Modulation scheme for filterless switching amplifiers

Details Modulation scheme for filterless switching amplifiers Modulation scheme for filterless switching amplifiers

1999-11-16

2001-04-03

Pascal, Robert (Department: 2817)

Amplifiers

Modulator-demodulator-type amplifier

C330S251000, C330S146000

Reexamination Certificate

active

06211728

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
Cross-reference is made to commonly assigned patent application entitled “Concept and Methods to Enable Filterless, Efficient Operation of Class-D Amplifiers,” filed herewith, the teaching of which are incorporated herein by reference.
Field of the Invention
The present invention is generally related to switching amplifier circuits, and more particularly to Class-D amplifiers.
BACKGROUND OF THE INVENTION
Switching amplifiers, also know as Class-D amplifiers as the name implies, have an output that is switched or pulse-width modulated (PWM) at a frequency much higher than the frequency of interest. For example, in audio applications, these amplifiers will switch at typical frequencies of 250 kHz, while the audio band is limited to 20 kHz. These amplifiers are analogous to switching regulators, and receive similar benefits and disadvantages of such devices when compared to their linear counterparts. The main advantage of these switching amplifiers is their efficiency and its derivatives, i.e., lower thermal dissipation, battery life, smaller power supplies, size, weight, etc. The main disadvantage is cost and complexity.
Present modulation schemes utilizing either half or full H-bridge output stage topologies which switch in a “binary” fashion. In such a switching method, there are two valid states for the bridge, neglecting dead time, which are shown at
10
in FIG.
1
and FIG.
2
. Although simpler to implement, the disadvantage of this solution is that there is always a current
12
provided to the load due to the voltage differential always provided across nodes OUTP and OUTN, shown at
14
, illustrated in FIG.
3
. For operation near zero crossing, or no audio signal, the majority of the current used is wasted, and is a drop in efficiency. As shown in
FIG. 3
, an output squarewave with a 50% duty cycle will spend 50% of the time period decaying the current in the inductor, and 50% of the period to re-establish the current in the opposite direction, this resulting in a time averaged current of zero.
Furthermore, this will require that the load be inductive. Consider a pure resistive load. Switching the H-bridge in a “binary” fashion would place the power supply voltage across the load (plus parasitics of the switch). Unlike the current waveform
12
shown in
FIG. 3
, the resulting current would be a squarewave with a magnitude equal to the power supply divided by the resistance of the load. For example, an H-bridge using a 5V supply driving a 40&OHgr; load would see a current of about 1 Amp, and this is with no signal. Although the electrical equivalent of a speaker is somewhere between purely resistive and purely inductive, this would still prevent filterless operation of Class-D amplifiers in audio applications as the main benefit of efficiency is lost. Today, the problem is solved by providing some current limiting device in series with the speaker, usually a post-filter comprised of inductors and thereby creating a current flow shown at
12
in FIG.
3
. The typical circuit topology is shown at
16
in FIG.
4
.
SUMMARY OF THE INVENTION
The present invention acheives technical advantages as a circuit and a method achieving filterless Class-D operation, increased efficiency, and reduced cost by delivering current to the load only when needed, and once delivered, maintain the current by not decaying or wasting energy in removing the current. This is accomplished by using a terinary modulation scheme. This scheme is implemented in an H-bridge configuration, where there are four states of operation as the scheme implies. The modulation scheme generates PWM signals, whereby the edges of the voltage signal at a node OUTP move away from each other, and the edges of the voltage signal at a node OUTN move towards each other. The voltage difference of these two signals, which appears across the load, is narrow pulses. These pulses also have the desired affect of doubling the single-ended PWM frequency.
The present invention allows for filterless operation of Class-D amplifiers, which becomes a huge cost, board, and implementation savings. Cost of a filter for Class-D amplifier at least matches silicon cost, thus making Class-D solutions at least 2× solution cost more than their linear counterparts. Eliminating the filter is currently the only way to be cost competitive with linear amplifiers. Also, the modulation scheme allows for higher efficiency and reduced board space, where fractions of the filter can be used.


REFERENCES:
patent: 4554215 (1985-11-01), Aiello
patent: 4673889 (1987-06-01), Cini et al.
patent: 6016075 (2000-01-01), Hamo
patent: 6028476 (2000-02-01), Schweighofer
patent: 6034565 (2000-03-01), Schweighofer

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