Power supply protection circuit for an in-vehicle electronic device

Abstract

Provided is a power supply protection circuit for an in-vehicle electronic device, for protecting a power supply of a circuit to be protected which is installed in the in-vehicle electronic device, including a battery power supply for the in-vehicle electronic device, a diode whose anode terminal is connected therewith, a transistor whose collector terminal is connected with a cathode terminal of the diode and whose emitter terminal is connected with a power supply voltage supplying terminal of the circuit to be protected, and a Zener diode provided between the battery power supply and a ground terminal to be connected in series with a resistor having an anode terminal connected with the ground terminal and a cathode terminal connected with the resistor and a base terminal of the transistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram showing a structure of a power supply protection circuit for an in-vehicle electronic device according to an embodiment of the present invention; and

FIG. 2 is a graph showing a characteristic between a power supply voltage Vbat of a battery power supply 1 and a power supply voltage Vdd of a circuit to be protected 3 as shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a circuit diagram showing a structure of a power supply protection circuit for an in-vehicle electronic device according to the embodiment of the present invention. A circuit to be protected for which power supply voltage protection is necessary is a CMOS process IC for a thermal flowmeter.

The power supply protection circuit for an in-vehicle electronic device as shown in FIG. 1 includes a diode 2 whose anode terminal is connected with a battery power supply 1 for the in-vehicle electronic device, a transistor 4 whose collector terminal is connected with a cathode terminal of the diode 2 and whose emitter terminal is connected with a power supply voltage supplying terminal of a circuit to be protected 3, and a Zener diode 6 which is provided between the battery power supply 1 and a ground terminal so as to be connected in series with a resistor 5 which has an anode terminal connected with the ground terminal and a cathode terminal connected with the resistor 5 and a base terminal of the transistor 4.

In the power supply protection circuit for the in-vehicle electronic device as shown in FIG. 1, assume that a power supply voltage of the battery power supply 1 is expressed by Vbat, a forward drop voltage of the diode 2 is expressed by Vf1, a collector-emitter saturation voltage of the transistor 4 and a base-emitter voltage are expressed by Vce(sat) and Vbe, respectively, a power supply voltage of the circuit to be protected 3 is expressed by Vdd, a resistance value of the resistor 5 and a consumption current thereof are expressed by R1 and I1, respectively, and a Zener voltage of a Zener diode 6 is expressed by Vz1. Then, the power supply voltage Vdd of the circuit to be protected 3 is as follows.

1) When Vbat ≦ 0 V,
Vdd = 0 V.
2) When 0 < Vbat ≦ Vz1:
I) in case where R1 × I1 + Vbe > Vf1 + Vce(sat),
Vdd = Vbat − R1 × I1 − Vbe; and
II) in case where R1 × I1 + Vbe < Vf1 + Vce(sat),
Vdd = Vbat − Vf1 − Vce(sat).
3) When Vbat > Vz1,
Vdd = Vz1 − Vbe.

At this time, a relationship between the power supply voltage Vbat of the battery power supply 1 and the power supply voltage Vdd of the circuit to be protected 3 is as shown in FIG. 2.

Therefore, the power supply voltage Vdd of the circuit to be protected 3 is limited to a voltage range of from 0 V to (Vz1−Vbe). At this time, when the Zener voltage Vz1 of the Zener diode 6 is selected so as to satisfy a withstanding voltage of the CMOS process IC which is the circuit to be protected 3, a power supply protection circuit for the IC can be constructed.

At this time, the consumption current flowing through the resistor 5 is sufficiently smaller than a circuit current flowing into the circuit to be protected 3 through the diode 2 and the transistor 4. Therefore, it is unnecessary to set the resistor 5 and the Zener diode 6 to high-voltage withstanding specifications. On the other hand, the transistor 4 requires a withstanding voltage. However, a size thereof is smaller than that of a conventional power Zener diode, so that an inexpensive transistor can be used.

As described above, the power supply protection circuit for the in-vehicle electronic device as shown in FIG. 1 is composed of small power parts without using an expensive power Zener diode. In addition, as shown in FIG. 2, the power supply voltage Vdd of the circuit to be protected 3 is limited to a range of from a minimum voltage of 0 V to a maximum voltage of (Vz1−Vbe) to limit both an overvoltage and a negative voltage as in a conventional case. Thus, it is possible to improve a protective function and realize a reduction in size of a circuit part, with the result that an inexpensive power supply protection circuit can be provided.

In particular, when the comparison with specifications using the power Zener diode is made, a manufacturing cost can be reduced by approximately ½ times. In addition, it is possible to narrow an area necessary for mounting by approximately ½ times and to lower a mounting height by approximately ½ times, thereby realizing a reduction in size.

The voltage drop of the power supply protection circuit is small. Therefore, even when the power supply voltage Vbat of the battery power supply 1 becomes a lower voltage, it is advantageous to ensure the circuit power supply voltage. That is, when the power supply voltage Vbat of the battery power supply 1 becomes the lower voltage, the power supply voltage Vdd of the circuit to be protected 3 depends on one of

(Vf1+Vce(sat))

and

(R1×I1+Vbe)

with a higher voltage and does not become a value obtained by adding both voltage values. Thus, even when the power supply voltage Vbat of the battery power supply 1 becomes the lower voltage, it is advantageous to ensure the circuit power supply voltage.

Claims

1. A power supply protection circuit for an in-vehicle electronic device, for protecting a power supply of a circuit to be protected which is installed in the in-vehicle electronic device, comprising: a diode whose anode terminal is connected with the power supply of the in-vehicle electronic device;a transistor whose collector terminal is connected with a cathode terminal of the diode and whose emitter terminal is connected with a power supply voltage supplying terminal of the circuit to be protected;a resistor whose one end is connected with the power supply of the in-vehicle electronic device; anda Zener diode which is provided between other end of the resistor and a ground terminal to be connected in series with the resistor and which includes an anode terminal connected with the ground terminal and a cathode terminal connected with the resistor and a base terminal of the transistor.

2. A power supply protection circuit for an in-vehicle electronic device according to claim 1, wherein: the circuit to be protected comprises a CMOS process IC; andthe Zener diode includes a Zener voltage set to a value satisfying a withstanding voltage of the IC.