Abstract:
Systems and methods for providing a boost converter with an improved stability are disclosed. A sample and hold circuit is connected to the output of the boost converter. That sample and hold circuit holds the output voltage before the main switch of the boost converter turns ON and holds the voltage while the main switch is ON. Thus a high frequency oscillation can be eliminated, an increased control bandwidth without stability problems can be achieved, and no complicated additional circuit is required.

Description:
BACKGROUND 
       [0001]    (1) Field of the Invention 
         [0002]    This invention relates generally to DC-to-DC converters and relates more specifically to boost switching regulators having a highly stabilized output. 
         [0003]    (2) Description of the Prior Art 
         [0004]    A boost converter (step-up converter) is a power converter with an output DC voltage greater than its input DC voltage. It is a class of switching-mode power supply (SMPS) containing and at least one energy storage element. Filters made of capacitors (sometimes in combination with inductors) are normally added to the output of the converter to reduce output voltage ripple. 
         [0005]    The key principle that drives the boost converter is the tendency of an inductor to resist changes in current. When being charged it acts as a load and absorbs energy (somewhat like a resistor), when being discharged, it acts as an energy source (somewhat like a battery). The voltage it produces during the discharge phase is related to the rate of change of current, and not to the original charging voltage, thus allowing different input and output voltages. 
         [0006]      FIG. 1  prior art shows the basic principle of a Boost converter consisting of two distinct states depending of the state of main switch S:
       In the On-state, the main switch S is closed, resulting in an increase in the inductor current;   In the Off-state, the main switch S is open and the only path offered to inductor current is through the fly back diode D, the capacitor C and the load R. This results in transferring the energy accumulated during the On-state into the capacitor.       
 
         [0009]    Feedback and control circuitry can be deployed with the circuit to regulate the energy transfer and maintain a constant output within normal operating conditions. 
         [0010]    From a power management standpoint, one of the critical blocks that hinder the complete integration of switching DC-DC converters is the frequency compensation circuit, whose design is based on the values of off-chip LC filter components. Since these LC filter values vary, because of various design requirements, manufacturer tolerances, and/or parameter drifts, integration of a compensation circuit implies a non-optimal control design and a lower bandwidth solution. 
         [0011]    The direct impact of a non-optimal compensation circuit is reflected in the transient response performance of the regulator, which is critical for voltage accuracy and stability in portable applications when driving switching loads. The poor transient response can be offset by increasing the size of the output capacitor, requiring more PCB real estate and cost. 
         [0012]    It is a challenge for engineers to design the higher bandwidth control loop without causing instability program. 
         [0013]    There are known patents or patent publications dealing boost converters: 
         [0014]    U.S. Patent Publication (US 2009/0001943 to Szlezak et al.) discloses a boost converter circuit that includes a power supply, an inductor coupled to the power supply to receive current from the power supply, a diode coupled to receive current from the inductor and coupled to provide current to a load as an output, an inductor switch coupled to a node between the inductor and the diode for selectively switching current from the inductor anyway from the diode, and a ramp circuit. The ramp circuit is coupled to the node between the inductor and the diode, and is configured to selectively sample a voltage at the node between the inductor and the diode via a sampling switch and use the sampled signal to produce a stabilization ramp to stabilize the output. 
         [0015]    U.S. patent (U.S. Pat. No. 7,528,590 to Wei) proposes a DC-to-DC boost converter circuit receiving a DC input voltage and converting it to a DC output voltage at a different voltage level than the DC input voltage. The DC-to-DC boost converter includes a switching power converter for receiving the input voltage on an input and converting the input voltage to an output as the DC output voltage in response to pulse control signals. A switching controller generates the pulse control signals during a switching cycle. The switching controller further includes pulse-skipping circuitry for generating a pulse width modulated signal to the switching power converter. A pulse width of the pulse width modulated signal is decreased responsive to a voltage level of an output voltage of the DC to DC boost converter being less than a control saw tooth waveform and the pulses width of the pulse width modulated signal is increased responsive to the voltage level of the output voltage of the DC to DC boost converter being greater than the control saw tooth waveform. 
         [0016]    U.S. patent application (U.S. Pat. No. 7,202,694409 to Eberlein) proposes circuits and methods to sense the current through a coil of an integrated switching converter, applicable to boost and to buck converters. The present invention uses a “replica biasing” technique to avoid a resistor for current measurement. The current through a pass device is mirrored into a replica, having a scale of n and being much smaller in size, of said pass device. The current through the replica is mirrored to another branch of the circuit and back again to achieve a fast stabilization of the current. The current through the replica is mirrored again to an output branch of the circuit, which conducts exactly a fraction 1/n of the current flowing through the pass device. The self-biasing current loop of the invention adapts quickly to the actual current level through the pass device of the switching converter. Accuracies better than 5% are achieved over a wide range of dynamic range. 
       SUMMARY 
       [0017]    A principal object of the present invention is to eliminate high frequency (sub-harmonic) oscillations of the output of boost converters. 
         [0018]    A further object of the invention is to increase a high control bandwidth without compromising stability 
         [0019]    A further object of the present invention is to keep the circuit design simple and at low cost. 
         [0020]    Moreover an object of the present invention is to achieve the required voltage regulation with a smaller output capacitor. 
         [0021]    In accordance with the objects of this invention, a method to achieve a boost converter having an improved stability has been disclosed. The method invented, comprises the steps of: (1) providing a boost converter, (2) deploying a sample and hold circuit connected to an output voltage of the boost converter, (3) using the sample and hold circuit output as feedback voltage, and (4) using a main switch drive signal to control the sample and hold circuit. 
         [0022]    In accordance with the objects of this invention a boost converter having an improved stability has been achieved. The boost converter invented comprises: an output capacitor, and a sample and hold circuit, controlled by a control signal, providing a feedback signal, wherein the sample and hold circuit is connected to an output of the boost converter, and wherein the circuit is enabled to hold an output voltage while a main switch of the boost converter is ON, wherein the feedback voltage is increased immediately after a duty cycle is changed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    In the accompanying drawings forming a material part of this description, there is shown: 
           [0024]      FIG. 1  prior art shows the basic principle of a Boost converter consisting of two distinct states 
           [0025]      FIG. 2  shows output voltage and feedback voltages of a boost converter with a Sample &amp; Hold circuit with a resistive divider. 
           [0026]      FIG. 3  shows output voltage and feedback voltages of a boost converter with a Sample &amp; Hold circuit without a resistive divider 
           [0027]      FIG. 4  shows a boost output stage and a feedback network with a Sample &amp; Hold circuit 
           [0028]      FIG. 5  illustrates the output and current waveform of a boost converter without a sample&amp;hold circuit with a duty cycle change event at four cycles. 
           [0029]      FIG. 6  shows the output and current waveform of the boost converter with sample &amp; hold circuit. 
           [0030]      FIG. 7  illustrates a flowchart of a method invented to provide a boost converter with improved stability. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    Methods and circuits to alleviate/remove the high frequency oscillation of pulse width modulation (PWM) controlled boost converters capable of a high frequency control loop are disclosed. In the preferred embodiments of the invention described sample &amp; hold circuits between the output and pulse-width modulated (PWM) control circuit of a boost converter solve the problem of high frequency oscillations. The most common control method for boost converters is pulse-width modulation (PWM). 
         [0032]    The output of the boost converter is sampled before a main switch of the boost converter is on, and holds its voltage during the time the boost main switch is on. 
         [0033]    Any analog sample &amp; hold circuit could be used with the present invention, but most simple and practical way to implement this is one switch (transistor) and one capacitor circuit. Two examples of a simple implementation of the present invention are shown in  FIG. 2  and  FIG. 3 . 
         [0034]      FIG. 2  shows output voltage and feedback voltages of a boost converter with a Sample &amp; Hold circuit with a resistive divider. In  FIG. 2 , the sample &amp; hold circuit  20  is inserted between after an output voltage resistive divider  21  and the output is used as a feedback signal for an internal control circuit (compensation circuit). If there is no resistive divider, the sample and hold circuit  20  is placed at the converter output shown in  FIG. 3 . If the internal control circuit input impedance is not high, a high speed and high input impedance analog buffer is needed between sample &amp; hold circuit and internal control circuit. 
         [0035]    The drive signal of the boost main switch can be utilized by changing its polarity as a control (timing) signal of this sample &amp; hold circuit. However, as long as we can achieve the above sample &amp; hold behavior, any timing signal can be used. 
         [0036]    In the PWM control loop of a boost converter, the converter tends to be unstable/causes oscillation when the control feedback loop has high bandwidth and has a certain open loop gain around the switching frequency. This makes a high-speed control loop design difficult, and sometimes a larger output capacitor has to be used to satisfy the required line &amp; load transient response. 
         [0037]      FIG. 4  shows a boost output stage and a feedback network with a sample &amp; hold network. Furthermore  FIG. 4  depicts an inductor L of the boost converter, an output capacitor  41  having an ESR  42 , an inverting diode  43 , an input diode  44 , an output diode  45 , a voltage divider  47 , and a switch  46 . In this preferred embodiment of the invention, the drive PWM signal for the boost converter is used after changing its polarity, to control the sample and hold circuit  40 . 
         [0038]      FIG. 5  shows a typical output voltage waveform of a boost converter without a sample &amp; hold circuit having a duty cycle change at the fourth cycle. First three cycles, the converter is in steady state. When the boost main switch is on, i.e. when the PWM signal shown in  FIG. 5  is high, inductor current is increased, and output voltage is decreased due to the nature of boost converter. Because of the equivalent series resistance (ESR) of the output capacitor, there is voltage jump when the main switch is on and off. This voltage gap is depends on the product of ESR and inductor current. 
         [0039]    At the fourth cycle, the duty cycle is changed from ˜25% to ˜50% in  FIG. 5 . As a result, the inductor current increases immediately after a change of duty ratio, however, the output voltage does not increase immediately. The average voltage is even decreased at the fourth cycle. This is so-called ‘right half plane zero’ effect, and causes instability. This instability can be explained as follows:
       a) The control circuit tries to increase the output voltage by some reason.   b) It generates wider duty ratio PWM signal than in previous cycle.   c) The output voltage is not increased for a few cycles   d) The control circuit tries to further increase the duty ratio (wider PWM signal) in order to increase the output voltage.   e) The output voltage is not increased again due to the duty cycle change   f) d)˜e) can be repeated (positive feedback) and cause large overshoot/instability       
 
         [0046]    Even to make matters worse, ordinary PWM control circuits control on time (high pulse) according to the output voltage. If the control circuit has high bandwidth, this on-time control is mainly dependent on the output voltage when the main switch is on. Therefore, the ‘right half plane zero’ effect is reinforced by the ESR of the output capacitor. 
         [0047]      FIG. 6  shows the waveform of the internal feedback voltage of a boost converter with a sample &amp; hold circuit  40  of the present invention. The dotted line shows the output voltage. The conditions are exactly same as in  FIG. 5 . The sample &amp; hold circuit  40  holds the voltage at the end of main switch off period and holds it when the main switch turns ON (PWM signal becomes high) and holds the voltage during the main switch is ON (PWM signal is high), so the feedback signal does not have a voltage jump due to the ESR  42  of the output capacitor  41 . Also, at cycle  4 , the feedback voltage is increased immediately after duty cycle change. Even the control circuit has high bandwidth, the positive feedback does not occur, and instability problem is solved. The objective of this sampling &amp; hold circuit  40 , shown in  FIG. 4 , is to remove the voltage jump from the feedback signal. In  FIG. 5 , there is no sampling &amp; hold circuit, so the output voltage becomes a feedback signal, and feeds into the control circuit. Therefore, the control circuit can see the voltage jump. However, in  FIG. 6  the sampling and hold circuit  40  holds the higher voltage when the main switch is on, and removes the voltage gap. This way, no gap signal becomes a feedback signal and feeds into the control circuit. 
         [0048]      FIG. 7  illustrates a flowchart of a method to provide a boost converter with an improved stability implemented utilizing a simple circuitry. 
         [0049]    Step  70  of the method of  FIG. 7  illustrates the provision of a PWM boost converter. Step  71  depicts deploying a sample and hold circuit connected to an output voltage of the boost converter. In a preferred embodiment of the invention the sample and hold circuit is optionally connected to the output of the boost converter via a resistive divider. Step  72  describes using the sample and hold circuit output as feedback voltage; and step  73  describes using a main switch drive signal to control the sample and hold circuit. 
         [0050]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.