Switch Control Module of Switch Mode Power Supply

Abstract

A switch control module for a switch mode power supply comprising a biased switch, an active switch and a control unit. The biased switch comprises a first node and a second node. The first node is coupled to a primary side winding. The active switch is connected to the second node. The control unit controls the ON/OFF states of the active switch and the biased switch is biased to be turned on initially. In this way, the active switch and the control unit are less likely to be damaged by voltage spikes generated by leakage in the primary side winding.

Claims

1. A switch control module, applied for a switch mode power supply, said switch power supply comprising a primary side winding and a secondary side winding, and said switch control module comprising: a biased switch, comprising a first node and a second node, and said first node coupled to said primary side winding; an active switch, connected to said second node; and a control unit, coupled to said active switch for controlling a switch state of said active switch; where said biased switch is biased to a turn-on state.

2. The switch control module of claim 1, wherein said biased switch further comprises a first switch unit; said biased switch is biased to turn on said first switch unit; when said control unit controls said active switch to turn off, the voltage at said second node will be raised to approaching a maximum voltage and turning off the said first switch unit.

3. The switch control module of claim 2, wherein first switch unit is a metal-oxide-semiconductor field-effect transistor; the drain and source of said first switch unit act as said first node and said second node, respectively; the gate of said first switch unit receives a bias voltage; and the difference between said bias voltage and the threshold voltage of said first switch unit is said maximum voltage.

4. The switch control module of claim 1, wherein said active switch comprises a second switch unit; and said control unit outputs a switch control signal to a control terminal of said second switch unit for controlling the turn-on or turn-off of said second switch unit.

5. The switch control module of claim 4, wherein said active switch comprises a snubber coupled to said second node; and said snubber guides a spike absorption current to flow through said biased switch when said second switch unit is turned off by said control unit.

6. The switch control module of claim 5, wherein said snubber comprises a current source and a third switch unit; and said current source and said third switch unit are connected in series for coupling to said second node.

7. The switch control module of claim 5, wherein said control unit is coupled to said control terminal of said second switch unit via a third switch unit; said snubber comprises a first coupling device and a second coupling device; said first coupling device is coupled between said control unit and said control terminal of said second switch unit; and said second coupling device is coupled between said control terminal of said second switch unit and said second node.

8. The switch control module of claim 7, wherein said second switch unit is turned on when said switch control signal is at a first level; said second switch unit is turned off when said switch control signal is at a second level; and said third switch unit is turned off when said switch control signal changes from said first level to said second level.

9. The switch control module of claim 4, wherein said active switch comprises a fourth switch unit coupled between said second node and a power source terminal; and said power source terminal is coupled to a voltage stabilizing circuit for generating a direct-current power source using the voltage at said second node.

10. The switch control module of claim 9, wherein said voltage stabilizing circuit comprises an output capacitor.

11. The switch control module of claim 9, wherein said fourth switch unit is selectively turned on in a supply duration when said second switch unit is turned off.

12. The switch control module of claim 9, wherein said power source terminal is coupled to said control unit for providing said direct-current power source to said control unit.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0010] FIG. 1 shows a schematic diagram of the switch mode power supply according to the prior art;

[0011] FIG. 2A shows a schematic diagram of the architecture for the switch control module for the switch mode power supply according to an embodiment of the present application;

[0012] FIG. 2B shows a schematic diagram of the architecture for the switch control module for the switch mode power supply according to another embodiment of the present application;

[0013] FIG. 3A shows a schematic diagram of a partial circuit of the switch control module for the switch mode power supply according to the first embodiment of the present application;

[0014] FIG. 3B shows a schematic diagram of another partial circuit of the switch control module for the switch mode power supply according to the first embodiment of the present application;

[0015] FIG. 4 shows a schematic diagram of a partial circuit of the switch control module for the switch mode power supply according to the second embodiment of the present application;

[0016] FIG. 5 shows the signals of the switch control module according to the second embodiment of the present application;

[0017] FIG. 6 shows a schematic diagram of a partial circuit of the switch control module for the switch mode power supply according to the third embodiment of the present application;

[0018] FIG. 7 shows the signals of the switch control module according to the third embodiment of the present application; and

[0019] FIG. 8 shows a schematic diagram of the application architecture for the switch control modules according to the various embodiments of the present application.

DETAILED DESCRIPTION OF THE INVENTION

[0020] In the specifications and subsequent claims, certain words are used to represent specific devices. A person having ordinary skill in the art should know that hardware manufacturers might use different nouns to call the same device. In the specifications and subsequent claims, the differences in names are not used for distinguishing devices. Instead, the differences in functions are the guidelines for distinguishing. In the whole specifications and subsequent claims, the word “comprising” is an open language and should be explained as “comprising but not limited to”. Besides, the word “couple” comprises any direct and indirect electrical connection. Thereby, if the description is that a first device is coupled to a second device, it means that the first device is connected electrically to the second device directly, or the first device is connected electrically to the second device via other devices or connecting means indirectly.

[0021] Please refer to FIG. 2A, which shows a schematic diagram of the architecture for the switch control module for the switch mode power supply according to an embodiment of the present application. The switch mode power supply comprises a winding unit 9, which comprises a primary side winding N.sub.P and a secondary side winding N.sub.S. Furthermore, the leakage inductance L.sub.LK represents the nonideal component of the primary side winding N.sub.P. The primary side winding N.sub.P and the secondary side winding N.sub.S are normally regarded as a transformer T.sub.1. The primary side winding N.sub.P can receive an input power source V.sub.IN, which is normally formed by rectifying the external alternate-current power source. The switch control module for the switch mode power supply connects a biased switch SW.sub.A to an active switch SW.sub.B in series. The biased switch SW.sub.A is connected between the primary side winding N.sub.P and the active switch SW.sub.B. Then the active switch SW.sub.B is coupled to the ground. The biased switch SW.sub.A can be formed by the switch transistor adopted in the prior art. Nonetheless, according to the prior art, the control unit controls the switch state of the switch transistor. On the contrary, according to the various embodiments of the present application, the biased switch SW.sub.A is pre-biased to the turn-on state. Since the biased switch SW.sub.A and the active switch SW.sub.B are connected in series, when the active switch SW.sub.B is turned off, no current will flow through the biased switch SW.sub.A. At this moment, the node voltage of the biased switch SW.sub.A is in the turn-off state correspondingly. In other words, the switch state of the biased switch SW.sub.A is controlled by the switch state of the active switch SW.sub.B.

[0022] To elaborate, in FIG. 2A, the biased switch SW.sub.A and the active switch SW.sub.B are connected in series to the low side of the primary side winding N.sub.P. Nonetheless, as shown in FIG. 2B, according to another embodiment of the present application, the biased switch SW.sub.A and the active switch SW.sub.B are connected in series to the high side of the primary side winding N.sub.P. The biased switch SW.sub.A is connected between the primary side winding N.sub.P and the active switch SW.sub.B. Then the active switch SW.sub.B is coupled to the input voltage V.sub.IN. Since the biased switch SW.sub.A and the active switch SW.sub.B are connected in series to the primary side winding N.sub.P, the placement of the components does not influence the operation. Thereby, in the subsequent embodiments of the present application, the architecture shown in FIG. 2A is used as an example for description. Nonetheless, the present application is not limited to the architecture.

[0023] When the switch control module for the switch mode power supply according to the present application is operating, the control unit will turn on the active switch SW.sub.B periodically. Because the biased switch SW.sub.A is biased to the turn-on state, the primary side winding N.sub.P will store the energy from the input power source V.sub.IN. When the active switch SW.sub.B is turned off, the current will no longer flow through the biased switch SW.sub.A and the active switch SW.sub.B. At this moment, the primary side winding N.sub.P will transfer energy to the secondary side winding N.sub.S to discharge the secondary side winding N.sub.S and form the output voltage V.sub.OUT at the output for the load.

[0024] To elaborate, please refer to FIG. 3A, which shows a schematic diagram of a partial circuit of the switch control module for the switch mode power supply according to the first embodiment of the present application. In the first embodiment, the biased switch SW.sub.A comprises a first switch unit SW.sub.1, which can be, for example, a metal-oxide-semiconductor field-effect transistor (MOSFET). Alternatively, the first switch unit SW.sub.1 can be selected from a bipolar junction transistor (BJT), a unijunction transistor (UJT), or a silicon controlled rectifier (SCR). The present application is not limited by the above examples. The drain of the first switch unit SW.sub.1 acts as a first node n.sub.1 connecting to the primary side winding N.sub.P; the source of the first switch unit SW.sub.1 acts as a second node n.sub.2 connecting to the active switch SW.sub.B; and the gate of the first switch unit SW.sub.1 receives a bias voltage V.sub.Z. The bias voltage V.sub.Z can be provided with ease by a Zener diode ZD.sub.1 and a bias resistor R.sub.1. The bias resistor R.sub.1 is coupled to the input power source V.sub.IN for providing a minimum breakdown current to the Zener diode ZD.sub.1 and producing the bias voltage V.sub.Z. If the first switch unit SW.sub.1 has a positive threshold voltage Vth, once the bias voltage V.sub.Z is greater than the threshold voltage Vth, the first switch unit SW.sub.1 will be biased to the turn-on state. In this condition, if the active switch SW.sub.B is turned off for disallowing currents to flow through the biased switch SW.sub.A, the voltage of the second node n.sub.2 will be raised to a maximum voltage V.sub.Clamp with a value of V.sub.Z-Vth. Once the second node n.sub.2 reaches the maximum voltage V.sub.Clamp, the switch unit SW.sub.1 will be turned off.

[0025] According to the present embodiment, because the switch state of the biased switch SW.sub.A is controlled by the switch state of the active switch SW.sub.B, the active switch SW.sub.B still needs to couple to a control unit 3 for controlling the switch state of a second switch unit SW.sub.2. Normally, a pulse-width modulation circuit will be adopted to generate a switch control signal. The switch control signal is output to the second switch unit SW.sub.2 for adjusting its switch state and starting or stopping energy storage in the primary side winding N.sub.P. In practice, a person having ordinary skill in the art can understand that the duty cycle of the switch control signal can be adjusted according to the feedback voltage of the output voltage V.sub.OUT for controlling the output voltage V.sub.OUT accurately. Since these control methods are normal schemes in the field, the details will not be described in detail.

[0026] On the other hand, please refer to FIG. 3B. According to the first embodiment, a depletion-mode (D-mode) GaN MOSFET can be selected to be the first switch unit SW.sub.1 of the biased switch SW.sub.A. Thanks to its negative threshold voltage Vth, simply connecting the gate to the ground or controlling it at a proper reference voltage level, the first switch unit SW.sub.1 will be biased to the turn-on state without additional bias components. Under this condition, if the active switch SW.sub.B is turned off for disallowing currents to flow through the biased switch SW.sub.A, the maximum voltage V.sub.Clamp that the second node n.sub.2 can be raised is −Vth. Once the second node n.sub.2 reaches −Vth, the first switch unit SW.sub.1 will be turned off.

[0027] In the following, FIG. 3A and FIG. 3B are first used to illustrate the first technical effects given by the switch control module for the switch mode power supply according to the present embodiment. Notice that no matter the threshold voltage Vth of the first switch unit SW.sub.1 of the biased switch SW.sub.A is positive or negative, an extremely low maximum voltage V.sub.Clamp can be applied to the second node n.sub.2 of the active switch SW.sub.B. The normal threshold voltage Vth is approximately in the order of one or two digits of volts. In other words, when the control unit 3 turns off the active switch SW.sub.B for stopping energy storage in the primary side winding N.sub.P, the voltage spike generated by the leakage inductance L.sub.LK of the primary side winding N.sub.P will raise the voltages at the first and second nodes n.sub.1, n.sub.2. The voltage at the second node n.sub.2 will be raised to around the maximum voltage V.sub.Clamp then the first switch unit SW.sub.1 will be turned off. Since the active switch SW.sub.B and the control unit 3 are located in the low-voltage operation region B, they can be manufactured using low-voltage components. Besides, by maintaining low voltage operation, they are less influenced and vulnerable by the voltage spikes generated by the leakage inductance L.sub.LK.

[0028] Please refer to FIG. 4, which shows a schematic diagram of a partial circuit of the switch control module for the switch mode power supply according to the second embodiment of the present application. Based on the first embodiment, the present embodiment adds a snubber to the active switch SW.sub.B. The snubber is coupled to the second node n.sub.2, and spontaneously guides a spike absorption current to flow through the first switch unit SW.sub.1 when the active switch SW.sub.B is turned off by the control unit 3. To elaborate, in the present embodiment, the snubber comprises a current source I.sub.Snubber and a third switch unit SW.sub.3. The current source I.sub.Snubber and the third switch unit SW.sub.3 can be connected in series, coupled to the second node n.sub.2, and connected in parallel with the second switch unit SW.sub.2. Likewise, since the current source I.sub.Snubber and the third switch unit SW.sub.3 are connected in series, the placement of the components does not influence the operation.

[0029] The control unit 3 will output a switch control signal V.sub.CTL to the control terminal of the second switch unit SW.sub.2. If the second switch unit SW.sub.2 is also a MOSFET, the control unit 3 is coupled to the gate of the second switch unit SW.sub.2 for outputting the switch control signal V.sub.CTL. For example, FIG. 5 shows the signals of the switch control module. In the example, the second switch unit SW.sub.2 will be turned on when the switch control signal V.sub.CTL is high and off when the switch control signal V.sub.CTL is low. When the second switch unit SW.sub.2 is turned off, although the voltage spike generated by the leakage inductance L.sub.LK of the primary side winding N.sub.P will raise the voltage at the first node n.sub.1, the voltage at the first node n.sub.1 will be coupled to the second node n.sub.2 via a parasitic capacitance C.sub.P1 for raising the voltage at the second node n.sub.2. The maximum voltage of the first node n.sub.1 can be raised to the voltage of the input power source V.sub.IN, which is approximately equal to N times the output voltage V.sub.OUT with N being the turn ratio of the primary side winding N.sub.P to the secondary side winding N.sub.S. However, the voltage at the second node n.sub.2 will turn off the first switch unit SW.sub.1 around the maximum voltage V.sub.Clamp. At this moment, to lower the voltage rising rate at the first and second nodes n.sub.1, n.sub.2, the third switch unit SW.sub.3 can be turned on for allowing the current provided by the current source I.sub.Snubber to flow through the first switch unit SW.sub.1. Consequently, the first switch unit SW.sub.1 will not be turned off immediately and the current source I.sub.Snubber can dissipate the energy stored in the leakage inductance L.sub.LK. In addition to slowing the rising rate of the voltages at the first and second nodes n.sub.1, n.sub.2, if the current provided by the current source I.sub.Snubber is sufficient, the maximum voltage at the first node n.sub.1 can be reduced and thus reducing effectively the voltage spikes caused by the leakage inductance L.sub.L of the primary side winding N.sub.P. Thereby, the influence of voltage spikes on the components in the high-voltage operation region A can be further reduced.

[0030] On the other hand, the active switch SW.sub.B can further include a fourth switch unit SW.sub.4, which can be coupled between the second node n.sub.2 and a power source terminal V.sub.OP. The power source terminal V.sub.OP can be simply coupled to an output capacitor C.sub.OP or a complete voltage stabilizing circuit for generating a direct-current power source by using the voltage at the second node n.sub.2. In addition, the power source terminal V.sub.OP can be coupled to the control unit 3 or any other circuit components requiring a direct-current power source. When the fourth switch unit SW.sub.4 is turned on, the direct-current power source formed at the second node n.sub.2 can be supplied to the control unit 3 via the power source terminal V.sub.OP. As shown in the figure, the other current source lop is used for representing the operation current drawn from the power source terminal V.sub.OP by the control unit 3 or other circuit components. Thereby, the power consumption of the switch mode power supply can be reduced effectively.

[0031] When the second switch unit SW.sub.2 is turned on in a duration T.sub.ON, the voltage at the second node n.sub.2 will be pulled low. To supply power to the control unit 3 with a more stable power source, the fourth switch unit SW.sub.4 should preferably be turned on for a supply duration T.sub.CH when the second switch unit SW.sub.2 is turned off (namely, in a T.sub.OFF duration). The supply duration T.sub.CH is equivalently the charging time to the output capacitor C.sub.OP by the voltage at the second node n.sub.2. Besides, the supply duration T.sub.CH can be determined according to the power consumption of the control unit 3 or other circuit components requiring a direct-current power source.

[0032] It is noteworthy that the total power P.sub.absorb of spontaneously absorbing the energy stored in the leakage inductance L.sub.LK according to the second embodiment can be roughly expressed by the following equation, where I.sub.Snubber is the current provided by the current source I.sub.Snubber as described above; V.sub.n1 is the voltage at the first node n.sub.1; and V.sub.n2 is the voltage at the second node n.sub.2:

[00002]$P_{\mathrm{absorb}}\cong \left(I_{\mathrm{Snubber}}+\frac{C_{\mathrm{OP}}\times {\mathrm{\Delta V}}_{\mathrm{OP}}}{T_{\mathrm{CH}}}\right)\times \left(V_{n_1}-V_{n_2}\right)$

[0033] Accordingly, if the operation current drawn by the control unit 3 or other circuit components requiring a direct-current power source from the power source terminal V.sub.OP is sufficient, the maximum voltage at the first node n.sub.1 is actually lowered, which further effectively reduces the voltage spikes caused by the leakage inductance L.sub.LK of the primary side winding N.sub.P. In general, this happens to the switch mode power supplies with lower power or the cases when the power source terminal V.sub.OP supplies to numerous components. For these scenarios, the switch control module for the switch mode power supply according to the second embodiment of the present application requires no snubber. In other words, the current source I.sub.Snubber and the third switch unit SW.sub.3 as described above are no longer required. Thereby, the voltage spikes caused by the leakage inductance L.sub.LK of the primary side winding N.sub.P can be reduced effectively.

[0034] Moreover, in practice, the present application absorbs the energy stored in the leakage inductance L.sub.LK by using the voltage difference between the two terminals of the biased switch SW.sub.A and resulting in the generation of heat. As described above, the biased switch SW.sub.A can be formed by the switch transistor adopted by the switch mode power supply according to the prior art, meaning that the first switch unit SW.sub.1 is itself an existing external component. In general, the switch mode power supply will include heat dissipating structures for the switch transistors. Thereby, no additional heat dissipating structure is required for the first switch unit SW.sub.1. Namely, in practice, no additional external component or heat dissipating structure is required for the switch control module for the switch mode power supply according to the various embodiments of the present application for absorbing the energy stored in the leakage inductance L.sub.LK and hence the overall manufacturing costs can be reduced significantly.

[0035] Furthermore, according to the U.S. Pat. No. 10,622,879, as described above, the energy generated by the leakage inductance of the primary side winding is directly used to charge a capacitor for providing an operation current to the control unit. Nonetheless, according to the second embodiment, the voltage at the second node n.sub.2 is used to generate the direct-current power source. As described above, the voltage at the second node n.sub.2 at most will be raised to around the maximum voltage V.sub.Clamp. Thereby, the second embodiment is suitable for switch mode power supplier with high power without using electronic components with medium to high voltage tolerance to manufacture the control unit 3. Consequently, the application range of the switch control module is increased significantly.

[0036] Please refer to FIG. 6, which shows a schematic diagram of a partial circuit of the switch control module for the switch mode power supply according the third embodiment of the present application. The difference between the third embodiment and the second is that, according to the third embodiment, the snubber disposed in the active switch SW.sub.B requires no current source. According to the present embodiment, the snubber comprises a coupling resistor R.sub.S1 and a coupling capacitor C.sub.S1. The control unit 3 is coupled to the control terminal of the second switch unit SW.sub.2 via a third switch unit SW.sub.3. The coupling resistor R.sub.S1 is coupled between the control unit 3 and the control terminal of the second switch unit SW.sub.2; the coupling capacitor C.sub.S1 is coupled between the control terminal of the second switch unit SW.sub.2 and the second node n.sub.2. Thereby, a driving voltage V.sub.DRV received by the control terminal of the second switch unit SW.sub.2 will be influenced by the control unit 3 and the second node n.sub.2 concurrently.

[0037] For example, please refer to FIG. 7, which shows the signals of the switch control module according the third embodiment of the present application. In this example, the second switch unit SW.sub.2 will be turned on when the driving voltage V.sub.DRV is high and turned off when the driving voltage V.sub.DRV is low. When the switch control signal V.sub.CTL changes from the high voltage level to the low voltage level, it will be output to the second switch unit SW.sub.2 via the third switch unit SW.sub.3 for shutting off the second switch unit SW.sub.2 immediately. Nonetheless, according to the present embodiment, the third switch unit SW.sub.3 is turned off when the switch control signal V.sub.CTL changes from the high voltage level to the low voltage level. At this moment, the switch control signal V.sub.CTL must pull low the driving voltage V.sub.DRV via coupling resistor R.sub.S1. Unfortunately, in the process of turning off the second switch unit SW.sub.2 as the driving voltage V.sub.DRV is lowering, although a voltage spike generated by the leakage inductance L.sub.LK of the primary side winding N.sub.P can pull up the voltages at the first and second nodes n.sub.1, n.sub.2, the voltage at the second node n.sub.2 will be coupled to the control terminal of the second switch unit SW.sub.2 via the coupling capacitor C.sub.S1 and influencing the driving voltage V.sub.DRV. Thereby, as shown in the figure, given the influence on the driving voltage V.sub.DRV as described above, the control result is equivalent to not turning off the second switch unit SW.sub.2 and maintaining in an incomplete shutoff state temporarily. Consequently, a spike absorption current will be formed to flow through the first switch unit SW.sub.1. According to the present embodiment, although no current source is disposed to form the snubber like the second embodiment, a spike absorption current still can be guided spontaneously to flow through the first switch unit SW.sub.1 when the active switch SW.sub.B is turned off by the control unit 3 by disposing coupling components to determine the driving voltage V.sub.DRV output to the second switch unit SW.sub.2. Thereby, the purpose of lowering voltage spike can still be achieved.

[0038] Please refer to FIG. 8, which shows a schematic diagram of the application architecture for the switch control modules according to the various embodiments of the present application. The input power source V.sub.IN in the previous description is generally formed by rectifying external alternate-current power source. FIG. 8 shows the circuit architecture of how to rectify an external alternate-current power source AC. A voltage stabilizing capacitor C.sub.X is connected to one side of the external alternate-current power source AC for filtering and stabilizing the voltage of the external alternate-current power source AC. Next, the voltage stabilizing capacitor C.sub.X the will be connected to an input capacitor C.sub.Bulk via a rectifier 92. The voltage across the input capacitor C.sub.Bulk is rectified by the rectifier 92 so that the input capacitor C.sub.Bulk can provide the input power source V.sub.IN as described above to the winding unit 9.

[0039] A person having ordinary skill in the art should know well that the external alternate-current power source AC is relatively a high voltage for human body, making safety concern on the voltage across the voltage stabilizing capacitor C.sub.X. To meet high-standard safety regulations, a normal switch mode power supply must include an additional discharge circuit for spontaneously releasing the charges stored in the voltage stabilizing capacitor C.sub.X after the external alternate-current power source AC is removed (such as unplugging). Unfortunately, such a discharge circuit needs to adopt a high-voltage device in the integrated-circuit fabrication process, leading to extra manufacturing costs.

[0040] On the contrary, the switch control module for the switch mode power supply according to the present embodiment of the present application requires no additional discharge circuit. To elaborate, since the voltage at the second node n.sub.2 can supply power to the control unit 3 indirectly, only one remove detection unit 4 is required to judge if the external alternate-current power source AC has been removed. The remove detection unit 4 is coupled to the control unit 3 for controlling the control unit 3 to continue to switch the active switch SW.sub.B when the external alternate-current power source AC is judged to be removed. Thereby, the energy in the input capacitor C.sub.Bulk can be released by the active switch SW.sub.B continuously and the energy in the voltage stabilizing capacitor C.sub.X can be transferred to the input capacitor C.sub.Bulk via the rectifier 92. The above operation is equivalent to releasing the charges stored in the voltage stabilizing capacitor C.sub.X continuously. The power to the control unit 3 will be continued until the voltage across the input capacitor C.sub.Bulk approaches zero. In other words, according to the various embodiments of the present application, without no addition discharge circuit, the charges stored in the voltage stabilizing capacitor C.sub.X can be released spontaneously and hence effectively lowering the overall manufacturing costs of switch mode power supply. Note that according to the switch mode power supply according to the prior art as shown in FIG. 1, the power should be supplied to the control unit for controlling the switch state of the switch transistor SW.sub.1. The power of this control unit is normally supplied by an auxiliary winding by inducing the energy in the secondary side winding N.sub.S of the winding unit 9. Unfortunately, the secondary side winding N.sub.S normally stops drawing current as soon as the external alternate-current power source AC is removed. Consequently, using the auxiliary winding according to the prior art cannot maintain the operation of the control unit when the external alternate-current power source AC is removed.

[0041] The second switch unit SW.sub.2, the third switch unit SW.sub.3, or the fourth switch unit SW.sub.4 in the various embodiments as described above can be manufactured, likewise, by a MOSFET. Alternatively, they can be selected from BJT, UJT, SCR, or other power switching devices. Nonetheless, the present application is not limited by the above examples.

[0042] To sum up, according to the switch control module for the switch mode power supply in the above embodiments, the active switch SW.sub.B controls the switch state of the biased switch SW.sub.A connected in series via a node (the second node n.sub.2 described above). Thereby, when the primary side winding N.sub.P of the winding unit 9 stops storing energy, a voltage spike generated by the leakage inductance L.sub.LK of the primary side winding N.sub.P can raise the voltage of the node to around a maximum voltage V.sub.Clamp then the biased switch SW.sub.A will be turned off. Thereby, the control unit 3 controlling the active switch SW.sub.B can be manufactured using low-voltage components. In addition, by maintaining low-voltage operations, the influence and damage caused by the voltage spike generated by the leakage inductance L.sub.LK can be avoided.

[0043] According to some embodiments, the active switch SW.sub.B comprises a snubber for spontaneously guiding a spike absorption current to flow through the biased switch SW.sub.A for absorbing the energy stored in the leakage inductance L.sub.LK when the active switch SW.sub.B is controlled to turn off. According to some embodiments, the voltage at the node can generate a direct-current power source for supplying power to the control unit coupled to the active switch SW.sub.B or to other circuit components requiring direct-current power source. Thereby, the power consumption of switch mode power supply can be reduced effectively. Besides, since the voltage at the node can be raised at most to around the maximum voltage V.sub.Clamp, the present application is suitable for switch mode power supplies with higher power, not requiring electronic components with medium to high voltage tolerance for the control unit. Consequently, the application range of the switch control module is increased significantly.

[0044] In practice, the present application absorbs the energy stored in the leakage inductance L.sub.LK by using the biased switch SW.sub.A and resulting in the generation of heat. As described above, the biased switch SW.sub.A itself can be an existing external component of switch mode power supply. In general, the switch mode power supply will include heat dissipating structures. Thereby, in practice, no additional external component or heat dissipating structure is required for the switch control module for the switch mode power supply according to the various embodiments of the present application for absorbing the energy stored in the leakage inductance L.sub.LK and hence the overall manufacturing costs can be reduced significantly.

[0045] The foregoing description is only embodiments of the present application. Those equivalent changes or modifications made according to the shape, structure, feature, or spirit described in the claims of the present application are within the scope and range of the present application.