Drive circuit for a power switch component

A drive circuit for a power switch component.

TECHNICAL FIELD

The present invention relates to a driving circuit for driving, for example, a power mosfet.

BACKGROUND AND RELATED ART

For gate driving of large power train components, such as power mosfets, a number of solutions are known in the art. For example a number of different integrated circuits exists that are suitable for this purpose. These devices are expensive, and have specific properties, so that they are not interchangeable. This means that when a particular type of integrated circuit from a particular vendor has been selected for an application it cannot easily be replaced by another type of integrated circuit.

Drive circuits based on bipolar transistors are also known. Such drive circuits are known to be more complex to achieve the same result.

There is a desire to achieve a negative voltage on the gate of the transistor that drives the output signal to saturate it completely. In the prior art solutions based on bipolar transistors or integrated circuits this is achieved by adding an additional voltage source for providing the negative voltage, or using inverted driving stages in common emitter style, which results in a more complex design.

SUMMARY OF THE INVENTION

The invention aims to provide a more flexible drive circuit for a power switch component.

According to the present invention a drive circuit for a power switch component is provided, said drive circuit comprising an input terminal for receiving a drive pulse, a first semiconductor component and a second semiconductor component, each semiconductor component having a controllable terminal, a voltage amplifying terminal and a current amplifying terminal, the current amplifying terminals of said first and second transistors being interconnected, the voltage amplifying terminal of the first semiconductor component being connected to a first power rail and the voltage amplifying terminal of the second semiconductor component being connected to a second power rail, the first power rail being at a higher potential than the second power rail, the controllable terminals of the first and second semiconductor components being interconnected, a resistor and a diode being connected in parallel between said controllable terminals and the input terminal, and a capacitor being connected between said controllable terminals and said current amplifying terminals, wherein each of said semiconductor components has a diode connected between its current amplifying terminal and its voltage amplifying terminal.

The drive circuit according to the invention is based on transistors, which are substantially equivalent independently of the manufacturer or supplier. Hence, the selection of a drive circuit does not result in dependency of a particular type of component from a particular vendor. Also, the drive circuit can be implemented at a relatively low cost.

With the drive circuit according to the invention a saturated drive voltage to both negative and positive supply rail voltage can be achieved using only one power supply rail. The output voltage can be driven up to the rail voltage and down to the negative rail voltage even if the drive voltage is lower than the positive rail voltage, and negative rail voltage, using only one power source. The delay time of the drive circuit is adjustable. Further, the change in current with respect to time, di/dt, and the change in voltage with respect to time, dv/dt, can be made very fast.

In a first preferred embodiment the first and second semiconductor components are mosfets. In this case, the controllable terminals are the gates of the mosfets, the voltage amplifying terminals are the drains and the current amplifying terminals are the sources. The diodes are body-drain diodes, which are inherent in the mosfets.

In a second preferred embodiment the first and second semiconductor components are bipolar transistors. In this case the controllable terminals are the bases of the transistors, the voltage amplifying terminals are the collectors and the current amplifying terminals are the emitters. Bipolar transistors do not have inherent body-drain diodes in the way that mosfets do; therefore, separate diodes, connected in the same way as the body-drain diodes of the mosfets, must be provided when using bipolar transistors.

In a third embodiment the first and second semiconductor components are Isolated Gate Bipolar Transistors (IGBT). In this case the controllable terminals are the gates of the transistors, the voltage amplifying terminals are the drains and the current amplifying terminals are the sources. As for the bipolar transistors, separate diodes connected, in the same way as the body-drain diodes of the mosfets, are provided.

The potentials of the first and second power rails may be selected in different ways. For example, the second power rail may be neutral while the first power rail has a positive potential. Alternatively, the first power rail may neutral and the second power rail has a negative potential. The important thing is that the first power rail has a higher potential than the second power rail.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1illustrates a drive circuit according to a preferred embodiment of the invention. The drive circuit is arranged in this example to drive a power switch component3. The driven component could be, for example, a mosfet, a bipolar transistor or, an Isolated Gate Bipolar Transistor (IGBT). The drive circuit comprises a first, n-channel, mosfet5and a second, p-channel, mosfet7that are connected to each other by the gates and the sources. Diodes9,11, referred to as body-drain diodes, from the source to the drain of the n channel mosfet5and from the drain to the source of the p channel mosfet7, are inherent in the mosfets. The drain of the n-channel mosfet5is connected to a positive branch13of a power source. The drain of the p-channel mosfet7is connected to a neutral branch15of the power source. The gates of the mosfets5,7are connected to a first end of a resistor17connected in parallel with a diode19whose anode is facing the gates. The other end of the resistor17, and the cathode of the diode19are connected to an input terminal21arranged to receive a control signal for the drive circuit1. Between the first end of the resistor17and the anode end of the diode19on one side and the sources of the mosfets5,7on the other side a capacitor23is connected.

It should be understood that instead of a positive rail13and neutral rail15, the rails13,15could have any potential as long as the first rail13has a higher potential than the second rail15. For example, the rail13could be neutral and the rail15could have a negative potential. Hence, the terms “positive” and “neutral” in relation to the power rails13,15in this description could be replaced by “higher potential” and “lower potential”.

When a positive pulse, referred to the neutral branch15of the power source, is provided on the input terminal21the gate voltage of the n-channel mosfet5will increase, and it will start to conduct. By the time the mosfet5starts to conduct the same time the capacitor23will already be charged, and will bootstrap the voltage back to the gates and saturate the n-channel mosfet. This will give the output side of the buffer at the interconnected sources of the mosfets5,7a very clean and rapid drive up to the positive rail13of the power source. The saturated drive will continue as long as the capacitor23is not discharged through the resistor17. That means that the maximum drive voltage to the gate of the power switch3is applied when needed to overcome the heavy charge to fully saturate in short time, thus minimize switching losses of the power switch3. The short distinct delay provided by the capacitor23and resistor17before the n-channel mosfet5starts to conduct can be used to prevent cross-conduction in different types of dc/dc converter topologies. Because of the bootstrap provided by the capacitor23a large positive feedback with a high dynamic gain will be provided, thus minimizing the positive slope time, instead giving a fixed delay of the signal.

When a negative going signal is applied at the input terminal21the diode19will start to conduct and drain gate of the n-channel mosfet5so it will stop conducting and when the voltage drops more the p-channel mosfet7will start to conduct and drain the gate of the power switch3. At the same time the capacitor23will drain the gate of the p-channel mosfet7and conduct a current so that a negative going voltage over gate to source over p-channel mosfet7is developed. This will saturate the negative going output signal of the driving stage completely to lower potential branch15at the output of the drive circuit. The dynamic gain is high due to the large positive feedback capacitor23is applying. This results in a fast going negative slope that goes to negative voltage. The nature of the negative going slope is of great use to ensure that the driven component3is properly drained. This is useful both for bipolar and mosfet transistors.

In the embodiment shown inFIG. 1mosfet transistors5,7are used in the drive circuit1. As mentioned above, such transistors have inherent diodes between the source and the drain. Instead of mosfet transistors, bipolar transistors could be used.

In this case, diodes would have to be provided between the emitter and the collector, in a corresponding way to the body-drain diodes9,11illustrated inFIG. 1.FIG. 2show three curves a, b and c, which are typical curves of the voltage between lower potential branch15and signal processes in the circuit. The vertical lines labelled t1-t3signify points in time where different events occur in the circuit.

Curve a illustrates the signal over input terminal21referred to lower potential branch15. At a first point in time t1a positive going signal relative to the lower potential branch15is applied to the input terminal21. At a third point in time t3the signal on the input terminal21goes negative again.

Curve b illustrates the voltage on the gates of the n-channel mosfet5and the p-cannel mosfet7relative to the lower potential branch15. At the first point in time t1, when a positive going signal referred to lower potential branch15is applied at input terminal21, capacitor23starts to charge through the resistor17making a positive going slope. The time it takes to reach the conducting threshold of n-channel mosfet5is referred to as a dead time. When the n-channel mosfet's5conducting threshold is reached it starts to conduct, at the second point in time t2. Capacitor23will now act as a positive feedback element and bootstrap a positive voltage higher than the signal at input terminal21back to n-channel mosfet5gate and make it fully saturate to positive branch13. This will cut down the switching time, therefore a very sharp positive going signal between lower potential branch and gate of the n-channel mosfet5is observed for the gate of the power switch3. A fully saturated signal will be available as long as capacitor23does not discharge too much through the resistor17. At the third point in time t3the signal on the input terminal21goes negative and together with the body-drain diodes11,19will create a negative going voltage over the gate of the n-channel mosfet5diode19speed up the process.

When the p-channel mosfet7reaches its conducting threshold it starts to conduct and the capacitor23will now act as a positive feedback element and bootstrap a negative voltage relative to the negative signal at input terminal21back to the p-channel mosfet7gate and make it fully saturate to negative branch15. This will cut down switching time, therefore a very sharp negative going signal between higher potential branch13and gate of p-channel mosfet7is observed for the gate of the power switch3.

Curve c illustrates the signal at the gate of the power switch3referred to lower potential branch15. As can be seen, at the second point in time t2the gate voltage of the power switch3, that is, the output voltage from the driving circuit1, increases sharply and stays at a high level until the third point in time t3, when the signal on the input21becomes low.

As will be appreciated, the signals shown inFIG. 2are ideal signals. In reality a certain amount of time is needed for a signal to become high or low. Using the driving circuit according to the invention, however, these times are significantly reduced compared to the prior art.