Pulse-width modulated circuit for driving a load

• US 5,070,292 A
• Filed: 11/13/1989
• Issued: 12/03/1991
• Est. Priority Date: 11/13/1989
• Status: Expired due to Term

• Current Assignee: Hitachi Medical Corporation (Hitachi, Ltd.)
• Original Assignee: Performance Controls, Inc. (Fujifilm Holdings Corporation)
• Inventors: Goff, Jerry K.
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): MARTIN, DAVID S
• Application Number:
  US07/435,258
• Time in Patent Office: 750 Days
• Field of Search: 318/798-802, 318/806, 318/807-811, 318/812, 318/254, 318/599, 388/819, 388/811-815, 388/829, 388/831, 388/907.2, 388/915
• US Class Current: 318/811
• CPC Class Codes: H02M 7/5387 in a bridge configuration

Abstract

A pulse-width modulated circuit applies a voltage across a load in accordance with a command signal. The command signal is converted into first and second trains of pulses, by electronically comparing the command signal, and its inverse, with a triangular dither signal. These two pulse trains, and two pulse trains obtained by taking the complements of the original pulse trains, are applied to a pair of bridge circuits. The load is connected to both bridge circuits. Both bridge circuits include electronic switching devices actuated by the pulses. The bridge circuits are configured such that when a pulse is present, from either the first or the second pulse train, no net voltage appears across the load. At all other times, substantially the entire power supply voltage is applied across the load. Compared to pulse-width modulated circuits of the prior art, having similar pulse frequencies and power supplies, the present circuit reduces the undesirable effects of current ripple through the load by a factor of four. This reduction is due to the use of two alternating pulse trains, which doubles the effective frequency of pulses, and to the fact that the maximum voltage excursion of the signal across the load is no greater than the absolute value of the maximum power supply voltage available. As a result, the present circuit has a much higher bandwidth than circuits of the prior art having comparable voltage and frequency parameters.

Claims

1. A circuit for providing a current through a load, the current being generated by a power supply having a predetermined maximum voltage, the current through the load being determined by a command signal, the circuit comprising:

(a) means for converting the command signal into a first train of pulses, the pulses of the first train having a width which is representative of the value of the command signal, the first train of pulses being called signal x,

(b) means for inverting the command signal, and for converting the inverted command signal into a second train of pulses, the pulses of the second train having a width which is representative of the value of the command signal, the second train of pulses being called signal x'"'"',

(c) means for providing a signal which is the complement of signal x, the latter being called signal y,

(d) means for providing a signal which is the complement of signal x'"'"', the latter being called the signal y'"'"',

(e) a pair of bridge circuits, the first bridge circuit including four switching means, two of the switching means being controlled by signal x and two of the switching means being controlled by signal y, the second bridge circuit also including four switching means, two of the switching means of the second bridge circuit being controlled by signal x'"'"' and two of the switching means of the second bridge circuit being controlled by signal y'"'"',

(f) the load being connected to both bridge circuits such that the load receives pulses of voltage applied through electrical paths formed in the bridges, wherein the first bridge is connected across two terminals extending from the load, and wherein the second bridge is connected across the same two terminals, wherein both the first and second bridges are connected to each other at the same terminals which extend from the load, wherein the pulses of voltage are of a frequency which is substantially double the frequency of pulses of signal x, and wherein the magnitude of the peak-to-peak voltage across the load is no greater than the magnitude of the maximum voltage of the power supply.

2. The circuit of claim 1, wherein the load is connected to the bridges through a plurality of inductors.

3. The circuit of claim 1, wherein both bridges are capable of defining circuit paths such that both bridges can apply the voltage of the power supply across the load in either of two possible directions.

4. The circuit of claim 1, wherein the switching means comprise electronic switches.

5. A circuit providing a current through a load, the current being generated by a power supply having a predetermined maximum voltage, the current through the load being determined by a command signal, the circuit comprising:

(a) means for converting the command signal into first and second trains of pulses, the pulses of the first and second pulse trains being out of phase with each other, the widths of the pulses of the first and second pulse trains being proportional to the instantaneous value of the command signal, and

(b) at least two bridge circuits connected to receive said first and second pulse trains, for selectively applying the voltage of the power supply across the load, the bridge circuits comprising means for applying zero effective voltage across the load when a pulse from either of said first or second pulse trains is detected, and for applying substantially the full voltage of the power supply appears across the load at all other times,

wherein a first bridge circuit is connected across two terminals extending from the load, and wherein a second bridge circuit is connected across the same two terminals, wherein both the first and second bridge circuits are connected to each other at the same terminals which extend from the load.

6. The circuit of claim 5, wherein the logic means includes means for generating the complements of the first and second pulse trains.

7. The circuit of claim 6, wherein the logic means also comprises at least one bridge circuit, the bridge circuit having switching means, each switching means connected to be turned on and off by pulses form one of said first and second pulse trains or their complements, the bridge circuit comprising means for instanteously providing current paths from the power supply through the load and back to the power supply.

8. The circuit of claim 5, wherein at least two of the inductors are magnetically coupled.

9. The circuit of claim 8, wherein each bridge circuit is connected across the load through two inductors, and wherein the inductors connected to different bridge circuits are magnetically coupled, and wherein the inductors connected to the same bridge circuit are not magnetically coupled.

10. A circuit for providing a current through a load, the current being generated by a power supply having a predetermined maximum voltage, the current through the load being determined by a command signal, the circuit comprising:

(a) means for converting the command signal into first and second trains of pulses, the pulses of the first and second pulse trains alternating with each other, the widths of the pulses of the first and second pulse trains being proportional to the instantaneous value of the command signal, and

(b) at least two bridge circuits connected to receive said first and second pulse trains, for selectively applying the voltage of the power supply across the load, the bridge circuits comprising means for applying substantially the full voltage of the power supply across the load when a pulse from either of said first and second pulse trains is detected, and for applying zero effective voltage across the load at all other times,

wherein a first bridge circuit is connected across two terminals extending from the load, and wherein a second bridge circuit is connected across the same two terminals, wherein both the first and second bridge circuits are connected to each other at the same terminals which extend from the load.

11. The circuit of claim 10, wherein the logic means includes means for generating the complements of the first and second pulse trains.

12. The circuit of claim 11, wherein the logic means also comprises at least one bridge circuit, the bridge circuit having switching means, each switching means connected to be turned on and off by pulses from one of said first and second pulse trains or their complements, the bridge circuit comprising means for instanteously providing current paths from the power supply through the load and back to the power supply.

13. The circuit of claim 10, wherein at least two of the inductors are magnetically coupled.

14. The circuit of claim 13, wherein each bridge circuit is connected across the load through two inductors, and wherein the inductors connected to different bridge circuits are magnetically coupled, and wherein the inductors connected to the same bridge circuit are not magnetically coupled.

15. A method of providing a current through a load in response to a command signal, comprising the steps of:

(a) converting the command signal into first and second trains of pulses, the width of each pulse in the pulse trains corresponding to the instanteous value of the command signal, the pulses of the first train being out of phase with the pulses of the second train,

(b) generating third and fourth pulse trains, the third and fourth pulse trains being the complements of the first and second pulse trains, respectively,

(c) applying the first and sthird pulse trains to a first bridge circuit, the first bridge circuit including switching means connected to be controlled by the first and third pulse trains, the switching means of the first bridge circuit being arranged such that the voltage of the power supply is applied across the load, by the first bridge circuit, the polarity of the applied voltage being determined by the first and third pulse trains, and

(d) simultaneously applying the second and fourth pulse trains to a second bridge circuit connected to the first bridge circuit, the second bridge circuit including switching means connectd to be controlled by the second and fourth pulse trains, the switching means of the second bridge circuit being arranged such that the voltage of the power supply is applied across the load, by the second bridge circuit, the polarity of the applied voltage being determined by the second and fourth pulse trains, wherein the first bridge is connected across two terminals extending from the load, and wherein the second bridge is connected across the same two terminals, wherein both the first and second bridges are connected to each other at the same terminals which extend from the load.

16. A circuit for providing a current through a load in response to a command signal, the current being generated by a power supply comprising means for producing a voltage, the circuit comprising:

(a) means for converting the command signal into a plurality of pulse trains, the pulse trains being out of phase with each other, the widths of the pulses of the pulse trains being proportional to the instantaneous value of the command signal, and

(b) a plurality of bridge circuits, each bridge circuit including switching means which are actuated by pulses from the pulse trains, wherein different bridges are controlled by different pulse trains, each bridge circuit being connected across teh load through inductors, each bridge circuit comprising means for selectively applying the voltage of the power supply across the load, depending on the presence or absence of a pulse from one of the pulse trains, wherein all of the bridges are connected across the same two terminals extending from the load, wherein all of the bridges are connected to each other at the same terminals which extend from the load.

17. The circuit of claim 16, wherein there are two bridge circuits, and wherein the inductors connected to different bridge circuits are magnetically coupled.

18. The circuit of claim 17, wherein each bridge circuit is connected to the load through two inductors, and wherein the inductors connected to the same bridge circuit are not magnetically coupled.

19. The circuit of claim 17, wherein the pulses from the first and second pulse trains are 180°

out of phase.

20. A circuit for controlling a brushless motor, the motor having at least three windings, each winding having first and second ends, the first end of each winding being electrically connected to the first end of every other winding, the second end of each winding being connected to a bridge circuit, each bridge circuit having four switches, wherein there is one bridge circuit for each winding,

wherein each bridge circuit is connected to means for comparing a dither signal with a command signal, the output of the comparing means comprising pulses which control the switches in each bridge circuit,

wherein each pair of bridge circuits comprises logic means for selectively applying a voltage of a power supply across a load, the load being defined by the windings connected to the bridge circuits of a given pair, the logic means comprising means for applying the full voltage of the power supply across the load when a specified condition is detected, and for applying zero voltage across the load at all other times, wherein the specified condition is defined as the presence or absence of a pulse from the comparing means.