GB2309344A - HF FET oscillator for electrodeless backlight - Google Patents

Description

AN OSCILLATOR This invention relates to an oscillator, and particularly, though not exclusively, to an oscillator for driving electrodeless backlights.

A number of low voltage oscillators have been described which use power FETs (field effect transistors) as the active devices. The power supply voltage used is typically 12 or 28 volts.

A known characteristic of power FETs is their high input capacitance, which must be charged and discharged on every clock cycle of the oscillator. As an example, transistor type RFD14N05 (which is manufactured by Harris) has an onresistance of 0. O.1Q, and a maximum drain voltage of 50 volts, which suits it for use in 12 volt inverters or oscillators, supplying perhaps several tens of watts in a push-pull configuration. However, the charge required to switch on the gate of a single RFD14N05 is typically 25 nanocoloumbs. If this charge comes directly from a +12 volt supply at the 10 MHz rate required for a 10 MHz inverter or oscillator, then the gate drive power alone is 3 watts per transistor, or 6 watts per transistor pair.

As well as the difficulty of driving the gate electrodes, there is a further known problem relating to the loop phase shift of self oscillating circuits. As is well known, a stable oscillator has a loop phase shift which is either zero or an integral multiple of 2x. Since at 10 MHz there is a significant phase delay through the transistors, the gate drive circuit has to incorporate a compensating phase advance, which must involve a resistive loss. This in turn adds to the gate drive losses.

According to the invention, there is provided an oscillator, capable of delivering power from a d.c. power supply to a load at frequencies above 100 kHz, comprising a pair of field effect transistors operating in anti phase in a resonant circuit, and a voltage limiter which in operation introduces a phase shift to the resonant circuit and returns energy to the power supply. This arrangement is very simple and results in reduced gate drive losses.

The invention will now be described, by way of example only, with reference to the accompanying drawing in which Figure 1 shows a circuit diagram of a circuit arrangement according to the invention.

The components shown in the Figure 1 are as listed in table 1 below.

TABLE 1 Reference Number Component Type Rating/Serial Number 1 Inductive Load 2 Capacitor 100pf, 6kV 3 Transformer (1) Winding 1 turn 4 Transformer (1) Winding 3 turns 5 Transformer (1) Winding 3 turns 6 Transformer (1) Winding 3 turns 7 Transformer (1) Winding 3 turns 8 Transformer (2) Winding 7 turns 9 Transformer (2) Winding 7 turns 10 Capacitor 47nf 11 Capacitor 47nf 12-15 Diodes BAT49 16-18 FETs IRLL 014 19 Transistor 2N2222 20 Resistor 5 ohms Transistors 16 and 17, together with adjacent components 2 - 11, 14 and 15 comprise the oscillator. Transistors 18 and 19, and components 12, 13 and 6, allow the oscillator to be turned on and off at will. Apart from that, the latter components take no part in the oscillation.

The component 1, as well as being an inductor, incorporates the load. In practice, it consists of a spiral coil adjacent a glass backlight envelope. Current in 1 causes a gas discharge in the envelope to strike, resulting in the emission of light in operation. Power absorbed by the load causes component 1 to have a corresponding resistive component.

Components 1 and 2 are automatically driven very close to resonance by the phase shifts in the circuit, and thus define the operating frequency. Feedback involves the reactive component, capacitor 2, which feeds a current through reactive winding 3, which in turn couples to 4, 5 and 7. The two transformers shown (one comprising windings 3 - 7, the other of windings 8 and 9) were bi-filar wound on 9.4 mm o.d.

toroids of 4C65 ferrite, manufactured by Philips. Other reactive components in the feedback loop are the input capacitance of transistors 16 and 17, and the magnetising inductance of the transformer having windings 3 - 7.

Diodes 14 and 15 conduct on each half cycle of the oscillation, and return oscillator energy to the power supply (the terminals labelled 30 in the Figure are connected to the zero volt output of a d.c. power supply (not shown) whilst the terminals labelled 40 are connected to the + 12 volt output). This causes a phase shift in the gate wave forms. Diodes 14 and 15 have a subsidiary function in that, due to the transformer action of the transformer comprising components 3 - 7, they effectively limit the gate drive voltage to transistors 16 and 17, thus protecting the transistors. However, their primary function according to the invention is to provide a phase shift. Components 10, 11, 7, 14, and 15 comprise a diode clipper circuit 35 being inductively coupled to the oscillator.

On initial start up, before the oscillator has entered the large signal mode characterised by conduction of diodes 12 and 13, the circuit should oscillate at roughly the same frequency. This is ensured by arranging for the magnetising inductance of the first transformer (3 - 7) to resonate with the input capacitance of the two transistors 16, 17 at a frequency somewhat above the intended oscillator frequency; the exact value is not critical.

Transistor 19 allows the oscillator to be started controllably, by applying a positive going pulse of roughly 50 ns width to its gate. This injects current through 6, thereby causing one of transistors 16 or 17 to turn on. Other ways of starting the oscillator, such as biasing the gates of 16 and/or 17, will be obvious to one skilled in the art.

Transistor 18 allows the oscillator to be stopped controllably. By applying a positive level to the gate of 18, it turns on 18 and shorts the gates of transistors 16 and 17 to 0V.

This remote stop and start system is intended for backlight control, since controlling the on/off ratio of the oscillator, at a repetition rate of perhaps 200 Hz, conveniently controls the brightness. In practice the brightness can be varied over a range of at least 1000:1 using an appropriate pulse time modulation scheme. Control means 33 is provided to introduce electrical control signals to the oscillator when it is being used as part of such a pulse time modulation drive system for an electrodeless discharge lamp. Such signals from outputs 32 and 31 switch the oscillator on and off respectively as required.

Oscillator frequencies in the range 1 - 20 MHz are preferred, very preferably in the range 5-15 MHz.

Claims (6)

1. An oscillator, capable of delivering power from a d.c. power supply to a load at frequencies above 100 kHz, comprising a pair of field effect transistors operating in anti-phase in a resonant circuit, and a voltage limiter which in operation introduces a phase shift to the resonant circuit and returns energy to the power supply.

2. An oscillator as claimed in claim 1 in which the voltage limiter is inductively coupled to the resonant circuit.

3. An oscillator as claimed in claim 1 or claim 2 in which the voltage limiter comprises a diode clipper.

4. A circuit arrangement, comprising an oscillator as claimed in any preceding claim, means for starting the oscillator in response to a first signal, and means for stopping the oscillator in response to a further signal

5. A circuit arrangement as claimed in claim 4 in which the means for starting the oscillator comprises a transformer magnetising inductance which resonates with the input capacitances of the pair of field effect transistors at a frequency above the oscillation frequency of the oscillator in operation.

6. A pulse time modulation drive system for a discharge lamp, comprising a circuit arrangement as claimed in claim 4 or claim 5 and control means for introducing said first and said further signals to said circuit arrangement according to a pulse time modulation scheme.