Description:A DOUBLE INTEGRAL SLIDING MODE CONTROL FOR A DC-DC FORWARD CONVERTER

FIELD OF THE INVENTION
The present invention relates to control circuits and, more particular to a PWM based non-linear control circuit for active clamping in a forward type switching mode DC-DC converter.
BACKGROUND OF THE INVENTION
An upgraded control circuit is required in present linear control methods considering the dynamic performances and regulation under perturbation in load and supply voltage variations. Theavailable PWM controller circuit does not maintain good dynamic response, better steady state input voltage and load regulation ability for any type of converter. These circuits do not ensure stability at all operating conditions of loads and inputs. The conventional circuits cause overshoot in voltage response during start-up. They require different way for tuning controller gain. Currently, the sliding mode controller based on non-linear control displays better dynamics for variations in load and input voltage in comparison with the PWM based control methods.
One of the systems known in the art provides a sliding mode control for DC-DC converters provides a design to give results in terms of robustness toward load and input voltage variations, while maintaining a dynamic response at least comparable to standard current control techniques. But the application in this paper is limited to buck-boost converters alone.

SUMMARY OF THE INVENTION
Accordingly, one aspect of the present invention relates to afixed frequency double integral sliding mode voltage controllerfor a dc-dc forward converter.
An embodiment presents a method for providing a double integral sliding mode control for a dc-dc forward converter. The method comprises of an input DC voltage in the range of about 15V to about 75V.A duty cycle and a plurality of gain parameters is determined based on the input voltage. A logic state of the converter as a function of a sliding surface, where the sliding surface is estimated as a product of a sliding coefficient and a plurality of control state parameters are obtained. An output voltage dependent on the input voltage, the duty cycle, the logical state of the converter and a ratio of the number of turns of a transformer is estimated. The gain parameters and the control state parameters provide the sliding mode control when altered.The duty cycle is a ratio of first voltage to a second voltage, the first voltage is obtained from a control signal, the second voltage is obtained from a peak amplitude of a ramp signal generated from pulse width modulation. The alteration of the gain parameter and the control state parameter enables efficient clamping of the voltage.The efficient clamping of the voltage results in the regulation of the voltage.The regulation of voltage yields in high precision operation of the converter required for space operations. The operating power range of the converter is in a range of about 100W to about 200W. The frequency of operation is in the range of about 100kHz to about 300kHz. The output voltage is in the range of about 2V to about 10V. The double integration sliding mode provides a fast dynamic response with a settling time of 0.00001s to 0.00004s.
In one embodiment, a fixed frequency PWM based sliding mode controller; the discrete control signal of gate is replaced by a smooth function known as the equivalent control signal .The equivalent control signal is derived from aconcept of duty cycle control.
In another embodiment, a fixed frequency PWM based double integral sliding mode voltage controller is derived.

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 double integral sliding mode control for a dc-dc forward converter, according to an embodiment of the invention.
FIG. 2 and FIG.3 shows an active clamping forward converter with integration and double integration nonlinear sliding mode controller with output power 150 W and switching frequency 200 kHz to operate at an output voltage of 5 V dc and an input voltage range of 18 V to 50 V dc, which is compatible with space grade dc supply voltage requirements, according to another 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.
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 double integral sliding mode control for a dc-dc forward converter, according to an embodiment of the invention. In an example embodiment, the circuit comprises of a forward converter (1), a double integral sliding mode voltage controller (2), a PWM (3), a driver circuit (4) and an active clamping circuit (5). The forward converter(1) comprises of an input voltage , an output filter inductor ,a main switch , an auxiliary switch a clamping capacitor (6), an isolation transformer(7), a magnetizing inductance , an output capacitor , a rectifier diode and a freewheel diode . The active clamping circuit (5) comprises the clamping capacitor (6) and the isolation transformer (7). The forward converter (1) is connected to the double integral sliding mode voltage controller (2). The forward converter (1) and the double integral sliding mode voltage controller (2) are supplied with the input voltage . The double integral sliding mode voltage controller (2) to the PWM (3). The PWM (3) is connected to the driver circuit(4). Output signals from the driver circuit (4) are connected to the main switch and the auxiliary switch . The active clamping circuit (5) comprises the auxiliary switch and the clamping capacitor . The forward converter (1) has two operating modes.
During mode 1 operation of the forward converter (1), the main switch is turned on and the auxiliary switch is turned off. The input DC voltage source supplies energy, at the range of about 15V to about 75V, to the magnetizing inductance of the isolation transformer (7). The magnetizing inductance transfers energy to secondary of the inductor and voltage at secondary is rectified through diode . The freewheeling diode is in reverse biased condition. The increase in supply current flows through the output filter inductor , the output capacitor and releases energy to output load terminals.
During mode 2 operation of the forward converter (1) the main switch is turned off and the auxiliary switch is turned on. During off condition of power switch , the free wheel diode is forward biased due to the energy stored in the inductor . The output filter inductor supply energy to load terminal through the output capacitor and the freewheeling diode . The active clamp circuit (5) absorbs transient energy from leakage inductance to reduce voltage stress of the main switch .
Output voltage on the capacitor is derived from energy balance on the inductor during on and off periods of the main switch and is written as:



; (a)
Where, is duty cycle of the forward converter (1) and is primary turns to secondaryturns ratio of the isolation transformer (7).
Voltage on the clamp capacitor (6) is derived from energy balance on the magnetizing inductor during on and off periods of the main switch and the auxiliary switch is written as:




; (b)
General sliding mode control law of dc-dc converter is employed using a single switch to adopt a switching function such as
(c)
According to equation (c) the control law is
(d)
Where is the logic state of theconverter (1) main switch T1 and is the instantaneous state variable’s trajectory treated as sliding surface.
The sliding surface is described as
(e) Where, , , and represents the sliding coefficients which act as control parameters.A logic state of the converter as a function of a sliding surface, where the sliding surface is estimated as a product of a sliding coefficient and a plurality of control state parameters are obtained.

The controlled state variables required in this method of sliding mode controller are output voltage error , voltage error dynamics and integral of voltage error and double integral of voltage error respectively. The output voltage is regulated to follow their reference voltage.
The controlled state variables , , and are expressed as,


Where, is voltage across the output filter inductor L during ON an OFF condition of power switch, is inductor current, is output capacitor current and is output load current.
During ON condition of the converter (1)

During OFF condition of the converter (1)

Hence considering both the ON ( ) and OFF ( ) state of the converter (1), the condition of in variable can be replaced as


(f)
Where; is reference value of output voltage, is dynamic resistance of the load, is output voltage of the converter (1), is the input voltage of the converter (1) and ß is feedback scaling ratio.

In fixed frequency PWM based sliding mode controller; the discrete control signal of gate can be replaced by a smooth function known as the equivalent control signal .The equivalent control signal is effectively derived from a concept of duty cycle control.
The equivalent control signal of the double integral SM voltage controller (2) can be obtained by equating the time differentiation of equation (3) to zero and its inequality .
(g)
Time differentiation of equation (f) gives the dynamic model of the double integral SM voltage controller (2) system as



(h)
Control signal of the PWMbased double integral SM controller (2) is derived based on equivalent control approach by solving equation (g) and its inequality
From (g)

Using inequality
(i)
Multiplying equation (i) with gives

(j)

In general, the fixed frequency PWM based control of dc-dc converter system; the instantaneous duty cycle is expressed as
(k)
where; is the control signal to the PWM controller and is the peak amplitude of the fixed frequency triangular or ramp signal. Since is continuous and bounded by 0 and 1, hence it may be written in the form
.
From equation (k), the control signal and carrier or ramp signal needed for implementing the fixed frequency forward converter using duty ratio control concept d, where gives
and
(l) Where,
; and (m)
are the gain parameters in the PWM (3) controller whose values are fixed. The values of , and are chosen such that it ensures stability at all operating conditions of input voltage and load. The duty cycle and the gain parameters are determined based on the input voltage. The output voltage dependent on the input voltage, the duty cycle, the logical state of the converter and a ratio of the number of turns of a transformer is estimated.
Realization of fixed frequency PWM based nonlinear sliding mode operation can easily be achieved by implementing control equations (l) and (m) into the PWM controller. The alteration of the gain parameter and the control state parameter enables efficient clamping of the voltage. The efficient clamping of the voltage results in the regulation of the voltage.
The derived control signal and ramp or carrier signal is not suitable for direct realization in experimental circuit due to high value of voltage and current magnitudes. Thus, for practical implementation, it is essential to introduce suitable scaling factor to scale down into standard chip level. The PWM gating signal is generated by comparing control signal with a ramp or carrier signal .
The double integration sliding mode controller (2) circuit for the forward converter (1) gives a dynamic state space model. It provides an increased order of the controller improving the steady-state accuracy of the system and decreasesthe stability problem. The presence of finite steady-state output voltage regulation error in forward dc-dc converter (1) employing integral sliding mode nonlinear voltage controller can be suppressed through an additional double integral term into the controller state variables. The sliding mode controller (2) that provides a regulated output voltage.

FIG. 2 and FIG.3 shows an example of an active clamping forward converter with integration and double integration nonlinear sliding mode controller circuit with output power 150 W and switching frequency 200 kHz to operate at an output voltage of 5 V dc and an input voltage range of 18 V to 50 V dc, which is compatible with space grade dc supply voltage requirements, according to another embodiment of the present invention.The circuit comprises of an active clamping forward transformer/converter (8), a step load changing (9), a voltage sensing circuit (10), a current sensing circuit (11), a PWM controller (12), a type-2 PI controller (13), a gate pulse delay circuit (14), a driver circuit(15).
The active clamping forward transformer/converter (8) is connected across an input voltage source of 18V-50V DC. The voltage sensing circuit (10) and current sensing circuit (11) is connected to the active clamping forward transformer/converter (8). The PWM controller(12) is connected to the input voltage source. The type-2 PI controller (13) and the gate pulse delay circuit (14) are connected to the PWM controller (12). The driver circuit (15) are connected to the gate pulse delay circuit (14).
The active clamping forward transformer/ converter (8) comprises a main switch (8a) and an auxiliary switch (8b). The PI controller (13) comprises of components R7,R8,C2 and Ccomp. The PI controller (13) is constructed with high band width external op-amp circuit.The op-amp circuit is constructed using a proportional and integral term. The PWM controller (12) is configured as fixed frequency nonlinear sliding mode controller.
An isolated output voltage (Vo) and a reference voltage (Vref) is derived from the PWM controller (12) is fed to the PI controller (13). An output (Vcon) of the PI controller (13), the active clamping forward transformer/ converter (8) output voltage (Vo) and a sensed current Ico, from an output capacitor (Co) is fed to an error amplifier of the PWM controller (12). The error amplifier of the PWM controller (12) is configured as a summing amplifier. A gate signal g1and g2are generated by the gate pulse delay circuit (15) by controlling the main switch (8a) and the auxiliary switch (8b) in the active clamping forward transformer/ converter (8). The PI controller (13) function of an op-amp is replaced with a proportional function by implementingan active clamping forward converter (8). The double integration sliding mode have fast dynamic response. The double integration sliding mode control gives a settling time of 0.00001s to 0.00004s while integration sliding mode controlgives 0.0001s.
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 control circuit using sliding mode controller for active clamping forward controllers based on PWM.
Another problem solved by the invention is to provide a control circuit for fixed frequency PWM based double integral sliding mode nonlinear voltage controller for active clamping forward converter with synchronous rectifier and a nonlinear controller compatible with space grade dc supply voltage requirements. The regulation of voltage yields in high precision operation of the converter required for space operations. The purpose of the invention is to achieve an inexpensive hardware implementing a PWM control on equivalent control approach, the controller is configured using readily available UC2825 PWM controller IC.
An advantage gained by the invention is that the method can be applied to all types of forward dc-dc converter especially in SMPS system. Another advantage gained by the invention is that the circuit provides the fast-settling time, low chattering magnitude, minimum peak overshoot and ensures a stable operation of the converter. Another aspect of the invention provides a negligible overshoot in voltage response during start-up compared to a conventional control method.
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.

, Claims:WE CLAIM:
1. A method for providing a double integral sliding mode control for a dc-dc forward converter, the method comprising of:
providing an input DC voltage in the range of about 15V to about 75V;
determining a duty cycle and a plurality of gain parameters based on the input voltage;
obtaining a logic state of the converter as a function of a sliding surface, wherein the sliding surface is an estimated as a product of a sliding coefficient and a plurality of control state parameters; and
estimating an output voltage, the output voltage dependent on the input voltage, the duty cycle, the logical state of the converter and a ratio of the number of turns of a transformer,
wherein altering the gain parameters and the control state parameters provide the sliding mode control.
2. The method as claimed in claim 1, wherein the duty cycle is a ratio of first voltage to a second voltage, wherein the first voltage is obtained from a control signal, the second voltage is obtained from a peak amplitude of a ramp signal generated from pulse width modulation.
3. The method as claimed in claim 1, wherein the alteration of the gain parameter and the control state parameter enables efficient clamping of the voltage.
4. The method as claimed in claim 1, wherein the efficient clamping of the voltage results in the regulation of the voltage.
5. The method as claimed in claim 1, wherein the regulation of voltage yields in high precision operation of the converter required for space operations.
6. The method as claimed in claim 1, wherein the operating power range of the converter is in a range of about 100W to about 200W.
7. The method as claimed in claim 1, wherein the frequency of operation is in the range of about 100kHz to about 300kHz.
8. The method as claimed in claim 1, wherein the output voltage is in the range of about 2V to about 10V.
9. The method as claimed in claim 1, wherein the double integration sliding mode provides a fast dynamic response with a settling time of 0.00001s to 0.00004s.

Bangalore RANI MADANAN
23 January 2024 (INTELLOCOPIA IP SERVICES)
AGENT FOR APPLICANT