GB2056195A - High frequency converter having starter - Google Patents

Abstract

The converter includes direct voltage (DV) input supply terminals providing a centre-tapped (19) DV supply, a pair of solid-state switching devices (20, 21) connected in series across the supply terminals, a load circuit including a supersonic frequency transformer (30), an oscillatory circuit including at least one capacitor (78, 79), and an oscillatory inductor unit (51, 52) including at least one inductor connected to the primary winding (37) of the transformer in a circuit between the centre-tapping of the supply and a point between the switching devices, and synchronising means synchronising the operation of the switching devices to the oscillation of the oscillatory circuit to allow the circuit to oscillate freely for part of a cycle to transfer energy from the inductor to the capacitor, to charge the latter to a certain condition in which the voltage across the inductor is substantial and that across one of the switching devices is negligable, and only then actuate the said switching device to connect the inductor across half of the DV supply, whereupon the inductor receives and stores energy from the said supply. A saturable starting inductor (75) in parallel with a free-wheeling diode (76) and a resistor (77), is connected between and in series with the switching devices. The converter may be used for charging a battery (45). <IMAGE>

Description

SPECIFICATION High frequency charger switchless starter This invention relates to high frequency converters and is particularly applicable, although not limited, to such converters incorporated in electric battery charging apparatus supplied from a mains supply. In the case of a high powered charger the weight and bulk of a transformer for mains frequency will be very substantial but if the frequency involved is increased to a supersonic value, the weight and bulk are greatly reduced. In the case of a battery driven vehicle this may make it practical to carry a charger on the vehicle.

The present applicants' British Patent specification No. 26880/77 (Case EPS 287) describes converter circuits incorporating an oscillatory circuit and solid-state switching devices synchronised with it so that each switching device turns on at a favourable moment when the voltage across it is negligable. The circuits described require a starting circuit incorporating a further solid-state switching device. An object of the present invention is to provide a circuit which will be self starting without the necessity for a solid-state switching device.

The present invention is concerned with a converter including direct voltage input supply terminals providing a centre-tapped direct voltage (DV) supply, a pair of solid-state switching devices connected in series across the supply terminals, a load circuit including a supersonic frequency transformer, an oscillatory circuit including at least one capacitor and an oscillatory inductor unit including at least one inductor connected to the primary winding of the transformer in a circuit between the centre-tapping of the supply and a point between the switching devices and synchronising means synchronising the operation of the switching devices to the oscillation of the oscillatory circuit to allow the circuit to oscillate freely for part of a cycle to transfer energy from the inductor to the capacitor, to charge the latter to a certain condition in which the voltage across the inductor is substantial and that across one of the switching devices is negligable, and only then actuate the said switching device to connect the inductor across half of the DV supply, whereupon the inductor receives and stores energy from the said supply.

The present invention is characterised by a saturable starting inductor, in parallel with a freewheeling diode, connected between and in series with the switching devices. A resistor is preferably also connected in parallel with the starting inductor.

Preferably the oscillatory circuit comprises a pair of capacitors connected in series across the supply with their junction connected to the junction between the load and oscillatory circuit and the saturable starting inductor.

In the accompanying drawing the single figure is a circuit diagram of one specific embodiment of the invention, as applied to the circuit of Figure 3 of the present applicants' British Patent Specification No.7907036 (Case EPS 316).

Further details may be as described in that specification.

The circuit comprises DV supply terminals 10 and 11 derived from an AC supply 1 5 and an input rectifier 16. A pair of supply capacitors 17 and 1 8 are connected in series across the terminals 10 and 11 to provide a centre-tapping 19.

A pair of main transistors 20 and 21 are also connected across the supply terminals 10 and 11 in series with one another and with a starting saturable inductor 75 connected between them. A resistor 76 and a freewheeling diode 77 are connected in parallel with the starting saturable inductor.

An oscillatory circuit comprising an oscillatory inductor unit 34 is connected between the centretapping 1 9 of the supply and one end 25 of the starting inductor 75.

With a single oscillatory inductor 34 it has been found that changes in load current cause the power supply to become slightly unbalanced, thus imposing a bias on the inductor whose current then tended to become unidirectional over several cycles until differences in the supply capacity loadings caused them to correct, and then over correct, thus causing oscillation.

In the arrangement shown, in order to allow for lack of balance or symmetry of the circuit, the parallel inductor unit 34 comprises two branches, each branch comprising a small inductor 51 or 52 in series with a diode 53 and 54, the diodes being poled so as to conduct in opposite directions. If desired each diode may be snubbed by a series combination of a resistor 55 and capacitor 56.

This arrangement ensures that there is a good supply of energy available to charge the commutating capacitor.

In parallel with the oscillatory inductor unit is a saturable inductor 33 in series with the primary winding 37 of a supersonic frequency transformer 30 having a centre-tapped secondary winding 44 connected to a load comprising diodes 43, an inductor 46 and a battery 45 to be charged.

The oscillatory circuit also comprises one or more capacitors effectively connected in parallel with the inductor unit. These may comprise a single capacitor connected directly in parallel with the inductor unit but it is preferred to provide capacitors associated more closely with the transistors 20 and 21. The prior specifications referred to described various arrangements in which the capacitor of the oscillatory circuit is connected directly across one of the transistors.

This has the advantage that if the transistor has a very short turn-off time there may be a considerable rate of change of current in stray inductances, which, if not by-passed, may cause a voltage spike to appear across the transistor at the instant of switch-off and may exceed the sustaining voltage of the transistor and result in secondary breakdown.

The preferred arrangement employs two capacitors 78 and 79 connected in series across the supply terminals 10 and 11 with their junction connected to the junction 25 between the starting inductor 75 and the oscillatory inductor unit 34.

Thus the capacitor 79 is directly in parallel with the transistor 21, while the capacitor 78 is in parallel with the transistor 20 in series with the starting inductor 75. Thus rapid changes of current across both transistors are by-passed.

Since the supply capacitors 17 and 18 offer negligable impedance at supersonic frequencies the capacitors 78 and 79 are effectively in parallel with the oscillatory inductor unit.

The general operation of the circuit is described in more detail in the specifications referred to above and is briefly as follows. When the transistor 20 is conducting, half the supply voltage is applied across the inductor unit 34 of the oscillatory circuit and the primary winding 37 of the transformer 30, and current builds up in the inductor. When the transistor 20 is turned off the current is inevitably high but the inductor current is diverted to the capacitors 78 and 79 and this limits the rate of rise of voltage across the transistor 20. With both transistors cut off the oscillatory circuit performs a free vibration for nearly half a cycle. The capacitors 78 and 79 effectively discharge and charge in the opposite direction so that the voltage across the transistor 21 is low.A synchronising circuit responds to the state of the oscillatory circuit and turns on the transistor 21 when the voltage across it is negligable. With the transistor 21 conducting, the current builds up in the oscillatory inductor unit 34 in the opposite direction, until the transistor 21 turns off and the inductor current is diverted to the capacitors 78 and 79 which again limits the rate of rise of voltage across the transistor 21. With both transistors off the oscillatory circuit executes nearly half a cycle of free vibration effectively discharging the capacitors 78 and 79 and charging them in the opposite direction so that the voltage across the transistor 20 is negligable and only then the synchronising circuit turns on that transistor.

In the prior circuits, a separate starting circuit including a switching device is provided for initiating the operation described above.

In the present circuit with the saturable starting inductor 75, resistor 76 and freewheel diode 77, the oscillator is self-starting without the need for any separate switching device, the operation being as follows.

Starting is effected by switching on the transistor 20 with the potential at the junction 25 of the capacitors 78 and 79 being one half of the DV supply. In the absence of the reactor 75, the transistor 20 would be called upon to directly discharge the capacitor 78 and charge the capacitor 79, with no means of current limitation, device damage will occur. The reactor 75 is provided, which in its unsaturated state exhibits a high impedance, and thus discharge of the capacitor 78, charge of the capacitor 79 takes place either through the resistor 76 or through the unsaturated reactance of the inductor 75. It is an essential requirement that this process be completed prior to reactor saturation, and the inter-relationship between the components is chosen to ensure this. After a short time the inductor becomes saturated and effectively shortcircuits the resistor 76.

When the transistor 20 is turned off current in the inductor 75 is diverted to flow through the freewheeling diode 77 and this can continue to keep the inductor 75 in or close to saturation throughout the remainder of the cycle.

In this way the starting circuit effectively shortcircuits itself once the converter is oscillating.

Claims (4)

1. A converter including direct voltage (DV) input supply terminals providing a centre-tapped DV supply, a pair of solid-state switching devices connected in series across the supply terminals, a load circuit including a supersonic frequency transformer, an oscillatory circuit including at least one capacitor, and an oscillatory inductor unit including at least one inductor connected to the primary winding of the transformer in a circuit between the centre-tapping of the supply and a point between the switching devices, and synchronising means synchronising the operation of the switching devices to the oscillation of the oscillatory circuit to allow the circuit to oscillate freely for part of a cycle to transfer energy from the inductor to the capacitor, to charge the latter to a certain condition in which the voltage across the inductor is substantial and that across one of the switching devices is negligable, and only then actuate the said switching device to connect the inductor across half of the DV supply, whereupon the inductor receives and stores energy from the said supply, characterised by a saturable starting inductor in parallel with a free-wheeling diode, connected between and in series with the switching devices.

2. A converter as claimed in Claim 1 including a resistor connected in parallel with the starting inductor.

3. A converter as claimed in Claim 1 or Claim 2 in which the oscillatory circuit includes a pair of capacitors connected in series across the supply terminals with their junction connected to the junction of the oscillatory inductor unit and the starting inductor.

4. A converter as specifically described herein with reference to the accompanying drawing.