Switching circuit
An H-bridge switching circuit for an automated external defibrillator comprises an SCR (D 8 ) in one of the high legs of the circuit and a control means (D 1 -D 7 ) associated with the SCR which is operative to switch the SCR on automatically in response to a voltage change across the SCR corresponding to the switching device in the diagonally opposite leg of the H-bridge turning on. The control means comprises a capacitor (D 1 ) and the voltage on the capacitor changes when the diagonally opposite switching device turns on, the change in capacitor voltage lagging the change in voltage across the SCR and the SCR being turned on when the difference between the capacitor voltage and the voltage across the SCR exceeds a predetermined threshold.
1. An external defibrillator comprising an H-bridge for delivering bipolar electrotherapy to a patient, the H-bridge having:
(a) a respective solid state switching device in each of its four legs, the switching device in at least one of the high legs of the H-bridge including an SCR,
(b) an input for applying an external control signal to the switching device in the low leg diagonally opposite said at least one high leg, wherein turning on the switching device in said diagonally opposite low leg causes a voltage increase across the SCR in said at least one high leg, and
(c) a control circuit in said at least one high leg, the control circuit comprising a capacitor connected in parallel with the anode-gate path of the SCR, the capacitor having a time constant which generates a voltage spike at the gate of the SCR in response to said voltage increase across the SCR, said spike turning on the SCR,
whereby the SCR is turned on without the application of an external control signal to said high leg control circuit.
2. The defibrillator of claim 1 , wherein the voltage spike is applied to the gate of the SCR via a voltage threshold device connected in series between the capacitor and the gate.
3. The defibrillator of claim 2 , wherein the voltage threshold device comprises a diac.
4. The defibrillator of claim 2 , wherein the control circuit further comprises a diode connected between the capacitor and the gate in parallel with the voltage threshold device.