Description:MAGNETIC ISOLATED VOLTAGE FEEDBACK CIRCUIT FOR PWM CONTROLLER

FIELD OF THE INVENTION
The present invention relates to feedback circuits and, more particular to a magnetic isolated voltage feedback circuit for PWM controller with no external auxiliary power supply for secondary of the isolator.

BACKGROUND OF THE INVENTION
A feedback control circuit is required in a power electronics based isolated dc-dc converter, as a PWM control lies only on its primary side, where the sensed secondary voltage is feedback to the PWM controller on the primary side. The isolation circuit used for this purpose has feedback integrated circuits which uses optocouplers. However, the use of optocouplers causes limited band width, degradation problem and damage induced by the radiation environment.
The conventional circuits such as analog magnetic isolation circuit for voltage feedback loop in forward DC-DC converter does not ensure stability at all operating conditions of loads and inputs. It causes an overshoot in voltage response during start-up. Tuning controller gain is difficult. In these circuits, eliminating low frequency noise by desynchronization of two frequencies is difficult.
One of the systems known in the art provides an Analog Magnetic Isolator for Space Power Applications, which depicts a similar circuit. The design presented in this scientific paper does not implement a circuit which has radiation hardened elements required for space application purpose. Here a synchronized carrier frequency for analog modulator employed is derived from external oscillator and not synchronized with the PWM switching frequency derived from output of the converter.
SUMMARY OF THE INVENTION
The present invention is intended to solve problems as indicated by the above presented background and state-of-the-art. In particular, the invention presents a magnetic isolated voltage feedback circuit for PWM controller.
Another problem solved by the invention is replacing the optocoupler and isolated feedback generator IC circuit such as UCC2901/1901, ADUM3901 and complex magnetic isolation circuit with discrete ones and those which are readily available in radiation-hardened version. The purpose of the invention is hence to realize a circuit and a method to overcome the above stated problems, applicable in all present and future application of dc-dc converter. An advantage gained by the invention is that it maintains high bandwidth, controller is easily tuneable, by the fact that it comprises readily available components and can be realized very economically. Another advantage gained by the invention is that the circuit design is based on fully discrete and custom circuit eliminating optocoupler based design and can be modified to required input/output voltage ranges. The invention provides a galvanically isolated feedback circuit that consists of very small coupling transformer, a modulator, and an AC-DC rectifier circuit. Also, the overshoot voltage during the start-up is negligible.
Accordingly, one aspect of the present invention relates to a magnetic isolated voltage feedback circuit for forward dc-dc converter in a SMPS system. In one embodiment, the circuit comprises of an isolation transformer having a primary winding and a bias winding on the primary part, magnetically coupled to a secondary winding, wherein during ON period, voltage across the primary winding of the isolation transformer induces voltage in the secondary, which in turn generates an isolated feedback voltage. The isolated feedback voltage together with a bias voltage from the bias winding determines generation of gating pulse signals to control a MOSFET switch to regulate a SMPS system.
One terminal of the primary winding of the isolation transformer is connected electrically in series with a current sense circuit, an EMI filter and a transient voltage protection unit. An input of the current sense circuit is connected to an output of the EMI filter and an output of the Transient voltage protection unit is connected to an input of the EMI filter. A MOSFET switch, is connected to second terminal of the primary winding, which in turn is grounded. An output from the EMI filter is connected to input of an under and over voltage shutdown unit. An output from the current sense circuit is connected to input of an input over current protection. A terminal of the bias winding is connected to an input of a rectifier and filter unit. Another terminal of the bias winding is grounded. Output of the input over current protection, the under and over voltage shutdown unit and the rectifier and filter unit is connected to an input of a PWM controller and driver circuit. An output of the PWM controller and driver circuit is connected to gate terminal of the MOSFET switch. On secondary part of the isolation transformer both terminals of the secondary winding is electrically connected to a rectifier. An output of the rectifier is connected to an output filter. An output of the rectifier and the output filter is connected to a magnetically isolated voltage feedback circuit. An output from the magnetically isolated voltage feedback circuit is connected to the PWM controller and driver circuit. During ON period, voltage across the primary winding of the isolation transformer induces voltage in the secondary which in turn generates an isolated feedback voltage. The isolated feedback voltage is obtained by the magnetic feedback voltage isolation circuit comprising an Op-amp amplifier with P or PI or PID controller circuit, a Modulator circuit based on transistor and a magnetic isolation transformer. The magnetic isolation transformer is driven by the op-amp error amplifier with controller output driving base of transistor Q1 at one end and transistor Q2 on other end, also is synchronized with the secondary pulse of a forward converter. The isolated feedback voltage is coupled with PWM switching frequency resulting in amplitude modulation with synchronization of carrier frequency and PWM switching frequency, eliminating low frequency noise from desynchronization of two frequencies.
To further clarify advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended figures. It is appreciated that these figures depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope.

BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described and explained with additional specificity and detail with the accompanying figures in which:
FIG.1 shows a magnetic isolated voltage feedback circuit for PWM controller, according to an embodiment of the invention.
FIG. 2 shows a magnetic isolated voltage feedback circuit for PWM controller replacing optocoupler ICs, according to an embodiment of the present invention.
Further, skilled artisans will appreciate that elements in the figures are illustrated for simplicity and may not have been necessarily been drawn to scale. Furthermore, in terms of the design of the circuits, one or more components of the circuit may have been represented in the figures by conventional symbols, and the figures may show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the figures with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

DETAILED DESCRIPTION OF THE INVENTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the figures and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof. The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures or components proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other sub-systems or other elements or other structures or other components or additional devices or additional sub-systems or additional elements or additional structures or additional components. Appearances of the phrase "in an embodiment", "in another embodiment" and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
It will be understood that when an element or layer is referred to as being “on,” “connected to,” or “coupled to” another element or layer, it can be directly on, connected to, or coupled to the other element or layer or intervening elements or layers that may be present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
Spatially relative terms, such as “detection,” or “capture, and the like, may be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the structure in use or operation in addition to the orientation depicted in the figures.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, circuits and examples provided herein are illustrative only and not intended to be limiting.
Embodiments of the present invention will be described below in detail with reference to the accompanying figures.
FIG. 1 shows a magnetic isolated voltage feedback circuit for PWM controller, according to one embodiment of the present invention. In an example embodiment, the circuit comprises of an isolation transformer (1) having a primary winding and a bias winding on the primary part, magnetically coupled to a secondary winding. One terminal of the primary winding is connected electrically in series with a current sense circuit (2), an EMI filter (3) and a transient voltage protection unit (4). An input of the current sense circuit (2) is connected to an output of the EMI filter (3) and an output of the transient voltage protection unit (4) is connected to an input of the EMI filter (3). The transient voltage protection unit (4) prevents sudden variation of input voltage transients. The EMI filter (3) eliminates the common mode and differential mode noise entring to input supply. The current sense cricuit (2) protects the system during overload condition. A MOSFET switch (5) is connected to second terminal of the primary winding, which in turn is grounded. The MOSFET (5) switches the isolation transformer (1) input voltage high level to low level output. An output from the EMI filter (3) is connected to input of an under and over voltage shutdown unit (6). The under and over voltage shutdown unit (6) protects system from under voltage and over voltage condition of input supply voltage. An output from the current sense circuit (2) is connected to input of an input over current protection (7). The input over current protection unit (7) protects the system during overload condition. A terminal of the bias winding is connected to an input of a rectifier and filter unit. The rectifier and filter unit (8) converts AC to rectified DC for a bias signal. Another terminal of the bias winding is grounded. Output of the input over current protection (7), the under and over voltage shutdown unit (6) and the rectifier and filter unit (8) is connected to an input of a PWM controller and driver circuit (9). An output of the PWM controller and driver circuit (9) is connected to gate terminal of the MOSFET switch (5). The PWM controller and driver circuit (9) generates gating pulse signals to the MOSFET switch (5) according to derived feedback voltage signal from a magnetic isolated feedback circuit unit (12).
On secondary part of the isolation transformer (1), both terminals of the secondary winding is electrically connected to a rectifier (10). The rectifier (10) converts the secondary side AC to rectified DC for regulated output. An output of the rectifier (10) is connected to an output filter (11). An output of the rectifier (10) and the output filter (11) is connected to a magnetic isolated voltage feedback circuit unit (12). An output from the magnetic isolated voltage feedback circuit unit (12) is connected to the PWM controller and driver circuit (9) to implement a closed loop circuit. The magnetic isolated voltage feedback circuit unit (12) comprises of an op-amp error amplifier with P or PI or PID controller circuit, a modulator circuit based on transistor and a magnetic isolation transformer.
During ON period, voltage across the primary winding of the isolation transformer (1) induces voltage in the secondary which in turn generates an isolated feedback voltage. The isolated feedback voltage together with a bias voltage from the bias winding determines generation of gating pulse signals to control the MOSFET switch (5) to regulate a SMPS system providing a feedback control loop.
FIG. 2 shows a magnetic isolated voltage feedback circuit for PWM controller replacing optocoupler ICs, according to another embodiment of the present invention. Here, magnetic isolated voltage feedback circuit unit (12) used for SMPS has been implemented using an op-amp error amplifier (12a) with a P or PI or PID controller circuit, a modulator circuit based on transistor Q1 (12b), Q2(12c) and a magnetic isolation transformer (12d).
The output voltage Vo (12e) is compared with a reference voltage (Vref_Con) and fed to the PID controller or type 3 compensator comprising resistor Rd, capacitor Cd, resistorR1, resistor R4, capacitor C4 and op-amp (12a). An error amplifier with controller output drives base of the transistor Q1 (12b) and is connected to one end of the magnetic isolation transformer (12d).
The transistor Q2 (12c) drives other end of the magnetic isolation transformer (12d) which is synchronized with a secondary pulse of a forward converter derived from rectifier output. In this isolation circuit, feedback is coupled with PWM switching frequency (the synchronized carrier frequency is equal to a PWM switching frequency). Hence an amplitude modulation can be performed with synchronization of carrier frequency and PWM switching frequency. Also, a low frequency noise from desynchronization of two frequencies can be eliminated.
A rectified output of the magnetic isolation transformer (12d) acts as a demodulator or sample-and-hold circuit. A modulated signal from transformer secondary after rectifying and filtering using diode D2 and capacitor C2 is fed back to a PWM controller circuit for implementing a closed loop control. The magnetic coupling transformer can be configured as a flyback-type converter.
While specific language has been used to describe the disclosure, any limitations arising on account of the same are not intended. As would be apparent to a person in the art, various working modifications may be made to the method in order to implement the inventive concept as taught herein.
The figures and the forgoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of processes described herein may be changed and are not limited to the manner described herein. Moreover, the actions of any flow diagram need not be implemented in the order shown; nor do all of the acts necessarily need to be performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope of embodiments is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of embodiments is at least as broad as given by the following claims.
Reference Numerals
Isolation Transformer 1
Current sense circuit 2
EMI filter 3
Transient voltage protection 4
MOSFET switch 5
Under and over voltage shutdown 6
Input overcurrent protection 7
Rectifier and filter 8
PWM controller and driver circuit 9
Rectifier 10
Output filter 11
Isolated magnetic feedback circuit 12
Op-amp 12a
Transistor Q1 12b
Transistor Q2 12c
Magnetic isolation transformer 12d
Output voltage 12e
, Claims:We Claim:
1. A magnetic isolated voltage feedback circuit for PWM controller comprising:
an isolation transformer (1) having a primary winding and a bias winding on primary part, magnetically coupled to a secondary winding, wherein during ON period, voltage across the primary winding of the isolation transformer (1) induces voltage in the secondary which in turn generates an isolated feedback voltage;
the isolated feedback voltage together with a bias voltage from the bias winding determines generation of gating pulse signals to control a MOSFET switch (5) to regulate SMPS, characterized in that, the isolated feedback voltage is obtained by a magnetic feedback voltage isolation circuit unit(12) comprising:
an op-amp (12a) error amplifier with P or PI or PID controller circuit;
a modulator circuit based on transistors Q1 (12b), Q2 (12c); and
a magnetic isolation transformer (12d).

2. The circuit as claimed in claim 1, wherein the magnetic isolation transformer (12d) is driven by the op-amp (12a) error amplifier with controller output driving base of transistor Q1 (12b) at one end and transistor Q2 (12c) on other end, which is synchronized with the secondary pulse of a forward converter.
3. The circuit as claimed in claim 1, wherein the magnetic isolated feedback voltage unit(12) is coupled with PWM switching frequency resulting in amplitude modulation with synchronization of carrier frequency and PWM switching frequency, eliminating low frequency noise from desynchronization of two frequencies.

Bangalore RANI MADANAN
(IN/PA/3054)
12 May 2023 (INTELLOCOPIA IP SERVICES)
AGENT FOR APPLICANT