DE10013939A1 - Electric circuit with polarity inversion protection e.g. for vehicle electronics, has N-channel and small signal MOSFET transistors with parallel connected zener diodes and drains supplied by positive supply line - Google Patents

Abstract

The circuit has a polarity reversal protector in the positive supply line to the circuit with an N-channel MOSFET transistor (1) and a small signal MOSFET transistor (2), both with parallel connected zener diodes (5,6). The drain connections of both transistors are supplied by the positive supply line. The N-channel MOSFET 's gate is supplied via a power bridge circuit and is connected to the small signal MOSFET's source connection.

Description

The invention relates to an electrical circuit with reverse polarity protection.

In the field of automotive electronics, there is polarity reversal for all electrical consumers Provide protection that ensures that the electronic assemblies are accidentally reverse polarity connection of the car battery. Especially power electronics Circuits with half-bridge or full-bridge circuits are special with reverse polarity endangered, because if the operating voltage is reversed, they will short-circuit due to their internal Cause diodes. Without a protective device, these circuits or the Zulei lines destroyed because of the high currents that occur. A backup is as Protective device is not permitted in cars, since the Battery the vehicle must still be roadworthy and all of its electrical equipment should be ready for operation again immediately.

The simplest circuit for reverse polarity protection is a rectifier diode in series. This can be in the positive as well as in the ground supply. But has a diode two major disadvantages. It only allows current to flow in one direction, leading to operation of an actuator or other reactive loads can be very problematic (missing Recovery capability). In addition, the diode has a functional voltage drop, which leads to losses in the diode at high currents. Usage is also common a relay that only picks up when the supply is positive and closes the circuit. In Applications with high currents, high ambient temperatures or high vibrations often this solution cannot be used.

A variant that replaces both of the above solutions uses a MOS-FET as a switch. An N-channel MOSFET is mostly used here, since N-channel types are used for same area, a lower on-resistance and correspondingly lower on switching losses have P-channel types. The N-channel MOSFET becomes line operated, since the gate voltage is easily available here.  

This structure, however, protects the ground potential from polarity reversal Circuit raised by the voltage drop across the MOSFET. This reduces the sturgeon strength of the inputs, since these usually refer to the ground potential as reference potential tial. In addition, compliance with the standards for electromagnetic ver Inertia can be made considerably more difficult because each jump in current has a voltage jump in the entire circuit protected against reverse polarity. To ver this avoid, the N-channel MOSFET must be supplied by a positive supply voltage become. No. 5,434,739 describes such an arrangement, the required gate Voltage for the N-channel MOSFET is supplied by a charge pump.

It is an object of the present invention to reverse polarity protection for electrical switching to provide circles, which the immunity of the inputs of the protected Receives circuit and does not increase the power consumption of the circuit to be protected.

The object is achieved in that in the positive supply reverse polarity protection is provided for the circuit, that the reverse polarity protection is an N-channel MOSFET transistor with parallel connection Zener diode and a small signal MOSFET transistor with parallel Zener Diode has that the drain connections of the two transistors for supply by the positive supply line are provided and that the gate connection of the N channel MOSFET transistor provided for supply by a power bridge circuit and is connected to the source terminal of the small signal MOSFET transistor.

Because the reverse polarity protection is fed directly from the positive supply line becomes an undesirable increase in the ground potential before polarity reversal protective power bridge circuit prevented. So the immunity remains Received inputs of the power bridge circuit undiminished. By exploiting The existing gate voltages of the power bridge circuit will be a separate one Provision of a gate voltage for the N-channel MOSFET by means of a charge pump unnecessary. This saves components and avoids increased power consumption. The Both Zener diodes offer the advantage that they are the gate connections of the two transistors Protect the reverse polarity protection against overvoltage.  

The embodiment according to claim 2 ensures that the N-channel MOSFET transistor if the operating voltage is reversed, the power bridge circuit opens Operating voltage separates.

The embodiment according to claim 3 ensures that the internal diode of the N channel MOSFET transistor is conductive only when the operating voltage is correctly polarized and at reverse polarity does not affect the shutdown process.

An embodiment of the invention is described in more detail below with the aid of a figure explained.

Fig. 1 shows a circuit diagram of a power bridge circuit with a reverse polarity protection according to the Invention.

The reverse polarity protection consists of an N-channel MOSFET transistor 1 with a Zener diode 5 connected in parallel and a small-signal MOSFET transistor 2 with a Zener diode 6 connected in parallel. Their drain connections are each supplied with voltage U _B by the positive supply line. In addition, the gate connection of the small-signal MOSFET transistor 2 is connected to the ground potential via a series resistor 7 .

As shown in Fig. 1, the gate voltage of the N-channel MOSFET transistor 1 is obtained from a so-called bootstrap supply of the power bridge by peak value rectification with two diodes 3 , 4 . As a result, the N-channel MOSFET transistor 1 always receives sufficient gate voltage as long as one of the upper switches of the power bridge is also supplied with gate voltage.

A known problem with the so-called bootstrap supply is starting the circuit. However, this problem is of minor importance for reverse polarity protection, since the inner diode of the N-channel MOSFET transistor 1 is conductive when the operation is not reversed and can supply the following circuit even if the bootstrap capacitances are discharged. As soon as current flows in the power bridge, at least one of the upper switches of the power bridge must also be supplied. Thus the N-channel MOSFET transistor 1 then also has sufficient gate potential. As a result, there are no forward losses in its inner diode.

The Zener diodes 5 , 6 are provided as a protective circuit for the respective gates of the transistors 1 , 2 of reverse polarity protection against overvoltage and do not contribute to the actual function.

The small-signal MOSFET 2 is always switched through via a series resistor 7 when the operating voltage U _{B is} reversed. As a result, the gate of the N-channel MOSFET transistor 1 is short-circuited and the latter opens. This ensures that the polarity reversal is ensured. The internal diode (body diode) of the N-channel MOSFET transistor 1 is only conductive with correct polarity of the operating voltage U _B , so it does not interfere with the switch-off in the event of reverse polarity. The prerequisite for this is, however, that the N-channel MOSFET transistor 1 is selected to match the switches of the bridge. The decisive parameters are the maximum current and the threshold voltage of the transistors.

Claims (3)

1. Electrical circuit with reverse polarity protection, characterized in
reverse polarity protection is provided in the positive supply line of the circuit,
that the reverse polarity protection has an N-channel MOSFET transistor ( 1 ) with a Zener diode ( 5 ) connected in parallel and a small-signal MOSFET transistor ( 2 ) with a Zener diode ( 6 ) connected in parallel,
that the drain connections of the two transistors ( 1 , 2 ) are provided for feeding through the positive supply line and
that the gate connection of the N-channel MOSFET transistor ( 1 ) is provided for supply by a power bridge circuit and is connected to the source connection of the small-signal MOSFET transistor ( 2 ). 2. Electrical circuit according to claim 1, characterized in that a series resistor ( 7 ) is provided for switching through the small-signal MOSFET transistor ( 2 ) when the voltage of the supply line is reversed and thus to short-circuit the N-channel MOSFET transistor ( 1 ), whereby this opens. 3. Electrical circuit according to claim 1 or 2, characterized in that the N-channel MOSFET transistor ( 1 ) with respect to maximum current and threshold voltage has comparable data as the power switch of the power bridge circuit.