Basic stimulate transformer principle to if the graph is shown 1 times,pursue instead, the low power that has electric segregation to inputting output in need (1W ～ 60W) switch power source uses an occasion, stimulate transformer instead (Flyback Converter) it is a kind of the most commonly used develop attacks structure (Topology) . Simple, reliable, low cost, coming true easily is the good point that surges transformer to highlight instead.

 

 

Next, the design stage that referenced graph shows 2 times, one pace design stimulates transformer instead

 

1.Step1: Initialization system parameter

 

——Input voltage limits: Vinmin_AC reachs Vinmax_AC

——Mains frequency: Fline (home is 50Hz)

——Output power: (be equal to the sum of each output power)

 

 

——Efficiency of preliminary estimation transformer: η (when low pressure is outputted, η takes 0.7 ～ 0.75, when high pressure is outputted, η takes 0.8 ～ 0.85) basis beforehand appraise efficiency, estimation power input:

 

 

To multichannel output, definition KL (N) the ratio that outputs power and output general power for N road:

 

 

When only way is outputted, KL (N) =1.

 

 

2.Step2: Input capacitance Cbulk certainly

 

The extraction of Cbulk value and power input are concerned, normally, to wide input voltage (85 ～ 265VAC) , take 2 ～ 3μF/W; Right narrow limits inputs voltage (176 ～ 265VAC) , take 1μF/W can, capacitance charges occupy sky comparing Dch to be taken commonly 0.2 can.

 

 

Be in a design to stimulate transformer instead in the Vinmin_DC of the least tension after rectification commonly, can calculate by Cbulk Vinmin_DC:

 

 

3.Step3: Affirmatory the biggest occupy sky comparing Dmax

 

Stimulate transformer to have two kinds of operation pattern instead: Electric electrification sorts progressive die type (CCM) with inductance electric current intermittent mode (DCM) . Two kinds of mode have actor drawback each, relative to character, DCM mode has better switch character, 0 electric current close sub commutate diode, because of the issue that the diode of this nonexistent CCM mode resumes reversely. In addition, fall with power social estate, the energy that stores than CCM mode as a result of the transformer of DCM mode is little, the transformer size of reason DCM mode is smaller. But, compare CCM pattern and character, DCM mode makes the RMS of primary electric current increases, this will increase what MOS is in charge of to guide a loss, can add the voltaic stress of sub output electric capacity at the same time. Accordingly, CCM mode often is recommended to use the occasion that outputs in low-pressure big electric current, DCM mode often is recommended to use the occasion that in high pressure small electric current outputs.

 

Graph 4 stimulate transformer instead

 

Stimulate transformer instead to CCM mode and character, the voltage gain prep only by that inputs output takes sky comparing decision. And DCM mode stimulates transformer instead, the voltage gain that inputs output is by what occupy sky comparing and laden condition to decide at the same time, this makes the circuit design of DCM mode becomes more complex. But, if we are in the critical point of DCM mode and CCM mode (BCM mode) , input voltage lowest (Vinmin_DC) , be fully loaded with a condition to fall, design DCM mode stimulates transformer instead, can make the problem becomes simplification. Then, no matter stimulate transformer to work instead at CCM mode, still be DCM mode, we can undertake designing according to CCM mode.

 

If pursue 4 (B) be shown, when MOS canal closes, collective overlay is in input voltage Vin and NVo of sub reflex voltage the DS of MOS two end. The biggest after occupying sky comparing Dmax to decide, reflection voltage Vor (namely NVo) , VD of the biggest voltage and MOS are in charge of susceptive of sub commutate diode susceptive is the biggest voltage Vdsmax, can get by next type:

 

 

Adopt formula (5) (6) (7) , knowable, dmax takes a cost smaller, vor is smaller, then the stress that MOS manages is less, however, the voltage stress that sub commutate manages increases however. Accordingly, below the condition that we ought to measure in the enough abundant that makes sure MOS is in charge of, increase Dmax as far as possible, will reduce the voltage stress that sub commutate manages. The extraction of Dmax is worth, 80% what ought to make sure Vdsmax does not exceed MOS to manage compression social estate; In the meantime, stimulate transformer instead to pilot of mode of peak value electric current, below CCM mode condition, when occupying sky comparing to exceed 0.5, can produce second harmonic concussion. Integrated consideration, to compression the value is 700V (NCP1015) MOS canal, in the design, it is advisable that Dmax does not exceed 0.45.

 

 

4.Step4: Affirmatory transformer is primary inductance Lm

 

Stimulate instead to CCM mode, when input voltage changes, transformer may transfer from CCM mode DCM mode, to two kinds of mode, all fall in the most abominable condition (lowermost input voltage, be fully loaded with) the primary inductance Lm that designs transformer. By decision leaving type:

 

 

Among them, fsw is the working frequency that stimulates transformer instead, KRF is voltaic ripple coefficient, its define following plan institute to show:

 

 

To DCM mode transformer, set timing KRF=1. To CCM mode transformer, KRF<1, right now, the extraction of KRF is worth the mean effective value that can affect primary electric current (RMS) , KRF is smaller, RMS is smaller, the loss that MOS runs can be jumped over small, cross small KRF to be able to increase the volume of transformer however, set timing need to be measured repeatedly. Generally speaking, design CCM pattern stimulate transformer instead, wide when pressing an input (90 ～ 265VAC) , KRF takes 0.25 ～ 0.5; Narrow when pressing an input (176 ～ 265VAC) , KRF takes 0.4 ～ 0.8 can.

 

Once Lm is affirmatory, the Idspeak of voltaic peak value that has shed MOS canal and Idsrms of mean effective value also decide subsequently:

 

 

Among them:

 

 

In the design, need to make sure Idspeak does not choose MOS to be in charge of the biggest electric current to be worth more than 80% , idsrms uses what computational MOS is in charge of to guide a loss Pcond, rdson guides for what MOS is in charge of electrify block.

 

 

 

5.Step5: The magnetism core with right choice and transformer are primary the v/LIT all over the ground number of inductance

 

In design of switch power source, ferrite magnetism core is the core of a kind of magnetism with the widest application, can be become by treatment a variety of appearance, in order to satisfy different applied requirement, like multichannel height of output, physics, optimize cost to wait.

 

In be being designed actually, because be full of too much variable, the choice of magnetism core does not have very rigid limitation, optional room is very big. Among them means of a kind of type selecting is, we can consult the type selecting manual that magnetism core supplier gives out undertakes type selecting. If did not consult suitably, can express below reference:

 

 

After core of magnetism of make choice of, through its Datasheet searchs Ae value, reach magnetization curve, decide B of △ of magnetic flux amplitude, secondary coil circles number decides by next type:

 

 

Among them, when DCM mode, △ B takes 0.2 ～ 0.26T; When CCM, △ B takes 0.12 ～ 0.18T.

 

 

6.Step6: Determine the v/LIT all over the ground number of each output

 

Decide first advocate v/LIT all over the ground number of road feedback winding, the v/LIT all over the ground number of other winding with advocate road winding circles number serves as reference can. Advocate feedback loop winding circles number is:

 

 

Criterion the v/LIT all over the ground number of winding of the others output is:

 

 

The Na of v/LIT all over the ground number of auxiliary coil winding is:

 

 

7.Step7: Decide the line way of every winding

 

The voltaic RMS that has flowed according to every winding is worth means of affirmatory winding line.

 

 

RMS of electric current of primary inductance winding:

 

 

RMS of sub winding electric current decides by next type:

 

 

ρ is current density, unit: A/mm2, normally, when growing when winding coil quite (> 1m) , coil current density takes 5A/mm2; When winding when loop length is shorter, coil current density takes 6 ～ 10A/mm2. When the electric current that has shed coil is bigger, can use the means that much group filament circles, in order to reduce the effect of collect skin effect.

 

 

Among them, ac is the summation of area of cut of all winding lead, KF is fill factor, take 0.2 ～ commonly 0.3.

 

The window area that checks magnetism core (if pursue 7 (A) be shown) , be more than the result that 21 computation give formula can.

 

 

8.Step8: Choose appropriate rectification canal for every output

 

The biggest retrorse voltage is worth the susceptive of output commutate canal of every winding VD (N) with IDrms of mean effective value (N) as follows:

 

 

Retrorse compression is worth chosen diode and rated guide connects electric current to need to satisfy:

 

 

9.Step9: Choose appropriate filter for every output

 

N road outputs capacitance Cout (N) Icaprms of ripple electric current (N) for:

 

 

The chosen ripple electricity that exports capacitance is worth Iripple to need to satisfy:

 

 

Output voltage ripple decides by next type:

 

 

Occasionally, the tall ESR of individual capacitance, make transformer very inaccessible the low ripple output that we mean is characteristic, can output end much paralell connection through be in right now a few capacitance, or the ripple noise that the method that adds filter of one class LC will come to improve transformer. Attention: The turn frequency of LC filter should be more than 1/3 switch frequency, the load that allow a gender may be taken in applying actually considering switch power source, l shoulds not be too big, the proposal does not exceed 4.7μH.

 

 

10.Step10: Clamp draw circuit design

 

If the graph is shown 8 times, stimulate the instant that transformer closes in MOS instead, the leakage that the syntonic aiguille that the output electric capacity that by transformer leakage feeling LLK and MOS provide causes adds in MOS to be in charge of extremely, if do not try to restrict, the life that MOS runs will sell at a discount greatly. Because this needs to take step, this aiguille sponge.

 

 

In stimulating transformer to design instead, commonly used graph 9 (A) shows circuit serves as the clamp that stimulates transformer instead draw circuit (RCD clamp absorb) .

 

RClamp is decided by next type, among them Vclamp presses Vor tower above than reflexing report commonly 50 ～ 100V, LLK is transformer primary leakage feeling, it is with actual measurement accurate:

 

Clamp of graph 9 RCD absorb

 

CClamp is decided by next type, among them the 5% ～ that Vripple takes Vclamp commonly 10% it is more reasonable:

 

 

Output power is less (20W is the following) when, clamp diode can be used slow restore diode, be like 1N4007; Conversely, need to use fast restore diode.

 

 

11.Step11: Compensate circuit design

 

System of switch power source is typical closed-loop control system, when the design, those who compensate circuit debug held quite big workload. Current and popular on market stimulate controller instead, great majority uses peak value electric current to control control mode. The power level small signal that mode of peak value electric current stimulates instead can simplify for first-order system, so its compensation circuit is designed easily. Normally, the circuit of Type II compensation that use Dean Venable puts forward is enough.

 

Before the design compensates circuit, need to inspect compensation object above all (power level) small signal is characteristic.

 

If the graph is shown 8 times, from IC in-house comparator turn over photograph end to disconnect, arrive from control the transfer function of output (the transfer function that controls a target namely) for:

 

 

The appendix gave out respectively the power level that CCM mode and DCM mode surge transformer instead transfer function model. NCP1015 job is in DCM mode, pass case to be from what control output:

 

 

Among them:

 

 

Vout1 gives priority to a road to output volts d.c. , the attenuation coefficient that K outputs signal to be inputted to voltaic comparator for error amplifier (to NCP1015 character, k=0.25) , m is ascendant slope of primary electric current, ma compensates slope for brae compensatory (compensate without brae as a result of NCP1015 interior, namely Ma=0) , idspeak issues electric current of primary peak value to give a condition. Then we can use Mathcad (or Matlab) the Bode plan that scale power level sends letter:

 

 

On the foundation that inspects power level to pass case Bode to pursue, we can undertake annulus road was compensated.

 

Preamble is mentioned, to mode of peak value electric current stimulate transformer instead, use Dean Venable Type II compensates circuit can, following plan institute show typical wiring pattern:

 

 

Normally, to reduce output ripple noise, output end can add filter of a diminutive LC, if the graph is shown 10 times, l1, C1B forms 2 rank low the stability that connects filter to be able to affect annulus way, of L1, C1B introduce, make the annulus road analysis of transformer becomes complex, not only influence power level sends case character, still meet those who affect compensation network send case character. However, after the analysis that build a model knowable: If the turn frequency of L1, C1B is more than the 5 times above of bandwidth Fcross, so its are negligible to the influence of annulus road not plan, in be being designed actually, proposal L1 does not exceed 4.7μH. Then we simplify when the analysis, direct L1 direct short circuit can, derivation should compensate the transfer function G(s) of the network to be:

 

 

Among them:

 

 

CTR is transmitted for the electric current of smooth Ou than, rpullup is smooth Ou resistance is helped on sub side (corresponding NCP1015, rpullup=18kΩ ) , cop is the parasitism capacitance of smooth Ou, concern with the size of Rpullup. Graph 13 (originate the data manual of Sharp PC817) the frequency that is smooth Ou answers character, can see, when RL (namely Rpullup) when be 18kΩ , will bring a the limit that makes an appointment with 2kHz left and right sides, so the size of Rpullup can affect the bandwidth of transformer directly.

 

 

K Factor (K factor law) it is Dean Venable is in what 80 time put forward 20 centuries, provided a kind of means that compensates network parameter certainly.

 

 

If the graph is shown 14 times, the times part of K that puts the utmost Wp of network of Type II compensation Fcross, the 1/k part that Wz put Fcross at 0 o’clock. Graph the compensation network of 12 has 3 parameter to need consideration: RLed, cz, cpole, will calculate with K Factor below these parameter:

 

 

——-Decide the Fcross of annulus road bandwidth after compensation: When moving voice load through limitation (△ Iout) output voltage crosses impulse (or next impulse) △ Vout, decide annulus road bandwidth by next type:

 

 

——-Inspect the character sending letter of power level, the intermediate frequency that compensates a network certainly takes gain (Mid-band Gain) :

 

 

——-Decide K of Dean Venable factor: The phasic abundant after the choice is compensated measures PM (take 55° ～ 80° commonly) , by formula the 32 phase shift that get Fcross handles power level (can calculate by Mathcad) PS, the phasic Boost that compensates a network to need promotion is:

 

 

Criterion K decides by next type:

 

 

——-Compensation network the utmost (Wp) put the times part of buy K at Fcross, can give Cpole by computation leaving type:

 

 

——-Compensation network at 0 o’clock (Wz) put the times part of buy 1/k at Fcross, can give Cz by computation leaving type:

 

 

3 emulation test and verify

 

The computer emulates what can replace a system not only a lot of trival artificial analysises, reduce labor intensity, the bigger error that because analytic law is brought in approximate processing,avoids, still can debug mutual complement with objective, of utmost reduce design cost, shorten development cycle.

 

This exemple uses UC3843 of classical electric current controller (as similar as NCP1015 control principle) , build stimulate transformer instead. Among them, compensating parameter all uses transformer and annulus means the computational parameter that the example of above paragraphs gives out.

 

Emulation test condition: Low pressure is inputted (90VAC, two-way is fully loaded with)

 

1. principle pursues

 

Graph graph of 17 emulation principles

 

2.Time domain of transient state signal is analysed

 

 

From the graph 18 can see, the lowermost voltage on lowermost Cbulk is 97.3V, be worth 98V with theory roughly conform to.

 

 

3.Communicate analysis of signal frequency domain

 

 

4.Form of dynamic negative carrier wave checks

 

Test condition: Low pressure is inputted, be fully loaded with, advocate the road outputs voltaic 0.1A—1A—0.1A, removed 2.5ms, the test outputs voltage weaveform.

 

 

4PCB designs guidance

 

1.Road of annulus of PCB Layout— big electric current

 

The area that road of annulus of big electric current surrounds should extremely likely small, walk along a line to want wide.

 

 

2.PCB Layout— is high frequency (Di/dt, Dv/dt) take a line

 

A. Rectification 2 class, clamp absorb diode, MOS canal and transformer cite a base, these high frequency place, down-lead should as far as possible short, right angle avoids when Layout;

B. The drive signal that MOS provides, check sheds the check of resistor to shed signal, the distance taking a line to control IC is shorter better;

C. Check sheds the distance of resistor and MOS and GND to answer as far as possible short.

 

 

3.PCB Layout— ground connection

 

Primary ground contact is regular:

 

A. After GND of all small signal and the GND that control IC are linked together, receive Power GND repeatedly (namely big signal GND) ;

B. Feedback signal should walk along IC independently, the GND of the GND of feedback signal and IC is linked together.

Sub ground contact is regular:

A. Ground of output small signal and after be linked together, the negative pole with what output capacitance is linked together;

B. The ground that outputs sampling resistor wants with fiducial source (TL431) the ground is linked together.

 

 

5.PCB Layout— example

 

 

6, summary

 

The article introduced to stimulate the design stage of transformer instead in detail, and the item that when PCB is designed, ought to notice, the means test and verify that uses software to emulate the rationality of the design. In the meantime, in appendix part, gave out respectively mode of peak value electric current surges the power level below requirement of CCM mode and DCM mode work instead transfer function.

 

Appendix: Small signal of level of power of mode of peak value electric current

 

Stimulate instead to CCM mode, its control output pass case to be:

 

 

The voltaic core of mode of peak value electric current, it is a kind of data collects a system substantially, power level passes case to be comprised by two parts Hp(s) and Hh(s) series connection, among them

 

 

Hh(s) forms for sampling of voltaic annulus electric current 2 rank sampling link (put forward by Ray Ridley) :

 

 

Among them:

 

 

Go up in type, PO is output general power, k outputs the attenuation coefficient that signal inputs to voltaic comparator for error amplifier, vout1 is feedback advocate the road outputs voltage, rs sheds resistor for primary side check, d is transformer occupy sky comparing, n is primary coil NP and advocate of Ns1 of road feedback coil v/LIT all over the ground than, m is ascendant slope of primary electric current, ma compensates slope for brae compensatory, esr is the resistor of equivalent series connection that outputs capacitance, cout is the sum of output electric capacity.

 

Attention: CCM mode stimulates transformer instead, send letter from what control output, by formula 40 knowable, have right half plane at 0 o’clock, it is in promotion amplitude while, brought the phasic attenuation of 90° , we did not want this at 0 o’clock, should make sure bandwidth frequency does not exceed when the design right half plane at 0 o’clock the 1/3 of frequency; By formula 41 knowable, if do not add brae to compensate (Ma=0) , when occupying sky comparing to exceed 50% , voltaic annulus concussion, expression is drive bulk wave, namely second harmonic concussion. Accordingly, when design CCM mode stimulates transformer instead, need to add brae compensation.

 

Stimulate instead to DCM mode, control output pass case to be:

 

 

Among them:

 

 

Vout1 gives priority to a road to output volts d.c. , k outputs the attenuation coefficient that signal inputs to voltaic comparator for error amplifier, m is ascendant slope of primary electric current, ma compensates slope for brae compensatory, idspeak issues electric current of primary peak value to give a condition.