CN104852612A - Switching power supply based on UC3842 - Google Patents

Abstract

The invention discloses a switching power supply based on a UC3842. The switching power supply comprises a UC3842, a transformer T1, a plurality of capacitors, a plurality of resistors, a plurality of diodes, optocouplers U2 and TL431. Alternating current is connected with a rectifier bridge through a fuse FU1 and a filter. The DC voltage output by the rectifier bridge supplies power for three circuits. In a first circuit, the DC voltage output by the rectifier bridge charges a capacitor C8 through a starting resistor R1, and the anode of the C8 supplies power for the VCC end of the UC3842. In a second circuit, the DC voltage output by the rectifier bridge supplies power for the fifth end of the transformer T1 through the starting resistor R1, a resistor R4, and a clamping diode D7. In a third circuit, the DC voltage output by the rectifier bridge is connected in series with a backward diode D through a resistance-capacitance filtering circuit, and then is connected in parallel with the second end and the third end of the transformer T1. Using the switching power supply can achieve objectives of good power supply stability, high loading capacity, high efficiency, high reliability, and low cost.

Description

A kind of Switching Power Supply based on UC3842

Technical field

The present invention relates to switch power technology field, be specifically related to a kind of Switching Power Supply based on UC3842.

Background technology

UC3842 is by the high-performance Single-end output formula current control mode pulse width modulator chip of UNIRODE company of U.S. Development and Production, the advantages such as this IC has pin few (8 pin), outward element is few, wiring is simple, reliability is high, cost is extremely low, be generally used for single-end flyback converters, output power limit is at below 100W.About UC3842, it can use inner E/A error amplifier to form voltage close loop, utilizes amperometric determination comparator to form current closed-loop.8 pin are reference voltage end, and reference voltage is 5V, and load capacity is 50mA; 7 pin are integrated package working powers, can at 8-40V; 4 pin connecting resistance and electric capacity, determine sawtooth waveforms frequency; 5 pin are ground; 6 pin are that integrated package recommends output, draw, fill with current capacity; 3 pin are overcurrent test side, and tested current conversion is that voltage accesses this pin; 2 pin are pressure feedback port, are the input of error amplifier, generally access from switched power output through electric resistance partial pressure sampling; 1 pin is that error amplifier exports, and as loop compensation use, mainly through adding RC network between pin 1 and input, forming closed loop, ensureing the stability of loop.Fig. 1 shows a kind of typical Switching Power Supply structured flowchart, mainly comprises current rectifying and wave filtering circuit, DC/DC translation circuit, output filter circuit and control circuit.Current Switching Power Supply ubiquity load capacity is poor, the problems such as efficiency is low, and cost is relatively high.

Summary of the invention

The present invention proposes a kind of Switching Power Supply based on UC3842, is optimized the existing Switching Power Supply based on UC3842.

The invention provides a kind of Switching Power Supply based on UC3842, comprise: UC3842, transformer T1, multiple electric capacity, multiple resistance, multiple diode, optocoupler U2 and TL431, wherein, alternating current connects rectifier bridge through fuse FU1 and filter, and rectifier bridge output dc voltage is three loop power supplies; Wherein, the first loop is rectifier bridge output dc voltage is that electric capacity C8 charges by starting resistance R1, C8 just very UC3842 VCC end power supply; Second servo loop is that rectifier bridge output dc voltage to be powered through 5 ends that resistance R4 and clamp diode D7 is transformer T1 by starting resistance R1; Tertiary circuit is that rectifier bridge output dc voltage is after rc filter circuit is connected with backward diode D5, be connected in parallel on 2 of transformer T1,3 ends, wherein, the anode of backward diode D5 is connected with the collector electrode of field effect transistor Q1 after being connected with 2 ends of T1, the gate pole of field effect transistor Q1 is held with the 6 pin OUT of UC3842 and is connected, the drain electrode of field effect transistor Q1 is exported and is connected to the ground by resistance R6, and the drain electrode of field effect transistor Q1 simultaneously is also connected through the ISEN end of resistance R7 and UC3842; Further, the outlet side of transformer T1 is connected to the ground through R13, optocoupler U2 former limit control end, TL431 successively, wherein the positive and negative electrode of optocoupler U2 output respectively with the CMPEN of UC3842, be connected to the ground.

Preferably, wherein rc filter circuit is composed in parallel by resistance R3 and electric capacity C7.

Preferably, after wherein UC3842 starts, the end-of-job of R1, produces voltage power for UC3842 to feed back winding.

Preferably, when wherein UC3842 normally works, the voltage of 7 pin is 14V.

Preferably, wherein clamp diode D7 adopts FR107 type diode.

Beneficial effect of the present invention: the present invention is based on UC3842 and carry out Switching Power Supply design, is optimized the existing Switching Power Supply based on UC3842, reaches the objects such as power good is good, load capacity is strong, efficiency is high, reliability is high, cost is low.

Accompanying drawing explanation

Fig. 1 is a kind of structured flowchart of typical Switching Power Supply.

Fig. 2 is the circuit theory diagrams of the Switching Power Supply of the embodiment of the present invention.

Current diagram when Fig. 3 is UC3842 startup and the work of the embodiment of the present invention.

Fig. 4 is the transformer primary side inductive current waveform schematic diagram of the embodiment of the present invention.

Fig. 5 and Fig. 6 is the emulation reference waveform schematic diagram of the former secondary inductance electric current of embodiment of the present invention full load and output current wave respectively.

Embodiment

Below in conjunction with accompanying drawing and specific embodiment, technical scheme of the present invention is described in detail.

Fig. 2 shows the circuit theory diagrams of Switching Power Supply of the present invention; this switching power circuit mainly comprises the elements such as UC3842, transformer T1, electric capacity, resistance, optocoupler and TL431; circuit connecting relation is as follows: alternating current (85-265V) connects rectifier bridge (being made up of four diodes) through fuse FU1 overcurrent protection and filter; rectifier bridge output dc voltage; to three loop power supplies; wherein

First loop: rectifier bridge output dc voltage is charged to electric capacity C8 by starting resistance R1, C8 positive pole holds power supply to the VCC of UC3842;

Second servo loop: rectifier bridge output dc voltage is by starting resistance R1 and power to 5 ends of many tapped transformers T1 through R4, D7;

Tertiary circuit: rectifier bridge output dc voltage is after rc filter circuit (being composed in parallel by R3, C7) and backward diode D5 are connected, be connected in parallel on 2 of many tapped transformers T1,3 ends, wherein the anode of D5 is connected with field effect transistor Q1 collector electrode after being connected with 2 ends of T1, Q1 gate pole is held with the 6 pin OUT of UC3842 and is connected, Q1 drain electrode is exported and is connected to the ground by resistance R6, and Q1 drain electrode is simultaneously connected through the ISEN end of resistance R7 and UC3842;

The 12V outlet side of transformer T1 is connected to the ground through R13, optocoupler U2 former limit control end, TL431 successively, and wherein optocoupler U2 output positive and negative electrode is connected with ground with the CMPEN of UC3842 respectively, and all the other elements help out.

Below describe the course of work of switching power circuit of the present invention, be described mainly through following six aspects.

1, start-up circuit

Power supply provides electric current to charge to electric capacity C8 by starting resistance R1, when C8 voltage reaches the starting resistor threshold value 16V of UC3842, UC3842 starts working and provides driving pulse, exports pulse pull switch pipe and works, output signal as high-low voltage pulse by its 6 end.During high voltage pulse, field effect transistor conducting, electric current by the former limit of transformer T1, simultaneously energy storage in transformer T1.

Wherein, UC3842 inside is provided with undervoltage lockout circuit, and it opens and closes threshold value and is respectively 16V and 10V, and before unlatching, the electric current that UC3842 consumes is within 1mA.After supply voltage is connected, when 7 terminal voltages of UC3842 rise to 16V, UC3842 starts working, and after starting normal work, its current sinking is about 15mA.In order to reduce the power consumption on R1, raise the efficiency, after making UC3842 startup, the work of R1 just terminates, and residue task gives feedback winding, by feedback winding generation voltage for UC3842 powers.Wherein charging capacitor gets 100uF, and R1 is:

$R1=\frac{\sqrt{2}{U}_{\mathrm{in}\max }-\mathrm{Vcc}}{1\mathrm{mA}}=\frac{\sqrt{2}\×265-16}{1\mathrm{mA}}=358\mathrm{K\Ω}$

Preferably, the present invention gets R1=350K Ω.

2, auxiliary winding

After UC3842 starts, if feedback winding can not provide enough voltage, circuit will start constantly, occurs phenomenon of having the hiccups.For this reason, by 13-15V design, when UC3842 is normally worked, the voltage of 7 pin maintains about 13V.Preferably, the present invention gets 14V.

3, current circuit

When load current overrate or short circuit; field effect transistor Q1 electric current increases; voltage (being greater than 1V) on R6 feeds back to 3 pin; by internal current amplifier, 6 pin output duty cycles are reduced; output voltage declines, until make UC3842 quit work, does not have trigger impulse to export; field effect transistor Q1 is ended, thus reaches the object of protection power tube.After short circuit phenomenon disappears, power supply can recover normal work automatically.

Wherein, transformer primary side inductance peak current I _p=1.52A, gets drain saturation current I _dsmaxduring for 2.13A, circuit overcurrent protection, like this when circuit is short-circuited or other abnormal electric current caused rises, when the voltage on R6 is greater than 1V, the PWM comparator of UC3842 exports high level, makes PWM latch reset, closes and exports.

4, voltage circuit

When making output voltage too high because of certain reason, on sample resistance R13, voltage increases, the electric current that the LED side of optocoupler U2 flows through increases, triode side output current becomes large, the variable resistor being equivalent to C19 parallel connection diminishes, and the gain of error amplifier output and pin 1 diminishes, and causes 6 pin to export pulse duty factor and diminishes, until output voltage stabilization is at 12V, vice versa.

Wherein, TL431 reference voltage is 2.5V, and current-limiting resistance R12 is 10K, then electric current $I_{R12}=\frac{2.5}{{10}^4}=0.25\mathrm{mA},$ So divider resistance $R=\frac{12-2.5}{0.25}=38\mathrm{k\Ω},$ Preferably, divider resistance is selected to be composed in series by R10=36K and R11=2K.

Wherein, R13 gets 1K, makes to flow through optocoupler LED current optocoupler U2 normally works.

5, the determination of timing resistor and electric capacity

Oscillator frequency is determined by the resistance Rt electric capacity Ct be connected on 4 pin of UC3842, first determines Rt, such as, is chosen as 13K, $f=\frac{1.8}{\mathrm{Rt}*\mathrm{Ct}},$ Then $\mathrm{Ct}=\frac{1.8}{13*{10}^3*6*{10}^4}=2.2\mathrm{nF}.$

6, RCD absorption circuit

Need the size measuring leakage inductance when calculating uptake pathway, normally an inductance is multiplied by a coefficient and estimates, now get 5% of an inductance, i.e. I _lK=773 μ H × 5%=38.65 μ H,

V _clamp＝0.9V _BR(DS)-V _inmax＝0.9×600-371＝169V，

Wherein, V _{bR (DS)}for switching tube drain-source breakdown voltage, V _inmaxfor input direct-current maximum.

$R_2=\frac{2(V_{\mathrm{clamp}}-V_{\mathrm{or}})\·V_{\mathrm{clamp}}}{L_{\mathrm{LK}}{I_{\mathrm{ds}-\mathrm{peak}}}^2{f}_s}=\frac{2*(169-69)*169}{38.65*{10}^{-6}*{1.52}^2*6*{10}^4}=6.3\mathrm{k\Ω}$

$C_3>\frac{V_{\mathrm{clamp}}}{{\mathrm{\ΔV}}_{\mathrm{clamp}}{R}_2{f}_s}=\frac{169}{16.9*10*{10}^3*60*{10}^3}=16.7\mathrm{nF}$

Wherein mains ripple Δ V _clampget V _clamp* 10%.

Preferably, R ₂get 10K Ω, C ₃>0.1 μ F.

Preferably, clamp diode D7 selects reverse breakdown voltage higher than the drain-source breakdown voltage of switching tube, and the fast recovery diode that reverse recovery time is short as far as possible, such as select FR107 type, reverse breakdown voltage is 1000V.

Below briefly introduce transformer parameter correlation computations process of the present invention.

When exchanging minimum input, minimum direct current voltage is 85*1.1=94V.When exchanging maximum input, maximum direct voltage is 265*1.4=371V, and switching frequency is set to 60KHz.Because UC3842 is current control mode pulse width modulator, in order to chip operation is more reliable, get D _max=0.45.According to (V _dcmin-V _ds) * D _max=V _or* (1-D _max), wherein V _dsfor switch conduction pressure drop, be taken as 10V, obtain $V_{\mathrm{or}}=\frac{(94-10)*0.45}{1-0.45}=69V;$

If wherein select drain-source withstand voltage to be the field effect transistor MOSFET of 600V, then the voltage margin of MOSFET is V _mos=600-371-69=160V;

Input power $P_{\mathrm{in}}=\frac{P_0}{\mathrm{\η}}=\frac{36}{0.8}=45W;$

Magnetic core model is determined:

$\mathrm{AP}=\frac{2*P_0*D_{\max }*{10}^6}{\mathrm{\ΔB}*{\mathrm{\η}}_T*K_u*J*f},$

Wherein, Δ B gets 0.2-0.25T, η _Τfor transformer efficiency, get 0.95-0.98; K _ufor transformer window utilization factor, get 0.1-0.5; J is current in wire density, gets 3-5A/mm ²; Have:

$\mathrm{AP}=\frac{2*36*0.45*{10}^6}{0.2*0.95*0.2*4*6*{10}^4}=3553{\mathrm{mm}}^4$

Core material selects PC40 type ferrite, and model is EI35;

By continuous current mode Model Design, obtaining current ripple factor Krp=Ir/Ip=0.6.Transformer primary side inductive current waveform is as Fig. 3; There is former limit peak current I _pfor:

$I_p=\frac{{2P}_{\mathrm{in}}}{V_{\mathrm{dc}\min }*D_{\max }*(3-K_{\mathrm{rp}})}=\frac{2*45}{94*0.45*1.4}=1.52A,$

Primary current effective value I _rmsfor:

$I_{\mathrm{rms}}=I_p*\sqrt{D_{\max }*({K_{\mathrm{rp}}}^2/3-K_{\mathrm{rp}}+1)}=1.52*\sqrt{0.45*0.52}=0.73A,$

Then can calculate former limit inductance value L by following formula _p:

$\left\{\begin{array}{c}{L}_p=\frac{V_{\mathrm{dc}\min }*D_{\max }}{f*(I_{p2}-I_{p1})}=\frac{94*0.45}{6*{10}^4*(1.52-0.608)}=773\mathrm{uH}\\ I_{p2}=I_p\\ I_{p1}=I_{p2}*(1-K_{\mathrm{rp}})=0.4*1.52=0.608A\end{array}\right.;$

Determine former limit number of turn N _p:

$N_p=\frac{L_p*I_p}{B_{\max }*\mathrm{Ae}}=\frac{773*{10}^{-6}*1.52}{0.2*1.01*{10}^{-4}}=58T;$

Determine vice-side winding number of turn N _s1:

$n_1=\frac{V_{\mathrm{or}}}{V_0+V_F}=\frac{69}{12+1}=5.3,N_{s1}=\frac{N_p}{n1}=\frac{58}{5.3}=10.9,$ Get 11T;

Determine auxiliary umber of turn N _sf:

$n_2=\frac{V_{\mathrm{or}}}{V_{\mathrm{cc}}+V_{\mathrm{sF}}}=\frac{69}{14+1}=4.6,N_{\mathrm{sf}}=\frac{N_p}{n_2}=\frac{58}{4.6}=12.6,$ Get 13T;

Calculate winding conducting wire wire diameter:

Former limit wire diameter D ₁:

$S_1=\frac{I_{\mathrm{rms}}}{J}=\frac{0.73}{4}={0.1825\mathrm{mm}}^2,D_1=\sqrt{\frac{4*S_1}{\mathrm{\π}}}=\sqrt{\frac{4*0.1825}{3.14}}=0.482\mathrm{mm};$

Secondary wire diameter D ₂:

$S_2=\frac{I_{0\max }}{J}=\frac{3}{4}={0.75\mathrm{mm}}^2,D_2=\sqrt{\frac{4*S_2}{\mathrm{\π}}}=\sqrt{\frac{4*0.75}{3.14}}=0.9775\mathrm{mm};$

Calculate air gap I _g:

$I_g=\frac{0.4*\mathrm{\π}*{N_p}^2*\mathrm{Ae}*{10}^{-8}}{L_p}=\frac{0.4*3.14*{58}^2*1.01*{10}^{-8}}{773*{10}^{-6}}=0.2552\mathrm{cm}=0.55\mathrm{mm},$

Wherein Ae unit is cm ², I _gunit be cm, L _punit is H.

Consider kelvin effect, copper conductor penetration depth can be calculated by following formula:

$\mathrm{\δ}=\sqrt{\frac{\mathrm{\ρ}}{\mathrm{\π}{\mathrm{\μ}}_{0}f}}=\sqrt{\frac{1}{\mathrm{\π}{\mathrm{\μ}}_0\mathrm{\γf}}},$

Wherein ρ is the conductivity of the resistivity of copper conductor, γ copper conductor, μ ₀for the magnetic permeability of air, f is switching frequency.100 DEG C time, ρ=2.3*10 ^-8Ω m, select wire diameter time make d<2 δ, adopt stranded conductor and around.

Wherein, former limit can adopt diameter to be the wire of 0.3mm, and secondary diameter D is that the wire of 0.4mm does sub-thread coiling.

S＝π*(D/2) ²＝3.14*(0.4/2) ²＝0.1256mm ²，

S'＝π*(D/2) ²＝3.14*(0.3/2) ²＝0.0706mm ²；

Its limit, Central Plains wire number of share of stock is S2/S=0.1825/0.0706=2.58, get 3 strands and around; Secondary wire number of share of stock is S1/S=0.75/0.1256=5.97, get 6 strands and around; Auxiliary winding adopts 0.3mm sub-thread.

Fig. 5 and Fig. 6 respectively illustrates the emulation reference waveform schematic diagram of the former secondary inductance electric current of Switching Power Supply full load of the present invention and output voltage waveforms, and wherein RMS is root mean square, and duty is duty ratio, and frequ is frequency.As can be seen from Fig. 5 and Fig. 6, the response of the design is rapid, and output accuracy is high, good stability.

It is below the list of the adoptable circuit element of the embodiment of the present invention.

Code name

Element title

Quantity

Specifications parameter

Code name

Element title

Quantity

Specifications parameter

C1

Cap

1

104/600V

FU1

Fuse 1

1

2A/250V

C2

Cap

1

104/600V

KBP206

Bridge1

1

RS210

C3

Cap

1

222/1KV

L1

TRANS1

1

15mH/2A

C4

Cap

1

222/1KV

Q1

MOSFET-N

1

K1507

C5

Cap Pol2

1

400V/68uF

R1

Res2

1

350K/2w

C7

Cap

1

0.1uF/400V

R2

Res2

1

22/0.25w

C8

Cap Pol2

1

120uF/50v

R3

Res2

1

10K/0.25w

C10

Cap

1

2700pF/50v

R4

Res2

1

5.1/0.25w

C12

Cap

1

1.8nF/50v

R5

Res2

1

100K/0.25w

C13

Cap

1

0.01uF/50v

R6

Res2

1

0.47/2w

C14

Cap Pol2

1

220uF/50v

R7

Res2

1

1K/0.25w

C15

Cap Pol2

1

220uF/50V

R8

Res2

1

10K/0.25w

C17

Cap

1

104/50v

C18

Cap

1

200pF/50v

R10

Res2

1

36K/02.5w

C19

Cap

1

0.01uF/50v

R11

Res2

1

2K/0.25w

C20

Cap

1

820pF/600v

R12

Res2

1

10K/0.25w

D5

FR107

1

FR107

R13

Res2

1

1K/0.25w

D6

1N4148

1

1N4148

R14

Res2

1

100K/0.25w

D7

FR107

1

FR107

R15

Res2

1

4.7K/0.25w

D8

STPR1620

1

STPR1620

T1

Component_1

1

EI35

D10

FR305

1

FR305

U1

UC3842AN

1

UC3842AN

D12

TL431ACLP

1

TL431A

U2

Optoisolator1

1

pc817A

Above, be described in detail in conjunction with specific embodiments to technical scheme of the present invention, described specific embodiment understands thought of the present invention for helping.The derivation that those skilled in the art make on the basis of the specific embodiment of the invention and modification also belong within scope.

Claims (5)

1. based on a Switching Power Supply of UC3842, it is characterized in that, comprising: UC3842, transformer T1, multiple electric capacity, multiple resistance, multiple diode, optocoupler U2 and TL431, wherein,

Alternating current connects rectifier bridge through fuse FU1 and filter, and rectifier bridge output dc voltage is three loop power supplies; Wherein,

First loop is rectifier bridge output dc voltage is that electric capacity C8 charges by starting resistance R1, C8 just very UC3842 VCC end power supply;

Second servo loop is that rectifier bridge output dc voltage to be powered through 5 ends that resistance R4 and clamp diode D7 is transformer T1 by starting resistance R1;

Tertiary circuit is that rectifier bridge output dc voltage is after rc filter circuit is connected with backward diode D5, be connected in parallel on 2 of transformer T1,3 ends, wherein, the anode of backward diode D5 is connected with the collector electrode of field effect transistor Q1 after being connected with 2 ends of T1, the gate pole of field effect transistor Q1 is held with the 6 pin OUT of UC3842 and is connected, the drain electrode of field effect transistor Q1 is exported and is connected to the ground by resistance R6, and the drain electrode of field effect transistor Q1 simultaneously is also connected through the ISEN end of resistance R7 and UC3842; Further,

The outlet side of transformer T1 is connected to the ground through R13, optocoupler U2 former limit control end, TL431 successively, wherein the positive and negative electrode of optocoupler U2 output respectively with the CMPEN of UC3842, be connected to the ground.

2., as claimed in claim 1 based on the Switching Power Supply of UC3842, it is characterized in that, wherein said rc filter circuit is composed in parallel by resistance R3 and electric capacity C7.

3. as claimed in claim 1 based on the Switching Power Supply of UC3842, it is characterized in that, after wherein UC3842 starts, the end-of-job of R1, produces voltage for UC3842 power to feed back winding.

4., as claimed in claim 1 based on the Switching Power Supply of UC3842, it is characterized in that, when wherein UC3842 normally works, the voltage of 7 pin is 14V.

5. as claimed in claim 1 based on the Switching Power Supply of UC3842, it is characterized in that, wherein said clamp diode D7 adopts FR107 type diode.