Floating Drive Technique for Reverse Battery Protection

This invention relates to a technique for protecting integrated circuits and other semiconductor devices against damage when the battery by which they are powered is connected in reverse. In particular, this invention relates to a technique of protecting integrated circuit devices in a motor vehicle against damage from a reverse-connected battery.

The introduction of integrated circuit and semiconductor devices to motor vehicles has greatly increased the control capability available to the designer. At the same time, however, it has created a need to protect these components against a reverse-connected battery. Many semiconductor devices such as microcomputers, for example, contain a diode connected between the supply voltage and ground. When the battery is properly connected (i.e., with the negative terminal connected to chassis ground) these diodes are reverse biased. When the battery is reversed, however, these diodes become forward-biased. The resulting high currents will almost certainly destroy the device. Thus, all such devices must be protected against a reverse-connected battery.

In many motor vehicles a Schottky diode is connected in series with the load, so that it is forward-biased when the battery is properly connected. If the battery is reversed, the diode becomes reverse-biased and the loads are protected from reverse currents or negative voltages. A disadvantage of this technique is that during normal operation a voltage drop exists across the forward-biased diode and heat energy is generated. For example, a 60 volt Schottky diode might generate a 0.8 volt drop, and with a current flow of 20 amps about 14 watts of heat energy would be generated. This heat must be transferred away from the diode. Heat sinks are becoming more difficult to find in motor vehicles, however, because more parts are being made of plastic. The metal surfaces in the engine compartment are generally too hot to serve as heat sinks.

The most attractive solution to this problem would be a device which approximates as closely as possible an ideal diode, with an equivalent resistance in the forward direction of no more than 50 milliohms. One possibility would be to connect a low on-resistance N-channel power MOSFET in series with the load, with its source connected to the battery and its drain connected to the load. Properly driven, during normal operation the power MOSFET's low resistance channel would shunt any current away from the intrinsic drain-body diode, producing a low on-state voltage drop. The diode formed by the drain-to-body junction is in parallel to the MOSFET's source-to-drain terminals whenever the source-body short is employed in a MOSFET. The source-body short is common in a vertical DMOS. When the battery is reversed, the MOSFET would be shut off, leaving the intrinsic drain-to-body diode reverse-biased. However, to turn the MOSFET on, its gate must be biased at least 8 volts above its source, which is connected to the battery. A charge pump or similar means is necessary to produce a gate-source voltage of a magnitude sufficient to guarantee that the MOSFET will be fully turned on. This charge pump must therefore have one terminal connected directly to the battery.

If a semiconductor device is used to provide the gate-source voltage, this device is susceptible to the same reverse-battery problems as the other devices in the motor vehicle. That is, if the gate driver is connected directly to the battery's plus and minus terminals (i.e., between the vehicle's "hot" and ground terminals), it will be subjected to a reverse potential if the battery is hooked up in reverse. In many self-isolated and junction-isolated IC technologies, a reverse battery connection will forward-bias many junctions in the IC, flood the substrate with minority carriers, or cause latch-up, excessive heating or other undesirable effects.

In accordance with this invention, a power MOSFET is connected in series with a battery-driven load. The source of the MOSFET is connected to the battery; the drain is connected to the load. The MOSFET's gate is driven by a "floating" driver which is not connected across the terminals of the battery, except via a high resistance signal path incapable of high reverse currents. Instead, the floating driver is connected only to the positive terminal of the battery and contains no logic or control circuitry which is powered by a DC conduction path to ground. The gate driver is associated with a ground-referenced "low-side" controller that is not directly connected to the battery's "hot" line but is powered from the protected side of the power MOSFET. Thus, neither the gate driver nor the control circuitry is susceptible to being reverse-biased in the event that the battery is connected in reverse. The gate driver contains a device that shorts the gate to the source of the MOSFET, thereby turning it off, if the battery is reversed. In a preferred embodiment, this device is a depletion mode MOSFET.

In accordance with another aspect of this invention, a sensing device detects when the voltage of the battery (properly connected) falls to the point where the protective MOSFET is not fully turned on. The sensing device generates a signal which causes an appropriate corrective action to be taken-for example, switching off all or a portion of the load.

FIGS. 1A and 1B illustrate circuit diagrams of a power MOSFET according to this invention in normal operation and in reverse-battery operation, respectively.

FIG. 2 illustrates a block diagram showing a floating gate driver and a low side controller in accordance with the invention.

FIG. 3 illustrates a circuit diagram of the floating gate driver and the low side controller.

FIG. 4 illustrates an alternative embodiment of the invention which uses a comparator to drive the gate of the protective MOSFET.

FIG. 1 shows a power MOSFET 10 connected in series between a battery 11 and a load 12. MOSFET 10 is an N-channel power MOSFET such as the model SMP60N06-18 manufactured by Siliconix, Inc. A diode 13 represents the body to drain diode in MOSFET 10. Load 12 represents the entire "load" of, for example, an automobile electrical system, including all active devices, switches, and protective devices. Load 12 may contain, for example, diodes or other elements for protecting various components against large positive or negative voltage transients. These elements operate in conjunction with power MOSFET 10, which protects the various electronic components in the load against a reversed battery condition.

FIG. 1A shows MOSFET 10 in normal operation, with its source connected to the positive terminal of battery 11 and its drain connected to load 12. The negative terminal of battery 11 is connected to ground; its positive terminal provides a voltage V.sub.BAT (normally 12 volts). To ensure that MOSFET 10 is turned on, its gate terminal must be maintained at a voltage V.sub.G which is at least 8 volts above V.sub.BAT. In this situation, current flows through the N-channel of MOSFET 10, which is a low resistance path, and diode 13 is effectively shunted out.

FIG. 1B illustrates MOSFET 10 when battery 11 has been connected in reverse. The gate and source of MOSFET 10 have been shorted (V.sub.GS =0) and MOSFET 10 has therefore been turned off. Diode 13 is reverse biased, and accordingly there is practically no current flow through MOSFET 10. The entire voltage drop of battery 10 is therefore sustained by MOSFET 10, and the various devices within load 12 are protected against the reverse potential.

FIG. 2 illustrates a block diagram of the circuitry used to control the source-to-gate voltage V.sub.GS of MOSFET 10. A "floating" gate driver 20 is connected between the positive terminal of battery 11 and the gate of MOSFET 10. A low side controller 21 is connected between ground and the drain of MOSFET 10. In this embodiment gate driver 20 and controller 21 are located on separate ICs which are connected together at pins designated 1, 2, 3 and SUB. The SUB pins connect to the substrates of floating gate driver 20 and controller 21.

FIG. 3 illustrates some of the circuitry within gate driver 20 and controller 21. A voltage regulator 30 is connected to the "protected" side of MOSFET 10. Under normal circumstances, the load is initially provided by forward-biasing diode 13. After start-up, the charge pump circuit increases the drive on MOSFET 10 and shunts the current from diode 13 through the MOSFET's channel. Voltage regulator 30 provides a voltage of, for example, 5 V to an oscillator 31. The two outputs of oscillator 31 drive a charge pump which includes capacitors C.sub.1 -C.sub.3 and diodes D.sub.3 -D.sub.6. One output of oscillator 31 is connected to capacitors C.sub.1 and C.sub.3 ; the other output is connected to capacitor C.sub.2. When the outputs of oscillator 31 provide signals approximately 180.degree. out of phase with each other, in a manner well known in the art charge is transferred in stages from capacitor C.sub.1 to capacitor C.sub.2 to capacitor C.sub.3, and then to the gate of MOSFET 10. Assuming that V.sub.BAT =12V, the charge pump would ordinarily deliver a V.sub.G .apprxeq.34V, or a V.sub.GS of about 22V. However, since a V.sub.GS of 22V might cause breakdown of the gate insulation layer in MOSFET 10, a zener diode D.sub.2 clamps V.sub.GS at a maximum of 16V.

If battery 11 is connected in reverse, the gate and source of MOSFET 10 are shorted by a depletion mode MOSFET M.sub.1. The source of depletion mode MOSFET M.sub.1 is connected to the source of MOSFET 10; the drain of depletion mode MOSFET M1 is connected to the gate of MOSFET 10. The gate and body of MOSFET M.sub.1 are shorted together and are connected via a diode D.sub.1 to battery 11 and via a relatively large resistor R.sub.1 to ground. As is typical with depletion mode MOSFETs, MOSFET M.sub.1 is turned on when its V.sub.GS =0, and turns off only when its source is biased below its gate by an amount equal to or greater than the pinch-off voltage V.sub.T for the device (i.e., V.sub.GS .ltoreq.V.sub.T). The purpose of the ground connection is essentially to provide an input signal (not shown) to the high side. Since this ground connection is only a signal path, the current can be limited by a large value of resistor R.sub.1.

The operation of this circuit will be described with reference to three possible situations. The first situation is where the battery is connected and functioning properly, i.e., typically V.sub.BAT =12V. Since the source of MOSFET M.sub.1 is connected to the high side of battery 11, and its gate is grounded, V.sub.GS for MOSFET M.sub.1 is equal to approximately -12V. MOSFET M.sub.1 has a pinch-off voltage of about -5V, so that at a V.sub.GS =-12V, MOSFET M.sub.1 is turned off. Thus, as described above, the charge pump drives the gate of MOSFET 10 until V.sub.GS for MOSFET 10 is equal to about 16V. Thus, current flows through the N channel of MOSFET 10, and MOSFET 10 typically shows a resistance of only about 18-50 milliohms.

The second situation is where battery 11 has been connected in reverse. In this situation, the V.sub.GS of MOSFET M.sub.1 is approximately +12V, and therefore MOSFET M.sub.1 is turned on. This shorts the gate and source of MOSFET 10, turning MOSFET 10 off and preventing the semiconductor components of load 12 from seeing the reverse voltage. In dynamic terms, MOSFET M.sub.1 shorts the gate and source of MOSFET 10 and thereby takes all of the energy out of the gate capacitor of MOSFET 10 before the reverse voltage appears across MOSFET 10. Thus, when the reverse voltage appears across MOSFET 10, that device is already turned off.

MOSFET M.sub.1 is protected against the reverse voltage because its inherent source-body diode is connected in series with a relatively large resistor R.sub.1. The value of resistor R.sub.1 may be, for example, 10K ohms. As noted above, gate driver 20 and controller 21 are on different chips. The oscillator 31 and other components in the controller 21 are not affected by the forward-biasing of the source-body diode in MOSFET M.sub.1. Moreover, even if MOSFET M.sub.1 snaps back or latches up, that will short the gate to the source of MOSFET 10 even faster. So, even in the unlikely event that gate driver 20 temporarily fails to function properly, it will merely turn MOSFET off faster. The current-limiting resistor R.sub.1 and capacitors C.sub.1 -C.sub.3 prevent any adverse consequences to the circuitry in the controller 21. The high side circuit can be understood as a circuit with no low-resistance connection between its substrate and ground. It is therefore incapable of strongly forward-biasing its internal substrate junctions. The low resistance path needed to drive the gate of MOSFET 10 is provided by the AC coupling of capacitors C.sub.1, C.sub.2 and C.sub.3. These capacitors represent a DC "open" connection incapable of continuous current.

The third situation is where battery 11 is connected properly, but the output of battery 11 drops to, for example, 4 or 5V. Since this is near the pinch-off voltage of MOSFET M.sub.1, MOSFET M.sub.1 is slightly on and is allowing some leakage from the gate of MOSFET 10. MOSFET M.sub.1 is not fully on, however. Moreover, the V.sub.GS of MOSFET 10 is only in the neighborhood of 8-10V in this situation, and therefore MOSFET 10 approaches turn off, where it may saturate. In this situation, a large voltage drop may develop across MOSFET 10. At high currents the resulting high power dissipation may overheat and damage MOSFET 10. If V.sub.GS of MOSFET 10 falls below about 8V above ground (or 4V above V.sub.BAT), MOSFET 10 will turn off and all of the current will flow through diode 13. This will dissipate a considerable amount of power.

The solution to this problem lies in under-voltage lockout (UVL) circuit 32. UVL circuit 32 monitors the voltage V.sub.DD across load 12. When V.sub.DD drops below, for example, 6V, UVL circuit 32 sends out a signal which instructs the system to begin shutting off some of the load elements within load 12. It continues to do this until the load current falls to the point where it can be supported by the available driving voltage V.sub.GS at the gate of MOSFET 10. Otherwise, with a substantial portion of the current flowing through diode 13, MOSFET 10 will begin to heat up, and this will create a heat dissipation problem. UVL circuit 32 thus acts as a sensing device to determine when V.sub.DD is within the "danger region" between 0 and +6V and where MOSFET 10 is either off or only partially on.

FIG. 4 illustrates an alternative embodiment which uses a comparator 41 to drive the gate of MOSFET 10. MOSFET 10, battery 11 and load 12 are connected in series as in FIG. 3. An oscillator 40 drives a charge pump which includes an inverter 42, capacitors C.sub.10 and C.sub.11 and diodes D.sub.10, D.sub.11 and D.sub.12. While the charge pump in this embodiment contains only three diodes, as compared with four diodes in the embodiment of FIG. 3, it will be apparent that various kinds of charge pumps employing various numbers of diodes or other elements may be used to satisfy specific requirements. The charge produced by the charge pump is stored on a capacitor C.sub.12, the low side of which is connected to the positive terminal of battery 11. A zener diode D.sub.14 limits the voltage across capacitor C.sub.12 to about 15 V. This charge is delivered to a power input 41A of comparator 41, the other power input 41B of comparator 41 being connected to the positive terminal of battery 11. A conduction path is formed between the positive terminal of battery 11 and ground by a diode D.sub.13 and resistors R.sub.10 and R.sub.11. The common point between diode D.sub.13 and resistor R.sub.10 is connected to the negative input of comparator 41.

Thus when battery 11 is connected properly, the voltage at the positive input of comparator 41 exceeds the voltage at the negative input of comparator 41 by the amount of the voltage drop across forward-biased diode D.sub.13 (about 0.7 V). As a result, the voltage stored on capacitor C.sub.12, which is limited to about 15V by zener diode D.sub.14, is delivered at the output of comparator 41 to the gate terminal of MOSFET 10. Assuming that this voltage V.sub.GS is greater than about 8V, MOSFET 10 is turned on and provides a low on-resistance conduction path between battery 11 and load 12. If battery 11 is reversed, the polarity of the voltages applied at the inputs of comparator 41 is reversed (i.e., the negative terminal of battery 11 is connected directly to the positive input of comparator 41 and the positive terminal of battery 11 is connected to the negative input of comparator 41 via a diode D.sub.16 and a resistor R.sub.10). Comparator 41 supplies a V.sub.GS =0V to the gate of MOSFET 10. MOSFET 10 is therefore turned off and intrinsic diode 13 blocks a reverse flow of current through load 12. Because the reversed battery condition leads to a DC current from V.sub.BAT to ground, which must flow through high value resistors R.sub.10 and R.sub.11, no substantial current can flow.

Another conduction path between the positive terminal of battery 11 and ground is formed by a diode D.sub.15, a resistor R.sub.12 and a capacitor C.sub.13. In normal operation, diode D.sub.15 charges capacitor C.sub.13 through resistor R.sub.12, which serves as a current-limiting resistor. Capacitor C.sub.13 serves as a voltage reference. When the battery voltage falls rapidly, the polarity of the voltage across diode D.sub.15 is reversed and this in effect disconnects the battery from capacitor C.sub.13. Resistor R.sub.11 provides a relatively long time constant for discharging capacitor C.sub.13 when the battery is disconnected or when the battery remains reversed. Resistor R.sub.10 limits current through diode D.sub.17 and Zener diode D.sub.14 when the battery is reversed. Diode D.sub.16, connected in parallel with capacitor C.sub.13, prevents capacitor C.sub.13 from being charged by the reversed battery, which could be harmful to an electrolytic capacitor.

The embodiments described above are intended to be illustrative and not limiting. Many alternative embodiments in accordance with this invention will be apparent to those skilled in the art. All such alternative embodiments are included within the broad scope of this invention, as defined in the following claims.