Control circuit of LLC resonant converter and LLC resonant converter using the same
The present invention provides a control circuit of an LLC resonant converter including a minimum switching frequency variable circuit of varying a minimum switching frequency corresponding to an AC input voltage of the LLC resonant converter; and a first pulse signal generating unit and a second pulse signal generating unit to generate a first pulse signal and a second pulse signal based on a switching frequency limited by the minimum switching frequency variable circuit.
1. A control circuit of an LLC resonant converter comprising:
a minimum switching frequency variable circuit for varying a minimum switching frequency by varying a time constant of a switching frequency variable circuit corresponding to an AC input voltage of the LLC resonant converter; and
a first pulse signal generating unit and a second pulse signal generating unit to generate a first pulse signal and a second pulse signal based on a switching frequency limited by the minimum switching frequency variable circuit.
2. The control circuit of the LLC resonant converter according to claim 1 , wherein the minimum switching frequency variable circuit comprises a minimum switching frequency adjustable transistor to operate corresponding to an AC input voltage of the LLC resonant converter; and
the time constant of the minimum switching frequency variable circuit is varied according to an operation of the minimum switching frequency adjustable transistor together with a switching capacitor and a switching resistor.
3. The control circuit of the LLC resonant converter according to claim 2 , wherein the minimum switching frequency variable circuit comprising:
the minimum switching frequency adjustable transistor;
a reference resistor connected to the minimum switching frequency adjustable transistor in series;
a switching resistor connected to the minimum switching frequency adjustable transistor and the reference resistor in parallel; and
a switching capacitor connected to the switching resistor in parallel.
4. The control circuit of the LLC resonant converter according to claim 3 , wherein. corresponding to the AC input voltage of the LLC resonant converter, when the minimum switching frequency adjustable transistor Or is ON, the time constant becomes
[
(
R
ref
+
R
trans
)
//
R
osc
]
C
osc
=
(
R
ref
+
R
trans
)
R
osc
R
ref
+
R
trans
+
R
osc
C
osc
,
corresponding to an AC input voltage of the LLC resonant converter, when the minimum switching frequency adjustable transistor is OFF, the time constant become R OSC C OSC , and
wherein the Rref is a resistance value of the reference resistor Rref, the Rtrans is a resistance value of an equivalent resistor of the minimum switching frequency adjustable transistor Qr during the operation of the minimum switching frequency adjustable transistor Qr, the R OSC is a resistance value of the switching resistor R OSC , and the C OSC is a capacitance of the switching capacitor C OSC .
5. The control circuit of the LLC resonant converter according to claim 2 , wherein the minimum switching frequency adjustable transistor comprises a first input resistor and a second input resistor; and
an AC input voltage of the LLC resonant converter is voltage dropped by the first input resistor and the second input resistor and applied to a base terminal of the minimum switching frequency adjustable transistor.
6. The control circuit of the LLC resonant converter according to claim 2 , wherein the first pulse signal generating unit and the second pulse signal generating unit generate the first pulse signal and the second pulse signal, alternately.
7. The control circuit of the LLC resonant converter according to claim 6 , wherein the first pulse signal generating unit and the second pulse signal generating unit set a dead time between the first pulse signal and the second pulse signal not to allow the first pulse signal and the second pulse signal to become high at the same time.
8. The control circuit of the LLC resonant converter according to claim 5 , wherein the minimum switching frequency variable circuit further comprises a third input resistor provided with one end located between the first input resistor and the second input resistor and the other end connected to a base terminal of the minimum switching frequency adjustable transistor.
9. The control circuit of the LLC resonant converter according to claim 1 , wherein the minimum switching frequency variable circuit comprises a switching capacitor and a variable resistor for varying corresponding to an AC input voltage of the LLC resonant converter to vary the time constant of the minimum switching frequency variable circuit.
10. An LLC resonant converter provided with a first switching transistor, a second switching transistor, a transforming unit, a rectifying unit and a control circuit of the LLC resonant converter, comprising:
the control circuit of the LLC resonant converter comprises a minimum switching frequency variable circuit for varying a minimum switching frequency by varying a time constant of a switching frequency variable circuit corresponding to an AC input voltage of the LLC resonant converter; and a first pulse signal generating unit and a second pulse signal generating unit to generate a first pulse signal and a second pulse signal based on a switching frequency limited by the minimum switching, frequency variable circuit,
wherein the first pulse signal and the second pulse signal are applied to the first switching transistor and the second switching transistor.
11. The LLC resonant convener according to claim 10 , wherein the first pulse signal and the second pulse signal are applied to a gate terminal of the first switching transistor and a gate terminal of the second switching transistor.
12. The LLC resonant converter according to claim 10 , wherein the minimum switching frequency variable circuit includes a minimum switching frequency adjustable transistor to operate corresponding to an AC input voltage of the LLC resonant converter; and
the time constant of the minimum switching frequency variable circuit is varied according to an operation of the minimum switching frequency adjustable transistor together with a switching capacitor and a switching resistor.
13. The LLC resonant converter according to claim 12 , wherein the minimum switching frequency variable circuit comprising:
the minimum switching frequency adjustable transistor;
a reference resistor connected to the minimum switching frequency adjustable transistor in series
a switching resistor connected to the minimum switching frequency adjustable transistor and the reference resistor in parallel; and
a switching capacitor connected to the switching resistor in parallel.
14. The LLC resonant converter according to claim 13 , wherein, corresponding to the AC input voltage of the LLC resonant converter, when the minimum switching, frequency adjustable transistor Qr is ON, the time constant becomes
[
(
R
ref
+
R
trans
)
//
R
osc
]
C
osc
=
(
R
ref
+
R
trans
)
R
osc
R
ref
+
R
trans
+
R
osc
C
osc
,
corresponding to an AC input voltage of the LLC resonant converter. when the minimum switching frequency adjustable transistor is OFF, the time constant become R OSC C OSC , and
wherein the Rref is a resistance value of the reference resistor Rref, the Rtrans is a resistance value of an equivalent resistor of the minimum switching frequency adjustable transistor Qr during the operation of the minimum switching frequency adjustable transistor Qr, the R OSC is a resistance value of the switching resistor R OSC , and the C OSC is a capacitance of the switching capacitor C OSC .
15. The LLC resonant converter according to claim 12 , wherein the minimum switching frequency adjustable transistor comprises a first input resistor and a second input resistor; and
an AC input voltage of the LLC resonant converter is voltage dropped by the first input resistor and the second input resistor and applied to a base terminal of the minimum switching frequency adjustable transistor.
16. The LLC resonant converter according to claim l 2 , wherein the first puke signal generating unit and the second pulse signal generating unit generate the first pulse signal and the second pulse signal, alternately.
17. The LLC resonant converter according to claim 16 , wherein the first pulse signal generating unit and the second pulse signal generating unit set a dead time between the first pulse signal and the second pulse signal not to allow the first pulse signal and the second pulse signal to become high at the same time.
18. The LLC resonant converter according to claim 15 , wherein the minimum switching frequency variable circuit further comprises a third input resistor provided with one end located between the first input resistor and the second input resistor and the other end connected to a base terminal of the minimum switching frequency adjustable transistor.
19. The LLC resonant converter according to claim 10 , wherein the minimum switching frequency variable circuit comprises a switching capacitor and a variable resistor for varying corresponding to an AC input voltage of the LLC resonant converter to vary the time constant of the minimum switching frequency variable circuit.