PATENT DOCUMENT

Publication Number: US-9042132-B2
Application Number: US-201113280885-A
Country: US
Kind Code: B2

Title: Noise suppression circuit for power adapter

Abstract:
A noise suppression circuit for a power adapter is disclosed. The noise suppression circuit can reduce or eliminate adapter-induced noise that could interfere with an electronic device powered by the adapter. In one example, the noise suppression circuit can include an active circuit to detect and attenuate or cancel the induced noise. In another example, the noise suppression circuit can include an RLC circuit in parallel with the adapter choke to suppress the induced noise at the operating frequencies of the powered electronic device. In still another example, the noise suppression circuit can include a modified adapter Y capacitor connection so as to bypass the adapter choke, thereby reducing or eliminating the choke&#39;s induced noise.

Claims:
What is claimed is: 
     
       1. A noise suppression circuit for a power adapter comprising:
 an operational amplifier coupled between an alternating current (AC) neutral line on a first side of the adapter and a direct current (DC) ground line on a second side of the adapter, the first side of the adapter and the second side of the adapter separated by a transformer, wherein: 
 an output and a first input of the operational amplifier are coupled to the AC neutral line, 
 a second input of the operational amplifier is coupled to the DC ground line, and 
 the operational amplifier is configured to cause current to flow from the DC ground line to the AC neutral line, or from the AC neutral line to the DC ground line, based on a noise between the AC neutral line and the DC ground line so as to attenuate the noise between the AC neutral line and the DC ground line, the noise being induced into a DC line on the second side of the adapter. 
 
     
     
       2. The circuit of  claim 1 , wherein the operational amplifier is configured to cause the current to flow by detecting the noise between the AC neutral line and the DC ground line and using the detected noise to control the current flow from the DC ground line to the AC neutral line or from the AC neutral line to the DC ground line. 
     
     
       3. The circuit of  claim 2 , further comprising:
 a capacitance multiplier configured to detect the noise; and 
 a feedback capacitor configured to control the current flow so as to attenuate the noise between the AC neutral line and the DC ground line. 
 
     
     
       4. The circuit of  claim 3 , wherein the capacitance multiplier comprises:
 a sense capacitor configured to simulate a larger capacitance so as to detect the noise, wherein operational amplifier is coupled to the sense capacitor to help simulate the larger capacitance in the sense capacitor. 
 
     
     
       5. The circuit of  claim 1 , wherein the operational amplifier is coupled to a transformer output on the second side. 
     
     
       6. The circuit of  claim 1 , wherein the operational amplifier is coupled to the first side at a first point where the adapter voltage is substantially free of the induced noise and to the second side at a second point where the adapter voltage has the induced noise. 
     
     
       7. A noise suppression circuit for a power adapter comprising:
 an RLC circuit in parallel with a component of the adapter, the RLC circuit and the component being on a first side of the adapter, the RLC circuit configured to suppress noise induced in the component at select frequencies of an adapter voltage on a second side of the adapter, wherein: 
 the first side of the adapter and the second side of the adapter are separated by a transformer, and 
 the RLC circuit comprises:
 a first resistor, a first inductor and a first capacitor in parallel with the component and coupling an AC live line on a first side of the component and a second side of the component, and 
 a second resistor, a second inductor and a second capacitor in parallel with the component and coupling an AC neutral line on a first side of the component and a second side of the component. 
 
 
     
     
       8. The circuit of  claim 7 , wherein the component of the adapter is a choke, the choke inducing the noise. 
     
     
       9. The circuit of  claim 8 , wherein the RLC circuit is configured to lower choke impedance at the select frequencies so as to suppress the induced noise at the select frequencies. 
     
     
       10. The circuit of  claim 7 , wherein the component of the adapter is a Y capacitor, the Y capacitor inducing the noise. 
     
     
       11. The circuit of  claim 7 , wherein the select frequencies coincide with operating frequencies of an electronic device powered by the adapter. 
     
     
       12. The circuit of  claim 1 , wherein the first input comprises an inverting input of the operational amplifier, and the second input comprises a non-inverting input of the operational amplifier. 
     
     
       13. A power adapter for noise suppression comprising:
 a plug configured to connect to a power source; 
 a connector configured to connect to a device; 
 a power circuit configured to receive a first voltage from the power source, to transform the first voltage into a second voltage compatible with the device, and to send the second voltage to the device to power the device; and 
 a noise suppression circuit configured to suppress noise induced in the power circuit, the noise suppression circuit comprising;
 an operational amplifier coupled between an alternating current (AC) neutral line on a first side of the adapter and a direct current (DC) ground line on a second side of the adapter, the first side of the adapter and the second side of the adapter separated by a transformer, wherein: 
 an output and a first input of the operational amplifier are coupled to the AC neutral line, 
 a second input of the operational amplifier is coupled to the DC ground line, and 
 the operational amplifier is configured to cause current to flow from the DC ground line to the AC neutral line, or from the AC neutral line to the DC ground line, based on a noise between the AC neutral line and the DC ground line so as to attenuate the noise between the AC neutral line and the DC ground line, the noise being induced into a DC line on the second side of the adapter. 
 
 
     
     
       14. The adapter of  claim 13 , wherein the device comprises a touch sensitive device configured to sense a proximity of an object to the device and to perform an action based on a sense signal indicative of the sensed proximity. 
     
     
       15. The adapter of  claim 14 , wherein the noise suppression circuit is configured to suppress the induced noise so as to avoid adversely affecting the sense signal. 
     
     
       16. A system for noise suppression comprising:
 an electronic device configured to generate signals; and 
 a power adapter configured to power the electronic device, the adapter lacking a plug ground prong and including a noise suppression circuit configured to suppress noise induced in the adapter from interfering with the generated signals of the device, the noise suppression circuit comprising:
 an operational amplifier coupled between an alternating current (AC) neutral line on a first side of the adapter and a direct current (DC) ground line on a second side of the adapter, the first side of the adapter and the second side of the adapter separated by a transformer, wherein: 
 an output and a first input of the operational amplifier are coupled to the AC neutral line, 
 a second input of the operational amplifier is coupled to the DC ground line, and 
 the operational amplifier is configured to cause current to flow from the DC ground line to the AC neutral line, or from the AC neutral line to the DC ground line, based on a noise between the AC neutral line and the DC ground line so as to attenuate the noise between the AC neutral line and the DC ground line, the noise being induced into a DC line on the second side of the adapter. 
 
 
     
     
       17. The system of  claim 16 , wherein the electronic device is a touch sensitive device configured to generate sense signals indicative of a proximity of an object to the touch sensitive device, and
 wherein the noise suppression circuit is configured to suppress the induced noise and prevent the induced noise from interfering with the sense signals. 
 
     
     
       18. The system of  claim 16 , wherein the electronic device is an audio device configured to generate audio signals for listening by a user, and
 wherein the noise suppression circuit is configured to suppress the induced noise and prevent the induced noise from interfering with the audio signals. 
 
     
     
       19. The system of  claim 16 , wherein the electronic device is a video device configured to generate video signals for viewing by a user, and
 wherein the noise suppression circuit is configured to suppress the induced noise and prevent the induced noise from interfering with the video signals.

Description:
FIELD 
     This relates generally to power adapters and more particularly to noise suppression circuits for power adapters. 
     BACKGROUND 
     Noise can be a persistent concern for electronic devices because the noise can come from a variety of sources (both internal and external) and can adversely affect the devices&#39; desired signals. In some circumstances, the noise can be related to the power adapters used to power the electronic devices. 
       FIGS. 1A and 1B  illustrate two types of power adapters that are typically used to power electronic devices. In the example of  FIG. 1A , “Type B” power adapter  110  can include plug  112  to plug into a power source, e.g., a wall outlet, to receive a voltage, e.g., AC voltage. The plug  112  can have three prongs, including live voltage prong  112 - a , neutral prong  112 - b , and ground prong  112 - c . The adapter  110  can also include connector  113  to connect to an electronic device to send a voltage, e.g., DC voltage, to power the device. The adapter  110  can further include power circuit  111  to receive the voltage from the power source via the plug  112 , transform the received voltage into a voltage that is compatible with the connected device, and send the compatible voltage to the device via the connector  113 . In the example of  FIG. 1B , “Type A” power adapter  120  can be the same as the Type B adapter  110 , except Type A adapter plug  122  has two prongs rather than three. The two prongs of the plug  122  can include live voltage prong  122 - a  and neutral prong  122 - b . The Type A adapter&#39;s power circuit  121  can receive voltage from a power source via the plug  122 , transform the received voltage into a voltage compatible with a connected electronic device, and send the compatible voltage to the device via connector  123  to power the device. 
     As the Type B adapter  110  has ground prong  112 - c  that can couple to earth ground, noise induced in the adapter can be shunted to ground rather than into components of a connected electronic device. On the other hand, the Type A adapter  120  does not have a ground prong and therefore can induce noise that can be introduced into the connected electronic device that the adapter powers. 
     Because many electronic devices use the Type A adapter, the challenge is to suppress induced noise in those devices while using that adapter. 
     SUMMARY 
     This relates to a noise suppression circuit for a power adapter to reduce or eliminate adapter-induced noise from being introduced into an electronic device powered by the adapter. This noise suppression circuit can be particularly helpful with power adapters, e.g., Type A adapters, which lack a ground prong in the adapter plug that could advantageously handle induced noise. In one example, the noise suppression circuit can include an active circuit to detect and attenuate or cancel the induced noise. In another example, the noise suppression circuit can include an RLC circuit in parallel with the adapter choke to suppress the induced noise at the operating frequencies of the connected electronic device. In still another example, the noise suppression circuit can include a modified adapter Y capacitor connection so as to bypass the adapter choke, thereby reducing or eliminating the choke&#39;s induced noise. By using a noise suppression circuit for a power adapter, electronic devices can operate properly with the power adapter because the devices&#39; desired signals carry little or no adapter-induced noise that would interfere with the devices&#39; operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate exemplary power adapters that can be used to power electronic devices according to various embodiments. 
         FIG. 2  illustrates an exemplary power system having a power adapter coupled to a touch sensitive device according to various embodiments. 
         FIG. 3  illustrates an exemplary power adapter noise suppression circuit having a feedback circuit according to various embodiments. 
         FIG. 4  illustrates an exemplary power adapter noise suppression circuit having an RLC tuned circuit in parallel with a choke according to various embodiments. 
         FIG. 5  illustrates an exemplary power adapter noise suppression circuit having a modified connection for a Y capacitor according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of example embodiments, reference is made to the accompanying drawings in which it is shown by way of illustration specific embodiments that can be practiced. It is to be understood that other embodiments can be used and structural changes can be made without departing from the scope of the various embodiments. 
     This relates to a noise suppression circuit for a power adapter that can be used to suppress adapter-induced noise that adversely affects desired signals in a device powered by the adapter. In some embodiments, the noise suppression circuit can include an active circuit to detect and attenuate or cancel the induced noise in the adapter. In some embodiments, the noise suppression circuit can include an RLC tuned circuit in parallel with the adapter choke to suppress the choke&#39;s induced noise at the operating frequencies of the connected electronic device. Alternatively, the RLC tuned circuit can be in parallel with the adapter Y capacitor to similarly suppress the capacitor&#39;s induced noise at the operating frequencies of the connected electronic device. In some embodiments, the noise suppression circuit can include a modified connection for the adapter Y capacitor so as to bypass the adapter choke, thereby reducing or eliminating the choke&#39;s induced noise. 
     By providing noise suppression circuitry in the power adapter, the adapter can suppress induced noise that would otherwise be introduced into desired signals of the device powered by the adapter. A power adapter with noise suppression circuitry can be particularly useful when powering touch sensitive devices because the devices depend on sense signals, which can be very sensitive to noise, to perform various functions on the devices. 
     Although various embodiments herein describe AC power adapters, it is to be understood that other types of adapters are also possible candidates for noise suppression. 
       FIG. 2  illustrates an exemplary power system having a touch sensitive device powered by an adapter with a noise suppression circuit according to various embodiments. In the example of  FIG. 2 , power system  200  can include power adapter  210  plugged into wall outlet  230  to receive AC voltage Vac and connected to touch sensitive device  240  to send DC voltage Vdc to power the device. The power adapter  210  can be a Type A adapter or any other adapter that lacks a ground prong according to various embodiments. The touch sensitive device  240  can detect an object, e.g., user&#39;s hand  250 , proximate to the device, generate a sense signal indicative of the object&#39;s proximity, and perform various functions based on the sense signal. 
     When the touch sensitive device  240  is connected to a power adapter  210  that lacks a ground prong, e.g., a Type A adapter, the device can be vulnerable to noise induced by the adapter that can interfere with the sense signals. The touch sensitive device  240  can generate stimulation signals to stimulate drive circuitry (not shown) to drive the device. In response to the stimulation signals, sense circuitry (not shown) of the touch sensitive device  240  can generate sense signals, where the relative strength of the sense signals can be a function of the proximity of the object to the device. The induced noise in the adapter  210  can be introduced directly into the sense signals due to the noise being on the system&#39;s isolated DC ground (to which the sense signals are referenced) relative to the user, causing the noise to couple to the generated sense signals. The resulting sense signals can be erroneous, thereby causing the touch sensitive device  240  to perform an erroneous function based on the noisy sense signals. In some embodiments, the noise in the sense signals can be exacerbated by a poorly grounded object, such as the user&#39;s hand  250 , touching or hovering over the touch sensitive device  240  at multiple locations. 
     Noise suppression circuit  215  for the power adapter  210  can suppress that induced noise so that its interference with the sense signals is reduced or eliminated. Exemplary noise suppression circuits will be described in detail in  FIGS. 3 through 5 . 
     Although various embodiments herein refer to touch sensitive devices, it is to be understood that other electronic devices can also be used with a noise suppression circuit for a power adapter. 
       FIG. 3  illustrates an exemplary noise suppression circuit that can be used in the system of  FIG. 2 . In the example of  FIG. 3 , power adapter  310  can include a power circuit to supply power to touch sensitive device  240  and a noise suppression circuit to suppress noise that could interfere with the device&#39;s sense signals. The power circuit can be a switched-mode power circuit, including AC live voltage line  301  to supply AC live voltage Vac from wall outlet  230 , AC neutral line  302  coupled to AC neutral of the wall outlet, choke  361  to suppress electromagnetic interference (EMI) emissions from the adapter, switching node  363  to help regulate the AC voltage Vac, transformer  362  to transfer the AC voltage Vac between primary and secondary sides of the power circuit, Y capacitor Cy to further suppress EMI emissions and to act as a safety element to attenuate stray voltages, rectifier diode D 1  to convert the AC voltage Vac to DC voltage Vdc, capacitor C 10  to smooth the DC voltage Vdc, and DC voltage line  303  to transmit the DC voltage Vdc to the touch sensitive device to power the device. The power circuit can couple to primary AC ground (illustrated by a triangle) on the primary side of the adapter  310  and to secondary DC ground (illustrated by parallel horizontal lines) on the secondary side of the adapter. The choke  361  and the Y capacitor Cy can induce noise that can propagate through the adapter  310  and the touch sensitive device  240  so as to interfere with the device&#39;s sense signals. In some embodiments, the noise induced in the choke  361  and the Y capacitor Cy can be due in part to parasitic capacitances at the switching node  363  in the adapter  310  or parasitic winding capacitances at the transformer  362  in the adapter that can appear as noise at the choke and the Y capacitor. 
     The noise suppression circuit for the adapter  310  can employ active noise suppression to suppress induced noise in the adapter. To do so, the noise suppression circuit can include an active circuit, e.g., a feedback circuit, acting as a capacitance multiplier (capacitor C 1  and operational amplifier  364 ) to effectively detect the induced noise, and a feedback capacitor (capacitor C 3 ) to effectively feed back the detected noise so as to attenuate or cancel the noise between the primary ground and the secondary ground. The noise suppression circuit can be coupled to the AC neutral line  302  at a point where the AC voltage Vac is clean, i.e., relatively free of induced noise, and to the secondary side output of the transformer  362  at the noisy secondary side ground via the (+) input to the operational amplifier  364 . The capacitance multiplier can include capacitor C 1  and operational amplifier  364  with resistor R and capacitor C 2 , where the capacitor C 1 , with the help of the operational amplifier, can simulate a larger capacitor so as to sense the induced noise in the AC voltage Vac by sensing the differential voltage between the primary side ground and secondary side ground. There can a tendency for the AC voltage Vac at the secondary side to be higher than at the primary side, indicative of the induced noise. Accordingly, feedback capacitor C 3  can feed back a current (including the induced noise) so as to reduce that differential voltage and, in the process, to couple the feedback noise into the AC voltage so as to attenuate or cancel out noise induced by the choke  361  and the Y capacitor Cy. 
     In operation, the power adapter  310  can receive AC voltage Vac from a power source, e.g., a wall outlet, transmit the AC voltage Vac through the choke  361 , the transformer  362 , the switching node  363 , the Y capacitor Cy, the rectifier diode D 1 , and the smoothing capacitor C 10  to process and convert the AC voltage Vac to DC voltage Vdc as described previously, and send the DC voltage Vdc to a connected touch sensitive device to power the device. While transmitting the AC voltage Vac, the power adapter  310  can also actively suppress induced noise in the AC voltage Vac using the active circuit as described previously so that the secondary DC ground provided to the connected touch sensitive device has little or no noise that could interfere with the device&#39;s sense signals. In some embodiments, the noise suppression circuit can include the capacitor C 1  having a capacitance of about 220 pF, the operational amplifier  364 , a gain-bandwidth product of about 12 MHz and a slew rate of about 400V/μs, the amplifier resistor R, a resistance of about 100 kΩ, the amplifier capacitor C 2 , a capacitance of about 47 pF, and the feedback capacitor C 3 , a capacitance of about 220 pF. The feedback current can be about 2 mA, at a peak voltage Vpk of about 5V and the touch frequency of about 300 kHz. Accordingly, at a touch frequency of 100 kHz, the noise suppression circuit can suppress induced noise in the adapter  310  by as much as 30 dB and, at a touch frequency of 300 kHz, by as much as 10 dB. 
     It should be understood that the power adapter  310  of  FIG. 3  has been simplified for explanatory purposes, but can include additional and/or other components capable of powering an electronic device and suppressing noise according to various embodiments. 
       FIG. 4  illustrates another exemplary noise suppression circuit that can be used in the system of  FIG. 2 . In the example of  FIG. 4 , the noise suppression circuit for power adapter  410  can employ selective frequency filtering to select particular frequencies for induced noise suppression. In some embodiments, the selected frequencies can coincide with the frequencies of the stimulation signals used by the touch sensitive device  240  for driving the device to sense a proximate object and to generate sense signals indicative thereof. The power adapter  410  can include choke  461  to suppress EMI emissions, as described previously in  FIG. 3 . To suppress EMI emissions, the choke  461  can have higher impedance to block higher frequency signals (which the EMI signals are) while passing lower frequency signals. However, at the same time, the choke  461  can induce substantial noise in the AC voltage Vac that can interfere with the connected touch sensitive device&#39;s sense signals. On the other hand, if the choke  461  were operated at low or zero impedance, the noise induced by the choke can be reduced significantly. However, at such impedance, the choke&#39;s EMI suppression can degrade. 
     The noise suppression circuit of  FIG. 4  can balance EMI suppression with noise suppression at the choke so as to reduce noise at the connected touch sensitive device&#39;s sense signals without substantially degrading EMI suppression. To do so, the noise suppression circuit can include RLC circuit  471  having resistor R, inductor L, and capacitor C in parallel with the choke  461 . The RLC circuit  471  can provide, at the selected frequencies, in effect a low choke impedance in parallel with the choke  461  so as to reduce induced noise at the selected frequencies, i.e., the operating frequencies for the touch sensitive device. At the other frequencies, the RLC circuit  471  can have little effect such that the choke  461  can perform normal EMI suppression. Accordingly, at the selected frequencies, the lower choke impedance can be used to avoid inducing noise in the AC voltage Vac, while at the other frequencies where induced noise is not at issue for the touch sensitive device, the higher choke impedance can effectively suppress EMI emissions. 
     The power circuit of the adapter  410  of  FIG. 4  can be the same as the power circuit of the adapter  310  of  FIG. 3 , having (in addition to the illustrated components) the switching node  363 , the Y capacitor Cy, the rectifier diode D 1 , the smoothing capacitor C 10 , and the DC voltage line  303 . 
     In operation, the power adapter  410  can operate in a similar manner as the power adapter  310  of  FIG. 3  to convert the AC voltage Vac to the DC voltage Vdc. While transmitting the AC voltage Vac, the power adapter  410  can also suppress induced noise in the AC voltage Vac using the RLC circuit as described previously so that the DC voltage Vdc sent to the connected touch sensitive device has little or no noise that could interfere with the device&#39;s sense signals. In some embodiments, the RLC circuit  471  can be tuned to suppress noise in the narrow touch frequency band of 100-300 kHz. For example, the RLC circuit resistor R can have a resistance of about 3.6 kΩ, the inductor L, an inductance of about 3.8 mH, and the capacitor C, a capacitance of about 73 pF. The RLC circuit  471  can provide, at the selected frequencies of 100-300 kHz, a lower choke impedance that is about 5 times lower than an existing choke impedance at the 9.6 mH choke. According, at the selected frequencies, the noise suppression circuit can suppress induced noise in the adapter  410  by as much as 10 dB. 
     In an alternate embodiment, rather than having the RLC circuit in parallel with the choke, the RLC circuit can be in parallel with the Y capacitor Cy, which can also induce noise that could interfere with the sense signals, to help suppress the Y capacitor induced noise in a similar manner as the choke. 
     In another alternate embodiment, a first RLC circuit can be in parallel with the choke to help suppress the choke&#39;s induced noise, and a second RLC circuit can be in parallel with the Y capacitor to help suppress the Y capacitor&#39;s induced noise. 
       FIG. 5  illustrates another exemplary noise suppression circuit that can be used in the system of  FIG. 2 . In the example of  FIG. 5 , the noise suppression circuit of power adapter  510  can employ a modified connection for Y capacitor Cy to suppress induced noise in the adapter. Rather than couple the Y capacitor Cy at the primary AC ground as in  FIG. 3  (illustrated by point (M)), the Y capacitor can be coupled to the AC neutral line  502  upstream of choke  561  at a point where the AC voltage Vac is relatively free of induced noise (illustrated by point (M′)). This can effectively bypass the choke  561 , thereby reducing the choke&#39;s induced noise contribution to the AC voltage Vac. 
     The power circuit of the adapter  510  of  FIG. 5  can be the same as the power circuit of the adapter  310  of  FIG. 3 , having (in addition to the illustrated components) the switching node  363 , the rectifier diode D 1 , the smoothing capacitor C 10 , and the DC voltage line  303 . 
     In operation, the power adapter  510  can operate in a similar manner as the power adapter  310  of  FIG. 3  to convert the AC voltage Vac to the DC voltage Vdc. While transmitting the AC voltage Vac, the power adapter  510  can also suppress induced noise in the AC voltage Vac using the modified Y capacitor Cy connection to bypass the choke  561  as described previously so that the DC voltage Vdc sent to the connected touch sensitive device has little or no noise that could interfere with the device&#39;s sense signals. In some embodiments, the modified Y capacitor Cy can have a capacitance of about 2200 pF. Accordingly, at touch frequencies between 100-300 kHz, the noise suppression circuit can suppress induced noise in the adapter  510  by as much as 10 dB. 
     In addition to suppressing noise in sense signals, a noise suppression circuit according to various embodiments can be used for certain kinds of EMI suppression, audio signal noise suppression, video signal noise suppression, and the like. 
     Although embodiments have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the various embodiments as defined by the appended claims.

Metadata:
Filing Date: 20111025
Publication Date: 20150526
Grant Date: 20150526
Priority Date: 20111025
Inventors: RICHARDS PETER W.
HOTELLING STEVEN PORTER
Assignee: APPLE INC
CPC Classifications: [{"code": "H02M1/44", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02M1/44", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 47018484