Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2002-02-28
2003-08-12
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S309000
Reexamination Certificate
active
06605975
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a level shift circuit in a power element drive integrated circuit.
2. Description of the Prior Art
As power element drive integrated circuits for home electric appliances and industries, in place of conventional photocouplers, high-voltage integrated circuits (HVIC) have been popularly used. Depending on applications, a voltage of several 100 V which is close to a power supply voltage of a power element is applied to the HVIC itself. For this reason, a high-withstand-voltage circuit is indispensable. Therefore, the circuit is manufactured by high-voltage processes for semiconductors.
In many cases, this HVIC is directly driven by a signal from a microcomputer. For this reason, in order to transmit a low-potential signal (about 5 V) of a microcomputer, a level shift circuit for converting the signal into a signal having a high potential (several 10 V) is constituted. As the level shift circuit, a high-withstand-voltage signal transmission element is used, and has a withstand voltage higher than the power supply voltage of the power element.
As this high-withstand-voltage element, a MOS transistor is mainly used. However, in an operation state in which a voltage of several 100 V is applied, when an overcurrent flows in the high-withstand-voltage MOS transistor, the element may be overheated to be broken.
This point will be described below by using a conventional HVIC level shift circuit shown in FIG.
6
. This circuit is constituted by an HVIC power supply VCC (
1
), a power element power supply HV (
2
), an offset power supply VS (
3
) using an output voltage as a power supply, a floating power supply VB (
4
) having a negative electrode connected to the offset power supply VS (
3
), a high-withstand-voltage MOS transistor (
5
), a drive inverter (
7
) for the high-withstand-voltage MOS transistor (
5
), a resistor (
9
) for pulling up the drain of the high-withstand-voltage MOS transistor (
5
) to the voltage of the floating power supply VB (
4
), and a diode (
10
) inserted between the drain of the high-withstand-voltage MOS transistor (
5
) and the VS (
3
).
When a P-ch MOS transistor of the drive inverter (
7
) is turned on by an input signal to pull up the gate voltage of the high-withstand-voltage MOS transistor (
5
), the high-withstand-voltage MOS transistor (
5
) is turned on. A drain current at this time is supplied from the floating power supply VB (
4
) through the resistor (
9
). When this drain current is sufficient, a drain potential Vds of the high-withstand-voltage MOS transistor (
5
) exceeds the threshold value of the inverter (power element) connected to the output of the high-withstand-voltage MOS transistor (
5
), and a drive signal is sent to the gate of the power element.
When the offset power supply VS (
3
) is boosted to several 100 V, the diode (
10
) inserted between the offset power supply VS (
3
) and the drain of the high-withstand-voltage MOS transistor (
5
), and the drain potential Vds is clamped to VS−VF (forward voltage of the diode (
10
)). When the current sink capability of the high-withstand-voltage MOS transistor (
5
) is sufficient, a drain current is supplied from the offset power supply VS (
3
) through the diode (
10
).
The current sink capability of the high-withstand-voltage MOS transistor (
5
) increases depending on a voltage Vgs applied across the gate and the source. For this reason, in a state in which a voltage of several 100 V is applied across the drain and the source, when a voltage (15 to 20 V) which is close to the power supply Vcc is applied across the gate and the source, an overcurrent flows in the high-withstand-voltage MOS transistor (
5
), and element breakdown may occur.
Since the capacitance (gate capacitance) between the gate and the source of the high-withstand-voltage MOS transistor (
5
) is considerably large, when a very short input signal having a time which is taken to cause the voltage of the high-withstand-voltage MOS transistor (
5
) to reach the gate voltage at which the high-withstand-voltage MOS transistor (
5
) operates is input, the high-withstand-voltage MOS transistor (
5
) is not turned on. As a result, a response speed is low.
SUMMARY OF THE INVENTION
The present invention provides a level shift circuit which can prevent element breakdown of a high-withstand MOS transistor and which increases a response speed of the high-withstand-voltage MOS transistor.
According to the present invention, a level shift circuit for converting a level signal of several volts into a signal of a high level and transmitting the high-level signal is characterized by comprising
voltage reduction means for reducing an overvoltage applied across the gate and the source of a high-withstand-voltage element used in a signal level conversion unit.
REFERENCES:
patent: 5057721 (1991-10-01), Miyazaki et al.
patent: 5896043 (1999-04-01), Kumagai
patent: 5905621 (1999-05-01), Drapkin
Mitsubishi Denki & Kabushiki Kaisha
Tran Toan
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