ZA200206610B - Drive circuit and method for MOSFET. - Google Patents

Description

DRIVE CIRCUIT AND METHOD FOR MOSFET

TECHNICAL FIELD

This invention relates to a circuit and method for improving the switching speed of insulated gate semiconductor devices such as metal oxide field effect transistors (MOSFET’s), more particularly power MOSFETs.

BACKGROUND ART

Capacitance inherent in transistor junctions limits the speed at which a voltage within a circuit can switch. It is also well known that the

Miller effect has an influence on the capacitance at the gate of devices of the aforementioned kind.

Prior art teaches a number of methods of alleviating the Miller effect in high frequency transistor switching circuits, for example by reducing source impedance or reducing feedback capacitance, or both. ~

Even with such improvements, an output of a MOSFET such as an

IRF740 typically switches through 200 volts in a rise time of . approximately 27ns at a peak current of 10 amperes and in a fall

C WO 01/63763 PCT/ZA01/00024 . | 2 i time of approximately 24ns. These times may be too long for many . applications.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method and circuit for improving the rise and/or fall times of insulated gate semiconductor devices with which the applicant believes the aforementioned disadvantages will at least be alleviated.

SUMMARY OF THE INVENTION

- | According to the invention there is provided a method of driving an insulated gate semiconductor device comprising as a first terminal a gate and further comprising at least second and third terminals, wherein charge storage means and switching means, preferably fast switching means, are connected in a circuit to the gate of the semiconductor device to apply a charge pulse to the gate of the semiconductor device so as to switch the semiconductor device between one of an on state and an off state and the other of the on state and the off state, the duration of the pulse being such that the pulse is substantially complete prior to switching of the semiconductor device.

AMENDED SHEET

. WOo163763 PCT/ZA01/00024 x The insulated gate semiconductor device may be a metal oxide ] semiconductor field effect transistor (MOSFET), such as a power

MOSFET.

Alternatively, the insulated gate semiconductor device may be an insulated gate bipolar transistor.

The charge pulse may raise a gate voltage of the insulated gate semiconductor device three to four times beyond a maximum rating of the semiconductor device.

The switching means may be connected between the charge storage means and the gate of the semiconductor device.

The charge storage means may be connected in parallel with the switching means and the insulated gate semiconductor device.

Alternatively, the charge storage means may be connected in series with the switching means and the insulated gate semiconductor device.

The fast switching means may comprise one of: a SIDAC, a break-over diode, a bipolar transistor, another insulated gate semiconductor device

AMENDED SHEET i . WO 01/63763 PCT/ZA01/00024 . and a high voltage fast switching device.

The fast switching means may be electronically controllable.

The charge storage device may be a capacitor.

An inductor may be provided in the circuit between the fast switching means and the gate.

The inductor may be connected in series with the switching means.

The circuit may be integrated on a single chip.

The chip may further comprise additional circuitry also integrated thereon.

According to another aspect of the invention, there is provided a circuit for driving an insulated gate semiconductor device comprising as a first terminal a gate and further comprising at least second and third terminals, the circuit comprising charge storage means and switching means connected to the gate of the semiconductor device to apply a charge pulse to the gate of the semiconductor device so as to switch

AMENDED SHEET

) 4(a) . the semiconductor device between one of an on state and an off state . and the other of the on state and the off state, the duration of the pulse being such that the pulse is substantially complete prior to switching of the semiconductor device.

The invention also extends to a method of driving an insulated gate semiconductor device comprising a gate, wherein charge storage means and switching means are connected in a circuit to the gate of the semiconductor device to apply from the charge storage means to the gate a charge pulse causing a voltage on the gate which is beyond a oo maximum rating of the semiconductor device.

The voltage may be three to four times beyond the maximum rating of the semiconductor device.

Yet further included within the scope of the present invention is a a circuit for driving an insulated gate semiconductor device comprising a gate, the circuit comprising charge storage means and switching means connected to the gate of the semiconductor device for applying from the charge storage means to the gate a charge pulse causing a voltage on the gate which is beyond a maximum rating of the semiconductor device.

AMENDED SHEET

" . WOO01/63763 PCT/ZA01/00024

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings wherein: figure 1 is a basic circuit diagram of a triggering circuit according to the invention for a MOSFET; figure 2 is a diagram of one embodiment of the circuit comprising a SIDAC as fast switching device; figure 3 includes a diagram in dotted lines of gate voltage against time of normal specified operation of the MOSFET and a diagram in solid lines of operation according to the method of the invention; figure 4 includes a diagram in dotted lines of gate current against time of normal specified operation of the MOSFET and a diagram in solid lines of operation according to the method of the invention; figure 5 is a diagram of another embodiment of the circuit according to the invention;

AMENDED SHEET figure 6 is a waveform of voltage against time at a first terminal of a charge storage capacitor in figure 5; figure 7 is a waveform of voltage against time at the source of the MOSFET in figure 5; figure 8 is a diagram of a triggering circuit for an insulated gate bipolar transistor; figure 9 is a waveform of voltage against time at a first terminal of a charge storage capacitor in the circuit in figure 8; figure 10 is a waveform of voltage against time at an emitter of the transistor in figure 8; and oo oo figure 11 is a basic circuit diagram of yet another embodiment of the triggering circuit.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In figure 1, there is shown a basic diagram of a triggering circuit 10 according to the invention for an insulated gate semiconductor device 12 such as a metal oxide semiconductor field effect transistor (MOSFET).

In the diagram a power MOSFET is shown and which is available from International Rectifier under the trademark HEXFET number . IRF740. A 10% to 90% rise time of an output voltage is specified in publicly available user data sheets of the device as being about 27ns and a corresponding fall time is specified as being in the order of 24ns. These times may be too long for some applications of the

MOSFET. The turn-on delay time is specified at 14ns and the turn- off delay time as 50ns.

The triggering circuit 10 comprises a charge storage device in the form of a capacitor 14 having first and second terminals 14.1 and 14.2 respectively. The first terminal 14.1 is connected in a circuit 17 to a fast switching device 16. An optional inductor 18 is connected oo : | between the fast switching device 16 and a gate 20 of the MOSFET. a

The drain and source of the MOSFET are shown at 22 and 24 respectively.

The fast switching device 16 may be any suitable device having a switching speed faster than the data sheet specified turn-on delay time and/or turn-off delay time of the MOSFET, preferably better than 2ns. Such devices may include a SIDAC, a break-over diode, a suitably configured bipolar transistor arrangement, or any other suitable fast switching device or circuit.

To switch the MOSFET on, the fast switching device is switched on electronically which rapidly transfers sufficient charge from the capacitor 14 to the gate 20 of the MOSFET, to switch the MOSFET on.

Time diagrams for the circuit in figure 1 are shown in figures 3 and 4. The diagrams in broken lines indicate normal specified operation of the MOSFET 12. Hence, diagram 30 in figure 3 shows the gate voltage of the MOSFET during conventional switching on. The

MOSFET is switched on at 32 and the diagram illustrates a turn-on delay time of about 34ns. The associated gate current is shown at 34 in figure 4.

The diagrams for the method according to the invention are shown at 36 and 38 in figures 3 and 4 respectively. At 40 in figure 3, the aforementioned rapid transfer of charge from capacitor 14 through switch 16 and consequent build up of voltage on the gate of the

MOSFET are shown. The subsequent fall in the gate voltage shown at 42 is attributable to the aforementioned Miller effect. What is clear though is that the device switches on at 44, after a mere 4ns.

The associated current at the gate 20 is shown at 38 in figure 4. . Initially, during the charge transfer stage, the gate current is high

. and thereafter it drops to a negligible level. It is also believed that . with drain currents within the data specification of the MOSFET, switching losses with the switching method and circuit according to the invention are also reduced.

The value (C) of the capacitor 14, the voltage (V,.) required on the capacitor before switching and hence the breakthrough voltage of the switching device 16, the gate threshold voltage (V,) of the MOSFET 12 and the gate charge (Q,) required for complete switching of the

MOSFET are related according to the following a equation eer eed

In figure 2 a circuit diagram of the triggering circuit 10 is shown wherein the first switching device 16 is a SIDAC.

A periodic voltage is applied across a capacitor 14, in parallel with a

SIDAC 16 and a MOSFET 12. Initially, during a first half cycle, the voltage supplied at the input 19 is insufficient to switch the SIDAC 16 on and the capacitor 14 is hence charged up. When the supplied voltage reaches the threshold of the SIDAC 16 it switches on, resulting in a closed circuit from the capacitor 14 to the gate 20 of

AMENDED SHEET

} the MOSFET 12, partially discharging the capacitor 14 and hence charging the gate 20. The result is that a charge will now be shared between the capacitor 14 and the gate 20, so that some voltage, preferably above the gate threshold voltage relative to ground, is applied to the gate.

Using this method, the gate voltage may for short intervals be driven approximately three to four times beyond the maximum rating of some MOSFET’s 12 without destroying the device. :

Ba Similarly, when during the other half cycle the gate voltage exceeds the reverse threshold of the SIDAC 16 and current is conducted in the opposite direction, the gate voltage of the MOSFET 12 drops to substantially below the threshold voltage of the MOSFET 12 shortly after the charge dissipates from the gate 20 of the MOSFET 12. As a result, the MOSFET 12 will turn off and the drain current will no longer flow.

In figure 5, an alternative and self-oscillating triggering circuit for the

MOSFET 12 is shown. Components thereof corresponding to components of the circuits in figures 1 and 2 are designated utilizing

AMENDED SHEET like reference numerals. In this embodiment, the fast switching means 16 comprises a bipolar transistor arrangement.

The voltage waveform at 50 is shown in figure 6. The voltage waveform at source 24 is shown in figure 7. From the latter waveform it can be seen at 52 that the source 24 of the aforementioned MOSFET 12 switches between an “off”-state to an “on” -state through about 400V in a rise time t, of about 4ns, which is substantially quicker than the specified rise time of 27ns.

Similarly, and as shown at 54 it switches from the “on” -state to the “off” -state in a fall time t, of about 15ns, which is also substantially } oo | | shorter than a specified fall time of about 24ns.

In figure 8, the same triggering circuit 10 is shown for an insulated gate bipolar transistor 60 having a gate 62, a collector 64 and an emitter 66. The transistor is an IRG4PC50W device which is being manufactured and sold by International Rectifier. The waveform at 68 in figure 8 is shown in figure 9 and the waveform at emitter 66 adjacent load 70 is shown in figure 10.

From the latter waveform it can be seen at 72 that the emitter 66 - switches between an “off”-state and an “on”-state through about

400V in a rise time t, of about 4ns, which is substantially less than a specified rise time of 33ns. in figure 11 a further embodiment of the triggering circuit is shown.

The switching means comprises a low output impedance, high voltage, fast switching driving circuit 116. The device 116 must be able to switch between OV and Vd in a first time period shorter than a specified turn-on delay time of the device 12. Vd is preferably bigger than 20xVt. Devices of this nature are available on the market. oo it will be appreciated that there are many variations in detail on the triggering circuit and method according to the invention, without departing from the scope and spirit of the appended claims.

Claims (21)

1. © Wwootes7es PCT/ZA01/00024 oC 13

   . CLAIMS:

   } 1. A method of driving an insulated gate semiconductor device comprising as a first terminal a gate and further comprising at least second and third terminals, wherein charge storage means and switching means are connected in a circuit to the gate of the semiconductor device to apply a charge pulse to the gate of the semiconductor device so as to switch the semiconductor device between one of an on state and an off state and the other of the on state and the off state, the duration of the pulse being such that the pulse is substantially complete prior to switching of the semiconductor device.
2. 2. A method according to claim 1 wherein the insulated gate semiconductor device is a metal oxide semiconductor field effect transistor (MOSFET).
3. 3. A method according to claim 2 wherein the MOSFET is a power MOSFET.
4. 4. A method according to claim 1 wherein the insulated gate semiconductor device is an insulated gate bipolar transistor. ANENDED SHEET
5. . 5. A method according to any preceding claim wherein the charge , pulse raises a gate voitage of the insulated gate semiconductor device three to four times beyond a maximum rating of the semiconductor device.
6. 6. A method according to any preceding claim wherein the switching means is connected between the storage means and the gate of the semiconductor device.
7. 7. A method according to any preceding claim wherein the charge : storage means is connected in parallel with the switching means and the insulated gate semiconductor device.
8. 8. A method according to any one of claims 1 to 6 wherein the charge storage means is connected in series with the switching means and the insulated gate semiconductor device.
9. 9. A method according to any preceding claim wherein the switching means is a SIDAC.
10. 10. A method according to any one of claims 1 to 8 wherein the switching means is selected from a break-over diode, a bipolar AMENDED SHEET

    B WO 01/63763 PCT/ZA01/00024 } transistor, a further insulated gate semiconductor device and a high voltage fast switching device.
11. 11. A method according to any preceding claim wherein the switching means is electronically controllable.
12. 12. A method according to any preceding claim wherein the charge storage means comprises a capacitor.
13. 13. A method according to any preceding claim wherein an inductor is provided between the switching means and the gate.
14. 14. A method according to any one of claims 1 to 12 wherein an inductor is connected in series with the switching means.
15. 15. A method according to any preceding claim wherein the circuit is integrated on a single chip.
16. 16. A circuit for driving an insulated gate semiconductor device comprising as a first terminal a gate and further comprising at least second and third terminals, the circuit comprising charge storage means and switching means connected to the gate of "NDED SHEET

    . the semiconductor device to apply a charge pulse to the gate of the semiconductor device so as to switch the semiconductor device between one of an on state and an off state and the other of the on state and the off state, the duration of the pulse being such that the pulse is substantially complete prior to switching of the semiconductor device.
17. 17. A method of driving an insulated gate semiconductor device comprising a gate, wherein charge storage means and switching means are connected in a circuit to the gate of the : semiconductor device to apply from the charge storage means to the gate a charge pulse causing a voltage on the gate which is beyond a maximum rating of the semiconductor device.
18. 18. A method as claimed in claim 17 wherein the voltage is three to four times beyond the maximum rating of the semiconductor device.
19. 19. A circuit for driving an insulated gate semiconductor device comprising a gate, the circuit comprising charge storage means and switching means connected to the gate of the semiconductor device for applying from the charge storage AMENDED SHEET

    ' © WO 01/63763 PCT/ZA01/00024 ' 16(a) . means to the gate a charge pulse causing a voltage on the gate which is beyond a maximum rating of the semiconductor device.
20. 20. A method of driving an insulated gate semiconductor device substantially as herein described with reference to the accompanying diagrams.
21. 21. A circuit for driving an insulated gate semiconductor device - substantially as herein described with reference to the : accompanying diagrams. AMENDED SHEET