High side active clamp charging circuit

1. A power supply comprising:

a power supply voltage input;

a transformer comprising a primary winding coupled to the power supply voltage input;

a first switch coupled to the primary winding and to the power supply voltage input;

an active clamp circuit coupled in parallel with the primary winding and comprising a second switch;

a clamp switch control coupled to the second switch and comprising a voltage input;

a bootstrap circuit coupled to the clamp switch control and comprising a bootstrap voltage storage device coupled to the voltage input of the clamp switch control;

a charging circuit comprising:

a first voltage input coupled to the primary winding;

a voltage output coupled to the bootstrap voltage storage device;

a resistor network configured to generate a charge voltage in response to an input voltage supplied to the first voltage input by the primary winding during a first portion of a switching cycle of the first switch; and

a charge voltage storage device coupled to the resistor network and configured to:

store at least a portion of the charge voltage during the first portion of the switching cycle; and

supply the at least a portion of the charge voltage to the bootstrap voltage storage device via the voltage output during a second portion of the switching cycle.

2. The power supply of claim 1 , wherein the charging circuit is configured to supply the charge voltage to the bootstrap voltage storage device during the first portion of the switching cycle.

3. The power supply of claim 2 , wherein the bootstrap circuit further comprises an internal power supply coupled to the charge voltage storage device and configured to provide a power supply voltage to charge the bootstrap voltage storage device during the first portion of the switching cycle.

4. The power supply of claim 3 , wherein the power supply voltage is less than the input voltage.

5. The power supply of claim 1 , wherein the charging circuit further comprises:

a second voltage input coupled to a low-side node coupled between the primary winding and the first switch; and

a voltage reference device coupled between the resistor network and the second voltage input.

6. The power supply of claim 5 , wherein the voltage reference device comprises a Zener diode.

7. The power supply of claim 1 , further comprising:

a first switch control coupled to the first switch;

a second switch control coupled to the second switch; and

a controller coupled to the first and second switch controls and configured to:

cause the first switch control to control the first switch into an on state during the first portion of the switching cycle;

cause the second switch control to control the second switch into an off state during the first portion of the switching cycle;

cause the first switch control to control the first switch into an off state during the second portion of the switching cycle; and

cause the second switch control to control the second switch into an on state during the second portion of the switching cycle.

8. The power supply of claim 1 , wherein the transformer further comprises a secondary winding; and

wherein the power supply further comprises a filter coupled to the secondary winding and configured to provide an output voltage to a load, the output voltage proportional to an input power supply voltage provided to the power supply voltage input.

9. The power supply of claim 8 , wherein the filter comprises:

a pair of diodes;

an inductor; and

a capacitor.

10. The power supply of claim 1 , wherein the bootstrap voltage storage device comprises a capacitor; and

wherein the charge voltage storage device comprises a capacitor.

11. The power supply of claim 10 , wherein the bootstrap voltage storage device has a greater capacitance than the charge voltage storage device.

12. A charging circuit for an active clamp forward converter, the active clamp forward converter comprising an active switch coupled to a primary winding of a transformer, a filter circuit coupled to a secondary winding of the transformer, a high-side active clamp coupled in parallel with the primary winding, a first switch control coupled to the active switch, a second switch control coupled to the high-side active clamp, and a bootstrap circuit coupled to the second switch control, the charging circuit comprising:

a first voltage input coupled to the primary winding;

a voltage output coupled to the bootstrap circuit;

a resistor network configured to generate a charge voltage in response to an input voltage supplied to the first voltage input by the primary winding during a first portion of a switching cycle of the active switch; and

a charge voltage storage device coupled to the resistor network and configured to:

store at least a portion of the charge voltage during the first portion of the switching cycle; and

supply the at least a portion of the charge voltage to the bootstrap circuit via the voltage output during a second portion of the switching cycle.

13. The charging circuit of claim 12 , wherein the charging circuit is configured to supply the charge voltage to the bootstrap circuit during the first portion of the switching cycle.

14. The charging circuit of claim 12 , wherein the charging circuit further comprises:

a second voltage input coupled to the primary winding and the active switch; and

a voltage reference device coupled between the resistor network and the second voltage input.

15. The charging circuit of claim 14 , wherein the voltage reference device comprises a Zener diode.

16. An active clamp forward converter comprising:

a first switch control coupled to a first active switch;

a transformer comprising a primary winding coupled to the first active switch;

an active clamp coupled in parallel with the primary winding;

a second switch control coupled to the active clamp;

a bootstrap circuit coupled to the second switch control; and

a charging circuit comprising:

a first voltage input coupled to the primary winding;

a voltage output coupled to the bootstrap circuit;

a resistor network configured to generate a charge voltage in response to an input voltage supplied to the first voltage input by the primary winding during a first portion of a switching cycle of the first active switch; and

a charge voltage storage device coupled to the resistor network and configured to:

store at least a portion of the charge voltage during the first portion of the switching cycle; and

supply the at least a portion of the charge voltage to the bootstrap circuit via the voltage output during a second portion of the switching cycle.

17. The active clamp forward converter of claim 16 , wherein the charging circuit is configured to supply the charge voltage to the bootstrap circuit during the first portion of the switching cycle.

18. The active clamp forward converter of claim 16 , wherein the bootstrap circuit comprises a capacitor configured to provide a supply voltage to the second switch control; and

wherein the charge voltage storage device, in being configured to supply the at least a portion of the charge voltage, is configured to supply the at least a portion of the charge voltage to the capacitor.

19. The active clamp forward converter of claim 16 , wherein the charging circuit further comprises:

a second voltage input coupled to the primary winding and the first active switch; and

a voltage reference device coupled between the resistor network and the second voltage input.

20. The active clamp forward converter of claim 16 , wherein the bootstrap circuit comprises a second active switch; and

further comprising a controller coupled to the first and second switch controls and configured to:

cause the first switch control to control the first active switch into an on state during the first portion of the switching cycle;

cause the second switch control to control the second active switch into an off state during the first portion of the switching cycle;

cause the first switch control to control the first active switch into an off state during the second portion of the switching cycle; and

cause the second switch control to control the second active switch into an on state during the second portion of the switching cycle.