Reducing voltage stress in a flyback converter design
Disclosed is flyback converter having a controller that performs a startup switching process when the flyback converter is powered up, and then performs normal switching afterward. The controller includes a pulse generator to generate a control signal for normal switching. During startup switching, the controller may generate a control signal by output every N th pulse from the pulse generator. In another embodiment, the controller may generate pulses based on a sense signal provided from an input section of the flyback converter.
1. A method for adjusting a duty cycle of switching of a switch in a flyback converter, wherein the flyback converter further includes a transformer having (i) a primary winding and (ii) a secondary winding, wherein the switch is coupled to the primary winding of the transformer, the method comprising, while applying an input voltage to the primary winding of the transformer;
alternately switching the switch between a closed position and an opened position in accordance with a first duty cycle, wherein while the switch is in the open position, no current flows in the secondary winding of the transformer, and wherein while the switch is in the closed position, current flows in the secondary winding of the transformer which causes an output voltage of the flyback converter to increase;
applying the output voltage to a load based on the current flowing through the secondary winding;
while the switch is being alternately switched between the closed position and the opened position in accordance with the first duty cycle, monitoring the output voltage of the flyback converter to determine whether the output voltage has exceeded a voltage threshold, wherein the voltage threshold is predetermined and set to limit the output voltage supplied to the load;
in response to the output voltage being determined to have exceeded the voltage threshold, alternately switching the switch between the open position and the closed position in accordance with a second duty cycle, wherein the second duty cycle is greater than the first duty cycle;
generating a first control signal having the first duty cycle;
generating a second control signal having the second duty cycle, wherein the second control signal is separate from the first control signal, wherein alternately switching the switch between the closed position and the opened position in accordance with the first duty cycle comprises alternately switching the switch based on the first control signal, and wherein alternately switching the switch between the closed position and the opened position in accordance with the second duty cycle comprises alternately switching the switch based on the second control signal;
controlling a level of current flowing through the secondary winding according to the first control signal during a first period of time;
controlling the level of current flowing through the secondary winding according to the second control signal during a second period of time, wherein the second period of time is subsequent to the first period of time;
generating a first plurality of pulses during the first period of time, wherein the first plurality of pulses are output at a predetermined frequency;
generating a second plurality of pulses during the second period of time, wherein the second plurality of pulses are output at the predetermined frequency;
during the first period of time, outputting every predetermined number of pulses from of the first plurality of pulses as the first control signal during the first period of time;
during the second period of time, outputting every pulse from each of the second plurality of pulses as the second control signal at the time subsequent to the first period of time;
comparing a first threshold value to the level of the current flowing through the switch;
based on a result of the comparing of the first threshold value to the level of the current flowing through the switch, generating a first pulse to be output as the first control signal;
incrementing the first threshold value to provide a second threshold value;
comparing the second threshold value to the level of the current flowing through the switch; and
based on the comparing of the second threshold value to the level of the current flowing through the switch, generating a second pulse to be output as the first control signal.
2. The method of claim 1 , wherein:
while the switch is in the closed position, a magnetizing current is created in a core of the transformer; and
alternately switching the switch between the closed position and the opened position in accordance with the first duty cycle or the second duty cycle to prevent the magnetizing current in the core of the transformer from exceeding a saturation level of the transformer.
3. The method of claim 1 , wherein the switch is alternately switched between the closed position and the opened position in accordance with the first duty cycle during a startup period in which the flyback converter is powered up from a power off state.
4. The method of claim 1 , further comprising:
reading a memory for a predetermined value; and
determining the predetermined number of pulses based on the predetermined value.
5. The method of claim 1 , further comprising:
comparing the voltage supplied to the load to the voltage threshold; and
controlling the switch with the second control signal when the voltage supplied to the load exceeds the voltage threshold.
6. A circuit comprising:
a transformer having a first winding and a second winding, wherein the second winding supplies a voltage to a load, wherein the first winding is a primary winding of the transformer;
a transistor connected to the first winding, wherein the transistor is configured to enable and disable a current flow through the first winding;
a comparator configured to compare the voltage supplied to the load to a voltage threshold; and
a controller connected to the transistor, wherein the controller is configured to
a) set the voltage threshold to limit the voltage supplied to the load,
b) generate a first control signal having a first duty cycle,
c) generate a second control signal having a second duty cycle, wherein the second duty cycle is greater than the first duty cycle,
d) based on a first output of the comparator and during a first period of time, (i) select the first control signal rather than the second control signal, and (ii) control the transistor with the first control signal,
e) based on a second output of the comparator, select the second control signal rather than the first control signal,
f) at the time subsequent to the first period of time, control the transistor with the second control signal,
g) compare a first threshold value to a level of current flowing through the transistor,
h) based on a result of the comparing of the first threshold value to the level of the current flowing through the transistor, generate a first pulse to be output as the first control signal,
i) increment the first threshold value to provide a second threshold value,
j) compare the second threshold value to the level of the current flowing through the transistor, and
k) based on the comparing of the second threshold value to the level of the current flowing through the transistor, generate a second pulse to be output as the first control signal.
7. The circuit of claim 6 , wherein:
the first winding is configured to generate a magnetically induced current in a core of the transformer; and
the controller is configured to control the transistor such that the magnetically induced current does not exceed a threshold level during the first period of time, wherein the threshold level is a saturation current level of the transformer.
8. The circuit of claim 6 , further comprising a pulse generator configured to (i) generate a first plurality of pulses during the first period of time, and (ii) a second plurality of pulses during a second period of time, wherein:
the second period of time is subsequent to the first period of time;
the first control signal comprises every Nth pulse of the first plurality of pulses generated by the pulse generator during the first period of time, where N is an integer greater than or equal to two; and
the second control signal comprises each of the second plurality of pulses generated by the pulse generator during the second period of time.
9. The circuit of claim 8 , further comprising a memory configured to store a predetermined value,
wherein the controller is configured to determine the integer N based on the predetermined value.
10. The circuit of claim 6 , wherein the controller is configured to:
monitor a level of current flowing through the transistor;
generate a plurality of pulses to be output as the first control signal; and
based on the level of the current flowing through the transistor, controlling pulse widths of the plurality of pulses.
11. A flyback converter comprising:
the circuit of claim 6 ; and
a feedback loop connected between the load and the controller, wherein the feedback loop provides the voltage supplied to the load to the controller.
12. A method for adjusting a duty cycle of switching of a switch in a flyback converter, wherein the flyback converter further includes a transformer having (i) a primary winding and (ii) a secondary winding, wherein the switch is coupled to the primary winding of the transformer, the method comprising, while applying an input voltage to the primary winding of the transformer:
alternately switching the switch between a closed position and an opened position in accordance with a first duty cycle, wherein while the switch is in the open position, no current flows in the secondary winding of the transformer, and wherein while the switch is in the closed position, current flows in the secondary winding of the transformer which causes an output voltage of the flyback converter to increase;
applying the output voltage to a load based on the current flowing through the secondary winding;
while the switch is being alternately switched between the closed position and the opened position in accordance with the first duty cycle, monitoring the output voltage of the flyback converter to determine whether the output voltage has exceeded a voltage threshold, wherein the voltage threshold is predetermined and set to limit the output voltage supplied to the load;
in response to the output voltage being determined to have exceeded the voltage threshold, alternately switching the switch between the open position and the closed position in accordance with a second duty cycle, wherein the second duty cycle is greater than the first duty cycle;
generating a plurality of pulses;
determining whether the voltage supplied to the load is less than a first threshold;
if the output voltage supplied to the load is less than the first threshold, (i) transitioning an output of a first switching device to a first state while generating a first series of pulses, and (ii) supplying the first series of pulses to the switch, wherein the first series of pulses are generated based on the plurality of pulses and during a first period of time;
if the voltage supplied to the load is greater than or equal to the first threshold, (i) setting a current threshold, and (ii) during a second period of time, generating a second series of pulses based on the current threshold and the plurality of pulses, wherein the second period of time is subsequent to the first period of time;
determining whether a level of the current flowing through the switch is greater than a second threshold;
if the level of the current flowing through the switch is greater than the second threshold, (i) transitioning the output of the first switching device to a second state, (ii) skipping a predetermined number of the plurality of pulses, and (iii) incrementing the second threshold; and
if the level of the current flowing through the switch is less than or equal to the second threshold, transitioning the output of the first switching device to the second state.
13. A circuit comprising:
a transformer having a first winding and a second winding, wherein the second winding supplies a voltage to a load;
a transistor connected to the first winding, wherein the transistor is configured to enable and disable a current flow through the first winding;
a comparator configured to compare the voltage supplied to the load to a voltage threshold; and
a controller connected to the transistor, wherein the controller is configured to
a) set the voltage threshold to limit the voltage supplied to the load,
b) generate a first control signal having a first duty cycle,
c) generate a second control signal having a second duty cycle, wherein the second duty cycle is greater than the first duty cycle,
d) based on a first output of the comparator and during a first period of time, (i) select the first control signal rather than the second control signal, and (ii) control the transistor with the first control signal,
e) based on a second output of the comparator, select the second control signal rather than the first control signal, and
f) at the time subsequent to the first period of time, control the transistor with the second control signal,
wherein
the controller comprises a multiplexer and a logic gate,
the multiplexer is configured to (i) based on the first output of the comparator, select the first control signal, and (ii) based on the second output of the comparator, select the second control signal,
the logic gate receives an output of the multiplexer and a brown out signal,
the brown out signal indicates whether a brown out condition exists, and
an output of the logic gate controls the transistor.
14. A circuit comprising:
a transformer having a first winding and a second winding, wherein the second winding supplies a voltage to a load;
a transistor connected to the first winding, wherein the transistor is configured to enable and disable a current flow through the first winding;
a comparator configured to compare the voltage supplied to the load to a voltage threshold;
a controller connected to the transistor, wherein the controller is configured to
a) set the voltage threshold to limit the voltage supplied to the load,
b) generate a first control signal having a first duty cycle,
c) generate a second control signal having a second duty cycle, wherein the second duty cycle is greater than the first duty cycle,
d) based on a first output of the comparator and during a first period of time, (i) select the first control signal rather than the second control signal, and (ii) control the transistor with the first control signal,
e) based on a second output of the comparator, select the second control signal rather than the first control signal, and
f) at the time subsequent to the first period of time, control the transistor with the second control signal;
a first switching device configured to (i) receive a pulse signal, a skip value, the voltage threshold, and a current value, and (ii) output a first series of pulses based on the pulse signal, the skip value, the voltage threshold, and the current value,
wherein
the skip value indicates a number of pulses in the pulse signal to skip,
the threshold value is a current threshold of the transistor, and
the current value is a level of current flowing through the transistor;
a second switching device configured to (i) receive the pulse signal and the voltage supplied to the load, and (ii) output a second series of pulses based on the pulse signal and the voltage supplied to the load; and
a multiplexer configured to (i) based on the first output of the comparator, select the output of the first switching device, or (ii) based on the second output of the comparator, select the output of the second switching device,
wherein the transistor is controlled based on the output of the multiplexer.