Noise suppression circuit for power adapter

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's induced noise.

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' desired signals. In some circumstances, the noise can be related to the power adapters used to power the electronic devices.

FIGS. 1A and 1Billustrate two types of power adapters that are typically used to power electronic devices. In the example ofFIG. 1A, “Type B” power adapter110can include plug112to plug into a power source, e.g., a wall outlet, to receive a voltage, e.g., AC voltage. The plug112can have three prongs, including live voltage prong112-a, neutral prong112-b, and ground prong112-c. The adapter110can also include connector113to connect to an electronic device to send a voltage, e.g., DC voltage, to power the device. The adapter110can further include power circuit111to receive the voltage from the power source via the plug112, transform the received voltage into a voltage that is compatible with the connected device, and send the compatible voltage to the device via the connector113. In the example ofFIG. 1B, “Type A” power adapter120can be the same as the Type B adapter110, except Type A adapter plug122has two prongs rather than three. The two prongs of the plug122can include live voltage prong122-aand neutral prong122-b. The Type A adapter's power circuit121can receive voltage from a power source via the plug122, transform the received voltage into a voltage compatible with a connected electronic device, and send the compatible voltage to the device via connector123to power the device.

As the Type B adapter110has ground prong112-cthat 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 adapter120does 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'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' desired signals carry little or no adapter-induced noise that would interfere with the devices' operation.

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'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'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'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. 2illustrates 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 ofFIG. 2, power system200can include power adapter210plugged into wall outlet230to receive AC voltage Vac and connected to touch sensitive device240to send DC voltage Vdc to power the device. The power adapter210can be a Type A adapter or any other adapter that lacks a ground prong according to various embodiments. The touch sensitive device240can detect an object, e.g., user's hand250, proximate to the device, generate a sense signal indicative of the object's proximity, and perform various functions based on the sense signal.

When the touch sensitive device240is connected to a power adapter210that 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 device240can 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 device240can 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 adapter210can be introduced directly into the sense signals due to the noise being on the system'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 device240to 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's hand250, touching or hovering over the touch sensitive device240at multiple locations.

Noise suppression circuit215for the power adapter210can 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 inFIGS. 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. 3illustrates an exemplary noise suppression circuit that can be used in the system ofFIG. 2. In the example ofFIG. 3, power adapter310can include a power circuit to supply power to touch sensitive device240and a noise suppression circuit to suppress noise that could interfere with the device's sense signals. The power circuit can be a switched-mode power circuit, including AC live voltage line301to supply AC live voltage Vac from wall outlet230, AC neutral line302coupled to AC neutral of the wall outlet, choke361to suppress electromagnetic interference (EMI) emissions from the adapter, switching node363to help regulate the AC voltage Vac, transformer362to 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 D1to convert the AC voltage Vac to DC voltage Vdc, capacitor C10to smooth the DC voltage Vdc, and DC voltage line303to 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 adapter310and to secondary DC ground (illustrated by parallel horizontal lines) on the secondary side of the adapter. The choke361and the Y capacitor Cy can induce noise that can propagate through the adapter310and the touch sensitive device240so as to interfere with the device's sense signals. In some embodiments, the noise induced in the choke361and the Y capacitor Cy can be due in part to parasitic capacitances at the switching node363in the adapter310or parasitic winding capacitances at the transformer362in the adapter that can appear as noise at the choke and the Y capacitor.

The noise suppression circuit for the adapter310can 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 C1and operational amplifier364) to effectively detect the induced noise, and a feedback capacitor (capacitor C3) 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 line302at a point where the AC voltage Vac is clean, i.e., relatively free of induced noise, and to the secondary side output of the transformer362at the noisy secondary side ground via the (+) input to the operational amplifier364. The capacitance multiplier can include capacitor C1and operational amplifier364with resistor R and capacitor C2, where the capacitor C1, 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 C3can 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 choke361and the Y capacitor Cy.

In operation, the power adapter310can receive AC voltage Vac from a power source, e.g., a wall outlet, transmit the AC voltage Vac through the choke361, the transformer362, the switching node363, the Y capacitor Cy, the rectifier diode D1, and the smoothing capacitor C10to 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 adapter310can 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's sense signals. In some embodiments, the noise suppression circuit can include the capacitor C1having a capacitance of about 220 pF, the operational amplifier364, 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 C2, a capacitance of about 47 pF, and the feedback capacitor C3, 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 adapter310by 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 adapter310ofFIG. 3has 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. 4illustrates another exemplary noise suppression circuit that can be used in the system ofFIG. 2. In the example ofFIG. 4, the noise suppression circuit for power adapter410can 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 device240for driving the device to sense a proximate object and to generate sense signals indicative thereof. The power adapter410can include choke461to suppress EMI emissions, as described previously inFIG. 3. To suppress EMI emissions, the choke461can have higher impedance to block higher frequency signals (which the EMI signals are) while passing lower frequency signals. However, at the same time, the choke461can induce substantial noise in the AC voltage Vac that can interfere with the connected touch sensitive device's sense signals. On the other hand, if the choke461were operated at low or zero impedance, the noise induced by the choke can be reduced significantly. However, at such impedance, the choke's EMI suppression can degrade.

The noise suppression circuit ofFIG. 4can balance EMI suppression with noise suppression at the choke so as to reduce noise at the connected touch sensitive device's sense signals without substantially degrading EMI suppression. To do so, the noise suppression circuit can include RLC circuit471having resistor R, inductor L, and capacitor C in parallel with the choke461. The RLC circuit471can provide, at the selected frequencies, in effect a low choke impedance in parallel with the choke461so 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 circuit471can have little effect such that the choke461can 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 adapter410ofFIG. 4can be the same as the power circuit of the adapter310ofFIG. 3, having (in addition to the illustrated components) the switching node363, the Y capacitor Cy, the rectifier diode D1, the smoothing capacitor C10, and the DC voltage line303.

In operation, the power adapter410can operate in a similar manner as the power adapter310ofFIG. 3to convert the AC voltage Vac to the DC voltage Vdc. While transmitting the AC voltage Vac, the power adapter410can 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's sense signals. In some embodiments, the RLC circuit471can 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 circuit471can 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 adapter410by 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's induced noise, and a second RLC circuit can be in parallel with the Y capacitor to help suppress the Y capacitor's induced noise.

FIG. 5illustrates another exemplary noise suppression circuit that can be used in the system ofFIG. 2. In the example ofFIG. 5, the noise suppression circuit of power adapter510can 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 inFIG. 3(illustrated by point (M)), the Y capacitor can be coupled to the AC neutral line502upstream of choke561at a point where the AC voltage Vac is relatively free of induced noise (illustrated by point (M′)). This can effectively bypass the choke561, thereby reducing the choke's induced noise contribution to the AC voltage Vac.

The power circuit of the adapter510ofFIG. 5can be the same as the power circuit of the adapter310ofFIG. 3, having (in addition to the illustrated components) the switching node363, the rectifier diode D1, the smoothing capacitor C10, and the DC voltage line303.

In operation, the power adapter510can operate in a similar manner as the power adapter310ofFIG. 3to convert the AC voltage Vac to the DC voltage Vdc. While transmitting the AC voltage Vac, the power adapter510can also suppress induced noise in the AC voltage Vac using the modified Y capacitor Cy connection to bypass the choke561as 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'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 adapter510by 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.