Electromagnetic radiation control for isolated power transfer product
A power transfer device includes an oscillator circuit of a DC/AC power converter responsive to an input DC signal and an oscillator enable signal to generate an AC signal. The oscillator circuit includes a first node, a second node, and a circuit coupled between the first node and the second node. The circuit includes a cross-coupled pair of devices. The oscillator circuit further includes a variable capacitor coupled between the first node and the second node. A capacitance of the variable capacitor is based on a digital control signal. A first frequency of a pseudo-differential signal on the first node and the second node is based on the capacitance. The power transfer device further includes a control circuit configured to periodically update the digital control signal. A second frequency of periodic updates to the digital control signal is different from the first frequency.
1. A power transfer device comprising:
an oscillator circuit of a DC/AC power converter responsive to an input DC signal and an oscillator enable signal to generate an AC signal comprising:
a first node;
a second node;
a circuit coupled between the first node and the second node, the circuit including a cross-coupled pair of devices; and
a variable capacitor coupled between the first node and the second node, a capacitance of the variable capacitor being based on a digital control signal, and a first frequency of a pseudo-differential signal on the first node and the second node being based on the capacitance; and
a control circuit configured to periodically update the digital control signal,
wherein a second frequency of periodic updates to the digital control signal is different from the first frequency.
2. The power transfer device, as recited in claim 1 ,
wherein the control circuit is configured to generate the oscillator enable signal based on a feedback signal received across an isolation barrier, the oscillator enable signal regulating an output DC signal of the power transfer device, and
wherein the control circuit is configured to generate the digital control signal based on the feedback signal.
3. The power transfer device, as recited in claim 1 , wherein the control circuit comprises a second oscillator circuit configured to generate a clock signal oscillating asynchronously to the pseudo-differential signal and the digital control signal is generated based on the clock signal.
4. The power transfer device, as recited in claim 3 , wherein the control circuit further comprises a triangular modulator responsive to the clock signal, the digital control signal is generated based on an output of the triangular modulator, and the capacitance has a triangular variation with respect to time.
5. The power transfer device, as recited in claim 1 , wherein the control circuit comprises a linear-feedback shift register, the digital control signal is generated based on an output of the linear-feedback shift register, and the first frequency varies pseudo-randomly.
6. The power transfer device, as recited in claim 5 , further comprising:
a select circuit responsive to a select signal to provide a feedback signal received across an isolation barrier or a locally-generated oscillating signal asynchronous to the AC signal to control the linear-feedback shift register.
7. The power transfer device, as recited in claim 1 , wherein the second frequency is at least 50% less than the first frequency.
8. The power transfer device, as recited in claim 1 , wherein the oscillator circuit further comprises a first conductive coil coupled to the first node and the second node to form a primary side circuit of the power transfer device.
9. The power transfer device, as recited in claim 1 , wherein the oscillator circuit is configured to operate as a Class-D power amplifier.
10. The power transfer device, as recited in claim 1 , further comprising:
a cascode circuit comprising transistors having a first conductivity type and a first breakdown voltage, the cascode circuit being coupled to the first node, and the second node,
wherein the pseudo-differential signal has a peak voltage of at least three times an input voltage level of the input DC signal, wherein each of the cross-coupled pair of devices has a breakdown voltage that is just over the input voltage level.
11. A method for controlling electromagnetic radiation of a power transfer device, the method comprising:
converting an input DC signal into a first AC signal, wherein the converting comprises:
biasing an oscillator circuit with the input DC signal;
selectively enabling the oscillator circuit according to an oscillator enable signal to generate the first AC signal having a first frequency; and
periodically updating a capacitance of the oscillator circuit using a digital control signal, the first frequency being different from a second frequency of periodic updates to the digital control signal.
12. The method, as recited in claim 11 , further comprising:
generating the digital control signal based on a feedback signal received across an isolation barrier.
13. The method, as recited in claim 11 , further comprising:
generating a pseudorandom digital signal; and
generating the digital control signal based on the pseudorandom digital signal.
14. The method, as recited in claim 13 , further comprising:
selecting between a feedback signal received across an isolation barrier and a locally-generated oscillating signal asynchronous to the first AC signal to control update of the pseudorandom digital signal, the selecting being based on a frequency of the feedback signal.
15. The method, as recited in claim 11 , further comprising:
generating an oscillating signal that is asynchronous to the first AC signal; and
generating the digital control signal based on a triangularly modulated version of the oscillating signal.
16. The method, as recited in claim 11 , further comprising:
generating the oscillator enable signal based on a feedback signal received across an isolation barrier, the oscillator enable signal controlling an output DC signal of the power transfer device.
17. The method, as recited in claim 11 , wherein the converting further comprises:
converting the first AC signal into a second AC signal, the first AC signal being electrically isolated from the second AC signal;
generating an output DC signal by rectifying the second AC signal, the output DC signal being electrically isolated from the input DC signal;
generating a feedback signal based on the output DC signal; and
communicating the feedback signal across an isolation barrier using a communications channel.
18. The method, as recited in claim 11 , wherein the second frequency is at least 50% less than the first frequency.
19. A method for controlling electromagnetic radiation of a power transfer device, the method comprising:
varying a first frequency of power transfer across an isolation barrier based on an N-bit digital control signal, where N is an integer greater than one; and
updating a value of the N-bit digital control signal at a second frequency, the value of the N-bit digital control signal being updated using a pseudo-random value, the second frequency being different from the first frequency.
20. The method, as recited in claim 19 , wherein the power transfer is controlled based on a feedback signal received across the isolation barrier.