Capacitively coupled power converter

[0002] 1. Field of the Invention

[0003] The present invention is directed, in general, to power conversion and, more specifically, to a power converter having a capacitively coupled feedback circuit for minimizing circuit costs and providing bi-directional communication in the power converter.

[0004] 2. Description of the Related Art

[0005] A power converter is a power processing circuit that converts an input voltage or current waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage into a specified output voltage. Power converters generally include power transfer components (e.g., transformers and switches) and control circuitry which governs the operation of the power transfer components to achieve desired operating characteristics.

[0006] It is often important (e.g., for safety reasons) that the primary and secondary sides of the converter be isolated from one another. Safety regulations (such as ULTM. requirements in the United States) often require some isolation between the power supply mains and electronic equipment. As a result, the transfer of control information from the secondary to primary sides is usually done via an isolation device (e.g., a transformer in which two windings are coupled by a permeable core, an opto-coupler, or a discrete light emitting device located adjacent to a discrete light detecting device) which is included in the control circuitry.

[0007] FIG. 1 is an illustration of a prior art power converter 100 which illustrates one technique for providing feedback from secondary to primary by using a second (small-signal) transformer T2. The voltage V_inon the primary side can be either an AC or DC voltage. The feedback control signal can be either amplitude-modulated (as in the case of the Unitrode UC3901) or pulse-triggered (as in the case of Intersil ICL7675/7676). In the latter arrangement, compensation could be implemented entirely on the secondary side, with only the final PWM signal being sent to the primary side.

[0008] FIG. 2 is an illustration of a prior art power converter 200 which utilizes an opto-coupler 202 for providing DC isolation for the control loop. Opto-couplers are typically used where safety regulations require galvanic isolation between the AC line voltage and the outputs of a switch-mode power supply. In the circuit of FIG. 2, the power transformer T1 is used to transfer the desired energy from the primary side to the secondary side, while maintaining DC isolation. The optical coupler 202 is used to feed the output signal back to the primary side. The voltage V_inon the primary side can be either an AC or DC voltage. The reference voltage V_refand a compensator 204 (or filter) is implemented on the secondary side. Additional compensation is usually necessary on the primary side. The use of opto-couplers is applicable to many power converter topologies, such as forward converter, flyback converter, etc.

[0009] One disadvantage associated with transformer isolation, as shown in FIG. 1, is that the isolation transformer, T2, is optimally designed to transfer high frequency signals (e.g., gate signals), and is ill-suited to transfer direct current (DC) signals whose characteristic frequencies can be highly variable. Another disadvantage associated with transformer isolation is that the transformer is susceptible to saturation. To prevent this occurrence, reset circuitry is often required.

[0010] Optical coupler isolation, as shown in FIG. 2 introduces its own unique set of disadvantages, such as low bandwidth (due to its limited frequency response), non-linearity and poor temperature stability and unidirectional communication capability. Accordingly, additional compensation circuits are needed on the secondary side.

[0011] A further disadvantage associated with both prior art approaches for providing feedback is that both methods require relatively expensive isolation devices (e.g., transformers, opto-couplers).

[0012] A still further disadvantage associated with the prior art approaches is that it is very difficult to design the compensator and reference voltage on the primary side. As such, the reference circuit and compensator cannot be integrated into the primary controller. This is especially true when the primary controller is implemented in a microcontroller or DSP. In this case, a filter and a reference voltage are still required on the secondary side. This limitation increases product costs and reduces flexibility.

[0013] Accordingly, there is a need in the art for a feedback approach that overcomes the afore-stated disadvantages of the prior art.

[0014] The present invention is embodied in a fly-back converter employing feedback and electrical isolation in a unique fashion using inexpensive components. While a fly-back converter is used to illustrate the teachings of the present invention, the principles of the invention may be applied to other power converter topologies. Instead of using optical or transformer coupling to provide electrical (DC) isolation as is well known in the prior art, the present invention uses a capacitive coupling network in the feedback loop.

[0015] The capacitive coupling network or circuit of the present invention has applicability to digital and analog embodiments. In an analog embodiment, the capacitive coupling network provides a more effective solution in terms of both cost and size than prior art optical and transformer coupling networks. In a digital embodiment, the capacitive coupling network provides a flexible solution than afforded by the prior art by providing a bi-directional communication capability between the primary and secondary sides of the converter for transmitting control data and commands there-between. Further, the digital embodiment advantageously removes the need for either the primary or secondary side micro-controller as used in the prior art. A still further advantage is that the digital information is transferred bi-directionally substantially without corruption therefore there is no linearity or temperature sensitivity problems. It is further noted that both the analog and digital implementations maintain DC isolation.

[0016] Other objects and advantages of the invention will become apparent from the following description of the presently preferred embodiments of the invention taken in conjunction with the drawings.

[0017] The present invention may be more fully understood from the detailed description given below and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for explanation and understanding only.

[0018] FIG. 1 is a is a schematic diagram of a representative prior art isolated power converter which uses transformer coupling to isolate the power converter input from its output;

[0019] FIG. 2 is a is a schematic diagram of a representative prior art isolated power converter which uses optical coupling to isolate the power converter input from its output;

[0020] FIG. 3ais a functional illustration of a digital embodiment of an isolated power converter constructed in accordance with the principles of the present invention;

[0021] FIG. 3bis an illustration of the circuit of FIG. 3ashowing the signals for performing primary side compensation;

[0022] FIG. 3cis an illustration of the circuit of FIG. 3ashowing the signals for performing secondary side compensation;

[0023] FIG. 4 is an illustration of a digital embodiment of a multiple-output flyback converter for performing bi-directional communication in accordance with the present invention;

[0024] FIG. 5 is an illustration of an analog embodiment of a capacitive coupler in accordance with the present invention;

[0025] FIG. 6 is an illustration of a preferred configuration of a capacitive coupler 600 in accordance with the present invention; and

[0026] FIG. 7 is a timing diagram of a simulation result illustrating the transmission of serial data from the secondary to the primary side through the capacitive coupler shown by FIG. 5.

[0027] The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment (by way of example, and not of limitation), in which:

[0028] Digital Implementation

[0029] FIGS. 3a-cand FIG. 4 illustrate a digital implementation of the capacitively coupled feedback network or circuit. It is contemplated that said circuit could be utilized as a standalone circuit or network, or instead be incorporated or integral with either a primary side or a secondary side module. For example, the circuit could be integral with any of the primary side 302 elements of FIG. 3a, or with any of the secondary side 304 elements of FIG. 3a.

[0030] FIGS. 3a-cillustrate techniques for effecting primary side and secondary side compensation and FIG. 4 illustrates the bi-directional transmission capability of the capacitively coupled network for transmitting digital data (i.e., command and control information).

[0031] FIGS. 3a-cillustrate techniques for effecting primary side and secondary side compensation for the digital implementation of the inventive capacitive coupling network.

[0032] FIG. 3ais a functional illustration of a digital implementation of a general power converter configuration 300 including the inventive capacitive coupling feedback circuit 306 of the present invention. FIGS. 3band 3cillustrate those signals involved in the power converter configuration of FIG. 3afor performing primary side and secondary side output regulation, respectively.

[0033] Referring now to FIG. 3a, the isolated power converter 300 includes a primary side 302. The primary side 302 further includes a primary power stage 303 and a primary side controller 308. The power converter 300 also includes a transformer T1, and a secondary side 304 which further includes a secondary power stage 305, an A/D converter 312 and a secondary controller 310. The elements recited above are conventional and well known in the art. In addition to the aforementioned elements, the converter 300 further includes an inventive feedback circuit 306. The feedback circuit 306 is embodied as a capacitive coupler for coupling the secondary 304 and primary 302 sides while maintaining electrical (i.e., DC) isolation. The inventive capacitive coupling feedback circuit 306 permits feedback compensation to be implemented on either the primary side 302 or the secondary side 304. Both primary and secondary side feedback compensation approaches are discussed below with reference to FIGS. 3band 3c.

[0034] Primary Side Compensation for the Digital Embodiment

[0035] As discussed in the background, it is very difficult to implement primary side compensation using conventional feedback structures (e.g., transformer, optical-coupling). The present invention achieves primary side compensation by employing the inventive capacitive coupling feedback circuit 306 of the present invention.

[0036] FIG. 3bis an illustration of the circuit of FIG. 3ashowing the signals for performing primary side compensation. In the circuit of FIG. 3b, a feedback signal V_fbgenerated as an output signal from the secondary side controller 310 is transferred via the inventive capacitive coupling feedback circuit 306 to the primary side controller 308 for regulating the output voltage V_o. It is noted that primary side compensation can be achieved with or without the secondary side controller 310 which is an incidental component for performing primary side compensation. As shown, the feedback (voltage regulation) signal V_fbis transferred from the secondary side 304 to the primary controller 308 on the primary side 302. The primary controller 308 uses the transferred signal V_fb(trans)as input to generate a gate signal V_gateto be supplied to the primary power stage 303 for controlling a switching element therein.

[0037] The capacitive coupling feedback circuit 306 of the present invention, capacitively couples or transfers the feedback signal V_fbto the primary side 302 while maintaining direct current isolation between the primary 302 and secondary 304 side of the transformer T1. Further, as a consequence of utilizing the inventive coupling network 300, there is no concern for linearity or temperature sensitivity variations associated with opto-couplers or saturation concerns associated with transformer coupling.

[0038] Secondary Side Compensation for the Digital Embodiment

[0039] FIG. 3cis an illustration of the circuit of FIG. 3aincluding those required signals for performing secondary side compensation. In the circuit of FIG. 3c, a final gate signal V_gateis generated on the secondary side 304, and is transferred, via the inventive capacitive coupling network 306, to be received as a transferred gate signal V_gate(trans)on the primary side 302 as an input to the primary controller 308 for regulating the output voltage V_o. It is noted that secondary side compensation can be achieved with or without the primary side controller 308 which is an incidental component for performing secondary side compensation.

[0040] It is therefore shown that the capacitive coupling feedback circuit 306 of the present invention, couples or transfers the gate signal V_gateto the primary side 302 while maintaining direct current isolation between the primary 302 and secondary 304 side of the transformer T1. Further, as a consequence of utilizing the inventive coupling network, there is no concern for linearity or temperature sensitivity variations.

[0041] In addition to performing primary and secondary side compensation, the inventive capacitive coupling feedback circuit 306 provides a further capability of bi-directional communication between the primary 302 and secondary 304 side of the transformer T1. This capability is illustrated with reference to FIG. 4.

[0042] Bi-directional Communication for the Digital Embodiment

[0043] FIG. 4 illustrates, by example, an exemplary power converter configuration 400 to illustrate the bi-directional communication capability of the inventive capacitive coupling feedback circuit 306 of the present invention. In particular, FIG. 4 illustrates a multiple-output flyback converter 400 generally including primary 402 and secondary 404 sides and further includes the inventive capacitive coupling network 406.

[0044] In operation, bi-directional digital data (e.g., output information and/or commands) are passed, via the inventive capacitive coupling network 406, between the primary 402 and secondary 404 sides of the converter 400. One example of primary to secondary communication occurs when the primary must inform the secondary of a drop in the line voltage such that the secondary side can shut down safely. An example of secondary 404 to primary 402 communication is the ability of the secondary 404 to inform the primary 402 of a desire to change modes (e.g., go to an energy saving mode).

[0045] As shown in the circuit of FIG. 4, primary-side 410 and secondary-side 412 micro-controllers are included to carry out the generation, issuance and receipt of the commands.

[0046] Analog Implementation

[0047] FIG. 5 is a functional illustration of an analog embodiment of an isolated power converter 500 constructed in accordance with the principles of the present invention. The isolated power converter 500 includes a primary side 502. The primary side 502 further includes a primary power stage 503, a primary side controller 508, and a frequency-to-voltage converter 509. The power converter 500 also includes a transformer T1, and a secondary side 504 which further includes a secondary power stage 505, an error amplifier 512 and a voltage-to-frequency converter 510. The elements recited above are conventional and well known in the art. In addition to the aforementioned elements, the converter 500 further includes the inventive capacitively coupled feedback circuit 506, which is identical in function and structure to the capacitively coupled feedback circuit 506 illustrated in FIGS. 3a-c.

[0048] In the current analog embodiment, the feedback circuit 506 facilitates feedback control between the primary 502 and secondary 504 sides of the converter 500. The circuit 500 generally operates by converting an error voltage to a frequency for transmission over the capacitive coupling network from the secondary to the primary side as an analog signal. Once received on the primary side, the analog frequency signal is converted back to its original corresponding voltage to control the duty cycle of a primary side power switch.

[0049] The following is a more detailed explanation of the analog embodiment of the circuit 500 of FIG. 5. An error amplifier 512 compares the output voltage Vo, with a reference voltage V_—REF, to produce an error voltage, V_E, to be supplied as input to the voltage-to-frequency converter 510. The voltage-to-frequency converter 510 converts the supplied error voltage V_Eto a corresponding frequency F_Eto be transmitted across the capacitively coupled feedback circuit 506. Voltage to frequency conversion techniques are well known in the art.

[0050] DC isolation is maintained by converting the error voltage V_Eto a corresponding analog frequency F_Ewaveform which is suitable for transmission over the capacitively coupled network 506. The transmitted frequency F_Eis then received on the primary side 502 by a frequency-to-voltage converter 509 to convert the transmitted frequency F_Eback to the original error voltage V_E. The converted error voltage V_Eis supplied as input to the primary-side controller 508 to adjust the duty cycle of a power switch, which is an element of the primary power stage 503.

[0051] Illustrative Embodiment

[0052] FIG. 6 is an illustration of a preferred configuration of a capacitive coupler 600 in accordance with the present invention. Elements 602 and 604 are identical to those shown in FIG. 3, i.e., 302, 304, FIG. 4, i.e., 402, 404, FIG. 5, i.e., 502, 504 and FIG. 6, i.e., 602, 604 and have the same functionality as previously described. As shown, capacitors C1 and C2 permit bi-directional communication between a primary and secondary side of the converter while maintaining DC isolation. Diodes D1 and D2 are included to compensate for some expected DC bias drift due to a change in the DC bias during a changing pulse pattern. The diodes D1 and D2 serve to prevent the DC bias drift resulting from the pulse changes by removing the DC bias altogether (i.e., shunting the DC bias to ground). Other compensation methods are well within the scope of the present invention and may include, for example, using a logic buffer or voltage comparator. Resistors R1 and R2 provide a load to the secondary and primary side controllers, respectively.

[0053] Simulation Results

[0054] This section describes the performance of the capacitive coupler 600 in sending serial (binary) data from the secondary controller 504 to the primary controller 602 of FIG. 6.

[0055] Referring now to FIGS. 7aand 7b, which illustrate the simulation result of sending serial data from the secondary side to the primary side of the controller of FIG. 3c. As shown, the serial data is sent in two sections, a first section defined herein as the ‘header’ section which is a data identification section describing the type of data will follow in the corresponding ‘body’ section which can include, for example, binary encoded voltage values.

[0056] FIG. 7aillustrates the transmission of a first header section, header 1, from the secondary side, followed by a corresponding body section, body 1, followed by the transmission of a second header section, header 2, followed by its corresponding body section, body 2. The header section may, for example, consist of a single bit for a simple feedback control implementation, or multiple bits for extra two-way or one-way communication in another implementation. In the simulation result of FIG. 7, two bits are shown which could represent, for example, feedback transfer data, e.g., bit sequence “1,0”, or commands to be transferred from the secondary to the primary, e.g., bit sequence “0,1”, or commands to be transferred from primary to secondary, e.g., bit sequence “1,1”. As shown in FIG. 7, the data is transferred from primary to secondary through the inventive capacitive coupler without corruption.

[0057] While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will be apparent to those skilled in the art. It is therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.