Patent ID: 6259297
Filing Date: 2001-07-10
Classification: H03F,H03K

Abstract:
A protection circuit for a bipolar power transistor for preventing the operating point of the bipolar power transistor from leaving a useful operating area, the output of the bipolar power transistor being drawn from the emitter thereof, said protection circuit comprising:a current source which drives the base terminal of the bipolar power transistor;a sense resistor connected between an input supply voltage and the collector of said bipolar power transistor;a first circuit branch connected between the collector of said bipolar power transistor and a common output node, comprising a first diode and a first current source, said first diode being connected to the collector terminal of the bipolar power transistor, said common output node coinciding with the output of said bipolar power transistor;a second circuit branch connected between said input supply voltage and said common output node, including a second diode and a second current source;a third circuit branch, connected between said input supply voltage and said common output node, including a pair of emitter-coupled transistors having emitters which are connected to said input supply voltage via a resistor and bases which are connected to the first and second diodes, and a current mirror connected to the collectors of the pair of emitter-coupled transistors;a fourth circuit branch comprising a plurality of series-connected diodes connected in series to a first resistor and in parallel to a second resistor, a current mirror circuit connected to the first and second resistors, and a third current source connected to the current mirror;a Zener diode, which is connected between said first and fourth circuit branches; anda transistor connected between the base terminal of said bipolar power transistor and said common output node and having a base terminal which is connected to the current mirror of said third circuit branch.