Method and apparatus for power converter for class D audio power amplifiers

• US 7,839,215 B2
• Filed: 06/12/2009
• Issued: 11/23/2010
• Est. Priority Date: 06/16/2008
• Status: Active Grant

• Current Assignee: RGB Systems Incorporated
• Original Assignee: RGB Systems Incorporated
• Inventors: Mendenhall, Eric
• Primary Examiner(s): Pascal; Robert
• Assistant Examiner(s): Nguyen; Hieu P
• Application Number:
  US12/483,736
• Publication Number:
  US 20090309658A1
• Time in Patent Office: 529 Days
• Field of Search: 330/10, 330/251, 330/207.A, 330296-297, 307/130, 363/41, 363 97- 98, 363131-134
• US Class Current: 330/251
• CPC Class Codes: H02M 3/33592 having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
• CPC Class Codes: H02M 5/4585 having a rectifier with controlled elements
• CPC Class Codes: H03F 2200/03 the amplifier being designed for audio applications
• CPC Class Codes: H03F 2200/504 the supply voltage or current being continuously controlled by a controlling signal, e.g. the controlling signal of a transistor implemented as variable resistor in a supply path for, an IC-block showed amplifier
• CPC Class Codes: H03F 2203/21193 the supply voltage of a power amplifier being continuously controlled, e.g. by an active potentiometer
• CPC Class Codes: H03F 3/217 Class D power amplifiers; Switching amplifiers
• CPC Class Codes: H03F 3/2171 with field-effect devices H03F3/2173 - H03F3/2178 take precedence
• CPC Class Codes: H03F 3/2173 of the bridge type
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

A method and apparatus for power conversion in a class D amplifier is provided. The power conversion is achieved using synchronous rectifiers in a regulated half bridge power supply, taking the sum of the positive and negative rails as feedback, in order facilitate energy transfer between positive and negative output rails. This minimizes the effects of off side charging and rail sag, as well as achieving good line regulation, while allowing use of very small, low value output capacitors.

Claims

1. A method comprising:

applying positive and negative supply voltages to a class D power amplifier;

comparing a difference between the positive and negative supply voltages;

producing a pulse width modulated control signal based on the difference;

controlling synchronous rectifiers based on the pulse width modulated control signal.

2. The method of claim 1 further comprising:

transferring charge through the synchronous rectifiers to compensate for off side charging.

3. The method of claim 1 further comprising:

allowing current flow through a MOSFET of a synchronous rectifier of the synchronous rectifiers, wherein the MOSFET is coupled in parallel with a diode, wherein the current flow occurs in a direction blocked by the diode.

4. The method of claim 1 wherein the controlling the synchronous rectifiers comprises:

causing a first bidirectionally conductive device to conduct in response to first pulses of a first polarity of the pulse width modulated control signal; and

causing a second bidirectionally conductive device to conduct in response to second pulses of a second polarity of the pulse width modulated control signal.

5. The method of claim 1 wherein the applying the positive and negative supply voltages to the class D power amplifier comprises:

coupling the positive and negative supply voltages to the class D power amplifier via a coupled inductor wherein a first winding for the positive supply voltage is inductively coupled to a second winding for the negative supply voltage.

6. The method of claim 5 wherein the applying the positive and negative supply voltages to the class D power amplifier further comprises:

filtering the positive and negative supply voltages, respectively, with a first and second capacitor following the coupled inductor, the first capacitor being connected between the positive supply voltage and ground and the second capacitor being connected between the negative supply voltage and ground.

7. The method of claim 1 wherein controlling the synchronous rectifiers comprises:

coupling the pulse width modulated control signal to the synchronous rectifiers via a transformer.

8. Apparatus comprising:

a power supply circuit comprising synchronous rectifiers,

a class D amplifier coupled to the power supply circuit; and

a power supply control circuit coupled to the power supply circuit, the power supply control circuit providing a pulse width modulated control signal to control the power supply circuit based on a difference between positive and negative supply voltages provided by the power supply circuit.

9. The apparatus of claim 8 wherein the pulse width modulated control signal controls the synchronous rectifiers of the power supply circuit.

10. The apparatus of claim 9 wherein transfer of charge through the synchronous rectifiers compensates for off side charging of capacitors on the positive and negative supply voltages resulting from operation of the class D power amplifier.

11. The apparatus of claim 10 wherein the synchronous rectifiers comprise:

bidirectionally conductive devices in parallel with diodes.

12. The apparatus of claim 11 wherein the bidirectionally conductive devices comprise:

metal oxide semiconductor field effect transistors (MOSFETs).

13. The apparatus of claim 12 further comprising:

a transformer coupling the pulse width modulated control signal to the gates of the MOSFETs.

14. The apparatus of claim 8 further comprising:

a coupled inductor, wherein the coupled inductor couples the positive and negative supply voltages from the power supply circuit to the class D amplifier, the coupled inductor comprising a first winding in series with the positive supply voltage and a second winding in series with the negative supply voltage, the first winding inductively coupled to the second winding.

15. The apparatus of claim 14 further comprising:

a first capacitor coupled from the first winding to ground, and;

a second capacitor coupled from the second winding to the ground.

16. Apparatus comprising:

a regulator comprising a comparator and a voltage reference, the comparator coupled to a positive supply voltage and a negative supply voltage of a class D power amplifier for comparing a comparison voltage obtained from a difference between the positive supply voltage and the negative supply voltage to a reference voltage of the voltage reference; and

a pulse width modulator, the pulse width modulator coupled to the regulator for producing a pulse width modulated control signal to control the positive supply voltage and the negative supply voltage by controlling synchronous rectifiers of a power supply circuit, the power supply circuit providing the positive supply voltage and the negative supply voltage.

17. The apparatus of claim 16 further comprising:

a transformer to couple the pulse width modulated control signal to the synchronous rectifiers.

18. The apparatus of claim 17 further comprising:

an optocoupler to couple the regulator to the pulse width modulator.

19. The apparatus of claim 18 further comprising:

a first capacitor coupled from the positive supply voltage to ground; and

a second capacitor coupled from the negative supply voltage to the ground, wherein the second capacitor is subject to off side charging when the class D amplifier is operating from the positive supply voltage.

20. The apparatus of claim 18 further comprising:

a voltage divider for obtaining the comparison voltage from the difference between the positive supply voltage and the negative supply voltage.