FR2528253A1 - Overvoltage protection circuit for electronic components - has zener diode and PTC resistor connected in series and in good thermal contact - Google Patents

Abstract

The protection circuit has two components - a zener diode and an element having a positive temperature coefficient of resistance. The zener diode has voltage - current characteristics such that beyond a certain point, the voltage across its terminals remains stable. The two components are connected in series across the variable supply voltage, and are in intimate thermal contact. The load is connected across the zener diode terminals. The zener diode may alternatively be a varistor (VDR). This or the diode is bonded directly on to the PTC resistor or thermistor. Silicone grease may ensure a good thermal contact between the two components.

Description

 The invention relates to the protection of electronic circuits against parasitic overvoltages.

 In general, electronic systems comprising transistors of the MOS or CMOS type or microprocessors are very sensitive to overvoltages appearing on their supply circuits.

They actually cause breakdowns at the junctions, thereby causing deterioration of equipment in operation.

 These pulses of sometimes significant duration are generated during power supply circuit breakers, often encountered in motor vehicles for example.

 It also happens that a battery connection "buffered" on a charger breaks, causing a significant increase in voltage for a period of time that can be long.

 To avoid equipment failure, a circuit comprising a resistor and a zener diode is therefore commonly used, the electronic circuits then being connected to the terminals of the diode. Therefore, whatever the disturbances present upstream, the supply voltage can never exceed that fixed by the zener diode.

 But, as is shown later, in the case where the consumption of the circuits is important, the resistance placed in series must be low to avoid a voltage drop too great in normal operation, which, in the case of a overvoltage will cause an overheating of the zener diode. This is usually remedied by replacing the series resistance (CTP) with a positive temperature coefficient resistor, which by heating up during the overvoltage, causes an increase in its own resistance and consequently a decrease of the power in the zener diode, thus reducing its size and therefore its price.

Unfortunately, it is found that for an overvoltage between the zener voltage and twice that, the dissipation of power in the resistor is lower than in the zener diode, and the resulting heating also too low to ensure an increase of significant resistance.

 The system according to the invention consists in closely coupling zoning diode and CTP so that the heating of the zener causes a premature switchover of the PTC and the desired increase of the resistance.

The invention will be better understood in view of the accompanying drawings in which
- Figure 1 shows the circuit diagram protection normally used.

 FIG. 2 illustrates by a graph the power variation at the terminals of the zener diode and of the resistance as a function of the supply voltage Vb.

 FIG. 3 gives some examples of embodiment of the device according to the invention.

 The diagram of the protection device (FIG.1) comprises a PTC thermistor 1 and a zener diode 2. The disturbed voltage applied to the input is called Vb and the output voltage Va is equal to Vb minus the voltage drop in the resistance 1; it should be noted that this voltage Va can never exceed the voltage Vz (zener voltage) and therefore, at worst, the circuit supplied with Va must be able to withstand this voltage.

 We will take, for example, the case of an electronic circuit mounted in a motor vehicle, battery-powered and consuming 1 A at 13.5V. The circuit can withstand, without damage, a voltage of 18Vs the zener diode is chosen equal to this voltage. The resistance is equal to O, SRet therefore dissipates 0.5w.

When Vb> Vz the power dissipated in resistor 1 and zener 2 is given by the following formulas
Wr = (Vb - Va) 2 in the resis
tance
R tance
and Wz = Va (Vb - Va) 2 in the zen
R assuming that the permanent power dissipated in the resistor is negligible and that its temperature coefficient is zero.

 It is shown in fig. 2 Wr and Wz according to Vb.

We see that Wr = Wz for Vb = 2 Vz and that the maximum difference between Wr = Wz is 160 w for a 28V overvoltage with powers dissipated in the resistance and the zener diode of 200 w and 360 w. Which is considerable.

We see that for 2Vz> Vb? Vz, and now replacing the pure resistance by a PTC thermistor, the power in the zener is likely to be largely in surplus while it is still relatively low in the resistance which does not cause it to switch to a much higher value and therefore an effective limitation of the power in the zener diode.

 According to the invention, it will mechanically couple the zener diode and CTP so as to obtain a premature switchover of the resistor, and thereby an automatic limitation of the power.

 It will be noted that the zener diode can be replaced by a VDR varistor, without changing the invention.

 Some examples of embodiments are given in FIG. 3.

 FIG. 3a shows a zener diode "chip" 1 packed on the PTC 2.

 Figure 3b shows a PTC 1, soldered to a VDR 2. The number of outputs being, of course, reduced to 3.

 Any combination of elements of different shapes is feasible, by welding or pressing, the essential is to obtain between the two components good thermal conduction. Figure 3c shows a PTC 1 and a VDR 2 joined by a clip 3 whose inner part is insulating. (4)

Claims (6)

REVENI) I CATIONS  1 - Device for protecting electronic circuits against spurious overvoltages, of the type comprising on the one hand, an element having a characteristic of current voltage such that beyond a certain level, the voltage at its terminals remains stable, and on the other hand a thermistor with a positive temperature coefficient, the two elements being connected in series with respect to the disturbed voltage and the operating voltage being collected across the threshold component,  Characterized by a thermal coupling between the two elements,  2 - Device according to claim 1>  Characterized by the fact that the threshold element is a zener diode in the form of a "chip".  3 - Device according to claim 1,  Characterized by the fact that the threshold element is a VDR varistor.  4 - Device according to claim 2,  Characterized by the fact that the "chip" is directly "crowded" on the PTC thermistor.  5 - Device according to claim 2 or 3>  Characterized by the fact that the two elements are welded to each other.  6 - Device according to claim 2 or 3,  Characterized by the fact that the two elements are held against each other by any clamping device, silicone grease possibly ensuring between them a good thermal contact.