Design of the latest high power single chip switch

Design of new high-power single-chip switching module power supply

1 performance characteristics and technical indicators

single chip switching power supply is a new type of switching power supply chip that became popular in the world in the 1990s, but the early single-chip switching power supply chip is widely used in low-power switching regulated power supply. The third generation single chip switching power supply top GX series IC is a new high-power single chip switching power supply chip launched by American power integration company in early 2000. It not only maintains the basic characteristics of top switch II, that is, it integrates power MOSFET, PWM controller, fault detection and other control circuits on a single CMOS chip, but also integrates many built-in and user configurable functions; The maximum output power at single voltage input is increased to 250W, and the electromagnetic compatibility performance is also greatly improved. The main performance characteristics and technical indicators of the third generation single chip switching power supply top GX series IC are:

(1) TOPSwitch GX series has the following new built-in functions:

① soft start: its function is to reduce the overshoot when opening and the impact on components

② frequency jitter: reduce the peak value of EMI

③ higher duty cycle allows larger output power and smaller input filter capacitor

④ zero load regulation capability, high efficiency under light load, and low power consumption during standby

⑤ lagging overheating shutdown enables the device to automatically recover from failure

(2) TOPSwitch GX series also adds user configurable pins for detecting over-voltage and under voltage, setting external current limits, and selecting two switching frequencies (132KHZ and 66KHz). Electric undervoltage protection, overvoltage shutdown, and sampling of circuit feedback voltage to reduce output ripple and minimize duty cycle at high input voltage. Through the external current limiting pin, the current of the power MOSFET can be accurately reduced and the high efficiency can be achieved cheaply

(3) in addition, TOPSwitch GX series also adopts energy-saving ecosmart technology, which can save energy especially under standby and no-load conditions. The functions of reducing energy consumption include remote on/off function, that is, controlling the opening and closing of power supply by remote control, and realizing external synchronization; Automatically reduce the working frequency at low load. These functions have exceeded the standard of American EnergyStar

because TOPSwitch GX series has many of the above characteristics, it can be widely used in medium and low power switching regulated power supplies. It not only has simple circuit, but also has high reliability, so it has a wide application prospect in medium and low power electronic equipment

2 pin function

the pin arrangement of TOPSwitch GX series is shown in Figure 1. This series has two structures: y package and P, G package. It can be seen from the figure that for the y-sealed TOPSwitch GX series, its D, C, s pins are named the same as the original TOPSwitch series switches, which are drain, control and source respectively. The functions of the newly added L, x, f pins are as follows: Laboratory of building materials testing:

L: it is the undervoltage and overvoltage detection end of the input voltage, and it also has the function of remote control. As shown in Figure 2, if 2m resistance is connected between the L end and the positive end of the input voltage, Then the undervoltage protection value is:

Figure 1 Top GX series switch pin diagram

(a) to 220 7C package (b) dip 8b and SMD 8b package

figure 2Y package TOPSwitch GX series electric voltage detection circuit

VUV = Iuv rl=100v (1)

the overvoltage protection value is:

VOV = IOV rl=450v (2)

in addition, the L end also has the function of remote control, as long as the L The remote control function can be realized by adding a triode or optocoupler between the C ends. The specific circuit is shown in Figure 3

x: set the adjusting end for the external current. If different resistance values are connected between the X terminal and the source, the switching current is limited to different values. If r2=12k, the current flowing through the switch is set to 69% of the rated value; If r2=6k, it is 90% of the rated value; If r2=25k, it is 43% of the rated value. It can be seen that when R2 value increases, the current allowed to flow through the switch will decrease

f: it is the switching frequency selection terminal. When f terminal is connected to the source, its switching frequency is 132KHZ; when it is connected to the control terminal, its switching frequency is 66KHz

in addition, TOPSwitch GX series can also be used as a general three terminal top device. At this time, as long as l, x, f terminals are connected to the source at the same time. But at this time, the configurable functions of new users are lost

for TOPSwitch GX series switches in P and G packages, there are no X and f terminals, but the new m terminal has the same function as the L terminal in Y package

3120w, 24V switching power supply module circuit design

3 1 design requirements

(1) input voltage: 85V ~ 265vac, 50Hz

(2) output voltage: 16V ~ 24VDC

(3) output current: 5.0A

(4) electric adjustment rate: 1%

3.2 design steps

according to the design requirements, the design steps are illustrated by the schematic diagram on Figure 4

(1) device selection

top248y is selected as the switching device according to the design requirements. At this time, top248y works at the upper limit of output power, and the current is set at the maximum value, that is, the X end is directly connected to the source. The overvoltage value is set at 4 and 50VDC at the same time. If the input voltage exceeds this value, TOPSwitch GX will turn off automatically until the input voltage returns to the normal value, and TOPSwitch GX will resume starting automatically. The frequency selection terminal f is also directly connected to the source. At this time, the working frequency of the switch is set at 132KHZ

(2) design requirements of pulse transformer

the relationship between current and voltage in the primary inductance (i.e. excitation inductance) LM of pulse transformer is approximately: im= (3)

where: uo is the voltage at both ends of the primary inductance

is the switching pulse width

according to formula (3), the primary inductance value of pulse transformer should be appropriate, generally between 300 h and 3000 H. If the output power is large, the lower limit should be taken; Otherwise, take the upper limit. The transformer is mainly used to change the performance of various materials. The primary inductance should not be too small, which will cause too large drain current of MOSFET in TOPSwitch GX series, increase the switching loss, and easily cause overcurrent protection action, making it difficult to start the power supply. Similarly, the primary inductance should not be too large, which cannot meet the requirements of output power

(3) design of secondary circuit of power supply

the secondary circuit mainly selects rectifier tube and filter capacitor. The selection of rectifier tube should be based on the output current and voltage, and its maximum value is:

irlcmax  2io=2 5=10a (4)

figure 3Y encapsulated TOPSwitch GX series remote control circuit

figure 4120W, 24V switching regulated power supply schematic diagram

vrlcmax  (5) n= Dmin (6)

where: VO is the output voltage

io is the output current

vinmax is the maximum input DC voltage

dmin is the minimum duty cycle of the switch

n is the transformation ratio of pulse transformer

set vinmax=375v; VO=24V； Dmin=0.18 is substituted into equation (6) to obtain the transformation ratio of the pulse transformer: n 4

at this time, the primary excitation current of the pulse transformer is: im==1.25a (7)

this value is much smaller than the drain current of top248y (7.2A)

under the condition of low output voltage, the secondary rectifier of power supply generally adopts Schottky diode to reduce the loss of diode. When the output voltage is high, a fast recovery diode (such as D2 in Figure 4) is required; When the switching frequency is high, the ultrafast recovery diode should be used as the rectifier to reduce the influence of its reverse current on the primary

the capacity of filter capacitor C7 should meet the requirements of output voltage ripple, and L1 and C8 should be able to effectively filter out the noise impact caused by the switch

(4) design of feedback adjustment circuit

the feedback adjustment circuit adopts the adjustment circuit composed of optocoupler and adjustable three terminal voltage regulator TL431, such as the output voltage adjustment circuit composed of VR2, R5, R7, R10, R11 and R6 in the circuit in Figure 4. R5, as the current limiting resistor of the optocoupler, cannot affect the gain of the detection loop of the circuit. At the moment of starting, detect the output current of the optocoupler, so as to change the current of control terminal C and realize pre adjustment, so as to ensure that the power supply reaches the specified adjustment value at low voltage and full load starting. R9, C10, C11, R4 and C4 form a loop compensation circuit

4 problems needing attention in design

TOPSwitch GX series switching power supply can easily meet the design requirements if it is properly designed; If the design is improper, some abnormal phenomena will appear. The following problems should be paid attention to when designing the power supply:

(1) because the output power of the power supply is large, the leakage induction of the pulse transformer is required to be as small as possible, especially in the case of low voltage and high current. The primary and secondary windings of the pulse transformer shall be wound alternately. Even so, the energy stored in the leakage inductance of the pulse transformer may still exceed the power capacity of the transient suppression diode VR1. Therefore, R2, R3 and C2 are used in parallel with VR1 to partially lose the energy in the leakage inductance on R2 and R3, so as to ensure the working reliability of VR1; At the same time, the voltage is clamped at 200VDC to ensure that the drain voltage of the internal MOSFET of the device is lower than 700vdc when the power supply is started and overloaded

(2) the equivalent series resistance of the output filter capacitor should be as small as possible, especially in the case of low voltage and high current, otherwise the reliability of the power supply will be greatly reduced due to the increase of capacitance loss

(3) the output end of the optocoupler should be close to the control end C, and the filter

wave capacitor of the control end C should be close to the source; In addition, the connecting line between the multifunctional terminal L, x, f or m and the source should also be as short as possible, and at the same time, it should be far away from the drain to reduce the power noise

5 conclusion

the third generation TOPSwitch GX series has increased many functions compared with the second generation TOPSwitch series, and the output power has also been greatly improved. Practice has proved that the single chip switching module power supply designed with the third generation TOPSwitch GX series has simple circuit structure, good anti-interference performance, high reliability and high cost performance, so it has a wide application prospect in medium and low power electronic equipment