# Patent

## Title
Power converter for a switching power supply and manner of operation thereof

## Abstract

A switching power supply comprises a power converter that includes a transformer, a low side switch and a high side switch. The low side switch draws current from a supply voltage through a primary winding of the transformer. The high side switch discharges current from the primary winding of the transformer to a snubber capacitor. The controller synchronously controls the opening and closing of the low side switch and the high side switch. The power converter can be included in a flyback converter. The power converter can generate a regulated output voltage.

## Background
BACKGROUND OF THE INVENTION
The present invention relates to the field of switching power supplies. More particularly, the present invention relates to a flyback power converter for a switching power supply and manner of operation thereof.
An off-line switching power supply receives power from an alternating-current (AC) power source and provides a voltage-regulated, direct-current (DC) output that can be used to power a load. An exemplary off-line power supply includes a power factor correction (PFC) stage and a DC-to-DC converter stage. The PFC stage receives the AC input signal, performs rectification and maintains current drawn from the AC source substantially in phase with the AC voltage so that the power supply appears as a resistive load to the AC source. The DC-to-DC converter stage receives the rectified output of the PFC stage and generates the voltage-regulated, DC output which can be used to power the load. The rectified output of the PFC stage is typically at higher voltage and is more loosely regulated than the output of the DC-to-DC stage.
A flyback power converter (or, more simply, a flyback converter) can be employed in a DC-to-DC power converter. A flyback converter employs a transformer that transfers energy from the input of the flyback converter to its output and provides electrical isolation between the input and output of the flyback converter. An input voltage, such as the rectified output voltage of a PFC stage, is applied across the transformer primary winding by closing a switch; as a result, a primary winding current flows and magnetic flux in the transformer increases, storing energy in the transformer. When the switch is opened, the voltage is removed and the primary winding current falls while magnetic flux drops. As a result, a current is induced in a secondary winding of the transformer. This induced current charges an output capacitor to generate an output voltage for powering a load.
Switching power supplies can be subjected to a variety of loading conditions. It is important for such power supplies to operate efficiently so as to minimize power usage. Therefore, what are needed are improved techniques for a switching power supply that accommodate different loading conditions and that achieve efficient operation. What are further needed are such techniques for a switching power supply that employs a flyback power converter.
SUMMARY OF THE INVENTION
In accordance with an embodiment, a switching power supply comprises a power converter that includes a transformer, a low side switch and a high side switch. The low side switch draws current from a supply voltage through a primary winding of the transformer. The high side switch discharges current from the primary winding of the transformer to a snubber capacitor. The controller synchronously controls the opening and closing of the low side switch and the high side switch. The power converter can be included in a flyback converter. The power converter can generate a regulated output voltage.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:
FIG. 1 illustrates a block schematic diagram of a two-stage, off-line power supply in accordance with an embodiment of the present invention;
FIG. 2 illustrates a flyback converter suitable for use in a DC-to-DC converter in accordance with an embodiment of the present invention;
FIG. 3 illustrates a voltage waveform for a flyback converter in accordance with an embodiment of the present invention;
FIG. 4 illustrates a voltage waveform for a flyback converter in accordance with an embodiment of the present invention;
FIG. 5 illustrates a flyback converter and control circuitry in accordance with an embodiment of the present invention;
FIG. 6 illustrates a controller integrated circuit for a DC-to-DC converter in accordance with an embodiment the present invention;
FIG. 7 illustrates a high-voltage resistor in accordance with an embodiment of the present invention;
FIG. 8 illustrates a two-way, high-voltage resistor in accordance with an embodiment of the present invention;
FIG. 9 illustrates control circuitry for a flyback converter in accordance with an embodiment of the present invention;
FIG. 10 illustrates additional control circuitry for a flyback converter in accordance with an embodiment of the present invention;
FIG. 11 illustrates a differential signal converter for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention;
FIG. 12 illustrates a graph of switching frequency vs. power for a flyback converter in accordance with an embodiment of the present invention;
FIG. 13 illustrates an oscillator for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention; and
FIG. 14 illustrates a comparator for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The present invention is directed toward a flyback power converter for a switching power supply and manner of operation thereof. The flyback converter can be employed in an offline switching power supply. In accordance with an embodiment of the present invention, the flyback converter employs two synchronously-operated transistor switches on the transformer primary side. A first of the transistor switches couples the transformer primary winding to a ground node and is referred to herein as the “low side” switch. A second of the two transistor switches couples the transformer primary winding to an input voltage via a snubber capacitor and is referred to herein as the “high side” switch. Switching is controlled in a feedback loop to generate a regulated DC output voltage at the transformer secondary side. In accordance with an embodiment of the present invention, the flyback converter can be operated in a frequency-control feedback loop or a current-control feedback loop, depending upon loading conditions. In accordance with a further embodiment, the flyback converter seamlessly transitions between frequency control and current control feedback loops. In accordance with a further embodiment of the invention, one or both of the low side and high side switches are operated in accordance with zero volt switching (ZVS). These and other aspects of the present invention are described herein.
FIG. 1 illustrates a block schematic diagram of a two-stage, off-line power supply 100 in accordance with an embodiment of the present invention. As shown in FIG. 1, a power factor correction (PFC) stage 102 has an input coupled to alternating-current (AC) source. The PFC stage 102 performs rectification on the AC input signal and maintains current drawn from the AC source substantially in phase with the AC voltage so that the power supply 100 appears as a resistive load to the AC source.
The PFC stage 102 generates a loosely regulated voltage, VDC, which is provided as input to a DC-to-DC converter 104. Using the input VDC, the DC-to-DC converter stage 104 generates a voltage-regulated, DC output, VO, which can be used to power a load. The level of VDC is preferably at a higher voltage and is more loosely regulated than the output VO of the DC-to-DC converter stage 104. The nominal level of the output, VDC, of the PFC stage 102 may be, for example, approximately 380 volts DC, while the voltage-regulated output VO of the DC-to-DC converter stage 104 may be, for example, approximately 12.0 volts DC.
FIG. 2 illustrates a flyback converter 150 in accordance with an embodiment of the present invention. The flyback converter is suitable for use in a DC-to-DC converter of a switching power supply, such as the DC-to-DC converter 104 of FIG. 1. The flyback converter 150 receives an input voltage from source VIN that can be a PFC stage output, VDC, or that can be received from some other source, such as an electromagnetic interference (EMI) filter.
As shown in FIG. 2, the input voltage source VIN is coupled to a first terminal of a capacitor CSN and to a first terminal of a primary winding of a transformer T1. The capacitor CSN functions as a snubber capacitor. A voltage VCSN having polarity as shown in FIG. 2 is formed across the capacitor CSN. A second terminal of the primary winding of the transformer T1 is coupled to a first terminal of a switch SW1 (“low side” switch) and to a first terminal of a switch SW2 (“high side” switch). A voltage V is formed at a node between the low side switch SW1 and the high side switch SW2 and at the second terminal of the primary winding of the transformer T1. A second terminal of the switch SW1 is coupled to a first ground node. A second terminal of the switch SW2 is coupled to a second terminal of the capacitor CSN. The switches SW1 and SW2 can be implemented as power MOSFET transistors; thus, a body diode is shown associated with each of switches SW1 and SW2. The switch SW1 is controlled by a signal LOWOUT while the switch SW2 is controlled by a signal HIGHOUT.
The low side switch SW1 and the high side switch SW2 are each preferably implemented by a corresponding MOSFET.
A first terminal of a secondary winding of the transformer T1 is coupled to an anode of a Zener diode D1. A cathode of the diode D1 is coupled to a first terminal of a capacitor C1. A second terminal of the secondary winding of the transformer T1 is coupled to a second terminal of the capacitor C1 and to a second ground node. The first and second ground nodes are preferably isolated from each other.
The flyback converter 150 is operated by opening and closing the switches SW1 and SW2. The transformer T1 transfers energy from the input of the flyback converter 150 to its output and provides isolation between the input and output of the flyback converter 150. In operation, when the switch SW1 is closed (the switch is turned “ON”), voltage source VIN is applied across the primary winding of the transformer T1. As a result, a current in the primary winding and a magnetic flux in the transformer T1 increases, which stores energy in the transformer T1. When the switch SW1 is then opened (the switch is turned “OFF”), the current in the primary winding and the magnetic flux drops. As a result, a current is induced in the secondary winding of the transformer T1 that charges the capacitor C1 with energy to generate an output voltage VO for powering a load.
The amount of energy transferred to the load can be controlled by adjusting the switching duty cycle of the switch SW1 (e.g., by controlling peak input current), the switching frequency of the switch SW1, or both. Controlling the duty cycle is referred herein to as peak current control, whereas, controlling the switching frequency is referred to herein as frequency control.
When the switch SW1 is opened and the switch SW2 is in the closed position (the switch SW2 is “ON”), the current in the primary winding of the transformer T1 can pass through the switch SW2 to the snubber capacitor CSN. Alternatively, when the switch SW1 is opened and the switch SW2 is in the open position (the switch SW2 is “OFF”), the current in the primary winding of the transformer T1 can pass through the body diode of the switch SW2.
The high side switch SW2 is preferably controlled such that it is open (OFF) when the low side switch SW1 is closed (ON). Then, when the switch SW1 is opened (OFF) and the energy from the transformer T1 has been largely discharged to the output capacitor C1, the voltage V will be equal to VCSN. Under these conditions, the switch SW2 is briefly closed (ON). The switch SW2 is thus operated under zero volt switching (ZVS) conditions. Closing the switch SW2 discharges the level of V to that of VIN. Then, once V is substantially equal to VIN, the switch SW2 is opened (OFF). The voltage V continues to fall after the switch SW2 is opened, such that when the switch SW1 is closed, the voltage across it is zero or nearly zero. Thus, the switch SW1 is also preferably operated under zero volt switching (ZVS) conditions. The cycle then repeats.
To summarize, during a switching cycle, the low side switch SW1 is turned off; then the high side switch SW2 is turned on and then off immediately before the low side switch SW1 is turned on. The cycle is then repeated (i.e. SW1—off, SW2—on, SW2—off, SW1—on, SW1—off, . . . ). The high side switch SW2 is thus turned on then off once for before each low side switch SW1 turn on. Also, the high side switch SW2 is turned on then off once for each cycle of the low side switch SW1 (while the low side switch SW1 is off). In other words, each switch is turned on and then off while the other switch is off.
In an embodiment, both switches SW1 and SW2 are operated under ZVS, regardless of load. Thus, they are both operated under ZVS from no load to full load.
The flyback converter 150 has a resonant switching frequency. The resonant frequency is dependent upon physical characteristics of the flyback converter 150, including the inductance value of the transformer T1 primary winding and parasitic capacitance of the switches SW1 and SW2. When the switch SW2 is closed, this introduces the capacitance of the snubber capacitor CSN and therefore effectively changes the resonant frequency of the flyback converter 150 while the switch SW2 is closed.
The diode D1 coupled to the transformer T1 secondary winding operates as a freewheeling diode, allowing current in the secondary winding of the transformer T1 to charge the capacitor C1, and preventing discharge of the capacitor C1 through the transformer T1. The diode D1 can be replaced with a switch that is operated synchronously with the switches SW1 and SW2 (synchronous rectification).
FIG. 3 shows a voltage waveform illustrating quasi-resonant operation of a flyback converter in accordance with an embodiment of the present invention. The waveform represents the level of V for two switching cycles. As shown in FIG. 3, the switch SW1 is initially closed (ON) at time t0 so that the level of V is essentially zero volts. This causes current to flow in the primary winding of the transformer T1 which charges the primary winding of the transformer T1 with energy. The switch SW1 is then opened (OFF) at time t1. As a result, the level of V rapidly rises to a level above that of VIN. Current then passes through the body diode of the switch SW2 while energy from the transformer T1 induces a current in the secondary winding of the transformer T1 which charges the output capacitor C1. Then, when the voltage V is equal to VCSN, or nearly equal to VCSN, the switch SW2 is briefly closed (ON) at time t2. This discharges the level of V to that of VIN. Then at time t3 the switch SW2 is opened (OFF) and the switch SW1 is closed (ON). This causes the level of V to fall to zero volts while a current again flows in the primary winding of the transformer T1 and the switching cycle repeats.
Because the voltage V is equal to or nearly equal to VCSN when the switch SW2 is closed, the switch SW2 is operated under zero volt switching (ZVS) conditions. The waveform shown in FIG. 3 assumes that the switch SW2 is closed (ON) as soon as the voltage V first becomes equal to VCSN, or nearly equal to VCSN. This is referred to herein as “quasi-resonant” or “first hill” switching. If the closing of the switch SW2 is delayed, the voltage V will tend to oscillate. Zero volt switching can be maintained under these conditions if the switch SW2 is closed at a time when V is equal to or nearly equal to VCSN during oscillation of the voltage V. This is referred to herein as “valley switching.”
FIG. 4 shows a voltage waveform illustrating valley switching operation of a flyback converter in accordance with an embodiment of the present invention. The waveform of the voltage V shown in FIG. 4 is equivalent to that shown in FIG. 3 except that the voltage V oscillates prior to closing of the switch SW2. FIG. 4 shows two oscillations occurring prior to the closing of switch SW2 at time t2. It will be apparent that greater or fewer oscillations can occur while maintaining zero volt switching so long as the switch SW2 is closed at a time when V is equal to or nearly equal to VCSN.
The flyback converter 150 can selectively operate in accordance with quasi-resonant switching (as in FIG. 3) or valley switching (as in FIG. 4), depending upon conditions such as switching frequency, loading conditions, component values, and so forth, in order to regulate the output voltage.
By controlling the high side switch SW2 synchronously with ZVS, this provides for more efficient operation, for example, by avoiding losses caused by non-ZVS switching, and allows for operation at higher switching frequencies than otherwise which also tends to increase efficiency of the flyback converter.
FIG. 5 illustrates a flyback converter 150 and control circuitry in accordance with an embodiment of the present invention. The flyback converter 150 of FIG. 2 is shown in FIG. 5 along with additional circuitry. In particular, a “low driver” controller 152 generates a signal LOWOUT that controls (opens and closes) the switch SW1. The low driver controller 152 can control the switch SW1 using frequency control and/or peak current control in a feedback loop so as to regulate the output voltage VO. A “high driver” controller 154 generates a signal HIGHOUT that controls (opens and closes) the switch SW2.
As shown in FIG. 5, a resistive divider and photo-couple network 156 is coupled to the flyback converter 150 output and includes resistors R1, R2, and R3, capacitor C2, photo-diode P1A, and shunt regulator U1. The photo-diode P1A is optically coupled to phototransistor P1B. The phototransistor P1B is coupled to a compensation resistor R4 and capacitor C3. A voltage signal VEAO is generated across the compensation resistor R4 and capacitor C3. The signal VEAO is representative of an error signal (a difference between the level of VO and a desired level for VO) and is also representative of a level of input power to the flyback converter 150. The signal VEAO is electrically isolated from the output voltage VO and is instead referenced to the ground level of the primary side of the transformer T1.
The transformer T1 can include a second secondary winding. As shown in FIG. 5, a first terminal of the second secondary winding of the transformer T1 is coupled to an anode of a diode D2. A cathode of the diode D2 is coupled to a first terminal of a capacitor C4. A second terminal of the second secondary winding of the transformer T1 is coupled to a second terminal of the capacitor C4 and to the first ground node. A voltage VCC is formed across the capacitor C4 and can be used for powering control circuitry of the flyback converter 150. A resistive divider includes resistors R5 and R6 and generates a voltage signal ZCD that is representative of the level of VCC. The signal ZCD is also representative of the level of V.
As also shown in FIG. 5, a current sensing resistor RSENSE is coupled between the second terminal of the transistor switch SW1 and the first ground node. A current sensing signal ISENSE is formed across the resistor RSENSE.
The low driver controller 152 receives as inputs the signals ZCD, ISENSE, VEAO as well as an oscillator signal OSC and uses these signals to generate the signal LOWOUT for controlling the transistor switch SW1 as explained herein. Briefly, the signal VEAO represents the load power and is used to regulate the output voltage in a feedback loop based on either peak current control or switching frequency control. The signal ISENSE represents the current in the transformer T1 and is used to the control peak current in the transformer primary winding during switching. The oscillator signal OSC is used for controlling the timing of switching. The signal ZCD is representative of the level of V and is used to turn on the switch SW1.
The low driver controller 152 generates a differential signal READYHIGHON which is used by the high driver controller 154 for controlling the transistor switch SW2 as explained herein. Briefly, the signal READYHIGHON informs the high driver controller 154 that it can (i.e. has permission to) turn on the switch SW2, though the high driver controller 154 determines the timing of turning on the switch SW2. The signal READYHIGHON is preferably a differential signal because the low driver controller 152 and the high driver controller 154 have different ground reference nodes. In particular, the low driver controller 152 is referenced to the first ground node, whereas, the high driver controller 154 preferably uses the voltage V as its reference.
As also shown in FIG. 5, a first terminal of a first high-voltage resistor RHV1 is coupled to the second terminal of the capacitor CSN. A second terminal of the resistor RHV1 is coupled to the high driver controller 154. This provides the high driver controller 154 with a signal CS that is representative of the voltage VCSN. A first terminal of a second high-voltage resistor RHV2 is coupled to the input voltage VIN. A second terminal of the resistor RHV2 is coupled to the high driver controller 154. This provides the high driver controller 154 with a signal RVIN that is representative of the voltage VIN. The voltage V signal is also coupled to the high driver controller 154. The high driver controller 154 uses the signals RVIN, CS, V, and READYHIGHOUT to generate the signal HIGHOUT that controls (opens and closes) the switch SW2 as explained herein. Briefly, when V is greater than VIN and CS is substantially equal to V, the high driver controller 154 turns on the switch SW2. The switch SW2 stays on until V is substantially equal to VIN and then the switch SW2 is turned off.
Also shown in FIG. 5, the voltage VCC can be used as a power supply for powering elements of the low driver controller 152. A voltage VBOOT can be used as a power supply for powering elements of the high driver controller 154. The voltage VBOOT can be obtained by drawing current from VCC, for example, via a diode which then charges a capacitor CVBOOT.
A switching cycle is performed as follows. The low side switch SW1 is turned on. Then, once the peak current in the primary winding of the transformer T1 is reached, as indicated by the current sensing signal ISENSE, the low side switch SW1 is turned off. The peak current depends on the level of VEAO: (1) when VEAO is less than a threshold (e.g. 2.5 volts), then the flyback converter is in frequency control mode and the peak current is essentially a fixed value (though the peak current is preferably gradually reduced as VEAO falls in order to increase efficiency and inhibit audible noise in burst mode); (2) when VEAO is greater than the threshold (e.g. 2.5 volts), then the flyback converter is in current control mode and the peak current depends on VEAO (and the switching frequency is clamped). Once the low side switch SW1 turns off, the voltage V flys up, eventually reaching a level above the input voltage VIN. The low side driver 152 then activates sending the READYHIGHON signal to the high side driver 154. The READYHIGHON signal is activated at a time that depends upon the switching frequency. After receiving the READYHIGHON signal, the high side driver 154 determines that V is greater than VIN and, in response to this determination, the high side driver 154 turns on the high side switch SW2. The high side switch SW2 remains on until the level of V falls to the level of VIN, as which time, high side driver 154 turns off the high side switch SW2. When the level of V falls to zero, the low side switch SW1 can be turned on again.
The waveforms of FIGS. 3 and 4 apply equally to the current control and frequency control modes, though the time scale will change, dependent upon the mode of operation.
FIG. 6 illustrates an integrated circuit (IC) controller for a DC-to-DC converter in accordance with an embodiment the present invention. In a preferred embodiment, the IC controller is implemented as an IC package 200 that includes the low driver controller 152 as a first monolithic IC chip and the high driver 154 as a second monolithic IC chip, both included in the same 18-pin IC package. In an embodiment, the switch SW2 is integrated into the high driver controller 152 IC chip. Also, in an embodiment, the resistors RHV1 and RHV2 is included in the IC package. One or both of the resistors RHV1 and RHV2 can be integrated into the high driver controller 152 IC chip. Further, the resistor RHV2 can be partially integrated into the high driver controller 152 IC chip. As explained herein, each of the two IC chips has a different ground reference. Communication between the two chips is via the differential signal READYHIGHON.
FIG. 6 shows signals assigned to each of the 18 pins:

    
    
        Pin 1 CS
        Pin 2 N/C
        Pin 3 VIN 
        Pin 4 N/C
        Pin 5 ZCD
        Pin 6 OTP
        Pin 7 OCS
        Pin 8 RESET
        Pin 9 VEAO 
        Pin 10 ISENSE 
        Pin 11 VSSD 
        Pin 12 LOWOUT
        Pin 13 VCC 
        Pin 14 N/C
        Pin 15 RVIN 
        Pin 16 VBOOT 
        Pin 17 HIGHOUT
        Pin 18 V
    
    


Pin 2, pin 4, and pin 14 are not used and are labeled “N/C” or “no connection.” A diode is connected between pin 13 and pin 16. OTP can be an over-temperature protection pin that provides a current to an external thermistor, the voltage on which can then be compared to a reference, such as 1.0 volt, to detect an over-temperature condition. VSSD is a ground pin. A reset pin RESET can be used to reset the ICs of the package after entering a protection mode. The reset can be accomplished by pulling the RESET pin to a voltage that is less than a reference voltage such as 2.5 volts.
FIG. 7 illustrates a high-voltage resistor RHV1 in accordance with an embodiment of the present invention. A first terminal of the high-voltage resistor RHV1 is coupled to receive the signal VCSN. Within the high-voltage resistor RHV1 the first terminal is coupled to a first terminal of a MOSFET M1 and to a first terminal of a resistor R7. A second terminal of the MOSFET M1 is coupled to the signal V. A second terminal of the resistor R7 is coupled to the signal RVIN that is representative of the voltage VIN. The resistor R7 can be, for example, 10 Megaohms. The resistor RHV1 can be integrated into the high driver controller 152 IC chip or can be included in a separate chip within the IC package.
FIG. 8 illustrates a two-way, high-voltage resistor RHV2 in accordance with an embodiment of the present invention. A first terminal of the high-voltage resistor RHV2 is coupled to the input voltage VIN. Within the high-voltage resistor RHV2 the first terminal is coupled to a first terminal of a MOSFET M2 and to a first terminal of a resistor R8. A second terminal of the MOSFET M2 is coupled a second terminal of the resistor R8, to a first terminal of a MOSFET M3 and to a first terminal of a resistor R9. A second terminal of the MOSFET M3 is coupled to a second terminal of the resistor R9 and to the signal CS. The resistors R8 and R9 can each be, for example, 10 Megaohms. The resistor RHV2 can be fully or partially integrated into the high driver controller 152 IC chip. If partially integrated, the MOSFET M2 and the resistor R8 can be integrated, while the MOSFET M3 and the resistor R9 can be included in a separate chip within the IC package. Alternatively, the MOSFET M3 and the resistor R9 can be integrated, while the MOSFET M2 and the resistor R8 can be included in a separate chip within the IC package. Alternatively, MOSFET M3 and the resistor R9 can be included a first separate chip within the IC package, while the MOSFET M2 and the resistor R8 can be included in a second separate chip within the IC package.
FIG. 9 illustrates control circuitry of the high driver 154 in accordance with an embodiment of the present invention. A signal RVIN, which approximates the signal VIN, is compared by a comparator CMP1 to the signal V. An output of the comparator CMP1 is inverted by an inverter 158 to form a logic signal V>VIN. The signal V>VIN is coupled to an input of a NAND gate 160, to an inverted set input S-bar of a flip-flop FF1, to an input of a NAND gate 161 and to an input of a NAND gate 162.
The signals at the input of the comparator CMP1 are shown as approximately equal VIN and V. These input signal levels may be adjusted (e.g. by a current source adding or removing current from each comparator input node) in order to compensate for signal path delays during high-frequency operation and to limit their amplitudes (e.g., by diode clamping).
The signal VCSN is coupled to a first terminal of a resistor R10 and to a first terminal of a MOSFET M4. The signal VCSN may also be amplitude limited by diode clamping. A second terminal of the resistor R10 is coupled to a first input terminal of a comparator CMP2. A second terminal of the MOSFET M4 is coupled to the signal V and to a reference node of the comparator CMP2. The signal V is coupled to a second input terminal of the comparator CMP2. The comparator CMP2 compares the signal VCSN to the signal V to form a current-limit signal ILIMIT. The signal ILIMIT is provided via an inverter 164 to an input of the NAND gate 160 and to an inverted set input S-bar of a flip-flop FF2. The signal ILIMIT indicates that the level of VCSN is equal to V and that the switch SW2 can be opened.
The signal ILIMIT is coupled to an inverted set input S-bar to a flip-flop FF3. An output Q of the flip-flip FF3 is coupled to an input of a one-shot circuit 166. An inverted output of the one-shot circuit 166 is coupled to an inverted reset input R-bar to the flip-flip FF2. An inverted output Q-bar of the flip-flop FF2 is coupled to an input of the NAND gate 160.
The signal READYHIGHON is coupled to an input of the NAND gate 160, to a first inverted reset input R1-bar to the flip-flop FF1, to an input of a delay 168 and to an input of the NAND gate 162. An inverted under-voltage lockout signal UVLO-bar is coupled to a first inverted reset input R1-bar to the flip-flop FF3 and to second inverted reset input Rs-bar to the flip-flop FF1. An output Q of the flip-flop FF1 is coupled to an input of the NAND gate 162. An output of the delay 168 is coupled to an input of the NAND gate 162.
An output of the NAND gate 160 is coupled to a first inverted set input S1-bar of a flip-flop FF4. An output of the NAND gate 162 is coupled to a second inverted set input S2-bar of the flip-flop FF4. An output Q of the flip-flop FF4 is coupled to an input of a gate 170. An output of the gate 170 is coupled to a second reset input to the flip-flop FF3. A non-inverted output of the gate 170 forms the signal HIGHOUT. The non-inverted output of the gate is coupled to a second input of the NAND gate 161. An output of the NAND gate 161 is coupled to a first input of a NAND gate 171. The inverted under-voltage lockout signal UVLO-bar is coupled to a second input of the NAND gate 171. An output of the NAND gate 171 is coupled to an inverted reset input R-bar to the flip-flop FF4.
Elements of FIG. 9 detect occurrence of the “first hill” for performing quasi-resonant switching as shown in FIG. 3. The signal READYHIGHON informs the high driver controller 154 that it can (i.e. has permission to) turn on the switch SW2. The high driver controller 154 then determines the timing of turning on the switch SW2: when V is greater than VIN (as indicated by the signal V>VIN) the high driver controller 154 turns on the switch SW2.
The flip-flop FF1 and the delay block 168 are used to delay turning on the switch SW2 so as to avoid turning on the switch prematurely. The UVLO signal inhibits switching in case of an under-voltage condition.
The switch SW2 stays on until V is substantially equal to VIN and then the switch SW2 is turned off. This is determined when VCSN is substantially equal to V, as indicated by the signal ILIMIT. The generated signal HIGHOUT is used to control the switch SW2.
FIG. 10 illustrates control circuitry of the low driver 152 in accordance with an embodiment of the present invention. As shown in FIG. 11, the low driver 152 includes a current control section 172, a frequency control section 174, a timer section 176, switching logic 178 and a switch driver 180.
Within the current control section 172 of the low driver 152, the signal VEAO is coupled to a first input to a comparator CMP3. A second input to the comparator CMP3 receives a first reference voltage (e.g. 2.5 volts) while a third input to the comparator CMP3 receives a second reference voltage (e.g. 2.0 volts). The comparator CMP3 generates a signal “VEAO>2.5v−bar” by comparing the signal VEAO to the first and second reference voltages; the signal “VEAO>2.5v−bar” is activated when VEAO rises above the first reference and is deactivated when the signal VEAO falls below the second reference. Thus, the comparator CMP3 performs its comparison with hysteresis. The comparator CMP3 determines whether low driver controller 154 performs switching based on peak current control or based on frequency control. When VEAO rises above 2.5 volts, switching is by peak current control; when VEAO falls below 2.0 volts, switching is by frequency control. Thus, the logic level of “VEAO>2.5v−bar” determines whether the switching is based on peak current control or frequency control.
The signal ISENSE is coupled an input to a first amplifier 182 and to an input to a second amplifier 184. The amplifier 182 can have, for example, a gain of 15, while the amplifier 184 can have a gain of, for example 7.5. The output of the amplifier 182 is coupled to a first input to a comparator CMP4 via a switch S1. The output of the amplifier 184 is coupled to the first input of the comparator via a switch S2. The signal VEAO is coupled to a second input to the comparator CMP4. The signal “VEAO>2.5v−bar” is coupled to control the switch S2 and to control the switch S1 via an inverter 186. Thus, one of the switches S1 are S2 is closed while the other is opened dependent upon the signal “VEAO>2.5v−bar”. Accordingly, the outputs of the amplifiers 182 and 184 are selectively coupled to the first input of a comparator CMP4 dependent upon the level of VEAO. An output of the comparator CMP4 is coupled to an input to switching logic 178.
Under current control, the amplifier 182 having higher gain is active so as to magnify the effect of ISENSE in comparison to VEAO by comparator CMP4. Under frequency control, the amplifier 184 is active which employs lower gain so as to reduce the effect of ISENSE in the comparison which causes the frequency control section 174 to primarily control switching.
Within the frequency control section 174 of the low driver 152, the signal ISENSE is coupled to an input to an amplifier 188. The amplifier 188 can have, for example, a gain of 7.5. An output of the amplifier 188 is coupled to a first input to a comparator CMP5. A second input to the comparator is coupled to a reference voltage, which can be, for example, approximately 2.5 volts. An output of the comparator CMP5 is coupled to an input to switching logic 178.
The current control section 172 and the frequency control section 174 control the timing of turning off the low side switch SW1 within each switching cycle via switching logic 178.
The timer section 176 of the low driver 152 controls switching frequency as well as the timing for turning on the low side switch SW1 for each switching cycle. Within the timer section 176, the signal VEAO is coupled to a first input to an oscillator 190. The signal “VEAO>2.5v−bar” from the comparator CMP3 is coupled to a second input of the oscillator 190. The oscillator 190 generates a periodic ramp signal that is coupled to an input of timer block 192.
The timer block generates a logic signal HON and a logic signal ONSET which are coupled to the switching logic 178. The signal HON is used to generate the signal READYHIGHON for the high driver controller 154. For peak current control, the signal HON is generated at fixed intervals. The signal ONSET is used to turn on the low side switch SW1. For example, a timer of 3.33 microseconds can be reset for each switching cycle; 500 nanoseconds prior to expiration of the timer, the signal HON is activated. And, upon expiration of the timer, and once ZCD is greater than zero, then the signal ONSET can be activated. Once ONSET is activated, then the low side switch SW1 can be closed upon a valley in signal ZCD (since ZCD represents V) so as to operate the switch SW1 under zero volt switching (ZVS) conditions. The 500 nanosecond difference ensures that HON is activated prior to ONSET.
The signal ZCD is referenced to the same ground level as the low side driver controller 152. The signal ZCD is also representative of the level of V. Thus, signal ZCD is used by the low side driver controller 152 as a proxy for V in order to operate the switch SW1 under ZVS conditions.
For current control, rather than a fixed timer interval of 3.33 microseconds, for example, the timer interval is varied dependent upon the level of VEAO. Thus, the timer interval affects the switching frequency for regulating the output voltage in a feedback loop.
The timer interval of 3.33 microseconds corresponds to a switching frequency for peak current control of 300 kHz. In an embodiment, the switching frequency fclamp can be clamped at 500 kHz, 300 kHz, 145 kHz, or some other selected frequency by appropriate selection of timing components.
An output of the switching logic 178 is coupled to a driver 180. The driver 180 generates the signal LOWOUT.
FIG. 11 illustrates a differential signal converter 200 for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention. The differential signal converter 200 converts the single-ended signal HON to a differential logic signal READYHIGHON. The logic signal HON is coupled to an input of a first inverter 202. An output of the first inverter 202 is coupled to an input to a second inverter 204 and to control a MOSFET M5. An output of the second inverter 204 is coupled to control a MOSFET M6. A MOSFET M7 and a current source 206 are coupled in series with the MOSFET M5. A MOSFET M8 and a current source 208 are coupled in series with the MOSFET M6. A current source 208 is coupled in series with MOSFET M9 and MOSFET M10. A reference current passes through the MOSFETS M9 and M10. The signal HON activates one of the MOSFETS M5 or M7 dependent upon the level of HON. The reference current is mirrored in the MOSFET M8 or in the MOSFET M7 dependent upon which of the MOSFETS M5 or M7 is active. The state of the differential signal READYHIGHON is dependent upon which of the MOSFETS M5 or M7 is active. Thus, the converter 200 converts the logic signal HON to the differential logic signal READYHIGHON.
FIG. 12 illustrates a graph of switching frequency vs. power for a flyback converter in accordance with an embodiment of the present invention. Switching frequency fsw is plotted on the x-axis while input power, as measured by the signal VEAO, is plotted on the y-axis. As shown in FIG. 12, when VEAO is below 0.75 volts, this indicates very light load conditions. In this mode, the flyback converter is preferably operated in “burst” mode. In such a burst mode, switching can be paused between “bursts” of switching for increased efficiency. Once the level of VEAO rises above 0.75 volts, the flyback converter can be operated in frequency control mode in which the switching frequency is modulated in a feedback loop to regulate the output voltage VO. Once the level of VEAO surpasses a threshold of, for example, 2.5 volts, then the switching frequency is clamped to predetermined value fclamp and the flyback converter enters a current control mode. In this current control mode, the current level, as sensed by the signal Isense, is controlled in a feedback loop to regulate the output voltage. As the power level rises, the switching frequency can additionally be reduced, as shown in FIG. 12 as the power level approaches full load though voltage regulation is still primarily performed through the current control feedback loop.
As shown in FIG. 12, the switch frequency vs. power curve is discontinuous. When transitioning from frequency control to current control (e.g., when VEAO rises above 2.5 volts), the switching frequency is suddenly increased and the peak current in the transformer primary winding is at the same time reduced. Conversely, when transitioning from current control to frequency control (e.g., when VEAO falls below 2.0 volts), the switching frequency is suddenly reduced and the peak current in the transformer primary winding is at the same time increased. In both modes, negative feedback is employed to regulate the output voltage. It is therefore important that operation of the flyback converter remains stable as is transitions between the two modes of operation.
FIG. 13 illustrates an oscillator for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention. FIG. 13 shows additional details of the oscillator 190 of FIG. 10. As shown in FIG. 13, the signal VEAO is coupled to a first input to an amplifier AMP1 via a switch S3. A reference voltage of, for example, 2.5 volts, is coupled to a second input of the amplifier AMP1 via a switch S4. A third input to the amplifier AMP1 is coupled to a first terminal of an adjustable resistor ROSC1 and to a first terminal of a resistor ROSC2. An output of the amplifier AMP1 is coupled to a control terminal of MOSFET M11. An output terminal of the MOSFET M11 is coupled to the first terminal of the resistor ROSC1 and to the first terminal of the resistor ROSC2. A second terminal of the resistor ROSC1 is coupled to a ground node via a switch S5. A second terminal of the resistor ROSC2 is coupled to the ground node via a switch S6.
A supply voltage VCC is coupled to an input terminal of a MOSFET M12 and to an input terminal of a MOSFET M13. An output terminal of the MOSFET M12 is coupled to a control terminal of the MOSFET M12, to a control terminal of the MOSFET M13 and to an input terminal of the MOSFET M11. An output terminal of the MOSFET M13 is coupled to a first terminal of an adjustable capacitor CT, to a first input terminal (inverting) to a comparator CMP6 and to a first input terminal (non-inverting) to a comparator CMP7. A second terminal of the adjustable capacitor CT is coupled to the ground node. A second input terminal to the comparator CMP6 is coupled to a reference voltage Vrefh. A second input terminal to the comparator CMP7 is coupled to a reference voltage Vrefl. An output of the comparator CMP6 is coupled to an inverted set input S-bar of a flip-flop FF5. An output of the comparator CMP7 is coupled to an inverted reset input R-bar of the flip-flop FF5. An output Q of the flip-flop FF5 is coupled to control a switch S7. The switch S7 is coupled to across the capacitor CT.
The switches S3 and S5 are controlled by the signal “VEAO>2.5v” while the switches S4 and S6 are controlled by the logic signal “VEAO>2.5v−bar”. Thus, when VEAO is greater than the 2.5 volt threshold, the switches S3 and S5 are closed and the switches S4 and S6 are open; when VEAO is below the 2.0 volt threshold, the switches S4 and S6 are closed and the switches S3 and S5 are open. As explained herein, the signals “VEAO>2.5v” and its inverse “VEAO>2.5v−bar” are generated with hysteresis.
The oscillator 190 generates a periodic ramp signal RTCT across the capacitor CT. The transistors M12 and M13 form a current mirror such that the current through the transistor M13 charges the capacitor CT. When the voltage across the capacitor CT reaches Vref, the capacitor CT is discharged by closing the switch S7 until the voltage across the capacitor CT falls below Vrefl. The switch S7 is then opened.
The frequency of the ramp signal RTCT is changed dependent upon of the state of the logic signal “VEAO>2.5v”. More particularly, when VEAO is less than 2.0 volts (the signal “VEAO>2.5v” is a logic “0”), the flyback converter operates in the frequency control mode in which the switching frequency and is dependent upon the level of VEAO. This is accomplished by closing the switch S3 so that VEAO is coupled to the amplifier AMP1 which turns on the MOSFET M11 in relation to the level of VEAO. The level of current in the current mirror of MOSFET M12 and M13 is, therefore, affected by the level of VEAO which, in turn, affects the rate of charging the capacitor CT and the frequency of the ramp signal RTCT. The frequency of the ramp signal RTCT is the same as the switching frequency of the flyback converter. Thus, in this frequency control mode, the switching frequency is controlled in a feedback loop to regulate the output voltage where the switching frequency is dependent upon VEAO.
The frequency control mode continues unless VEAO rises above 2.5 volts. When VEAO rises above 2.5 volts and the signal “VEAO>2.5v” becomes a logic “1” then the switch S3 is opened and the switch S4 is closed which couples a fixed reference voltage to the input of the amplifier AMP1 so that the current that charges the capacitor CT is essentially constant. This causes the switching frequency for the flyback converter to be essentially constant; in this mode, current is controlled in feedback loop to regulated the output voltage.
The frequency of the ramp signal RTCT and, thus, the switching frequency of the flyback converter is dependent upon the value of CT as well as the resistor ROSC1 and ROSC2. In the current control mode, the switch S6 is closed so that the resistor ROSC2 affects the switching frequency whereas the switch S5 is opened so that the resistor ROSC1 does not affect the switching frequency. In the frequency control mode, the switch S6 opened so that the resistor ROSC2 no longer affects the switching frequency and the switch S5 is closed so that the resistor ROSC1 does affect the switching frequency.
The values of CT, ROSC1 and ROSC2 are selected so as to appropriately set the nominal switching frequency in the frequency control mode, as well as the essentially fixed switching frequency in the current control mode. Additionally, the values of the resistor ROSC1 and the capacitor CT can preferably be fine-tuned, e.g. by laser or fuse trimming in order to ensure that there is a smooth transition between the frequency control and current control modes. For this purpose, the resistor ROSC1 is preferably incorporated into the IC package shown in FIG. 6.
Component selection for the oscillator can include first selecting a value for the resistor ROSC2, which sets the clamping frequency fclamp. Then, the capactor CT, which is preferably internal to the low driver controller IC 152, is trimmed to fine-tune the clamping frequency. Finally, the internal resistor ROSC1, which is also preferably internal to the low driver controller IC 152, is trimmed to fine-tune the switching frequency at the transition between current control and frequency control modes of operation.
FIG. 14 illustrates a comparator for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention. The comparator of FIG. 14 can be used in place of the comparator CMP5 shown in the frequency mode control section 174 of FIG. 10. As shown in FIGS. 10 and 14, the comparator accepts as input the signal ISENSE×7.5 which is compared to a reference voltage of 2.5 volts for generating the signal OFF. The signal OFF is used to turn off the main switch SW1. The comparator of FIG. 14 additionally accepts as input the signal VEAO. The signal VEAO reduces the effective level of the reference voltage thereby generating the signal OFF sooner and therefore reducing the switching frequency. This is useful to reduce switching noise in burst mode.
The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the embodiments disclosed. Accordingly, the scope of the present invention is defined by the appended claims.

## Claims

1. A switching power supply comprising:
a flyback power converter configured to perform DC-to-DC power conversion, the flyback power converter having a transformer, a low side switch for drawing current from a supply voltage through a primary winding of the transformer, and a high side switch for discharging current from the primary winding of the transformer to a capacitor; and
a controller configured to synchronously control the opening and closing of the low side switch and the high side switch so as to form a regulated output voltage in a feedback loop, the output voltage being formed from a current induced in a secondary winding of the transformer wherein, during each switching cycle, the high side switch is held open while the low side switch is closed and then opened and the low side switch is held open while the high side switch is closed and then opened, and wherein a first voltage is generated at a node between the low side switch and the high side switch during operation, and wherein the high side switch is closed when the supply voltage is greater than the first voltage and the high side switch is opened when the first voltage supply voltage falls to a level of the supply voltage.

2. The switching power supply according to claim 1, wherein a controller integrated circuit package comprises the controller.

3. The switching power supply according to claim 2, wherein the controller integrated circuit package comprises a first monolithic integrated circuit chip that comprises a low side driver controller configured to control the low side switch and a second monolithic integrated circuit chip that comprises a high side driver controller configured to control the high side switch.

4. The switching power supply according to claim 3, wherein the controller integrated circuit package comprises a first high voltage resistor and a second high voltage resistor.

5. The switching power supply according to claim 4, wherein the first high voltage resistor couples the supply voltage to the high side driver controller and the second high voltage resistor couples a voltage from the capacitor to the high side driver controller.

6. The switching power supply according to claim 5, wherein one or both of the first high voltage resistor and the second high voltage resistor are fully or partially incorporated into the second monolithic integrated circuit chip.

7. The switching power supply according to claim 1, wherein the high side switch is closed and then opened immediately before the low side switch is closed.

8. The switching power supply according to claim 1, wherein the first voltage oscillates for at least one cycle before the high side switch is closed.

9. The switching power supply according to claim 1, wherein one or both of the high side switch and the low side switch is operated in accordance with zero volt switching.

10. The switching power supply according to claim 1, wherein both of the high side switch and the low side switch are operated in accordance with zero volt switching regardless of loading of the power supply.

11. The switching power supply according to claim 1, wherein a period that the high side switch is closed is shorter than a period that the low side switch is open.

12. A switching power supply comprising:
a flyback power converter configured to perform DC-to-DC power conversion, the flyback power converter having a transformer, a low side switch for drawing current from a supply voltage through a primary winding of the transformer, and a high side switch for discharging current from the primary winding of the transformer to a capacitor; and
a controller configured to synchronously control the opening and closing of the low side switch and the high side switch so as to form the regulated output voltage in a feedback loop, the output voltage being formed from a current induced in a secondary winding of the transformer, wherein the controller is configured to selectively operate in: a frequency control mode in which frequency of switching is controlled in a feedback loop for regulating the output voltage; and a current control mode in which peak current in the primary winding of the transformer for each switch cycle is controlled in a feedback loop for regulating the output voltage.

13. The switching power supply according to claim 12, wherein during each switching cycle, the high side switch is held open while the low side switch is closed and then opened and wherein the low side switch is held open while the high side switch is closed and then opened.

14. The switching power supply according to claim 13, wherein a period that the high side switch is closed is shorter than a period that the low side switch is open.

15. The switching power supply according to claim 13, wherein the high side switch is closed and then opened immediately before the low side switch is closed.

16. The switching power supply according to claim 13, wherein one or both of the high side switch and the low side switch is operated in accordance with zero volt switching.

17. The switching power supply according to claim 13, wherein both of the high side switch and the low side switch are operated in accordance with zero volt switching regardless of loading of the power supply.

18. The switching power supply according to claim 13, wherein a first voltage is generated at a node between the low side switch and the high side switch during operation of the low side switch and wherein the high side switch is closed when the supply voltage is greater than the first voltage and wherein the high side switch is opened when the first voltage supply voltage falls to a level of the supply voltage.

19. The switching power supply according to claim 18, wherein the first voltage oscillates for at least one cycle before the high side switch is closed.

20. The switching power supply according to claim 12, wherein the controller operates in the frequency control mode below a first load power threshold and wherein the controller transitions to the current control mode when load power exceeds the first load power threshold.

21. The switching power supply according to claim 20, wherein the controller transitions from the current control mode to the frequency control mode when load power falls below a second load power threshold, the second load power threshold being lower than the first load power threshold, thereby the controller transitions between the frequency control mode and the current control mode based on load power and with hysteresis.

22. The switching power supply according to claim 21, wherein the switching frequency changes suddenly upon the controller transitioning between the current control mode and the frequency control mode.

23. The switching power supply according to claim 22, wherein the controller comprises a monolithic integrated circuit chip configured to control the low side switch and wherein switching frequency is controlled by an oscillator and wherein the oscillator is tunable by a trimmable capacitor and a trimmable resistor incorporated into the monolithic integrated circuit chip.

24. The switching power supply according to claim 12, wherein the peak current has an initial value in the frequency control mode and wherein the peak current is reduced from its initial value under light load conditions.

25. A switching power supply comprising:
a flyback power converter configured to perform DC-to-DC power conversion, the flyback power converter having a transformer, a low side switch for drawing current from a supply voltage through a primary winding of the transformer, and a high side switch for discharging current from the primary winding of the transformer to a capacitor; and
a controller configured to synchronously control the opening and closing of the low side switch and the high side switch so as to form the regulated output voltage in a feedback loop, the output voltage being formed from a current induced in a secondary winding of the transformer wherein, during each switching cycle, the high side switch is held open while the low side switch is closed and then opened and the low side switch is held open while the high side switch is closed and then opened, and wherein a controller integrated circuit package comprises the controller and the controller integrated circuit package comprises a first monolithic integrated circuit chip that comprises a low side driver controller configured to control the low side switch and a second monolithic integrated circuit chip that comprises a high side driver controller configured to control the high side switch and further wherein the low side driver controller and the high side driver controller have different ground reference voltages and the low side driver controller communicates with the high side driver controller via a differential signal.

26. The switching power supply according to claim 25, wherein the controller integrated circuit package comprises a first high voltage resistor and a second high voltage resistor.

27. The switching power supply according to claim 26, wherein the first high voltage resistor couples the supply voltage to the high side driver controller and the second high voltage resistor couples a voltage from the capacitor to the high side driver controller.

28. The switching power supply according to claim 27, wherein one or both of the first high voltage resistor and the second high voltage resistor are fully or partially incorporated into the second monolithic integrated circuit chip.

29. The switching power supply according to claim 25, wherein the differential signal informs the high side driver that the high side driver has permission to close the high side switch.

30. A switching power supply comprising:
a flyback power converter configured to perform DC-to-DC power conversion, the flyback power converter having a transformer, a low side switch for drawing current from a supply voltage through a primary winding of the transformer, and a high side switch for discharging current from the primary winding of the transformer to a capacitor; and
a controller configured to synchronously control the opening and closing of the low side switch and the high side switch so as to form the regulated output voltage in a feedback loop, the output voltage being formed from a current induced in a secondary winding of the transformer wherein, during each switching cycle, the high side switch is held open while the low side switch is closed and then opened and the low side switch is held open while the high side switch is closed and then opened, and wherein a controller integrated circuit package comprises the controller and the controller integrated circuit package comprises a first monolithic integrated circuit chip that comprises a low side driver controller configured to control the low side switch and a second monolithic integrated circuit chip that comprises a high side driver controller configured to control the high side switch and further wherein the low side driver controller and the high side driver controller have different ground reference voltages and the low side driver controller receives a signal that is representative of a voltage at a node between the high side switch and the low side switch in order to close the low side switch under zero volt switching conditions.

31. The switching power supply according to claim 30, wherein the controller integrated circuit package comprises a first high voltage resistor and a second high voltage resistor.

32. The switching power supply according to claim 31, wherein the first high voltage resistor couples the supply voltage to the high side driver controller and the second high voltage resistor couples a voltage from the capacitor to the high side driver controller.

33. The switching power supply according to claim 32, wherein one or both of the first high voltage resistor and the second high voltage resistor are fully or partially incorporated into the second monolithic integrated circuit chip.

34. The switching power supply according to claim 30, wherein the signal that is representative of a voltage at a node between the high side switch and the low side switch is formed by a current induced in a second secondary winding of the transformer.