POWER MOSFET DRIVER WITH CROSS-CONDUCTION CURRENT REDUCTION

Abstract: An output circuit includes a totem-pole output circuit for driving a power MOSFET (FETI). A pull-up circuit and a pull-down circuit drive the output node low or high as required. The circuit prevents the pull-up (20) and pull-down (30) circuits from simultaneously conducting current. The pull-up circuit has a pull-up threshold voltage and the pull-down circuit has a pull-down threshold voltage such that the pull-up circuit is turned off before the pull-down circuit is activated when the output node is switched from a high state to a low state and further wherein the pull-down circuit is turned off before the pull-up circuit is activated when the output node switches from a low state to a high state.

Description: POWER MOSFET DRIVER WITH CROSS-CO DUCTION CURRENT REDUCTION Field of the Invention This invention relates to an integrated power MOSFET driver with cross-conduction current reduction. Background of the Invention In conventional bipolar totem-pole drivers for power MOSFE 's it is possible for both the pull-up and pull-down circuits to conduct at the same time during transients. Spikes of current flow from the positive supply to the ground, resulting in excessive power losses at high switching frequencies. The switching transient spikes can also deleteriously affect the operation of other sensitive circuits. The circuit described in this patent eliminates transient cross-conduction current by preventing the pull-up and pull-down circuits from conducting simultaneously. Other design considerations for the circuit described include reduced switching time and means of producing temperature independent, non- overlapped conduction intervals for cross-conduction elimination. Summary of the Invention An output circuit in accordance with the present invention includes a totem-pole output circuit for driving a power MOSFET. A pull-up circuit and a pull-down circuit drive the output node low or high as required. During normal dc operation only one of the pull-up or pull-down circuits is turned on. When the pull-up is on and the pull-down is off the output node is high. When the pull-down is on and the pull- up is off the output node is low. Since the output circuit is intended for driving MOSFET's, the totem-pole transistors carry no dc current. If both the pull-up and pull-down transistors are on at the same time, excessive power will be lost, especially at high frequencies. A circuit built according to the present invention prevents the pull-up and pull-down circuits from conducting simultaneously. The pull-up circuit has a pull-up threshold voltage and the pull-down circuit has a pull-down threshold voltage. The pull-up circuit is turned off before the pull-down circuit is turned on during output high-to-low transition. The pull-down circuit is turned off before the pull-up circuit is turned on during output low-to-high transition. The pull-up circuit is held off during output high-to-low transition by having a pull-up threshold lower than the pull-down threshold. One or more conducting diodes coupled in series, one diode having its anode coupled to the output node and the cathode of the other diode coupled to the input of pull-up circuit, hold the pull-up Darlington off. Since the threshold of the pull-down circuit is higher than that of pull-up circuit, the pull-down circuit is cut off before the pull-up circuit drives the output from a low to a high state. A resistor is used to speed up output low-to-high transition. Brief Description of the Drawing Figure 1 shows a circuit schematic according to the preferred embodiment of the present invention. Detailed Description of the Preferred Embodiment Figure 1 shows a schematic diagram of a circuit implementation for the preferred embodiment of the present invention. The circuit includes a power supply node V__, a circuit ground, an input node A and an output node B having an output voltage V . An input voltage is coupled to the input node A. The output node B drives the gate of an n-channel MOSFET transistor FETl. The drain of the MOSFET transistor FETl is connected to a load circuit and the source to ground. The input A is coupled to two resistors Rl and R2. The other terminal of the resistor Rl is coupled to the base of a Schottky NPN bipolar transistor Ql. The emitter of transistor Ql is coupled to ground. The collector of transistor Ql is coupled to a first terminal of resistor R3, to the cathode of Schottky diode D-_, the anode of Schottky diode D- and to a current source I . The current source I χ is designed to provide a V dependant current. The current source I, is also coupled to the voltage supply V__. The other terminal of the resistor R2 is coupled to the base of a Schottky NPN transistor Q3. The emitter of transistor Q3 is coupled to ground. The collector of transistor Q3 is coupled to the emitter of Schottky NPN transistor Q2 and to the base of a Schottky-clamped output NPN transistor Q5. The base of transistor Q2 is coupled to the cathode of Schottky diode D- and the anode of Schottky diode D ± . The collector of transistor Q2 is coupled to the anode of a diode Ql , the cathode of a diode Q6 and to a current source I_. The diodes Q6 and Q7 are typically formed of the base-emitter junction of a transistor whose collector is shorted to the base node. The current source I 2 is also coupled to the power supply node V_ c . The anode of diode Q6 and the cathode of Ql are both connected to the output B and to the collector of transistor Q5. The emitters of transistor Q5 are coupled to the ground node. The other terminal of the resistor R3 is also coupled to a resistor R4 and to the base of a Schottky NPN transistor Q4. The other terminal of the resistor R4 and the emitter of transistor Q4 are each connected to ground. The collector of transistor QA is connected to the cathode of diode Qll, the base of an NPN transistor Q8 and to a current source I 3 . The current source I 3 is also connected to the power supply V . The collector of transistor Q8 is also connected to the power supply The anode of diode Qll is connected to the cathode of diode Q10, a resistor R5, the emitter of transistor Q8 and the base of a large output NPN transistor Q9. The anode of diode Q10 the other terminal of the resistor R5 and the emitters of transistor Q9 are all connected to the output B. The diodes Q10 and Qll are typically formed of the base- emitter junction of a transistor whose collector is shorted to the base node. The collector of transistor Q9 is connected to the voltage supply V . Transistors Q8 and Q9 form a first Darlington circuit 20. A second Darlington circuit 30 is formed from diode Q6, transistors Q2 and Q5. The second Darlington circuit is a composite Darlington. The diode Q7 clamps the diode Q6 to prevent Q6 from being damaged from Zenering during an output low to high transition. DC OPERATION When V in is high, both transistors Ql and Q3 turn on, pulling their collectors low. The current from the current source I. is shunted through Ql to the ground. Since the collector of Ql is pulled low, the transistors Q2 and Q4 both turn off. With transistor Q4 off, current from current source I 3 provides the base drive to the first Darlington circuit 20, pulling the output B high. Once the gate is fully charged, the first Darlington carries only negligible current. Further, because transistor Q3 is on and the base of the transistor Q2 is low, the second Darlington circuit 30 is turned off. The current sources I 2 and I 3 saturate and provide only negligible current. Notice that the first Darlington circuit 20 is controlled by the transistor Q4. When V in is low, both transistors Ql and Q3 are off. With the collectors of the transistors Ql and Q3 high, the transistors Q2, Q4 and Q5 are all turned on. With the transistor Q4 on, the collector of transistor Q4 is pulled very close to ground turning the first Darlington circuit 20 (transistors Q8 and Q9) off. The transistor Q2 is the controlling transistor for the composite second Darlington circuit 30. When the transistor Q2 is on and the transistor Q3 is off, the transistor Q5 conducts, discharging the gate of the MOSFET device FETl and pulling the output node B to the ground. When the output is at its dc low state, the current from the current source I 3 flows through the transistor Q . The current rom the current source I 2 flows through the diode Q7 and the transistors Q2 and Q5. The transistor Ql acts as an invertor to the input v\ . Accordingly, the transistors Q2 and Q3 are always in an opposite condition from one another, i.e., when the transistor Q3 is on, the transistor Q2 is off and vice-versa. TRANSIENT OPERATION The switching threshold V-,, for the first Darlihgton circuit is the potential required at node C to turn on the first Darlington circuit 20. This 5 potential is the base emitter voltage V BE4 of the transistor Q4 multiplied by the action of the voltage divider circuit of R3 and R4. The resistance of res-istor R3 controls the dead band. As shown in the equation, this resistance is used as a ratio. 10 Because all resistors are simultaneously formed as an integrated circuit the ratio does not change even though specific impedances may. Thus, V T1 = V BE4 (- + R 3 / R 4> Similarly, the second Darlington circuit has a 15 switching threshold V τ2 whereby the collector of the transistor Ql must rise above the base emitter voltage of transistor Q5 plus the base emitter voltage of transistor Q2 plus the diode voltage of diode D 2 . Thus, 2 * -0 u V V T2 = VBE5 + V V BE2 + VD2 Assuming that the output, node B, is initially high and V. is switched from high to low, the transistor Ql turns off. Part of the current from the current source charges the parasitic capacitance present 5 at the collector of the transistor Ql. Before node C is charged to V l , Q4 remains off. The remainder of the current flows into R3. When the collector of the transistor Ql rises to V BE4 (1+R_/R 4 ) , the transistor Q4 turns on. Since I χ is V BE dependent, V χχ and V χ2 0 all have similar temperature dependence. The non¬ conducting time interval is relatively independent of temperature. Once Q4 begins to conduct, the collector voltage of the transistor Q4 falls. When output node B is high and the collector of 5 Q4 begins to fall the diodes Q10 and Qll start to conduct, holding the transistors Q8 and Q9 off. The diodes Q10 and Qll also function as anti-Zenering devices for the transistors Q9 and Q8 by clamping the reverse voltages across the transistor Q8 and Q9 base-emitter junctions to V βE . The gate capacitance of FETl will only be slightly discharged by the diodes Q10 and Qll due to limited base drive to the relatively small transistor Q4. When the collector voltage of transistor Ql reaches the switching threshold V 2 of transistor Q2, the transistor Q5 conducts and discharges the gate capacitance of the MOSFET FETl to ground. Since the transistors Q8 and Q9 are held in an off condition by transistor Q4 before the transistor Q5 turns on, cross-conduction current due to simultaneous conduction of the transistors Q9 and Q5 is eliminated. Assuming that the output voltage V Q is initially low and V in is switched from low to high, the transistor Ql turns on, pulling the collector of transistor Ql towards ground. Since switching threshold V T2 of the transistor Q2 is higher than that of the transistor Q4, the transistor Q2 turns off before the transistor Q4, cutting off base drive to the transistor Q5. Transistor Q4 stays on until the voltage at the collector of the transistor Ql falls below V 4 (1+R 3 /R 4 ). As the base drive of the transistor Q4 is removed, the voltage on the collector of transistor Q4 rises. This causes the transistors Q8 and Q9 conduct. The MOSFET gate capacitance is charged towards V . The Schottky diode D χ and the transistor Q3 speed up the turn-off transient of the transistors Q2 and Q5, respectively. The resistor R4 shortens the turn-off time of the transistor Q4 by shunting the parasitic base-collector junction capacitance C. displacement current to ground as the transistor Q4 is switched off. The result is shorter low-to-high propagation delay time t dJ£L and faster output rise time t r . Since the base drive of the transistor Q5 is cut off before the collector voltage of the transistor Q4 is allowed to rise, there is again no cross-conduction current during output low-to-high transition.

Claims: C L A I M S 1. An output circuit comprising a totem-pole output circuit with an output node having a high state and a low state having a pull-up circuit having a pull-up threshold voltage and a pull-down circuit having a pull-down threshold voltage comprising means for preventing the pull-up and pull-down circuits from simultaneously conducting current such that the pull- up circuit is turned off before the pull-down circuit is turned on when the output node switches from a high state to a low state and further wherein the pull-down circuit is turned off before the pull-up circuit is turned on when the output node switches from a low state to a high state. 2. The circuit according to claim 1 wherein the pull-up circuit is turned off before the pull-down circuit is turned on by holding the pull-up circuit off when the output node goes from a high state to a low state by one or more diodes coupled in series from the output node to an input of the pull-up circuit. 3. The circuit according to claim l wherein the pull-down threshold is greater than the pull-up threshold so that the pull-down circuit is shut off before the output node is driven from a low state to a high. . The circuit according to claim 3 wherein the pull-up circuit has a resistor coupled to shunt the parasitic capacitance current to ground to speed up output low-to-high transition. 5. The output circuit according to claim 1 further comprising a V BE -dependent current supply for generating a non-conducting dead time interval relatively independent of temperature. 6. An output circuit for driving a power MOSFET circuit comprising a totem-pole output circuit having a pull-up means having a first threshold and a pull- down means having a second threshold and means for controlling the first and second threshold such that the pull-up and pull-down means are not activated at the same time.

Inventors: MA BING FONG, MA, BING, FONG
Applicants: MICRO LINEAR CORP, MICRO LINEAR CORPORATION
Application year: 1992
Main classification: H03K17
IPC codes: H02M1, H03K17, H03K19