CA1246141A - Boost feedforward pulse width modulation regulator - Google PatentsBoost feedforward pulse width modulation regulator
- Publication number
- CA1246141A CA1246141A CA 475252 CA475252A CA1246141A CA 1246141 A CA1246141 A CA 1246141A CA 475252 CA475252 CA 475252 CA 475252 A CA475252 A CA 475252A CA 1246141 A CA1246141 A CA 1246141A
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- Patent type
- Prior art keywords
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
- H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
- H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
level with respect to the instantaneous level of a sawtooth waveform, under the influence of a feedforward portion [from 96] of the input voltage .
Description Boost Feedforward Pulse Width Modulation Regulator Technical Field This invention is directed toward the art o~
feedforward circuits and more particularly to the art of feedforward circuits for pulse width modulatecl regulators in DC power supplies.
Background Art Pul~e width modulation is a well known technique for DC voltage regulation, in which a constant output voltage i~ ePficiently maintained despite wide variation in input voltage and output current.
Voltage regulators employing pulse width modulation techniques are widely used in power supplies driving complex electronic systems.
There are various kinds of pulse width modulators, but generally speaking they all employ semiconduc~or switching to genera~e output rectangular voltage and current pulses ~hich are 20, eefectively switched by an inductor-capacitor filter network to produce a constant ou~put DC voltage level. The magnitu~e of the output voltage is controlled by the duty ratio of the semiconductor switch.
~5 To maintain this output constant voltage level, a negative feedback arrangement is conventionally employed. This calls for a fixed fraction of the output voltage to be compared with a stable voltage reference, and developing an error signal which then effectively controls the duty ratio of the semiconductor switch.
The conven~ional switching power stage operates a transistor between on and off states, and smooths output pulses from the transistor to an average DC
value which is a function of the on or off time.
Smoothing the output pulses is accornplished by conventional filter circuitry which averages out the amplitudes of the switching pulses to produce a constant output voltage.
Feedback of an error signal is accomplished by an error amplifier and comparator operating at a nominal switching rate which is many times the power line frequency, for example in the vicinity of 20 IcHz to 200 kHz. The error ampliier essentially operates to force the error signal to zero and force the output of the switcher (fed back through a re~istor, ~or example,) to equal a reference voltage. When the output voltage of the switcher drops too low, the error amplifier turns on the switching transistor;
when the output voltage rises too high, the switching transistor is turned off.
Reliable oscillation and regulation in these circuits are accomplished by either holding the pulse rate of the switcher constant and permitting only the pulse wid~l:h (the "on" time) to vary, or holding the pulse wid~h constant and permitting only the pulse rate to vary.
Switching regulators as discussed above can be implemented utilizing a series or shunt switching element, according to common knowledge in the art.
One way to obtain a variable pulse width is to compare the output of the error amplifier to a triangular wave, where the switching ~ransistor on time is determined by the tima that the triangular wave is less than the output of ~he error amplifier.
~ 3 --Although these circuits of the prior art are effective for many applications, the response time to variations in the input voltage is too slow. In many cases, the duty cycle of the switching transistor is S thus too slow to compensate affectively ~or the change in input voltage.
Accordingly, it is an object of this invention to establish a switching regulator circuit arrangement which is effective for rapid response to input voltage fluctuation~.
It i9 a further object of the instant invention to feed forward a portion of the input voltage in a switching regulator circuit arrangement in order to influence the duty cycle of the switching transistor Of the arrangement toward enhanced responsiveness to changes in the input voltage.
It is another object of the instant invention to cancel the e~fect of an input voltage change in a switching regulator circuit arrangement.
Disclosura of Invention The invention herein accomplishes the objects indicated above, including the establishment of more ef~ective, swifter response to input voltage changes by ~eeding forward a portion of the input voltage to a critical point in the eeedback network controlling the duty cycle of the switching transistor.
In particular, the input voltage is fed forward to a point beyond the output of the error amplifier.
The output of the error amplifier feeds through a selected resistor, and the input voltage feeds through another selected resistor, permitting the two to combine at a common input terminal to the ~æ~
comparator and thereby to form the control voltage.
The comparator compares the control input from the two resistors to a sawtooth voltage level, and output~ a signal to change the state of the switching transistor, whenever the level of the sawtooth waveform rises above or below the control voltage.
If the control voltage against which the sawtooth is compared changes, the switching transistor "on" time changes, effectively changing the length of its duty cycle in order to keep the output voltage constant.
Brief Description of Drawing The invention i5 best understood by reference to the drawing, which is in several ~igures, wherein:
F'igs. lA and 1~ show respectively a buck feedforward circuit o~ the prior art, and associated voltage waveforms;
Figs. 2A and 2~ are respectively a boost feedforward circuit according to one preferred version of the invention and associated voltage waveforms;
Figs. 3A and 3B are respectively a boost feedforward circuit according to another preferred version of the invention and voltage waveforms associated therewith.
Best Mode for Carrying Out the Invention Fig. 1 shows a conventional buck feedforward circuit of the prior art for step-down voltage regulation between input and output terminals, respectively 12 and 13. The circuit includes an input capacitor 17 to filter input noise, which is connected to a suitable switching transistor 23.
~11 41 The switching transistor 23 operates on a duty cycle to be discussed below, alternately switching between "on" and "off" states in accordance with a signal to its base or control side 23(1). The emitter output of the switching transistor 23, which in this case has been selected to he an "npn"
transistor, is connected to an inductor 31, which in turn is connected to a capacitor 39 leading to ground. The output of the switching transistor 23 is additionally connected to the cathode of a diode 41 having a grounded anode.
When the switching transistor 23 is "on" and conducts, diode 41 is reverse biased and does not conduct. The current from the transistor 23 thus lS pa3ses through inductor 31, charging capacitor 39 and passing to output 53.
When transistor 23 switches of, the inductor 31 maintains the current level existing while transistor 23 was "on". The inductor does so by drawing current from ground through now ~orward biased diode 41.
Then as the current through the inductor 31 begins to diminish, capacitor 39 picks up and begins to supply the output 53 with a compensating level of current to maintain the output at substantially the same level.
The level of the output voltage at terminal 13 is monitored by a sampling or voltage divider network including in series to ground respectively resistors 63 and 67 with an output tap 69 therebetween. A
selected portion of the output voltage is thus provided to an error amplifier 71 effective ~or comparing that portion of voltage with a reference voltage level.
The control voltage output o~ error amplifier 71 is fed to a comparator 83 which eontrols the switching of transistor 23 under direction of the error amplifier control voltage, Vc, from error amplifier 71, and a sawtooth voltage waveform produced by sawtooth generator 94 including an emitter grounded transistor 9S, a resistor 96 to an input voltage connection 98, and ramp generating capacitor 99. The sawtooth voltage is created by 5awtooth generator 94 acting upon a pulse pattern established by oscillator 101 feeding the base of transistor 95.
The output voltage from this conventional arrangement is that portion of the input voltage defined by the ratio of the time during which the switching transistor 23 is "on" to th0 period o~ the sawtooth waveform established by oscillator 101.
Comparator 83 insures that transistor 23 is "on"
whenever the output or control voltage of the error amplifier 71 exceeds the level of the sawtooth voltage waveform.
Correction for chang~s in the input voltage 98 is accomplished in the configuration of ~ig. 1 by connecting the integrator resistor 96 to the input voltage rather than to a constant bias voltage. For a buck regulator, this cau~es essentially perfect correction with respect to input voltage changes. In such a buck regulator, the output voltage is proportional to the product of the input voltage and the on time for a constant period oscillator. qy connecting resistor 96 to the input voltage, the saw amplitude and slope is made proportional to the input voltage, effectively forcing the on time to be inversely proportional to the input voltage, at a constant control voltage, Since the output voltage is directly proportional to the input voltage times the on time, which is now inversely proportional to the input voltage, any variation in the input voltage is completely and immediately cancelled, keeping the output voltage constant. This feedforward correction technique is well known and is not being claimed as part of the invention herein.
More particularly, feedforward correction for changes in the input voltage 98 is accomplished in the buck coneiguration by connecting the sawtooth generator resistor 96 to the input voltage. In a buck regulator the output voltage is proportional to the product of th~ input voltage and on time ~or a constant period oscillator, i.e., VOUt = Vin ton/T (1) where VOUt is the output voltage 13, Vin is the input voltage 98, ton is the transistor switch 23 on time and T is the period of the oscillator 101 as shown in Figure lA. For exact ~eedforward the required on time is obtained ~rom equation (1), giving:
ton = T VOUt/Vin (2) ~y connecting resistor 96 to the input voltage, 2S the saw amplitude and slope is made proportional to the input voltage forcing the on time to bs inversely proportional to the input voltage for a constant control voltage, Vc. From the con~rol signal waveforms shown in Fig. 1 the on time is given by:
ton = Vc T/Vsaw and the peak saw amplitude v5aW is given by:
VsaW = Vin T/(RT CT
The on time is then:
ton = Vc (RT CT)/Vin which i9 in the form required by equation (2), i.e., the on.time is inversely proportional to the input voltage. Solving for the constants gives:
Vc RT CT = T Vout (6) For a desired period, output voltage and control voltage, the required sawtooth generator time constant RT CT is given by equation (6). Since the output voltage is directly proportional to the input voltage times the on time (equation (1)) and the on time i9 now inversely proportional to the input lS voltage (equation (5)), any variation in the input voltage is completely and immediately cancelled, keeping the output voltage constant. This feedforward correction technique is well known and is not being claimed as part of this patent.
Fig. 2A shows a boost feedforward inverted drive circuit according to the invention herein which is effective for boosting the input DC voltage level to a higher regulated DC voltage level. As in Fig. l, the circuit employs a switching transistor 23 with an on/off ratio determined by a control signal at its control side or ba~e 23(1). During the "on" period, transistor 23 conducts current from the input through inductor 31 to ground. When the transistor 23 is turned off, the current through inductor 31 travels through diode 41, charging capacitor 39.
9 _ The remainder of the circuitry in Fig. 2 is similar to the circuit in Fig. 1, with comparator 83 again controlling the state of transistor 23 in view of the amount by which the sawtooth ~oltage level rises above (rather than below) the control voltage level, Vc. However, the control voltage level against which the sawtooth is compared is subject to the level of the input voltage and changes therein as transmitted through selected intervening resistors 201 and 208, which are respectively connected to the input voltage and the output of the error amplifier 71 on one end, and to an input with respect to comparator 83 on the other end. Sawtooth generator 94, including transistor 95, re~istor 96, and capacitor 97, operates as before in conjunction ~ith oscillator 101, except that in this case according to the invention herein, resistor 96 is connected to a selected constant voltage VT, maintaining the sawtooth at a constant slope and at a fixed or established peak amplitude.
For boost circuit with inverted drive as shown in Fig. 2A, the control signals are as follows.
toff Vc T/Vsaw (7) Substituting into equation (8) gives the required control voltage, V = V V /V (8) c saw ln out The feedforward circuit shown in Figure 2 gives Vc in this form, i.e., V = VA R3/(R3 + R4) Vin 4 3 4 Solving for the circuit constants gives R4/(R3 + R4) = VSaW/Vout (10) and VA = (11) 6~
With these circuit constants the off time is made proportional to the input voltage and any variation in the input ~oltage is cancelled, keeping the output voltage constant.
Fig. 3A shows a preferred way of carrying out the invention just described in terms of Fig. 2. In partic~lar, a semiconductor chip, such as a SGl524, SGl525, or SGl526, available from Silicon General in Garden Grove, California, can be employed as a regulating pulse width modulator 301 to provide the control signal governing the operation of transistor 23. The sawtooth provided by the Silicon General chip has a constant amplitude and a constant period.
The chip provides an inverter 301' which requires the 15 use Oe resistor 303 to establish the control vol~age, Vc ~
The boost feedforward converter of the invention herein requires a dieferent kind of feedforward correction ~han the huck converter of the prior art in Fig. l, because in the boost converter, the output voltage is directly proportional to the input voltage and inversely proportional to the o~f time for a constant period oscillator, i.e., out in T/to~f (12) where VOUt is the output voltage, Vin is the input voltage, T is the period and toff is the off time of transistor switch 23 in Fig. 3A. Accordingly, for exact feedforward the required off time from equation tl2) is tof~ = Vin T/Vout (13) In order to essentially perfectly cancel variations in the input voltage, the feedforward correction under the invention must establish an off time proportional to the input voltage.
This is accomplished in the boost converter of this invention by using resistor 201 connected to the input voltage, to sum the input vol.tage with control voltage Vc. Thus, ~hen the input ~oltage increases, the control voltage decreases in proportion to the input voltage for a constant error ampli~ier output voltage VA, causing the off time to increase in direct proportion to the input voltage. From the control signal waveforms shown in Fig. 3s, the on time is again given by ton = VC T/Vsaw (14) and to~f = T - ton = ~ (1 Vc/Vsaw) tl5) Substituting into equation (13) and solving eor the required con~rol voltage ~ives Vc = VsaW (l-Vin/Vout) (16) The feedforward circuit shown in Fig. 3 gives Vc in exactly this ~orm, i.e., V = -V~ R5/R4 -Vin R5/R3 + R3llR4llR5 R (17) Setting equation (17) equal to equation (16) and solving for the circuit constants gives R5/R3 = VSaW/VOUt (18) and saw A ~ ~ 4 ~ VR (19) With these circuit constants then, the off time is made proportional to the input voltage, i.e., to~f = Vin T/Vout Since the output voltage is directly proportional to the input voltage divided by the off time and the off time is directly proportional to the input 6~
voltage, any variation in the input voltage is essentially completely and immediately cancelled, keeping the output voltage constant. This feedforwrd technique for boost converters i5 considered novel as claimed herein. In contrast to the prior art where th~ saw amplitude had to vary in a prescribed manner, here the saw amplitude should be kept constant and the control volta~e should be varied.
The description above is likely to induce individuals skilled in the art to develop variations or related embodiments of the invention, which nonetheless fall within the scope thereof.
Accordingly, reference to the claims which follow is ur~ed, as these de~ine with particularity the metes 15 and bounds of the invention addre99ed herein.
switching means including input, output and control sides, for passing power in successive on and off states, inductive means including input and output sides, for maintaining a current throughout between said circuit input and output;
diode means for transferring current with respect to said inductive means, said diode and inductive means being electrically connected in series, and said diode means being reverse biased during the on states of said switching means;
capacitor means for storing charge, including respective output and grounded sides, the output side of said inductor and capacitor means being electrically connected at the circuit output;
sampling means for sampling a selected portion of the output voltage at said circuit output;
error means including an error output for establishing an error difference between said selected portion of said output voltage and a selected reference voltage;
sawtooth means for establising a sawtooth waveform of characteristic slope and peak amplitude;
comparator means including sawtooth and error inputs and an output, for establishing the on and off times of said switching means with respect to the difference between the respective levels of said sawtooth and error inputs; and feedforward means for modifying the level of said error output in terms of changes in the level at said circuit input, whereby said feedforward means is effective for modifying the duty cycle of said switching means without affecting the slope and peak amplitude of said sawtooth waveform.
a) periodically switching between on and off states of a switching means for passing power, including input, output and control sides b) electrically connecting an inductive means including input and output sides, for maintaining a current throughout between said circuit input and output;
c) electrically connecting a diode means for supplying current with respect to said inductive means, said diode and inductive means being electrically connected in series, and said diode means being reverse biased during the on states of said switching means;
d) electrically connecting a capacitor means for storing charge, including respective output and grounded sides, the output side of said capacitor means being electrically connected at said circuit output;
e) electrically connecting at said circuit output a sampling means for sampling a selected portion of the output voltage at said circuit output;
f) providing an error means including an error output for establishing an error difference between said selected portion of said output voltage and a selected reference voltage, g) establishing a sawtooth means for providig a sawtooth waveform of characteristic slope and peak amplitude, including an oscillator for establishing a switching period;
h) providing a comparator means including sawtooth and error inputs and an output, for establishing the on and off times of said switching means with respect to the difference between the respective levels of said sawtooth and error inputs:
i) electrically connecting a feedforward means for modifying the level of said error output in terms of changes in the level at said circuit input, whereby said feedforward means is effective for modifying the duty cycle of said switching means without affecting the slope and peak amplitude of said sawtooth waveform.
Priority Applications (2)
|Application Number||Priority Date||Filing Date||Title|
|US06594137 US4536700A (en)||1984-03-28||1984-03-28||Boost feedforward pulse width modulation regulator|
|Publication Number||Publication Date|
|CA1246141A true CA1246141A (en)||1988-12-06|
Family Applications (1)
|Application Number||Title||Priority Date||Filing Date|
|CA 475252 Expired CA1246141A (en)||1984-03-28||1985-02-27||Boost feedforward pulse width modulation regulator|
Country Status (6)
|US (1)||US4536700A (en)|
|JP (1)||JPS60218125A (en)|
|CA (1)||CA1246141A (en)|
|DE (1)||DE3509713A1 (en)|
|FR (1)||FR2562285B1 (en)|
|GB (1)||GB2156549B (en)|
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