JP2012050328A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor device that suppresses self-turn-on of a MOSFET, and enhances the power supply efficiency, without causing an increase in drive loss, in a power supply unit of synchronous rectification type.SOLUTION: In a power supply unit, the gate size of a MOSFET (2) for rectification, constituting a synchronous rectification circuit, is made smaller than the gate size of a MOSFET (3) for commutation. Furthermore, a threshold of the MOSFET for commutation is made higher than a threshold of the MOSFET for rectification. For example, the threshold of the MOSFET for commutation is set to be 0.5 V higher than the threshold of the MOSFET for rectification, the threshold of the MOSFET for rectification is set to be 1.5 V or less, and the threshold of the MOSFET for commutation is set to be 2.0 V or more.

Description

  The present invention relates to a power supply device, and more particularly, to a synchronous rectifier circuit and a power supply device used for electronic equipment and the like.

  2 is known as a power supply device of the prior art, and a control signal output from a drive unit 70 is a DC power input from a DC input power source 60 to an input unit 51 including an input capacitor 61. Is switched by the switching unit 52, and power is supplied to the load 66 from the output unit 53 including the diode 63 and the output filter 55. The voltage or current output to the load 66 is detected by the detection unit 67, and the detected value and the control target value of the load 66 set by the setting unit 68 are compared by the comparison calculation unit 69. A control signal based on the comparison result is output to the switching unit 52.

  A specific circuit configuration of the power supply device of FIG. 2 is shown in FIG. The switching unit 52 includes an active element 62 (for example, a transistor or a MOSFET). The output unit 53 includes a commutation diode 63 and an output filter 55 including a choke coil 64 and a capacitor 65. The control unit 54 includes a comparison calculation unit 69, a setting unit 68, and a drive unit 70. Further, the control unit 54 includes an oscillation circuit (not shown), and outputs a pulse signal from the driving unit 70 to the active element 62. As a result, the DC voltage Vin from the DC input power supply 60 applied to the active element 62 is switched.

  In the power supply device of FIG. 3, when the active element 62 is on, DC power is charged to the choke coil 64 and the capacitor 65 and supplied to the load 66. When the active element 62 is off, the energy charged in the choke coil 64 and the capacitor 65 is supplied to the load 66 through the commutation diode 63.

  At this time, the control unit 54 monitors the output voltage Vo detected by the detection unit 67 in the comparison calculation unit 69, compares this with the control target value set by the setting unit 68, and based on the comparison result from the drive unit 70. The control signal is output to the switching unit 52. As a result, the active element 62 is controlled to be turned on / off, and the power supplied to the load is controlled to coincide with the control target value. The output voltage V0 at this time is expressed by the following (Equation 1).

V0 = Vin × (Ton / T) (Equation 1)
Where Vin is the input DC voltage, T is the period of the pulse signal output from the drive unit 70, and Ton is the period T during which the active element 62 is on. That is, Ton / T represents the duty ratio.

  FIG. 5 shows another conventional technique, which is a synchronous rectification type power supply device using a MOSFET 3 on the commutation side. As shown in FIG. 6, in this power supply device, the current-voltage characteristic of the MOSFET becomes linear depending on the gate voltage, so that the voltage drop is smaller than that of the diode.

FIG. 7 schematically shows the feedback capacitance Crss and the gate-source capacitance Ciss of the commutation MOSFET 3 of the synchronous rectification type power supply device. A phenomenon in which the commutation MOSFET 3 in the off state is turned on when the rectifying MOSFET 2 is turned on will be described with reference to FIG. When the commutating MOSFET 3 is turned off and the rectifying MOSFET 2 is turned on, the drain voltage of the commutating MOSFET 3 is suddenly changed to the voltage Vin of the input power supply 1, so that the gate-source capacitor Ciss is charged through the feedback capacitor Crss, The commutation MOSFET 3 that should be turned off is turned on. That is, the gate-source voltage Vgs of the commutation MOSFET 3
Vgs = (Crss / Ciss + Crss) × dVds (Expression 2)
When the value exceeds the threshold value Vth, self-turn-on occurs. Here, dVds represents the amount of change in the drain-source voltage of the commutation MOSFET 3.

  A semiconductor integrated circuit such as a microprocessor is assumed as a load of the synchronous rectification type power supply device shown in FIGS. In recent years, the operating voltage of a semiconductor integrated circuit tends to decrease, and the output voltage of a power supply needs to be reduced accordingly. Under the condition that the voltage of the DC input power source is constant, the ON time Ton of the rectifying MOSFET 2 in the equation (1) is shortened, and conversely, the ON time of the commutation MOSFET 3 is lengthened to lower the output voltage.

  Unlike an ideal switch, a MOSFET used for a switching power supply such as a synchronous rectification type power supply device generates a loss. This loss can be divided into a loss occurring in the on state, that is, a conduction loss, and a loss occurring when switching from the on state to the off state or from the off state to the on state, that is, a switching loss.

  In a power supply device with a low output voltage, the rectification MOSFET 2 with a short on-time has a dominant switching loss, and the commutation MOSFET 3 with a long on-time has a dominant conduction loss.

  The conduction loss is proportional to the on-resistance which is the resistance of the MOSFET in the on state, and the switching loss is proportional to the feedback capacitance. Therefore, the total loss is reduced by using an element having a small feedback capacitance for the rectifying MOSFET 2 in which the switching loss is dominant and using an element having a small on-resistance in the commutation MOSFET 3 in which the conduction loss is dominant.

  As shown in FIG. 9, a conventional technique is known in which a parallel circuit of a resistor 21 and a diode 22 is inserted into the gate in order to reduce the speed at which the rectifying MOSFET 2 is turned on. Since the rectifying MOSFET 2 is the resistor 21, the rise of the gate voltage is delayed, and the change amount dVds of the drain voltage Vds shown in the equation (2) becomes small, so that self-turn-on is less likely to occur. On the other hand, the turn-off gate charge is extracted at high speed because the diode 22 is passed.

  Further, as shown in FIG. 10, a conventional technique is known in which a capacitor 23 and a discharge resistor 24 are connected to the gate of the commutation MOSFET 3. Since this method drives the gate voltage via the capacitor 23, when the gate terminal 25 is changed from the positive potential to the ground potential, the gate potential 26 of the commutation MOSFET 3 is transferred to minus, and when the rectifying MOSFET 2 is turned on, The commutation MOSFET 3 is less likely to self-turn on.

  In the conventional power supply device shown in FIG. 3, a diode which is a passive element is used on the commutation side of the output unit 53. The commutation diode 63 has current-voltage characteristics as shown in FIG. 4, and the forward voltage becomes saturated when the current exceeds a certain value. This saturation voltage is about 0.9 V to 1.3 V for high-speed diodes, and about 0.45 V to 0.55 V for Schottky diodes. Therefore, there is a problem that power loss occurs and power conversion efficiency deteriorates. It was. Furthermore, since the power loss is large and the junction temperature of the element rises, the larger the output current is, the more commutation diodes 63 (two or three, etc.) are connected in parallel to distribute the power loss per element. There is also a problem that it is necessary to suppress the junction temperature.

  5 and FIG. 7, when the self-turn-on occurs, the rectifying MOSFET 2 and the commutation MOSFET 3 are simultaneously turned on, causing excessive loss, causing a factor of efficiency deterioration, and causing the element to deteriorate due to the worst heat generation. It may be destroyed.

  Unlike the ideal switch, the MOSFET used in the conventional power supply device shown in FIGS. 5 and 7 generates a loss. In general, a MOSFET has a relationship that an element having a small on-resistance has a large feedback capacitance, and an element having a small feedback capacitance has a large on-resistance. FIG. 8 is a diagram showing the relationship between on-resistance and feedback capacitance, and there is a trade-off relationship between the two. Therefore, since the commutation MOSFET 3 selects an element having a low on-resistance, there is a problem that the feedback capacitance Crss becomes large and self-turn-on is likely to occur.

  In the prior art shown in FIG. 9, since the turn-on of the rectifying MOSFET 2 is delayed, the turn-on loss of the rectifying MOSFET 2 is increased.

  The conventional technique shown in FIG. 10 has a problem that the drive loss of the commutation MOSFET 3 is increased due to the charge / discharge loss of the capacitor 23.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problem, and to provide a low-loss power supply device that suppresses self-turn-on without causing an increase in drive loss.

  The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

  The following is a brief description of an outline of typical inventions disclosed in the present application.

  That is, the power supply device of the present invention includes a commutation MOSFET and a rectification MOSFET which are insulated gate power semiconductor elements constituting a synchronous rectification circuit, and the threshold value of the commutation MOSFET is the threshold of the rectification MOSFET. Higher than threshold.

  The effects obtained by the representative ones of the inventions disclosed in the present application will be briefly described as follows.

  That is, the power supply device of the present invention can suppress the self-turn-on of the MOSFET without increasing the drive loss, and can improve the power supply efficiency.

Explanatory drawing of the MOSFET characteristic of the power supply device of this invention. The schematic block diagram of the power supply device of a prior art. The schematic block diagram of the power supply device of a prior art. The figure which shows the relationship between the voltage drop of a diode, and an electric current. The schematic block diagram of the power supply device of a prior art. The figure which shows the voltage drop of a diode and MOSFET, and the relationship of an electric current. Explanatory drawing of the parasitic capacitance of MOSFET for commutation. The figure which shows the relationship between on-resistance and feedback capacity. Schematic of rectification MOSFET used for the power supply device of a prior art. Schematic of rectification MOSFET used for the power supply device of a prior art. Explanatory drawing which shows the relationship between the threshold value and efficiency of MOSFET of the power supply device of this invention. Explanatory drawing which shows the relationship between the threshold value and loss of MOSFET of the power supply device of this invention. Explanatory drawing of the current of the MOSFET of the power supply device of this invention, and a voltage waveform.

  Hereinafter, details of the present invention will be described with reference to the drawings. The power supply device of the present invention has the same circuit configuration as that of FIG. As shown in FIG. 5, the power supply device of the present invention includes a rectifying MOSFET 2 and a commutation MOSFET 3, and a DC input power supply 1 is connected to the drain terminal of the rectifying MOSFET 2. The source terminal of the rectifying MOSFET 2 is connected to one end of the choke coil 4 and the drain terminal of the commutation MOSFET 3. The other end of the choke coil 4 is connected to one end of a capacitor 5 (for example, an electrolytic capacitor) and a load resistor. The capacitor 5 is connected to the ground terminal.

  FIG. 1 shows threshold values of the rectifying MOSFET 2 and the commutation MOSFET 3 used in the power supply device of the present invention. In FIG. 1, the horizontal axis represents the gate voltage, and the vertical axis represents the drain current. In the power supply device of the present invention, the threshold value of the commutation MOSFET 3 and the rectification MOSFET 2 are different, and the threshold value of the commutation MOSFET 3 is higher than that of the rectification MOSFET 2.

  Here, the threshold value is defined in this specification as a gate-source voltage when a drain current of 1 mA flows under a condition where the drain-source voltage is 10V.

  FIG. 11 shows the relationship when the threshold Vth of the commutation MOSFET 3 is taken on the horizontal axis and the power supply efficiency η is taken on the vertical axis. As shown in FIG. 11, the power supply efficiency increases as the threshold value increases.

  FIG. 12 shows the relationship between the threshold value of the power supply device of the present invention and the loss component of the power supply. The breakdown of each bar graph of FIG. 12 is, in order from the top, the conduction loss of the rectification MOSFET 2, the turn-on loss of the rectification MOSFET 2, the turn-off loss of the rectification MOSFET 2, the drive loss of the rectification MOSFET 2, the conduction loss of the commutation MOSFET 3, and the commutation MOSFET 3 Self turn-on loss, drive loss of commutation MOSFET 3, conduction loss of diodes 2A and 3A, and recovery loss of diodes 2A and 3A.

  As shown in FIG. 12, the total loss decreases as the threshold value increases. Looking at the breakdown of the components, when the threshold value is low (2.39 V), the self-turn-on loss of the commutation MOSFET 3 is large, but when the threshold value is high (3.39 V), the self-turn-on loss of the commutation MOSFET 3 is large. No longer occurs. On the other hand, as the threshold value increases, the conduction loss of the commutation MOSFET 3 increases. However, since the decrease amount of the self turn-on loss is larger than the increase amount of the conduction loss, the total loss of the power supply device is reduced, and the power supply efficiency is improved as shown in FIG.

  FIG. 13 shows changes in drain voltage, drain current, and gate voltage of the commutation MOSFET 3 when the rectification MOSFET 2 of the power supply device of the present invention is turned on. FIG. 13A shows the case where the threshold value is 2.39V. At this time, the drain voltage rises, the gate voltage rises through the feedback capacitance, and the drain current flows because it exceeds the threshold value. Since the drain current flows when the drain voltage is high, a large loss occurs. FIG. 13B shows the case where the threshold value is 2.89V. At this time, the drain voltage rises, the gate voltage rises through the feedback capacitance, and the drain current flows because it exceeds the threshold value. However, the magnitude of the drain current is smaller than that in the case of FIG. Therefore, the generated loss is also reduced. FIG. 13C shows the case where the threshold value is 3.39V. At this time, the gate voltage rises but no drain current flows. That is, no loss has occurred.

  As described above, in the power supply device of the present invention, self-turn-on can be suppressed and power supply efficiency can be improved without causing an increase in drive loss.

  Next, how much the difference between the threshold value of the rectifying MOSFET 2 and the threshold value of the commutation MOSFET 3 is preferable will be described. Since the threshold values of the rectifying MOSFET 2 and the commutation MOSFET 3 vary within a range of about ± 0.5 V with respect to the design values in the mass production line, considering this variation, the threshold of the commutation MOSFET 3 is set to the rectification MOSFET 2. Therefore, it is desirable to increase it by 0.5V or more.

  Next, specific numerical values of threshold values of the rectifying MOSFET 2 and the commutating MOSFET 3 will be described. In order to reduce the switching loss of the rectifying MOSFET 2, it is desirable to increase the transconductance gm. In order to increase the transconductance gm, it is effective to lower the threshold value of the rectifying MOSFET 2, specifically, it is desirable that the threshold value of the rectifying MOSFET 2 is 1.5 V or less. The threshold value shall be 2.0V or more.

  In order to reduce the loss of the power supply device of the present invention more surely, it is desirable that the threshold value of the commutation MOSFET 3 is higher than the rectification MOSFET 2 by 1.0 V or more. This is because when the wiring inductance of the power supply device is large, when the rectifying MOSFET 2 is turned on, the jumping of the drain voltage of the commutating MOSFET 3 becomes large, and self-turn-on is easily performed. Specifically, this corresponds to the case where the total wiring inductance of the main circuit of the power supply device exceeds 10 nH.

  As described above, the embodiment of the present invention has been described using the n-type MOSFET as the rectifying MOSFET 2, but it goes without saying that a p-type MOSFET can also be used.

1,60 ... DC input power supply 2 ... Rectifying MOSFET
3 ... MOSFET for commutation
4, 64 ... Choke coil 5 ... Output capacitor 6 ... Load resistor 7, 61 ... Input capacitor 21, 24 ... Resistor 22 ... Diode 23 ... Capacitor 25 ... Gate terminal 26 ... Gate potential 51 ... Input unit 52 ... Switching unit 53 ... Output Unit 54 ... Control unit 55 ... Output filter 62 ... Active element 63 ... Commutation diode 65 ... Capacitor 66 ... Load 67 ... Detection unit 68 ... Setting unit 69 ... Comparison calculation unit 70 ... Drive unit

Claims (8)

A first insulated gate power semiconductor element including a first terminal and a second terminal connected to the high potential side of the DC input power supply;
A second insulated gate type power semiconductor device including a third terminal and a fourth terminal connected to the low potential side of the DC input power source;
A connecting portion between the first terminal and the third terminal;
The capacitance value of the feedback capacitance between the gate of the first insulated gate power semiconductor element and the second terminal is between the gate of the second insulated gate power semiconductor element and the third terminal. Smaller than the value of the feedback capacitance,
A semiconductor device, wherein an absolute value of a threshold voltage of the first insulated gate power semiconductor element is lower than an absolute value of a threshold voltage of the second insulated gate power semiconductor element.   The second insulated gate power semiconductor element is turned off during an on period of the first insulated gate power semiconductor element, and the second insulated gate power semiconductor is turned off during an off period of the first insulated gate power semiconductor element. 2. The semiconductor device according to claim 1, wherein a semiconductor element is turned on, and an on period of the first insulated gate power semiconductor element is controlled to be shorter than an on period of the second insulated gate power semiconductor element.   The semiconductor device according to claim 1, wherein a voltage lower than a voltage of the DC input power supply is output from the connection portion.   2. The semiconductor device according to claim 1, wherein an absolute value of a threshold voltage of the second insulated gate power semiconductor element is 0.5 V or more higher than an absolute value of the threshold voltage of the first insulated gate power semiconductor element.   The absolute value of the threshold voltage of the first insulated gate power semiconductor element is 1.5 V or less, and the absolute value of the threshold voltage of the second insulated gate power semiconductor element is 2.0 V or more. The semiconductor device according to claim 1.   The semiconductor device according to claim 1, wherein the first terminal is a source terminal, the second terminal is a drain terminal, the third terminal is a drain terminal, and the fourth terminal is a source terminal. .   The semiconductor device according to claim 1, wherein the first insulated gate power semiconductor element is a rectifying MOSFET, and the second insulated gate power semiconductor element is a commutation MOSFET.   The semiconductor device according to claim 7, wherein the rectifying MOSFET is an n-type MOSFET.

Images