Claims:WE CLAIM:

1. A Flyback switching mode power supply (SMPS) (200), comprising:
a Transformer (210), comprising
a Primary winding (211) at a Primary side (PP) connected to an input power supply receiving an input voltage (Vin); and
a Main Secondary winding (221) at a Secondary side (SS) outputting an output Voltage and a Secondary Bias winding (223) at the Secondary side (SS); wherein the polarity of the Main Secondary winding (221) and the Secondary Bias winding (223) are same but opposite to the polarity of the Primary winding (211);
an output current sensor (250) connected at the Main Secondary winding (221) at a Secondary side (SS) for measuring output current;
an ‘‘Bias for Current control’’ winding (225) at the Secondary side (SS) for controlling output Current, wherein the polarity of the ‘‘Bias for Current control’’ winding (225) is opposite to both the Main Secondary winding (221) and the Secondary Bias winding (223) but same as the polarity of the Primary winding (211);
a Voltage control block (220) powered up by the Secondary Bias winding (223), generating a feedback Voltage signal based on the output voltage;
a Current control block (230) powered by the ‘‘Bias for Current control’’ winding (225), generating a feedback Current signal based on the output Current;
at least one high speed Electronic switch (240) selectively switched between an ‘ON’ state and an ‘OFF’ state based on the feedback Current signal and the feedback Voltage signal;
Wherein during a normal operation, a control Voltage is induced in the ‘‘Bias for Current control’’ winding (225) due to the input Voltage at the Primary winding (221), and also during a higher output Current or short circuit condition at the output, the induced control Voltage is used to Powerup the Current control block (230), to generate the feedback signal and at least one high speed Electronic switch (240) is switched to the required `ON` /’OFF’ state , based on the feedback signal from the Current control block (230), so as to control the output Current in the Main Secondary winding (221).

2. The Flyback switching mode power supply (SMPS) as claimed in claim 1, wherein the Transformer (210) comprises a magnetic core made of ferrite.
3. The Flyback switching mode power supply (SMPS) as claimed in claim 1, further comprises an Current control power supply.
4. The Flyback switching mode power supply (SMPS) as claimed in claim 1, further comprises a plurality of isolating opto-couplers added at the Secondary side of the Transformer for feeding back the feedback Voltage/ Status from Main Secondary side to Primary side.

5. A method of protecting a Flyback switching mode power supply (SMPS) (200), comprising a Transformer (210) and at least one high speed Electronic switch (240) selectively switched between an ‘ON’ state and an ‘OFF’ state to control an output Current of the Flyback SMPS, during Over load /Short circuit condition, the method comprising:
determining the output Current in a Main Secondary winding (221) at a Secondary side (SS) of the Transformer using an output current sensor (250);
inducing a control Voltage at an ‘Bias for Current control’ winding (225) due to input Voltage applied at a Primary winding (211) of the Transformer (210), during a normal operation;
generating a feedback Voltage signal at a Voltage control block (220) Powered up by a Secondary Bias winding (223) when the output Voltage exceeds a predetermined threshold voltage;
operating a Current control block (230) Powered up by ‘Bias for Current Control’ winding (225) using the induced control voltage at the ‘Bias for Current Control’ winding (225);
generating a feedback signal at the Current control block (230) when the output Current exceeds a predetermined threshold Current;
receiving the feedback signal from the Current control block (230) at the high speed Electronic switch (240);
modifying the switching of at least one high speed Electronic switch (240) from an ‘ON’ state and an ‘OFF’ state upon receiving the feedback signal to control the output Current in the Main Secondary winding (221).
, Description:TECHNICAL FIELD
[0001] The present invention relates to switching mode power supplies, and more particularly relates to a Flyback switching mode power supply and a method of protecting the same from over Current during output short circuit condition.

BACKGROUND
[0002] Switched-mode power supplies (SMPS) are employed in a variety of applications to convert incoming supply (typically alternating or direct Voltage) to an output Voltage (typically direct Voltage) for consumption by a load. Many well-known configurations may be employed to implement a SMPS, such as buck, boost, buck-boost, Flyback, etc. In each of these configurations, a switch employed by the SMPS, which is selectively controlled (i.e., Turn ‘ON time’ and ‘Turn OFF time’) to dictate the output generated.
[0003] Changes in the load and or fault conditions can result in the SMPS generating excessive output Currents (i.e., over-Current condition). To prevent excessive Currents from damaging components, prior art SMPSs have included over-Current protection circuits that limit the ‘Turn ON’ time of the switch compared to ‘Turn OFF time’ in response to the monitored Current exceeding a threshold value. However, for short-circuit faults, the Current built-up during the minimum ‘Turn ON’ time of the switch most often exceeds the Current discharged during the ‘Turn OFF’ time. As a result, the Current supplied by the SMPS continues to increase with each successive cycle. Further, these over-Current protection circuits are adequate for normal Ac line input Voltage variations. However, for wide input voltages, such as output from solar panels, the conventional Current control technique are not able to limit the Current within its maximum rating, resulting in failure of SMPS components at short circuit faults.

SUMMARY
[0004] In one aspect of the present new disclosure 200, a Flyback switching mode power supply (SMPS) is disclosed. The Flyback switching mode power supply (SMPS) comprises a Transformer, a Voltage control block 220 coupled to the Secondary Bias winding 223 , generating a feedback Voltage signal based on the output voltage, a Current control block 230, Powered by an additional Bias Voltage, named from here on as ‘‘Bias for Current control’’225 , generating a feedback signal proportional to the output Current and at least one high speed Electronic switch selectively switched between an ‘ON’ state and an ‘OFF’ state based on the status of Voltage and Current of the Main secondary output . The Transformer comprises a Primary winding at a Primary side PP connected to an input power supply receiving an input voltage; and a Main Secondary winding at a Secondary side SS outputting an output voltage, and a Secondary Bias winding at the Secondary side SS, for controlling the output voltage. The polarity of the Main Secondary winding and the Secondary Bias winding are same but opposite to the polarity of the Primary winding, shown by symbol Dot. The Flyback SMPS further comprises of an Bias for Current control winding’ for effective output Current control on the Secondary side SS, for controlling output Current, wherein the polarity of this said Bias for Current control winding is opposite to both the Main Secondary winding and the Secondary Bias winding but same polarity to the polarity of the Primary winding and this is indicated by Dot. During a normal operation, a control Voltage is induced in this said winding, due to the input Voltage at the Primary winding, and available as “Bias Voltage” for current control Block [230], during a higher output load Current or short circuit condition. This induced Control Voltage in the ‘Bias for Current control’ winding, is used to Powerup and operate the Current Control block to generate the feedback signal to switch at least one high speed Electronic switch to the required ‘ON’ /’OFF’ state. Based on the feedback signal from the Current control block, the output Current at Pout in the Main Secondary winding in controlled by the switching device.

[0005] In another aspect of the present disclosure, a method of protecting a Flyback switching mode power supply (SMPS) is disclosed. The Flyback switching mode power supply (SMPS) comprises a Transformer and at least one high speed Electronic switch selectively switched between an ‘ON’ state and an ‘OFF’ state to control an output Current of the Flyback SMPS, during over load or short circuit condition at the Main Secondary Pout. The method comprises determining the output Current in a Main Secondary winding at a Secondary side of the Transformer, and inducing Voltage in ‘Bias for Current control’ winding in block 225 to Powerup the Current control block 230. During normal operation, Voltage control block 220 senses a feedback Voltage signal through block 221 connected to the Main Secondary winding through diode D1. When the Main Secondary Output Voltage exceeds a predetermined threshold voltage, this control block 220, regulates to Maintain the output Voltage at a set value of Vout by controlling the ON/OFF period of Electronic switch. When the output Current in Main Secondary Iout, exceeds a predetermined threshold Current detected by Current sensor 250, this feedback signal proportional to the Main Secondary Output Current, is fed to the Current control block 230. After getting processed in the said block 230, this signal modifies the ON/OFF switching of the high speed Electronic switch in such a way that output Current from Main Secondary is forced to reduce to a very small value, thus saving the Electronic switch and other components from failure.

BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The detailed description is described with reference to the accompanying figures.
[0007] Figure 1, illustrates a schematic diagram of a conventional Flyback switching mode power supply (SMPS).
[0008] Figure 2, illustrates a schematic diagram of a Flyback switching mode power supply (SMPS) (Flyback converter) in accordance with an exemplary embodiment of the present disclosure.
[0009] Figure 3, illustrates a flow chart for the method of protecting a Flyback switching mode power supply (SMPS) in accordance with another embodiment of the present disclosure.
[0010] Figure 4 illustrates the comparison of test results with the presented method of protecting the Flyback SMPS against the conventional method of Current control in a tabular form.

DETAILED DESCRIPTION

[0011] A Flyback switching mode power supply (SMPS) comprising an over Current control block and a method of protecting the same are disclosed. The Flyback SMPS comprises an ‘Bias for Current control’ winding 225 at the Secondary side, designed to have an opposite polarity to the Main Secondary winding 221 shown by sign DOT. The Voltage induced at the ‘Bias for Current control’ winding is used to Powerup the Current control block during short circuit conditions and the Flyback SMPS can be protected from the over Current.
[0012] Referring to figure 1, illustrates a schematic diagram of a conventional Flyback switching mode power supply (SMPS). The conventional Flyback SMPS 100 comprises a Transformer 110. The Transformer 110 comprises a Primary winding P1 wound with the polarity opposite to the polarity of Secondary windings S1 and S2 as shown by Dot. This Dot sign indicates the start of the winding. The Secondary winding S1 is the Main Secondary winding carrying output power Pout to the load and the Secondary Bias winding S2 is used to control the output Voltage of the Main Secondary winding S1 by switching the Primary winding through at least one Electronics switch SW1. The Secondary Bias winding S2 is also used to control the output Current of the Main Secondary winding in the conventional Flyback SMPS using a Current sensor block 150, which is controlling the Current/Voltage control circuit block 103. The Secondary winding S1 induces the required Voltage in Secondary Bias winding S2 for Powering up the Current control block 103 during normal opera-tion. However, during short circuit at the Secondary winding S1, there is no Voltage or grossly inadequate Voltage is induced in this said winding S2, since secondary windings S1 and S2 are designed to have same polarity. Due to this said inadequate Bias Voltage, Current control circuit is not Powered up with the required Voltage limit, hence unable to carry out the intended over current limiting function, leading to flow of large uncontrolled Current in the Main Secondary Circuit Pout, causing damages to the switching components.
[0013] Referring to figure 2, illustrates a schematic diagram of a Flyback switching mode power supply (SMPS) Flyback converter in accordance with an exemplary embodiment of the present disclosure. The Flyback SMPS 200 comprises including a Transformer 210, a Voltage control block 220, a Current control block 230 and a high speed Electronic switch 240. The Transformer 210 comprises a Primary winding 211 at a Primary side PP, a Main Secondary winding 221 and a Secondary Bias winding 223 at Secondary side SS. The Primary winding 211 is connected to an input power supply, receiving an input Voltage Vin. The Main Secondary winding 221 outputs a rectified output power Pout through a rectifier diode D1 with an output Voltage Vout and an output Current Iout. The polarity of the Main Secondary winding 221 is opposite to the polarity of Primary winding 211 as shown in Fig 2 with sign Dot. The output voltage at the rectifier diode D1 is taken for the Voltage control block 220 for generating feedback signal. The Secondary Bias winding 223 is used for controlling the output Voltage Vout. The said winding 223 polarity is designed with opposite to the polarity of Primary winding as per Flyback principle, but have same polarity as the polarity of the Main Secondary winding 221, so that the Voltage through the Secondary Bias winding 223, follows the output Voltage at the Main Secondary winding 221. The Secondary Bias winding 223 is connected to a rectifier diode D2 and the rectified voltage at D2 is given to the Voltage control block 220 for generating feedback voltage signal.
[0014] The Transformer 210 further comprises an ‘Bias for Current control’ winding 225 at the Secondary side SS for controlling output Current I out. The polarity of the ‘Bias for Current control’ winding 225 is opposite to both the Main Secondary winding 221 and the Secondary Bias winding 223, but same to the polarity of the Primary winding 211. In other words, the said winding 225 is designed to have out of phase (180 degrees) with both the Main Secondary winding 221 and the Secondary Bias winding 223, and in phase with the Primary winding 211. Hence the said winding 225, follows the input Voltage at the Primary winding 211. The Transformer 210 may comprise a magnetic core made of ferrite material. The Bias for Current control’ winding 225 is connected to a rectifier diode D3 and the rectified voltage at D3 is given to the Current control block 230 for generating feedback current signal.
[0015] The Voltage control block 220 is powered by the Secondary Bias winding 223, for controlling the output Voltage at the Main Secondary winding 221, generating a feedback Voltage signal proportional to the output Voltage Vout. This feedback Voltage signal is generated when the output Voltage is higher than a predetermined threshold voltage, which in turn controls the switching ON and OFF time of Electronic Switch, thereby Maintaining the output Voltage on the Main Secondary within its limits. The Current control block 230 is Powered up by the ‘Bias for Current control’ winding 225 for controlling output Current at the Main Secondary winding 221, generating a feedback signal proportional to the output Current. This said feedback signal is generated when the output Current is higher than a predetermined threshold Current, or due to an over current /short circuit condition occurred at the output of Main Secondary. The high speed Electronic switch 240 relates to the Primary winding 211. The said Electronic switch 240 is selectively switched between an ‘ON’ state and an ‘OFF’ state, based on the status of the Output Voltage and Output Current. The ‘OFF’ state of the switch disconnects the input path to the Primary winding 211 based on feedback Voltage signal at the Voltage control block 220 or based on the feedback signal at the Current control block 230, whereas the ‘ON’ state of the switch connects the circuit input path to the Primary winding 211 so that the rectified output power Pout is the required output from the Main Secondary winding. The Flyback SMPS 200 further may comprise an Current control power supply and a plurality of isolating Opto-Couplers. The plurality of isolating Opto-Couplers may be added at the second side SS of the Transformer 210 for transferring back the feedback on Output Current and Voltage status, to the Primary side from the low voltage and isolated secondary side.
[0016] During a normal operation of the Flyback SMPS 200, a control Voltage Vc is induced in the ‘Bias for Current control’ winding 225 due to the input Voltage at the Primary winding 211, as the said winding 225 has same polarity as that of Primary winding 211 and the high speed Electronic switch 240 is switched in ‘on/ off’ state. The “‘Bias for Current control’” Voltage Vc is always available even during “off switching Period “of the high speed Electronic switch 240 since Voltage is induced every time when Primary winding is turned ON. During a higher Current flowing condition or on a short circuit condition, output current is detected by an output sensor 250 producing a proportional Voltage signal to modify the switching on/off period of the high speed Electronic switch 240. The modified switching speed of Electronic switch 240 is controlled by Current control 230, so that the output Current at Pout gets automatically restricted to a very low value, thereby limiting the output Current in the Main Secondary winding 221. By this method, the Flyback SMPS can be protected from the over Current flow during overload/short circuit conditions.
[0017] In another embodiment of the present disclosure, a method of protecting a Flyback switching mode power supply (SMPS) comprising a Transformer and at least one high speed Electronic switch selectively switched between an ‘ON’ state and an ‘OFF’ state to control an output Current of the fly back SMPS, during short circuit condition is disclosed. Referring to figure 3, illustrates a flow chart for the method of protecting a Flyback switching mode power supply (SMPS) in accordance with the same embodiment of the present disclosure.
[0018] The method comprises the step of determining the output Current in a Main Secondary winding at a Secondary side of the Transformer, inducing a Voltage in ‘Bias for Current control’ winding in order to power up Current control block, due to input Voltage applied at a Primary winding of the Transformer during a normal operation and generating a feedback Voltage signal at a Voltage control block connected with a Secondary Bias winding when the output Voltage exceeds a predetermined threshold voltage. The method further comprises operating a Current control block powered up by an ‘Bias for Current control’ winding , using the induced Bias voltage, generating a feedback signal at the Current control block when the output Current exceeds a predetermined threshold Current, activating a Hiccup mode to protect the flyback SMPS from Over load /Short circuit current, receiving the feedback signal from the Current control block at the high speed Electronic switch and modifying the switching of at least one high speed Electronic switch from an ‘ON’ state and an ‘OFF’ state upon receiving the feedback Current signal to so as to control the output Current in the Main Secondary winding.
[0019] The method of protecting the Flyback SMPS was tested by loading output initially and then by performing short circuit test under wide input Voltage variation ranging from 250v dc to 850v dc. Tests were also conducted with the conventional method of Current control and results were compared with the presented method of protecting the Flyback SMPS. Under normal operating conditions, the default output Voltage is 12 volts and the output Current is 1 amps, and the over load /short circuit Current was done during tests. Referring to figure 4, illustrates the comparison of test results with the presented method of protecting the Flyback SMPS against the conventional method of Current control in a tabular form. From the table, it is observed that the Current control block operated correctly in both the methods for the input Voltage only up to 550v dc. For the input voltages greater than 550v dc, the conventional method failed to control and limit the Current to a low value by switching to “Hiccup mode” (Turn on for a short time and Turn off for a large time and keep on repeating) to save the Flyback SMPS from destruction. But the presented control method with the Flyback SMPS could control and limit the output Current to a low value by switching to Hiccup mode, even for the input voltages greater than 550v dc thereby saving the Flyback SMPS from destruction. The Flyback SMPS withstood up to the input Voltage 800v dc, called as maximum withstand Voltage of the Flyback SMPS. Thus, for the wide input Voltage range, the short circuit Current protection can be achieved using the Flyback SMPS.
[0020] Although the present disclosure has been described in the context of certain aspects and embodiments, it will be understood by those skilled in the art that the present disclosure extends beyond the specific embodiments to alternative embodiments and/or uses of the disclosure and obvious implementations and equivalents thereof. Thus, it is intended that the scope of the present disclosure described herein should not be limited by the disclosed aspects and embodiments above.