from the output voltage from flowing back through the switching power converter. The switching power converter may be any converter that includes a synchronous rectifier, such as a flyback converter, a forward converter, a buck converter, in a single-ended, double-ended and/or multi-phased configuration.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to circuits for controlling the operation of synchronous rectifiers during pre-bias conditions.

Description of the Related Art

Switching power converters are currently used in numerous applications of power systems. These converters are devices that convert a direct current (DC) input voltage into a predetermined DC output voltage. A typical switching power converter has an energy storage section, a switching control circuit such as a pulse width modulator (PWM), a primary switch, and a rectifier. The energy storage section is responsive to the selective application of the input voltage to produce a current and the output voltage. The switching control circuit, primary switch and rectifier control the application of the input voltage to the energy storage section to set the value of the output voltage.

Synchronous rectification has become a desirable alternative to passive rectifiers in power converters due to the increased power conversion efficiency that results at least in part from reduced power losses and higher density. However, while diodes only permit current flow in one direction, synchronous rectifiers such as MOSFETs, or other equivalent semiconductor switches permit current flow in either direction

In a typical buck converter, the primary switch and synchronous rectifier control the transfer of energy in response to the switching control circuit, which produces a waveform that toggles between HIGH and LOW at a duty cycle to set the value of the output voltage. During normal or steady state operation, the switch and rectifier are in opposition to each other and respond to the waveform such that when the primary switch is CLOSED and the synchronous rectifier is OPEN the energy storage section is charging and when the primary switch is OPEN and the synchronous rectifier is CLOSED the energy storage section is discharging the output.

A problem common in many switching power converters employing synchronous rectification is therefore the drawing of current from a pre-existing voltage, or pre-biased output voltage, also known as reverse bias or back bias, during certain sequences such as startup or shutdown conditions. Pre-biased voltage may come from other power sources in a non-isolated system, or may come from a load. During a soft-start condition, the synchronous rectifiers may have a high duty ratio for the duration of the output voltage rise time of the power supply. Where a pre-biased voltage exists, a reverse current may exist, which may cause the output voltage to drop and correspondingly disturb other elements in the system.

U.S. Pat. No. 6,618,274 entitled “Synchronous rectifier controller to eliminate reverse current flow in a DC/DC converter output” discloses a control scheme for a synchronous rectifier converter that prevents substantial reverse current flow in all modes of operation without disabling the synchronous rectifiers. Rather than disable the synchronous rectifier altogether to stop the flow of reverse current in light-load, startup, or shutdown conditions, the secondary synchronous rectifier is always enabled, operating either in the fully-synchronous mode or the partially-synchronous mode. The transition between the two operating modes is determined by sensing a system parameter. For example, this parameter can be based on the amount of reverse current that would disrupt the bus to which the converter output is connected, or it could be based on the heat created by the reverse current flow in the power converter when heat dissipation is a concern. In the partially synchronous mode, a duty cycle of the synchronous rectifier switch is modified to turn off the synchronous rectifier before the output current goes negative. The control scheme effectively limits substantial reverse current flow while also improving efficiency by eliminating the need for discrete diodes, yet retaining the benefit of synchronous rectification throughout the operating range of the converter.

U.S. Pat. No. 6,912,138 entitled “Synchronous rectifier control circuit” discloses a synchronous rectifier control circuit for controlling a synchronous rectifier of a power converter. In one embodiment, the conduction of the synchronous rectifier is controlled in proportion of the differentiated output voltage. This embodiment involves increasing the rate of a voltage level of the control signal to the synchronous rectifier.

U.S. Pat. No. 8,373,403 entitled “Circuit for Controlling Synchronous Rectifiers During Start-up into Pre-Bias Output Voltage” discloses a power supply that includes circuitry for gradually enabling switching rectifiers dining a startup condition without drawing current from a pre-biased power supply output. A driver provides a control signal to a synchronous rectifier. A driver supply circuit is coupled across the driver and has a first input receiving pulse signals provided by a pulse modulation controller, an output providing supply voltage to the driver, a second input receiving driver supply input voltage, and circuitry defining a time constant. The circuitry includes a first switching element that turns on when pulse signals are provided and a second switching element connected to the output. The time constant is associated with a rise time for the power supply, and defined by selected component values, such that the second switching element only becomes fully conductive after elapsing of the time constant.

SUMMARY OF THE INVENTION

The present invention provides a pre-bias control circuit for a switching power converter that detects the slope of the output voltage over time and outputs an OPEN command when the slope detected is more NEGATIVE than a pre-defined threshold and a pre-charge current that flows back through the switching power converter has reached a maximum value. In response, the synchronous rectifier switch OPENs overriding the typical control waveform to control the energy from the output voltage flowing back through the switching power converter.

In one embodiment, the pre-bias control circuit has an AND gate to determine the control of the synchronous rectifier

In one embodiment, the pre-bias control circuit has a slope measurement circuit and a slope detection circuit.

In one embodiment, the slope measurement circuit includes a slope amplification circuit.

In one embodiment, the pre-defined threshold is set such that the pre-bias control circuit is controlled and non-responsive to noise.

The switching power converter may be any converter that includes synchronous rectifiers, such as a flyback converter, a forward converter, a buck converter, in a single-ended, double-ended and/or multi-phased configuration.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements of a conventional power converter. For example, certain power converters require a transformer reset mechanism. However, such reset mechanisms are not described herein. Those of ordinary skill in the art will recognize, however, that these and other elements may be desirable in a typical power converter. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein.

All circuit components are assumed ideal for the purpose of describing the present invention. In addition, as used herein, the term “ON” is used synonymously with “CLOSED,” and the term “OFF” is used synonymously with “OPEN” when referring to the state of a semiconductor switch. Also, as used herein, a semiconductor switch is “ON” when the switch reaches a low-impedance state after the control signal to the switch reaches a suitable voltage level to initiate turn-on of the switch. Similarly, a switch is “OFF” when the switch reaches a high-impedance state after the control signal reaches a suitable level to initiate turn-off of the switch. Additionally, as used herein, a slope is “NEGATIVE” relative to the polarity of the output voltage. For example, if the output voltage is +5V or 5 volts in the positive direction, the “NEGATIVE” slope would be in the negative direction. Alternatively, if the output voltage is a −5V or 5 volts in the negative direction, the “NEGATIVE” slope would be in the positive direction. In addition, as used herein, a waveform is “HIGH” when the signal is ON or producing a logic “1”. Similarly, a signal or waveform is “LOW” when the signal is OFF or producing a logic “0”.

During normal conditions, the primary switch and the synchronous rectifier switch operate in opposition to each other to charge and discharge the energy storage section to produce the desired output voltage from the input voltage. However, during pre-bias conditions, there is a potential to disturb or damage components of the switching power converter with uncontrolled current flowing back into the converter. To limit the current flow back into the power converter, the pre-bias control circuit overrides the commanded state of the synchronous rectifier switch in order to control the discharging of the energy storage section by holding the synchronous rectifier switch OPEN as current and voltage conditions are met.

Referring now toFIG. 1, one embodiment of the switching power converter100includes a buck converter102, a switching control circuit104and a pre-bias control circuit106.

The switching control circuit104creates a waveform that controls the switching frequency of the switching power converter100. The waveform is rectangular having a period and a duty cycle. The frequency of the switching power converter100is equal to the inverse of the period. The output voltage (Vout)110is the result of the input voltage (Vin)108multiplied by the duty cycle. In one embodiment, the switching control circuit104may be configured to include a pulse width modulator (PWM) integrated chip. In another embodiment, the switching control circuit104may include an internal clock that sets the duty cycle or operating frequency of the switching power converter. In another embodiment, the switching control circuit104may include an error amplifier in combination with the PWM integrated chip.

The buck converter is a typical DC-DC switching power converter that steps down voltage while stepping up current from the input voltage108to the output voltage110. The illustrated buck converter102includes a primary switch112showing the ideal intrinsic body diode114, a synchronous rectifier switch116showing the ideal intrinsic body diode118, and an energy storage section120. The energy storage section120in a buck converter102includes an inductor (Lo)122and an output capacitor (Cout)124. The primary switch112has an input, an output and a control. The input of the primary switch112is connected to the input voltage108, the output is connected to the input of the inductor122and the control is connected to the output of the switching control circuit104. The synchronous rectifier switch116has an input, an output and a control. The input of the synchronous rectifier switch116is connected to ground125, the output is connected to the input of the inductor122and the control is connected to the output of the pre-bias control circuit106. The pre-bias control circuit106has two inputs and an output. The first input of the pre-bias control circuit106is connected to the output of the switching control circuit104through an inverter126. The second input of the pre-bias control circuit106is connected to the output voltage110. The function of the pre-bias control circuit106will be described below.

During normal operation the primary switch112and the synchronous rectifier switch116operate in opposition to each other responsive to the output of the switching control circuit104such that when the primary switch112is CLOSED and the synchronous rectifier switch116is OPEN the energy storage section120is charging responsive to the application of the input voltage108such that the inductor122will have a voltage impressed across it equal to (Vin−Vout). With a constant voltage across the inductor122, its current increases. When the primary switch112is OPEN and the synchronous rectifier switch116is CLOSED the energy storage section120is discharging such that the voltage polarity across the inductor122immediately reverses trying to maintain the previous current. Since the polarity of the inductor122is reversed, the current will ramp down until the switching control circuit104commands the switches112,116again such that the voltage and current flow within the circuit is controlled.

However, during pre-bias conditions, the switching control circuit104will command the synchronous rectifier switch116CLOSED while the output voltage110is greater than the input voltage108. At this time, the inductor's122current is flowing back through the circuit as the voltage polarity across the inductor122is reversed. While the output voltage110is greater than the input voltage108, this pre-charge current (Ipre)109is uncontrolled by the switching control circuit104or the energy storage section120but is controlled by the pre-bias control circuit106as described below. The maximum pre-charge current (Ipremax) value can be calculated with the circuit's parameters as shown in the following equation:

The pre-bias control circuit106detects the slope of the output voltage110over time (rate of change) and produces an OPEN command when the slope detected is more NEGATIVE than a pre-defined threshold and the pre-charge current109has reached the maximum value, Ipremax, the synchronous rectifier switch116responds to the OPEN command and overrides the output of the switching control circuit104by commanding the synchronous rectifier switch116OPEN to control the energy from the output capacitor124flowing back through the switching power converter100. The pre-bias control circuit106is continually monitoring the output voltage110, pre-charge current109and overriding the switching control circuit104as the more NEGATIVE slope is detected while reaching the maximum pre-charge current of the switching power converter.

FIG. 2is a schematic of the pre-bias control circuit106ofFIG. 1according to one embodiment of the present invention. The pre-bias control circuit106may control one or more synchronous rectifier switches of a switching power converter. In one embodiment of the present invention, the illustrated pre-bias control circuit106includes a slope measurement circuit230, a slope detection circuit232, an inverter126and a rectifier gating circuit234, in this case an AND gate234. The slope measurement circuit230differentiates the output voltage110of the switching power converter over time. The slope detection circuit232is responsive to the output of the slope measurement circuit230by generating a logic ‘0’ when a NEGATIVE slope is detected with a value more negative than a pre-defined threshold voltage and the pre-charge current109has reached the maximum value; and a logic ‘1’ when a non-NEGATIVE slope is detected with a value more positive than a pre-defined threshold voltage without reaching the maximum pre-charge current of the switching power converter. By using the logic AND gate234, the output of the slope detection circuit232overrides the switching control circuit104waveform, such that the output of the pre-bias control circuit106will be a logic “1” when the switching control circuit104waveform is LOW and the slope detection circuit output is a logic “1”. The output of the pre-bias control circuit106will be a logic “0” if either the switching control circuit104waveform is HIGH or the slope detection circuit output is a logic “0”. In another embodiment, the output of the slope measurement circuit230is amplified and filtered to remove noise before being used by the slope detection circuit232. In another embodiment, the gating rectifier circuit is a combination of multiple logic gates with one output controlling the synchronous rectifier switch.

One embodiment of the pre-bias control circuit106ofFIG. 2is shown inFIG. 3where the slope measurement circuit230ofFIG. 2is a passive resistor-capacitor differentiator circuit including a differentiator capacitor340and a differentiator resistor (Rdiff)342. The differentiator circuit is used to extract the slope of the output voltage110. An offset voltage (Vofst)348is developed across the differentiator resistor342to prevent false tripping of the differentiator circuit. This offset voltage348can impose itself across the differentiator resistor342to develop a current source function that can be used to charge the differentiator capacitor340. This current source function can further be used to command a controlled amount of pre-charge current that is adjustable by circuit parameter selection, which allows the switching power converter to operate with pre-bias conditions present.

Further, inFIG. 3, the slope detection circuit232ofFIG. 2is a non-inverting hysteresis comparator circuit. The operation of the non-inverting hysteresis comparator circuit is known in the art and, therefore, not described herein. The comparator circuit consists of a comparator352two resistors344,345and a reference voltage (Vref)346and produces a hysteresis voltage (Vhys). The component values of the resistors344,345and the reference voltage346are configured to act as a voltage divider such that the output is equal to the desired pre-defined threshold (Vofst)348. In one embodiment, the pre-defined threshold is chosen for system requirements including but not limited to noise reduction, thermal sensitivities and the synchronous rectifier switch's response time. For example, the pre-defined threshold may be zero or a finite value determined by the magnitude of the switching power converter's noise. In another embodiment, the pre-defined threshold is set to limit the maximum pre-charge, second quadrant (negative) current flow in the switching power converter.

Considering the implementation of the pre-bias control circuit106ofFIG. 3and the reference numbers therein, the operation of the switching power converter is now described with reference toFIGS. 4aand 4b.FIG. 4arepresents the waveform407generated by the switching control circuit104in a soft start mode. This is represented by the expanding duty cycle, D,403interval as a function of time. In normal operation, this waveform407would be controlled in such a manner as to ramp the switching power converter output voltage110as shown inFIG. 4b. If the output voltage110is monotonically rising405, the output of the pre-bias control circuit106will not override the waveform407of the switching control circuit104inFIG. 4a.

Still using the implementation of the pre-bias control circuit106inFIG. 3and the reference numbers therein, refer toFIGS. 5a-ewhereFIG. 5ais the waveform563depicting the inductor's122pre-charge current109,FIG. 5bis the waveform570depicting the voltage (Vsn) at the primary switch112output,FIG. 5cis the waveform581depicting the output of the comparator circuit (slope detection circuit),FIG. 5dis the waveform585depicting the output voltage110of the switching power converter over several cycles, andFIG. 5eis the waveform591depicting the voltage at the comparator's352positive input. Now consider the non-zero pre-bias voltage case where the output voltage110is greater than zero but lower than the steady state operating point of the switching power converter, and the duty cycle is near zero. In this case, the inputs to the comparator352have the positive input greater than the negative by the level of the offset voltage. As such, the output of the comparator352is a logic “1”. The switching control circuit104will be LOW, the synchronous rectifier switch116will be commanded ON, and the rectifier gating circuit234will allow the waveform from switching control circuit104to pass. This grounds the left end of the inductor122and a pre-charge current109will build.

Due to the hysteretic nature of the pre-bias control circuit, the operation can be described in terms of various states. Within a set of operating conditions set by the switching power converter voltages and circuit values, a quasi-periodic function can be described and state variables within the period of operation can be found. On the initial cycle, the switching power converter must overcome the offset voltage but on subsequent cycles, the bounding control limits are set by the hysteresis value of the comparator circuit.

The number of cycles is illustrative inFIGS. 5a-eand is not indicative of the total number of cycles for a switching power converter. Cycles following the initial cycle are described inFIG. 5das Vpre, Vpre_n, Vpre_n+1, and Vpre_n+2. On each cycle, the output voltage110will be discharged by a prescribed amount as determined by the circuit parameter values.

The following operation is described for cycles after the initial cycle where the pre-bias control circuit bounding limits are set by the hysteresis voltage586. As shown inFIG. 5a, one pre-bias control circuit cycle (Tpre)560has a start (tprestart)561and an end (tpreend)569. Tpre is split into two basic periods: the period (Ton)562the pre-bias control circuit106is allowing the switching control circuit104to control the synchronous rectifier switch116and the period (Toff)568the pre-bias control circuit106is overriding the switching control circuit104and commanding the synchronous rectifier switch116OPEN. Ton562is further broken into two modes—ton1564and ton2566, described in detail below.

As shown inFIG. 5a, a pre bias control circuit cycle560begins at tprestart561when the comparator's positive input reaches the upper value (Vhys/2+Vref)592of the comparator circuit. This is a result of the differentiator capacitor340being charged by the effective current source developed by the offset voltage and the differentiator resistor (Vofst/Rdiff). As shown inFIG. 5a, the ton1564mode is now active and the pre-bias control circuit106is allowing the switching control circuit104to control the synchronous rectifier switch116. Referring toFIGS. 5a, 5c, and5d, the comparator's352output is a logic “1”582commanding the synchronous rectifier switch116CLOSED as the inductor's122pre-charge current109builds572while discharging588the output voltage110. When the maximum pre-charge current574is reached, the comparator's352positive input reaches the minimum value of the comparator circuit (Vref−Vhys2)590and the comparator's output is a logic “0”584which causes the synchronous rectifier switch116to be commanded OPEN. This ends ton1564and starts the period ton2566which represents the reset time for the inductor122. Due to flyback action, the energy stored in the inductor122during ton1564is returned to the input via the primary switch's ideal intrinsic body diode114. This is shown inFIG. 5bby the primary switch output's voltage (Vsn) clamping to the voltage input source level580. Since the voltage is reversed on the inductor122the pre-charge current109ramps down576. Ton2566ends when the pre-charge current109in the inductor122reaches zero amps578. Since interval ton2566represents charge extracted from the output capacitor124, the output continues to discharge by a value of Vhys2587which is the undershoot voltage with respect to the output voltage110that occurs due to the reset of the inductor122and is equal to the pre-charge current109multiplied by the period ton2566divided by two times the value of the output capacitor124. This becomes the starting point for the next cycle. As stated, toff568defines the period the pre-bias control circuit106is overriding the switching control circuit104and commanding the synchronous rectifier switch116OPEN. It is set by the time it takes for the positive input of the comparator352to return to the upper value (Vhys/2+Vref)592of the comparator circuit. The action that defines this period is due to the charging of the differentiator capacitor340by the effective current source (Vofst/Rdiff). At the conclusion of toff, one pre-bias control circuit cycle560completes at tpreend569.

The described cycles of the pre-bias control circuit106repeat until either the output capacitor124is completely discharged or the duty cycle of the switching power converter is sufficiently high. Once this condition is achieved the switching control circuit104begins to command the primary switch112to a finite duty cycle developing a non-NEGATIVE slope of the output voltage110further causing the positive input of the comparator352to always be above the negative input; thus allowing the pre-bias control circuit to pass the switching control circuit104waveform to the synchronous rectifier switch116until the switching power converter is unable to maintain a monotonic output with a non-NEGATIVE slope then the pre-bias control circuit106will reactivate automatically.

The pre-bias control circuit106may be employed for any switching power converter topology utilizing synchronous rectifiers. For example,FIG. 6is a schematic of a switching power converter topology commonly referred to as a forward converter including the pre-bias control circuit106. The switching power converter ofFIG. 6includes two synchronous rectifiers616a,616b. The first synchronous rectifier switch616arectifies the voltage across the secondary winding623and the second synchronous rectifier switch616bacts as the freewheeling rectifier. An output filter, comprising the output capacitor124and an inductor122filters the output voltage110. In a forward converter, energy is transferred from the primary winding621to the secondary winding623of the transformer627during the CLOSED period of the primary switch112. The operation of forward converters is known in the art and, therefore, not described herein. As shown inFIG. 6, the drive signal from the switching control circuit104to the pre-bias control circuit106may be inverted by an inverter because the synchronous rectifiers may alternatively conduct.

Another example of a different topology employing the pre-bias control circuit106is shown inFIG. 7. The switching power converter is in the flyback mode. A flyback converter stores energy from the input voltage108during the CLOSED period of the primary switch112and that energy is released to the output voltage110during the OPEN period of the primary switch112. Energy stored in the output capacitor124is supplied to the output voltage110during the CLOSED period of the primary switch112. The switching power converter includes a synchronous rectifier switch116coupled to the secondary winding of the transformer for rectifying the voltage across the secondary winding. The pre-bias control circuit106detects the slope of the output voltage110over time and produces an OPEN command when the slope detected is more NEGATIVE than a pre-defined threshold and the pre-charge current109has reached a maximum value, the synchronous rectifier switch116responds to the OPEN command and overrides the output of the switching control circuit104by commanding the synchronous rectifier switch116OPEN to prevent the synchronous rectifier switch116from conducting current in the second quadrant.

The flyback, forward and buck converters shown previously are examples of the types of switching power converters that may employ the pre-bias control circuit106and associated methods of the present invention. As stated previously, any switching power converter topology utilizing synchronous rectification may employ the pre-bias control circuit106method. This includes, but is not limited to; single ended and double-ended converters, half bridge and full bridge converters, integrated forward/flyback converter, etc. In addition, the pre-bias control circuit may be used to control multiple synchronous rectifiers in, for example, interleaved or multi-phased converters.