Lossless clamp circuit for DC-DC converters
View Patent ↗A lossless clamping circuit for use in a synchronous rectifier DC-DC converter in which at least one transistor switch (MOSFET) periodically connects two terminals of an output transformer winding across a main output and return terminal to provide a DC current at a selected voltage for a load connected between the output and reference terminals. A diode and an auxiliary capacitor are connected in series across the MOSFET's drain source so that the capacitor accumulates a charge in accordance with the reverse recovery current in the winding leakage inductance when the MOSFET is turned off and an auxiliary DC-DC converter is connected to the auxiliary capacitor for converting the accumulated charge across the capacitor to a DC current at a selected output voltage. Preferably the output of the auxiliary DC-DC converter is connected to the main output terminal to supplement the output power.
1. In a power recovery circuit for use in a DC-DC converter, the converter having an output winding ( 12 ), a main DC output terminal connected to one terminal of the winding through an output inductor (L 01 ) and an output capacitor (C 0 ) connected between the main output terminal and a return voltage with a MOSFET transistor switch having its drain and source terminals connected between the return and another terminal of the winding and a controller ( 18 ) for supplying the gate source drive current to the MOSFET independently of the output winding ( 12 ) to provide a synchronously rectified current to the main output terminal, the power recovery circuit comprising:
a) a rectifier (D 3 ) and an auxiliary capacitor (C 1 ) connected in series across the transistor's drain source whereby the auxiliary capacitor accumulates a charge of the reverse recovery current in the winding leakage inductance when the transistor switch ceases to conduct; and
b) an auxiliary DC-DC converter connected between the auxiliary capacitor and an auxiliary output terminal to convert charge across the auxiliary capacitor to a direct current at a selected voltage at the auxiliary output terminal ( 24 ).
2. The power recovery circuit of claim 1 wherein the auxiliary output terminal is arranged to supply current to other circuit components.
3. The power recovery circuit of claim 1 wherein the auxiliary output terminal is connected to the main output terminal.
4. The power recovery circuit of claim 3 wherein the converter includes a second output inductor (L 02 ) connected between said other winding terminal and the main output terminal and a second transistor switch having its drain source electrodes connected in series between said one winding terminal and the return voltage and wherein the power recovery circuit further includes a second rectifier (D 4 ), the second rectifier and the auxiliary capacitor being coupled across the drain source electrodes of the second transistor switch to increase the auxiliary capacitor's accumulated charge in accordance with the reverse recovery current in the winding leakage inductance when the second transistor switch ceases to conduct.
5. The power recovery circuit of claim 1 wherein the converter includes a second output winding with the output inductor connected to one terminal of the second winding and a second transistor switch having its drain source electrodes connected in series with the other terminal of the second winding and the return voltage so that the first and second transistor switches provide synchronously rectified current to the output terminal during both halves of the voltage waveform in the output winding and wherein the power recovery circuit includes a second rectifier, the second rectifier and auxiliary capacitor being coupled across the second transistor switch's drain source electrodes to increase the auxiliary capacitor's accumulated charge in accordance with the reverse recovery current in the second winding leakage inductance when the second transistor switch ceases to conduct.
6. In a lossless clamping circuit for use in a synchronous rectifier DC-DC converter in which at least one transistor switch, under the control of a PWM controller, periodically connects at least two terminals of an output transformer winding across a main output and return terminal for current flow through the winding in one direction to provide a DC current at a selected voltage for a load connected between the main output and reference terminals, the PWM controller being independent of the transformer winding, the clamping circuit comprising:
a) a diode and an auxiliary capacitor connected in a series relationship across the drain source terminals of the transistor switch so that the capacitor accumulates a charge in accordance with the reverse recovery current in the winding leakage inductance when the transistor switch ceases to conduct to clamp the peak voltage across the drain source electrode of the transistor switch at a maximum preselected value; and
b) an auxiliary DC-DC converter connected to the auxiliary capacitor for converting the accumulated charge on the auxiliary capacitor to a DC current at a selected output voltage.
7. The lossless clamping circuit of claim 6 wherein the converter includes a second transistor switch, under the control of said PWM controller, which periodically connects said two winding terminals across the output and reference terminals for current flow through the winding in the opposition direction and wherein the clamping circuit includes a second diode with the second diode and auxiliary capacitor being connected in a series across the drain source electrodes of the second transistor switch to increase the auxiliary capacitor's accumulated charge in accordance with the reverse recovery current in the winding leakage inductance when the second transistor switch ceases to conduct.
8. The lossless clamping circuit of claim 6 wherein the output transformer includes a second output winding with a second transistor switch, under the control of said PWM controller, periodically connecting the two terminals of the second output winding across the main output and return terminals for current flow in the opposite direction to provide current to the main output terminal during each half wave of the input voltage and wherein the clamping circuit further includes a second diode with the second diode and auxiliary capacitor connected in series across the drain source terminals of the second transistor switch to increase the auxiliary capacitor's accumulated charge in accordance with the reverse recovery current in the second winding's leakage inductance when the second transistor switch ceases to conduct.
9. The lossless clamp of claim 6 wherein the output of the auxiliary DC-DC converter is connected to the main output terminal.
10. The lossless clamp of claim 6 wherein the output of the auxiliary DC-DC converter is arranged to supply current to other components.
11. The lossless clamp of claim 7 wherein the output of the auxiliary DC-DC converter is connected to the main output terminal.
12. The lossless clamp of claim 7 wherein the output of the auxiliary DC-DC converter is arranged to supply current to other components.
13. The lossless clamp of claim 8 wherein the output of the auxiliary DC-DC converter is arranged to supply current to other components.