Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Utility Patent
1999-09-28
2001-01-02
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S134000
Utility Patent
active
06169681
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to power supplies for class D or switching amplifiers. More specifically, the present invention provides circuits and methods for reducing the effects of power supply pumping in single-ended switching amplifiers.
Power supply pumping in single-ended switching amplifiers is a phenomenon which results in an imbalance between the positive and negative rails of the power supply. Power supply pumping results from current being sent back into one or the other of the supply rail causing an increase in the magnitude of that supply rail and an imbalance between the two rails. This has a negative effect on amplifier performance, as well as enhances the potential for power supply bus runaway in which the imbalance quickly grows to destructive proportions.
A block diagram of a typical switching amplifier
100
is shown in
FIG. 1. A
representative schematic for an output stage
102
and filter
101
is shown in FIG.
2
. Switch
102
is built around high side and low side FETs
202
and
204
, respectively, and drives load
206
via a low pass filter comprising series inductor
208
and capacitor
210
. Diodes
212
and
214
and capacitors
216
and
218
are part of the power supply circuitry which generates the ±V
BB
supply rails.
When the output voltage of the switching amplifier goes negative, a current flows from ground, through load
206
and inductor
208
and into output switch
102
(as shown by the dashed arrows), alternately flowing back to either the positive or negative power supply rail depending upon which of FETs
202
and
204
is on. Under these conditions and when low side FET
204
is on, the current discharges capacitor
218
. However, when high side FET
202
is on, capacitor
216
must accept the current. This charges the positive supply rail above its desired regulation voltage, continuing until high side FET
202
is turned off and low side FET
204
is turned on. Pumping of the negative supply occurs in a similar manner during positive swings of the amplifier's output voltage.
As mentioned above, this phenomenon causes undesirable fluctuation in the balance of the amplifier's power supply rails increasing the likelihood of a catastrophic power supply bus runaway and potentially degrading the output signal. One approach to balancing the supply rails of such an amplifier are described in a 1989 Motorola Semiconductor Application Note by Donald E. Pauly entitled
High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs
, the entirety of which is incorporated herein by reference for all purposes. A schematic of the solution described is shown in FIG.
3
.
A complementary pair of TMOS power MOSFETs
302
and
304
are configured in series between the ±V
BB
supply rails and driven with the same 20 kHz square wave. If there is an imbalance between the supply rails, the square wave delivered to inductor
306
includes a net average dc component. Thus, if the magnitude of +V
BB
is higher than that of −V
BB
, more energy will be stored in inductor
306
during the time MOSFET
302
is conducting than when MOSFET
304
is conducting. When MOSFET
302
turns off and MOSFET
304
turn on, the extra energy stored in inductor
306
is transferred to −V
BB
through MOSFET
304
thereby reducing the voltage on the +V
BB
rail and increasing the voltage on the −V
BB
rail. In this way, balance between the two supply rails is restored. Similarly, if the magnitude of −V
BB
is higher than that of +V
BB
, the extra energy stored in inductor
306
during the conduction time of MOSFET
304
is transferred to the +V
BB
rail during the conduction time of MOSFET
302
, thereby restoring balance.
A significant disadvantage of this solution is the constant circulation of current through inductor
306
and MOSFETs
302
and
304
. For example, in the specific implementation discussed in the Motorola reference cited above, the current delivered to the inductor for supply voltages of ±44 volts is a ±5.5 amp triangle wave. In addition, imbalances between the supply rails may result in an additional 3.5 amps of direct current, potentially causing saturation of the inductor core. Obviously, in terms of power dissipation EMI, and device size, this solution has some drawbacks.
In view of the foregoing it is desirable to provide a technique by which the effects of power supply pumping are reduced which is not characterized by the drawbacks of previous solutions.
SUMMARY OF THE INVENTION
According to the present invention, a power supply balancing circuit is provided which maintains the balance between opposing power supply buses without dissipating an unacceptable amount of energy. Power dissipation is much lower than the solution described above because power only flows when an imbalance between the supply rails exists. According to a specific embodiment, two balancing MOSFETs are provided in series between the power supply rails. The balancing MOSFETs are alternately switched on and off so that the positive rail and the negative rail are alternately coupled to the primary of a balancing transformer. The balancing transformer is an inverting transformer thus generating an inverted signal on the secondary. Two recycling diodes are coimected in series between the power supply rails which, under balanced conditions, are reversed biased. The secondary of the balancing transformer is coupled to the junction between the two recycling diodes. Thus, when each of the rails is connected to the primary of the transformer, the voltage at the junction of the diodes is of roughly equal magnitude to the rail voltage but of opposite polarity.
When, for example, the positive rail voltage is connected to the primary of the balancing transformer through one of the balancing MOSFETs, a negative voltage of substantially the same magnitude is generated at the secondary, and thereby at the cathode of the recycling diode coupled to the negative rail (the low-side recycling diode). If the imbalance between the positive and negative rails is sufficient to cause the low-side diode to be forward biased, current flows from the positive rail “through” the balancing transformer and into the negative rail, thereby restoring balance between the two rails. An imbalance where the negative rail has a greater magnitude than the positive rail is ameliorated in a similar manner. Thus, only if the magnitude of either of the rail voltages exceeds the other by more than one diode drop will the balancing mechanism of the present invention come into play. That is, where the rails are sufficiently balanced, virtually relatively little current e.g., magnetizing current) flows in the balancing MOSFETs. According to another specific embodiment, the turns ratio of the balancing transformer is adjusted to negate the effect of he recycling diodes such that neither rail may exceed the other.
According to another embodiment of the invention, the balancing transformer is also used to provide power to low voltage power supplies. That is, the balancing transformer has at least one additional secondary winding with an appropriate turns ratio with the primary winding and rectification circuitry to a generate a low power dc bias voltage. According to various embodiments there are multiple such secondary windings and bias voltage supplies. In this way, both high power and low power supplies are consolidated into a single power supply.
According to yet another embodiment of the invention, the balancing transformer is also used to implement a control loop for controlling operation of the power supply. That is, the balancing transformer has an additional secondary winding which is galvanically isolated from the other transformer windings and which generates a feedback signal to the power supply controller. According to a specific embodiment, the feedback signal is a ratio of the rail voltages. In the embodiments discussed above in which low power bias voltages are generated from secondaries of the balancing
Keller Richard
Kemp, III Alson R.
Beyer Weaver & Thomas LLP
Laxton Gary L.
Tripath Technology Inc.
Wong Peter S.
LandOfFree
Power supply topology to reduce the effects of supply pumping does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Power supply topology to reduce the effects of supply pumping, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Power supply topology to reduce the effects of supply pumping will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2551034