Control circuit and control method for switch circuit and switching-mode power supply circuit

Abstract

The present invention provides a control circuit and a control method for a switch circuit and a switching-mode power supply circuit. The control method comprises following steps: detecting an output voltage or an output current, and adjusting an upper limit value and a lower limit value of an inductor current according to a result of comparing the output voltage or the output current with the corresponding reference; and sampling the inductor current, and controlling a main switch transistor in the circuit to be switched off when a sampling current rises to the upper limit value, and controlling the main switch transistor to be switched on when the sampling current drops to the lower limit value. In the present invention, the inductor current is fast in response without overshoot, the output voltage drops very little, there is no overshoot in a process of voltage recovery, and circuit transient response is fast.

Claims

1. A control method for a switch circuit, comprising following steps: detecting an output voltage or output current in a circuit, and increasing an upper limit value and a lower limit value of an inductor current when the output voltage is lower than a preset reference voltage or the output current is lower than a preset reference current; and decreasing the upper limit value and the lower limit value of the inductor current when the output voltage is higher than the preset reference voltage or the output current is higher than the preset reference current; and sampling the inductor current in the circuit to obtain a sampling current, and controlling a main switch transistor in the circuit to be switched off when the sampling current rises to the upper limit value of the inductor current; and controlling the main switch transistor to be switched on when the sampling current drops to the lower limit value of the inductor current; wherein detecting the output voltage or the output current in the circuit, and increasing the upper limit value and the lower limit value of the inductor current when the output voltage is lower than the preset reference voltage or the output current is lower than the preset reference current, and decreasing the upper limit value and the lower limit value of the inductor current when the output voltage is higher than the preset reference voltage or the output current is higher than the preset reference current comprise the following steps: detecting the output voltage in a switch circuit of a constant voltage output, and increasing the upper limit value and the lower limit value of the inductor current when the output voltage is lower than the preset reference voltage; and decreasing the upper limit value and the lower limit value of the inductor current when the output voltage is higher than the preset reference voltage; and detecting the output current in a switch circuit of a constant current output, and increasing the upper limit value and the lower limit value of the inductor current when the output current is lower than the preset reference current; and decreasing the upper limit value and the lower limit value of the inductor current when the output current is higher than the preset reference current; wherein detecting the output voltage in the switch circuit of the constant voltage output, and increasing the upper limit value and the lower limit value of the inductor current when the output voltage is lower than the preset reference voltage, and decreasing the upper limit value and the lower limit value of the inductor current when the output voltage is higher than the preset reference voltage comprise the following steps: acquiring a sampling voltage of the output voltage in real time; comparing the sampling voltage with the preset reference voltage, and obtaining a compensation voltage according to a comparison result; converting the compensation voltage to obtain a command current; and adjusting the upper limit value and the lower limit value of the inductor current according to a preset compensation current value and the command current; wherein converting a compensation voltage to obtain a command current, adjusting a lower limit value of an inductor current according to the command current, the lower limit value of an inductor current plus a preset compensation current value to obtain the upper limit value of an inductor current, the switching frequency can be controlled by controlling the preset compensation current to achieve fixed frequency.

2. The control method for the switch circuit according to claim 1, further comprising following steps: after sampling the inductor current in the circuit to obtain a sampling current, determining whether the sampling current is smaller than zero, and if yes, switching off a synchronous rectifier in the circuit.

3. The control method for the switch circuit according to claim 1, further comprising following steps: after sampling the inductor current in the circuit to obtain a sampling current, determining whether the sampling current is smaller than zero, and if yes, switching off a synchronous rectifier in the circuit.

4. The control method for the switch circuit according to claim 1, further comprising following steps: after sampling the inductor current in the circuit to obtain a sampling current, determining whether the sampling current is smaller than zero, and if yes, switching off a synchronous rectifier in the circuit.

5. A control circuit for a switch circuit, comprising a command current acquiring circuit, an adjuster, an inductor current control circuit, and a drive circuit; wherein the command current acquiring circuit is configured to detect an output voltage or an output current in a circuit, convert the output voltage or the output current into a command current, and output the command current to the adjuster; the adjuster is configured to adjust an upper limit value or a lower limit value of an inductor current according to the command current and a preset compensation current value; and the inductor current control circuit is connected with the adjuster, the drive circuit, and a sampling terminal of the inductor current; the inductor current control circuit is configured to sample the inductor current to obtain a sampling current, generate a switching signal according to the sampling current and the upper limit value and the lower limit value of the inductor current, and control a main switch transistor by the drive circuit, the main switch transistor is controlled to be switched off when the sampling current rises to the upper limit value of the inductor current, and the main switch transistor is controlled to be switched on when the sampling current drops to the lower limit value of the inductor current; wherein converting a compensation voltage to obtain a command current, adjusting a lower limit value of an inductor current according to the command current, the lower limit value of an inductor current plus a preset compensation current value to obtain the upper limit value of an inductor current, the switching frequency can be controlled by controlling the preset compensation current to achieve fixed frequency.

6. The control circuit for the switch circuit according to claim 5, wherein the command current acquiring circuit comprises a load voltage sampling unit, a voltage comparing unit, and a voltage-to-current converting unit; the load voltage sampling unit is connected with the voltage comparing unit and the voltage-to-current converting unit in sequence; and the load voltage sampling unit is configured to sample a voltage between two ends of a load to obtain a sampling voltage, and output the sampling voltage to the voltage comparing unit; and the voltage comparing unit is configured to compare the sampling voltage with a preset reference voltage, output a voltage signal obtained by comparison to the voltage-to-current converting unit for conversion to obtain a command current, and output the command current to the adjuster.

7. The control circuit for the switch circuit according to claim 5, wherein the adjuster is an adder or a subtracter; a first input terminal of the adder or the subtracter is connected with an output terminal of a voltage-to-current converting unit, a second input terminal of the adder or the subtracter is connected with a preset compensation current value, and an output terminal of the adder or the subtracter is connected with the inductor current control circuit.

8. The control circuit for the switch circuit according to claim 7, wherein when the adjuster is the adder, the command current is the lower limit value of the inductor current; and when the adjuster is the subtracter, the command current is the upper limit value of the inductor current.

9. The control circuit for the switch circuit according to claim 5, wherein the inductor current control circuit comprises a zero-crossing comparison circuit; and the zero-crossing comparison circuit is configured to determine whether the sampling current is smaller than zero, and output a control signal to switch off a synchronous rectifier when the sampling current is determined to be smaller than zero.

10. The control circuit for the switch circuit according to claim 6, wherein the inductor current control circuit comprises a zero-crossing comparison circuit; and the zero-crossing comparison circuit is configured to determine whether the sampling current is smaller than zero, and output a control signal to switch off a synchronous rectifier when the sampling current is determined to be smaller than zero.

11. A switching-mode power supply circuit, comprising an inductor and a main switch transistor, and further comprising the control circuit for the switch circuit according to claim 5.

12. A switching-mode power supply circuit, comprising an inductor and a main switch transistor, and further comprising the control circuit for the switch circuit according to claim 6.

13. A switching-mode power supply circuit, comprising an inductor and a main switch transistor, and further comprising the control circuit for the switch circuit according to claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A is a schematic structural diagram of a BUCK (step-down) circuit in the prior art;

(2) FIG. 1B is a schematic structural diagram of a BOOST (step-up) circuit in the prior art;

(3) FIG. 2 is a schematic diagram of waveforms of a clock signal of an existing switch circuit in a fixed frequency mode and a turn-on signal TON of a main switch transistor in the prior art;

(4) FIG. 3A is a schematic diagram of a change relationship among a load current, the clock signal, the turn-on signal TON of the main switch transistor, an inductor current, a feedback voltage FB, and a compensation voltage VC when the load current in the existing switch circuit jumps from a small value to a large value in the prior art;

(5) FIG. 3B is a schematic diagram of a change relationship among the load current, the clock signal, the turn-on signal TON of the main switch transistor, the inductor current, the feedback voltage FB, and the compensation voltage VC when the load current in the existing switch circuit jumps from a large value to a small value in the prior art;

(6) FIG. 4A is a schematic flow chart of a control method for a switch circuit according to an embodiment of the present invention;

(7) FIG. 4B is a schematic flow chart of a control method for a switch circuit according to another embodiment of the present invention;

(8) FIG. 5A is a schematic flow chart of a control method for a switch circuit including a synchronous rectifier instead of a freewheeling diode according to an embodiment of the present invention;

(9) FIG. 5B is a schematic flow chart of a control method for a switch circuit including a synchronous rectifier instead of a freewheeling diode according to another embodiment of the present invention;

(10) FIG. 5C is a schematic flow chart of a control method for a switch circuit including a synchronous rectifier instead of a freewheeling diode according to still another embodiment of the present invention;

(11) FIG. 6 is a schematic diagram of a change relationship between an inductor current and a turn-on signal TON of a main switch transistor in the control method for the switch circuit as shown in FIGS. 4A-4B or FIGS. 5A-5C;

(12) FIG. 7 is a schematic structural diagram of a control circuit for a switch circuit according to one embodiment of the present invention;

(13) FIG. 8 is a schematic structural diagram of a control circuit for a switch circuit according to another embodiment of the present invention;

(14) FIG. 9 is a schematic diagram of waveforms of a load current, a turn-on signal TON of a main MOS, an inductor current, a feedback voltage FB, and a compensation voltage VC in the control circuit for the switch circuit as shown in FIG. 6;

(15) FIG. 10 is a schematic diagram of waveforms of a load current, a turn-on signal TON of a main MOS, an inductor current, a feedback voltage FB, and a compensation voltage VC in the control circuit for the switch circuit as shown in FIG. 7; and

(16) FIG. 11 is a schematic diagram of a change relationship between the inductor current and the compensation voltage VC in the circuit when the control circuit for the switch circuit as shown in FIG. 7 and FIG. 8 operates in a discontinuous conduction mode (DCM).

DETAILED DESCRIPTION OF THE INVENTION

(17) With reference to the accompanying drawings, the foregoing and additional technical features and advantages of the present invention are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present invention and not all embodiments.

(18) An embodiment of the present invention provides a control method for a switch circuit, as shown in FIG. 4A, and the method includes following steps:

(19) S100: detecting an output voltage or an output current in a circuit, and increasing an upper limit value and a lower limit value of an inductor current when the output voltage is lower than a preset reference voltage or the output current is lower than a preset reference current; and decreasing the upper limit value and the lower limit value of the inductor current when the output voltage is higher than the preset reference voltage or the output current is higher than the preset reference current; and

(20) S200: sampling the inductor current in the circuit to obtain a sampling current, and controlling a main switch transistor in the circuit to be switched off when the sampling current rises to the upper limit value of the inductor current; and controlling the main switch transistor to be switched on when the sampling current drops to the lower limit value of the inductor current.

(21) As shown in FIG. 6, in the switch circuit, the inductor current has an upper limit and a lower limit. When the inductor current rises to the upper limit of the current, the main switch transistor is switched off; when the inductor current is equal to (equivalent to reaching) the lower limit of the current, the main switch transistor is switched on. Based on the above control principle, the present invention controls the output current by regulating the upper limit and the lower limit of the inductor current, thereby controlling the output voltage. This control mode is fast in transient response, and overshoot and drop of the output voltage recover relatively fast when a jump occurs in an output load.

(22) Specifically, using the control method for the switch circuit provided by the present invention, when the output voltage or the output current jumps from small to large, the inductor current can reach the lower limit value of the inductor current earlier than in a steady state, so that the turn-off time of the main switch transistor can become shorter and the turn-on time can become longer. Furthermore, there is no limitation on the maximum turn-on time in this control method, so that the inductor current can be increased more rapidly. Therefore, when the load current is increased, the inductor current is fast in response without overshoot, the output voltage drops little, there is no overshoot in the process of voltage recovery, and the circuit is fast in transient response. On the contrary, when the output voltage or the output current jumps from large to small, the inductor current cannot reach the lower limit value of the inductor current, so that the main switch transistor can be switched off all the time until the inductor current reaches the lower limit value of the inductor current. Further, there is no limitation on the minimum turn-on time in this control method, so that there is no limitation on the turn-off time of the main switch transistor, and the inductor current can decrease more rapidly. Therefore, when the load current decreases, the inductor current is fast in response without overshoot, overshoot of the output voltage is little, and the circuit is fast in transient response.

(23) Further, another embodiment of the present invention provides a control method for a switch circuit, as shown in FIG. 4B, and the method includes following steps:

(24) S100: by detecting the output voltage in the switch circuit of a constant voltage output, the upper limit value and the lower limit value of the inductor current are increased when the output voltage is lower than the preset reference voltage. The upper limit value and the lower limit value of the inductor current are decreased when the output voltage is higher than the preset reference voltage, and by detecting the output current in the switch circuit of a constant current output, the upper limit value and the lower limit value of the inductor current are increased when the output current is lower than the preset reference current. The upper limit value and the lower limit value of the inductor current are decreased when the output current is higher than the preset reference current.

(25) Further, S200 in FIG. 4B is same as S200 in FIG. 4A and thus is not described here again.

(26) In the switch circuit of the constant voltage output, Step S100 of FIG. 4B may be implemented through the following specific steps:

(27) S110: acquiring a sampling voltage of the output voltage in real time;

(28) S120: comparing the sampling voltage with the preset reference voltage, and obtaining a compensation voltage according to a comparison result;

(29) S130: converting the compensation voltage to obtain a command current; and

(30) S140: adjusting the upper limit value and the lower limit value of the inductor current according to a preset compensation current value and the command current.

(31) The above command current is generally a voltage signal, which is obtained by the compensation voltage multiplied by a certain proportion and then added or subtracted a certain value. Acquiring the command current may be achieved by connecting a proportional circuit to a circuit such as an adder or a subtracter.

(32) Furthermore, refer to FIG. 5A-FIG. 5C, wherein S100 of FIG. 5A is same as S100 of FIG. 4A and thus is not described here again; S100 of FIG. 5B is same as S100 of FIG. 4B and thus is not described here again; S100 of FIG. 5C comprise S100 of FIG. 4B and S110-S140 mentioned above and thus is not described here again. For such a case where an auxiliary switch transistor in the switch circuit is a synchronous rectifier instead of a freewheeling diode, S200 of FIG. 5A-FIG. 5C provided by the present invention comprises S200 of FIG. 4A or FIG. 4B and further includes following steps: after sampling the inductor current in the circuit to obtain the sampling current, determining whether the sampling current is smaller than zero, and if yes, switching off the synchronous rectifier in the circuit.

(33) When the switch circuit is in the discontinuous conduction mode (DCM) and the lower limit value of the inductor current is smaller than zero, if the auxiliary switch transistor is a freewheeling diode, the inductor current is reduced to zero, and no negative current may occur. The inductor current remains at zero all the time, the compensation voltage rises, and both the upper limit and the lower limit of the inductor current increase. When the lower limit of the inductor current increases more than zero, namely, when the sampling current is equal to the lower limit value of the inductor current, the main switch transistor is switched on; and if the auxiliary switch transistor is a synchronous rectifier, the inductor current is reduced to zero, and the synchronous rectifier is switched off (negative current may occur if the synchronous rectifier is not switched off). The inductor current remains at zero all the time, the compensation voltage rises, and both the upper limit value and the lower limit value of the inductor current increase. When the lower limit of the inductor current increases more than zero, namely, when the sampling current is equal to the lower limit value of the inductor current, the main switch transistor is switched on.

(34) Based on the same inventive concept, the present invention further provides a control circuit for a switch circuit, which includes a command current acquiring circuit, an adjuster, an inductor current control circuit and a drive circuit.

(35) The command current acquiring circuit is configured to detect an output voltage or an output current in the circuit, convert the output voltage or the output current into a command current, and output the command current to the adjuster. The adjuster is configured to adjust an upper limit value and a lower limit value of an inductor current according to the command current and a preset compensation current value. The inductor current control circuit is connected with the adjuster, the drive circuit, and a sampling terminal of the inductor current in the circuit. The inductor current control circuit is configured to sample the inductor current in the circuit to obtain a sampling current, generate a switching signal according to the sampling current and the upper limit value and the lower limit value of the inductor current, and control a main switch transistor in the circuit by the drive circuit. When the sampling current rises to the upper limit value of the inductor current, the main switch transistor is controlled to be switched off, and when the sampling current drops to the lower limit value of the inductor current, the main switch transistor is controlled to be switched on.

(36) The above adjuster may be an adder or a subtracter. When the adjuster is the adder, the command current is the lower limit value of the inductor current; and when the adjuster is the subtracter, the command current is the upper limit value of the inductor current. A specific implementation is as below.

(37) Referring to FIG. 7, a control circuit for a switch circuit provided by an embodiment of the present invention includes a command current acquiring circuit, an adder U02, an inductor current control circuit U06, and a drive circuit U07.

(38) The command current acquiring circuit is configured to detect an output voltage or an output current in the circuit, convert the output voltage or output current into a command current, and output the command current to the adder U02. The command current acquiring circuit includes a load voltage sampling unit (including resistors R02 and R01 in the figure), an operational amplifier U00, and a voltage-to-current conversion circuit U01.

(39) The adder U02 is configured to increase the upper limit value and the lower limit value of the inductor current when the output current (namely, the load current) jumps from small to large, and reduce the upper limit value and the lower limit value of the inductor current when the output current jumps from large to small. The inductor current control circuit U06 is connected with the adder U02, the drive circuit U07, and a sampling terminal of the inductor current in the circuit.

(40) As shown in FIG. 7, the inductor current control circuit U06 samples the current of the inductor L00. Therefore, the sampling terminal of the inductor current in the circuit may be arranged at the input terminal of the inductor L00, or may be arranged at a terminal of the main switch transistor M00 or a terminal of the diode D00.

(41) After voltage division by the voltage dividing resistors R01 and R02, the output voltage V.sub.out of the switch circuit obtains a voltage FB and inputs the voltage FB to a negative input terminal of the operational amplifier U00, and a positive input terminal of the operational amplifier U00 is connected with a preset reference voltage Vref1. A command current is obtained at the output terminal VC of the operational amplifier U00 through the voltage-to-current conversion circuit U01. The command current controls the lower limit of the inductor current. Generally, in a drive control circuit, the inductor current obtained by sampling is a voltage signal. Therefore, the output (namely, the command current) of the voltage-to-current conversion circuit U01 is also a voltage signal. The lower limit value of the inductor current may be a value smaller than zero. The command current controlling the lower limit of the inductor current is inputted to an input terminal of the adder U02, and a preset compensation current i is connected to another input terminal of the adder U02. The preset compensation current i may be a fixed value or may be a variable value. The adder U02 adds the lower limit value of the inductor current and the preset compensation current i to obtain the upper limit value of the inductor current. Both the upper limit and the lower limit of the inductor current are connected to the input of the inductor current control circuit U06, the inductor current control circuit U06 outputs a switching signal, and the switching signal drives the main switch transistor M00 through the drive circuit U07.

(42) It should be supplementarily explained that when the freewheeling diode D00 in the switch circuit is a synchronous rectification MOS transistor, the control signal of the synchronous rectification MOS transistor is also driven by the drive circuit U07. The operational amplifier U00 regulates the output voltage by regulating a value of the output terminal VC thereof, so that the voltage FB is equal to the preset reference voltage Vref1, thereby controlling the output voltage.

(43) Specifically, when the output voltage is low, the voltage FB is smaller than the preset reference voltage Vref1, the VC becomes larger, and the lower limit of the inductor current becomes larger. The upper limit value of the inductor current is obtained by adding the lower limit value of the inductor current and the preset compensation current i together. The inductor current control circuit U06 obtains the switching signal according to control values of the upper limit and the lower limit of the inductor current. By driving the switch of the main switch transistor M00 by the drive circuit U07, the inductor current is increased, and the output voltage is increased. On the contrary, when the output voltage is high, the voltage FB is larger than the preset reference voltage Vref1, the VC becomes smaller, and the lower limit value of the inductor current becomes smaller. The upper limit value of the inductor current is obtained by adding the lower limit value of the inductor current and the preset compensation current i together. The inductor current control circuit U06 obtains the switching signal according to the control values of the upper limit and the lower limit of the inductor current. By driving the switch of the main switch transistor M00 by means of the drive circuit U07, the inductor current is decreased, and the output voltage is decreased.

(44) When the preset compensation current i is a fixed value, the switching frequency is not a fixed frequency; and if a fixed frequency is needed, the switching frequency can be controlled by controlling the preset compensation current i.

(45) Another implementation manner by means of the subtracter is as shown in FIG. 8. The voltage-to-current conversion circuit U01 generates the upper limit value of the inductor current, and inputs the upper limit value to an input terminal of the subtracter U12. The preset compensation current i is also inputted to another input terminal of the subtracter U12. The lower limit value of the inductor current is obtained by subtracting the preset compensation current i from the upper limit value of the inductor current. Both the upper limit value and the lower limit value of the inductor current are inputted to the inductor current control circuit U06.

(46) A load jump in the BUCK circuit is taken as an example for description in the following.

(47) When a load current jumps from small to large, the output voltage V.sub.out is reduced, and FB, as a division voltage of the output voltage, is also reduced. Therefore, the output voltage VC of the operational amplifier rises. After the voltage VC rises, both the upper limit value and the lower limit value of the inductor current become larger simultaneously, the inductor current reaches the lower limit value of the inductor current earlier than in a steady state, so that the turn-off time of the main switch transistor M00 becomes shorter immediately and the turn-on time becomes longer immediately. Furthermore, there is no limitation on the maximum turn-on time in this control method, so that the inductor current can be increased more rapidly. Therefore, when the load current is increased, the inductor current is fast in response without overshoot, the output voltage drops little, there is no overshoot in the process of voltage recovery, and the circuit is fast in transient response. The load current, the turn-on signal TON of the main switch transistor, the inductor current, the feedback voltage FB, and the compensation voltage VC are as shown in FIG. 9.

(48) When the load current jumps from large to small, the output voltage V.sub.out rises, and FB, as the division voltage of the output voltage, also rises. Therefore, the voltage VC of the operational amplifier is reduced. After the voltage VC is reduced, both the upper limit value and the lower limit value of the inductor current become smaller simultaneously, and the inductor current cannot reach the lower limit value of the inductor current, so that the main switch transistor M00 is switched off all the time until the inductor current reaches the lower limit value of the inductor current. Further, there is no limitation on the minimum turn-on time in this control method, so that there is no limitation on the turn-off time of the main switch transistor, and the inductor current can be decreased more rapidly. Therefore, when the load current is decreased, the inductor current is fast in response without overshoot, the overshoot of the output voltage is little, and the circuit is fast in transient response. The load current, the turn-on signal TON of the main MOS, the inductor current, the feedback voltage FB, and the compensation voltage VC are as shown in FIG. 10.

(49) Furthermore, for such a case where an auxiliary switch (the freewheeling diode D00) in the switch circuit is a synchronous rectifier, in the control circuit for the switch circuit provided by the present invention, the inductor current control circuit U06 includes a zero-crossing comparison circuit. The zero-crossing comparison circuit is used for determining whether the sampling current is smaller than zero and outputting a control signal to switch off the synchronous rectifier in the switch circuit when the sampling current is smaller than zero.

(50) Referring to FIG. 11, when the switch circuit is in the discontinuous conduction mode (DCM) and the lower limit value of the inductor current is smaller than zero, if the auxiliary switch transistor is a freewheeling diode, the inductor current is reduced to zero, and no negative current may occur. The inductor current remains at zero all the time, the compensation voltage VC rises, and the lower limit of the inductor current increases. When the lower limit of the inductor current increases more than zero, namely, when the sampling current is equal to the lower limit value of the inductor current, the main switch transistor M00 is switched on; and if the auxiliary switch transistor is a synchronous rectifier, the inductor current is reduced to zero, and the synchronous rectifier is switched off (negative current may occur if the synchronous rectifier is not switched off). The inductor current always remains at zero, the compensation voltage VC rises, and the lower limit value of the inductor current increases. When the lower limit of the inductor current increases more than zero, namely, when the sampling current is equal to the lower limit value of the inductor current, the main switch transistor M00 is switched on.

(51) Based on the same inventive concept, the present invention further provides a switching-mode power supply circuit, which includes an inductor, a main switch transistor, and the above control circuit for the switch circuit.

(52) In the present invention, the output current is controlled by regulating the upper limit and the lower limit of the inductor current, thereby controlling the output voltage. This switching-mode power supply circuit of the present invention is fast in transient response, and when a jump occurs in the output load, overshoot and drop of the output voltage recover faster.

(53) In the above specific embodiments, the objectives, the technical solutions and the beneficial effects of the present invention are further described in detail. However, it should be understood that the above embodiments are merely the specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. It is particularly pointed out that for those skilled in the art, all modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention shall fall within the scope of protection of the present invention.