51 Research and Design, Electronic Engineers website - Embedded Systems, MCU, DSP, EDA, Test and Measurement, Components, Communications, Power, Microelectronics, Semiconductors

    Abstract: To raise the low power instead to stir up switching power supply’s complete machine efficiency large scale, may select the vice-side synchronized rectification technology to substitute for the original Short base diode rectifier. It enhances one of low pressure direct-current output switch voltage-stabilized source performance most efficacious devices.

    Key word: Instead stirs up the converter; Vice-side synchronized rectification controller STSR3; High efficiency converter

2.7 initialization time (tant) prevents the primary side and the vice-side altogether condition breakover

Realizes a synchronized rectification main difficult problem, is guaranteed that controls driving signal which IC sends out correctly unmistakable, by? Stops presents overlapping “altogether condition the breakover” between the vice-side synchronous rectifier and the primary side switching valve. Its schematic drawing obvious chart 16 profile. When primary side MOSFET breakover, Figure 16 voltage Vs favors the negative polarity. If time vice-side synchronized MOSFET shutdown has some detentions, then and vice-side between will present a short circuit link in the primary side. In order to avoid the situation occurrence which this kind did not hope, before the primary side MOSFET breakover, synchronized MOSFET must be the closure, this indicated that has the essential establishment ration “the initialization” time tant.

    Figure 17 when has given the normal work situation which launches in detail, CK clock signal and between OUTGATE output driving signal relational graph fixed time. Chip interior’s tant fixed time has provided the pre-placed time which needs, thus has avoided altogether condition the breakover appearance. Power supply condition use foot SETANT, tant has three kind of different choice values according to Table 1. In the foot SETANT external connection resistance voltage divider power supply, may obtain this foot voltage value which and the pre-placed time in table 1 needs.

    In the chip numerical control unit has these initialization time, is the radio impulse number which contains during the switching period completes through the computation. Because this system has the digital performance, will lose several in the counting process, will thus cause in the output driving signal to have the beat. Table 1 pre-placed time value is a mean value, has considered this kind of beat factor. Figure 18 has given the OUTGATE shutdown period beat profile.

2.8 idling and underloading active status

When dutyfactor <14%, the STSR3 internal characteristic can cause OUTGATE to close, and cut-off chip interior majority electric circuit power supplies, thus reduces component’s power loss. Under this kind of condition, converter’s low output current, is completes by the synchronized MOSFET body diode. When dutyfactor >18%, IC starts once more, therefore has 4% lag quantity. When the primary side PWM controller has the sharp-edged condition when the extremely light output load, this characteristic still could maintain the STSR3 system to work correctly.

    Outputs the driver to have withstands the big electric current ability, the source peak value reaches 2A, after adding the radiator, may reach 3A. Therefore the synchronized MOSFET switch is extremely quick, permission parallel several MOSFET reduces the breakover loss. In the power supply period’s high level is Vcc, therefore the chip only actuates to have logic level electronics grid threshold MOSFET.

2.9 transient state characteristics and actual profile

When the load has the sweeping change, the dutyfactor may in several switching periods from the low value extremely quick diastrophism be the high value, vice versa. But OUTGATE gives the pre-placed time, is acts according to the compute switch cycle (frequency), but non-basis dutyfactor. Even if in dutyfactor fast change time, it can also provide the initialization time correctly, thus provides the correct actuation throughout for synchronized MOSFET. Figure 19 gave the dutyfactor from 50% to turn 80% in one cycle, returned to 50% when immediately survey profile. Figure 20 has given the pre-placed time which OUTGATE provides correctly, saw from the chart is 131ns.

    2.10 synchronized rectification controller STSR3 model application circuit diagram

Figure 21 has given the STSR3 model application circuit wafer test chart. This electric circuit may substitute instead stirs up in converter’s rectification diode, carries on the synchronization with the external clock detector, may use in each type instead stirring up the converter, for example AC/DC or DC/DC. In chart some electric circuits are not essential, for example, when the primary side switch cuts off, if does not have the ringing to appear, then R24, D15, R25 and C11 may erase. Models the shell seal with TO220 synchronized MOSFET to be possible to assemble on the circuit wafer. ST Corporation provides suitably to make the synchronized rectification the MOSFET product model, gauge Geleg in table 2.

Table 2 ST Corporation provide uses in the synchronous rectifier ultra low breakover resistance specially the MOSFET new product specification

This circuit wafer, can instead stir up-like in the converter, the diode rectification change is the MOSFET synchronization rectification very easily.

    2.11 main chip STSR3 printed wiring board design layout

Any kind of high frequency switching power supply, needs a good PCB design layout, realizes the complete machine system performance highest target, and solves the disturbance radiation conduction problem. On the circuit wafer primary device’s emissions position, the pin walk the line and the width and so on, is the main topic. This article will give some basic rules, enables the PCB designer to manufacture the good STSR3 circuit wafer layout.

Figure 21

    Draws a line when PCB, all electric currents walk the line to reduce and to overstrike, cause the line resistance and the parasitic inductance are the minimum values as far as possible, promotes system’s efficiency and reduces the disturbance the radiation conduction. The electric current returns’s way arrangement is another has the decision significance topic. Signal grounding SGLGND and power grounding PWRGND should distinguish the wiring, and meets the chip the signal skintled brick work. Printed wiring board various primary devices layout as shown in Figure 22.

Figure 22

    Because foot INHIBIT meets the chip interior - 25mV comparator, it is sensitive to the wiring, must therefore cause on the board to meet INHIBIT the segment to reduce as far as possible. As the experience, the signaling current walks the line to be far away from the pulse current or the split-second-selection voltage walks the line, avoids between them presenting the coupling effect.

    Figure 23 gave frontage which welded from the primary device (i.e. crown end) to see board copper foil (according to 1:1 area size) drew the line layout; Figure 24 the piece gave at the back of the board (i.e. base end) the copper foil to draw the line, several garden shape perforation spot.

how do 2.12 use the STSR3 control panel to replace the original diode conveniently

Figure 25 has given with the simple method, in original instead stirs up on the converter, removes already the vice-side rectification diode which installs, exchanges the synchronized rectification STSR3 control panel’s schematic drawing. If original Vout is equal to or is bigger than 5V, receives the new board’s on Vs electricity line ball Vout; If it is lower than 5V, still received Vs the MOS drain electrode.

Figure 25

3 conclusions

Uses in controlling the synchronized rectification specially the new component to be published, it can enhance AC/DC or DC/DC instead stirs up-like converter’s efficiency. STSR3 regarding the primary side PWM controller is completely transparent, it works in vice-side. This component can work in? What topology, provides the correct driving signal for the synchronized MOSFET switching valve. The above introduction’s control panel in extant instead stirs up on willfully the converter, can realize the synchronized rectification by the simple effective method. (continues)