Reverse phase control power switching circuit with overload protection
In one embodiment of the invention, a reverse phase control power switching circuit for controlling the flow of current through a load from an AC source comprises a sensing bridge in series with the load and the AC source. The conductive state of the sensing bridge is responsive to a pulse generator and a zero crossing detection and overload protection means. Phase control means coupled to the pulse generator determines turn off of the sensing bridge is reset by the zero crossing detection and overload protection means. Responsive to a load current condition exceeding a maximum current caused by an overload or a short, the zero crossing detection and overloading protection network terminates conduction of current through the sensing bridge. The sensing bridge starts conducting current to the load at the beginning of the AC cycle and continues to conduct to a desired AC voltage phase angle or earlier if an excessive current condition caused by an overload condition or a short occurs.
1. A phase control power switching circuit for controlling the phase angle of current flow through a load from an alternating current source, the circuit comprising:
a pair of voltage control switches in inverse series connection for controlling alternating current flow through the load;
maximal voltage sensing means coupled to the pair of voltage control switches to sense a maximal voltage between the pair of voltage control switches;
a zero crossing detection and overload protection network coupled to said maximal voltage sensing means and having the maximal voltage as an input;
control means having a phase control means for selectively adjusting the phase angle to drive the pair of voltage control switches to an off condition; and
pulse generator means adapted to generate a first voltage of a first value and a second voltage of a lower value wherein the second voltage can have a value of zero volts or greater and wherein the pulse generator means is coupled to drive the pair of voltage control switches from their on condition to their off condition,
wherein
the phase control means is coupled to selectively switch the output of the pulse generator means from the first voltage to the second voltage to drive the voltage control switches to their off condition,
the phase control means comprises a resistor in series with a capacitor wherein the voltage across the capacitor is applied to the pulse generator means, and
the zero crossing detection and overload protection network is coupled to discharge the capacitor after the pulse generator drives the voltage control switches to their off condition.
2. The phase control power switching circuit of claim 1 wherein the maximal voltage sensing means comprises first and second series connected diodes coupled in parallel to the pair of voltage control switches wherein the maximal voltage between the pair of voltage control switches is sensed between the first diode and the second diode.
3. The phase control power switching circuit of claim 2 wherein the control means is adapted to assume a first state when the maximal voltage sensed is less than a set voltage and a second state when the maximal voltage sensed is greater than the set voltage, wherein the pair of control switches are switched from their on condition to their off condition in accordance with the control means being in its second state.
4. The phase control power switching circuit of claim 3 wherein the control means assumes its second state and causes switching of the pair of voltage control switches to their off condition when an overheated voltage control switch causes the maximal voltage sensed to be greater than the set voltage.
5. The phase control power switching circuit of claim 1 wherein the voltage control switches are adapted to be driven to their off condition by the second voltage from the pulse generator.
6. The phase control power switching circuit of claim 1 wherein the time required to charge the capacitor to a set voltage is controllable.
7. The phase control power switching circuit of claim 1 wherein the phase control means is coupled to switch the output of the pulse generator to the first voltage when the capacitor is discharged to condition the pair of voltage control switches to assume their on condition at the next occurring zero crossing time of the AC voltage waveform.
8. The phase control power switching circuit of claim 1 wherein the pair of voltage control switches are MOSFETs.
9. The phase control power switching circuit of claim 1 wherein the pair of voltage control switches are IGBTs.