Low power charger design

1 Working principle Schematic diagram shown in Figure 1, it consists of the following components: C1, V1 ~ V4, C2 constitutes a filter rectifier circuit, transformer T is a high-frequency transformer, V5, R2, C11 constitutes the protection circuit of the power switch tube V7, NF is the winding that supplies the IC power supply. The single-ended output IC is UC3842, its 8 pin outputs 5 V reference voltage, 2 pin is inverting input, 1 pin is amplifier output, 4 pin is oscillation capacitor C9, resistor R7 input terminal, 5 pin is ground terminal, 3 pin is Overcurrent protection terminal, 6 feet for widening single pulse output, 7 pin for power input. R6 and C7 form negative feedback. When the IC starts, the starting voltage is supplied by R1. After the circuit starts, the potential generated by NF is rectified and filtered by V6, C4 and C5 to supply the IC working voltage. R12 is an overcurrent protection sampling resistor, and V8 and C3 form a flyback rectification and filtering output circuit. R13 is the internal load, and V9~V12 and R14~R19 form the LED display circuit. V5, V6 selects FR107, V8 selects FR154, V7 selects K792. When V7 is turned on, the rectified voltage is added to the primary winding Np of transformer T to become magnetic energy stored in the transformer. At the end of V7 conduction, Np winding The current reaches the maximum value: Ipmax: Ipmax = (E / Lp) ton where: E is the rectified voltage; Lp is the transformer primary winding inductance; ton is the V7 conduction time. At the moment of V7 shutdown, the secondary winding discharge current of the transformer is the maximum value Ismax. If the various losses are neglected, it should be: Ismax=nIpmax=n(E/Lp)ton. Where: n is the transformer ratio, n = Np / Ns, Np, Ns is the number of turns of the transformer primary and secondary windings.

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The high-frequency transformer should store the energy of the primary winding during V7 conduction and the secondary winding release energy during V7 shutdown: n(E/Lp)ton=(Uo/Ls)toff, where: Ls is the transformer secondary winding inductance; Uo is the output voltage; toff is the V7 off time.
Since Lp=n2Ls, then: (E/nLs)ton=(Uo/Ls)toffEton=nUotoffUo=(ton/ntoff)E, the above equation shows that the output voltage Uo is proportional to ton and inversely proportional to the turns ratio n and toff.
The energy WLp stored by the transformer during conduction is:

The larger the transformer Lp, the more energy it can store. Whether the energy stored in the transformer can be released during toff is related not only to the operating frequency f of the transformer, but also to the inductance of the secondary winding Ls, which is more related to the magnitude of the load.
The difference between the energy storage release time constant τ and the V7 turn-off time toff forms three operating states of the converter, which are described separately below:
(1) The state of toff=τ is a critical state. The waveform of each parameter is shown in Figure 1.
Figure 2 is a waveform diagram of toff = τ; in Figure 2, ub is the control voltage waveform of Vp; up is the transformer primary Np potential waveform; φ is the transformer flux variation waveform; uces is the V7 collector voltage waveform; ip, is the initial , secondary current waveform.

(2) The parameters of each parameter of toff>τ are shown in Fig. 3.
It can be seen from Fig. 3 that the V7 is turned off for a period of time when the magnetic flux φ is reset, the ip rises linearly, and the is linearly decreases.
The energy stored in the transformer is equal to the output energy of the circuit. (1/2) LPIpmax2f=Uo2/RL, Uo2=(1/2)LpIpmax2RLf Substituting Ipmax=(E/Lp)ton into the above equation, where RL is the circuit load resistance; T=1/f is the transformer operation cycle. In the formula, E, ton, T, Lp are fixed values, so the output voltage Uo varies with the magnitude of the load resistance RL. If the voltage drop of the rectifier device is ignored, the maximum output voltage should be: Uomax=(1/n)Up= (1/n) The back pressure of the EV7 should be: Ucc=E+ Up=E+nUo.
(3) toff < τ each parameter waveform shown in Figure 4.

It can be seen from Fig. 4 that the magnetic flux φ cannot be reset during toff, ip does not increase linearly from 0, and is decreases to less than 0. The operating state output voltage Uo should satisfy the following relationship:

The above equation shows that in the case of a large Lp, Uo is determined only by the transformer turns, the turn-on cutoff pulse width, and the power supply voltage E, regardless of the load resistance RL.
Among the above three working states, the second working state output voltage Uo varies with the magnitude of the load resistance, and we just use this feature to satisfy the charging characteristics of the charger. It can be seen from the circuit that the load resistance RL of the circuit is actually the equivalent internal resistance of the battery to be charged. When the battery power is emptied, the equivalent internal resistance RL is small, and as the charge amount increases, the equivalent internal resistance increases. The circuit output voltage Uo is the charging voltage, and the change thereof increases as the RL increases, so there is a charging characteristic curve as shown in FIG.
It can be seen from Fig. 5 that the charging current decreases as RL increases. Io=uo/RL charging voltage uo, charging current io are all changed with RL, RL curve is determined by the charging characteristics of the battery, so the charger with single-ended flyback circuit has good charging voltage and current. Follow-up When the battery is fully charged, the RL is also reduced to a certain limit, the charging voltage is saturated, and the charging current automatically enters the floating state. This greatly simplifies the control circuit for automatic charging. Compared with other charger circuits of the same performance, the cost is greatly reduced and the reliability is greatly improved.

2 Circuit design calculation (1) Design of high-frequency transformer Transformer is the main component of the converter. Its design content is mainly the selection of core, winding number and wire diameter.
The calculation formula of the main parameters of the transformer:
Output power Po=Uoio; input power Pi=Po/η; duty ratio D=ton/T; transformer efficiency is η=Po/Pi; load resistance RL=Uo/io.
Transformer input current maximum Ipmax=2Uo2/DηEminRL; transformer input current effective value Ipeff=Dip; determination of transformer operating frequency f:
Although f is high, the volume and weight can be reduced, but the V7 switching loss is increased. When f is low, the transformer volume is increased and the weight is increased. For comprehensive consideration, generally, f=50 kHz is selected.
When the battery is fully charged, the RL is also reduced to a certain limit, the charging voltage is also saturated, and the charging current automatically enters the floating state. This greatly simplifies the control circuit for automatic charging. Compared with other charger circuits of the same performance, the cost is greatly reduced and the reliability is greatly improved.
(2) The core size is selected because the circuit is a single-ended flyback circuit, so the excitation current is unidirectional, and the magnetic flux generated in the transformer core moves only along the hysteresis loop in the first quadrant, as shown in Fig. 6. Show.
According to the working state of the magnetic circuit in Fig. 6, the formula for calculating the core size is as follows:

Where: 104 is the conversion factor of the magnetic flux density unit; 10-6 is the conversion factor of the on-time unit; SC is the cross-sectional area of ​​the magnetic core, and the unit is cm. ; ΔB takes 0.7Bs (saturated magnetic density), the unit is T; the unit of ton is μs. The selected core window area So should be able to bypass the primary and secondary windings, so the following formula is related:

Where: Ko is the copper wire space factor, generally takes Ko = 0.2 ~ 0.5; Kc is the core duty factor, ferrite takes Kc = 1; j is the current density in the wire, generally take j = 2~3 A/mm2; 10-2 is the conversion factor of the wire cross-sectional area unit.
Transformer design capacity PT = EI, then:

The primary and secondary power relationship of the transformer is:
Ps=ηPTPo=Ps-PD
Where: Ps is the secondary output power of the transformer; Pd is the power loss of the output diode. If PD is omitted, then: Po=ηPT; SoSC=2Poton/η△BjKoKc (Po unit is cm4). So, SC is calculated according to the formula, and the core size and specification are selected.
(3) Calculation of winding turns

In order to meet the circuit requirements, E, ton should take the maximum value, and the single-ended flyback circuit transformer primary winding has both inductance. The required amount of inductance Lp (unit: μH) is calculated by the following equation: Lp = Eton / Ip.
Where: ton is μs. Calculate whether the number of Np winding turns can meet the inductance requirement by the following formula: Lp'=(0.4πNp2Sc×10-8)/(Lδ+Lc/μc) where: μc is the effective permeability of the core material; Lc is the magnetic The average length of the core magnetic circuit (cm); Lδ is the length of the air gap in the core (in cm). If Lp ≤ Lp', increase Np to meet the inductance requirement. Selection of transformer turns ratio: If the influence of secondary rectification voltage drop and transformer internal loss is not considered, then n=Ep/Eo, Ns=nNp/D, and NF=(Ns/Uo)Up can be calculated in the same way.
(4) Calculation of wire diameter selection If j=2.5 A/mm2 is taken:
d=0.7
From this, the wire diameter d (unit: mm) of each winding is calculated and the specification value is selected to check whether the core window area can bypass the windings. If the winding is not completed, the above design calculation is repeated.
(5) Check the secondary winding discharge constant, τs should be less than toffτs=Ls/RL=(Lp'/n2)/RL=Lp'/(n2RL)toff=T/2, T=1/f, so toff=1 /(2f), toff>τs is the principle of checking. If not, repeat the above calculations.

3 Selection of main components (1) Selection of power switch tubes. The withstand voltage of the switch tube should be greater than or equal to E+nUo, generally taking (2.5~4) Emax. The current of the switching power tube is determined by the following formula:

(2) For the selection of capacitors C2 and C3, the voltage of C2 should be greater than 1.1×220 V; the voltage of C3 depends on the output voltage.
The selection principle of C2 and C3 capacitance is: C2Rp=(4~5)T50; C3RL=(4~5)T. Where: T50 is the corresponding duty cycle when the frequency is 50 Hz;
Rp, C2 is the discharge equivalent resistance and capacitance; T is the period corresponding to the operating frequency of the transformer. From this, the capacitance can be estimated. The design of the new charger is shown in Figure 7.

4 circuit debugging
    (1) Converter operating frequency adjustment: R7 and C9 of IC4 pin can be adjusted to achieve the purpose of adjusting the working frequency;
(2) Adjustment of the on-time of the power switch tube: adjusting R3 and R5 can achieve the purpose of adjusting the ton;
(3) Adjustment of overcurrent protection working point: Adjusting R12 can achieve the purpose of adjusting the overcurrent protection working point.

5 Conclusion Using a single-ended flyback converter circuit to make a fully automatic charger is a summary of the practice of single-ended flyback converter circuits. With this circuit, more than 30 fully automatic chargers of 100 W or less have been designed and manufactured, and the effect is good. The application of the technology can save complicated control circuits and ICs, which not only reduces the cost, but also greatly improves the reliability and the comprehensive benefits.