Switching circuit with controllable active clamp function

1. A switching circuit with controllable active clamp function, comprising:

a first node;

a second node;

a third node;

a power switch, comprising:

a first end coupled to the first node;

a second end coupled to the second node; and

a control end for receiving a first control signal which includes a predetermined frequency and a state-switching frequency;

a driving circuit configured to:

provide the first control signal;

modulate the power switch to operate between an ON state and an OFF state with the predetermined frequency when the state-switching frequency is larger than the predetermined frequency; and

control the power switch to operate in the ON state or the OFF state when the state-switching frequency is equal to the predetermined frequency;

an active clamping circuit coupled between the first node and the third node and configured to provide a clamp voltage at the third node according to a voltage established across the active clamping circuit; and

an active clamping control unit configured to:

allow the clamp voltage to be transmitted to the control end of the power switch during a period when the power switch is modulated between the ON state and the OFF state with the predetermined frequency; and

prevent the clamp voltage from being transmitted to the control end of the power switch during a period when the power switch operates in the OFF state.

2. The switching circuit of claim 1 , wherein:

the driving circuit comprises:

a first logic unit configured to generate the first control signal; and

a second logic unit configured to generate a second control signal;

the active clamping control unit comprises an auxiliary switch which includes:

a first end coupled to the third node;

a second end coupled to the control end of the power switch; and

a control end for receiving the second control signal; and

a logic state of the first control signal is opposite to a logic state of the second control signal.

3. The switching circuit of claim 1 , wherein:

the driving circuit comprises:

a first logic unit configured to generate the first control signal; and

a second logic unit configured to generate a second control signal;

the active clamping control unit comprises an auxiliary switch which including:

a first end coupled to the third node;

a second end coupled to the control end of the power switch; and

a control end for receiving the second control signal; and

the second control signal is an inverting logic and time-delayed signal of the first control signal.

4. The switching circuit of claim 1 , wherein the active clamping control unit comprises:

a high-pass filter configured to detect the state-switching frequency in the first control signal;

an inverting logic circuit configured to generate a third control signal by inverting a phase of the first control signal when the high-pass filter detects the state-switching frequency; and

an auxiliary switch including:

a first end coupled to the third node;

a second end coupled to the control end of the power switch; and

a control end for receiving the second control signal.

5. The switching circuit of claim 1 , wherein the active clamping control unit comprises:

a high-pass filter configured to detect the state-switching frequency in the first control signal;

an inverting logic circuit configured to generate a third control signal by inverting a phase of the first control signal when the high-pass filter detects the state-switching frequency;

a delay circuit configured to generate a second control signal by time-delaying a phase of the third control signal; and

an auxiliary switch including:

a first end coupled to the third node;

a second end coupled to the control end of the power switch; and

a control end for receiving the second control signal.

6. The switching circuit of claim 1 , further comprising a resistor coupled between the driving circuit and the control end of the power switch.

7. The switching circuit of claim 6 , wherein the active clamping control unit further comprises a capacitor having a first end coupled to the third node and a second end coupled between the resistor and the control end of the power switch.

8. The switching circuit of claim 7 , wherein:

the capacitor and the resistor form a high-pass filter which is configured to allow the clamp voltage to be transmitted to the control end of the power switch when a frequency of the clamp voltage is not smaller than the predetermined frequency; and

the high-pass filter is configured to prevent the clamp voltage from being transmitted to the control end of the power switch when the frequency of the clamp voltage is smaller than the predetermined frequency.

9. The switching circuit of claim 1 , wherein the active clamping circuit comprises:

a Zener diode including:

an anode; and

a cathode coupled to the first node; and

a unidirectional diode including:

an anode coupled to the anode of the Zener diode; and

a cathode coupled to the third node.

10. The switching circuit of claim 1 , wherein the active clamping control unit is further configured to:

prevent the clamp voltage from being transmitted to the control end of the power switch during a period when the power switch is not modulated between the ON state and the OFF state with the predetermined frequency.

11. The switching circuit of claim 1 , wherein the active clamping control unit is further configured to:

determine whether the power switch is modulated between the ON state and the OFF state with the predetermined frequency according to a time inverted by the predetermined frequency; and

prevent the clamp voltage from being transmitted to the control end of the power switch when determining that the power switch is not modulated between the ON state and the OFF state with the predetermined frequency.