Power control circuit with active impedance to avoid...

Details Power control circuit with active impedance to avoid... Power control circuit with active impedance to avoid...

2001-10-01

2004-10-26

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Gating

Signal transmission integrity or spurious noise override

C327S384000

Reexamination Certificate

active

06809571

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power control circuit with sensing circuitry to sense information about a power device and with active impedance circuitry to prevent sensing of spurious information. More particularly, the invention can be implemented in a noise immune power control circuit to eliminate spurious measurements due to high voltage surge and other causes.
2. Description of the Related Art
Circuits for driving electrical devices such as motors typically include power devices such as power field effect transistors (FETs) across which output power is provided. The power devices can also, for example, be insulated gate bipolar transistors (IGBTs).
Circuit
10
in
FIG. 1
, for example, is a conventional motor controller circuit with low side power FET
12
and high side power FET
14
illustratively connected with diodes
16
and
18
in a half bridge between a DC bus supplying 1200 volts and a common ground. Circuit
10
includes integrated circuit (IC)
20
, a representative power IC for this and similar applications. On the low side, IC
20
includes driver
22
, comparator
24
, and buffer
26
, and on the high side, driver
30
, comparator
32
, and buffer
34
.
IC
20
has output pins for controlling power FETs
12
and
14
and also input pins for receiving information about operation of FETs
12
and
14
.
In circuit
10
, output pins LO and HO serve respectively as gate control pins for FETs
12
and
14
. The central node of the half bridge is connected to pin VS, and provides voltage potential to the output device, in this case a motor.
Low side desat/voltage feedback (DSL/VFL) input pin serves to receive information about operation of FET
12
, while high side desat/voltage feedback (DSH/VFH) input pin serves to receive information about operation of FET
14
. The DSL/VFL and DSH/VFH pins provide desat input indicating a short circuit condition across a power FET, in response to which circuit
10
switches into a soft shutdown mode. The DSL/VFL and DSH/VFH pins also provide voltage feedback input indicating voltage across a power FET, in response to which a microprocessor controller for circuit
10
can manage power output to increase system efficiency. The DSL/VFL and DSH/VFH pins are examples of sensing nodes for connecting to power devices like FETs
12
and
14
.
Information detected through desat and voltage feedback inputs of IC
20
can be understood from circuit
40
in
FIG. 2
, whose components represent either low or high side components of circuit
10
, as suggested by the pin labels. In
FIG. 2
, components on IC
20
are shown at left, while components on a board on which IC
20
is mounted are shown at right.
In circuit
40
, power FET
42
, for example, represents either FET
12
or FET
14
, with its gate connected either to the LO or the HO pin, which in turn receives gate control voltage from driver
44
, representing either driver
22
or driver
30
on IC
20
. Similarly, comparator
46
represents either comparator
24
or comparator
32
, and buffer
48
represents either buffer
26
or buffer
34
. Comparator
46
serves as part of sensing circuitry, providing a sense result signal in response to a sense input signal that includes information received at the DS/VF pin. The sense result signal at the output of comparator
46
includes information derived from the sense input signal about operation of FET
42
.
The DS/VF pin is connected to a power supply, either V
CC
or output pin VB, through resistance
50
, representing resistance
52
or resistance
54
in circuit
10
. Resistance
50
, which could for example be 100 Kohms, provides a sufficiently high impedance to reduce current.
Pin DS/VF senses the source to drain voltage across FET
42
through high voltage diode
60
, representing diode
62
or diode
64
in circuit
10
. Under normal conditions, diode
60
is forward biased and turns on when FET
42
is on; then, when FET
42
turns off, diode
60
becomes reverse biased and also turns off; then, when FET turns on again, diode
60
again becomes forward biased and turns on.
One function of diode
60
is to allow detection of a short circuit condition in which voltage across FET
42
is high even though FET
42
is gated on. When a short circuit condition occurs while FET
42
is on, damage to FET
42
must be prevented by turning it off through its gate signal. The short circuit condition causes the voltage at node
66
to rise, and diode
60
becomes reverse biased and turns off, indicating detection of the short circuit condition. As a result, current begins to flow through the path from V
CC
or VB through resistances
50
,
70
, and
72
to ground, in which resistances
70
and
72
are illustratively at 200 Kohms and 500 Kohms, respectively. The voltage at the DS/VF pin rises, and the voltage to the “+” input of comparator
46
also rises relative to the “−” pin. As a result, the output from comparator
46
goes high, indicating the short circuit condition turning off driver
44
, which then turns off FET
42
through the gate signal.
One problem with the circuitry in
FIGS. 1 and 2
, referred to herein as “the interference problem”, relates to high frequency noise from the DC bus. As shown in dashed lines, diode
60
behaves like capacitance
74
when it is off, allowing high frequency noise to reach comparator
46
. For example, capacitance
74
can pass both negative and positive spikes, as shown by the waveforms in circle
76
. A negative spike can pull down the voltage at the “+” input of comparator
46
, falsely indicating a short circuit condition.
Another problem, referred to herein as “the sensing problem”, relates to voltage feedback (VFB) information received through the DS/VF pin. To increase system efficiency, accurately timed VFB information indicating when FET
42
turns on and off should be provided to the controller, allowing it to make appropriate adjustments. In circuit
40
, VFB information can be obtained using the same circuitry that detects short circuit conditions, by comparing the DS/VF pin voltage to a threshold or reference voltage received at the “−” input of comparator
46
from voltage source
80
, representing either voltage source
82
or voltage source
84
on IC
20
, which could be implemented, for example, with a zener diode or other appropriate device. Comparator
46
in turn provides a signal with VFB information to a microprocessor controller through buffer
48
and the VFL or VFH output pin.
When FET
42
is turned off, the DS/VF pin is at a high voltage relative to the threshold and comparator
46
provides a high output to buffer
48
. Similarly, when FET
42
is turned on, the DS/VF pin is at a low voltage relative to the threshold and comparator
46
provides a low output to buffer
48
.
The sensing problem arises because spurious signals can be provided while FET
42
makes a transition from off to on, as illustrated in FIG.
3
. The upper waveform in
FIG. 3
shows the voltage across FET
42
, the middle waveform the voltage at the DS/VF pin relative to ground, and the lower waveform the VFB signal provided by comparator
46
through buffer
48
to the VFL or VFH pin.
From t0 to t1, FET
42
is turned off and the voltage across it has a high value of several hundred volts, as illustrated by segment
100
of the upper waveform. As a result, diode
60
is off, so that the voltage at the DS/VF pin is also high relative to ground, as illustrated by segment
102
, and comparator
46
provides a high signal indicating that its “+” input is at a higher voltage than its “−” input, as illustrated by segment
104
.
At t1, driver
44
begins to provide a high gate signal to FET
42
to turn it on. As a result, the voltage across FET
42
makes a rapid transition downward of several hundred volts in a few hundred nanoseconds, illustratively shown by segment
110
.
During the off-to-on transition of FET
42
, diode
60
remains temporarily off and therefore acts as a capacitor, so that a high frequ

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