LLC CONVERTER, AND METHOD FOR CONTROLLING AN LLC CONVERTER

Abstract: An LLC power converter has an output voltage which is determined by a frequency controller controlling the frequency of an on and off switching of the switches (5, 6) of the converter. In order to stabilize the output voltage at varying output currents, a control circuit feeds back the variation of the total current (I) in the converter to the frequency controller (22, 25).

Description: LLC converter, and method for controlling an LLC converter The present invention relates to an LLC converter comprising: a series arrangement of a first parallel arrangement of a first switch and a first diode, and a second parallel arrangement of a second switch and a second diode, the series arrangement being coupled between a first input terminal and a second input terminal for receiving a DC input voltage, a voltage on the first terminal being positive with respect to a voltage on the second terminal, the cathode of the first diode and the cathode of the second diode being directed towards the first input terminal; a series arrangement of a capacitor, a first inductor and a second inductor coupled in parallel to either the first diode or the second diode; a rectifier coupled to either the first inductor or the second inductor for supplying an output voltage; and a control circuit comprising: a switching control means for controlling the frequency of an on and off switching of the first switch and the second switch, and a difference detection means for detecting a difference between said output voltage and a reference value, and controlling said switching control means on the basis of said difference. The first diode and the second diode may be intrinsic to the first switch and the second switch, respectively. The invention further relates to a method for controlling an LLC converter comprising switches for generating a current in a resonant circuit formed by a series arrangement of a capacitor, a first inductor, and a second inductor, the method comprising: sensing an output voltage of the converter for obtaining a first control signal; and sensing the current of the converter. In the field of Switched Mode Power Supplies (SMPSs), the use of LLC converters is spreading. Examples of devices in which an LLC converter is applied are lighting devices, medical systems, displays systems, etc. The topology of the LLC converter has a number of advantages, such as a low Electromagnetic Interference (EMI), and a high efficiency. However, in practice a problem arises in that controlling the output voltage of the converter by adjusting the switching frequency of the switches may lead to large instabilities, as will be explained below. In a practical embodiment of an LLC converter, the switches are electronic switches, such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). Only one of the first switch and the second switch may conduct at any given time to avoid short- circuiting the power supply supplying the DC input voltage. The theoretical on-time of each of the first and second switches is 50%, assuming that both switches are driven symmetrically. In practice, the on-time of the switches will be less than 50% due to transition phenomena occurring during the off-on transition and the on-off transition of the switches. The LLC converter has a resonance frequency which is determined by the values of the inductors, the capacitor, and external conditions which are governed by the input and output voltages and the load condition. When the converter is driven below the resonance frequency, the current through the switched-off first switch was negative just before the moment of switching-of As a result, the first diode parallel to the first switch starts conducting current. Now, switching on the second switch during the time the first diode is conducting results in high switching losses. These losses are caused by the hard recovery of the first diode, which is called hard switching. Preferably, the first and second diodes are fast or even ultra fast types in order to limit the switching losses in the hard switching mode. If power MOSFETs, having intrinsic (parasitic) diodes obviating the use of the first and the second diodes, are used as switches, operating the converter in the hard switching mode will most probably destroy the MOSFETs due to the very poor switching behavior of the intrinsic diodes. Consequently, in practice the LLC converter is driven above the resonance frequency in order to avoid hard switching. In this soft switching mode, the current through the switched-off first switch is positive just before the moment of switching-off. As a result, the voltage on the connecting node between the first and the second switches commutates, and the second diode parallel to the second switch starts conducting current. The second switch may be switched on at the moment the second diode is conducting, so virtually no switching losses occur. At such operating conditions, the use of MOSFETs with their intrinsic diodes is most suitable. As to the control behavior of the LLC converter, the following is observed. In most cases a load should be supplied with a constant DC voltage. The LLC converter, however, generates an AC voltage which is to be rectified and filtered. A simple rectifier arrangement is a full bridge rectifier comprising four diodes, connected in parallel with a filter capacitor. A known control circuit for an LLC converter comprises a series connection of a sensing circuit generating a voltage by measuring the output voltage and/or output current of the converter, a voltage to frequency converter, and a driving stage for driving the switches of the converter. Additionally, an over-current protection circuit may be added. The control circuit may be designed such that the operating frequency is limited to ensure that the converter can only be operated in a predetermined frequency window avoiding e.g. hard switching. When frequency control is applied to the LLC converter, the output power, output voltage or output current can be controlled by controlling the switching frequency of the switches of the converter. As elucidated above, an LLC converter using power MOSFETS as switches normally is operated above the resonance frequency. In this operating area, the output power, output voltage or output current will decrease with increasing switching frequencies, whereas the output power, output voltage or output current will increase when the switching frequency is decreased. At the moment the LLC converter is stabilized for a given supply and load condition at a frequency F, the effective voltage across the resonance capacitor C has reached a value V. By way of example, as the load increases, resulting in an increased output current, the frequency setting of the converter is to be decreased in order to fulfil the new increased load and supply conditions. This reduction of frequency causes an instability problem, since the output power of the LLC converter is correlated to the effective voltage across the capacitor C multiplied by the switching frequency of the first and the second switch. By decreasing the switching frequency of the converter the output power will temporarily decrease, since the effective voltage V across the capacitor C is not changed yet. The control logic will react by again decreasing the frequency to a value which is too low. On the other hand, after some time the effective voltage across the capacitor C will increase to a value greater than V due to the new operating condition, which is closer to the resonance frequency, so more power can be generated. However, as a result of the undershoot of the frequency control an overshoot will occur, and the frequency must be increased again. Finally, the resonant voltage V across the capacitor C will stabilize on a new higher value if the instabilities are being damped. As a result the increased output power can be generated, but there is a certain transition time during which the converter is unstable. These effects, which mutatis mutandis also play a role when from a given stable supply and load condition the load is decreased instead of increased, cause the instabilities in the LLC converter, and cannot be prevented when a simple frequency control is used. Under worst case conditions the LLC converter remains unstable after a load transition, or is always unstable, when the LLC converter is frequency controlled. For a fixed input and output voltage, the LLC converter can be designed such that the frequency has a substantially fixed value for varying output power and output current values. In practice, small input voltage variations occur, resulting in a rather limited frequency sweep (i.e. the difference between the minimum and maximum frequency). As a result, the loop gain (i.e. the frequency to output current ratio at a fixed input and output voltage) of the LLC converter designed in this way is extremely high. Consequently, instabilities in the frequency control may occur due to the fact that a small frequency change results in a huge output current change. For a stable control behavior close to the resonance frequency of the converter the control circuit would have to be adapted for every change of operating conditions. However, for most applications this is not a feasible solution. On the other hand, frequency control can be used when the operating frequency is not close to the resonance frequency. However, in this situation the needed frequency sweep would be large in order to cover all input and output voltage variations. When current control is applied to the LLC converter, the output parameters of the converter can be controlled. In JP-A- 10225122 an LLC converter is proposed which is regulated by means of a current control. In addition, a slope compensation method is used to improve the stability of the control circuit. Such a method is known (e.g. from R. Redl, N.O. Sokal: "What a Designer Should Know About Current-mode Control", The Power Electronics Design Conference, Anaheim, California, 15/17 October 1985, pages 18-33) to improve the stability of a buck converter, which operates at a fixed frequency and is current controlled. In particular when such a buck converter is operated at a duty cycle higher than 50% the slope compensation is essential to avoid instabilities. In general, an advantage of current control of an LLC converter with respect to frequency control is an improvement of the stability of the converter. A drawback of current control resides in the fact that the total current, i.e. the resonant current and the output current, is measured. This implies that the current measurement is always affected by the resonant current and by changes of the resonant current. It is an object of the invention to provide an LLC converter, and a method for controlling an LLC converter, which avoid the instabilities in an LLC converter caused by a basic frequency control. This object is reached by the LLC converter according to the invention having a control circuit which further comprises a current variation detection means coupled to said difference detection means for correcting said frequency in dependence on variations of a total current through the series arrangement of the capacitor, the first inductor and the second inductor. The correction provided by the current variation detection means effectively prevents instabilities of the converter. As the converter frequency is shifted towards the resonance frequency, the resonance current starts increasing. The increase of the resonance current is detected by the current variation detection means, which will shift the frequency away from the resonance frequency. As a result of this corrective action, the output of the converter, e.g. the output voltage, is not affected by the start of an unstable behavior of the LLC converter. Thus, the instability is effectively damped by the control circuit of the converter according to the invention. The features as defined in claim 2 provide a simple, reliable implementation of the current variation detection means. The current sensing circuit may be adapted to measure a current directly, e.g. by means of a current transformer being included in the converter total current circuit or, advantageously, to measure a voltage which is associated with the current, in particular the effective voltage across the capacitor of the converter. The rectifier may be a half bridge of a full bridge diode rectifier or any other suitable type of rectifier. The filter may be a low-pass RC-filter. The differentiator may be a high-pass RC-filter. The features as defined in claim 4 provide a first simple preferred embodiment of the control circuit of the converter according to the invention. The features as defined in claims 5 and 6 provide a second preferred embodiment having a relatively slow outer control loop controlling the output voltage of the converter, and generating an input signal for a relatively fast inner loop. The input and output voltages of the converter may preset the inner loop in such a way that the outer loop hardly will have to take corrective actions. The above object of the invention is further reached by the method according to the invention, in which an output voltage of the LLC converter is sensed for obtaining a first control signal, the method being characterized by: determining the variation of the current for obtaining a second control signal; correcting the first control signal by the second control signal for obtaining a third control signal; and driving the switches of the converter with a frequency of on and off switching determined by the third control signal. As elucidated above by reference to control circuit components of an LLC converter, this control method provides an simple and effective stabilization of the converter. The methods as defined in any of claims 7-11 provide preferred steps for implementing the control method according to the invention. These and other aspects, features and advantages of the invention will be apparent from the appended drawings, in which non-limiting embodiments are shown, and in which: Fig. la shows a circuit diagram of an inverter part of an LLC converter; Fig. lb shows a circuit diagram of a rectifier circuit; Fig. 2 shows a circuit diagram of a control circuit for an LLC converter; Fig. 3 shows a circuit diagram of an embodiment of a section of the control circuit of Fig. 2; Fig. 4 shows a circuit diagram of an alternative embodiment or the section of Fig. 3; and Fig. 5 shows a circuit diagram of a second embodiment of a control circuit for an LLC converter. In the different Figures, the same reference numerals relate to the same components, or to components having the same or a similar function. Fig. la shows a basic circuit diagram of an inverter part 1 of an LLC converter, delimited by a box shown in dashed lines. In the inverter part 1 of the converter DC input power from a DC supply 2 is converted into AC output power being provided at output terminals 3 and 4. The inverter part 1 comprises a first high frequency switch 5 and a second high frequency switch 6 connected in series, a first diode 7 and a second diode 8 connected in parallel to the switches 5 and 6, respectively, and a series arrangement of a resonant capacitor 9, a resonant inductor 10, and a resonant inductor 11, the series arrangement being coupled in parallel to the second diode 8. The terminals of the supply 2 are coupled to the series arrangement of the switches 5 and 6. The output terminals 3 and 4 are coupled to the terminals of the inductor 11. It will be clear that the series arrangement of the capacitor 9 and inductors 10, 11 may also be coupled in parallel to the first diode 7 instead of the second diode 8. Further it will be clear that the output terminals 3 and 4 may also be coupled to the terminals of the inductor 10. In operation, the switches 5 and 6, e.g. MOSFET type electronic switches, are opened and closed with a frequency preferably above the resonant frequency of the arrangement of the capacitor 9 and the inductors 10, 11 to avoid so-called hard switching. In a symmetrical switching mode, each of the switches 5, 6 will be closed for the same time period, switch 5 being closed while switch 6 is open, and switch 6 being close while switch 5 is open. Fig. lb shows a rectifier circuit comprising a full bridge rectifier 12 well- known in the art containing four diodes 13, 14, 15 and 16 in series-parallel connection. A filter capacitor 17 is coupled in parallel to the rectifier 12. The rectifier 12 is coupled to input terminals 18 and 19, and output terminals 20 and 21. The input terminals 18, 19 of the circuit shown in Fig. lb may be coupled to the output terminals 3, 4, respectively, of the inverter part 1 shown in Fig. la, for providing a conversion of the AC output from the inverter part 1 to a DC output from the rectifier 12. The assembly of the circuits of Fig. la and lb forms an LLC converter with a DC output. Fig. 2 shows a control circuit comprising a driving stage 22 producing control signals for switching the switches 5, 6 (Fig. 1) through output terminals 23, 24, respectively. The frequency of the control signals is determined by a voltage to frequency converter 25 driving the driving stage 22. In turn, a voltage input to the voltage to frequency converter 25 is supplied by an error amplifier 26, comparing a set reference voltage supplied through a terminal 27 to an output signal supplied by an adder 28. The adder 28 supplies a sum signal of a first control signal (an output voltage of the converter supplied through an input terminal 29a), and a second control signal (supplied through an input terminal 29b). The second control signal is generated from the total output current I (the load current plus the resonant current) of the converter. In turn, the current I may at a given frequency, be represented by the voltage across the capacitor 9, the inductor 10 or the inductor 11. The current I is sensed by a current sensing circuit 58, rectified by a rectifier 30, filtered by a filter 31, differentiated by a differentiator 32, and scaled to a proper value by attenuator 29c before being supplied to the adder 28 through the input terminal 29b. At the moment the control of the LLC converter becomes unstable, the second control signal will generate a feedback signal preventing instabilities of the converter. Fig. 3 shows a circuit for generating the second control signal, comprising two input terminals 33, 34 coupled to a shunt resistor 35 which in turn is coupled in parallel to two input terminals of a bridge rectifier 36. A series arrangement of a resistor 37 and a capacitor 38 is coupled in parallel to two other terminals of the bridge rectifier 36. A bleeder resistor 39 is coupled in parallel to the capacitor 38. A common node between the resistor 37, the capacitor 38 and the resistor 39 is coupled to a series arrangement of a capacitor 40 and a resistor 41 leading to an output terminal 42 to be coupled to the adder 28 (Fig. 2). A signal associated with the total AC current of the converter is supplied to the input terminals 33, 34, generating an AC voltage across the resistor 35. This voltage is rectified by the rectifier 36 (providing a full wave rectification) and filtered by the resistor 37, the capacitor 38, and the resistor 39. Thus, the average DC voltage across the capacitor 38 changes according to the actual absolute value of the total current of the converter. This voltage variation across the capacitor 38 is fed through the capacitor 40 and the resistor 41 to the adder 28 (Fig. 2) and, consequently, to the error amplifier 26 (Fig. 2). As a result, any changes of the resonant energy in capacitor 9 (Fig. 1) immediately result in a corrective action of the control circuit. Fig. 4 shows an alternative circuit for generating the second control signal, comprising two input terminals 43 and 44, coupled to a shunt resistor 45, which in turn is coupled in parallel to a series arrangement of a diode 46, a first resistor 47 and a second resistor 48. The second resistor 48 in coupled in parallel to a capacitor 49. The node coupling the first resistor 47, the second resistor 48 and the capacitor 49 is connected to a series arrangement of a resistor 50 and a capacitor 51 leading to an output terminal 52 to be coupled to the adder 28 (Fig. 2). A signal associated with the total output current of the converter is supplied to the input terminals 43, 44, generating an AC voltage across the resistor 45. This voltage is rectified and filtered by the diode 46 (providing a half wave rectification), the resistors 47 and 48, and the capacitor 49. Thus, the average DC voltage across the capacitor 49 changes according to the actual absolute value or the total current of the converter. This voltage across the capacitor 49 is fed through resistor 50 and the capacitor 51 to the adder 28 (Fig. 2) and, consequently, to the error amplifier 26 (Fig. 2). As a result, any changes of the resonant energy in capacitor 9 (Fig. 1) immediately result in a corrective action of the control circuit. From the above description of the circuits of Figs. 3 and 4, it will be clear that the function of the respective circuits is identical, the only difference essentially lying in the implementation of the rectification of the AC voltage across the resistor 35 or 45, respectively. Fig. 5 shows in conformity with Fig. 2 a first error amplifier 26, providing an output voltage to a voltage to frequency converter 25, which in turn drives a driving stage 22. The driving stage 22 produces control signals having a frequency determined by the voltage to frequency converter 25 through output terminals 23, 24 for switching the switches 5, 6 (Fig. 1), respectively. A second error amplifier 53 has a first input terminal 54 to which a set voltage is supplied, and a second input terminal 55 to which the output voltage of the converter is supplied. The output of the second error amplifier 53 is coupled to a first input terminal 56 of the first error amplifier 26. An AC signal representing the total output current I (the load current plus the resonant current) of the converter is supplied to a current sensing circuit 58, a rectifier 30, a filter 31 , a differentiator 32 and an attenuator 57, and is then supplied to an adder 59. The adder 59 is further supplied with the output voltage of the converter through an input terminal 60, and with the input voltage of the converter through an input terminal 61. The signals input through the input terminals 60 and 61 may be attenuated to a suitable level by variable resistors 62 and 63, respectively. The output from the adder 59 is coupled to a second input terminal 64 of the first error amplifier 26. It is observed here, that the feedforward supply of the input voltage of the converter through the input terminal 61 is not essential, but leads to an optimisation of the control. The control logic thus defined for the LLC converter is based on an inner and an outer control loop: a relatively slow outer loop controlling the output voltage of the converter, and generating the input (set) signal for the relatively fast inner loop. The inner loop will be preset by the input and the output voltages of the converter. As a result, the outer loop hardly will have to take any corrective action. Changes of the input and output parameters of the LLC converter immediately result in a reaction of the inner loop. By designing the inner loop with a relatively small time constant, and the outer loop with a relatively large time constant, a stable control algoritm for an LLC converter is obtained. Accordingly, the present invention provides an LLC converter with a control circuit employing a control method for greatly enhancing the stability of the converter by determining the variation of the total current of the converter and using a corresponding control signal for additionally controlling the frequency of on and off switching of the switches of the converter.

Claims: CLAIMS: 1. An LLC converter comprising: a series arrangement of a first parallel arrangement of a first switch (5) and a first diode (7), and a second parallel arrangement of a second switch (6) and a second diode (8), the series arrangement being coupled between a first input terminal and a second input terminal for receiving a DC input voltage, a voltage on the first terminal being positive with respect to a voltage at the second terminal, the cathode of the first diode (7) and the cathode of the second diode (8) being directed towards the first input terminal, a series arrangement of a capacitor (9), a first inductor (10) and a second inductor (11) coupled in parallel to either the first diode (7) or the second diode (8), a rectifier (12) coupled to either the first inductor (10) or the second inductor (11) for supplying an output voltage, a control circuit comprising: a switching control means (22, 25) for controlling the frequency of an on and off switching of the first switch (5) and the second switch (6); and a difference detection means (26) for detecting a difference between said output voltage and a reference value, and controlling said switching control means (22, 25) on the basis of said difference, characterized in that the control circuit further comprises: a current variation detection means (58, 30, 31, 32, 29c; 57) coupled to said difference detection means (26) for correcting said frequency in dependence on variations of a total current through the series arrangement of the capacitor (9), the first inductor (10) and the second inductor (11). 2. The converter according to claim 1, characterized in that the current variation detection means comprises a series arrangement of a current sensing circuit (58), a rectifier (30), a filter (31) and a differentiator (32). 3. The converter according to claim 2, characterized in that the current sensing circuit (58) is adapted to sense the effective voltage across the capacitor (9). 4. The converter according to claim 1 , characterized in that the difference detection means comprises an error amplifier (26) having a first input supplied with said reference value, and a second input coupled to an output of an adder (28) having a first input supplied with said output voltage, and a second input coupled to the current variation detection means (58, 30, 31, 32, 29c). 5. The converter according to claim 1, characterized in that the difference detection means comprises a first error amplifier (26) having a first input (56) coupled to the output of a second error amplifier (53) having a first input (54) supplied with said reference value, and a second input (55) supplied with said output voltage, the first error amplifier (26) having a second input (64) coupled to an output of an adder (59) having a first input coupled to the current variation detection means (58, 30, 31, 32, 57), and a second input supplied with an input voltage of the converter. 6. The converter according to claim 5, characterized in that the adder (59) has a third input supplied with said output voltage. 7. A method for controlling an LLC converter comprising switches for generating a current in a resonant circuit formed by a series arrangement of a capacitor, a first inductor, and a second inductor, the method comprising: sensing an output voltage of the converter for obtaining a first control signal; and sensing the current of the converter, characterized by: determining the variation of the current for obtaining a second control signal; correcting the first control signal by the second control signal for obtaining a third control signal; and driving the switches of the converter with a frequency of on and off switching determined by the third control signal. 8. The method according to claim 7, characterized in that the third control signal is obtained by adding the first and the second control signal. 9. The method according to claim 7, characterized in that the third control signal is compared to a reference signal, a difference signal between the third control signal and the reference signal being used for generating said frequency of on and off switching. 10. The method according to claim 7, characterized by: determining a difference between the output voltage and a reference voltage for obtaining a fourth control signal; and correcting the third control signal by the fourth control signal for generating said frequency of on and off switching. 11. The method according to claim 10, characterized by: sensing an input voltage of the converter for obtaining a fifth control signal; and correcting the first control signal by the fifth control signal for generating said frequency of on and off switching.

Inventors: BLOM ANTON C, VAN DALFSEN AGE J, BLOM, ANTON, C., VAN DALFSEN, AGE, J.
Applicants: BLOM ANTON C, DALFSEN AGE J VAN, KONINKL PHILIPS ELECTRONICS NV, BLOM, ANTON, C., KONINKLIJKE PHILIPS ELECTRONICS N.V., VAN DALFSEN, AGE, J.
Application year: 2000
Main classification: H02M3
IPC codes: H02M3, H02M7