"When designing auxiliary power supply for industrial control, TinySwitch - Ⅲ series third generation micro switching power supply launched by American PI company can be used. At present, the universal AC input voltage range in the world is u = 85 ~ 265V. However, when 18V ＜ u ＜ 75V, the chip can not provide enough bias to maintain normal operation, which greatly limits the application of TinySwitch - Ⅲ series products in low voltage field. In order to solve the above problems and make TinySwitch - Ⅲ work normally under ultra-low AC input voltage, it is necessary to design a suspended high-voltage constant current source from the outside.
Performance characteristics of TinySwitch - Ⅲ series products
TinySwitch - Ⅲ series products include 14 models, including tny274p ~ tny280p, tny274g ~ tny280g, etc. This series of products mainly have the following characteristics:
● the maximum output power of TinySwitch - Ⅲ series products is 36.5w (tny280p / G type). By selecting the capacitance of BP / M terminal, the internal leakage limit current point can be set externally. In addition to tny274p / g, each model has three different limit current values for users to choose. The utility model has the advantages that when replacing with adjacent models, there is no need to redesign the high-frequency transformer and change the peripheral components.
● users can optimize the power supply design from the perspective of maximizing power efficiency and obtaining maximum output power. Higher peak power can be obtained by selecting higher leakage limit current point, or higher continuous output power can be obtained in open power module; The lower limit current value can improve the efficiency of sealed power adapter / battery charger.
● the efficiency of traditional pulse width modulation (PWM) switching power supply decreases significantly with the reduction of load. Tinyswitch-Ⅲ adopts on / off control mode, which can meet the requirements of standby power supply and no-load power saving standard, and the no-load power consumption is lower than 150MW; After adding bias winding, it can be reduced to less than 50MW.
● it has the functions of input undervoltage protection, output overvoltage protection and adaptive conduction time extension of power switch tube.
Working principle of TinySwitch - Ⅲ series micro switching power supply
The internal block diagram of TinySwitch - Ⅲ is shown in Figure 1, where s and D are the source and drain of the internal power MOSFET respectively (four sources are connected internally). EN / UV is a "enable / undervoltage" dual function lead out terminal, which can control the on and off of power MOSFET under normal conditions; If an external resistance is connected between this terminal and the DC input voltage, it can detect whether the input is undervoltage. BP / M is a bypass / multi-function terminal. When it is used as a bypass terminal only, it is connected to the ground (S pole) by 0.1 μ Bypass capacitance of F. BP / M terminal also has the characteristics of multi-functional terminal: first, change the capacity of bypass capacitor to set the leakage limit current point; Secondly, the terminal can also provide shutdown function. The specific method is to connect a voltage stabilizing tube between the output terminal of feedback voltage and BP / M terminal to realize output overvoltage protection.
Figure 1 Internal block diagram of TinySwitch - Ⅲ
TinySwitch - Ⅲ internally integrates a power MOSFET with a withstand voltage of 700V and an on / off controller. Different from the traditional PWM controller, it uses a simple on / off controller to adjust the output voltage. It mainly includes oscillator, 5.85v voltage regulator, 6.4v voltage stabilizing tube for clamping at bypass end, enabling detection and logic circuit, limit current state machine, undervoltage, overcurrent and overheating protection circuit and automatic restart counter.
The corresponding relationship between the limit current ilimit of TinySwitch - Ⅲ and the bypass capacitor CBP is shown in Table 1. Taking tny279p / g as an example, when the bypass capacitance CBP = 0.1 μ F, select the standard limit current ilimit ＝ 650ma (typical value, the same below); When bypass capacitance CBP = 1 μ F, select the lower limit current ilimit - = 550mA; When the bypass capacitance CBP = 10 μ F, select the higher limit current ilimit + = 750ma, and the difference between the three is 100mA in turn. The biggest advantage of this design is that it can ensure good compatibility between adjacent models. For example, ilimit - = 550mA of tny279p / g, which is exactly the limit current value of adjacent tny278p / g; Ilimit + = 750ma of tny279p / g, which is the limit current value of adjacent tny280p / g, and so on.
Circuit design of industrial control power supply with ultra wide input range
The circuit of 3W ultra wide input range industrial control power supply composed of TinySwitch - Ⅲ series product tny280p is shown in Figure 2. The remarkable feature of the power supply is that the AC voltage input range is very wide (18 ~ 265V), the output voltage is + 5V and the output current is 600mA. The power efficiency can reach 65%, and the no-load power consumption is less than 200MW when the AC input voltage is 230V. The application fields of the power supply include auxiliary power supply for industrial control.
Fig. 2 circuit of industrial control power supply with 3W ultra wide input range composed of tny280p
TinySwitch - Ⅲ series products can start and work normally with a minimum drain voltage of 50V. Generally, when the AC input voltage U > 85V, the chip can provide self supplied bias voltage. However, when 18V ＜ u ＜ 75V, the chip cannot provide enough bias to maintain normal operation, which greatly limits the application of TinySwitch - Ⅲ series products under low voltage. In order to solve the above problems and make TinySwitch - Ⅲ work normally under ultra-low AC input voltage, a suspended high-voltage current source needs to be added from the outside of tny280p to continue to supply power to the bypass terminal BP / m at low voltage. The circuit of suspended current source is shown in Figure 3. It includes 7.5V zener vdz1 (1n5236b), PNP transistor VT1 (ztx558), NPN transistor vt2 (ztx458), diodes VD2 and VD4, and resistors R4 ~ R6. Among them, VD2 is a half wave rectifier; VD4 is an isolation diode, which can isolate the current source from other circuits.
Fig. 3 circuit of suspended current source
Ztx558 and ztx458 are high back voltage transistors produced by Zetex semiconductors in the UK. Among them, ztx558 is a PNP type high back voltage transistor. The main parameters are the Collector Emitter reverse breakdown voltage U (BR) CEO = - 400V when the base is open, the collector base reverse breakdown voltage (i.e. collector reverse breakdown voltage) U (BR) CBO = - 400V when the emitter is open, the maximum collector current ICM = - 200mA, and the common emitter current amplification factor HFE ≤ 300, Maximum power consumption PCM = 1W. Ztx458 belongs to NPN type high back voltage transistor. The main parameters are u (BR) CEO = 400V, u (BR) CBO = 400V, maximum collector current ICM = 300mA, HFE ≤ 300, PCM = 1W.
The 18 ~ 265V AC voltage is rectified by VD2 half wave to obtain a pulsating DC bias voltage UB, which is added to the input of the suspended current source. The current source can supply about 600 to the BP / M terminal of tny280p in the whole input voltage range μ A constant current. First, assuming that the circuit only uses transistor vt2, it can be considered that the voltage regulator vdz1 provides a reference potential ub2 to the base of vt2. Since the sum of the transmitting junction voltage (ube2) of vt2 and the voltage drop (UR5) on R5 is equal to the stable voltage value UZ of the regulator, and ube2 is approximately a constant voltage when the ambient temperature is constant, UR5 is also a fixed voltage, and the constant current value can be set by R5. However, in fact, the range of AC input voltage is very wide, and the variation range of bias current provided by the regulator is very large, which will lead to the offset of the set constant current value. To overcome the above problems, PNP transistors VT1 and R4 need to provide a constant bias current. Let the transmitting junction voltage of VT1 be UbE1, and the constant bias current IB1 set by R4 = UbE1 / R4. Obviously, IB1 is not affected by the change of input voltage.
The relationship curve between bias voltage (UB) and total bias current (IB) is shown in Figure 4. As can be seen from the figure, vt2 provides a constant bias current IB2 at a lower input voltage, while VT1 provides a constant bias current IB1 at a higher input voltage. It can be divided into the following three situations:
Figure 4 Relationship curve between bias voltage and total bias current
1 when the bias voltage UB ≈ 50V (i.e. the DC input voltage UI ≈ 50V after rectification and filtering), vt2 provides tny280p with a constant bias current IB2. At this time, the total bias current IB = IB2.
2 when the bias voltage UB > 50V, the current flowing through vt2 will decrease linearly and the current flowing through VT1 will increase linearly. At this time, VT1 and vt2 jointly provide tny280p with a constant bias current. The total bias current IB = IB1 + IB2, where IB2 > IB1.
3 when the bias voltage reaches the maximum value (UB = 375v), VT1 mainly provides a constant bias current, IB = IB1 + IB2, but IB1 > IB2.
The total bias current provided by the circuit IB ≈ 600 μ A。
The 18 ~ 265V AC input voltage passes through the half wave rectifier filter circuit composed of VD1, C1 and C2 to obtain the DC input voltage UI, which provides high-voltage DC for the flyback switching power supply. C1 and C2 also form π filters with inductor L to reduce serial mode electromagnetic interference. Y capacitor C7 is used between the primary and secondary windings of high frequency transformer to filter out common mode interference. The primary clamping circuit consists of VD3 (1n4007gp), R1, R2 and C3. Rectifier vd5 adopts byv27-200 2A / 200V ultrafast recovery diode, and its reverse recovery time TRR is less than 25ns. The output voltage is set by the sum of the voltage drop of the LED in the voltage stabilizing tube vdz2 and the optical coupler PC817A. Vdz2 adopts 4.3v voltage stabilizing tube 1n5229b, the forward voltage drop of LED is approximately 1V, and the set no-load output voltage is 5.3v.
Tny280p adopts on / off control mode. It receives the feedback voltage of the secondary winding through the optocoupler, and then keeps the output voltage stable by enabling or prohibiting the on / off of the internal MOSFET. Once the current flowing from the en / UV terminal exceeds the shutdown threshold current (115 μ A) , the switching cycle will be skipped; When the current from the en / UV terminal is less than the off threshold current, the switching cycle will be enabled again.
Key points of design
1 high frequency transformer adopts ef20 ferrite core and primary winding Φ 32 turns of 0.33mm enamelled wire. For secondary winding Φ 8 turns of 0.40mm enamelled wire. Inductance of primary winding LP = 278 μ H (allowable error of ± 12%), maximum leakage inductance lp0 = 12 μ H。 The minimum resonant frequency of the primary winding is 1MHz.
2 because the input voltage range of the power supply is very wide, the inductance LP of the primary winding must be small enough (278 is actually selected) μ H) In order to make tny280p work at the edge of continuous mode. However, when the LP is small, the current rise rate di / dt will increase. If necessary, the chip with large output power in TinySwitch - Ⅲ series products can be selected.
3 the resistance of R1 in the clamping circuit should not be too small, otherwise the no-load power consumption will increase., Technology Zone
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