GB2524065A

GB2524065A - Converter

Info

Publication number: GB2524065A
Application number: GB1404494.5A
Authority: GB
Inventors: Martin Coates; Giorgio Marco Ponzo; Jose Santillan
Original assignee: TDK Lambda UK Ltd
Current assignee: TDK Lambda UK Ltd
Priority date: 2014-03-13
Filing date: 2014-03-13
Publication date: 2015-09-16
Anticipated expiration: 2034-03-13
Also published as: GB2524065B; GB201404494D0

Abstract

A half-bridge flyback converter for transferring energy from a voltage supply to one or more outputs comprises a controller 14 for controlling a pair of switches Ml, M2 to switch the converter between an on-state in which switch M1 is conducting and energy from the voltage supply is stored in an inductive component T, and an off-state in which switch M2 is conducting, and energy is delivered to the output(s). The converter switches periodically between the on-state and the off-state to regulate the voltage at each output, with an operating frequency f and a duty cycle D. When the duty cycle D is within a predefined range, the converter operates in a first mode of operation having a fixed duration off state and variable duration on-state. When the duty cycle D is outside of the range, for example during start-up, the converter operates in a second mode of operation having a fixed duty cycle D and a variable operating frequency f.

Description

CONVERTER

[OOlJ The present invention relates to an asymmetric half-bridge flyback converter.

[002J The asymmetric half-bridge flyback converter is a known circuit. For example, Us 5,808,879 describes an asymmetric half-bridge flyback converter used with fixed frequency control, whilst US 7,116,561 describes a nominally fixed on-time, variable off-time implementation. US 5,430,633 describes an active-clamp flyback converter which is also operated with nominally fixed on-time, variable off-time control.

[003J In these converters, the resonance of the transformer leakage inductance and the half-bridge capacitance is utilized, but the operating frequency is always significantly higher than the resonant freguency of the combined leakage and magnetising inductance of the transformer and the half-bridge capacitance. Thus, the resonance of the combined leakage and magnetising inductance of the transformer and the half-bridge capacitance is not utilised and provides no additional gain.

[004J According to one aspect of the present invention there is provided a half-bridge flyback converter for transferring energy from a voltage supply to one or more outputs, the converter comprising:-an inductive component T having a primary winding ISa and a secondary winding Llb for each output; a pair of switches Ml, P12 arranged as a half-bridge for connecting the primary winding Lla to the voltage supply; and a controller for controlling said switches to switch the converter between an on-state in which switch Ml is conducting and energy from the voltage supply is stored in the inductive component I, and an off-state in which switch Ml is not conducting, switch M2 is conducting, and energy is delivered to the output(s) v:a the secondary winding(s) Llb, wherein the controller controls the switches to switch periodically between the on-state and the off-state with an operating freguency f and a duty cycle D; wherein, when the duty cycle S is within a range defined by a first limit SM and a second limit DH>EJM, the controller operates the converter in a first mode of operation in which said off-state has a substantially constant duration and the duration of the on-state is varied to regulate the voltage at each output; and wherein, when the duty cycle U reaches or exceeds the second limit, or falls below the first limit, the controller operates the converter in a second mode of operation in which the duty cycle U is substantially constant and the operating frequency f is varied to regulate the voltage at each output.

[005j Thus, the converter operates in a first node (fixed off-time, variable on-time) for duty cycles within a given range, and operates in a second mode (fixed duty-cycle, variable operating frequency) for duty cycles outside that range.

[006j The first mode of operation corresponds to normal running of the converter. That is to say, when the or each output is charged to the reguired output voltage, and in the absence of fault and/or hold-up conditions.

[007) The second mode of operation applies, for example, during start-up of the converter when the or each output is charging to the required output voltage, under fault conditions, and during hold-up when the input supply to the circuit falls outside a normal range.

[008] In the second mode of operation, the resonance between the combined leakage and magnetising inductance of the inductive component T and the half-bridge capacitance is utilised. This allows for improved distribution of losses between switches Ml and M2, compared to operating with duty cycle control over a wider range of duty cycles. This also allows for improved cross regulation between multiple outputs, since the peak voltage across the leakage inductance of inductive component T is reduced, as compared with previous solutions.

[009] According to another aspect of the present invention, there is provided a method of operating a half-bridge flyback converter for transferring energy from a voltage supply to one or more outputs, the method comprising:-switching periodically between an on-state and an off-state with an operating frequency f and a duty cycle ID to regulate the voltage at each output; operating in a first mode of operation when the duty cycle 0 is within a range defined by a first limit DM>0 and a second limit DL, where DM<DJ-i<100%, wherein, in the first mode of operation, said off-state has a substantially constant duration and the duration of the on-state is varied to regulate the voltage at each output; and operating in a second mode of operation when the duty cycle D reaches or exceeds the second limit, or falls below the first limit wherein, in the second mode of operation, the duty cycle is substantially constant and the operating frequency f is varied to regulate the voltage at each output.

[0010] Preferably, the first limit DM is at least 20%.

Preferably, the first limit DM is at most 40%. More preferably the first limit ON is 30%.

[00IIJ Preferably, the second limit DH is at least 55%.

Preferably, the second limit DH is at most 65%. More preferably the second limit DLI is 60%.

[00l2J Preferably, when the duty cycle D reaches or exceeds the second limit, the controller changes to the second mode of operation with a substantially constant duty cycle substantially equal to said second limit DI-i.

[0013J Preferably, during start-up of the converter, when the output(s) are charging to the required output voltage(s), the controller operates the converter in said second mode of operation with a substantially constant duty cycle within a range defined by a third limit DL and a fourth limit DE, where DL is less than OF.

[0014J DL may be lower than SM.

[0015J DF may be higher than DM.

[0016J Preferably, DL is 20%.

[0017J Preferably, SF is 50%.

[0018J The present invention will now be described with reference to the accompanying drawings in which:- [00l9J Figure 1 shows the basic schematic of an asymmetric half-bridge flyback DC:DC converter; [0020J Figures 2a and 2b show approximate waveforms which illustrate the operation of an asymmetric half-bridge flyback DC:DC converter; [0021) Figure 3 is a flow diagram which illustrates the operation of a converter according to an embodiment of the present invention; and [0022) Figures 4 and 5 show the basic schematics for an asymmetric half-bridge flyback DC:DC converter with multiple outputs.

[0023) In the description of the invention, the following definitions are applied.

[0024) "On-time" of the converter means the period of time when switch Ml is conducting (on) . During this time, energy is stored in the transformer T. This is the energy storage phase of the converter. During on-time, the converter is said to be in an on-state.

[0025) "Off-time" of the oonverter means the period of time when switch Ml is not conducting (off) and switch M2 is conducting (on) . During this time, energy stored in transformer F during the on-time is delivered to output capacitor 02 and thence to the load. During off-time, the oonverter is said to be in an off-state.

[0026) "Dead time" means the period of time when switches Ml and M2 are both off.

[0027) "Duty-cycle" means the ratio of on-time to period (on-time+off-time) [0028) The definitions of "on-time" and "off-time" used herein are consistent with the terminology used to describe a conventional flyback converter with a single controlled switoh on the primary side.

[0029) Figure 1 shows the basic schematic of an asymmetric half-bridge flyback DC:DC converter.

[0030) A DC input voltage Vdc is supplied between a high voltage rail 10 and a low voltage rail 12. One side of a capacitor C is connected to the high voltage rail 10 and the other side of capacitor C is connected to the low voltage rail 12. The drain terminal of a first switch Ml is connected to a node a in the circuit, and the source terminal of switch Ml is connected to one end of a resistor Rl. The other end of resistor Ri is connected to the low voltage rail 12. The drain terminal of a second switch M2 is connected to the high voltage rail 10, and the source terminal of switch Ml is connected to node a. Switches Ml and M2 are typically MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) . The gate terminals of switches Ml and M2 are connected to a controller 14 which controls operation of the switches.

[0031) A transformer or coupled inductor T is indicated by a dotted boundary in figure 1. The transformer has a primary winding Ma which is inductively coupled to a secondary winding Llb. One end of primary winding Lla is connected to node a. The other end of primary winding Lla is connected to one side of a half-bridge capacitor Ci. The other side of capacitor Cl is connected to the lOw voltage rail at node b. As will be known to someone skilled in the art, Cl may be connected to the low voltage rail or the high voltage rail. Alternatively, capacitor Cl may be divided into 2 capacitors with one connected to each rail.

[0032) Transformer I has an associated leakage inductance which is represented in figure 1 by inductor Ileakage, connected in series with primary winding Lia between said winding and node a. Lleakage in figure 1 may also include an additional discrete inductor to provide a more controlled value.

[00331 One end of secondary winding Sib is connected to the anode of a diode Di. The cathode of diode Dl is connected to one side of an output capacitor 02. The other side of capacitor 02 is connected to the other end of secondary winding Sib. Secondary winding Sib is oriented relative to the primary winding Ida and diode Dl such that the voltage induced by current flow in Lla when switch Ml is conducting and switch M2 is not conducting reverse biases diode Dl, and such that the voltage induced when switch Ml is not conducting and switch M2 is conducting forward biases diode Dl.

[00341 Output capacitor 02 supplies an output voltage Vout to a load represented in the schematic of figure 1 by a resistor RL.

[0035J In use, switches Ml and M2 are switched by the controller to achieve and then maintain a desired output voltage Vout at the or each output.

[0036J The half-bridge capacitor Cl is nominally charged to a voltage whereby the difference between the DO input voltage, Vdc, and the voltage of Cl, V(Cl), approximately equals the reflected output voltage Vout.

[0037J That is to say, Vdc-V(Cl) Vout*N(Lla) /N (Llb) where N(Lx) is the number of turns associated with transformer winding x.

[0038J With these voltages, whenever switch M2 is closed there is approximately no vo.tage imposed across The leakage inductance Lieakage, and hence no current in the resonant circuit formed by the leakage inductance and the half-bridge capacitor Cl. This is approximately the operating point for operation at no output load.

[0039J If switches Ml and M2 are operated with a duty-cycle whereby Vdc*DutyVoutput*N (Lia) IN (Llb) then V(Cl) will be maintained at this operating point.

[0040J If the output load is then increased, the duty-cycle of the converter, which is also the duty-cycle of switch Ml, increases. This lowers the voltage on capacitor Cl, allowing a resonant current to flow when switch M2 is closed. This resonant current is transferred through the transformer to the secondary winding Llb and via the output diode Dl to the output.

[004lJ Figure 2a shows approximate waveforms for the converter operating in resonant mode, as described in more detail below. Mi is the signal that controls switch Mi (when signal Mi is high, switch Mi is closed) ; M2 is the signal that controls switch M2 (when signal M2 is high, switch M2 is closed) ; Ip is the current in the primary winding Lia of the transformer; and Vb represents the voltage at the mid-point of the half-bridge switches Ml and M2.

[0042J As can be seen from figure 2, the resonant current that flows into capacitor Ci when switch M2 is closed is balanced by a linearly rising current out of capacitor Cl when switch Mi is closed. This current ramp is controlled by the resonant impedance of the magnetising inductance of transformer Ml and the half-bridge capacitor Cl. Since the converter normally operates well above this resonant frequency, the waveform is linear rather than sinusoidal.

[0043) In fixed frequency mode, as described, for example, in Us 5,808,879, both resonances are at a half-period significantly longer than the conduction times of switches Ml arid M2. In this mode of operation, the effective impedance of the resonant circuits is high, because they are operating well above resonance. This results in significant variation in the voltage of Cl with load, which in turn results in poor cross-regulation.

[0044) In resonant mode, as represented by the waveforms shown in figure 2a, the conduction time of switch M2 is set close to the resonant half-period of the resonant circuit formed by the transformer leakage inductance (and any additional external inductance) and capacitor Cl. In this mode of operation, M2 switches off close to zero current, resulting in low turn-off losses for M2, but high turn-on losses for Ml, since there is typically insufficient energy stored in the leakage inductance to facilitate zero-voltage switching. In this respect, with zero-voltage switching, energy stored in the leakage inductance would charge the output capacitance of the switches, allowing device turn-on to occur with nearly zero-volts across the device.

[0045) As shown in figure 2a, switch Ml is switched on at time tl, off at time t2, and on again at time tS. switch M2 is switched on at time t3 and off at time t4. Typically, some dead-time exists between t2 and t3, and between t4 and 5, to allow for zero-voltage switching, and/or to prevent shoot-through, which Is a condition where both switches Ml and M2 are on at the same time due to differences in switching speed.

[0046) Typically, the transition of the mid-point of the half-bridge, Vb in figure 2a, has near zero-voltage switching -10 -for the transition from Ml conducting to M2 conducting (time t2 to t3) . The voltage transition from M2 conducting to Ml conducting is more hard-switched with the transition happening at time t5 when Ml starts to conduct. The degree of zero-voltage switching will vary with load and operating point (duty-cycle) for this topology.

[0047) The conduction time of switch M2 can be adjusted either side of the resonant half-period to improve switching losses.

[0048) Figure 2b shows the impact of adjusting the conduction time of switch M2. If this is shorter than the resonant half-period, F42 can turn-off with significant current that can lead to higher turn-off losses but lower turn-on losses for Ml. Due to the high di/dt at the end of the resonant half-period, the sensitivity to timing accuracy is high. If the conduction time of switch P42 is longer than the resonant half-period, P42 still turns off with current flowing, but di/dt is lower, since this is now defined by the higher magnetising inductance of the transformer, because the resonant action is compiLeted. Thus, timing sensitivity is lower. Turn-on losses for Ml will typically be worst if P42 turns off with zero-current. Typically, P42 is operated with a conduction time slightly greater than the resonant half-period when working in this resonant mode for energy delivery.

[0049) when operating in this mode, the amplitude of the resonant half cycle of current I is given by I=Vi/Zo (1) where Vi=Vdc-V(Cl) is the initial voltage across The leakage inductance when M2 is closed, and Zo is the characteristic impedance of the resonant circuit, where Zo=(Lleakage/Cl)A0.5 -11 - [0050J Equation (1) allows the required variation in the voltage across Cl with load to be calculated.

[005lJ In a typical asymmetric half-bridge flyback converter, the duty-cycle of switch M2 is maintained significantly below 100% duty cycle due to the cost associated with high duty-cycle gate-drivers for floating devices. Some controllers for this topology limit the duty-cycle of switch M2 to a maximum of 50%.

[0052J For fixed frequency operation with a maximum duty-cycle of 50%, the turns ratio of the transformer is required to be low, resulting in high ms currents on the primary side and relatively high switching losses. A converter of this type would typically operate at a low duty cycle of 20- 30%, reserving operation at 50% for use during hold-up. This results in unfavourable device stresses.

[0053J Where a larger duty-cycle range is achievable, the stresses on the switches Ml and M2 will vary significantly with operating point (duty-cycle) . For example, operating at near to 50% duty-cycle, power is delivered to the load for 50% of the time, with 2x output power being delivered in each resonant pulse. Whereas, if Ml is operated at 75% duty-cycle, then power is delivered 25% of the tine with 4x output power being delivered in each resonant pulse. This leads to significant conduction losses in all components. The high peak currents also lead to Cl having to charge significantly below its "no-load" point, resulting in high levels of cross regulation in multi-output designs.

[0054J The schematic diagram of figure 1 shows a single output. However, it will be understood that the circuit may have additional outputs, each comprising a respective secondary winding magneticafly coupled to primary winding -12 -Lla, a respective diode and a respective output capacitor, as illustrated in figures 4 and 5.

[0055J Figure 4 shows a simplified schematic for an asymmetric half-bridge flyback converter with 2 outputs. As drawn, all the leakage inductance is shown as existing on the primary side of the transformer, with all windings perfectly coupled. However, in reality, leakage inductance exists on all windings as shown in figure 5.

[0056J If output 1 is loaded and output 2 is unloaded, then no voltage is required across leakage inductance tk3, because no current must flow into the unloaded output. This can only happen if the voltage on output 2 rises to a voltage equal to the voltage on the secondary winding Llc. This increase of voltage on output 2 due to loading of output 1 is referred to as cross-regulation.

[0057J Operating close to 50% duty-cycle with low resonant currents reduces the level of cross-regulation seen in this topology. However, operation with a narrow duty-cycle range is only possible if a more complex control strategy is employed, because the duty-cycle range typically requires a range greater than 2:1 over the required input voltage and output load range of a typical converter.

[0058J The present invention concerns improved operation of the asymmetric half-bridge flyback converters illustrated in figures 1, 4 and 5. With the present invention, the converter is operated in different modes under different conditions, as described in more detail below, with reference to figure 3.

Start-Up Node [0059J In this mode, represented by block 301 in figure 3, -13 -the controller operates switches Ml and M2 with a fixed duty cycle D=1 and variable frequency f.

[0060) During start-up, the output(s) charge from Cv to the desired output voltage. The controller provides a "scftstart", operating from 1Mhz with fixed duty cycle D, preferably in the range 20-50%, down to the nominal operating frequency.

[00611 During operation in this mode, dead time is adjusted to provide a degree of soft switching across this range of frequencies.

[0062) At block 302 of figure 3, it is determined whether the output(s) have charged to the required output voltages.

When this is determined to be the case, the converter changes to normal running mode.

Normal Running Mode [0063) In this mode, represented by block 303 in figure 3, the controller operates switches Ml and M2 with a fixed off-time T and a variable on-time.

[0064) Thus, when the converter reaches the nominal operating frequency after start-up, the controller changes to fixed off-time, variable on-time mode to provide regulation within a duty cycle range of typically 30-60%, where it can maintain regulation over loads between zero and full load. In this mode, the fixed off-time is preferably set to equal the half-period of the resonant frequency defined by the leakage inductance Lleakage of the transformer T and the half-bridge capacitor 01.

[0065) At block 304 of figure 3, it is determined whether the duty cycle remains within a range defined by a lower -14 -limit DM and a higher limit OH, typically 30% and 60% respectively. Whilst this is the case, the convener remains in normal running mode.

[0066) If, at block 304, it is determined that the duty-cycle falls outside the defined range, it is then determined at block 305 whether the duty-cycle has fallen below the lower limit, or exceeds the higher limit.

[00671 In the case where the duty-cycle has fallen below the lower limit, the converter changes to low duty cycle mode, represented by block 306 in figure 3. In the case where the duty cycle exceeds the higher limit, the converter changes to high duty cycle mode, represented by block 308 in figure 3.

Low Duty-Cycle Mode [0068) In this mode, represented by block 306 in figure 3, the controller again operates switches Ml and M2 with a fixed duty cycle 0 and variable frequency f.

[0069) When the duty-cycle falls below the lower limit ON (eg 30%), the controller locks the duty cycle at the lower limit and enters a fixed duty cycle, variable frequency mode, in which duty cycle is maintained at an approximately constant level and the freguency is varied to maintain regulation of the output voltage(s).

[0070) At block 307 it is determined whether off-time is greater than or egual to the substantially constant off-time i applied during normal running mode (block 303) . Whilst the off-time is less than this fixed off-time, the converter continues in low duty-cycle mode. Once the off-time is determined to equal or exceed this fixed off-time, the flow returns to block 303 of figure 3, and the converter reverts -15 -to normal running mode.

}dicth Duty-Cycle Mode [0071) In this mode, represented by block 308 in figure 3, the ccntrcller again cperates switches Ml and M2 with a fixed duty cycle D and variable frequency f.

[0072) The controller switches to this mode when the duty cycle reaches an upper limit D1-i (for example, 60%) This may occur, for example, when the input voltage to the converter reduces, or when the converter load is high.

[0073) When this occurs, the controller locks the duty cycle at the upper limit and enters a fixed duty cycle, variable frequency mode, in which duty-cycle is maintained at an approximately constant level and the frequency is varied to maintain regulation of the output voltage(s).

[0074) In this mode, the converter energy delivery phase operates below the resonant frequency of the leakage inductance bleakage and the half-bridge capacitor Cl.

Accordingly, once this resonance has delivered its energy to the output, the resonance of the transformer magnetising inductance and the half-bridge capacitor continues on the primary side. This second resonance increases the AC voltage on the half-bridge capacitor and allows more energy to be stored during the energy storage phase and thus allows more power to be delivered by the converter.

[0075) At block 309 it is determined whether off-time is less than or equal to the substantially constant off-time i applied during normal running mode (block 304) . Whilst the off-time is greater than this fixed off-time, the converter continues in high duty-cycle mode. Once the off-time is determined to be less than or equal to this fixed off-time, -16 -the flow returns to block 303 of figure 3, and the converter reverts to normal running mode.

[0076) It will be understood that the embodiment illustrated above shows an application of the invention only for the purposes of illustration. In practice the invention may be applied to many different configurations, the detailed embodiments being straightforward for those skilled in the art to implement.

[0077) In figures 1, 4 and 5, the output(s) comprise diode(s) Dl, D2. More generally, the output of inductive component T may be rectified by any suitable rectifier(s).

[0078) In figures 1, 4 and 5, Cl is connected to the low voltage rail. However, it may be connected to the high voltage rail instead. Pdternatively, capacitor Cl may be divided into 2 capacitors with one connected to each rail.

Claims

-17 -C LA TM S1. A half-bridge flyback converter for transferring energy from a voltage supply to one or more outputs, the converter comprising:-an inductive component I having a primary winding ISa and a secondary winding Llb for each output; a pair of switches Ml, M2 arranged as a half-bridge for connecting the primary winding Lla to the voltage supply; and a controller for controlling said switches to switch the converter between an on-state in which switch Ml is conducting and energy from the voltage supply is stored in the inductive component T; and an off-state in which switch Ml is not conducting, switch M2 is conducting and energy is delivered to the output(s) via the secondary winding(s) Llb; wherein the controller controls the switches to switch periodically between the on-state and the off-state with an operating frequency f and a duty cycle 5; wherein, when the duty cycle C is within a range defined by a first limit DM>0 and a second limit DH, where DM<DH<100%, the controller operates the converter in & first mode of operation in which said off-state has a substantially constant duration and the duration of the on-state is varied to regulate the voltage at each output; and wherein, when the duty cycle C reaches or exceeds the second limit, or falls below the first limit, the controller operates the converter in a second mode of operation in which the duty cycle is substantially constant and the operating frequency f is varied to regulate the voltage at each output.
2. A half-bridge flyback converter according to claim 1 wherein the first limit CM is 205-40%.

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3. A half-bridge flyback converter according to any preceding claim wherein the first limit DM is 30%.
4. A half-bridge flyback converter according to any preceding claim wherein the seccnd limit DH is 55-65%.
5. A half-bridge flyback converter according to any preceding claim wherein the second limit DH is 60%.
6. A half-bridge flyback converter according to any preceding claim wherein, when the duty cycle D reaches or exceeds the second limit, the controller changes to the second mode of operation with a substantially constant duty cycle substantially equal to said second limit DH.
7. A half-bridge flyback converter according to any preceding claim wherein, during start-up of the converter when the output(s) are charged to the required output voltage(s), the controller operates the converter in said second mode of operation with a substantially constant duty cycle within a range defined by a third limit DL and a fourth limit OF, where DL<DF.
8. A half-bridge flyback converter according to claim 7 wherein DL<DM.
9. A half-bridge flyback converter according to claim 7 or 8 wherein DF<=DM.
10. A half-bridge flyback converter according to any of claims 7 to 9 wherein IDF=DM.
11. A half-bridge flyback converter according to any of claims 7 to 10 wherein DL=20%.

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12. A half-bridge flyback converter according to any of claims 7 to 11 wherein DF=5D%.
13. A method of operating a half-bridge flyback converter for transferring energy from a voltage supply to one or more outputs, the method comprising:-switching periodically between an on-state and an off-state with an operating frequency f and a duty cycle ID to regulate the voltage at each output; operating in a first mode of operation when the duty cycle D is within a range defined by a first limit DM>0 and a second limit DH, where DM<D}d<lO0% wherein, in the first mode of operation said off-state has a substantially constant duration and the duration of the on-state is varied to regulate the voltage at each output; and operating in a second mode of operation when the duty cycle D reaches or exceeds the second limit, or falls below the first limit wherein, in the second mode of operation, the duty cycle is substantially constant and the operating freguency f is varied to regulate the voltage at each output.
14. A half-bridge flyback converter for transferring energy from a voltage supply to one or more outputs, substantially as hereinbefore described with reference to the accompanying drawings.
15. A method of operating a half-bridge flyback converter for transferring energy from a voltage supply to one or more outputs, substantially as hereinbefore described with reference to the accompanying drawings.