An IC for a switching power supply is a specific IC for controlling a discrete high voltage switching transistor. The IC obtains its own power supply during operation by operating the high voltage switching transistor. At the start, however, the IC requires a starting current power supply, which is supplied by a start-up circuit. The start-up circuit is typically integrated in the same semiconductor substrate as the IC to decrease the number of components and simplify the power supply system.
The starting current, which is an alternating current (AC) at a voltage of 100 to 240V, is rectified. It is necessary for a normally-on type element of the higher voltage side of the start-up circuit to have a breakdown voltage of around 450V to supply the starting current to the start-up circuit. The normally-on type element is realized as a lateral high voltage JFET integrated in the same semiconductor substrate as the IC for the switching power supply. A design specification of the switching power supply is determined by current drive capability of the normally-on type element.
FIG. 7 is a circuit diagram of a conventional switching power supply. An AC voltage is applied to a rectifier 103 through a fuse 102 from an AC power source 101. A direct current (DC) voltage output from the rectifier is applied to a power source capacitor 104, which becomes a DC power source. The DC voltage from the power source capacitor 104 is applied to a drain terminal of a JFET 302 of a start-up circuit 133 through a power source terminal 301 of an IC 120 for a switching power supply. In addition, the DC voltage is applied to an NMOSFET 121 through a primary winding 106 of a transformer 105. A source terminal 303 of the JFET 302 is connected to a drain terminal of an NMOSFET 304 and coupled to a gate terminal 306 of the NMOSFET 304 through a resistor 305. The gate terminal 306 connects with an NMOSFET 134 of a power section 131 of a control circuit 129. A source terminal of the NMOSFET 304 is connected to the power section 131 of the control circuit 129 and to a smoothing capacitor 110 through a start-up circuit output voltage terminal 308.
The voltage level of the gate terminal 306 is designed to be greater than the threshold voltage of the NMOSFET 304 in the voltage level of the start-up circuit output voltage terminal 308 when the smoothing capacitor 110 has been charged to a predetermined voltage value. The voltage level of the source terminal 303 of the JFET 302 is determined by the voltage level of the gate terminal 306. When a power source voltage is applied from the power source capacitor 104, the NMOSFET 304 turns on and a starting current charges the smoothing capacitor 110 through the JFET 302 and the NMOSFET 304. When the smoothing capacitor 110 reaches the predetermined voltage value, the control circuit 129 starts and the NMOSFET 121 starts to operate by a control signal from a control section 132. When the NMOSFET 121 starts to operate, the current based on the voltage induced in a second winding 111b of the transformer 105 charges the smoothing capacitor 110 through a diode 112, and the NMOSFET 121 continues operating. The current from a secondary winding 111a of the transformer 105 charges an output capacitor 108 through a diode 107, and a DC voltage/current is output through an output terminal 109 from the output capacitor 108. In addition, the NMOSFET 134 of the power section 131 turns on after the control circuit 129 starts, and the NMOSFET 304 turns off due to the voltage level of the gate terminal 306 lowering. After the NMOSFET 304 turns off, the JFET 302 turns off since the voltage level of the source terminal 303 of the JFET 302 becomes high.
FIGS. 8A-8D schematically illustrate the JFET 302 of FIG. 7. FIG. 8A is a plan of the JFET 302 (40). FIG. 8B is an enlarged detail of a section A of FIG. 8A. FIG. 8C is a cross-sectional view taken along the line 8C-8C of FIG. 8B. FIG. 8D is a cross-sectional view taken along the line 8D-8D of FIG. 8B. A gate region 1 (a p-type well region) is selectively formed in a surface layer of a p-type substrate 30 so that radially outwardly extending recessed regions each of a predetermined width are formed circumferentially around a drift region 2 (a first n-type well region), which also extends into a portion of the recessed regions of the gate region 1. A plurality of (eight shown) source regions 3 (second n-type well regions) are formed and the source regions are in contact with the drift region 2 that extends into the portion of the gate region 1. A drain region 4 (a third n-type well region) is formed opposite to and apart from the source region 3. The source regions 3 are formed equally spaced circumferentially around the drift region, with the drain region 4 positioned centrally. The source regions 3 and the drain region 4 can be formed at the same time. In addition, a metal wiring 21, namely a gate electrode electric wiring, is formed to surround regions 2, 3, and 4 on the surface of the gate region 1. The metal wiring 21 is connected to a field plate 13 formed via a LOCOS oxide film 12 on the drift region 2, and to a ground GND.
The source contact region 7 is formed in the respective surface layer within the source regions 3 and the drain contact region 8 is formed in a surface layer within the drain region 4. The source regions 3 are connected to a metal wiring 23 via the source contact regions 7, and the drain region 4 is connected to a metal wiring 24 via the drain contact region 8. A depletion layer 11 spreads from p-n junctions associated with the gate region 1, the drift region 2, and the source regions 3, which are surrounded by the gate region 1. The depletion layer 11 is narrow on the side of the source regions 3 of a high impurity concentration, and wide on the side of the drift region 2 of a low impurity concentration. Therefore spreading of the depletion layer 11 can be changed by changing the impurity concentration of the drift region 2. The channel width W1 thus can be controlled.
In the above-mentioned JFET 302 (40), the structure for a high breakdown voltage is in charge of junctions relating to the gate region 1 and the drift region 2, and the structure for a high current is in charge of the source regions 3. Thus, the high breakdown voltage and a low on-resistance are compatible by sharing of roles. In addition, the gate region 1 of the JFET 40 is always grounded, and the depletion layer 11 spreads when the source regions 3 have a bias of a positive electric potential. When a certain drain voltage is applied, the drain current continues decreasing due to the formation of channels that pinches off with rise in potential of the source region 3, and intercepts (hardly flows) the drain current when the drift region 2 (a channel region) is cut off.
In addition, Japanese Patent Laid-Open No. 2001-7121 discloses that a cut-off voltage slightly depending on the drain voltage can be realized because lateral high breakdown voltage JFETs are connected in series. Further, Japanese Patent Laid-Open No. 2005-268319 discloses that increasing the starting current can be realized without sacrificing the breakdown voltage by making a portion of a drift layer of a lateral high breakdown voltage JFET have a high impurity concentration.
In configuring the start-up circuit 133 shown in FIG. 7, an input voltage of the start-up circuit 133 is applied to the JFET 302 through the power source terminal 301, and the starting current charges the smoothing capacitor 110 through the output terminal 308. The NMOSFET 304 is made of an enhancement type to prevent a reverse current from the output terminal 308. Because the NMOSFET 304 is an enhancement type, it is necessary for the voltage level of the gate terminal 306 to be not less than the threshold voltage of the NMOSFET 304 to make itself turn on. This voltage is led through the resistor 305 from the source terminal 303 of the JFET 302. Because the source terminal of the NMOSFET 304 is connected to the start-up circuit output terminal 308, the NMOSFET 304 causes a substrate bias effect by the voltage of the start-up circuit output terminal 308, thereby increasing the threshold voltage of the NMOSFET 304 as the voltage level of the start-up circuit output terminal 308 becomes higher. Therefore, it is necessary for the voltage led to the gate terminal 306 to be more than the threshold voltage of the NMOSFET 304 in the voltage level of the start-up circuit output terminal 308 when the smoothing capacitor 110 has been charged to a predetermined voltage by the starting current.
On the other hand, the channel width W1 in the JFET 302 (40) of the input side of the start-up circuit becomes narrower as the voltage level of the source terminal 303 becomes higher. The voltage level of the source terminal 303 of the JFET 302 thus needs to be low to pass a large current. Because as above mentioned in the circuit of FIG. 7, the gate terminal 306 of the NMOSFET 304 is set to a voltage not less than the threshold voltage for the voltage level of the start-up circuit output terminal 308 when the smoothing capacitor 110 has been charged to the predetermined voltage by the starting current. Further the drain of the NMOSFET 304 is connected to the source terminal 303 of the JFET 302 and the gate terminal 306 is coupled to the source terminal 303 of the JFET 302 through the resistor 305. Then, the voltage of the source terminal 303 of the JFET 302 approximately equals to that of the gate terminal 306 so that the voltage level of the source 303 of the JFET 302 becomes high in comparison with the voltage level of the source terminal of the NMOSFET 304 (voltage level of the starting current output terminal 308).
Comparing this case with a case where the voltage level of the source terminal of the NMOSFET 304 and the voltage level of source terminal 303 of the JFET 302 approximately equal, the starting current decreases or the charging voltage level of the smoothing capacitor 110 becomes low. Otherwise, the starting current decreases and at the same time the charging voltage level of the smoothing capacitor 110 becomes low.
Accordingly, there remains a need for an improvement that can be used for a start-up circuit of an IC for a switching power supply without the drawbacks mentioned above. The present invention addresses this need.
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