WO1997032389A1

WO1997032389A1 - Switch-mode power driver

Info

Publication number: WO1997032389A1
Application number: PCT/IB1997/000093
Authority: WO
Inventors: Eise Carel Dijkmans
Original assignee: Philips Electronics N.V.; Philips Norden Ab
Priority date: 1996-03-01
Filing date: 1997-02-07
Publication date: 1997-09-04

Abstract

The driver comprises a H-bridge (1) and a control circuit (2). The H-bridge comprises four switches (41, 42, 43, 44). The control circuit (2) comprises a pulse modulator (50) to generate a pulse signal (Vpwm) responsive to an input signal (Vi) and a polarity detector (60) to generate a polarity signal (Vpol) corresponding to the polarity of the input signal (Vi). The control means are conceived to drive a first switch (41) and a third switch (43) dependant on the pulse signal (Vpwm) and to drive a second switch (42) and a fourth switch (44) dependant on the polarity signal (Vpol) only. In the driver a second output terminal (22) is alternatively connected to one of the poles of a DC power source dependant on the polarity of the input signal only. As the polarity of the input will change with a lower frequency than the frequency of the pulse signal the resulting electromagnetic radiation will be less.

Description

Switch-mode power driver

The invention relates to a switch-mode power driver comprising a Hi- bridge circuit having a first input terminal and a second input terminal for connecting a DC power source, - a first output terminal and a second output terminal for connecting a load, a first switch for connecting the first output terminal to the first input terminal, a second switch for connecting the second output terminal to the first input terminal, a third switch for connecting the first output terminal to the second input terminal, a fourth switch for connecting the second output terminal to the second input terminal, and control means to control said switches, said control means comprising a pulse modulator having a input responsive to a input signal and an output for supplying a pulse signal. Such a driver is known from US 4,969,079. In the known driver the switches are embodied in the form of transistors and the control means are embodied in the form of a pulse width modulator. When the output of the pulse width modulator is high, the first switch and the fourth switch are closed and the second switch and the third switch are open. When the output of the pulse width modulator is low, the first switch and the fourth switch are open and the second switch and the third switch are closed. In this way an alternating voltage is applied to a circuit of a reactor in series with a load and a capacitor in parallel with the load. In case the load is positioned remote from the driver and the operating frequency of the pulse width modulator is in the radio frequency range, the leads from the driver to the load will cause unwanted electromagnetic radiation. It is an object of the invention to provide a switch-mode power driver with generates less electromagnetic radiation than the prior art driver. To achieve this object the switch-mode power driver according to the invention is characterized in that, the control means further comprise a polarity detector to generate a polarity signal corresponding to the polarity of the input signal, the control means being conceived to drive the first switch and the third switch dependant on the pulse signal and to drive the second switch and the fourth switch dependant on the polarity signal only. In the prior art driver both output terminals are alternatively connected to one of the poles of a DC power source dependant on the operating frequency of the pulse modulator so both output terminals give rise to electromagnetic radiation. In the driver according to the invention the second output terminal is alternatively connected to one of the poles of a DC power source dependant on the polarity of the input signal only. As the polarity of the input will change with a lower frequency than the frequency of the pulse signal the resulting electromagnetic radiation will be less. Another advantage of the switch-mode power driver in accordance with the invention is that only two switches are operated at the frequency of the pulse signal so that the switching losses are reduced in comparison width the prior art driver.

An embodiment of the switch-mode power driver in accordance with the invention is characterized in that the first switch and the third switch are operated in antiphase and that the second switch and the fourth switch are operated in antiphase. Due to these measures it is avoided that the power source is short circuited and that the current through the switches becomes so high they may be damaged. Also it is prevented that one of the output terminals is not connected at all which may give rise to large voltages on the output terminals when they are connected to an inductive load.

An embodiment of the switch-mode power driver in accordance with the invention is characterized in that the pulse modulator is conceived to generate a pulse signal corresponding to the absolute value of the input signal and the first switch and the third switch are driven dependant on the pulse signal and the polarity signal. It has been found that these measures can be realized with an analog circuit.

An embodiment of the switch-mode power driver in accordance with the invention is characterized in that - the fourth switch is closed when the polarity of the input signal is positive, the first switch is closed when the pulse signal is high and the polarity of the input signal is positive, the third switch is closed when the pulse signal is low and the polarity of the input signal is positive, - the second switch is closed when the polarity of the input signal is negative, the third switch is closed when the pulse signal is high and the polarity of the input signal is negative, the first switch is closed when the pulse signal is high and the polarity of the input signal is negative. These measures can be realized for instance with suitably programmed microprocessor.

An embodiment of the switch-mode power driver in accordance with the invention is characterized in that the control means comprise an XOR gate array having a first input and a second input, the pulse signal being applied to the first input and the polarity signal being applied to the second input, the XOR gate array further comprising a non- inverted output and an inverted output, the non-inverted output driving the first switch and the inverted output driving the third switch, the control means further comprising a buffer responsive to the output of the polarity detector, the buffer having a non-inverted output and a inverted output, the non-inverted output driving the second switch and the inverted output driving the fourth switch. With these measures the above mentioned embodiment is realized with a minimum of control logic.

An embodiment of the switch-mode power driver in accordance with the invention is characterized in that the switches are CMOS transistors. Due to this measure the power dissipation of the driver is optimized. The invention also relates to an apparatus for retrieving and/or recording information on a carrier by means of a head, which the apparatus comprises an actuator for positioning the head relative to the carrier and a switch-mode power driver according to the invention for driving the actuator. For instance in CD players switch-mode power drivers are used to drive the electromagnetic actuators. The leads to these actuators are usually in the form of flex foils which tend to radiate considerably. As four of such drivers are usually needed in a CD player, the driver according to the invention gives considerable improvement of the EMC characteristics, the power dissipation and the cost of the CD player. The same advantages can be obtained for a magnetic tape or a magnetic disc apparatus.

Embodiments of the invention are represented in the drawing and will be further described hereafter, in which:

Fig. l shows a schematic representation of an embodiment of the switch- mode power driver according to the invention,

Fig.2 shows various signals as a function of time and

Fig.3 shows a schematic representation of an apparatus according to the invention.

Fig.1 shows a schematic representation of an embodiment of the switch- mode power driver according to the invention. The driver comprises a H-bridge 1 and a control circuit 2. The H-bridge has a first input terminal 11 and a second input terminal 12 for connecting a DC power source 4 and a first output terminal 21 and a second output terminal 22 for connecting a load 31, 32, 33. Further, the H-bridge comprises four switches, a first switch 41 for connecting the first output terminal to the first input terminal, a second switch 42 for connecting the second output terminal to the first input terminal, a third switch 43 for connecting the first output terminal to the second input terminal and a fourth switch 44 for connecting the second output terminal to the second input terminal. The control circuit 2 comprises a pulse modulator 50 to generate a pulse signal V,^ responsive to a input signal V, and a polarity detector 60 to generate a polarity signal V, ,, corresponding to the polarity of the input signal V₍. The control means are conceived to drive the first switch 41 and the third switch 43 dependant on the pulse signal V_pwπι and to drive the second switch 42 and the fourth switch 44 dependant on the polarity signal V^, only. The input signal V, is fed to a rectifier 70. The output of the rectifier 70 is fed to the input of a pulse modulator 50. In this case the pulse modulator 50 is a pulse width modulator as described in US 4,969,079 but a pulse density modulator could be used as well. Due to the rectifier 70, the pulse modulator 50 generates a pulse signal V^ corresponding to the absolute value of the input signal V_s. The control circuit 2 further comprises an XOR gate array 80 with a first input 81 to which the pulse signal V_pwm is applied and a second input 82 to which the polarity signal V_p,,, is applied. The XOR gate array comprises a non-inverted output 83 which drives the first switch 41 and an inverted output 84 which drives the third switch 43. In this way it is achieved that the first switch 41 and the third switch 33 are driven dependant on the pulse signal V_pwm and the polarity signal V^ .

The control circuit 2 comprises a buffer 90 responsive to the output of the polarity detector 60 and has a non-inverted output 91 which drives the second switch 42 and an inverted output 92 which drives the fourth switch 44. Preferably, the switches 41, 42, 43, 44 are CMOS transistors. A load, in this case a circuit comprising a motor 31 and a coil 32 connected in series and a capacitor 33 parallel with the motor, is connected between the output terminals 21 and 22. The coil 32 and the capacitor 33 form high frequency filter to reduce electromagnetic radiation which may radiate from the a lead connected to the first output terminal 21 of the driver. Although it is normal practice to have a high frequency filter at the output of a switch-mode power driver it is to be noted that with the driver according to the invention only one such a filter is required to prevent electromagnetic radiation by the leads to the load whereas with the prior art driver two filters, one for each output terminal, would be required. It is known in the art to have a down converter followed by a H-bridge inverter. With the above embodiment a down converter and an inverter are realized with a minimum number of switches. At any moment only two switches are involved in the current path from the power source 4 to the load 31, 32, 33 which is favourable, especially in a battery operated apparatus, as each switch causes a voltage drop and a corresponding power dissipation.

Fig.2 shows various signals as a function of time. V; represents the input signal. | V_; J represents the signal at the output of the rectifier 70. V_pwm represents the signal at the output of the pulse modulator 50. V^, represents the polarity signal at the output of the polarity detector 60. S,, S₂, S₃ and S₄ represent control signals for the switches 41, 42, 43 and 44 respectively. When a control signal is high the corresponding switch is closed and when a control is low the corresponding switch is open. From Fig.2 it can be seen that the first switch 41 and the third switch 43 are operated in antiphase and that the second switch 42 and the fourth switch 44 are operated in antiphase. Further, Fig.2 shows that the fourth switch 44 is closed when the polarity of the input signal V_; is positive, the first switch 41 is closed when the pulse signal V_pwm is high and the polarity of the input signal V_; is positive, - the third switch 43 is closed when the pulse signal V_pwm is low and the polarity of the input signal V; is positive, the second switch 42 is closed when the polarity of the input signal V_f is negative, the third switch is closed when the pulse signal V_pwm is high and the polarity of the input signal Vj is negative, the first switch is closed when the pulse signal V_pwm is high and the polarity of the input signal Vj is negative.

Fig.3 shows a schematic representation of an apparatus according to the invention. The apparatus, in this case a CD-player 100 is capable of retrieving and/or recording information on a carrier 105 by means of a head 110. The CD-player 100 comprises an actuator 120 for positioning the head 110 in the radial direction of the carrier 105 and a rotation motor 130 for positioning the carrier 105 in its circumferential direction relative to the head 110. The CD-player comprises a servo circuit 140 which receives position signals from the head 110 and which generates input signals for two switch-mode power drivers 150 and 151 according to the invention for driving the actuator and the rotation motor.

Hereinbefore the invention has been described with an embodiment for a CD-player. It is to be noted that the driver according to the invention is also very suitable for driving actuators in magnetic Hard Disc drives as the requirements, especially for portable applications (single power supply, low power dissipation, low electromagnetic radiation) are similar.

Claims

1. A switch-mode power driver comprising a H-bridge (1) circuit having a first input terminal (11) and a second input terminal (12) for connecting a DC power source (4), a first output terminal (21) and a second output terminal (22) for connecting a load (31, 32, 33), a first switch (41) for connecting the first output terminal to the first input terminal, a second switch (42) for connecting the second output terminal to the first input terminal, - a third switch (43) for connecting the first output terminal to the second input terminal, a fourth switch (44) for connecting the second output terminal to the second input terminal, and control means (2) to control said switches, said control means comprising a pulse modulator (50) having a input responsive to a input signal (V;) and an output for supplying a pulse signal (V_pwm), characterized in that, the control means (2) further comprise a polarity detector (60) to generate a polarity signal (V,^ corresponding to the polarity of the input signal (VJ, the control means being conceived to drive the first switch (41) and the third switch (43) dependant on the pulse signal (V_p^ and to drive the second switch (42) and the fourth switch (44) dependant on the polarity signal only.

2. A switch-mode power driver according to claim 1, characterized in that the first switch (41) and the third switch (43) are operated in antiphase and that the second switch (42) and the fourth switch (44) are operated in antiphase.

3. A switch-mode power driver according to any one of the preceding claims, characterized in that the pulse modulator is conceived to generate a pulse signal (V_pwm) corresponding to the absolute value of the input signal (Vj) and the first switch (41) and the third switch (33) are driven dependant on the pulse signal (V^ and the polarity signal (V_pol).

4. A switch-mode power driver according to claim 3, characterized in that the fourth switch (44) is closed when the polarity of the input signal (V_j) is positive, the first switch (41) is closed when the pulse signal (V,_^) is high and the polarity of the input signal is positive, the third switch (43) is closed when the pulse signal is low and the polarity of the input signal is positive, the second switch (42) is closed when the polarity of the input signal is negative, - the third switch is closed when the pulse signal is high and the polarity of the input signal is negative, the first switch is closed when the pulse signal is high and the polarity of the input signal is negative.

5. A switch-mode power driver according to claim 4, characterized in that - the control means (2) comprise an XOR gate array (80) having a first input (81) and a second input (82), the pulse signal (V^ being applied to the first input and the polarity signal (V^ being applied to the second input, the XOR gate array further comprising a non-inverted output (83) and an inverted output (84), the non-inverted output driving the first switch (41) and the inverted output driving the third switch (43), the control means further comprising a buffer (90) responsive to the output of the polarity detector, the buffer having a non-inverted output (91) and a inverted output (92), the non-inverted output driving the second switch (42) and the inverted output driving the fourth switch (44).

6. A switch-mode power driver according to any one of the preceding claims, characterized in that the switches (41 , 42, 43, 44) are CMOS transistors.

7. An apparatus (100) for retrieving and/or recording information on a carrier (105) by means of a head (110), the apparatus comprising an actuator (120, 130) for positioning the head relative to the carrier and a switch-mode power driver (150) according to any one of the preceding claims for driving the actuator.