Note: Descriptions are shown in the official language in which they were submitted.
CA 02418123 2003-02-05
BACKGROUND OF THE INVENTION
1. Field ofthe Invention
The present invemion relates to power converters and more directly to those
utilizing bridge
configuration of same type of switching elements, i.e. N-channel MOSFETs, and
more specifically to
power converters having very high speed capability combined with very high
voltage and power.
2. Description of the Prior Art
In power conversion the bridge configuration is the one that can be used in an
extremely large number
of applications including those that use pulse width modulation to amplify low
frequency signals at high
e~ciency.
The applications can be divided into following major categories: AC to DC, DC
to DC, DC to
AC, and AC to AC converters.
The most common approach would be to choose a standard high side drive,
include a dead zone
processing circuit to avoid shoot through and use a series low valued resistor
between the supply and the
switching device for current sensing. It turns out that for each different
application a new design needs to
be created with additional waste of engineering time. Also, the prior art
solutions above 100V supply
voltage have so much variation in propagational delay that direct paralleling
of these solutions is not
permitted. In the case of a pulse width modulation input signal at modulation
extreme, most solutions
utilizing a bootstrap circuit will fall apart due to the finite value which
the holdup capacitor may have.
Also, chip solutions have to deal with the quadratic power loss increase as a
function of operating voltage
at a given frequency. The series current sensing resistor, especially at
higher power, becomes very
troublesome, not just as a result of the extra heat, but also the increase of
ESL of the bypassing circuit.
Due to the high dv/dt and di/dt involved in high speed, high power circuitry,
the design of the printed
circuit board becomes a major task for accomplishing a sound design.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, an object of this invention is to provide a topology for a
universal power converter
free from the defects encountered in the prior art bridge mode power
converters. Another object of the
invention is to provide a method of constructing a power converter of the
widest possible use without
fundamental change in the topology or even in the printed circuit board
design, i.e. to enable the bridge
mode converter to be truly universal use. In accordance with the present
invention, a universal power
converter is provided having a well defined, very low propagational delay,
typically 4 times less the
conventional solutions, to enable direct paralleling of converters without the
need for the reduction of
frequency of operation, having a well defined limit in minimum and maximum
duty cycle in the event
that the input signal has a duty cycle variation of 0 to 100 %, and also
having a lossless current sensing
method that tracks switching device on resistance change versus temperature to
ensure a safe over current
protection level at all allowable temperatures without dissipating extra power
or introducing any
inductance to the circuit. Yet another advantage ofthe present invention is
that the same printed circuit
board can be used for power levels of a few hundred watts to several kilowatts
economically by simply
reselecting the devices. The universal power converter of the present
invention has myriad applications
ranging from high fidelity audio to high power miniature welders, battery
chargers, etc.
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BRIEF DESCRIPTION OF DRAW INGS
FIG. 1 is a schematic circuit diagram showing a prior art power converter;
FIG. 2 is a schematic circuit diagram showing an example of the universal
power
converter according to the present invention; and
FIGS. 3A to 3H are waveform diagrams used for explaining the operation of the
example
of the invention shown in FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In order to better understand the present invention a prior art power
converter will be described
with reference to FIG.1 which is a half bridge power converter building block.
The PWM signal is
applied to signal input terminal 1 which is also the input for input buffer 2.
The output of input buffer 2 is
then connected to upper delay network 3 and lower delay network 4. The outputs
of upper and lower
delay networks 3 and 4, respectively, are connected to inputs of high/low side
drive and shut down circuit
5. The upper drive output of high/low side drive and shut down circuit 5 is
connected to the control
electrode of the upper switching element 6. The lower drive output of high/low
side drive and shut down
circuit 5 is connected to the control electrode of lower switching element 7.
Current sensing resistor 8 is
connected between supply terminal of the lower switching element 7 and
negative DC supply 12 and the
junction is then fed to the shut down input of high/low side drive circuit 5.
Bootstrap capacitor 9 is
connected to high/low side drive and shut down circuit 5 to supply power to
the upper switching element
6 when it is turned on. The output terminal of upper switching element 6 is
connected to the positive DC
supply 10. The output terminal of upper switching element 6 and output
terminal of lower switching
element 7 are connected together to output terminal 11.
With the prior art power converter shown in FIG. 1, if the PWM signal applied
to signal input
terminal 1 exceeds 100% duty cycle, the upper switching element 6 may lose its
supply, which is usually
stored in bootstrap capacitor 9, causing severe distortion.
The dead zone protection delay networks 3 and 4 are supplying the signal for
the upper and lower
drive outputs of high/low side drive and shut down circuit 5. This results in
a propagational delay of turn
offofthe upper and lower switching elements 6 and 7, respectively, as high as
100 nanoseconds with
large variation in operating conditions and from device to device. Therefore,
paralleling of such power
converters is only possible if the dead zones are selected very high which
would impair the quality of the
low frequency signal to be reproduced.
Current sensing resistor 8 is installed in series with negative DC supply 12
introducing power loss
and inductance which is an impairment from the bypassing point of view and can
easily introduce
distortion.
An example ofthe universal power converter according to the present invention,
which is free
from the defects above, will be described with reference to FIG. 2.
In the example of the invention shown in FIG. 2, clock input terminal 11 is
connected to first
upper delay network 13 and first lower delay network 14. PWM input terminal 12
is connected to an input
of logic circuit 15. Output of logic circuit 15 is connected to an input of
shut down circuit 16. The upper
drive output of shut down circuit 16 is connected to second upper delay
network 18. The lower drive
output of shut down circuit 16 is connected to second lower delay network 19.
The output of second
upper delay network 18 is connected to switched current source 20 and output
of switched current source
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20 is connected to input of first buffer 22. The output of first buffer 22 is
connected to input of upper
capacitive level shifter 23 and output of upper capacitive level shifter 23 is
connected to input of second
buffer 26. Output of second buffer 26 is connected to control electrode of
upper switching element 29.
Supply terminal of upper switching element 29 is connected to a power supply
terminal 28 which is
supplied with a positive DC voltage +VCC and supply terminal of lower
switching element 31 is
connected to a power supply terminal 32 which is supplied with a negative DC
voltage -VCC whose
absolute value is same as that of positive DC voltage +VCC. Output of second
lower delay network 19 is
connected to input of lower capacitive level shifter 24 and control input of
controlled switch 21.
Controlled switch 21 is connected to voltage sense circuit 25. Input of
voltage sense circuit 25 is
connected to a output terminal 30. Output of voltage sense circuit 25 is
connected to input of monostable
circuit 17 and output of monostable circuit 17 is connected to an input of
shut down circuit 16. Output of
lower capacitive level shifter 24 is connected to input of third buffer 27 and
output of third buffer 27 is
connected to control electrode of lower switching element 31. Output terminal
of lower switching element
31 and output terminal of upper switching element 29 are connected together to
output terminal 30.
The operation ofthe above-described circuit will now be explained. Clock input
signal Sl, such
as that shown in FIG. 3A, supplied to clock input terminal 11 is then input to
the first upper delay
network 13 which will be initiated at the positive going transition of the
clock input signal S1. The first
tower delay network 14 will be initiated at the negative going transition of
the clock input signal Sl at
clock input terminal 11 and will have such output as that shown in FIG. 3B.
Logic circuit 15 will
combine the two delayed clock signals such that if PWM signal input S2, such
as that shown in FIG. 3E,
which is then input to PWM input terminal 12, approaches the 100% modulation
angle or 0 modulation
angle and then a maximum and minimum pulse width, respectively, in both
directions ofthe modulation
shall be supplied at the output of logic circuit 15 as shown in FIGS. 4C and
4D respectively. Here T is
the period ofthe clock and Ot is the delay interval.
As logic circuit signal S3 is fed through the shut-down circuit 16, a disable
fimetion may be
facilitated, if monostable circuit 17 is triggered by voltage sense circuit 25
due to over current condition,
by rendering upper switching element 29 and lower switching element 31
inactive by producing zero
voltage at the c~trol electrodes of said devices. Shut down circuit 16 via
second upper delay network 18
drives a switched current source 20 that is configured such that under no
condition shall said switched
current source 20 saturate. This configuration then yields a very high speed
level translator, that has a
typical propagational delay similar to the high speed first buffer 22,
approximately 5 nanoseconds. Low
voltage logic level signal S4 at the output terminal of first buffer 22 is fed
to second buffer 26 via upper
capacitive level shifter 23. Second buffer 26 is of high current capacity such
that the propagational delay
can be maintained again at approximately 5 nanosecond while signal levels are
increased typically three
fold to drive the control electrode of upper switching element 29 with signal
such as that shown in FIG.
3F. As a final result, the delay from the output of first buffer 22 to control
electrode of upper switching
element 29 will be approximately 15 nanoseconds with a source and sink current
of 10 amperes or greater
thereby enabling very high speed drive of the high side upper switching
element 29 at rail voltages of
several hundred volts and output current of over 100 amperes (which is usually
associated with very large
control electrode to supply terminal capacitance).
The low side lower switching element 31 is driven in a similar fashion to
maintain high power
and high speed. Low voltage logic level signal S5, such as that shown in FIG.
3G, at the lower drive
output of shut down circuit 16 is fed to third buffer 27 via second lower
delay network 19 and via lower
capacitive level shifter 24. The resultant signal at the control electrode of
lower switching element 31 is
such as that shown in FIG. 3H.
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The operation of the short circuit protection of the above mentioned circuit
will now be
explained. The voltage drop across lower switching element 31 is directly
proportional to the current
flowing through it and is being sensed by voltage sense circuit 25. if current
across lower switching
element 34 exceeds defined limit, overload condition, voltage sense circuit 25
will turn on and trigger
monostable circuit 17 which, in tum, shall initiate shut down condition via
shut down circuit 16. The shut
down condition is maintained until monostable circuit 17 times out (i.e.
several hundred milliseconds).
This process then repeats until the overload condition is removed. Switch 21
is to keep voltage sense
circuit 25 in the off state during off states of lower switching element 31.
Thus, lossless current sensing is
achieved having the additional feature of moving the current limit to a lower
value when the junction
temperature increases, further increasing the reliability of the circuit.
Also, it is of great advantage that
series sensing resistance is nat used, the use ofwhich would result in a
decrease ofthe effectiveness of
bypassing the upper and lower switching elements 29 and 31 and would decrease
the efficiency of the
circuit.
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