MOSFET CIRCUITS

Field of the Invention

The present invention relates to a field of power electronics, and particularly to circuits that incorporate MOSFET devices.

Background

MOSFET (Metal Oxide Semiconductor Field Effect Transistor) devices are used widely in power electronics applications. They are a near-perfect switch that exhibit resistive characteristics and require very low drive power. A MOSFET device has three terminals: a drain, a source and a gate. A switch-on (or drive) voltage is applied between the gate and the source, typically +5 to +10 volts. In the on-state, the direction of conventional current flow is from the drain to the source.

All MOSFET devices exhibit the characteristic of a parasitic diode (also known as a body diode) existing in parallel with the drain-source conduction channel. The parasitic diode is an essential part of the construction and operation of a MOSFET device. In most circuit topologies this parasitic diode does not present a problem as the diode always is reverse biased. However, in the full- or half-wave bridge topologys, the parasitic diode does present a problem, particularly when the bridge is supporting a reactive load. That is, if the parasitic diode is biased into conduction in the forward direction, then when the

MOSFET device is switched off the parasitic diode is intrinsically slow, and the bridge circuit will have a reverse recovery time measured in 100's of nanoseconds. This reverse recovery time must be allowed to pass before another MOSFET device can be switched on, else a short circuit condition will exist. This effectively places an upper limit on the switching frequency of MOSFET circuits. This problem therefore is exacerbated as ever higher switching speeds are desired for MOSFET device circuits.

One solution to this problem is shown in Fig. 1, which is a full-wave bridge inverter circuit 10 having a DC supply 11. The four MOSFET devices 12 - 15 (Ql - Q4) indicate a respective parasitic diode 16 - 19, and a respective silicon Shottky diode 20 - 23 in a shunt arrangement across the respective drain (D) and source (S) terminals. Also shown are the power supply smoothing capacitor 29 (Cl), and a load having real and reactive components in the form of resistor 27 (Rl) and an inductor 28 (Ll), respectively. This

technique works well in low-voltage designs (e.g. of less than 40 volts), because the Shottky diodes 20 - 23 have a forward voltage that is less than the forward voltage of the respective parasitic diodes 16 - 19, and essentially zero reverse recovery time. Current thus preferentially flows through the relevant Shottky diode, in this case the diode 22 associated with the device 14 (Q3). The Shottky diodes 20 - 23 have essentially zero reverse recovery time, and thus do not produce a short circuit when the respective MOSFET device 12 - 15 (Ql - Q4) is being turned-on.

Silicon Shottky diodes are useful to about 150 volts DC. Therefore, for inverter or converter circuits with DC supply voltages of greater than, say 100 volts, another solution is required. Such a solution is shown in Fig. 2. Here, a full- wave bridge circuit 30 further includes Shottky diodes 32 - 35 in series with the respective source (S) terminal of the respective MOSFET device 12 - 15 (Ql - Q4). However, the losses in the series diodes

32 - 35 are severe, in that they must carry all of the load current and typically will have a forward voltage drop of about 0.5 to 1.5 volts. This will mean that for a square wave converter with a current of 10 A, the losses in the series diodes 32 - 35 could be about 30 watts, which is significant.

It is the object of the present invention to overcome or at least reduce one or more of the foregoing problems.

Summary

There is disclosed a MOSFET circuit including a circuit branch having a first MOSFET device with a drain-source voltage withstand rating of at least a maximum applied DC circuit voltage, and a second MOSFET device with a drain-source voltage withstand value substantially less than the said first device, said first and second MOSFET devices being arranged such that the respective gate terminals and the respective source terminals are connected.

Preferably, the drain-source voltage withstand value of said second device is of the order of the forward voltage of a diode. A Shottky diode can be connected in parallel with the respective drain terminals of the first and second devices. Furthermore, it is a reactive load that is switched by the circuit branch.

The circuit branch can be incorporated in switching circuits, including half- or full-wave bridge circuits.

Brief Description of the Drawings Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings wherein:

Figs. 1 and 2 show known full-wave bridge circuit arrangements incorporating MOSFET devices.

Figs. 3 and 4 show a full-wave bridge circuit, incorporating MOSFET devices, embodying the invention.

Fig. 5 shows a circuit branch in another embodiment.

Detailed Description

A MOSFET full-wave bridge inverter circuit 50 embodying the invention is shown in Figs. 3 and 4. It will be appreciated that numerous other embodiments of the invention are equally possible in circuits such as rectifiers, inverters and voltage converters, whether half- or full- wave types. Embodiments of the invention are generally applicable where a circuit has a turn-off reactive load current requiring commutation.

hi the circuit 50, the elements in common with the circuits 10 and 30 of Figs. 1 and 2 are represented by the same numeral. The switching drive circuitry for the gate terminals of the respective MOSFET devices is not shown, but would be readily understood by a person skilled in the art. However, explained breifly, MOSFET devices are switched ON by applying a voltage between the source and the gate terminals. To switch a MOSFET device OFF, the gate and source terminals are shorted together. The switching voltage typically is provided by an isolated source.

hi the circuit 50, the respective series diodes 32 - 35 of Fig. 2 are replaced by MOSFET devices 52 - 53 (Q2, Q4, Q6 and Q8). (Note that devices Q2, Q3 and Q4 of Fig. 2 are relabelled in Fig. 3 as Q3, Q5 and Q8, respectively). The DC supply voltage 11 is of the

order of 400 V and the devices 12 - 15 (Ql, Q3, Q5 and Q7) must be rated to withstand this voltage, plus a margin, and therefore, in one example, have a voltage withstand value, V _DS , of 650 V.

Taking, by way of example, the circuit branch formed by the device pair 12 (Ql) and 52 (Q2), it can be seen that they are in a reversed series configuration such that the two MOSFET devices have their respective gate (G) and source (S) terminals connected. The device 52 (Q2), is not required to withstand the full DC voltage 11, rather need only be rated at a voltage slightly above the forward voltage of Shottky diode 21 (Dl), which is always less than 1.8 volts. A practical small MOSFET device typically is rated at a voltage withstand value of about 20 volts, and thus can easily block the voltage drop of a Shottky diode. The device 52 (Q2) (and similarly the devices 54, 56 and 58 (Q3, Q6 and Q7)) therefore have a drain-source voltage withstand value that is substantially less that devices 12 - 15 (Ql, Q4, Q5 and Q8). In this way, the MOSFET device 12 (Ql) advantageously can have a R _DS(Oπ) value of 0.002 ohms, whereas the higher rated device 52 (Q2) will have an R _DS(Oπ ) value of about 0.2 ohms.

Assume now that devices 12, 52, 58 and 18 (Ql, Q2, Q7 and Q8) all are turned-on at the same time, resulting in current (of say 10 A) flowing through the load 27, 28 as particularly shown in Fig. 3. Assuming also the values of R _DS(Oπ) mentioned above, then the power dissipated by the devices 12, 18 (Ql and Q8) is 20 watts, whereas the power dissipated in devices 52, 58 (Q2 and Q7) is 0.2 watts.

Assume now, at a later time, that the conducting devices are to be turned-off by removing the gate-source drive voltage to devices 12, 52, 58, 18 (Ql, Q2, Q7 and Q8) then the inductive nature of the load means that current will continue to flow. Referring now particularly to Fig. 4, this commutation current can not flow through the respective parasitic diode 16 - 19 of devices 12, 53, 58, 15 (Ql, Q2, Q7 and Q8) since the lower- rated devices 13 (Q2) and 58 (Q7) are reversed, and therefore the respective body diode 60, 63 is blocking. The current thus is diverted to the Shottky diodes 21 (D2) and 22 (D3) to return to the DC supply 11.

The circuit arrangement of Figs. 3 and 4 has the benefit of the parasitic diodes (e.g., the diodes 16, 60 of the MOSFET devices 12 and 52 (Ql and Q2) never coming into a

conduction state. At any time, one of the 'pair' will be reverse biased, and the other will be short-circuited by its respective drain-source channel.

The energy losses in the circuit 50 are associated with the forward conduction of the Shottky diodes 20 - 23 (Dl - D4), but are only present during the turn-off event, therefore are relatively small. This can mean that real circuits incorporating MOSFET devices can operate with estimated switching efficiencies of perhaps 99.5%, as opposed to around 95% in the known circuits described.

Fig. 5 shows another embodiment of a circuit branch 70. Again, two MOSFET devices 72 (Q9) and 74 (QlO) are connected such that the respective gate terminals (G) and source (S) terminals are connected. A gate drive circuit 76 also is shown. The MOSFET device 72 (Q9) has a body diode 78. The MOSFET device 74 (QlO) similarly has a body diode 80. As is the case for the circuit of Figs. 3 and 4, the circuit branch 70 of Fig. 5 includes a Shottky diode 82 (D5) connected across the drain terminals (D). The MOSFET device 74 (QlO) has lower drain-source voltage withstand value than the MOSFET device 72 (Q9).

hi Fig. 5, a further low voltage Shottky diode 84 (D6) is shown connected across the drain and source terminals (D, S) of the MOSFET device 74 (QlO). The Shottky diode 84 (D6) protects the MOSFET device 74 (QlO) against overvoltage due to the displacement currents that can flow in the parasitic capacitances of the two MOSFET devices 72 (Q9) and 74 (QlO). This diode 84 (D6) will preferentially conduct over the internal body diode

80 of the MOSFET device 74 (QlO), as it is a low voltage diode with a typical "on" voltage of <0.5 V. Thus, the internal body diode 80 of the MOSFET device 74 (QlO) is never turned on, and thus is not subject to reverse recovery limitations.