Note: Descriptions are shown in the official language in which they were submitted.
CA 02243462 1998-07-17
WO 97/26705 PCT/GBg7/00099
AN OSCILLATOR
T'nis invention relates to an oscillator, and particularly, though not exclusively,
to an oscillator for driving electrodeless b7~k1ight~.
A number of low voltage oscillators have been described which use power
FETs (field effect tr~n~i~tors) as the active devices. The power supply voltage used is
t 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 on-
resistance of 0.1Q, and a m~xim-lm 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 re~luired 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 M~Iz 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
2~. Since at 10 MHz there is a significant phase delay through the transistors, the gate
drive circuit has to incorporate a comF.on~t~ng 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 lirniter which in operation introduces a phase shift to the
resonant circuit and returns energy to the power supply. This arr~n~ement is very
simple and results in reduced gate drive losses.
The invention will now be described, by way of example only, with reference
30 to the accoll~allying drawing in which
Figure 1 shows a circuit f~ m of a circuit arrangement according to the
invention.
The components shown in the Figure 1 are as listed in table 1 below.
CA 02243462 1998-07-17
WO 97/26705 PCT/GB97/00099
TABLE 1
Reference Number Component Type Rating/Serial Number
Inductive Load
2 Capacitor 100pf, 6kV
3 Transformer (1) Winding 1 turn
4 Transformer (1) Winding 3 turns
Transformer (1) Winding 3 turns
6 Transformer (1) Winding 3 turns
7 Transformer (1) Winding 3 turns
8 T~ r~ ller (2) Winding 7 turns
9 Tran~rull.. el (2) Winding 7 turns
Capacitor 47nf
11 Capacitor 47nf
12-15 Diodes BAT49
16-18 FETs IRL~ 014
19 Transistor 2N2222
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 b~ light envelope. Current in 1
causes a gas discharge in the envelope to strike, res~ll*n~ in the emission of light in
operation. Power absorbed by the load causes component 1 to have a correspondingresistive component.
Components 1 and 2 are automatically driven very close to resonance by the
phase shifts in the circuit, and thus define the ope~dting 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.
CA 02243462 1998-07-17
WO 97/26705 PCT/GB97/00099
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 forrns. Diodes 14 and 15 have a subsidiary function in that, due to
the transformer action of the transformer compri~ing components 3 - 7, they
effectively lirnit 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
10 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 atroughly the same frequency. This is ensured by arranging for the magnetising
in(l~lc~nce of the first transformer (3 - 7) to resonate with the input capacitance of the
15 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 tr~n~i~tors 16 or 17 to turn on. Other ways of starting the
20 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 in~n~lerl for baclclight control, since
controlling the on/off ratio of the oscillator, at a repetition rate of perhaps 200 Hz,
conveniently controls the bri~htn~s~. In practice the brightness can be varied over a
range of at least 1000:1 using an appropliate pulse time modulation scheme. Control
means 33 is provided to introduce electrical control signals to the oscillator when it is
30 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.
