Accurate feed-forward sensing in flyback-transformer based secondary controller
A secondary side controller for an AC-DC converter and method for operating the same are provided. Generally, the controller includes a single synchronous rectifier sense (SR-SNS) pin coupled to a drain of a SR on a secondary of a transformer to sense a voltage (V SRD ). In feed-forward (FF) mode V SRD is a sum of a voltage (V IN ) on a primary divided by a turn-ratio (N) of the transformer and an output bus voltage (V BUS ). A voltage-to-current (V2I) converter coupled to the SR-SNS pin and to the output bus removes V BUS from V SRD . A sample and hold (S/H) module coupled to the SR-SNS pin samples a voltage (V SAMP ) including information on V IN /N. A V IN /N V2I converter coupled to the S/H module converts V SAMP to a feed-forward current (I FF ), and a cancellation and signal module coupled thereto extracts information on V IN from I FF and generates signals to control the AC-DC converter.
1. A secondary side controller for an AC-DC converter, the secondary side controller comprising:
a single synchronous rectifier sense (SR-SNS) pin coupled to a drain of a synchronous rectifier (SR) on a secondary side of a transformer in the AC-DC converter to sense a voltage (V SRD ) on the drain, wherein V SRD comprises a sum of a voltage (V IN ) applied to a primary side of the transformer divided by a turn ratio (N) of the transformer and a voltage (V BUS ) on an output bus coupled to the secondary side, and
a V BUS voltage-to-current (V2I) converter coupled to the output bus and to the SR-SNS pin to remove from a voltage (V SR-SNS ) on the SR-SNS pin a component of V SRD arising from V BUS ;
a sample and hold (S/H) module coupled to the SR-SNS pin to sample and hold a voltage (V SAMP ) comprising information on V IN /N;
a V IN /N V2I converter coupled to the S/H module to convert V SAMP to a feed-forward current (I FF ) comprising information on V IN /N; and
a N cancellation and signal module coupled to the V IN /N V2I converter to extract from the I FF information on V IN and generate feed-forward (FF) signals used by the secondary side controller to control the AC-DC converter.
2. The secondary side controller of claim 1 , wherein the SR-SNS pin is one pin of an integrated circuit comprising the V BUS V2I converter, the S/H module, the V IN /N V2I converter and the N cancellation and signal module.
3. The secondary side controller of claim 1 , wherein the single SR-SNS pin is coupled to the drain of the SR through a voltage divider including elements both internal and external to the IC.
4. The secondary side controller of claim 3 , wherein the elements of the voltage divider includes an external resistive element (Rext) having a resistance based on V IN and N.
5. The secondary side controller of claim 4 , wherein the V BUS V2I converter is configured to sink a current (I VBUS ) drawn through Rext to remove from V SR-SNS the component of V SRD arising from V BUS .
6. The secondary side controller of claim 4 , wherein the V BUS V2I converter is a temperature compensated circuit configured so that I VBUS is constant across temperature.
7. The secondary side controller of claim 4 , wherein the V BUS V2I converter is coupled to the SR-SNS pin through the V IN /N V2I converter, and wherein the V BUS V2I converter is configured to subtract from IFF in the V IN /N V2I converter a current substantially equivalent to a current capable of generating V BUS across a resistance equal to Rext.
8. The secondary side controller of claim 1 , wherein the FF signals generated by the N cancellation and signal module include FF over voltage (FF_OV) and FF under voltage (FF_UV) signals.
9. The secondary side controller of claim 1 , configured to control the AC-DC converter to provide a DC output compatible with a universal serial bus type C standard (USB-C).
10. A system comprising:
a transformer including a primary side coupled to receive an input voltage (V IN ), and a secondary side having an output and a synchronous rectifier (SR) coupled between the DC output and the secondary side; and
a secondary controller comprising:
a single synchronous rectifier sense (SR-SNS) pin coupled to a drain of the SR to sense a voltage (V SRD ) thereon, V SRD comprises a sum of V IN divided by a turn ratio (N) of the transformer and a voltage (V BUS ) on the output;
a V BUS voltage-to-current (V2I) converter coupled to the output and to the SR-SNS pin to remove from a voltage (V SR-SNS ) on the SR-SNS pin a component of V SRD arising from VBUS;
a sample and hold (S/H) module coupled to the SR-SNS pin to sample and hold a voltage (V SAMP ) comprising information on V IN and N;
a V IN /N V2I converter coupled to the S/H module to convert V SAMP to a feed-forward current (I FF ) comprising information on V IN /N; and
a N cancellation and signal module coupled to the V IN /N V2I converter to extract from the I FF information on V IN and generate FF signals used by the secondary side controller to control the flyback converter.
11. The system of claim 10 , wherein the SR-SNS pin is one pin of an integrated circuit comprising the V BUS V2I converter, the S/H module, the V IN /N V2I converter and the N cancellation and signal module.
12. The system of claim 10 , wherein the single SR-SNS pin is coupled to the drain of the SR through a voltage divider including elements both internal and external to the IC.
13. The system of claim 12 , wherein the elements of the voltage divider includes an external resistive element (Rext) having a resistance based on V IN and N.
14. The system of claim 13 , wherein the V BUS V2I converter is configured to sink a current (I VBUS ) drawn through Rext to remove from V SR-SNS the component of V SRD arising from V BUS .
15. The system of claim 13 , wherein the V BUS V2I converter is coupled to the SR-SNS pin through the V IN /N V2I converter, and wherein the V BUS V2I converter is configured to subtract from IFF in the V IN /N V2I converter a current substantially equivalent to a current capable of generating VBUS across a resistance equal to Rext.
16. The system of claim 10 , further comprising a DC output compatible with a universal serial bus type C standard (USB-C) connector.
17. A method of operating a secondary side controller for an AC-DC converter, the method comprising:
coupling a voltage (V SRD ) on a drain of a synchronous rectifier (SR) on a secondary side of a transformer in the AC-DC converter to a single synchronous rectifier sense (SR-SNS) pin of the secondary side controller, V SRD comprises a sum of a voltage (V IN ) applied to a primary side of the transformer divided by a turn ratio (N) of the transformer and a voltage (V BUS ) on an output bus coupled to the secondary side;
sensing a voltage (V SR-SNS ) on the SR-SNS pin;
removing from the V SR-SNS on the SR-SNS pin a component of V SRD arising from V BUS ;
sampling and holding a voltage (V SAMP ) coupled from the SR-SNS pin, wherein VSAMP comprises information on V IN /N;
converting V SAMP to a feed-forward current (I FF ) comprising information on V IN /N; and
extracting from the I FF information on V IN and generating FF signals used by the secondary side controller to control the AC-DC converter.
18. The method of claim 17 , wherein the secondary side controller is implemented as an integrated circuit (IC), and wherein the single SR-SNS pin is coupled to the drain of the SR through a voltage divider including elements internal to the IC and an external resistive element (Rext).
19. The method of claim 18 , wherein removing the component of V SRD arising from V BUS comprises sinking a current (I VBUS ) drawn through Rext to remove from V SR-SNS the component of V SRD arising from V BUS .
20. The method of claim 18 , wherein sensing V SR-SNS on the SR-SNS pin comprises converting V SR-SNS to a current substantially equivalent to that necessary to generate V SR-SNS across a resistance equal to Rext, and wherein removing the component of V SRD arising from V BUS comprises subtract from I FF a current substantially equivalent to a current capable of generating V BUS across a resistance equal to Rext.