Short-circuit protection circuit, particularly for power...

Details Short-circuit protection circuit, particularly for power... Short-circuit protection circuit, particularly for power...

1999-05-07

2001-01-16

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific current responsive fault sensor

C361S101000, C361S086000

Reexamination Certificate

active

06175478

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a short-circuit protection circuit, particularly for power transistors.
BACKGROUND OF THE INVENTION
It is known that many integrated circuits use protections against short-circuits to control the maximum current that flows through the power transistors. In some applications it is important that the power transistor be able to supply a small current also when high voltages are applied to its terminals.
In single power-supply audio applications, during power-on transients the power transistor must charge the decoupling capacitor, which is connected between the loudspeaker and the output of the power stage, while the entire power supply voltage is applied to the terminals of the power transistor.
In such a situation, it is important to ensure that the power transistor is capable of supplying the current required to charge said capacitor, so that the amplifier can power-on correctly.
FIG. 1
illustrates a conventional protection circuit, in which the reference numeral
1
designates the power transistor and the reference numeral
2
designates a so-called sense resistor, which is suitable to sense the output current Io and is arranged in series to the power transistor
1
, a current mirror Q
1
, Q
2
with corresponding current sources Io and Iref which are series-connected, and finally a circuit branch which is connected between the ground and the bases of the transistors Q
1
and Q
2
and is constituted by a resistor
3
which is series-connected to a Zener diode
4
.
At a point that is intermediate between the transistor Q
2
and the current source Iref, a pin is provided for acquiring the signal for indicating intervention or lack thereof on the power transistor
1
in order to limit the maximum voltage across it.
The resistor
3
senses the voltage drop on the power transistor
1
. When he voltage across the transistor exceeds the value
Vds_Max
=
R
⁢
 
⁢
I
ref
⁢
A2
A1
+
V
Z
the protection circuit disconnects the power transistor
1
and no current can be supplied anymore: this limits the maximum supply voltage.
In the above relation, the terms A
2
and A
1
are, respectively, the areas of the transistors Q
2
and Q
1
, while Vds_Max is the maximum voltage between the drain and the source of the power transistor
1
and V
z
is the voltage across the Zener diode
4
.
SUMMARY OF THE INVENTION
The aim of embodiments of the present invention is therefore to provide a short-circuit protection circuit, particularly for power transistors, in which means are provided for determining the maximum and minimum values of the output current of the power transistor as a function of the voltage applied to its drain and source terminals.
Within the scope of this aim, an advantage of the present invention is that it provides a short-circuit protection circuit, particularly for power transistors, which occupies a reduced area.
Another advantage of the present invention is that it provides a short-circuit protection circuit, particularly for power transistors, which is highly reliable, relatively easy to manufacture and at competitive costs.
This aim, these advantages and others which will become apparent hereinafter are achieved by a short-circuit protection circuit, particularly for power transistors, having first means for mirroring the output current of a power transistor which are parallel-connected to said power transistor, and second mirroring means which are series-connected to said first mirroring means and are suitable to emit a current which is correlated to the current mirrored by said first mirroring means, for comparison with a reference current. The result of said comparison determines the need to intervene or not on said power transistor. The short-circuit protection circuit also includes means for sensing the voltage drop across said power transistor which are parallel-connected to said power transistor and to said first mirroring means, in order to adjust minimum and maximum values of the current in output from said power transistor, as a function of the voltage that is present across said transistor.


REFERENCES:
patent: 4355341 (1982-10-01), Kaplan
patent: 5008586 (1991-04-01), Miyazaki et al.
patent: 5272392 (1993-12-01), Wong et al.
patent: 5343141 (1994-08-01), Metro et al.
patent: 5369308 (1994-11-01), Schoofs et al.
patent: 5786970 (1998-07-01), Nao et al.
patent: 5903422 (1999-05-01), Hosokawa
patent: 0 772 273 (1997-05-01), None
patent: 2 214 745 (1989-09-01), None

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