Charge recovery for enhanced transistor drive
A control circuit for an electro luminescent display in which a flyback converter and an H-bridge are used to provide a high voltage alternating current to a capacitive load from a low voltage DC source. Each time the polarity of the capacitive load is reversed, the capacitive load discharges. This high voltage discharge is stored on a low voltage capacitor and subsequently used as an auxiliary power supply to power the switching elements of the control circuit.
1. A control circuit comprising:
a low voltage DC source;
a supply circuit including:
a converter including at least one converter switching element and arranged to receive a first voltage supplied from the low voltage DC source and to generate a pulsed direct current; and
switching elements arranged to define an H-bridge circuit including two parallel limbs that each include a first switching element in series with a second switching element and a node between the first and second switching elements;
a capacitive load connected between the respective nodes of the two parallel limbs and arranged to be charged to a voltage which is higher than that of the low voltage DC source by receiving the pulsed direct current from the H-bridge circuit; and
a discharge circuit arranged to receive a high voltage discharge current from the capacitive load and to generate a second voltage that is higher than the first voltage at least during charging of the capacitive load; wherein
the second voltage is used to drive the switching elements of the H-bridge circuit and the at least one converter switching element.
2. A control circuit according to claim 1 , wherein the discharge circuit is a controlled discharge circuit connected in parallel with the H-bridge circuit.
3. A control circuit according to claim 1 , wherein the discharge circuit includes a voltage limiting device arranged to limit the second voltage.
4. A control circuit according to claim 3 , wherein the voltage limiting device limits the second voltage to a nominal 12 V.
5. A control circuit according to claim 3 , wherein the discharge circuit includes a low voltage capacitor.
6. A control circuit according to claim 5 , wherein the voltage limiting device includes a zener diode connected in parallel with the low voltage capacitor.
7. A control circuit according to claim 1 , wherein the switching elements of the control circuit are MOSFETs.
8. A control circuit according to claim 1 , wherein the control circuit includes a smoothing capacitor connected in parallel with the capacitive load.
9. A control circuit according to claim 1 , wherein the capacitive load is an electroluminescent display.
10. A control circuit according to claim 1 , wherein the second voltage is at least 4 V.
11. A control circuit according to claim 1 , wherein the second voltage is at least 9 V.
12. A method supplying a capacitive load:
generating a pulsed direct current from a first voltage provided by a low voltage DC source using at least one generating switching element;
supplying the pulsed direct current to the capacitive load through switching elements arranged to define an H-bridge circuit that includes two parallel limbs that each include a first switching element in series with a second switching element and a node between the first and second switching elements, the capacitive load is connected between the respective nodes of the two parallel limbs and is arranged to be charged to a voltage which is higher than that of the low voltage DC source by receiving the pulsed direct current from the H-bridge circuit;
generating a second voltage that is higher than the first voltage at least during charging of the capacitive load, from a high voltage current that is discharged from the capacitive load; and
driving the switching elements of the H-bridge circuit and the at least one generating switching element with the second voltage.
13. A control circuit arranged to supply a pulsed direct current, the control circuit comprising:
a low voltage DC source providing a first voltage;
an H-bridge circuit including two parallel limbs that each include a first switching element in series with a second switching element and a node between the first and second switching elements;
a load connected between the respective nodes of the two parallel limbs and arranged to be charged to a voltage which is higher than that of the low voltage DC source by receiving the pulsed direct current; and
a converter connected to the low voltage DC source, arranged to generate the pulsed direct current, and including at least one converter switching element; wherein
the control circuit is arranged to drive the load and to receive current from the load such that the current received from the load is used to drive the switching elements of the H-bridge circuit and the at least one converter switching element; and
the first and second switching elements and the at least one converter switching element are driven at a second voltage higher than the first voltage at least during charging of the capacitive load.