Output overvoltage protection for a totem pole power factor correction circuit
During a light load (or no-load) operation of a totem pole power factor correction circuit (i.e., PFC), a pulse width modulation (PWM) controller can operate in a skip mode. Further, the PWM controller may disable portions of the PFC to reduce standby power consumption. In this mode, and in this disabled configuration, the output of the PFC may be peak charged over time to a voltage that could be damaging or destructive. This peak charging results from the PFC circuit's inability to fully charge/discharge EMI capacitors between half cycles of the input line voltage. The present disclosure provides circuits and methods to fully charge/discharge the EMI capacitors to prevent peak charging the output.
1. A method for controlling a power factor correction (PFC) circuit, the method comprising:
detecting that an input voltage is in a first half cycle having a first polarity;
applying a PWM signal to a first transistor of a fast-leg portion of the PFC circuit for a conversion process corresponding to the first half cycle;
detecting that the input voltage is at a first polarity change;
pausing the PWM signal; and
applying a pulse sequence to a second transistor of the fast-leg portion of the PFC circuit to change a voltage on an EMI capacitor from a first voltage corresponding to the first half cycle to a second voltage corresponding to a second half cycle having a second polarity opposite the first polarity.
2. The method for controlling a power factor correction (PFC) circuit according to claim 1 , wherein:
at the first polarity change the input voltage changes from a positive voltage to a negative voltage; and
the pulse sequence is applied on the second transistor to change the voltage on the EMI capacitor from approximately a ground voltage to approximately a bulk voltage at an output of the PFC circuit.
3. The method for controlling a power factor correction (PFC) circuit according to claim 1 , further comprising:
after the pulse sequence, applying the PWM signal to the second transistor of the fast-leg portion of the PFC circuit for a conversion process corresponding to the second half cycle;
detecting that the input voltage is at a second polarity change;
pausing the PWM signal; and
applying the pulse sequence to the first transistor of the fast-leg portion of the PFC circuit to change the voltage on the EMI capacitor from the second voltage corresponding to the second half cycle to the first voltage corresponding to the first half cycle.
4. The method for controlling a power factor correction (PFC) circuit according to claim 3 , wherein:
at the second polarity change the input voltage changes from a negative voltage to a positive voltage; and
the pulse sequence is applied on the first transistor to change the voltage on the EMI capacitor from approximately a bulk voltage at an output of the PFC circuit to approximately a ground voltage.
5. The method for controlling a power factor correction (PFC) circuit according to claim 1 , wherein the pulse sequence includes a plurality of pulses having substantially equal duty cycles.
6. The method for controlling a power factor correction (PFC) circuit according to claim 5 , wherein an ON period for each pulse of the plurality of pulses increases in succession in the pulse sequence.
7. The method for controlling a power factor correction (PFC) circuit according to claim 6 , wherein the pulse sequence includes four pulses and has a duration of less than 100 microseconds.
8. The method for controlling a power factor correction (PFC) circuit according to claim 1 , wherein the pulse sequence includes a first delay for a first period before the pulse sequence starts and a second delay for a second period after the pulse sequence ends.
9. The method for controlling a power factor correction (PFC) circuit according to claim 1 , wherein the conversion process is a skip-mode conversion.
10. The method for controlling a power factor correction (PFC) circuit according to claim 1 , wherein:
the EMI capacitor is coupled to a slow-leg node of a slow-leg portion of the PFC circuit; and
the voltage on the EMI capacitor is the voltage at the slow-leg node, the slow-leg node further coupled to a first slow-leg transistor and a second slow-leg transistor.
11. The method for controlling a power factor correction (PFC) circuit according to claim 10 , further including:
detecting a light-load condition at an output of the PFC circuit; and
disabling the slow leg portion of the PFC circuit by transmitting OFF switching signals to the first slow-leg transistor and the second slow-leg transistor.
12. The method for controlling a power factor correction (PFC) circuit according to claim 11 , wherein:
the light-load condition is detected at the output of the PFC circuit when a load has been decoupled from the output of the PFC circuit.
13. The method for controlling a power factor correction (PFC) circuit according to claim 1 , wherein each pulse in the pulse sequence incrementally changes the voltage on the EMI capacitor from the first voltage to the second voltage.
14. A power factor correction (PFC) system comprising:
an alternating current (AC) source configured to output a line voltage that changes a polarity at half cycles;
a PFC circuit configured to receive the line voltage at an input and having a fast-leg portion; and
a controller configured to:
detect that the line voltage received at the PFC circuit is in a negative half cycle;
transmit a PWM signal to a first transistor of the fast-leg portion of the PFC circuit for a conversion process corresponding to the negative half cycle, the conversion process generating a bulk voltage at an output of the PFC circuit;
detect that the line voltage is at a first polarity change;
pause the PWM signal; and
transmit a pulse sequence to a second transistor of the fast-leg portion of the PFC circuit to change a voltage on an EMI capacitor from a first voltage corresponding to the negative half cycle to a second voltage corresponding to a positive half cycle.
15. The power factor correction (PFC) system according to claim 14 , wherein the first voltage corresponding to the negative half cycle is approximately the bulk voltage and the second voltage corresponding to the positive half cycle is approximately a ground voltage.
16. The power factor correction (PFC) system according to claim 14 , wherein the controller is further configured to:
after the pulse sequence, transmit the PWM signal to the second transistor of the fast-leg portion of the PFC circuit for a conversion process corresponding to the positive half cycle;
detect that the line voltage is at a second polarity change;
pause the PWM signal; and
transmit the pulse sequence to the first transistor of the fast-leg portion of the PFC circuit to change the voltage on the EMI capacitor from the second voltage corresponding to the positive half cycle to the first voltage corresponding to the negative half cycle.
17. The power factor correction (PFC) system according to claim 14 , wherein:
the first transistor of the fast-leg portion is coupled between a positive output of the PFC circuit and a fast-leg switch node;
the second transistor of the fast-leg portion is coupled between the fast-leg switch node and a negative output of the PFC circuit; and
the fast-leg switch node is coupled to the AC source via an inductor.
18. The power factor correction (PFC) system according to claim 17 , wherein the PFC circuit further includes:
a slow leg portion including:
a first slow-leg transistor coupled between the positive output and a slow-leg switch node; and
a second slow-leg transistor coupled between the slow-leg switch node and the negative output;
a filter leg including:
a first EMI capacitor coupled between the positive output and a slow-leg switch node; and
a second EMI capacitor coupled between the slow-leg switch node and the negative output; and
a bulk output capacitor coupled between the positive output and the negative output.
19. The power factor correction (PFC) system according to claim 18 , wherein the controller is further configured to:
detect a light load condition of a load coupled between the positive output and the negative output; and
disable the first slow-leg transistor and the second slow-leg transistor.
20. The power factor correction (PFC) system according to claim 14 , wherein the pulse sequence includes a plurality of pulses having substantially equal duty cycles.
21. A controller for a power factor correction (PFC) circuit, the controller configured to:
detect that a line voltage received at the PFC circuit is in a negative half cycle;
transmit a PWM signal to a first transistor of a fast-leg portion of the PFC circuit for a conversion process corresponding to the negative half cycle, the conversion process generating a bulk voltage at an output of the PFC circuit;
detect that the line voltage is at a first polarity change;
pause the PWM signal; and
transmit a pulse sequence to a second transistor of the fast-leg portion of the PFC circuit to change a voltage on an EMI capacitor from a first voltage corresponding to the negative half cycle to a second voltage corresponding to a positive half cycle.