CONTROL METHOD AND APPARATUS FOR A FLYBACK CONVERTER

Abstract: A power supply comprising a flyback converter and a controller (20) is disclosed. The flyback converter drives a load when electrically coupled to an alternating current power source. The controller (20) controls a soft switching during each switching time period of the flyback converter. Upon an initial switching time period, the controller (20) determines each acceptable switching frequency for the subsequent switching time periods and selects one of the acceptable switching frequencies for soft switching the flyback converter over one or more switching time periods during a constant load.

Description: Control method and apparatus for a flyback converter BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to power supplies for electronic devices, e.g., televisions, desktop computers, computer monitors, laptop computers, compact disc players, digital video disc players, and audio components such as receivers and tuners. The present invention specifically relates to a method and apparatus for controlling a soft switching of a switch of a flyback converter. 2. Description of the Related Art A typical flyback converter includes a metal-oxide semiconductor field-effect transistor switch ("MOSFET switch") that is selectively turned on and off in view of regulating an output voltage of the flyback converter. Specifically, the MOSFET switch is transitioned from an OFF state - to an ON + state at one of the valleys of a drain voltage of the MOSFET switch in view of an output power being transmitted by the flyback converter to a load. First valley switching occurs when the output power is within an upper end of a load range of the flyback converter. Referring to FIG. 1 A, a gate voltage VQ I of the MOSFET switch transitions from a voltage off level VO FF to a voltage on level V OΝ when a first valley of a drain voltage V D I is detected. An off-time period T OFF (H L ) corresponds to an acceptable switching frequency for the flyback converter when the output power is within an upper end of the load range. High order valley switching occurs when the output power is within an intermediate portion or a lower end of the load range of the flyback converter. Referring to FIG. IB, a minimum off-time period T OF F (M I Ν) that extends beyond a first valley and a second valley of drain voltage V DI is determined, and gate voltage V GI transitions from voltage off level V OFF to voltage on level V ON upon a first detection of a third valley of a drain voltage VDI after an elapse of minimum off-time T O FF(MIN)- An off-time period TOF F ( ILZ L L ) corresponds to an acceptable switching frequency for the flyback converter when the output power is within the intermediate portion or the lower end of the load range. The first valley switching as shown in FIG. 1 A and the high order valley switching as shown in FIG. IB is predicated upon a selection of a specific valley of drain voltage V DI for transitioning gate voltage VG I from voltage off level V OFF to voltage on level V O N- However, this predication fails to recognize that two or more valleys of drain voltage V DI may correspond to acceptable switching frequencies for drain voltage V DI - This predication also fails to incorporate other performance parameters of the flyback converter such as an efficient power operation of the flyback converter. The electronic industry is therefore striving to improve upon a control of a soft switching of the MOSFET switch. SUMMARY OF THE INVENTION It is an object of the invention to provide an improved flyback converter. The invention is defined by the independent claims. The dependent claims define advantageous embodiments. The present invention relates to a control of a soft switching of a switch of a flyback converter. While the actual nature of the present invention covered herein can only be determined with reference to the claims appended hereto, certain features, which are characteristic of the embodiments disclosed herein, are described briefly as follows. A first form of the present invention is a power supply comprising a flyback converter including a switch operable to be switched between an on state and an off state. The power supply further comprises a controller that is operable during a first switching time period to determine at least one soft switching frequency for transitioning the switch from the off state to the on state during a second switching time period. The second switching time period is subsequent to the first switching time period. A second form of the present invention is a power supply comprising a flyback converter including a switch having a gate terminal, a drain terminal, and a source terminal. The power supply further comprises a controller electrically coupled to said gate terminal and said drain terminal. The controller is operable to provide a gate voltage to the gate terminal in response to a drain voltage at the drain terminal. The foregoing forms and other forms, features and advantages of the present invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 A is a first exemplary illustration of a prior art gate voltage waveform and a prior art drain voltage waveform of a MOSFET switch of a flyback converter under a high load; FIG. IB is a second exemplary illustration of a prior art gate voltage waveform and a prior art drain voltage waveform of a MOSFET switch of a flyback converter under an intermediate load or a light load; FIG. 2 is a schematic diagram of one embodiment of a flyback converter in accordance with the present invention; FIG. 3 is a block diagram of one embodiment of a controller in accordance with the present invention; and FIG. 4 is an exemplary illustration of a first switching time period and a second switching time period of a gate voltage waveform and a drain voltage waveform of a MOSFET switch of the FIG. 2 flyback converter. DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS Referring to FIG. 2, a flyback converter 10 as coupled to an alternating voltage source V A c, e.g. a wall outlet, is shown. Flyback converter 10 includes an inductor LI and a capacitor Cl. Inductor LI is electrically coupled to alternating voltage source V A c and a node Nl . Capacitor Cl is electrically coupled to node Nl and a node N2. Alternating voltage source V AC is also electrically coupled to node N2. Flyback converter 10 further includes a diode Dl, a diode D2, a diode D3, and a diode D4. Diode Dl is electrically coupled to node Nl and a node N3. Diode D2 is electrically coupled to node Nl and a node N4. Diode D3 is electrically coupled to node N2 and node N3. Diode D4 is electrically coupled to node N2 and node N4. An input voltage V IN is applied between node N3 and node N4, and an input current I ΓN flows into node N3. Flyback converter 10 further includes a capacitor C2, a capacitor C3, a resistor Rl, a diode D5, and a transformer having an inductor L2, an inductor L3, an inductor L4 and an inductor L5. Capacitor C2 is electrically coupled to node N3 and a node N5. Capacitor C3 is electrically coupled to node N3 and node N4. Resistor Rl is electrically coupled to node N3 and node N5. Inductor L2 is electrically coupled to node N3 and a node N6. Diode D5 is electrically coupled to node N5 and a node N7. Inductor L3 is electrically coupled to node N6 and node N7. Inductor L4 is electrically coupled to node N6 and node N7. A reflected secondary voltage V RS is applied between node N6 and node N7. Flyback converter 10 further includes a diode D6, and a capacitor C4. Inductor L5 is electrically coupled to diode D6 and a node N9. Diode D6 is also electrically coupled to a node N8. Capacitor C4 is electrically coupled to node N8 and node N9. An output voltage V O U T is applied between node N8 and node N9. A load Z can be applied to flyback converter 10 between node N8 and node N9. Flyback converter 10 further includes a MOSFET switch SI, a diode D7, and a capacitor C5. MOSFET switch SI has a drain terminal electrically coupled to node N7 and a source terminal electrically coupled to node N4. In other embodiments of a flyback converter in accordance with the present invention, other types of switches may be substituted from MOSFET switch SI as would occur to those with ordinary skill in the art. Diode D7 and capacitor C5 are electrically coupled to node N7 and node N4. Referring additionally to FIG. 3, a controller 20 is electrically coupled to a gate terminal of MOSFET switch SI, node 4, and node N7 to control a soft switching of MOSET switch SI. Controller 20 is preferably an electronic circuit comprised of one or more components that are assembled as a common unit. Alternatively, for the multiple component embodiments, one or more of these components may be distributed throughout a power supply system comprising flyback converter 10 and controller 20. Controller 20 may be comprised of digital circuitry, analog circuitry, or both. Also, controller 20 may be programmable, a dedicated state machine, or a hybrid combination of programmable and dedicated hardware. It is preferred that controller 20 include an integrated processing unit (not shown) operatively coupled to one or more solid-state memory devices (not shown). It is also preferred that this memory contain programming corresponding to a valley switching acceptance routine 30 (FIG. 5) and that this memory be arranged for reading and writing of data in accordance with the principals of the present invention. The memory may be either volatile or nonvolatile and may additionally or alternatively be of the magnetic or optical variety. To implement the principals of the present invention, controller 20 can further include any control clocks, interfaces, signal conditioners, filters, Analog-to-Digital (A/D) converters, Digital-to- Analog (D/A) converters, communication ports, or other types of operators as would occur to those having ordinary skill in the art. Referring additionally to FIG. 4, a switching time period T PERI and a switching time period T PER2 of gate voltage VQ 2 and drain voltage VD 2 are shown. Three valleys of drain voltage V D2 are shown during switching time period T PERI and during switching time period TP ER 2 for facilitating a description of a valley switching acceptance routine. During an on-time period T O N I of switching time period T P E R I, gate voltage V G2 equates voltage on level V O N and drain voltage V D2 equates a zero voltage level. During an off-time period T OF F I of switching time period T PE R I , gate voltage V G2 equates a voltage off level V OF F- During a demagnetization period T DEMI of switching time period T PERI , drain voltage V D2 initially equates a voltage level V LI which is a summation of input voltage VΓ , reflected secondary voltage V R and an overshoot voltage Vos- Overshoot voltage Vos dissipates and drain voltage V D2 thereafter equates a voltage level V L2 that is summation of input voltage VΓ N and reflected secondary voltage V R . During a decaying time period T D E CI of switching time period T PERI , drain voltage V D2 resonates as a function of inductor L3 and capacitor C5. Drain voltage V D2 further decays between voltage level V L2 and a voltage level V toward a voltage level V_ A . Voltage level V 3 is a differential of input voltage VΓ and reflected secondary voltage V R and voltage level V 4 approximates input voltage VΓ N . Decaying period T D E CI is terminated upon a transitioning of gate voltage V G 2 from voltage off level V OF F to voltage on level V ON at one of the valleys of drain voltage V D 2, such as, for example, a third valley of drain voltage V D2 as shown. A preferred valley switching acceptance routine operates as follows. Controller 20 implements a first stage, a second stage and a third stage of the routine during switching time period T PERI . Controller 20 implements a fourth stage, a fifth stage and a sixth stage of the routine during an on-time time period T ON 2- Controller 20 implements a seventh stage of the routine from an off-time period T OFF2 and may continue an implementation of the seventh stage over one or more subsequent switching time periods. During the first stage of the routine, controller 20 samples drain voltage V D2 over switching time period T PERI . AS a result, demagnetization period T DEMI is measured and drain voltage V D2 during demagnetization period T DEM I is measured. During the second stage of routine 30, controller 20 determines input voltage V Γ N and reflected secondary voltage V r during switching time period T P E RI . In one embodiment, input voltage V ΓN and reflected secondary voltage V r are computed from the following equations (1) and (2): TDEMI = ToNi(V IN Nr) (1) V D2 = V w + V r (2) where controller 20 knows demagnetization period T DEM U drain voltage V D2 , and on-time period TON i- During the third stage of the routine, controller 20 determines an average value of input current I ΓN for the third valley of drain voltage VD2- In one embodiment, the average value of input current I IN is computed from the following equation (3). Ir N = [0.5*ToNi 2 *(ViN/L 3 )]/[(T O Ni ,,! (l+((V IN /N r ))+((n-0.5)*T RE s)] (3) where n = 3 for the third valley switching and a resonance time period T RES is a function of inductor L3 and capacitor C5 as would occur to those with ordinary skill in the art. In another embodiment, a resistor is electrically coupled in series between the source terminal of MOSFET switch SI and node Ν4. Input current I IN is measured over switching time period T PERI as would occur to those with ordinary skill in the art. During the fourth stage of the routine, controller 20 predicts on-time period T ON2 for the first valley, the second valley and the third valley of drain voltage V D 2- In one embodiment, on-time period TQ N 2 is computed from the following equation (4): [0.5*(VIN/L 3 )*TONI ]-[(l+(V I NNr))*IiN*T ON1 ]-[(n-0.5)*T RE s*I IN ] = 0 (4) where the value for I ΠM is take from the third stage. For the first valley of drain voltage V D2 , integer n is 1 to obtain an on-time period TO N2(FV) - For the second valley of drain voltage V D 2, integer n is 2 to obtain an on-time period T O N2( SV ). For the third valley of drain voltage V D2> integer n is 3 to obtain an on-time period T O N2( T V> During the fifth stage of the routine, controller 20 computes a switching frequency fsi for a first valley switch during switching time period T PER2 , a switching frequency fs2 for a second valley switch during switching time period T PER 2, and a switching frequency fs 3 for a third valley switch during switching time period T PER2 - In one embodiment, switching frequency fsi, switching frequency fs 2 , and switching frequency fs 3 are computed in accordance with the following equation (5): fsN = l/[(ToN2*(l+((V IN /N r ))+(n-0.5))*T RE s)] (5) In computing switching frequency fsi, integer n is 1 for a first valley switching and T ON 2 equates T ON2(FV) as previously computed during the fourth stage. In computing switching frequency fs2, integer n is 2 for a second valley switching and T ON2 equates T ON2(SV) as previously computed during the fourth stage. In computing switching frequency fs 3 , integer n is 3 for a third valley switching and T ON2 equates T ON2 ( T V) as previously computed during the fourth stage. During the sixth stage of the routine, an acceptance criterion is applied to switching frequency fsi, switching frequency fs 2 and switching frequency fs 3 . In one embodiment, switching frequency fsi, switching frequency fs 2 and switching frequency fs 3 are compared to a switching frequency range specification for flyback converter 10. Switching frequency fsi is deemed acceptable if switching frequency fsi is within the switching frequency range specification. Switching frequency f$ 2 is deemed acceptable if switching frequency fs 2 is within the switching frequency range specification. Switching frequency fs 3 is deemed acceptable if switching frequency fs 3 is within the switching frequency range specification. During an optional seventh stage of the routine, one or more performance parameters are applied to each switching frequency accepted during the sixth stage. Controller 20 implements the seventh stage when two or more switching frequencies are deemed acceptable during the sixth stage. In one embodiment, an input power P I is computed for each accepted switching frequency and the switching frequency having the lowest input power P IN is selected as the switching frequency for subsequent switching time periods as long as the load Z remains constant. For example, when switching frequency fsi, switching frequency fs 2 and switching frequency fs 3 are all accepted during the sixth stage, a corresponding input power P IN is computed in accordance with the following equations (6) and (7): IIN (Π) = f s(n) * 0.5 *(VIN L 3 ) *T ON 2 (7) Where T ON represents a measured value of during a current switching time period. The switching frequency corresponding to the lowest computed PiN( n ) is tested during subsequent switching time periods and is utilized for soft switching if the testing verifies the computed PlN(n)- While the embodiments of the present invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the scope of the present invention. The scope of the present invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.

Claims: CLAIMS: 1. A power supply, comprising: a flyback converter (10) including a switch (Si) operable to be switched between an on state and an off state; and a controller (20) operable during a first switching time period (T PERI ) to determine at least one soft switching frequency for transitioning said switch (Si) from said off state to said on state during a second switching time period (T PER2 ), said second switching time period (T PEP Z ) being subsequent to said first switching time period (T PERI ). 2. The power supply system of claim 1 , wherein said controller (20) is further operable to apply acceptance criterion to each soft switching frequency of said at least one soft switching frequency. 3. The power supply system of claim 2, wherein said controller (20) is further operable to select a first soft switching frequency from said at least one soft switching frequency in view of said acceptance criterion. 4. The power supply system of claim 1 , wherein said controller (20) is further operable to apply at least one performance parameter to each soft switching frequency of said at least one soft switching frequency. 5. The power supply system of claim 4, wherein said controller (20) is further operable to select a first soft switching frequency from said at least one soft switching frequency in view of said at least one performance parameter. 6. A method (30) for soft switching a switch (Si) of a flyback converter (10), comprising: (S32) sampling a drain voltage (VD 2 ) of the switch (Si); and (S34) determining an input voltage (VΓN) of the flyback converter (10) as a function of said sampling of said drain voltage(Vo2)- 7. The method (30) of claim 6, further comprising: (S34) determining a reflected secondary voltage (V r )of the flyback converter (10) as a function of said sampling of said drain voltage (V D2 )- 8. The method (30) of claim 7, further comprising: (S36) determining an average value of an input current of the flyback converter (10) for a first switching valley of the said drain voltage (V D2 ) during a first switching time period (T PE R I ). 9. The method (30) of claim 8, further comprising: (S38) predicting an on time period (Torø) of a second switching time period (Tp ER .) for the switch (Si) for each valley of the drain voltage (V D2 ), said second switching time period (T PER∑ ) being subsequent to said first switching time period (T PERI ). 10. The method (30) of claim 9, further comprising: (S40) predicting a switching frequency for each valley of the drain voltage (V D2 ) during said second switching time period (T PE R2). 11. The method (30) of claim 10, further comprising: (S42) applying an acceptance criterion to each predicted switching frequency. 12. The method (30) of claim 11 , further comprising: (S44) applying at least one performance parameter to each switching frequency deemed to be acceptable in view of said acceptance criterion.

Inventors: GIANNOPOULOS DEMETRI, LEE NAI-CHI, LI QIONG M, GIANNOPOULOS, DEMETRI, LEE, NAI-CHI, LI, QIONG, M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H02M3
IPC codes: H02M3