US 20040228153 A1 Abstract This invention relates to new soft-switching techniques for minimizing switching losses and stress in power electronic circuits using inverter legs. By choosing the switching frequency with specific relationships with the resonant frequency of the power electronic circuits, the proposed switching technique enables the power electronic circuits to achieve soft switching under full load and short-circuit conditions at the defined frequencies for both capacitive and inductive loads. This technique can be applied to an electronic circuit with two switches connected in totem pole configuration between two dc voltage rails or commonly known as a power inverter leg or inverter arm. Examples of these circuits are class-D power converter, half-bridge power converters and full-bridge power converters or inverters. The proposed techniques allow inverter circuits with resistive, capacitive and inductive loads to achieve soft switching.
Claims(11) 1. A method of operating a power electronics circuit comprising an inverter and a load including a resonant tank, wherein said inverter is switched at a frequency f_{s }and said resonant tank has a resonant frequency f_{r}, wherein K<f_{r}/f_{s}<K+1 where K is an even-numbered integer. 2. A method of operating a power electronics circuit comprising an inverter and a load, wherein said load as first and second operating conditions associated with respective first (f_{r1}) and second (f_{r2}) resonant frequencies, f_{r2 }being greater than f_{r1}, and wherein said inverter is switched at first (f_{s1}) and second (f_{s2}) switching frequencies corresponding to said first and second operating conditions, f_{s2 }being greater than f_{s1}, and wherein K<f_{r2}/f_{s1}<K+1 where K is an even-numbered integer. 3. A method as claimed in _{a}), and wherein K−f_{s}f_{s1}<K+1 where K is an even-numbered integer. 4. A method as claimed in _{s2}>f_{a}. 5. A method as claimed in _{s2}>f_{r2}. 6. A method as claimed in 7. A method of operating a power electronics circuit comprising an inverter and a load, wherein said load as first and second operating conditions associated with respective first (f_{r1}) and second (f_{r2}) resonant frequencies, f_{r2 }being greater than f_{r1}, and wherein said inverter is switched at first (f_{s1}) and second (f_{s2}) switching frequencies corresponding to said first and second operating conditions, f_{s2 }being greater than f_{s1}, wherein an auxiliary resonant tank is provided between said inverter and said load and having a resonant frequency (f_{a}), and wherein K<f_{a}/f_{s1}<K+1 where K is an even-numbered integer, and wherein K<f_{r2}/f_{s1}<K+1, and f_{s2}>f_{r2 }and f_{s2}>f_{a}. 8. A method as claimed in _{a }is close to f_{r2}. 9. A method of operating a power electronics circuit comprising an inverter and a load, wherein said load as first and second operating conditions associated with respective first (f_{r1}) and second (f_{r2}) resonant frequencies, f_{r2 }being greater than f_{r1}, and wherein said inverter is switched at first (f_{s1}) and second (f_{s2}) switching frequencies corresponding to said first and second operating conditions, f_{s2 }being greater than f_{s1}, wherein an auxiliary resonant tank is provided between said inverter and said load and having a resonant frequency (f_{a}), wherein K<f_{r2}/f_{s1}<K+1 and f_{s1}>f_{a}. 10. A method of operating a power electronics circuit comprising an inverter and a load including a resonant tank, wherein an auxiliary resonant tank is provided between said inverter and said load whereby in the event of the load acting as a short-circuit during operation, current provided by said auxiliary resonant tank enables soft-switching of said inverter. 11. A power electronics circuit comprising an inverter, a load including a resonant tank, and an auxiliary resonant load provided between said inverter and said load.Description [0001] The present invention relates to methods for the soft-switching of power inverter legs, for example, though by no means exclusively, in electronic ballasts for high energy discharge lamps. [0002] Many power electronic circuits consist of inverter legs or arms. An inverter leg is shown in FIG. 1. Each inverter leg consists of two power switches (S [0003] Usually, the node between S [0004] Examples of typical loads are shown in FIG. 4 and FIG. 5. In FIG. 4, the overall load consists of a dc voltage blocking capacitor, a resonant inductor, a resonant capacitor and an equivalent resistive load. This is a commonly used circuit for an electronic ballast for a lamp and the resistive load represents the energy consuming lamp. The equivalent resistive load can also be a transformer coupled circuit with the energy-consuming load connected on the secondary transformer circuit via a rectifier (such as the system for a switched mode power supply). FIG. 5 shows multi-resonant circuit with an energy consuming load. This multi-resonant circuit is a alternative electronic ballast circuit for a high-intensity-discharge (HID) lamp, in which Lr2 and Cr form a relatively low-frequency (e.g. 50 kHz) resonant tank to create a high voltage to ignite the HID lamp and Lr1 and Cr form a relatively high-frequency (e.g. 400 kHz) resonant tank for operating the lamp under steady-state conditions, Here Lr2>Lr1. [0005] To understand the problems faced by existing technology, existing soft-switching techniques for power electronic circuits with inverter leg or legs will be described, using the half-bridge circuit in FIG. 4 as an example. The directions of the load current I [0006] In the example of FIG. 4, the de blocking capacitor Cb eliminates the dc component of the ac voltage generated by the inverter leg. The resonant tank consists of Lr and Cr and the dominant resonant frequency is f [0007]FIG. 7 shows three typical switching trajectories of a power switch. The y-axis is the current through the switch and the x-axis is the voltage across the switch. During the transition periods of the turn-on or turn-off processes, a power switch will withstand high transitional voltage (across the switch) and current (through the switch). This is called hard switching. Hard switching not only leads to switching loss and stress, but more importantly causes switching transients or spikes that are major source of electromagnetic interference (EMI). Such EMI problems may induce noise in the gating signals of the power switches, causing reliability problems. For example, if noise is induced in the gate of a nominally-off power switch and triggers the switch to turn on, the inverter leg may have a shoot-through or short-circuit situation. As one solution to this problem it is known to connect a snubber circuit consisting of resistor and capacitor to reduce the high di/dt and dv/dt of the switch so as to reduce the switching loss and stress. However, traditional snubber circuits are lossy because part of the switching loss is transferred from the switch to the snubber resistor. In order to achieve soft switching, it is necessary to create a zero voltage and/or zero current condition for the switch to turn on or off. If either the switch voltage or switch current is zero, the instantaneous product of switch voltage and current is zero. Thus, the switching loss becomes zero. In practice, it may not be possible to achieve absolute zero switch voltage and/or current. Instead, the switch voltage and/or current can be clamped to near-zero value. Such near-zero voltage and/or current zero-voltage and/or current switching may still be considered to be zero voltage or zero current. The general term for zero-voltage or zero-current switching is soft switching. [0008] The following conditions have to be met in order to achieve soft switching in circuits including an inverter leg. [0009] (A) For Zero-Voltage ‘Turn Off’ of Power Electronic Switches S [0010] Condition (1)—Parallel capacitance is needed across the power switches S [0011] Parallel capacitance across the switch can come from the power switches' device capacitance such as the drain-source capacitance of the power mosfet. External capacitor can be added across the switch if necessary. This is a well known technique for zero-voltage turn off of power electronic devices. [0012] (B) For zero-voltage ‘turn-on’ of power electronic switches S [0013] Condition (1*)—The tank current I [0014] For the inverter circuit example (FIG. 4), soft switching can be achieved if the overall load (including the resonant tank and resistive load) is inductive. The normal understanding in the prior art is that the frequency (fs) of the inverter's ac voltage V [0015] When S [0016] Similar arguments apply to the soft-turn-off process of S [0017] The main problem of the above soft-switching method for the inverter circuit is that fs must be greater than fr so that the overall load is inductive. If fs<fr, the overall load becomes capacitive and the soft-switching condition that “the current I [0018] Condition (2*). Tank current I [0019] It is necessary to find the current threshold for soft switching in the operating frequency region. When tie current is above the current threshold, soft switching can be achieved. The current i [0020] where Qs is the charge and Cs is the total equivalent capacitance across the power switch (e.g. drain and source of the power switch), Vg is the de inverter voltage and t [0021] If a resonant tank is used in the load circuit, the input circuit can be approximated as a sinusoidal current because of the filtering effect of the resonant tank. [0022] where I [0023] Based on (1) and (2), the input current must obey the following equation in order to create a zero-voltage condition for the power switch to achieve soft switching:
[0024] Therefore, equation (3) must be met as a necessary condition for soft switching. This equation provides a guideline to choose the appropriate t [0025] The present invention provides new soft-switching techniques for inverter bridges. According to the present invention there is provided a method of operating a power electronics circuit comprising an inverter and a load including a resonant tank, wherein said inverter is switched at a frequency f [0026] In particular, a first preferred method enables soft switching to be achieved in the inverter bridge with overall capacitive load or for inverter operating at a frequency below the dominant resonant frequency of the resonant tank(s). This may be considered a “pseudo inductive soft-switching” method. Within the nominal “capacitive” operating range (fs<fr), certain frequency regions may be defined that can be considered to be pseudo-inductive regions. Within the pseudo inductive regions, soft switching can be achieved even though the frequency range is within the capacitive region. A second preferred method includes the use of an additional and unloaded resonant tank that provides a current path to ensure soft-switching irrespective of the load condition. This additional resonant tank lowers the minimum inverter frequency at which soft switching can be achieved. Even if the inverter operates in the nominally capacitive region of the original resonant tank, the inductive effect of the additional resonant tank makes soft switching possible at a lower inverter frequency. [0027] According to conventional resonant circuit theory, a series resonant tank works in the “capacitive” region when the inverter operating frequency f [0028] The invention also provides a method of ensuring that there is a threshold current for enabling soft-switching in the event, for example, of the load acting as a short-circuit using an auxiliary resonant load. In particular the invention also extend to a method of operating a power electronics circuit comprising an inverter and a load including a resonant tank, wherein an auxiliary resonant tank is provided between said inverter and said load whereby in the event of the load acting as a short-circuit during operation, current provided by said auxiliary resonant tank enables soft-switching of said inverter. [0029] Some embodiments of the invention will now be described by way of example and with reference to the accompanying figures in which: [0030]FIG. 1 illustrates a typical inverter leg, [0031]FIG. 2 illustrates a single-phase full-bridge inverter, [0032] FIGS. [0033]FIGS. 4 and 5 illustrate half-bridge inverters different loads, [0034] FIGS. [0035]FIG. 7 illustrates typical switching trajectories of a power switch, [0036]FIG. 8 shows a half-bridge inverter loaded by a series resonant tank, [0037] FIGS. [0038]FIG. 10 illustrates a half-bridge inverter used in experimental verification of embodiments of the present invention, [0039] FIGS. [0040] FIGS. [0041] FIGS. [0042] FIGS. [0043]FIG. 15 shows a half-bridge inverter circuit for use in a method according to a second embodiment of the invention, [0044]FIG. 16 shows simulated (left) and measured (right) voltage and current waveforms in a test of the second embodiment of the invention, [0045]FIG. 17 shows simulated (left) and measured (right) voltage and current waveforms in a test of the second embodiment of the invention, [0046]FIG. 18 shows simulated (left) and measured (right) voltage and current waveforms in a test of the second embodiment of the invention, [0047]FIG. 19 shows simulated (left) and measured (right) voltage and current waveforms in a test of the second embodiment of the invention, [0048]FIG. 20 shows a half-bridge inverter circuit similar to FIG. 15 but in an alternate embodiment, [0049]FIG. 21 illustrates schematically the effect of the auxiliary resonant tank on the inductive region, [0050]FIG. 22 plots calculated auxiliary inductance upper limit against switching frequency, [0051]FIG. 23 plots calculated maximum auxiliary inductor current against switching frequency, [0052]FIG. 24 shows simulated (left) and measured (right) voltage and current waveforms in a test of an alternative form of the second embodiment of the invention, [0053]FIG. 25 shows simulated (left) and measured (right) voltage and current waveforms in a test of an alternative form of the second embodiment of the invention, [0054]FIG. 26 shows simulated (left) and measured (right) voltage and current waveforms in a test of an alternative form of the second embodiment of the invention, and [0055]FIG. 27 shows simulated (left) and measured (right) voltage and current waveforms in a test of an alternative form of the second embodiment of the invention. [0056] In preferred embodiments of this invention a novel pseudo-inductive soft-switching technique is provided that can be applied to the circuits described in FIGS. 1-5. The overall load Z can consist of different combination of resonant tank(s) and is not restricted to the forms shown in FIG. 4 and FIG. 5. [0057] A first embodiment of the present invention (which may be termed a “pseudo-inductive soft-switching” method) will now be described firstly by reference to theory, and then by experimental verification of the theory. [0058] The half bridge inverter loaded by series resonant tank shown in FIG. 8 is considered as an example to illustrate the soft-switching technique in the capacitive region based on the pseudo-inductive region concept. Based on the Fourier analysis approach, the input voltage V [0059] The rectangular ac voltage applied to the resonant tank is:
[0060] Normally a DC-blocking capacitor is used to remove the Dc component V [0061] V [0062] Define a variable to represent the ratio between f [0063] When 0<N<1, the resonant tank works at the inductive region as explained in the background section and the input current to the resonant tank lags the input voltage pulse. In other words, i [0064] Referring now to FIG. 9( [0065] Note the n is an odd number in the equation. [0066] From this equation, it can be found that when N equals to one of the odd numbers, such as 1,3,5 . . . , the factor
[0067] will be equal to zero when n equals to N, and thus the n-th harmonic component will make i [0068] acts as a resonant frequency, which is called a sub-resonant-frequency. When the switching frequency of the inverter f [0069] When the inverter switching frequency f [0070] The region of N>1 (i.e. f [0071] The capacitive regions (in which soft switching cannot be achieved) meet the following two conditions;
[0072] Under these conditions, i [0073] However, soft switching can be achieved within the pseudo inductive regions in the nominal capacitive region. By choosing an appropriate value of N (ratio of fr and fs), soft switching can be achieved in the capacitive region. In the capacitive region of N>1, when
[0074] i [0075] In summary, for N>1 (i.e. f [0076] (1) The Capacitive Regions: [0077] Odd integers<N<Even integers→capacitive characteristics, such as: 1<N<2, 3<N<4, 5<N<6, . . . [0078] (2) The Pseudo-Inductive Regions: [0079] Even integers<N<Odd integers→pseudo-inductive characteristics, such as: 2<N<3, 4<N<5, 6<N<7, . . . [0080] Soft switching achieved in the pseudo-inductive regions can be explained in an intuitive way. Consider N=fr/fs again. If N>1, there are more than one resonant period within the inverter switching period. If N is chosen to satisfy equation (9), the resonant current is in the positive half cycle when the top switch S [0081] Experimental Verification: [0082] The pseudo-inductive soft-switching technique is illustrated with a half-bridge power inverter circuit example (FIG. 10) that is suitable for electronic ballast of high-intensity-discharge (HID) lamp. There are two resonant frequencies in this system. Inductance Ls is much larger than inductance Lr. The operating procedure is as follows: [0083] (1) By operating the inverter frequency fs=fs [0084] (2) in the lamp's glow-to-arc transitional period, the lamp is close to a short-circuit situation (shorting the large inductor Ls). [0085] (3) Once the lamp arc is established, the lamp is like a resistor. The steady-state inverter frequency is then increased to a high value (fs=fs [0086] This circuit is good example to illustrate the usefulness of the invention. Before the lamp is ignited in stage (1) at a lower starting frequency fs [0087] In summary, the HID lamp ballast example has the following operating modes: [0088] a) The lamp behaves like an open circuit before ignition, when a relatively low inverter starting frequency fs [0089] b) The lamp behaves like a short circuit in the glow-to-arc transition during the ignition process, with the inverter operating at fs [0090] c) Immediately after the ignition process is completed, the lamp behaves like a resistive load at an inverter frequency of fs [0091] d) The inverter frequency is then increased to a relatively high values fs [0092] e) The lamp behaves like an open circuit when the lamp arc is broken due to acoustic resonance, when the inverter frequency is a relatively high fs [0093] Among these operating modes, modes (b) and (e) have the potential danger of hard switching. In mode (b), the sudden change of dominant resonant frequency to fr [0094] In this experimental system, the DC link is set at 310V. HIE-E27 150 W metal halide is selected for testing. The starting inverter frequency fs [0095] Test1: Confirmation of the Inductive Region (fs [0096] The large inductor Ls in FIG. 10 is shorted so that the equivalent resonant tank consists of Lr and Cr only. This high resonant frequency fr=fr [0097]FIG. 11( [0098] Test 2: Confirmation of Capacitive Region with the Steady-State Inverter Frequency fs [0099] Similarly to Test 1, the large inductor Ls in FIG. 10 is shorted so that the equivalent resonant tank consists of Lr and Cr only. When the steady-state inverter frequency fs=fs [0100] FIGS. [0101] Test 3: Confirmation of Capacitive Region with Starting Inverter Frequency fs [0102] When the HID lamp is in the glow-to-arc transition the starting inverter switching frequency fs [0103]FIG. 13( [0104] Test 4: Confirmation of Pseudo-Inductive Soft-Switching Technique in the Nominally Capacitive Region (fs [0105] Test 3 shows that if the starting inverter frequency fs [0106]FIG. 14( [0107] A second embodiment of the present invention uses an additional or auxiliary resonant tank. This embodiment will now be described again with regard to theory first of all, and then with experimental justification. There are two different versions of this embodiment: one in which the additional resonant tank has a relatively high resonant frequency, and another in which the additional resonant tank has a relatively low resonant frequency. When the additional resonant tank has a relatively high resonant frequency there are more than one resonant cycles in the resonant tank within one cycle of the inverter switching frequency and this may be called the resonant mode of operation. On the other hand, when the additional resonant tank is operated at a relatively low resonant frequency, the resonant tank is charged and discharged once within each inverter cycle. This may be termed a linear mode. [0108] The basic concept of the use of an additional resonant tank is illustrated with the use of FIG. 15 and FIG. 20. FIG. 15 shows a resonant circuit similar to the circuit example in FIG. 10, except that an additional unloaded resonant tank capacitor Ca and inductor La is added. An alternative implementation is to use the dc blocking capacitor Cb and La to form an additional unloaded resonant tank as shown in FIG. 16 [0109] (A) Additional or Auxiliary Parallel Resonant Tank (Resonant Mode) (FIG. 15) [0110] Through proper selection of the components' parameters of the auxiliary capacitor and auxiliary inductor, the auxiliary resonant tank can operate at an ‘inductive’ state in the high frequency range (namely the operating frequency fs [0111] The use of the auxiliary resonant tank for soft switching makes it easy to meet conditions 1* and 2*. For a specific selection of the auxiliary tank components' parameters, when the switching ratio is given, the maximum current through the auxiliary tank is determined accordingly. When this current is above the current threshold, soft switching can be achieved. Of course, superfluous inductive current undoubtedly ensures soft switching, but it gives rise to larger conduction loss in the power switches and higher switch's current ratings requirement. So components' parameters and switching frequency ratio should be carefully chosen in the consideration of soft switching current threshold and switch's conduction loss and current ratings. [0112] In the prototype circuit, the parameters are selected like this, C [0113] Once the lamp is turned on, the inverter switching frequency can be increased from fs=fs [0114] Experimental Verification [0115] Test 5: [0116] Simulated and experimental waveforms of V [0117] (i) Starting Inverter frequency fs [0118] (ii) Starting Inverter frequency fs [0119] (iii) Steady-state inverter frequency fs=fs [0120] (iv) Steady-state inverter frequency fs=fs [0121] All simulated and measured results confirm that soft switching can be achieved at the relatively low starting inverter frequency operation and the high inverter frequency operation under both open and short circuit conditions. These results verify the high reliability offered by the proposed auxiliary resonant branch and pseudo-inductive soft-switching method. [0122] (B) Alternative Implementation of Auxiliary Parallel Resonant Tank (Linear Charging and Discharging Mode) (FIG. 20) [0123] An alternative way to implement the auxiliary resonant branch is shown in FIG. 20. In this case, the dc blocking capacitor Cb, that is commonly used in high-frequency inverter to remove the dc voltage component, is employed as part of the auxiliary resonant tank. Because the size of Cb is relatively large (typically in the order of micro-Farad), the resonant frequency fa of Cb and La is relatively low. [0124] Consider the nature of the resonant tanks of the original resonant tank and the auxiliary one in FIG. 21. If the resonant frequency fa of the auxiliary resonant tank is chosen to be lower than the resonant frequency fr of the original resonant circuit, the use of the auxiliary resonant tank can widen the inductive frequency range of the overall circuit for achieving soft switching [0125] Equations (1)-(3) can be rewritten for FIG. 20 as follows: [0126] The current through the auxiliary inductor can be formulated by the following equation:
[0127] where Vg is the dc voltage of the inverter, Td is the dead time between S [0128] where Qs is the charge and Cs is the total equivalent capacitance across the drain and source of the power switch. This equation can be simplified as:
[0129] This means that the inductance of the auxiliary inductor cannot exceed a certain limit as shown in (12) in order to achieve soft switching. By using this equation, the needed auxiliary inductance can be determined. When the dead time T [0130] From this graph, it is clear that the auxiliary inductance should be set below 180 uH in this example if the ballast works up to about 500 kHz. When the value of the auxiliary inductor is set at 180 kHzμH, then the maximum current through the auxiliary branch can be determined by equation (10) and the relationship between this maximum current and the switching frequency can be shown by FIG. 23. [0131] In the prototype circuit (FIG. 20), the selected parameters are: C [0132] Test 6: [0133] Simulated and experimental waveforms of V [0134] (v) Starting Inverter frequency fs [0135] (vi) Starting Inverter frequency fs [0136] (vii) Steady-state inverter frequency fs=fs [0137] (viii) Steady-state inverter frequency fs=fs [0138] The simulating and experimental waveforms of V Referenced by
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