ELECTRICAL POWER CONVERTER WITH TRAPEZOIDAL OUTPUT
WAVEFORM
FIELD OF THE INVENTION
The present invention relates in general to an electrical power converter with trapezoidal output voltage waveform.
BACKGROUND OF THE INVENTION
An electrical power converter (EPC) is a device normally used for converting direct-current (DC) power to alternating-current (AC) power.
EPCs generating a sinusoidal output waveform require components with higher rating since sinusoidal output waveforms result in relatively sharp peaks near the center of each voltage half cycle in comparison with square waveforms for the same voltage and load. Further the DC source used in many EPCs is a battery. The peaks associated with sinusoidal waveform generation lead to inefficient battery utilization.
On the other hand if the output voltage has a rectangular or trapezoidal waveform, the output current and battery current waveforms approximate these voltage waveforms for resistive loads. Equipments generating rectangular or trapezoidal waveform normally have components with relatively lower ratings in comparison to equipment used with sinusoidal waveforms.
Rectangular output waveforms too, as is known to persons skilled in the art, are not preferred since EPCs generating rectangular waveform have a high leading current spike at the beginning of each half cycle. The high current spike generates noise which interferes with the normal operation of devices with capacitive and rectifier-capacitor loads such as computers or computer monitors.
Converters with trapezoidal output waveforms are therefore preferred for most loads. Fig. 1 describes a typical cycle of an AC trapezoidal voltage waveform. The trapezoidal waveform comprises:
(i) a Crest 101 having crest time to and absolute crest voltage peak value Vpk;
(ii) a Rising Edge 105 having a rise time tr;

(iii) a Falling Edge 107 having a fall time tf;
(iv) an Off Time t0ff;
(v) Half Cycle Time, T=to +tr + tf + W;
(vi) output RMS voltage value VRMS which is the Root Mean Square Voltage of the trapezoidal voltage waveform for the Full Cycle Time 2T. VRMS as would be evident to a person skilled in the art is a function of VPk, to, tr, tf and toff for the Full Cycle Time.
The above waveform will repeat itself every Full Cycle Time 2T.
While Fig. 1 describes the cycle of an AC trapezoidal voltage waveform, as would be evident to a person skilled in the art, a DC trapezoidal voltage waveform will have the same shape as the AC waveform for time T and will thereafter repeat itself every Half Cycle Time T.
Prior art patents describe EPCs that produce trapezoidal waveform. US patent number 4,716,509 describes a DC to DC converter comprising a Pulse Width Modulator (PWM) which has a trapezoidal output wave form shape. The EPCs of this patent however would not be suitable for use for reactive power loads such as Computers, Monitors, AC Fan, AC Motors etc.
US patent number 5,657,220 describes an EPC having a trapezoidal output voltage wave form shape which can be used with reactive loads. The patent describes an EPC which returns energy to the DC power source through flyback action of a transformer. The embodiments taught by this patent use complex electric circuit which increases initial capital and subsequent maintenance costs. The embodiments further are likely to be energy inefficient because of a large number of components. Further in the embodiments of this patent the DC power source is not isolated from the load since the AC output voltage of the EPC is electrically coupled to the DC power source. Faults or disturbances in the load would thus communicate to the DC power source and could damage it. There is also risk of electrical shock.
In real life applications the Load on the EPC tends to vary. (The term "Load" refers to one more devices drawing power from the EPC. The Load may be fully or partly resistive

or reactive.) It is however important, for the proper operation of the Load, that the VRMS at the Load be substantially constant.
There is thus a need for an EPC with trapezoidal output wave form shape having simple electrical circuit and wherein the DC power source and load are isolated from each other. Further there is also need for an EPC which dynamically responds to changes in load.
SUMMARY OF THE INVENTION
An object of the present invention is an EPC with trapezoidal output wave form shape having simple electrical circuit. Another object of the present invention is an EPC wherein the DC power source is isolated from the Load. Yet another objective of the present invention is an EPC which dynamically responds to changes in load.
An object of the present invention relates to a method of voltage regulation for a trapezoidal voltage waveform. Another objective of the present invention is maintaining substantially constant VRMS by dynamically varying the crest time to and Off Time W
These and other objects of the present invention will become apparent to those skilled in the art from the following summary of the invention, the detailed description of the invention and preferred embodiments and the accompanying drawings. The preferred embodiments and accompanying drawings are intended to describe particular instances of the invention and the invention is not limited to any of the preferred embodiments or accompanying drawings. As would be obvious to a person skilled in the art, the inventions can be embodied in other ways.
The EPC of this invention comprises:
a) A DC power source;
b) A Converter comprising a Power Conversion Controller;
c) A Clamp Switch;
d) A Bulk Capacitor;
e) A Bus;
f) An Inverter comprising a Floating PWM Controller;

g) A Feedback Circuit;
The EPC during operation is connected to a Load.
In EPC operation the Converter receives power from the DC power source and generates a substantially trapezoidal DC output waveform with RMS voltage which is substantially VRMS- The DC output waveform is achieved by suitably programming, as would be evident to a person skilled in the art, the Power Conversion Controller and Floating PWM Controller.
The VPk the rise time tr, fall time tf; and Half Cycle Time T of the trapezoidal output waveform are determined, as would be evident to a person skilled in the art, with reference to the total harmonic distortion (THD) permitted for the Load.
The Clamp Switch is connected in series with the Bulk Capacitor. When the Clamp Switch is in ON state the Bulk Capacitor discharges through the Clamp Switch to the Bus. The Clamp Switch is in ON state after time tr and retained in ON state till time tf. At all other times the switch is kept in OFF state. When the Clamp Switch is in OFF state the Bulk Capacitor cannot discharge.
The Inverter receives the substantially trapezoidal DC output waveform through the Bus and generates a substantially trapezoidal AC output waveform with RMS voltage which is substantially VRMS- This AC output waveform is also referred to as the output waveform of the EPC and the associated output voltage is also referred to as the output voltage of the EPC.
The Feedback Circuit dynamically senses the trapezoidal AC output waveform of the EPC and adjusts the Crest Time (and correspondingly the Off Time toff of the trapezoidal waveform while keeping Half Cycle Time T substantially constant) such that VRMs remains substantially constant.
In another embodiment of the EPC, the DC power source of the above embodiment is a battery.
The Converter of the above embodiment of the EPC, as would be evident to a person skilled in the art, can be either of the Push Pull type, Full Bridge type, Half Bridge type, Full Bridge Resonant type or Half Bridge Resonant type.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 describes a full wave trapezoidal output waveform.
Fig. 2 is a circuit diagram of the first embodiment of the present invention.
Fig. 3A and Fig. 3B describes voltage waveform of results at primary windings of a transformer of the first embodiment of the present invention.
Fig. 3C describes output voltage waveform at secondary winding of the transformer of the first embodiment of the present invention.
Fig. 3D describes the DC output voltage waveform before LC filter of the first embodiment of the present invention.
Fig. 3E describes the filtered DC output voltage waveform at Bus of the present invention.
Fig. 4A, Fig. 4B, Fig. 4C, and Fig. 4D describe the switching states of the four switches of Inverter of the present invention.
Fig. 4E describes the filtered output AC waveform of the present invention.
Fig. 5 is a circuit diagram of the second embodiment of the present invention.
Fig. 6 is a circuit diagram of the third embodiment of the present invention.
Fig. 7 is a circuit diagram of the fourth embodiment of the present invention.
Fig. 8 is a circuit diagram of the fifth embodiment of the present invention.
Fig. 9 is a circuit diagram of the sixth embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
This invention will further be described with reference to the embodiments hereinafter provided. The embodiments are by way of example only and are the invention can be embodied in many different ways as would be evident to a person skilled in the art.

Embodiment 1
An EPC according to the present invention comprising a Push-PuU Converter is shown in Fig. 2. The EPC of this embodiment comprises:
a) DC Power Source 1 with positive terminal 11 and negative terminal 12. The DC Power Source can optionally be a battery.
b) Converter 2, comprising:
(ii) Power Conversion Controller 25;
(iii) Transformer 21 with two primary windings 201 and 203 respectively, a center-tap 209 and a secondary winding 205;
(iv) Two switches 211 and 213. The switches can operate in the high frequency range;
(v) Rectifier diodes 221,223,225 and 227;
(vi) LC filter 23 comprising capacitor 231 and inductor 233;
(vii) Bleeder Resistance 235;
c) Bus 100;
d) Clamp Switch 251 comprising an anti parallel diode 253 which may optionally be a body diode and a switch 255;
e) Bulk Capacitor 271 having capacitance preferably in microfarads.
f) Inverter 3 comprising:
i) Floating PWM Controller 301; ii) four switches 311,313,315 and 317; iii) an optional LC filter 33;
g) Feedback circuit 4
A Load 5 can electrically be connected to the EPC as needed.
In EPC operation, switches 211 and 213, alternately couple DC Power Source 1 to primary windings 201 and 203 of Transformer 21 such that the output voltage waveform

of Fig. 3A and Fig. 3B results at the primary windings 201 and 203 respectively. This is achieved by suitably programming the Power Conversion Controller 25 and Floating PWM Controller 301 as hereinafter described.
The Power Conversion Controller 25 can be either a standard power conversion controller or a pre-programmed micro-controller controlled device. The Power Conversion Controller 25 receives instructions from the Floating PWM Controller 301 and varies crest time t0 in accordance with the instructions.
The Floating PWM Controller 301 can be a pre-programmed micro-controller controlled device.
The Floating PWM Controller 301 receives feedback of trapezoidal AC output waveform through Feedback Circuit 4. The Floating PWM Controller 301 instructs Power Conversion Controller 25 to increase or decrease, as would be evident to a person skilled in the art, the time period to while maintaining Half Cycle Time T substantially constant, so as to maintain the output voltage VRMS of the EPC substantially constant.
The AC output voltage waveform at secondary winding 205 is described in Fig. 3C.
The AC output voltage waveform of Fig. 3C, from secondary winding 205, is rectified using rectifier diodes 221, 223, 225 and 227 to give the DC output voltage waveform described in Fig. 3D. This rectification can be undertaken, as would be evident to a person skilled in the art, using any of the techniques of the prior art.
The DC output voltage waveform described in Fig. 3D is filtered using the LC filter 23. The LC filter filters out high frequency components. The capacitance of capacitor 231 should be significantly less than the capacitance of the bulk capacitor 271. The inductance of the inductor 233 is selected, as would be evident to a person skilled in the art, keeping in mind the anticipated high frequency components. The filtered voltage is described in Fig. 3E.
The Clamp Switch 251 is connected in series with the bulk capacitor 271. When the switch 255 of the Clamp Switch 251 is in ON state (i.e. the the Clamp Switch 251 is in ON state) the Bulk Capacitor 271 discharges through switch 255 to the Bus 100. The switch 255 is switched ON after time tr and retained in ON state till time tf. At other times

the switch 255 is kept OFF (i.e. the the Clamp Switch 251 is in OFF state). When the Clamp Switch is in OFF state the Bulk Capacitor cannot discharge. This ensures minimal distortion of the wave form in relation to the wave form of a resistive load (i.e. when the load current power factor is substantively unity).
The filtered voltage described in Fig. 3E passes to the Inverter 3.
The switching states of the four switches 311,313,315 and 317 of Inverter 3 is described in Fig. 4A, Fig. 4B, Fig. 4C, and Fig. 4D respectively.
The output voltage of Inverter 3 is described in Fig. 4E. The LC filter 33 is optional.
The Feedback Circuit 4 could be of analog or digital type as would be evident to a person skilled in the art. The Feedback Circuit 4 measures output voltage VRMS of the EPC and provides the measured value to the Floating PWM Controller 301 as described hereinbefore.
It would be noted that the Converter 2 of this embodiment comprises a push pull transformer. This invention can however be embodied using other Converters as would be hereinafter described.
It would be noted that in the subsequent embodiments only the Converter is different and other components and circuits remain the same.
It would also be noted that in the subsequent embodiments the voltage described in Fig. 3E will pass to the Inverter 3 from Converter 2.
Embodiment 2
The Converter 2 of the second embodiment of this invention comprises a Clamp Forward Converter as described in Fig. 5.
The Converter 2 comprises:
(i) Power Conversion controller 25 as in Embodiment 1;
(ii) Clamp forward type of transformer 50 with a primary winding 51 and a secondary winding 52;
(iii) two high frequency switches 250 and 253;

(iv) two diodes 251 and 252;
(v) rectifier 257;
(vi) LC filter 23 comprising capacitor 231 and inductor 233;
(vii) Bulk Capacitor 271;
(viii) Bleeder Resistance 235.
In EPC operation, switches 250 and 253 energize the primary winding 51 of Transformer 50 from DC Power Source 1 such that the output voltage waveform of Fig. 3E results at the Bus 100 which in then inverted by the Inverter 3. This is achieved by suitably programming the Power Conversion Controller 25 and Floating PWM Controller 301 as in the previous embodiment.
Embodiment 3
The Converter 2 of the fourth embodiment of this invention comprises a Half Bridge Converter in Fig. 6.
The Converter 2 comprises:
(i) Power conversion controller 25 as in Embodiment 1;
(ii) a Transformer 60 with a primary winding 61 and a secondary winding 62. The transformer is of the Half bridge type and the ratio of the primary to secondary windings is pre-determined in order to realise the desired output voltage;
(iii) two high frequency switches 260 and 261;
(iv) capacitor 262 and 263;
(v) rectifier diodes 221,223,225 and 227;
(vi) a LC filter 23 comprising capacitor 231 and inductor 233;
(vii) a Bulk Capacitor 271.
(viii) Bleeder Resistance 235.

In EPC operation, switches 260 and 261 alternately couple DC Power Source 1 to primary windings 61 of Transformer 70 such that the output voltage waveform of Fig. 3E results at the Bus 100 which in then inverted by the Inverter 3. This is achieved by suitably programming the Power Conversion Controller 25 and Floating PWM Controller 301 as in the previous embodiment
Embodiment 4
The Converter 2 of the third embodiment of this invention comprises a Full Bridge Converter as described in Fig. 7.
The Converter 2 comprises:
(i) Power Conversion Controller 25 as in Embodiment 1;
(ii) Transformer 70 with a primary winding 71 and a secondary winding 72. The transformer is of the Full bridge type and the ratio of the primary to secondary windings is pre-determined in order to realize the desired output voltage;
(iii) four high frequency switches 270,271,272 and 274;
(iv) rectifier diodes 221,223, 225 and 227;
(v) a LC filter 23 comprising capacitor 231 and inductor 233;
(vi) a clamp switch 251;
(vii) Bleeder Resistance 235.
In EPC operation, switches 270 and 272 ,271 and 274 alternately couple DC Power Source 1 to primary windings 71 of Transformer 70 such that the output voltage waveform of Fig. 3E results at the Bus 100 which in then inverted by the Inverter 3. This is achieved by suitably programming the Power Conversion Controller 25 and Floating PWM Controller 301 as in the previous embodiment.
Embodiment 5
The Converter 2 of the fifth embodiment of this invention comprises a Full Bridge Resonant Converter as described in Fig. 8.

The Full bridge Resonant Converter 2 comprises:
(i) Power conversion controller 25 as in Embodiment 1;
(ii) a Transformer 80 with a primary winding 81 and a secondary winding 82. The transformer is of the Full bridge Resonant type and the ratio of the primary to secondary windings is pre-determined in order to realise the desired output voltage.;
(iii) four high frequency switches 280,281,282 and 283;
(iv) resonant inductor 534 and resonant capacitor 535;
(v) rectifier diodes 221,223, 225 and 227;
(vi) a LC filter 23 comprising capacitor 231 and inductor 233;
(vii) a Bulk Capacitor 271.
(viii) Bleeder Resistance 235.
In EPC operation, switches 280 and 282 ,281 and 283 alternately couple DC Power Source 1 to primary windings 81 of Transformer 80 such that the output voltage waveform of Fig. 3E results at the Bus 100 which in then inverted by the Inverter 3. This is achieved by suitably programming the Power Conversion Controller 25 and Floating PWM Controller 301 as in the previous embodiment.
Embodiment 6
The Converter 2 of the sixth embodiment of this invention comprises a Half Bridge Converter as described in Fig. 9.
The Converter 2 comprises:
(i) Power conversion controller 25 as in Embodiment 1;
(ii) a Transformer 90 with a primary winding 91 and a secondary winding 92. The transformer is of the Half bridge Resonant type and the ratio of the primary to secondary windings is pre-determined in order to realise the desired output voltage.;
(iii) two high frequency switches 290 and 291;

(iv) resonant inductor 294 and resonant capacitor 295;
(v) rectifier diodes 221,223, 225 and 227;
(vi) a LC filter 23 comprising capacitor 231 and inductor 233;
(vii) a Bulk Capacitor 271.
(viii) Bleeder Resistance 235.
In EPC operation, switches 290 and 291 , alternately couple DC Power Source 1 to primary windings 91 of Transformer 90 such that the output voltage waveform of Fig. 3E results at the Bus 100 which in then inverted by the Inverter 3. This is achieved by suitably programming the Power Conversion Controller 25 and Floating PWM Controller 301 as in the previous embodiment.
Each of the above embodiments, as would be evident to a person skilled in the art, could be embodied in other ways. It is reiterated that the above embodiments are only by way of example and that the invention could be exemplified in many other ways obvious to a person skilled in the art.

We claim:
1. An Electric Power Converter (EPC) comprising:
a) A DC power source 1;
b) A Converter 2;
c) A Clamp switch 251;
d) A Bulk Capacitor 271;
e) A Bus 100;
f) An Inverter 3;
g) A Feed Back Circuit 4; wherein:

Converter 2 receives power from DC power source 1 and generates a substantially trapezoidal DC output waveform with RMS voltage which is substantially VRMS;
Inverter 3 receives the trapezoidal DC output waveform from Converter 2 and converts the substantially trapezoidal DC output waveform into a substantially trapezoidal AC output waveform with RMS voltage which is substantially VRMS;
Bulk Capacitor 271 discharges to the Bus 100 through the Clamp Switch 251 when the Clamp Switch 251 is in ON state.
2. The EPC of Claim 1 wherein the DC power source 1 is a battery.
3. The EPC of Claim 1 or Claim 2 wherein the Converter 2 is of Push Pull type .
4. The EPC of Claim 1 or Claim 2 wherein the Converter 2 is of the Full Bridge
type.
5. The EPC of Claim 1 or Claim 2 wherein the Converter 2 is of the Half Bridge type.
6. The EPC of Claim 1 or Claim 2 wherein the Converter 2 is of the Full Bridge Resonant type.
7. The EPC of Claim 1 or Claim 2 wherein the Converter 2 is of the Half Bridge Resonant type.
8. The EPC of any of the preceding claims wherein the EPC is electrically connected to Load 5.
9. An EPC as substantially described herein having reference to the accompanying drawings.