Alternating current power controller with DC transistor switching and an internal DC power supply

An alternating current power controller is provided that employs DC transistor switching for subcycle switching capability and to eliminate problems associated with SCRs and furthermore provides internal elements for generating the required DC control power from the AC source whether the AC load is on or off.

This invention relates to power controllers for controlling the supply of 
power from an AC supply to a load. 
Power controllers provide circuit breaker functions such as protection of 
the load and wiring from overload conditions and in addition provide 
on/off control of the conduction of the load circuit. In solid state 
electronic configurations, power controllers can be devised to provide for 
on/off control from a location remote from the load and associated with 
the load circuit by a low power control circuit, such power controllers 
being referred to as remote power controllers (RPCs). Present remote power 
controllers are available in two basic configurations respectively for AC 
and DC load circuits. 
DC RPCs are normally implemented with transistors as the switching elements 
because of their low saturation voltage drop which provides high 
efficiency. Also, they have the ability to turn off in a DC application, 
which is not the case for SCRs, and have very fast response to applied 
fault conditions. 
AC RPCs, however, are normally implemented with SCRs (silicon controlled 
rectifiers or thyristors) for the main switching elements for the reasons 
that SCRs are latching devices which require very little drive energy to 
sustain conduction even for overloads and hence provide high efficiency. 
In AC circuits, SCRs naturally commutate (turn themselves off) at zero 
current crossover which lessens electromagnetic interference. 
The qualities of SCRs can become a detriment for overload or fault 
conditions in AC RPCs. If the SCR is on and carrying load current and a 
fault occurs, the conducting SCRs and the RPC cannot be turned off until 
natural commutation occurs at the next zero current crossover. This is due 
to the latching nature of the SCR and cannot be changed no matter how fast 
the overload trip logic operates. During the overload, the peak current is 
limited only by system capability such as generator and conductor size and 
could be thousands of amperes. The duration of such a fault current could 
be up to about 270 electrical degrees if the fault is applied early in the 
period of the AC cycle. 
These qualities can, of course, be taken into account in selecting the SCRs 
for the power controller so that they have sufficient size and current 
rating. However, this has a major impact on the cost of the RPC so that 
the cost is essentially governed by the maximum half cycle surge current 
that is anticipated. For example, an AC RPC of a given rating such as two 
amperes for normal load current will cost substantially more if it is to 
be used in a system in which it might be subject to a short circuit 
current of 10,000 amperes rather than one used in a system where it will 
only be subject to a short circuit current of about 200 amperes. 
In addition, the SCR size and cost for a given surge current rating will 
increase as the frequency of the AC power decreases. For example, the SCR 
cost and hence the cost of the RPC will be higher for a 60 Hz. system than 
it is for an otherwise similar RPC for a 400 Hz. system. 
Efforts to minimize these drawbacks have generally been along the lines of 
using a current limiting resistor in series with the SCRs to keep the 
maximum fault currents to which the SCRs are subjected within reasonable 
levels. This makes the SCR to some degree independent of fault current 
capability and to a lesser degree the cost and size becomes somewhat 
independent of system frequency. Billings U.S. Pat. No. 3,879,652, Apr. 
22, 1975, discloses an example of an AC solid state power controller using 
SCRs of the type in which current limiting resistors have been used in 
series with the SCRs to limit their maximum fault current. 
It is desired to provide an AC RPC that is totally independent of system 
surge current levels and system frequency. 
Another aspect of RPCs that has been addressed in the art has been to try 
to achieve an RPC that is operable in either an AC or a DC load circuit. 
Such apparatus has been proposed in "Power Controller Breadboard and 
Development Requirements", a final report under United States Department 
of the Navy Contract N62269-74-C-0151, by Perkins et al., March 1975. Two 
power switch configurations are proposed which use only transistors as the 
switching elements and which avoid the inherent drawbacks of SCRs. In one 
version, inverse/parallel switches are arranged so that, in operation on 
an AC system, there is half cycle independent control with some resulting 
complexity. This arrangement does not require any isolated supply of AC to 
DC power but it does incur high losses in resistors associated with each 
of the inverse/parallel switches and an increased DC offset voltage. An 
alternative and preferred version is presented in the report which has 
only a single switch configuration associated with the line through full 
wave rectifiers so that it operates effectively as a DC switch even when 
associated with an AC system. However, this configuration as disclosed 
requires an isolated power supply for the DC switch and incurs a higher 
voltage drop in the DC mode unless terminals are provide for bypassing 
part of the rectifier in DC operation. 
The present invention came about as a result of efforts to provide an AC 
power controller without the problems associated with SCRs and hence using 
transistors as the primary power switches but in a configuration such that 
no isolated power supply is required. The result is an AC power controller 
with the intended performance while retaining the optional capability of 
utilization as a DC power controller with preferably separate terminals 
for such purpose. 
In accordance with the present invention, an AC power controller is 
provided in which a DC power controller with a transistor switch is 
utilized as the switching element and contains the features applicable to 
power controllers for control and status implementation. The arrangement 
is such that the DC power input and DC power output terminals of the DC 
transistor switch or power controller are connected through rectifiers 
respectively to terminals for connection to the AC supply and the AC load. 
Furthermore, these terminals are associated with a filter that smooths the 
ripple resulting from the rectified AC in order to provide the necessary 
DC power supply for the internal DC power controller circuits. Hence, what 
is achieved is a transistorized power controller operable on AC, while 
retaining capability for operation on DC, without requiring any isolated 
power supply. It therefore represents a simplified yet effective circuit 
for the performance of the objectives of independence from surge current 
levels and system frequency.

PREFERRED EMBODIMENTS 
FIG. 1 illustrates the basic building blocks of the present invention in a 
power controller 10 for controlling the supply of power from an AC supply 
12 to an AC load 14 in which the AC supply terminal 13 and the AC load 
terminal 15 are each connected through rectifiers 16 and 18 for full wave 
rectification to the power terminals 20 and 21 of a DC transistor switch 
22. The DC transistor switch preferably has associated with it the logic 
and control circuitry (not shown) of a DC power controller and may be 
implemented in accordance with known practice and will not be detailed 
herein. Examples of suitable DC power controllers are disclosed in a paper 
by D. A. Fox entitled, "Remote Power Controllers for the NASA Space 
Shuttle Orbiter" presented at the AIAA Conference of March, 1977, which is 
incorporated herein by reference. Such apparatus as is generally known 
provides a transistor switch for controlling the power in the load circuit 
in an arrangement such that the switch is responsive to applied signals to 
turn on or off the conductive path in the load circuit in response to 
inputs that may result from manual switch application or from the 
occurrence of faults in the load circuit. 
The arrangement of FIG. 1 further illustrates a filter 24 connected between 
one of the input terminals 20 to the transistor switch 22 and the AC power 
ground 26. This filter 24 accomplishes the needs to the DC transistor 
switch as far as its power supply is concerned so that no isolated power 
supply is required. 
In a sense, therefore, the present invention accomplishes its purpose in 
providing an improved AC RPC by utilizing a known type of transistorized 
DC RPC and modifying it by the rectifier and filter elements 16, 18 and 24 
to provide AC operation. 
Referring to FIG. 2, the rectifier diodes CR1 through CR4 convert the AC 
load current to DC which is applied to the DC RPC switch terminals 20 and 
21. The AC input terminal 13 is connected through CR1 and CR2 respectively 
in the forward and reverse directions (corresponding to rectifier 16 of 
FIG. 1) to the respective DC power input and power output terminals 20 and 
21. The AC output terminal 15 is connected through CR3 in the forward 
direction to the DC power input terminal 20 and through CR4 in the reverse 
direction to the DC power output terminal 21. The AC supply 12 and the AC 
load 14 have their other terminals connected in common to the AC ground 
point 26. 
A filter 24 is connected between one of the DC terminals, here the plus or 
DC input terminal 20, to the AC ground 26 and comprises a capacitor 24A 
between the DC power input terminal and the DC power ground 23 and a diode 
rectifier 24B is connected from that point to the AC ground 26. This 
provides the filtered DC voltage for the control and drive circuits as 
required by the DC power controller and is discussed in the aforementioned 
Fox paper, for example. The achievement of this combination provides an 
RPC that can operate over a voltage range of from 25 to 200 volts. 
Therefore, the ripple voltage across the filter capacitor 24A can be quite 
high and thus the capacitor can be relatively small. 
The switch voltage drop for this circuit at rated load is 2.7 volts which 
is comparable to an SCR RPC with a peak limiting resistor. Overall 
efficiency of the new circuit is also comparable to existing AC RPC 
designs. Unlike prior art AC power controller, such as that of the 
Billings patent, the cost and size of the RPC is independent of the 
required system surge current capability and system frequency since no 
SCRs are used and the switching is in the DC mode. 
The circuit provides standardization of design so that one fundamental 
circuit, the DC power controller, can provide 25 to 200 volt DC RPCs 
and/or 115 to 230 volt AC RPCs. It is therefore possible to build one 
device which can be programmed by connecting external terminals to provide 
any of the above ratings. 
As compared with other AC RPCs employing SCRs, this circuit avoids problems 
of having to provide capacitor voltage dividers, the problems of matching 
SCRs and the concern about their temperature ratings and the problems of 
selecting and matching other previously necessary components as those 
required herein may be readily provided without difficult matching 
problems. 
In a typical DC RPC such as that of the described Fox paper, the inverse 
trip time delay characteristic is in the form of 
##EQU1## 
where A=instant trip current 
B=ultimate trip current 
K=time constant 
and 
I.sub.L =load current. 
This can be shown to approximate a constant I.sup.2 t relationship. As the 
load current I.sub.L approaches the instant trip current level A, the trip 
time, T.sub.t, approaches zero or instant trip. In a typical 4,000 ampere 
surge system, this circuit can limit peak currents to two or three times 
the instant trip level due to the fast, instant, trip times of 1 to 10 
microseconds. 
It is therefore seen that a simple and effective AC RPC is provided that 
avoids the problems of the prior art. It will be understood that the 
invention may be practiced in various modified forms other than those 
specifically described or shown herein.