AC power converter

Details AC power converter AC power converter

1999-01-19

2002-02-12

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

C315S307000, C323S223000

Reexamination Certificate

active

06346778

ABSTRACT:

FIELD
The present invention relates to apparatus used in power control, such as apparatus for controlling the power level in an AC electrical circuit. Such an apparatus is referred to as an AC power converter. In one form, the present invention particularly relates to a power converter suitable for use in lighting dimmer control although other applications of the present invention are not to be excluded from the scope of the present application. A number of aspects of the power converter are disclosed.
BACKGROUND OF THE INVENTION
The prior art teaches us about power converters based on a variety of technologies. These techniques can be broadly divided into linear and switching types, and U.S. Pat. No. 5,500,575 discloses a number of examples of linear and switching power converter technologies. Two of these are “Phase Control” and PWM.
One converter disclosed in the prior art is a high frequency, switch-mode power converter operating on the Pulse Width Modulation or “PWM” principal. PWM converters may again be further subdivided into a variety of types: “Direct” converters, “Indirect” converters, “L-bridge” and “H-bridge” for example.
A number of problems exist with prior art PWM power converters.
For a PWM converter of the prior art, in normal operation, power is transferred from the mains to the load when the input voltage and current waveforms are substantially of the same polarity and when the main switch is “on”. This type of converter includes an output filter without which the converter output to the load would contain substantial harmonics of the switching frequency.
One problem of the prior art occurs when the load is reactive (Capacitive or Inductive). Referring to
FIG. 1A
,
1
B,
1
C and
1
D, a schematic of prior art power converter
1
is shown having an input
2
comprising active terminal
3
and neutral terminal
4
, input filter
5
, main-switch
6
. output filter
7
and load
8
. In the positive half cycle of the AC input waveform, current flows as shown by arrow
9
. Conversely, in the negative half cycle, current flows as shown by arrow
10
.
Consider the inductive load case and when the main-switch is turned “off”. Due to the residual energy stored in the inductor
8
, the current flowing in the load has a tendency to continue to flow even when the main-switch is “off”. However, because there is no path for the current flow, the energy stored in the inductor causes a voltage to develop across the load. Referring to FIG.
1
B. the arrow
11
indicates this voltage.
However, if this voltage is allowed to develop unchecked, it can rise to levels that can damage the components of the power converter: a voltage “spike”. One way to avoid this problem is to provide a path for the inductor current to continue to flow even when the main-switch is “off”. This current is often termed “free-wheeling current”. Typically a secondary switch device
12
(FIG.
1
C), also known as sub-switches is provided to carry this freewheeling current
13
and thus eliminate the voltage spike. Typically the sub-switches are arranged in pairs, one each for positive and negative half cycles. However, for simplicity,
FIG. 1C
shows only one such switch. The sub-switches of the prior art are typically operated at line frequency and when the load current polarity and input voltage polarity are the same, the prior art works satisfactorily.
However, when the load current and input voltage are of opposite polarity, as illustrated in
FIG. 1D
, the prior art no longer works satisfactorily. In the figure it can be seen that the “on” main-switch and “on” sub-switch form a short circuit across the AC Input
2
. In this circumstance special techniques are required to prevent this short circuit and consequent dangerous current spike.
These techniques are typically complex, bulky, inefficient, expensive or only partially successful. Furthermore, the output filter renders almost all loads to be reactive and thus exposed to this problem.
Canadian Patent 2107490 discloses one arrangement designed to address the problem of freewheeling currents. This arrangement necessitates the use of sub-switches which are controlled to switch on when the Main switch is switched off. In this way, the sub-switches provide the current path for the freewheeling current. One problem, however, with this technique is that the sub-switches must be of a similar specification to Main switch because they usually operate at the same frequency as the Main switch. This leads to relatively higher cost and lower efficiency.
In the prior art, another separate and distinct problem exists, namely that prior art circuits are usually designed to operate optimally at a particular load output. However, problems occur when the circuit is used at a relatively lower power output than that of the initial design.
The output filter as illustrated schematically in
FIG. 1A
usually comprises an inductor and capacitor. Under normal operation with large loads with low internal impedance, the charge that would otherwise accumulate on the filter's capacitor is discharged into the load. However, with small loads, the charge tends to accumulate on the capacitor with each PWM switching cycle. This is because the charging source impedance (the filter) is higher than the discharge impedance (the load) so that the capacitor voltage tends to approach line potential with each successive PWM pulse. In effect, when the main-switch is “off”, the load continues to be driven by the capacitor. The result is that with small loads and low to medium output level settings, the output of the converter is distorted and the transfer characteristic of the converter is impaired.
FIG. 2
illustrates the transfer characteristic for a power converter designed for an output Of Up to 3 KW. On the vertical axis, output voltage is illustrated, and in this example from 0 to 250 volts. It would be readily appreciated that the range of voltage is not limiting in describing the present invention. On the horizontal axis, a percentage of pulse width of PWM is illustrated, ranging from 0 to 255, being an eight-bit binary representation of 0 to 100%. As can be seen by the line denoted
13
, the characteristic is relatively linear for operation at 3 KW output. This is the intended transfer characteristic for the particular circuit plotted. Compare this, however, to line
14
illustrating operation at 25 W output, line 15 illustrating operation at 200 W output and line 16 illustrating operation at 650 W output. The transfer characteristic as represented by each of numerals
14
,
15
and
16
is not relatively linear. Thus, the output of the circuit is not linearly proportional to the percentage PWM, resulting in the output for line
14
(at 50 PWM) being approximately 160 volts rather than approximately 40 volts for line
13
.
U.S. Pat. No. 5,500,575 describes a means of using the sub-switches to discharge the filter capacitor under certain load conditions. The problem with this technique is that it is not considered progressive in operation, it is relatively complex to implement and requires high-speed sub-switches.
Still further problems are associated with the prior art in relation to detecting and minimising the problems resultant from surge currents circulating within the circuit. U.S. Pat. No. 5,500,575 discloses a form of current limiting however the current sensing is done at the load side of the output filter and is thus considered to be not as effective because it is affected by the filter time constant. Also, the prior art cannot protect the circuit, particularly the main switch, against over-current and/or short circuit in the sub-switches.
Another problem with the prior art concerns remote control.
Theatrical/Professional dimmers have been remotely controlled for many years, even in times preceding solid state phase control dimmers. In this context “Remote Control” refers to the ability to command the output level of the dimmer from a remote location. This has been accomplished in a number of ways, ranging from individual control wires for each channel carrying a v

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