Abstract: Widely uses in by the constant pressure/constant flow type switching power supply in the battery charger. But the traditional circuit design needs to use many primary devices, not only the electric circuit is complex, moreover the performance is bad. Introduced one kind of special-purpose LinkSwitch series constant pressure/constant flow type monolithic switching power supply, has the performance to be advanced, the electric circuit is simple, cost inexpensive and so on merits, suits constitutes below the 4W each kind of battery charger or the power source adapter.
Key word: Constant pressure/constant flow; Monolithic switching power supply; Battery charger; Input level; Circuit design
Introduction
The constant pressure/constant flow (CV/CC) the formula switching power supply is the most commonly used one kind of special power source. in September, 2002, PI Corporation took the lead to promote the LinkSwitch series (to contain LNK500, LNK501) the high efficiency constant pressure/constant flow type three end monolithic switching power supply chip, it was in the world first to substitute for the high integration rate which, the low cost, the high pressure power conversion chip specially below 4W the low power linear voltage-stabilized source developed. in March, 2004, this company developed the LNK520 constant pressure or the constant pressure/constant flow type three end monolithic switching power supply. By the LinkSwitch constitution’s constant pressure/constant flow type switching power supply, the price endures with the linear power source compares favorably with. It is younger than the traditional linear power source volume, the weight is light, especially qualify in low cost battery charger. This kind of switching power supply may under the constant pressure, the constant flow two kind of working patterns the automaticswitching, the ratio use TOPSwitch-ⅡThe constitution constant pressure/constant flow type special switching power supply may save 20 primary devices approximately. When the specified output is 3W, the LinkSwitch permission uses the EE13 magnetic core, by further reduces switching power supply’s volume.
The LinkSwitch series monolithic switching power supply may substitute the linear power source, widely uses in each kind of individual electronic installation (handset, on non-rope telephone, palm computer, digital camera, the MP3 player, electrically operated razor blade and so on) battery charger or power source adapter, may also apply, in the television and so on domestic electric appliances wait for an opportunity in the power source and each kind of accessory power supply.
1 LinkSwitch series monolithic switching power supply’s performance characteristic
1) the LinkSwitch serial products uses PI Corporation’s EcoSmartR technology, 700V power MOSFET, the PWM controller, the high pressure starts, electric circuit integrations and so on current limitation and overheated protection in a chip. This chip only then 3 pins, speaking of LNK500, LNK501, only need match 14 periphery primary devices, then constitutes has the constant pressure/constant flow (CV/CC) the input level special switching power supply. With when makes the power source adapter, the LinkSwitch work in the constant pressure area, may for the load provide the stable voltage, this time the constant flow area uses for to provide the over-load protection and to short-circuit the time to restart automatically moves the protection. When makes the battery charger, the LinkSwitch work in the constant flow area, the charge finished changes over to the constant pressure area automatically; Because if the load short-circuits in the charging-up causes the output voltage to drop to below 2V, enters restarts automatically moves the stage. Compares with the linear voltage-stabilized source, its power loss may reduce 70%.
2) LNK500, LNK501 when the wide scope input (exchanges 85~265V) the peak power output is 3W, exchanges time the 230V fixed input peak power output is 4W. Usually works the LinkSwitch design under not the continual pattern. May enhance the constant pressure output using the light pair feedback technology the precision and the stability, but uses the exterior stabilovolt to carry on two time constant voltage to be able to improve the constant current characteristic.
Figure 1
3) this component uses the novel primary constant pressure, the constant flow control plan, including primary electric circuits and so on clamp, feedback, internal electrical power supply and return route compensation, simplified peripheral circuit’s design enormously. LNK500/501 does not need the auxiliary winding and the exterior constant pressure/constant flow control circuit, controls the constant pressure/constant flow output completely by primary induced tension UOR.
4) has the consummation protection function, including the overheated protection, crosses the current protection, in the output short circuit situation over-load protection, the opening fail safe and the soft start function.
5) the power MOSFET drain electrode breakdown voltage is 700V, the limiting current fixed is 254mA, leaks - the source to pass condition the resistance for 28Ω (typical value), the biggest dutyfactor is 77%. Restarts moves the frequency is 300Hz automatically. Overheated protection temperature for 135℃, when the temperature drops to 70℃ chip only then rehabilitation.
2 LinkSwitch series monolithic switching power supply’s model application
By LNK501 constitution constant pressure/constant flow type battery charger’s electric circuit as shown in Figure 1. This battery charger’s major technique target is as follows: Constant pressure area specified output voltage Uo= 5.5V, constant flow area output current IOM=500mA, peak power output POM=2.75W; When exchange input voltage u=85~265V, power source efficiency η≥72%; When exchange input voltage u=230V or 115V, the idling power loss respectively is 260mW or 200mW.
RF (FusibleResistor) uses 10Ω, the 1W fuse resistor, when the input end has the short trouble can play the overflow protective function. BR is 1A, the 600V rectification bridge, also the available 4 1N4005 silicon electric valve replaces. C1, L and C2 constitution Pi filter. The LNK501 turn-on frequency is 42kHz, the permission use simple EMI filter filtration electromagnetic interference, moreover does not need between generally the primary returns end and the secondary returns end the parallel security electric capacity (also calls the Y electric capacity).
By 1A, 600V silicon diode VD1 (1N4937) and 0.1μF capacitor C4 is composed the clamp protection circuit, uses for to absorb the peak voltage which produces by the high-frequency transformer leakage inductance. Primary winding’s induced tension value (UOR) also said that the secondary reflection voltage, it has the following relationship with output voltage UO between: UOR=n (UO UF1) (in the formula n is primary and secondary number of windings compared to, UF1 is secondary electric valve’s pressure drop). This indicated that UOR can reflect the output voltage the height. Therefore, use sample electric capacity C4 obtains the feedback voltage can reflect UO similarly the change situation. The resistance R1 function is transforms the C4 on feedback voltage the feedback electric current (i.e. control end electric current IC), then adjusts LNK501 the output dutyfactor, realizes the constant voltage goal. May reduce the switch noise using R2. According to the actual need may also at the beginning, between the secondary returns end parallel 1000~2200pF, the pressure resistance value be 1.5kV security electric capacity C6, further suppresses the electromagnetic interference, concrete wiring method like chart 1 dashed line shows.
In constant pressure region, output voltage dutyfactor control. When IC>2mA, enters the constant pressure area, the output voltage and the dutyfactor also reduces; When IC=2.3mA, has carried on the current protection, causes the dutyfactor to drop to 30%. If UO falls to 2V below, then the C3 electric discharge, causes LNK501 to enter restarts automatically moves the stage, rapid the output current will limit below 50mA. If the actual output power surpasses POM, then UO↓→UOR↓→IC↓, thus has limited drain electrode electric current ID further enlargement. Because if the out-port has the short trouble to cause the output to continue to increase, then IC drops 0.9mA, the force control end electric capacity Cc electric discharge, LNK501 enters restarts automatically moves the stage. The above automatic protection function enhanced battery charger’s in work time security. In the idling or in the underloading situation, the chip power loss reduces along with the turn-on frequency reduces.
This constant pressure/constant flow type battery charger’s input level as shown in Figure 2. Figure 2 the solid line represents the limit value. Secondary electric valve VD2 uses 11DQ06 1A/60V the Short base diode, also available MBR160 replaces.
3 circuit design main point
3.1 peak power outputs
LinkSwitch when exchanges the 85~265V wide scope input, to exchange the 230V fixed input most greatly series power output (POM) respectively is 3W, 4W. In order to achieve the most greatly series power output, when designa circuit should pay attention to the following item.
1) designs cocurrent input voltage minimum value UImin≥90V. When exchange input voltage u=85~265V, inputs the filter electric capacity’s capacity to be possible according to 3μF/W the scale-up factor selects. Regarding exchanges 230V or the 115V fixed input voltage, may according to 1μF/W the scale-up factor selects.
2) LinkSwitch is designs specially in not the continual pattern puts down work
Does, this time the primary winding induced tension UOR scope is 40~60V. If designs the continual pattern, will cause the ring circuit work not to be unstable.
3) the secondary electric valve should use the Short base diode. Figure 1 VD2 uses 1A/60V the Short base diode.
4) may suppose the power source efficiency in advance η=70%.
5) the source pin must be good with the board on duplicate copper foil contact, guaranteed that sends out promptly the quantity of heat, causes the chip temperature not to surpass 100℃.
Besides receives factor and so on temperature condition, well ventilated condition, seal form, power source structure influences, in assigns under the condition the LinkSwitch peak power output also with the high-frequency transformer magnetic core’s size, the magnetic permeability, the primary inductance capacity, smallest parameters and so on input voltage, input filter electric capacity’s capacity, output voltage, electric valve pressure drop related, this will create the actual POM value and the design value is not equal.
3.2 high-frequency transformers
Under non-continual pattern, when power MOSFET shutdown, saved in high-frequency transformer’s energy transforms into the secondary output. If did not consider loses secondarily in situation, then high-frequency transformer’s biggest transformation power is
In the formula: LP is the primary inductance quantity;
I2LIMITf is a coefficient, it represents the primary limiting current square and the turn-on frequency product, its typical value is I2LIMITf=
PM corresponds the control end electric current indicated with IDCT that IDCT may use for to establish LinkSwitch power source’s maximum output power POM.
When the switching power supply starts from the constant flow area works, the primary inductance quantity (LP) to the peak power output (POPK) decides the function, this parameter should perform to control. If estimated the POPK value permission has ±20% change quantity, then LP value allowable deviation ±10%, magnetic core’s air gap δ≥0.08mm.
If uses the EE13 magnetic core, takes δ=0.08mm, the LP permission has ±10% deviations, primary induced tension UOR=40~60V, then the LinkSwitch peak power output may reach 2.7W. The choice size big magnetic core can promote induced tension UOR, then enhancement output. When for example, chooses the EE16 magnetic core, the peak power output is 3W. If does not consider time the idling power loss, but may also use the EE19 magnetic core, when exchanges the 230V input, UOR>70V, the output will enhance 5W. UOR also affects the constant flow input level curve the linearity. Before completing the design to inspect the constant flow input level.
3.3 influence input level factor
The LinkSwitch input level remainder decided by the LinkSwitch remainder as well as the exterior electric circuit. Uses when as shown in Figure 1 the electric circuit, the peak power output is 2.75W, the output voltage change quantity is ±10%, output current’s change quantity is ±20%. The influence input level factor mainly has the primary leakage inductance, in the secondary electric valve’s pressure drop and the output lead resistance. For example, when the primary leakage inductance achieves 50μH will cause when the idling the output voltage probably rises 30%. If increases the light pair feedback circuit and uses the exterior stabilovolt to carry on two times the constant voltage, then in the entire load scope’s output voltage change quantity may reduce ±5%.
3.4 essential primary device’s choices
Below take Figure 1 as an example, introduced that chooses the periphery essential primary device’s principle.
1) clamp diode VD1 should choose the pressure resistance is 600V even higher quickly restores or ultra-fast restores the diode, cannot use the ordinary low speed diode.
2) clamp electric capacity C4 may use 0.1μF, the 100V metallic membrane either the plastic film electric capacity, the capacity error in ±5%, ±10% or ±20% may, but does not recommend the use ceramics electric capacity, because this kind of electric capacity is big easily the temperature and the voltage change’s influence, to cause the output voltage fluctuation.
3) controls end electric capacity C3 to be possible to be the LinkSwitch on electricity process or restarts automatically moves the stage to provide the control voltage. When design battery charger, C3 should choose 0.22μF, the 10V capacitor, guaranteed that has the enough time starting circuit.
4) chooses feedback resistance R1 the principle is when the peak power output can cause 2.3mA the feedback electric current inflow control end. The R1 accurate value also needs to look designs UOR value. Increases the R1 value to be possible suitably to change battery charger’s input level, as shown in Figure 3.
Figure 3 the dashed line frame represents the model the input level curve, the solid line frame indicated that will increase the R1 value later actual work area, the dotted arrow expresses the constant pressure area and the constant flow area change direction separately. As seen in Figure 3, increases the R1 value to be able to enhance average output voltage UO (constant pressure curve upward movement), reduces average output current IO (constant flow curve toward left migration). Its change characteristic is RFB↑→UO↑→IO↓.
5) like needs to increase security electric capacity filtration secondary on the transient state disturbance, should the security electric capacity be parallel, in the primary circuit returns to the end and the secondary circuit returns to between the end. The security electric capacity’s capacity is very small, but the pressure resistance value must achieve 1.5kV only then to conform to the IEC standard.
3.5 oscilloscope’s wiring ways
When uses the oscilloscope observes LinkSwitch the output wave shape, if the probe meets source S, the grounding meets the primary returns end, then on the probe head distributed capacity’s electric charge possibly creates triggers by mistake, enables LinkSwitch to achieve the limiting current. The correct method is according to shown in Figure 4, meets LinkSwitch probe head’s grounding drain electrode D, directs current takes the reference point. Because on the drain electrode has the direct-current high pressure, therefore, enters the terminal in the power source to meet insulating transformer T1.
4 conclusions
The LinkSwitch series constant pressure/constant flow type three end monolithic switching power supply uses the primary constant pressure, the constant flow control plan, it does not need the auxiliary winding and the exterior constant pressure/constant flow control circuit, simplified peripheral circuit’s design enormously, to realize the low power CV/CC type switching power supply’s optimization design to create the advantage.
