A high-performance power supply is the energy guarantee for all electronic devices. Nowadays, electronic technology is developing rapidly, and the requirements for power supply are becoming more and more demanding, especially for some power sources with large current and wide voltage input range. The original series regulator power supply has been gradually eliminated due to its large size, low efficiency, and severe heat generation. Designers have turned their attention to and adopted a lightweight and efficient switching power supply circuit, and gradually extended to various fields of application. Note: LM2576---TO-220 package
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One of the LM2576 chips has excellent performance and can work in a wide power supply range; the upper limit of the common type is 40V, while the LM2576HV can reach 60V, and the output current is about 3A (with good heat dissipation conditions). Moreover, the chip has few external components and is easy to debug, so it is used by many people, especially on DC-DC converters.
However, since the input power of the DC-DC converter is mostly a chemical power source, that is, various types of batteries, and the battery has internal resistance, the voltages of the no-load and full-load terminals vary greatly, so in order to reduce the battery plate area when manufacturing a high-power battery, The manufacturer's strategy is often to increase the number of tandem individuals, so that the nominal voltage of the battery has a gradual increase trend. Take the 48V lead-acid battery for bicycle as an example. It is 58V at no load and 48V at full load. The 48V fuel cell has a no-load of 72V and a full load of 48V.
Obviously, the no-load voltage of the 48V lead-acid battery has exceeded the LM2576 upper limit, which is close to the LM2576HV input upper limit, while the 48V fuel cell no-load has exceeded the LM2576HV upper limit. Moreover, many applications require a DC-DC output of 5-15A, which forces the LM2576 output to expand. However, this is not extended to the required value by using a high-power tube to form an emitter follower. The reader does not have to try, the result is that the high-power tube can not withstand the power consumption and severe damage!
In order to make the system meet the requirements of high voltage input and high current output, of course, the simplest is to change the circuit structure and use the switching power supply of the high frequency transformer output. However, some chips have poor conversion efficiency, and the system size cannot be compared with the LM2576. Moreover, due to the complexity of production, the cost is also doubled, so many input and output power supplies do not need an electrically isolated power supply. Designers don't like to adopt it, but they want to keep the simple LM2576 platform and peripheral auxiliary circuits to achieve their performance improvement and make a cost-effective application.
The author accepts a task of controlling the turtle source at 48V fuel cell: its parameters are input voltage no load 72V to overcurrent 39V, output is 30V voltage regulation, output 7A. Moreover, there is a limitation in volume, and it is not allowed to install a large heat sink. Among them, such a high power supply voltage makes many components lose the user's land! To this end, it is necessary to make a completely new and perfect design for the expansion circuit of the input voltage and output current, in order to get out of the predicament!
Repeatedly deduced to get a specific design: the chip power supply with a simple string stable power supply 24V power supply, with VMOS tube as a switch tube to replace the chip on-chip switch; the chip is only used as a VMOS tube driver application.
The key problem to realize this solution is to require the circuit to effectively transmit the drive signal, and at the same time ensure that the external VMOS tube and the output end of the chip are well isolated, and the consequences of the chip and the tube are avoided!
After many experiments, the specific structure of the simple circuit is shown in the figure below. Among them, R1, Q1, D1, Q2 constitute a 24V simple voltage regulator circuit. C3 is the filter capacitor. This power supply provides a stable 23V supply to the LM2576. When the power input is lower than 25V, it can't be regulated, but it can still work at voltage above 20V. If you want to work at low power supply voltage, you can reduce the D1 voltage regulation value, the experiment can be as low as 10V application, without affecting the circuit operation, but Rl must adjust the resistance value to make the voltage regulator tube work normally. Since the power supply only provides the LM2576 chip with a small current output, the Q2 does not require a heat sink.
When the upper limit of the power supply does not exceed the upper limit of the chip input voltage (LM2576HV-60V, LM2576-40V), the voltage regulator circuit can be omitted, and the battery is directly connected to the LM2576 power supply terminal.
The LM2576 output is driven by R2 and R3 to drive Q7 on and off. When Q7 is turned on, current flows from VCC through D3, D2, R4, and Q7 to ground. D3 generates a voltage of 15V to supply the gate voltage of Q3 to Q5, and Q6 is fully turned off due to the conduction of D2. The high resistance at the cutoff does not constitute a bypass of the gate voltage of Q3 to Q5. Therefore, Q3 to Q5 are fully turned on, and C2 is charged. When Q7 is turned off, the VCC→D3→D2→R4 current loop is cut off, and the D3 has no voltage drop to make the gate voltage of Q3~Q5 disappear, and D2 is cut off, which does not constitute the reverse bias of Q6. Q6 is turned on due to the power supply of R6, which causes the Q3~Q5 gates to be short-circuited, which forces Q3~Q5 to turn off quickly. C2 is still charged by L4 to generate positive and negative induced voltages through L1.
This state is completed until Q7 is turned back on.
Q7 is driven by the R2 and R3 voltage dividers of the LM2576 output port. When it is high, it is turned on and the low level is turned off. Since the VMOS tube drive current is small, the 5551 drives 3 to 4 VMOSs without problems. If necessary, replace the TIP41 with more drive capability and drive more VMOS tubes.
LM2576-ADJ (ADJ is the output voltage adjustable type) voltage adjustment mechanism, R7, R5 resistance adjustment to form the adjustment system.
Its formula VOUT(V)=1.23*(R7/R5+1), the additional electronic switch does not affect its value. In the circuit, R7 is 42K, R5 is 1.8K, and the measured output power is 30.3V. It is very close to the calculated value.
Circuit debugging results: the quality is extremely good, basically meet the design requirements; the output voltage is stable, the heat is very small; in the case of no heat sink, 3 IRF9540 parallel output 4A work 3 hours, the pipe is not hot; and LM2576 and 20100 SCHOTT The base diode has no temperature rise at all. After the heat sink is mounted, a current of 7.0A can be output. The conversion efficiency is similar to the Im2576 single application. Its performance has met the extremely demanding military requirements: input voltage 22 ~ 80V; output current 1 ~ 7A; all-weather working temperature rise <27 ° C, the system enhances the application effect is quite successful.
In addition, this circuit can also be derived from many application circuits, such as can be used as a brush DC motor controller; R7 changed to a potentiometer to adjust the speed smoothly; R5 is connected to a suitable resistance negative temperature thermistor, so that it becomes a DC brushless fan controller; can automatically adjust the fan speed according to the temperature rise of the device... As an efficient, wide range power input, high power output platform; there are certainly many waiting for readers to develop!
IRF9540, TIP41, 20100 are all in TO-220 package
D1, D2, and D3 are all packaged in a 1005 package.
Ql, Q6, Q7 are SOT-23 packages
R1-R7 is a patch 0805 package
The LX 12X16 I-shaped magnetic core is wound with 1.35 lacquered copper wire, and the inductance is 47μH-100μH.
C1 and C3 are RB.3/. 6 package
C2 is RB. 2/. 4 package
C5 is a patch 1005 package 