GB2254496A

GB2254496A - Switched mode a.c. voltage converter

Info

Publication number: GB2254496A
Application number: GB9105142A
Authority: GB
Inventors: Michael Gybbon-Monypenny; Jack Alfred Nicholson
Original assignee: MACEY ELECTRONICS Ltd
Current assignee: MACEY ELECTRONICS Ltd
Priority date: 1991-03-12
Filing date: 1991-03-12
Publication date: 1992-10-07
Also published as: GB9105142D0

Abstract

A switched mode voltage converter for the control of an a.c. supply voltage has input terminals for connection to an a.c. supply 16 and output terminals for connection to a load such as a motor. A first bidirectional switch Sa, and an inductor L are connected in series between the first input terminal and the first output terminal. A second bidirectional switch Sb is connected between the common second input and output terminals and a node between the first bidirectional switch Sa and the inductor 11. A capacitor C2 is connected across the output terminals and control means 18 is provided to control the alternate switching of the bidirectional switches Sa, and Sb. The output voltage is thus dependent on the duty cycle of the bidirectional switches Sa and Sb. Each bidirectional switch comprises a diode bridge D1-D4 or D5-D8 and a FET Q1 or Q2. The control circuit 18 includes an integrated circuit ICI pulse width modulation controller. <IMAGE>

Description

SWITCHED MODE A.C. VOLTAGE CONTROLLER The present invention relates to a switched mode voltage controller for the control of an a.c. supply voltage.

Switched mode voltage controllers for switching regulators are widely used for the control of d.c. power supplies (see for example US 4713742). These voltage controllers have the advantage over conventional series pass regulators in that they have a lower power dissipation. An example of a simple prior art switched mode d.c. voltage controller is illustrated in Figure 4.

In such an arrangement a d.c. power source 1 is rapidly switched by a switch 2 under the control of controller 3 to provide a square wave output. This square wave output is filtered by a series inductor 4 and capacitor 5 to ground.

A commutating diode 6 is provided so that the output to load 7 represents a d.c. voltage with high frequency ripple which is dependent on the switching frequency of the switch 2. The output voltage to the load 7 can be varied by varying the duty cycle of the switch 2.

However, this circuit is limited to the regulation of d.c. power supplies.

The present invention provides a switched mode voltage controller for the control of an a.c. supply voltage, comprising input terminals for connection to an a.c. supply, output terminals for connection to a load, a first bidirectional switch and an inductor connected in series between a first input terminal and a first output terminal, a second bidirectional switch connected between common second input and second output terminals and a node between said first bidirectional switch and said inductor, a capacitor connected across said output terminals and control means to control the alternate switching of said bidirectional switches.

The provision of bidirectional switches in place of the unidirectional switch 2 and the suitably orientated diode 6, allows for the circuit to control the voltage of a.c. power supplies while maintaining the original frequency and waveform. The a.c. voltage waveform input to the controller need not be sinusoidal. Any a.c. voltage waveform can be controlled using the present invention.

Conveniently, the control means controls the duty cycle of the bidirectional switches to vary the voltage VO at said output terminals according to

where V. is the input voltage and t1 and t2 are the switching periods of said first and second bidirectional switches respectively. The control means can provide a pulse width modulated control signal of variable duty cycle and of high frequency with respect to the frequency of the a.c. supply. The frequency used can be as high as 2 MHz although preferably it is higher than the audible frequency of humans and domestic animals.

According to one aspect of the invention each bidirectional switch comprises a bridge rectifier input terminals thereof providing connection terminals of said bidirectional switch, and a field effect transistor connected across outward terminals thereof, the gate voltage of said field effect transistor being controlled by said control means.

The switched mode voltage controller can also include a series connected resistor and capacitor connected across said second bidirectional switch.

Examples of the present invention will now be described with reference to the drawings, in which: Figure 1 illustrates a switched mode a.c. voltage controller circuit according to one embodiment of the present invention; Figure 2 illustrates the type of bidirectional switches used according to one embodiment of the present invention; Figure 3 illustrates a switched mode fan speed controller circuit utilising an embodiment of the present invention; Figure 4 illustrates a switched mode d.c. voltage controller of the prior art.

Referring now to the drawings, in Figure 1 an input voltage V. is applied to first and second input terminals 10 and 11. A first bidirectional switch 5a is connected in series with an inductor L between the first input terminal 10 and a first output terminal 12. The second input terminal 11 is commonly connected to a second output terminal 13. A second bidirectional switch Sb is connected between the commonly connected second input and output terminals and a circuit node between the first bidirectional switch 5a and the inductor 11. The two bidirectional switches 5a and Sb are controlled by a control means 14 such that they are alternately switched.

The circuit also includes a series connected resistor R and capacitor C1 connected across the second bidirectional switch Sb. . Also, connected across the output terminals 12 and 13 is a second capacitor C2. Thus the circuit provides an output voltage VO across the output terminals 12 and 13 to which a load resistor RL can be connected.

Such a load could for instance be a motor. The output need not be restricted to connection to a resistive load. The output device could be capacitive or inductive in nature.

The operation of the circuit of Figure 1 will now be considered. First and second bidirectional switches Sa and Sb form a changeover, non-shorting contacts, switch.

The changeover frequency is high with respect to the a.c.

supply frequency, so that during a switching cycle the instantaneous values of the voltages at the output (VO) and the input (Vi), and also the load current (it) may be regarded as constant.

If t1 is the "on" time of the first bidirectional switch 5a and t2 is the "on" time of the second bidirectional switch Sb, then while the first bidirectional switch 5a is "on", the inductor current i changes by

while the second bidirectional switch 5b is "on", the load current iL changes by

In the steady state, (Ai)b = - (Ai)a from which

...(4) The above current changes are controlled by the correct choice of the inductance L and the switching period t1 and t2, The filtering capacitor C2 is chosen so that the output voltage ripple, produced by the current changes, is small enough for the intended usage, while allowing VO to follow V. according to equation (4).

In practice, there must be a finite interval between the "on" periods of the bidirectional switches 5a and 5b to avoid a short circuit. A snubber network comprising the resistor R and capacitor C1 connected in series across the second bidirectional switch Sb is required to limit the voltage change at the node between the two bidirectional switches Sa and 5b and the inductor L, which would otherwise occur during this interval.

Figure 2 illustrates the type of bidirectional switches 5a and Sb which can be used in the circuit of Figure 1. A bridge rectifier generally indicated by numeral 15 is connected such that its inputs A and B correspond to the circuit connections to switches 5a and Sb. Connected across the output of the bridge rectifier is a MOS-FET Q. The gate voltage G of the MOS-FET Q is controlled by the control means 14. Thus while the transistor is turned "on" by a suitable drive voltage to the gate, current can flow in either direction between points A and B.

Figure 3 illustrates a switched mode fan speed controller circuit utilising the switchmode a.c. voltage controller of one embodiment of the present invention. The circuit is designed to supply a driving voltage to an induction motor which drives a fan. The motor or fan speed is controlled by varying the voltage. In this circuit an unregulated input a.c. supply is applied at terminals L, N and E and is filtered by the radio frequency interference filter generally indicated by numeral 16, before being supplied to the two bidirectional switches 5a and Sb.

The components that comprise the first bidirectional switch 5a are diodes D1, D2, D3 and D4 forming the bridge rectifier and the first MOS-FET Q1. The components comprising the second bidirectional switch 5b are diodes D5, D6, D7 and D8 and the second MOS-FET Q2. The inductor L, series connected resistor R and first capacitor C1 and the second capacitor C2 are all connected in a similar manner to that illustrated in Figure 1.

The finite interval between the "on" periods of the bidirectional switches 5a and Sb can be achieved by utilising the slower "turn on" time compared to the "turn off" time of the gate drive circuit 19. The utilisation of this characteristic allows for both switches to be controlled by a single pulse from the control circuit 18 to the transformer T1. The gate drive of MOS-FET Q1 receives the inverse of the pulse received by MOS-FET Q2.

This arrangement is far more reliable than generating two pulses in the "active" part of the circuit. The gate drive circuit 19 operates as follows. For switching "on", the resistor in series with each gate in conjunction with the input capacitance of the gate, slows down the rise of the gate voltage. For switching "off", the parallel diode conducts allowing the gate voltage to fall more rapidly.

From the output of the radio frequency interference filter a d.c. supply is taken for the control circuit 18.

The d.c. supply to the control circuit is supplied by circuit 17 comprising a capacitor C3, stabilising Zener diode DZ1 and filtering capacitors C4 and C5. The control circuit 18 and the gate drive circuit 19 performs the function of the control means 14 of Figure 1, in that an output signal is supplied to transformer T1 from the output of pin 11 of integrated circuit IC1. The integrated circuit IC1 comprises a pulse width modulation circuit which together with the associated circuitry controls the duty cycle of the MOS-FETS Q1 and Q2. The duty cycle of the MOS-FETS Q1 and Q2 can be set by the potentiometer RV1.

The gate drive pulses to MOS-FETS Q1 and Q2 are supplied by transformer T1, with provision being made for the d.c. restoration of the pulses and for a suitable delay in switching the MOS-FETS on as discussed hereinabove.

Claims

1. A switched mode voltage controller for the control of an a.c. supply voltage, comprising input terminals for connection to an a.c. supply, output terminals for connection to a load, a first bidirectional switch and an inductor connected in series between a first input terminal and a first output terminal, a second bidirectional switch connected between common second input and second output terminals and a node between said first bidirectional switch and said inductor, a capacitor connected across said output terminals and control means to control the alternate switching of said bidirectional switches.

2. A switched mode voltage controller as claimed in Claim 1, wherein said control means controls the duty cycle of said bidirectional switches to vary the voltage VO at said output terminals according to

where V. is the input voltage and t1 and t2 are the switching periods of said first and second bidirectional switches respectively.

3. A switched mode voltage controller as claimed in Claim 2, wherein said control means provides a pulse width modulated control signal of variable duty cycle.

4. A switched mode voltage controller as claimed in any preceding claim, wherein each said bidirectional switch comprises a bridge rectifier connected by input terminals thereof providing connection terminals of said bidirectional switch, and a field effect transistor connected across output terminals thereof, the gate voltage of said field effect transistor being controlled by said control means.

5. A switched mode voltage controller as claimed in Claim 4, wherein said control means includes a gate drive circuit for said bidirectional switches, said gate drive circuit having a slower "turn on" time than "turn off" time and being responsive to a single control signal.

6. A switched mode voltage controller as claimed in any preceding claim, further including a series connected resistor and capacitor connected across said second bidirectional switch.

7. A switched mode voltage controller as claimed in any preceding claim, wherein said control means controls the switching frequency of said bidirectional switch to be very much higher than the frequency of said a.c. supply.

8. A switched mode voltage controller as claimed in Claim 7, wherein said switching frequency is higher than the audible frequency of humans and domestic animals.

9. A switched mode voltage controller as hereinbefore described with reference to Figures 1 to 3 of the drawings.