Digital control system and method for switched mode power supply

Description of equivalent WO 2004072589 A2

DIGITAL CONTROL SYSTEM AND METHOD FOR SWITCHED MODE

POWER SUPPLY

RELATED APPLICATION DATA This application relates to copending application Serial No. , for

DIGITAL SIGNAL PROCESSOR ARCHITECTURE OPTIMIZED FOR CONTROLLING SWITCHED MODE POWER SUPPLY, filed concurrently herewith, the subject matter of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power supply circuits, and more particularly to digital control systems and methods for switched mode power supply circuits.

2. Description of Related Art

Switched mode power supplies are known in the art to convert an available direct current (DC) or alternating current (AC) level voltage to another DC level voltage. A buck converter is one particular type of switched mode power supply that provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. It includes two power switches that are typically provided by MOSFET transistors. A filter capacitor coupled in parallel with the load reduces ripple of the output current. A pulse width modulation (PWM) control circuit is used to control the gating of the power switches in an alternating manner to control the flow of current in the output inductor. The PWM control circuit uses signals communicated via a feedback loop reflecting the output voltage and/or current level to adjust the duty cycle applied to the power switches in response to changing load conditions.

Conventional PWM control circuits are constructed using analog circuit components, such as operational amplifiers, comparators and passive components like

LA2-653092.1 - 1 - resistors and capacitors for loop compensation, and some digital circuit components like logic gates and flip-flops. But, it is desirable to use entirely digital circuitry instead of the analog circuit components since digital circuitry takes up less physical space, draws less power, and allows the implementation of programmability features or adaptive control techniques. A conventional digital control circuit includes an analog-to-digital converter (ADC) that converts an error signal representing the difference between a signal to be controlled (e.g., output voltage (V₀)) and a reference into a digital signal having n bits. The digital control circuit uses the digital error signal to control a digital pulse width modulator, which provides control signals to the power switches having a duty cycle such that the output value of the power supply tracks the reference. In order to keep the complexity of the PWM control circuit low, it is desirable to hold the number of bits of the digital signal to a small number. At the same time, however, the number of bits of the digital signal needs to be sufficiently high to provide resolution good enough to secure precise control of the output value. Moreover, the ADC needs to be very fast to respond to changing load conditions. Current microprocessors exhibit supply current slew rates of up to 20 A/μs, and future microprocessors are expected to reach slew rates greater than 350 A/μs, thereby demanding extremely fast response by the power supply.

Single stage (i.e., flash) ADC topologies are utilized in power supply control circuit applications since they have very low latency (i.e., overall delay between input and output for a particular sample). If a standard flash ADC device is used to quantize the full range of regulator output voltage with desired resolution (e.g., 5 mV), the device will necessarily require a large number of comparators that will dissipate an undesirable amount of power. Under normal operation, the output voltage V₀ of the regulator remains within a small window, which means that the ADC need not have a high resolution over the entire range. Accordingly, a "windowed" ADC topology permits high resolution over a relatively small voltage range tracked by a reference voltage (V_ref). Since the quantization window tracks the reference voltage V_ref, the signal produced by the ADC will be the voltage error signal. Thus, the windowed ADC provides the dual functions of the ADC and error amplifier, resulting in a further reduction of components and associated power dissipation.

Notwithstanding these advantages, a drawback with the windowed ADC topology is that the device can go into saturation due to transient load conditions that cause the window ranges to be exceeded. By way of example, a 4-bit windowed ADC has a least significant bit (LSB) resolution of roughly 5 mV. This means that an output voltage error of as low as ±40 mV pushes the ADC into saturation. The ADC would then continue to reflect the same error signal (i.e., maximum) even though the actual error could grow even larger, referred to as a "windup" condition of the digital control system. The reaction of the feedback loop in this windup condition can be difficult to predict, since without accurate information about the error size the digital control system no longer functions as a linear system. This behavior can be particularly harmful, since it can damage the load due to overcurrent and/or overvoltage, and can also damage the power supply itself. Thus, it would be advantageous to provide a system and method for digitally controlling a switched mode power supply that overcomes these and other drawbacks of the prior art. More specifically, it would be advantageous to provide a system and method for digitally controlling a switched mode power supply having a windowed ADC topology that can maintain accurate voltage regulation during ADC saturation caused by transient load conditions.

SUMMARY OF THE INVENTION

The present invention provides a switched mode power supply having a digital control system. More particularly, the power supply comprises at least one power switch adapted to convey power between input and output terminals of the power supply, and a digital controller adapted to control operation of the at least one power switch responsive to an output measurement of the power supply.

In an embodiment of the invention, the digital controller comprises an analog-to- digital converter providing a digital error signal representing a difference between the output measurement and a reference value, a digital filter providing a digital control output based on a sum of previous error signals and previous control outputs, an error controller adapted to modify operation of the digital filter upon an error condition, and a digital pulse width modulator providing a control signal to the power switch having a pulse width corresponding to the digital control output. The analog-to-digital converter further comprises a windowed flash analog-to-digital converter that provides a HIGH signal reflecting a negative saturation of the analog-to-digital converter and a LOW signal reflecting a positive saturation of the analog-to-digital converter. The digital filter further comprises an infinite impulse response filter providing the following transfer function G(z): ₌ PWM(z) ₌ C₀ + C_l - -T¹ + C₂^■ z-² + ... + C„ ^• -T"

VEd(z) \ - B_x - z^'l - B₂ - z^'2 - ... - B_n^■ z^~n wherein PWM(z) is the digital control output, VEd(z) is the error signal, Co...C_n are input side coefficients, and B-|...B_n are output side coefficients. The digital filter further comprises a range limiter adapted to clip the digital control output if upper or lower range limits are reached. The range limiter provides a limit signal to the error controller if the upper or lower range limits are reached. The error controller may provide an alternative digital control output to the digital pulse width modulator upon the error condition, or may replace at least one of the previous error signals and/or at least one of the previous control outputs with predetermined values upon the error condition.

In another embodiment of the invention, a method is provided for controlling a power supply having at least one power switch adapted to convey power between input and output terminals of the power supply. The method comprises the steps of receiving an output measurement of the power supply, sampling the output measurement to provide a digital error signal representing a difference between the output measurement and a reference value, filtering the digital error signal to provide a digital control output based on a sum of previous error signals and previous control outputs, modifying operation of the filtering step upon an error condition, and providing a control signal to the at least one power switch, the control signal having a pulse width corresponding to the digital control output. The sampling step further comprises providing a HIGH signal reflecting a negative saturation state and a LOW signal reflecting a positive saturation state. The filtering step further comprises filtering the digital error signal using an infinite impulse response filter having the transfer function G(z) described above.

A more complete understanding of the system and method for digitally controlling a switched mode power supply will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 depicts a switched mode power supply having a digital control circuit;

Fig. 2 depicts a windowed flash ADC that provides high and low saturation signals; and

Fig. 3 depicts a digital controller having an infinite impulse response filter and error controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method for digitally controlling a switched mode power supply. More specifically, the invention provides a method for digitally controlling a switched mode power supply having a windowed ADC topology that maintains accurate regulation during ADC saturation caused by transient load conditions. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more figures.

Fig. 1 depicts an exemplary switched mode power supply 10 having a digital control circuit in accordance with an embodiment of the present invention. The power supply 10 comprises a buck converter topology to convert an input DC voltage Vj_n to an output DC voltage V₀ applied to a resistive load 20 (R|_0ad). The power supply 10 includes a pair of power switches 12, 14 provided by MOSFET devices. The source terminal of the high side power switch 12 is coupled to the input voltage Vj_n, the source terminal of the low side power switch 14 is connected to ground, and the drain terminals of the power switches 12, 14 are coupled together to define a phase node. An output inductor 16 is coupled in series between the phase node and the terminal providing the output voltage V_0> and a capacitor 18 is coupled in parallel with the resistive load R|_0ad- Respective drivers 22, 24, alternatingly drive the gate terminals of the power switches 12, 14. In turn, the drivers 22, 24 are controlled by digital control circuit 30 (described below). The opening and closing of the power switches 12, 14 provides an intermediate voltage having a generally rectangular waveform at the phase node, and the filter formed by the output inductor 16 and capacitor 18 converts the rectangular waveform into a substantially DC output voltage V₀. The digital control circuit 30 receives a feedback signal from the output portion of the power supply 10. As shown in Fig. 1 , the feedback signal corresponds to the output voltage V₀, though it should be appreciated that the feedback signal could alternatively (or additionally) correspond to the output current drawn by the resistive load R|_oad or any other signal representing a parameter to be controlled by the digital control circuit 30. The feedback path may further include a voltage divider (not shown) to reduce the detected output voltage V₀ to a representative voltage level. The digital control circuit 30 provides a pulse width modulated waveform having a duty cycle controlled to regulate the output voltage V₀ (or output current) at a desired level. Even though the exemplary power supply 10 is illustrated as having a buck converter topology, it should be understood that the use of feedback loop control of the power supply 10 using the digital control circuit 30 is equally applicable to other known power supply topologies, such as boost and buck-boost converters in both isolated and non-isolated configurations, and to different control strategies known as voltage mode, current mode, charge mode and/or average current mode controllers. More particularly, the digital control circuit 30 includes analog-to-digital converter

(ADC) 32, digital controller 34, and digital pulse width modulator (DPWM) 36. In a preferred embodiment of the invention, the ADC 32 further comprises a windowed flash ADC that receives as inputs the feedback signal (i.e., output voltage V₀) and a voltage reference (Ref). The ADC 32 produces a digital representation of the voltage error signal (VEd_k) corresponding to the difference between the inputs (Ref - V₀). The digital controller 34 has a transfer function G(z) that transforms the voltage error signal VEd_k to a digital output provided to the DPWM 36, which converts the signal into a waveform having a proportional pulse width (PWMR). As discussed above, the pulse-modulated waveform PWM_k produced by the DPWM 36 is coupled to the gate terminals of the power switches 12, 14 through the respective drivers 22, 24.

Fig. 2 depicts an exemplary windowed flash ADC 40 for use in the digital control circuit 30. As discussed above, the ADC 40 receives as inputs the voltage reference Ref and the output voltage V₀. The voltage reference is applied to the center of a resistor ladder that includes resistors 42A, 42B, 42C, 42D connected in series between the reference voltage terminal and a current source connected to a positive supply voltage (VDD), and resistors 44A, 44B, 44C, 44D connected in series between the reference voltage terminal and a current source connected to ground. The resistors each have equal resistance values to define together with the current sources a plurality of equally spaced voltage increments (e.g., 5 mV) ranging above and below the voltage reference Ref. The magnitude of the resistance values and/or current sources can be selected to define the LSB resolution of the ADC 40. An array of comparators is connected to the resistor ladder, including a plurality of positive side comparators 46A, 46B, 46C, 46D and a plurality of negative side comparators 48A, 48B, 48C, 48D. The positive side comparators 46A, 46B, 46C, 46D each have a non-inverting input terminal connected to the output voltage V₀, and an inverting input terminal connected to respective ones of the resistors 42A, 42B, 42C, 42D. Likewise, the negative side comparators 48A, 48B, 48C each have a non-inverting input terminal connected to the output voltage V₀, and an inverting input terminal connected to respective ones of the resistors 44A, 44B, 44C, 44D. Negative side comparator 48D has a non-inverting input terminal connected to ground and the inverting input terminal connected to the output voltage V₀. It should be appreciated that a greater number of resistors and comparators may be included to increase the number of voltage increments and hence the range of the ADC 40, and that a limited number of resistors and comparators is shown in Fig. 2 for exemplary purposes only.

The ADC 40 further includes a logic device 52 coupled to output terminals of comparators 46A, 46B, 46C and 48A, 48B, 48C. The logic device 52 receives the comparator outputs and provides a multi-bit (e.g., 4-bit) parallel output representing the voltage error VEd_k. By way of example, an output voltage V₀ that exceeds the reference voltage Ref by one voltage increment (e.g., 5 mV) would cause the outputs of comparators 46B, 46A, 48A, 48B, and 48C to go high, while the outputs of comparators 46C, 46D and 48D remain low. The logic device 52 would interpret this as logic level 9 (or binary 1001 ) and produce an associated voltage error signal VEd_k. It should be understood that the voltage reference Ref is variable so as to shift the window of the ADC 40. If the output voltage V₀ exceeds the highest voltage increment of the resistor ladder, the output terminal of comparator 46D provides a HIGH saturation signal. Similarly, if the output voltage V₀ is lower than the lowest voltage increment of the resistor ladder, the output terminal of comparator 48D provides a LOW saturation signal.

Fig. 3 depicts a digital controller having a digital filter and error controller 62. The digital filter further comprises an infinite impulse response (IIR) filter that produces an output PWM'_k from previous voltage error inputs VEd_k and previous outputs PWM'_k. As discussed above, ADC 40 provides the voltage error inputs VEd_k. The digital filter outputs PWM'_k are provided to the digital pulse width modulator (DPWM) 36, which provides the pulse width modulated control signal (PWM_k) to the power supply power switches.

The IIR filter is illustrated in block diagram form and includes a first plurality of delay registers 72, 74, ..., 76 (each labeled z^"1), a first plurality of mathematical operators (multipliers) with coefficients 71 , 73, ..., 77 (labeled CO, C1 , ..., Cn), a second plurality of mathematical operators (adders) 92, 94, 96, a second plurality of delay registers 82, 84, ..., 86 (each labeled z^"1), and a third plurality of mathematical operators (multipliers) with coefficients 83, 87 (labeled B1 Bn). Each of the first delay registers 72, 74, 76 holds a previous sample of the voltage error VEd_k, which is then weighted by a respective one of the coefficients 71 , 73, 77. Likewise, each of the second delay registers 82, 84, 86 holds a previous sample of the output PWM'_k, which is then weighted by a respective one of the coefficients 83, 87. The adders 92, 94, and 96 combine the weighted input and output samples. It should be appreciated that a greater number of delay registers and coefficients may be included in the IIR filter, and that a limited number is shown in Fig. 3 for exemplary purposes only. The digital filter structure shown in Fig. 3 is an exemplary implementation of the following transfer function G(z): _^ PWM(z) _^ C₀ + C_x - z-¹ + C₂^• -T² + ... + C. • z-"

VEd(z) \ -B_l - z^~l - B₂ - z^~2 - ... - --?„ • -T"

The error controller 62 receives a plurality of input signals reflecting error conditions of the ADC 40 and the digital filter. Specifically, the error controller 62 receives the HIGH and LOW saturation signals from the ADC 40 reflecting that the output voltage V₀ is above and below the voltage window of the ADC, respectively. Each of the mathematical operators (adders) 92, 94, 96 provides an overflow signal to the error controller 62 reflecting an overflow condition (i.e., carry bit) of the mathematical operators. The digital filter further includes a range limiter 81 that clips the output PWM'_k if upper or lower range limits are reached. In that situation, the range limiter 81 provides the error controller 62 with a corresponding limit signal. The error controller 62 uses these input signals to alter the operation of the digital filter in order to improve the responsiveness of the digital filter to changing load conditions. The error controller 62 is coupled to each of the first plurality of delay registers 72, 74, 76 and second plurality of delay registers 82, 84, 86 to enable the resetting and/or presetting of the value stored therein. As used herein, "resetting" refers to the setting of the value to an initial value (e.g., zero), whereas "presetting" refers to the setting of the value to another predetermined number. Particularly, the error controller 62 can replace the previous samples of the voltage error VEd and output PWM'_k with predetermined values that change the behavior of the power supply. The digital controller further includes multiplexer 64 that enables selection between the PWM'_k output signal and a predetermined output signal provided by the error controller 62. A select signal provided by the error controller 62 determines which signal passes through the multiplexer 64. When the ADC 40 goes into HIGH or LOW saturation, the error controller 62 sets the PWM'_k signal to a specific predetermined value (or sequence of values that are dependent in part on the previous samples) by controlling the multiplexer 64. In order to recover smoothly from such a condition, the error controller can also alter the delayed input and output samples by reloading the first plurality of delay registers 72, 74, 76, and second plurality of delay registers 82, 84, 86. This will assure a controlled behavior of the feedback loop as the ADC 40 recovers from saturation.

By way of example, if the ADC 40 experiences a positive saturation, i.e., the LOW signal changing from a low state to a high state, the PWM'_k sample can be reset to zero to help to reduce the error. By resetting the PWM'_k sample to zero, the pulse width delivered to the high side power switch 12 of the power supply 10 goes to zero, effectively shutting off power to the resistive load 20 (see Fig. 1 ). In order to recover from this situation smoothly, the samples PWMV., PWM'k-2, ■■-, PWMVn can also be reset to zero or preset to another value in order to allow a smooth recovery. Likewise, if the ADC 40 experiences a negative saturation, i.e., the HIGH signal changing from a low state to a high state, the PWM'_k sample can be preset to a maximum value to increase the pulse width delivered to the high side power switch 12 to reduce the error. Also, when an internal numeric overflow of the digital filter occurs, the error controller 62 can take actions to prevent uncontrolled command of the power switches of the power supply, such as altering the input and output samples of the digital filters. Having thus described a preferred embodiment of a method for digitally controlling a switched mode power supply, it should be apparent to those skilled in the art that certain advantages of the system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.