Class D modulator with peak current limit and load impedance...

Details Class D modulator with peak current limit and load impedance... Class D modulator with peak current limit and load impedance...

1999-11-12

2001-05-08

Pascal, Robert (Department: 2817)

Amplifiers

Modulator-demodulator-type amplifier

C330S20700P

Reexamination Certificate

active

06229389

ABSTRACT:

This invention implements a new technique for active current limit in a class D self-oscillating variable frequency amplifier. This invention can also sense the load impedance and take appropriate action. The amplifier enters current limit when the speaker impedance is abnormally low and shuts down when the output is shorted.
BACKGROUND OF THE INVENTION
Most transistor amplifiers are either class A, AB or B. the output transistors are usually bipolar transistors that are operated in their linear region. Current limit is implemented using sense resistors in series with the output transistors to develop a voltage proportional to the output current. When this voltage is too high, the base current of the output transistors is limited, thus limiting the output current to a safe level. Linear amplifiers using MOSFETs employ similar circuits to limit the gate voltage on the output MOSFETs in order to limit the output current. These approaches do not work in class D amplifiers because the MOSFETs are either off or fully enhanced. The traditional solution is too offer over current latch off as opposed to current limit. In that solution sense resistors are placed in series with the MOSFETs. When the voltage across the sense resistors is too high, the amplifier shuts down. The drawback to that approach is that the impedance of speakers varies with frequency, and it may dip low enough at some frequencies to shut down the amplifier.
It is possible to regulate the output current with a separate control loop, which is independent of the audio loop. The goal of the second loop is to override the main control loop when the current is excessive. The disadvantage of this approach is that it is expensive and relatively difficult to stabilize due to the delays associated with filtering the voltage across the sense resistor to remove the noise associated with switching transitions.
SUMMARY
A better solution would limit current when the speaker impedance dips to an unsafe level or many speakers are connected in parallel by the end user, and to shut down the amplifier when the output is shorted. Some filtering is also desired before shutdown, so that if the output is only shorted for a brief instant, the amplifier does not shut down. With this approach nuisance shut downs are minimized, and the amplifier is fully protected.
The invention described here implements this protection scheme. Current limit is also gradually phased in so that the current clipping is soft. This improves the quality of the music by generating less higher order harmonics, while still providing full output protection.
This invention provides a simple and easy way to implement current limit in a variable frequency self oscillating class D modulator such as the modulator described in my co-pending patent application, Ser. No. 09/183,453, filed Oct. 30, 1998, which is incorporated by reference.
A method to sense the peak current in the MOSFETs and then generate a logic signal to indicate excessive current is described in my co-pending patent application Ser. No. 09/183,879, filed Oct. 30, 1998 which is incorporated by reference. This sensing circuit generates a logic signal as soon as the peak MOSFET current exceeds a preset threshold, so the delays normally associated with analog current limit circuits are eliminated and the new circuit is easily stabilized. The present invention works best with the sensing circuit described in that application but it will work with any circuit that can generate a logic signal in response to excessive current in the output MOSFETs such as the circuit
110
in FIG.
1
and the circuit in FIG.
5
.
Since current limit into a dead short is undesirable, this invention incorporates a load impedance sensor. When the impedance of the load falls below the safe level, the amplifier is shut down immediately.


REFERENCES:
patent: 5805020 (1998-09-01), Danz et al.
patent: 6016075 (1998-09-01), Hamo
patent: 6107875 (2000-08-01), Pullen et al.
patent: 6108182 (2000-08-01), Pullen

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