Flyback converter and method of operating the same
View Patent ↗The subject application provides a zero-voltage switching flyback converter comprising: a transformer having a primary winding and a secondary winding; a primary switch and a secondary switch for conducting the currents flowing in the primary winding and secondary winding respectively. A timing control method for operating the flyback converter are provided to accomplish zero-voltage switch by turning on the secondary switch twice within one switching power cycle.
1. A flyback converter comprising:
a transformer having a primary winding receiving an input voltage and a secondary winding providing an output voltage;
a primary switch coupled to the primary winding, having a source terminal, a drain terminal and a gate terminal;
a secondary switch coupled to the secondary winding, having a source terminal, a drain terminal and a gate terminal;
a zero-crossing detection (ZCD) circuit having a first input connected to a first end of the secondary winding for detecting a drain-to-source voltage of the secondary switch, a second input connected to a second end of the secondary winding for detecting the output voltage, the ZCD circuit is configured to generate a ZCD signal based on the drain-to-source voltage of the secondary switch and the output voltage;
a controller configured to obtain the ZCD signal from the ZCD circuit via a ZCD node and generate control signals to turn the primary switch and the secondary switch on and off alternately such that:
the primary switch is turned on to conduct a primary current in the primary winding to start a power cycle when a voltage across the drain terminal and the source terminal of the primary switch reaches a value less than a reference voltage value;
the primary switch is turned off when the primary current reaches a value greater than a reference current value;
the secondary switch is turned on, for a first time within the power cycle, to conduct a secondary current in the secondary winding after the primary switch being turned off for a non-overlapping delay;
the secondary switch is turned off, for the first time within the power cycle, when the secondary current reaches a zero-value;
the secondary switch is turned on, for a second time within the power cycle, in response to a falling edge of the ZCD signal occurring immediately after an event count reaches a corresponding count threshold;
the secondary switch is turned off, for the second time within the power cycle, following the secondary switch being turned on for a second-on time interval; and
the primary switch is turned on to initiate a next power cycle following the secondary switch being turned off for the second time within the power cycle for a second-off time interval.
2. The flyback converter according to claim 1 , wherein the reference voltage value is in a range from approximately 1V to approximately 10V.
3. The flyback converter according to claim 2 , wherein the reference voltage value is approximately 5V.
4. The flyback converter according to claim 1 , wherein the reference current value is set by a feedback loop circuit configured to compare the output voltage against a reference voltage and generate a feedback signal to the controller via a feedback node.
5. The flyback converter according to claim 1 , wherein the event count is obtained by counting number of valleys occurring in the ZCD signal and the corresponding count threshold is determined based on a load coupled to the flyback converter.
6. The flyback converter according to claim 1 , wherein the event count is obtained by counting number of peaks occurring in the ZCD signal and the corresponding count threshold is determined based on a load coupled to the flyback converter.
7. The flyback converter according to claim 1 , wherein the second-on time interval is given by
t
O
N
2
=
L
m
C
o
s
s
1
V
i
n
+
n
V
o
n
V
o
=
τ
r
e
s
2
π
V
i
n
+
n
V
o
n
V
o
,
where t ON2 is the second-on time interval, L m is the inductance of the primary winding, C oss1 is the equivalent capacitance between the drain terminal and the source terminal of the primary switch and τ res is the resonance time constant of ringing between L m and C oss1 , V in is the input voltage, V o is the output voltage, n is the ratio of number of turns of the primary winding to number of turns of the secondary winding.
8. The flyback converter according to claim 1 , wherein the second-off time interval is given by:
t
O
F
F
2
=
2
L
m
C
o
s
s
1
=
τ
r
e
s
π
,
where t OFF2 is the second-on time interval, L m is the inductance of the primary winding, C oss1 is the equivalent capacitance between the drain terminal and the source terminal of the primary switch and τ res is the resonance time constant of ringing between L m and C oss1 .
9. A method of operating a flyback converter having a transformer comprising a primary winding receiving an input voltage and a secondary winding providing an output voltage, a primary switch being coupled to the primary winding and having a source terminal, a drain terminal and a gate terminal, and a secondary switch being coupled to the secondary winding and having a source terminal, a drain terminal and a gate terminal, the method comprising:
turning on the primary switch by a controller to start a power cycle and conduct a primary current in the primary winding when a switching voltage reaches a value less than a reference voltage value, wherein the switching voltage is indicative of a drain-to-source voltage of the primary switch;
turning off the primary switch by the controller when the primary current reaches a value greater than a reference current value;
turning on, for a first time within the power cycle, the secondary switch by the controller to conduct a secondary current in the secondary winding after the primary switch being turned off for a non-overlapping delay;
turning off, for the first time within the power cycle, the secondary switch by the controller when the secondary current reaches a zero-value;
receiving a zero-current detection (ZCD) signal from a ZCD circuit via a ZCD node, wherein the ZCD signal is generated by the ZCD circuit based on a drain-to-source voltage of the secondary switch and the output voltage;
turning on, for a second time within the power cycle, the secondary switch by the controller in response to a falling edge of the ZCD signal occurring immediately after an event count reaches a corresponding count threshold;
turning off, for the second time within the power cycle, the secondary switch by the controller after the secondary switch being turned on for a second-on time interval; and
turning on the primary switch by the controller to initiate a next power cycle following the secondary switch being turned off for the second time within the power cycle for a second-off time interval.
10. The method according to claim 9 , wherein the reference voltage value is in a range from approximately 1V to approximately 10V.
11. The method according to claim 10 , wherein the reference voltage value is approximately 5 V.
12. The method according to claim 9 , wherein the reference current value is set by a feedback loop circuit configured to compare the output voltage against a reference voltage and generate a feedback signal to the controller via a feedback node.
13. The method according to claim 9 , wherein the event count is obtained by counting number of valleys occurring in the ZCD signal and the corresponding count threshold is determined based on a load coupled to the flyback converter.
14. The method according to claim 9 , wherein the event count is obtained by counting number of peaks occurring in the ZCD signal and the corresponding count threshold is determined based on a load coupled to the flyback converter.
15. The method according to claim 9 , wherein the second-on time interval is given by
t
O
N
2
=
L
m
C
o
s
s
1
V
i
n
+
n
V
o
n
V
o
=
τ
r
e
s
2
π
V
i
n
+
n
V
o
n
V
o
,
where t ON2 is the second-on time interval, L m is the inductance of the primary winding, C oss1 is the equivalent capacitance between the drain terminal and the source terminal of the primary switch, τ res is the resonance time constant of ringing between L m and C oss1 , V in is the input voltage, V o is the output voltage, n is the ratio of number of turns of the primary winding to number of turns of the secondary winding.
16. The method according to claim 9 , wherein the second-off time interval is given by:
t
O
F
F
2
=
2
L
m
C
o
s
s
1
=
τ
r
e
s
2
π
,
where t OFF2 is the second-on time interval, L m is the inductance of the primary winding, C oss1 is the equivalent capacitance between the drain terminal and the source terminal of the primary switch, and τ res is the resonance time constant of ringing between L m and C oss1 .