Abstract:
A four-quadrant buck converter is described having a common leg of an inductor in series with an output capacitor, one power supply for providing a positive voltage output signal and negative voltage output signal to two solid-state switches joined at a common node, an output transformer whose primary is connected across the output capacitor and a pulse width modulated control circuit for operating the switches to produce a predetermined voltage across said output capacitor and for regulating the current out of the transformer. The control circuitry operates in response to a voltage signal from the output of the power supply, a voltage representative of the voltage at the output of the converter, a high frequency ramp voltage, an internal oscillator, and a voltage representative of the RMS current flowing on the secondary side of the output transformer. The converter incorporates overcurrent protection, an under-voltage lockout, overshoot protection, a slow start-up, inexpensive RMS conversion and other useful functions and capabilities.

Description:
TECHNICAL FIELD 
     This invention is related to the general subject of power supplies and, in particular, to the subject of switch-mode power converters. 
     BACKGROUND OF THE INVENTION 
     Part of the xerography copying process requires a high voltage AC power supply provided by a switch mode power converter. Typically, a high voltage quasi-square waveform is generated using push-pull circuitry and then filtered by an inductor-capacitor low pass filter network (i.e., 500 Hz); U.S. Pat. No. 4,714,978 is an example. The resultant waveform is a distorted sinusoid. Usually, the output frequency of the AC converter is limited to around 400 Hz, due to the inherent losses in the xerography process. A pure sinewave is preferred for low noise content. As the duty cycle of the quasi-square waveform is varied, the distorted sinusoid varies in amplitude; unfortunately, the distortion content also varies. The voltage amplitude is varied by control circuitry to keep a regulated output current. A regulated current is preferred to insure uniform copy quality. This is all the more desirable since current is affected by the age of the components, temperature conditions, dirt, etc. 
     One modern converter which operates over a 50 percent duty cycle is described in Diaz et al U.S. Pat. No. 4,717,994 (and assigned to the assignee of the present invention). The control and operation of conventional switched-mode power supplies is covered in the paper &#34;Conceptually New High-Frequency Switched-Mode Power Amplifier Technique Eliminates Current Ripple&#34;, by Cuk and Erickson, Proceedings of POWERCON FIVE, May 4-6, 1978. de Sartre U.S. Pat. Nos. 4,694,386 and Murakami et al U.S. Pat. No. 4,195,335  describe power supplies which provide automatic start-up. Hamilton et al U.S. Pat. No. 3,879,647 describes a converter having a soft start capability. Finally, Sutton U.S. Pat. No. 4,586,119, describes a switching mode power supply which employs current and voltage feedback and sensing. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a unique four-quadrant high voltage DC to AC buck converter is described which is not only suitable for use in xerography but also useful as a Class D amplifier in motor control and in audio amplifier applications. In one basic embodiment, the converter comprises: switching and commutation means for switching current to a common node from a DC power supply using two switches, two capacitors in series with each other and across the power supply, a series capacitor and inductor for joining the common node to the junction of the two capacitors, an output transformer in parallel with the series capacitor, and control means for operating the switching and commutation means to produce a predetermined voltage across the series capacitor. Preferably, the control means produces a pulse width modulated control signal, regulates the output current, is generally responsive to RMS current flow, has a wide ranging duty cycle, a slow start capability, and includes overcurrent protection, under-voltage lockout protection, and overshoot protection on start-up. 
     Accordingly, one object of the present invention is to provide a high voltage AC power supply or converter which maintains a relatively constant current output and a uniform sinusoidal waveform over prolonged periods and under differing machine operating conditions. 
     Another object of the invention is to provide a converter which is lower in cost and does not make use of components that require large operating margins, breakdown potentials, or ratings. 
     Still another object of the present invention is to provide a converter that does not require expensive circuits to convert instantaneous current values to RMS equivalents. 
     Yet another object of the present invention is to provide a converter which includes pulse width modulation control combined with overcurrent protection, undervoltage lockout protection, and overshoot protection on start-up. 
     Another object of the present invention is to provide a converter with a wide ranging duty cycle and a slow start capability. 
     Finally, it is an object of the present invention to provide a unique four-quadrant buck converter that is adapted to pulse width modulation control. 
     Other features and advantages of the invention will become clear from the following detailed description, the accompanying drawings, and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of the power converter that is the subject of the present invention; 
     FIG. 2 is a representation of a sinusoidal waveform of the output of the converter of FIG. 1, and the pulse train used to produce it; 
     FIG. 3 is a representation of the frequency performance of the converter of FIG. 1; 
     FIG. 4 is a simplified schematic diagram of the power stage of the four-quadrant buck converter of FIG. 1; 
     FIG. 5 is a detailed schematic diagram of the converter of FIG. 1, and the associated control circuitry; 
     FIG. 6 is a schematic diagram of the Current Regulator, Oscillator, and Band Pass Filter; 
     FIG. 7 is a schematic diagram of the Gate Drive; 
     FIGS. 8, 8A, 8B, 9A and 9B depict the operation of the Gate Drive of FIG. 7 in response to changes in duty cycle; 
     FIG. 10 is a schematic diagram of the Overcurrent Protection section of the converter; and 
     FIG. 11 is a schematic diagram illustrating the operation of the Under Voltage Lockout section of the converter of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     While this invention is susceptible of embodiment in many different forms, there is shown in the drawings, and will herein be described in detail, one specific embodiment of the invention having several specific features. It should be understood, however, that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated and described. 
     Overview 
     FIG. 1 shows a block diagram of the DC to high voltage AC converter 20 that is the subject of the present invention. The power stage 22 is a four quadrant switching amplifier. The output of the power stage is stepped up by the output transformer &#34;T&#34; to the desired magnitude. The converter 20 employs a PWM Controller 24 having three feedback loops. One loop, the Current Loop, senses the output current and modulates the amplitude of a low frequency Oscillator 26; accordingly, this loop maintains a constant output current. A second loop, the Voltage Loop 28, senses the voltage waveform at the primary of the output transformer &#34;T&#34;. This loop maintains the input voltage waveform a pure sinusoid at all times. The third loop (inside block 22) makes it possible to have a two transistor (or any comparable electronic switch) four-quadrant power stage running off a single DC input power supply. The operation of this third loop will be explained later. 
     The output of the low frequency Oscillator 26 is pulse width modulated (See. FIG. 2.) at a much higher frequency by the PWM Controller 24. The pulse width contains both frequency and amplitude information. The high frequency pulses are then fed to the power stage 22 for power amplification. Demodulation is done by an averaging L-C filter (See FIG. 1) with a resonant frequency between the PWM frequency and the sinewave oscillator frequency. Averaging the high frequency pulses extracts the encoded sinewave while attenuating the high frequency pulses (See. FIG. 3). 
     FIG. 4 shows a simplified circuit diagram of the Four-Quadrant Power Stage 22. Its performance is that of two back-to-back buck converters joined together with the output filters combined, such that the output AC waveform appears across the capacitor C. The internal drain to source diode in each FET is used as the commutation diode. It requires positive and negative input voltages to operate. This converter can therefore be used as highly efficient AC power amplifier. Since converter stability is important when designing switching power amplifiers, feedback is used to compensate for any distortion due to power stage non-linearity and other variations, such as load and input voltage changes. 
     Power Stage 
     Turning to FIG. 5, the Power Stage 22 comprises of a buck type four-quadrant converter running off a single DC input source. This is made possible by using a unique feedback loop. FIG. 5 shows the circuitry. Two capacitors C1 and C2 divide the input voltage essentially in half. This half voltage point Vl, is taken as a &#34;ground&#34;; solid-state switches Q1, Q2, inductor L and capacitor C form a four-quadrant buck converter. The output of converter Vo appears across capacitor C. Note that the output Vo equals Vq times the duty cycle D or (Vq * D) minus Vl. Voltage point Vl is not low enough in impedance to handle much power, and will easily move up or down. This problem is solved by adding a feedback loop to keep Vl constant at all times. Amplifier A2 compares Vl to 1/2 Vin; if different, an error voltage is fed into the PWM control circuitry 24 which will bring Vl to exactly 1/2 Vin. Capacitors C1 and C2 should be chosen large enough such that, while the loop is responding, the capacitors will keep Vl from moving much. Thus, Vl will have a ripple which depends on the loop response time and the size of capacitors C1 and C2. 
     Transistors Q1 and Q2 are driven from a common gate drive transformer Td. When switch Q1 is &#34;on&#34;, switch Q2 is &#34;off&#34; and vice versa. Current in switches Q1 and Q2 will flow from drain to source, as well as from source to drain (i.e., internal diode). Thus, the internal source-drain diode must provided for fast recovery. Most new FETs now have fast recovery diodes. In addition, when one source-drain diode is conducing and the opposite transistor turns &#34;on&#34;, that source-drain diode will be turned &#34;off&#34; forcefully. Here a failure known as &#34;commutating failure&#34;, found in motor drives, can occur. Some new FETs have a &#34;source-drain diode commutating safe operating area&#34; specified (i.e., Motorola&#39;s MTP-3055D). Other manufacturers (i.e., Fairchild) are expected to have similar devices available with guaranteed safe commutating areas. 
     PWM Pulse Width Modulator 
     A pulse width modulator (PWM) is formed by amplifier A1 and comparator Com1. A 400 Hz input signal Vi is fed via a capacitor C4 into the non-inverting input of A1, with Vl used as a reference. Vi is compared to the output voltage Vo which appears across C (R4 and R5 provide proper scaling), and an error voltage appears at the output of A1. Comparator Com1 compares Ve to a high frequency (i.e., 100 KHz) ramp and outputs a pulse train whose pulse width is proportional to Ve, and thus Vi. The ramp sets the operating frequency. Its amplitude is set from 0 volts to about 5 percent above Vr. (See top of FIG. 10). Transistor Q3 (2N4401) clamps Ve to Vr; thus, the maximum pulse width is limited to approximately 95 percent. Q3 circuitry (i.e., R8 and R9) also limits minimum Ve to approximately 5 percent of Vr, such that the minimum duty cycle is limited to approximately 5 percent. 
     The high frequency pulses are amplified by switches Q1 and Q2, and demodulated by filter L and C, as explained before. An amplified Vi signal appears across C and the output transformer To steps it up. 
     The output transformer To cannot tolerate any DC voltage. For this reason the reference voltage for the PWM controller (i.e., amplifier A1) is chosen as Vl (via R6). In the absence of any input signal (i.e., Vi=0), amplifier A1 generates an error voltage if there is any difference between Vl and Vo. Since at DC, amplifier A1 has high gain, any DC voltage across C will generate a large error signal Ve and any DC voltage across C will be minimized. 
     Amplifier A2 adds a biasing factor to amplifier A1 reference (via resistor R3), only if Vl drifts away from 1/2 Vin. For Vin=0, the end result is that the voltage across C is zero and Vl equals 1/2 Vin; this corresponds to a Duty Cycle of 50 percent at the drain (i.e., Vq) of Q2. Since Vo is the average of Vq, we have that Vo=1/2 Vin which equals Vl; this is the loop equilibrium point. C3 and R7 provide compensation for optimum response. R2 and C2 slow the response of amplifier A2, such that amplifier A1 responds faster, and the effect of amplifier A2 is seen as a biasing effect only. 
     Oscillator--Variable Amplitude, Fixed Frequency 
     FIG. 6 shows the oscillator section and the Current Control Loop. The Oscillator 26 (See FIG. 1) consists of a Squarewave Oscillator 28 feeding into a 400 Hz Bandpass Filter 30. The Bandpass Filter 30 passes only the fundamental frequency and the output is a 400 Hz sinewave. The Squarewave Oscillator 28 uses an amplitude signal provided by a Peak and Averaging Circuit 32. 
     Comparator Com2 is the heart of the Squarewave Oscillator 28. Assume initially that C9 has no charge. The inverting input of the comparator Com 2 is low and R15 will take the comparator output up to Va, if R15 is much smaller than R16 and R19. The voltage at the non-inverting input will be 2/3 of Va, since R19 equals R18 and R17, and since R19 and R17 are practically in parallel. Capacitor C9 will charge via R16 until voltage at C9 reaches 2/3 of Va. At this time, comparator Com2 will switch states. Its output will now be low and R19 will be in parallel with R18, dropping the non-inverting input voltage to 1/3 Va. Now, R16 will discharge C9 until its voltage reaches 1/3 Va. Afterwards, the cycle starts over (see the waveforms at the lower left corner of FIG. 6). The voltage at C9 will oscillate between 1/3 and 2/3 of Va. Thus, the comparator output Vco will be a squarewave of amplitude Va. Its frequency will be determined by R16 and C9 (if R16 is much greater than R15), and will be independent of Va, since the comparator Com2 always switches when voltage at C9 reaches 1/3 Va and 2/3 Va. 
     The Bandpass Filter 30 consists of a standard second order bandpass filter with the following parameters: 400 Hz center frequency, unity Bandpass gain and a 60 Hz 3 dB band width. The Bandpass Filter 30 only passes the fundamental of the squarewave and outputs a sinewave at 400 Hz. 
     The amplitude of the sinewave is varied to keep the converter-output current I out  constant as the load or line changes. This is done with a current loop which controls the voltage Va by operating FET Q4 in the linear region. Since the output current is AC, it first needs to be converted to DC. A RMS to DC converter is preferable, but the cost is relatively high. Using the fact that the RMS value of a rectified AC waveform is somewhere between its average and its peak, a combination of averaging and peak detection can be used when rectifying the output. The output current is sensed by R10. Diodes D1 and D2 established a rectifying doubling circuit. Resistors R11 and R12, and capacitors C5 and C6 provide the proper peak-averaging combination. The voltage Vf at capacitor C6 is a DC equivalent to the RMS value of the output current and is representative of it. Amplifier A3 is the current loop error amplifier. Vf is fed into its inverting input and a reference, set by R14, is fed into its non-inverting input. The output of A3 controls Q4, a FET operated as a variable resistor; therefore, Va is controlled by Amplifier A3. If the load or line changes, A3 will change Va which changes the sinewave oscillator amplitude, which in turn changes the output voltage amplitude and, thus, regulates the output current. Thus, the output current is kept constant (at essentially the RMS value). Potentiometer R14 controls the current set point. R13 and C7 provide proper compensation. 
     Gate Drive 
     The Gate Drive for switches Q1 and Q2 must satisfy many requirements. First, it should be low cost. Secondly, it must also prevent switches Q1 and Q2 from conducting at the same time, since they are connected across Vin and simultaneous conduction could be catastrophic. Finally, the duty cycle of each switch should cover a wide range (i.e., from 5 percent to 95 percent). These requirements present a difficult design problem when using a transformer coupled drive. 
     FIG. 7 shows the Gate Drive used. To solve the problem of simultaneous conduction, which can occur when one FET is being turned &#34;on&#34; and the other is turned &#34;off&#34;, a delaying inductor L1 and L2 is added in series with the gate drive circuit. A diode D3 or D4 bypasses the delaying inductor L1 or L2, so that at turn &#34;off&#34; there is no delay. This allows the primary N1 of the drive transformer Td to be driven from a simple &#34;totem pole circuit&#34; (i.e., transistors Q5 and Q6). Its operation will now be described. 
     Assume that Q5 is &#34;on&#34;. This means the &#34;dots&#34; which mark the windings of Td are positive, and Q1 is &#34;off&#34; and Q2 is &#34;on&#34;. When Q6 turns &#34;on&#34; the voltage at the Td windings reverses. Q2 is turned &#34;off&#34; immediately, since diode D4 bypasses inductor L2. Q1 is not turned &#34;on&#34; immediately; inductor L1 will delay the gate drive voltage until it saturates, thus delaying Q1 turn-on until Q2 is completely &#34;off&#34;. This delay is in the order of 50 nanoseconds only. Thus, the inductors L1 and L2 need only withstand 50 nanoseconds at 10 volts or 500 nano volt-seconds. Using the equation: ##EQU1## the core area and turns can be found, where: dV=volts 
     dT=seconds 
     A=core area 
     N=turns. 
     The design problem of providing for a very wide duty cycle range will be explained with the aid of FIG. 8. The gate voltage Vg will vary its positive amplitude as a function of duty cycle. Because any transformer must be volt-second balanced, at low duty cycle (i.e., see FIG. 8A), Vg will be 9  volts high, providing good gate drive. But at a 90 percent duty cycle (see FIG. 8B), the gate drive will only be 1 volt, and the FET will never turn on- 
     Referring back to FIG. 7, this problem is solved by providing a level shift as a function of duty cycle (i.e., capacitors C12 and C13, and zener diodes Z1 and Z2). First assume a 90 percent duty cycle (i.e., FIG. 8B) at the gate drive of Q2. When Vg is negative, the diode Z2 will conduct and C13 will charge negatively to 8.3 volts. When Vg switches positive (i.e., 1 volt), the 8.3 volts at C3 will add to the 1 volt providing a 9.3 volt gate drive, which is sufficient for turn-on. On the other hand, Q1 will have a 10 percent duty cycle gate drive. When Vg is negative, the diode Z1 will charge C12 to 0.3 volts. When Vg is positive, the 0.3 volts will add to the 9 volts providing a 9.3 volt gate drive Vg&#39;. The end result is that no matter what the duty cycle is, the gate drive voltage will be constant at 9.3 volts (See FIGS. 9A and 9B). 
     Capacitor C11 blocks the DC preventing the transformer from saturating. The base of transistor Q15 is connected directly to the comparator Com1 output of the PWM (See FIG. 5). 
     Overcurrent Protection 
     Returning to FIG. 5, if the output of the output transformer To is shorted, the associated capacitor C will also be shorted, and the PWM control circuitry will &#34;see&#34; no output voltage. Therefore, the PWM control circuitry will attempt to compensate for this by going to either minimum or maximum duty cycle. The inductor L will then saturate after several switching cycles, inducing high currents in Q1 and Q2. Thus, over current protection is needed. 
     FIG. 10 shows the Overcurrent Protection Circuitry. Resistor Rs senses (See FIG. 5) the current at the ground leg of capacitor C2. Sensing it here has two advantages. The sensed voltage is referenced to ground and the sensed current is approximately equal to 1/2 the current through L resulting in lower losses. The voltage developed at Rs is filtered by resistor R24 and capacitor C15; this eliminates high frequency noise spikes. The sensed voltage Vs, which proportional to the inductor current Is, is then fed to the base-emitter junctions of transistors Q8 or Q14. If the sensed voltage exceeds approximately 0.6 volts, Q8 or Q14 will turn &#34;on&#34;. This triggers comparator Com3 which is configured as a monostable. If Vs is positive, Q8 will turn &#34;on&#34;; if Vs is negative the Q14 will turn &#34;on&#34;. Thus, the inductor current is sensed in either direction. A diode D21 in series with Q14 collector prevents Q14 collector from going negative once it turns &#34;on&#34;. The monostable is achieved by using positive feedback. The inverting input of Com3 is normally higher than the non-inverting input; therefore, the comparator output is normally &#34;low&#34;. When Q8 or Q14 turns &#34;on&#34;, the inverting input is pulled low causing the comparator output to switch &#34;high&#34;. C16 then pulls the non-inverting input higher than Vr, for a time determined by the values of resistor R25 and capacitor C16; this sets the monostable duration. A diode D22 in parallel with resistor R25 quickly charges C16 back to 1/2 Vr, so it is ready for the next trigger pulse. 
     The output of the monostable Com3 drives transistors Q10 and Q11, and FET Q4 (see FIG. 6) which are used to disable other circuits and thereby achieve overcurrent protection: 
     1. The output of the Squarewave Oscillator (Com2 in FIG. 6) is disabled by Q10; 
     2. Main FET Q2 is turned &#34;off&#34;; Q11 shorts its gate to ground (see FIG. 7). 
     3. Diode D3 and R27 charge capacitor C6 (see FIG. 6) providing a &#34;false&#34; current feedback voltage Vf, such that the Squarewave Oscillator input voltage Va (via A3 and Q4) will drop to &#34;O&#34;, and during restart it will ramp up slowly. 
     4. Q15 disables the gate drive to Q1 and Q2 by disabling power to transistors Q5 and Q6 of FIG. 7. Refer to the description of the Undervoltage Lockout circuit (FIG. 11) which is discussed below. 
     Undervoltage Lockout 
     In one specific application of the invention, the power supply has provision for a safety input signal called &#34;INTERLOCK&#34;. When this input is low, the power supply is disabled. When it is at 24 volts, it enables the supply. This INTERLOCK input is connected, as shown on FIG. 11, to a transistor Q12 to provide the power for the gate drive Vd. With the INTERLOCK input low, Vd is at zero volts and the gate drive looses power and the supply shuts down. There is one problem; as Vd is rising, the gate drive voltage may be insufficient, causing poor gate drive. 
     Therefore, the gate drive should be disabled until Vd is high and stable. This is done as follows: Zener diode Z3 keeps transistor Q14 &#34;off&#34;, until Vd is greater than 18 volts. When Q14 turns &#34;on&#34;, Q13 is turned &#34;on&#34; and Q13 collector is pulled up to Vd. Resistor Rb provides hysteresis by providing more Q14 base drive, preventing any oscillation. Q13 then supplies base drive to Q15, as well as Q5 and Q6, enabling the gate drive. 
     Resistors R30 and R31 precharge capacitor C11 to 1/2 Vin. To see why this is needed, suppose that C11 is fully discharged, and Q5 and Q6 start switching at 50 percent duty cycle. Eventually, C11 will charge to 1/2 Vin and the voltage at the primary winding N1 of the drive transformer Td will be an AC squarewave. But, while C11 is charging, the voltage at N1 will be unbalanced, being more positive than negative. This causes the gate drive (at switches Q1 and Q2) to be unbalanced also, and it is possible to have both switches Q1 and Q2 &#34;on&#34; at the same time. Precharging C11, before the gate drive is enabled, will prevent this problem. Diodes D7 and D8 prevent C11 from discharging when Vg is low. Note that the capacitor precharge level must be related to the initial duty cycle (i.e., 50 percent duty cycle, 50 percent precharge), to prevent initial volt seconds imbalance at Td, which brings us to the next protection circuit. 
     Slow Start--50 Percent Initial Duty Cycle 
     As was mentioned before, the 50 percent duty cycle operation corresponds to no pulse width modulation for a four-quadrant switching amplifier. So, ideally, the initial duty cycle should be 50 percent and then increase or decrease according to the input signal. 
     FIG. 5 shows a circuit that provides 50 percent initial duty cycle. Vl is set higher than 1/2 Vin by having resistor R33 about 20 percent higher than R32. With Vl higher than 1/2 Vin, the output of amplifier A2 will be &#34;low&#34;, causing the output of amplifier A1 also to go &#34;low&#34;. Q3 will be &#34;off&#34; and the duty cycle will be minimum. Because Vl is unbalanced (i.e., greater than 1/2 Vin) every time at start-up, the duty cycle will be minimum. 
     The voltage at the emitter of transistor Q16 is set by R34 and R35; therefore, the error voltage Ve (via diode D9) is clamped to approximately 1/2 Vr which forces the initial duty cycle to equal 50 percent. As the power supply is turned &#34;on&#34;, Q16 is turned &#34;off&#34; (i.e., its base grounded) through a connection (via diode D) to the undervoltage lockout circuit previously described (i.e., Q14 collector in FIG. 11). Thereafter, capacitor C18 will slowly charge to Vr via R36. This lets Ve slowly go &#34;low&#34;; thus, the duty cycle is slowly decreased until Vl equals 1/2 Vin at which time the voltage loop is closed. 
     Apart from the initial duty cycle having to be matched to the gate drive capacitor C11 (see FIG. 11) voltage precharge, 50 percent initial duty cycle prevents output overshoot at turn-on. Suppose Vl is more than 1/2 Vin (even 0.01 volts-), and suppose the slow start circuit is not present; the output of A2 will be &#34;low&#34;, the output of A1 will be &#34;low&#34;, and error voltage Ve will also be &#34;low&#34;. The initial duty cycle will be minimum, about 5%. Transistor switch Q2 will be &#34;on&#34; most of the time; since the PWM voltage loop has a finite response time, many high frequency switching cycles will pass before the voltage loop is closed. With Q2 mostly &#34;on&#34;, L and the primary winding of To will see a DC voltage approximately equal to 1/2 Vin. The output transformer To will then couple this voltage to the output, until it saturates. Thus, at the output we would have a large transient at turn-on. Inductor L will also saturate endangering Q1 and Q2. As Vl is brought equal to 1/2 Vin, the voltage loop will close and duty cycle will reach 50%. By contrast with the slow start circuit in place, the loop starts at 50 percent (not at some significantly lower value), decreases some to set Vl, equal to 1/2 Vin, and returns to 50 percent closed loop equilibrium. 
     Capacitor C18 and resistor R36 are chosen large enough, such that the duty cycle lowering is slower than the loop response time, preventing the inductor L and the output transformer To from saturating. The output voltage overshoot at turn-on is also reduced by an order of magnitude. 
     Ramp Generator 
     The ramp generator is diagramed in the lower right corner of FIG. 5. Assume C20 is initially discharged, the non-inverting input to comparator Com4 is &#34;low&#34;, and the inverting input is at reference voltage Vr. Therefore, the Com4 output is &#34;low&#34; and transistor Q18 is &#34;off&#34;. Capacitor C20 then charges through resistor R39. Vrr is chosen much higher than Vr, so that the C20 charging current is relatively constant and the voltage at C20 increases linearly. When the voltage at the non-inverting input of Com4 reaches Vr, the comparator switches &#34;high&#34; and Q18 discharges C20 completely. Resistors R37 and R38 are chosen, such that the peak voltage at C20 is approximately 5 percent higher than Vr. Having ramp peak voltage higher than Vr limits the maximum duty cycle of the PWM control circuitry (here that limit is approximately 95 percent). The ramp frequency is set by the values of capacitor C20 and resistor R39. 
     CONCLUSION 
     From the foregoing description, it will be appreciated that the invention represents a significant improvement in cost reduction and performance. It is powered by a single DC voltage, thus directly replacing a push-pull type converter. Its output voltage is essentially a non-distorted sinewave at any amplitude. Moreover, by reducing in size the low pass L-C filter, the overall cost is reduced by an order of magnitude. In addition, the electronic power switches Q1 and Q2 require a voltage rating five times lower than an equivalent push-pull type converter, thereby further reducing cost. 
     From the foregoing description, it will also be observed that simple variations and modifications may be effected without departing from the true spirit and scope of the novel concepts embodied in the present invention. For example, those skilled in the art will know and understand that the heart of the converter is basically a Class D amplifier. Moreover, there are many other applications, as a motor control and as a very efficient Audio Amplifier. Thus, it should be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.