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   Abstract: Multi-channel outputs the DC/DC module power source’s detailed design process introduction through one kind of UC3843 control low power, discussed in the multi-channel output module power source design to output the different place with emphasis with the single-channel, introduced in detail in DC/DC module power source commonly used new chip UC3843 peripheral circuit parameter design, has given in the multi-channel output module power source the transformer and the coupling inductance engineering design detailed process and satisfies each kind of question which each performance index should pay attention.

    Key word: DC/DC converter; Multi-channel outputs; Coupling inductance

Introduction

The DC/DC module power source has widely used in the microwave communication, the avionics, the ground-based radar, the fire fighting equipment, the medical instrument and so on many domains. And many application situation needs the multi-channel outputs. If in the monolithic integrated circuit intelligence controller, the monolithic integrated circuit power supply needs 5V, but transports puts the integrated circuit usually to need 12V. When design multi-channel output power sources, many places are different in the single-channel output, needs to consider the question are many, the difficulty is big. For instance, both must consider that the transformer base pin limit, many vice-side transformer designs, each road’s voltage-stabilizer circuit realizes, and must consider time each group underloading and the full load load relative regulation, the load overlapping adjustment characteristic. This article through one for the monolithic integrated circuit intelligence controller power supply’s 15W three group module power source’s design example, showed the multi-channel output power source’s design feature in detail.

Figure 1

1 power source’s design target

the 12V input, 5V/±12V three group output module power source’s design target like table 1 arranges in order.

Table 1 design target

2 power source’s principles of design

Figure 1 aims at the multi-channel power take-off valve power source schematic diagram which the monolithic integrated circuit motherboard electric power supply designs.

Figure 1 inductance L201, L202, L203 is the coupling inductance, L204 is the bias winding, as a result of the transformer base pin limit, is been from the coupling inductance.

The electric circuit uses the single end to stir up the transfer network, when converter connection power source, input DC voltage after resistance R601 and 12V stabilovolt D601 and triode V601 and V602 composition constant voltage voltage dropping electric circuit, starts UC3843. After entering the normal work, the bias winding L204 feed circuit starts to work, bias winding’s output after the diode D4 rectification, C601 filter outputs the 12V voltage, is higher than from the power line voltage, causes diode D602? , The starting circuit stops the work. The bias winding is UC3843(IC301) provides the working voltage (12V), the converter enters the normal work, under the PWM pulse-duration modulation way, after each group secondary’s output passes through each road’s diode rectification, the LC filter filter, produces each road’s direct-current output voltage. 5V output voltage after resistor R402 and R406 differential pressure, (IC401) the 2.5V reference voltage compares with programmable constant voltage source TL431, then the transmitted light coupler (IC101) feeds back to the UC3843 foot 2, steering impulse dutyfactor, stable 5V output. Coupling inductance L202 and L203 realize ±12V two groups constant voltages. Overflow protective resistor R101 and R102 examine switching valve’s overflow signal, sends in UC3843 the foot 3, blocks UC3843 the output signal, realizes the overflow protection.

Figure 2

3 design proposal choice

Sometimes the DC/DC module power source by the low power primarily, the power under 150W, the electric circuit analysis situs which uses by instead stirs up mostly with is stirring up the converter primarily, also uses the push-pull converter, the power source request volume is small, when design uses completely pastes the piece part.

3.1 master control chip selects

The master control chip uses new pulse-duration modulation integrated circuit UC3843, is one kind of current mode control special-purpose chip, Figure 2 is the UC3843 functional block diagram. It has the undervoltage locking circuit, the low quiescent current (1mA), the big electric current output, built-in can the crack reference voltage, the 500kHz operating frequency, the low R0 amplifier, the voltage relative regulation may reach 0.01V, the very close linear voltage-stabilized source’s relative regulation, low initial firing current 1mA, the starting circuit is only simple and so on characteristics.

3.2 constant voltage mode selection

Outputs to the single-channel, only adds the constant voltage feedback circuit in the out-port then, but to the multi-channel outputs, must regard the request to decide: If each group output voltage precision requests high, then each Lu Duying designs the independent closed loop constant voltage return route, like this designs the difficulty to be big; If only then a group is the important load, other groups loads are light, and to the output voltage accuracy requirement is not very strict, then only need give the important load to add the reaction control return route in the electric circuit, other two groups split-rings, depend upon the coupling inductance to realize the constant voltage.

3.3 multi-channel output filter inductance circles the system mode selection

In this example’s three groups outputs, 5V (Uo1) is the quite important load, the output current is biggest (2A), 12V is the operational amplifier electric power supply, the permission voltage in the 1~2V scope change, the electric current is small (0.25A), therefore, only in the 5V main road Canada reaction control return route, ±12V the auxiliary road’s constant voltage performance is realizes depending on the coupling inductance. In view of this example multi-channel output’s special details, output the filter inductance not to be suitable use the independent inductance, but should use the coupling inductance, soon three group output filter inductance circles on a magnetic core, only then the 5V main circuit is controlled, the input level is good, but ±12V two group bad influences are not big.

4 power source design process

4.1 UC3843 peripheral circuit design

4.1.1 turn-on frequency choice

The secondary power product operating frequency general choice between 100kHz~400kHz, this example establishment turn-on frequency is the 250kHz, UC3843 operating frequency may reach 500kHz, the foot 4 is the Rt/Ct saw-toothed oscillator’s resistance and electric capacity’s public end fixed time, speaking of UC3843,

In the formula: R is Figure 1 R304, its value is 6.8kΩ;

C is Figure 1 C302, its value is 1nF.

4.1.2 overflow protection circuit design

Figure 1 R101 and R102 are the overflow examination resistances, ISMAX≈1.0V/RS designs R101 and R102 according to the type, this resistance must suppose very much slightly, take reduces in the resistance the loss, Figure 1 design as two 10Ω resistances is parallel. The examination voltage sends in UC3843 the foot 3.

The foot 3 voltages are higher than the 1V overflow protection circuit on the movement, causes the foot 6 stop output rectangular wave, the electric circuit to stop the work. The foot 3 must meet RC filter to suppress switching valve’s peak electric current, Figure 1 this filter is composed of R103 and C306.

4.1.3 feedback error amplifier design

R302, R303 and C305 constitution integral regulator, resistance R302 and R303 proportional relationship influence system’s dynamic characteristic. R302 and the R303 ratio may change the UC3843 voltage error amplifier’s enlargement factor, regarding certain feedback voltage quantity, may cause the PWM regulator’s output pulse width to be different, thus the influence output voltage adjustment scope, namely in the influence target outputs dynamic response adjustment scope. Integrator’s electric capacity C305 size influence system’s adjustable speed, namely in the influence target outputs dynamic response time.

4.2 power component’s selection

Converter’s switch component uses power MOSFET, the basis single-valve converter computation voltage empirical formula, takes

In the formula: Udmax is leaks the source the greatest voltage;

D is a dutyfactor.

Therefore, the power MOSFET reverse voltage with is bigger than 144V elected, the electric current determined according to a high-frequency transformer winding’s maximum current. Figure 1 V101 selects pressure resistance 200V, electric current 9A IRF630.

4.3 high-frequency transformer’s designs

4.3.1 magnetic core’s selections

The multi-channel output transformer general request has the big window area, the DC/DC module power source may select, FEE, EUI magnetic cores and so on FEY, regarding is stirring up the converter, theoretically the transformer primary must have replacement winding Nr, here considered the transformer foot position the question, selects the high saturation induction density magnetism material, removes repositions the winding, like this causes each time the magnetic core magnetization curve lower part to work, avoids the magnetic core being saturated.

Determined first biggest magnetic induction intensity Bm, calculates and the primary election magnetic core model.

1) considered when high temperature saturation induction density Bs will drop, simultaneously to reduce time the high frequency work the magnetic core will lose, the maximum work magnetic induction intensity will elect is 0.2~0.25T generally. Here selects the high saturation induction density magnetism material RM2.2KD, its Bs is 0.44T.

2) the magnetic core model’s selection has two methods; first, basis type (3)

In the formula: Ae is the magnetic core cross-sectional area;

Aw is the magnetic core window area;

fs is the turn-on frequency;

δb biggest magnetic-flux density range of variation which permits for the magnetic material;

dc is the transformer winding conductor current density;

kc is winding’s in magnetic window space factor.

Second, gives the output and the magnetic core size relations according to the factory magnetic core material handbook. Here uses the second method to select the FEY15.3 magnetic core, its effective cross-sectional area is 18.7mm2.

4.3.2 computation ratio-turns

Uo=Uo1 UD=5.0 0.5=5.5V    (4)

In the formula: Uo1 is the 5V main road output voltage;

UD is the electric valve MBR1545 forward voltage drop, takes 0.5V.

In the formula: nearby n12 primarily Lu Yuanfu ratio-turn;

Ui=UminDmax=36×0.48=17.28V (Umin is power source lowest input voltage, Dmax takes 0.48);

Uo is the N2 output voltage.

Actual selects n12=4:1.

4.3.3 calculate and adjusts main Lu Fubian the number of windings

δBm is the magnetic flux increase, δBm=0.44-0.065=0.375T;

Ae is the magnetic core cross-sectional area, to the FEY15.3 magnetic core, Ae=0.187cm2.

Actual takes the N2=4 circle.

4.3.4 computation primary side number of windings

N1=N2×n12=4×4=16 circle    (7)

4.3.5 calculate other two auxiliary Lu Fubian the number of windings

In the formula: Uo2 is the 12V auxiliary road output voltage;

UD ‘ for the electric valve SK3B forward voltage drop, also takes 0.5V.

Actual takes the N3=N4=10 circle.

Circles when the system, because the primary side, host Lu Fubian the electric current is big, to reduce the leakage inductance, uses the double thread and the winding separately and three and the winding.

4.4 output electric valve design

In order to reduce the power loss, raises the power source efficiency, selects the Short base rectification diode. Outputs electric valve’s nominal current (IF) the value should for the output cocurrent current capacity definite value (Io) above 3 times, namely IF1>3Io; Electric valve’s reverse pressure resistance UR≥1.25PIVs, (PIVs=Uo UMAX, UMAX=2UACMAX, UACMAX is output biggest ripple voltage peak-to-peak value). Based on this principle, the Uo1 road electric valve uses MBR1545, reverse pressure resistance 45V, forward current 15A; Uo2 and Uo3 use SK3B, reverse pressure resistance 100V, forward current 3A. Here reverse pressure resistance chooses high, is advantageous in reducing on electric valve’s loss.

4.5 output coupling inductance design

In uses a group to be controlled, under other two group dependence coupling inductance constant voltage this control mode, to maintain at the auxiliary road output voltage adjustment in the voltage 1V stable region, when multi-channel outputs, advocates the output inductance and each group inductance want work in the inductance electric current continual condition. When design carries on the high-pressured leg first to the low pressure leg’s conversion, according to total output electric current according to single-inductor selection magnetic core, total wire cross-sectional area as well as wire size, number of windings. Namely first through circuit design determination output filter inductance value. In order to cause the inductance electric current to maintain the filter effect continuously, outputs the filter inductance to design in the continual condition, the inductance value which winds through inductor’s electric current to be bigger than the load current minimum value IOMIN, the inductance value is bigger than when IOMIN corresponds

In the formula: n is the transformer ratio-turn;

Uimax is the biggest input supply voltage;

δi for permission inductance electric current biggest ripple magnitude of current.

After definite filter inductance value, according to the inductance biggest stored energy value 0.5×L×I2, the basis type (10) chooses the magnetic model,

In the formula: IMAX is the inductance electric current biggest effective value;

ISP is the inductance electric current biggest peak value;

BMAX is the magnetic circuit magnetic-flux density maximum value.

Then basis type (11) definite inductance number of windings,

Finally the redistribution to various legs, obtains various coils number of windings and the size according to each group actual electric current and the secondary ratio-turn.

The electric current continual pattern inductance core may choose compared to the transformer magnetic core bad some magnetic core material, but in practical application, if two material price difference is not big, often uses with the transformer same material. Figure 1 inductance core was still FEY15.3.

The choice inductance’s number of windings first need to satisfy the inductance the number of windings compared to be equal to main transformer’s output winding’s number of windings compared to, because, if coupling inductance L201 and the L202 number of windings compare cannot guarantee and transformer’s ratio-turn are equal, will have the electric current flowing which between Uo1 and Uo2 will attach, will thus have the very big ripple in its output. Next, each road excellent by 2 or 3 obtains each road’s inductance number of windings in each group corresponding transformer number of windings. Figure 1 chooses the transformer number of windings 3 times, happen to may the double thread and circles the stuff window width. Finally obtains the Uo1 road output inductance number of windings is NL201=3N2=3×4=12 the circle.

    The Uo2 road output inductance number of windings are NL02=3N3=3×10=30 the circle, the Uo3 road output inductance number of windings are the NL203=NL02=30 circle.

Bias winding NL204 is UC3843 provides the 12V working voltage, its output voltage =Uo2, therefore number of windings NL04=NL202=30 circle.

In order to satisfy the load relative regulation, the mutual inductance must the very good coupling. Therefore when circles system each winding, should cover the entire skeleton’s width, moreover must use the same line diameter several wires to twine abreast in row, guaranteed that in the entire skeleton’s width, achieves the best coupling.

5 other matters needing attention

1) debugs when the multi-channel output power sources must separate the auxiliary road first, adjusts the good host road, guaranteed after main road work normal, adds on the auxiliary road adjustment again, may reduce the debugging difficulty.

Figure 5 and Figure 6

    2) to satisfy the design target, except must pay attention to when the full load load relative regulation, but must take into consideration time the underloading load relative regulation; In order to prevent time the idling the output voltage too high damage output electric valve, must give each group output to add on the false load, Figure 1 R5, R6, R7 is the corresponding road’s false load, the false load value is not suitable is too big, the size available experiment determined. Moreover advocates between the road and the auxiliary road’s false load must coordinate the adjustment, satisfies the auxiliary road’s voltage range in the target.

3) the main road idling output voltage may determine by the TL431 voltage-division resistance, when the main road idling output voltage is low may reduce R406, guarantees TL431 the 2.5V datum. When Table 2 are the R406 resistance number changes, obtains each group idling output voltage a group of data. After the main road idling output voltage and the false load determine, when appears the auxiliary road idling output voltage surpasses the indication range, but suitably changes the electric valve parameter, when if the auxiliary road 12V output is 12.7V surpasses target 12.5V, may use in exchange the forward voltage drop is the 0.7V electric valve replaces the forward voltage drop is the 0.5V electric valve. Outputs the false load also to be possible to adjust the idling output voltage.

Table when 2 R406 resistance number change each group output voltage change

4) when layout wiring, each primary device places in turn according to the schematic diagram order, the switching valve drain electrode and the transformer primary side’s segment must be as far as possible short, the UC3843 all periphery primary device must approach this integrated circuit as far as possible, particularly the decoupling electric capacity and the shunt capacitor must the cloth nearby the corresponding base pin, probably, in the transformer, the switching valve and so on give off heat nearby the primary device with to disperse through many holes the first floor the thermit welding to scrutinize enhance continually the radiation effect.

6 measured data and profile

Figure 3 is the UC3843 foot 4 output saw-tooth waves, whether can use in judging the UC3843 normal work.

Figure 4 is when an idling UC3843 foot 6 output rectangular waves.

Figure 5 is the main road Canada 25Ω load, when auxiliary road idling the switching valve leaks - the source profile, may see this time D approximately 0.35, the pulse width already started to adjust.

Table when 3 load fluctuations output voltage fluctuation

the 5V idling, ±12V full load 5.04 11.205-11.26 charts 6 is when the main auxiliary road entire full load the switching valve leaks - the source profile, may see this time D approximately 0.5, the pulse width transferred to a big way.

Table 3 are this three group output power source when main, the auxiliary road load fluctuation auxiliary road’s output voltage fluctuation measured data.

7 conclusions

The multi-channel outputs are more complex than the single-channel output design, must complete the UC3843 periphery primary device parameter, many vice-side high-frequency transformers, the coupling inductance with emphasis based on the design target and so on correct design, the coordination feedback loop’s adjustment, can guarantee each target comprehensively. This article designs the power source has used in the monolithic integrated circuit data acquisition control circuit.