Electrical transmission or interconnection systems – Switching systems – Condition responsive
Reexamination Certificate
1999-04-15
2001-05-22
Fleming, Fritz (Department: 2836)
Electrical transmission or interconnection systems
Switching systems
Condition responsive
C307S130000
Reexamination Certificate
active
06236122
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a load drive device wherein the load is driven with intermittent voltage applied to the load or with the current controlled; in particular, it relates to a load drive device of simple construction and high reliability.
BACKGROUND ART
Conventionally, as a load drive device wherein an inductive load such as for example the coils of a solenoid plunger or stepping motor were driven, a device has been employed whereby the mean voltage or mean current applied to the load is controlled by opening and closing a circuit opening/closure means inserted between this load and the power source and consisting of a semiconductor switch or the like. This device is known as a chopper control or PWM (pulse width modulation) control, and a typical conventional circuit layout is shown in FIG.
11
and FIG.
12
.
The circuit shown in
FIG. 11
is of a circuit construction called a high side switch for switching current on the power source side of the load; the circuit shown in
FIG. 12
has a circuit construction called a low side switch that switches current on the earthed side of the load.
FIG. 11
is constituted by providing two transistors TR
1
, TR
2
constituting switch means and two resistors R
0
, R
1
; a PWM signal of predetermined duty ratio is applied to the base of transistor TR
1
, a power source is connected to the emitter of transistor TR
2
; resistor R
0
is connected between the emitter and base of transistor TR
2
; resistor R
1
is connected between the collector of transistor TR
1
and the base of transistor TR
2
; and the emitter of transistor TR
1
is earthed. Also, a flywheel diode FD is connected in parallel with the inductive load L that is the subject of drive by this circuit; the cathode of flywheel diode FD is connected to the collector of transistor TR
2
; and the anode of flywheel diode FD is earthed.
With such a construction, when the PWM signal becomes high level, turning transistor TR
1
ON, in response to this, transistor TR
2
is turned ON, and the power source is applied to load L through transistor TR
2
, so that load current flows to earth from the power source through transistor TR
2
and load L. When this happens, due to the characteristic of the load L, this load current increases with time, finally becoming constant at a saturation point given by the power source voltage and the amount of resistance, not shown, in inductive load L.
However, when chopper control is exercised, with the object of controlling the mean current flowing in the load, the mean current value required in load L is lower than the current value at the saturation point, so, before the load current flowing in load L reaches this saturation point, the PWM signal goes from high level to low level, causing transistor TR
1
to go from ON to OFF and transistor TR
2
also to go from ON to OFF, cutting off the power source from load L.
In this connection, the ON/OFF timing of transistor TR
2
, i.e. the timing of high level/low level of the PWM signal is determined by for example ON/OFF ratio control (duty control) of transistor TR
2
based solely on the time or by detecting the load current flowing in load L by means of a current detector, not shown, in accordance with a predetermined standard such as constant-current control based on its instantaneous value or mean value.
In either case, when transistor TR
2
turns OFF and the power source is cut off from load L, the load current flowing in load L flows back through flywheel diode FD due to the inductive component of load L, and is gradually decreased by the electrical resistance of the circuit and the forward voltage of the flywheel diode FD.
In this condition, when the PWM signal again goes from low level to high level, causing transistor TR
1
to turn ON and transistor TR
2
to turn ON, the power source is once more connected to load L and the load current flowing in load L progressively increases as described earlier.
Thus, with this high side switch, the ON/OFF timing of transistor TR
2
is altered by means of the ON/OFF timing of the PWM signal applied to the base of transistor TR
1
, and the load current flowing in load L can thereby be controlled.
The advantages of this high side switch include:
(1) Load L is on the earth side of the switch (transistor TR
2
), so that voltage is not constantly applied to load L, so, even if a short circuit occurs when load L is not being driven, it is still safe; this arrangement is also on the safe side so far as electrical corrosion etc. due to moisture is concerned.
(2) Also, even if there is a short circuit in the wiring of the load whilst the load is being driven, the load L can be cut off by a switch (transistor TR
2
).
(3) Return wiring from load L can be dispensed with.
On the other hand, a drawback of this high side switch is that, due to the nature of the circuit, P-type elements such as a PNP bipolar transistor, P channel FET, or P channel IGBT etc. must be employed. In general, P type elements are inferior in characteristics to N type elements such as NPN transistors, N channel FETs, or N channel IGBTs in many respects such as their current amplification factor, voltage-withstanding ability, and saturation voltage, and are moreover costly. The efficiency of the switch circuit is therefore poor and it is uneconomic. And if the circuit is constructed using N type elements such as NPN transistors, N channel FETs or the like instead of the P type elements, the circuit construction will normally be an emitter follower or source follower circuit; the efficiency of this is poor in that the base-emitter voltage or gate-source voltage directly affects the collector-emitter voltage or drain-source voltage.
As a method for preventing this, in a high side switch circuit employing an emitter follower or source follower circuit of N type elements such as NPN transistors or N channel FETs, a circuit construction may be adopted wherein a drive power source is provided for base drive or gate drive independently of the main circuit, or, alternatively, in which the drive power source is stepped up in voltage from the main power source voltage by an amount matching the base-emitter voltage or gate-source voltage.
However, a large number of circuit elements are required in order to construct such an insulated power source and/or voltage step-up circuit, so this itself increases the cost of the device, and, furthermore, the failure rate will be increased in proportion to the increase in elements: thus the reliability of the circuit is lowered.
In contrast,
FIG. 12
shows a circuit layout, called a low side switch, in which switching of the load current is performed on the earthed side of the load.
In this circuit, a construction is adopted wherein transistor TR
1
constituting the switch means is provided on the earthed side of the load and the PWM signal of predetermined duty ratio is applied to the base of this transistor TR
1
; the circuit can therefore be made of even simpler construction than the high side switch.
When the PWM signal becomes high level and transistor TR
1
is turned ON, load current from the power source flows to earth through load L and transistor TR
1
. When this happens, due to the characteristics of the load L, the load current increases with time, finally becoming constant at a saturation point given by the source voltage and the amount of resistance, not shown, in inductive load L. The chopper control method, whereby a mean current value lower than this saturation point that is required for load L is maintained is essentially the same as described above with reference to the high side switch.
This method is superior in that it can be implemented with a simpler layout than the high side switch, and in that N type semiconductor elements can be employed for the switching elements, the drive voltages of these N type semiconductor elements being always referred to earth, so the emitter or source potential is fixed, etc., but it suffers from the problem that the current cannot be cut off if part of the load gets short-circuited to the earth s
Kato Koichi
Kawaji Yasushi
Maeda Kenichiro
Yoshida Daisuke
Fleming Fritz
Greer Burns & Crain Ltd.
Komatsu Ltd.
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