Dwayne Reid

If the DC/DC converter must be cheap and produced in small quantities, construct it using standard approaches and solutions are impossible. Integrated switches produced by companies, or, in the typical case, are not designed for a configuration with isolated outputs and require the use of expensive transformers, which, moreover, are difficult to purchase in small quantities. In addition, to provide feedback, integrated switch circuits typically require opto-isolators and corresponding control chips.

An alternative circuit that uses discrete components is capable of delivering up to 50 mA of current at 5 V. The input voltage of the circuit is 8 to 32 V. To eliminate the need for feedback components, the output stage is designed as a source circuit direct current. You can use a parallel regulator like the TL431, or even a simple zener diode.

Since the output voltage is less than the minimum input voltage, a simple 1:1 transformer can be used as an isolating transformer. Any pulse transformer will do general purpose. Such transformers, with a price of about $0.25, have low interwinding capacitance and excellent insulation between the windings.

A DC/DC converter requires a sawtooth voltage generator and a comparator that controls a powerful cascade. Feedback on the load current is taken from a resistor connected in series with the primary winding of the transformer. The ramp voltage generator is easy to make using an op amp with a few resistors and a capacitor, but to save components the op amp is replaced by the second half of a dual LM393 comparator. The circuit works great with symmetrical ramp voltage, so no protection diodes are needed. To reduce the likelihood of saturation of transformer T1, the maximum duty cycle of the ramp voltage generator is set to approximately 50%.

To minimize control losses due to current withdrawal by the transformer with feedback In terms of current, the ramp voltage generator should operate at a frequency of about 400 kHz and an amplitude of about 0.5 V. The exact frequency and amplitude values ​​depend on the reference voltage. In the circuit, the reference voltage source is made using a regular red LED, the forward voltage drop across which is quite stable under room conditions.

The output current of the PWM comparator is not enough for direct control transformer. Therefore, connected to its open collector output pnp transistor Q1 and resistor R12, forming a powerful output stage. In addition, the cascade acts as a voltage surge limiter. When, during a transient, the voltage surge exceeds the breakdown voltage of the comparator, the transistor opens and cuts off the surge.

Despite the completely unconventional design, the circuit performs very well, and the Spice model shows agreement with the results obtained on the breadboard. And, most importantly, the scheme is extremely cheap. The total cost of all components, including the transformer, when purchased in batches of several hundred pieces, does not exceed $0.60.

Isolated or non-isolated DC/DC converter: which to choose?

What is preferable - to develop, manufacture or buy a ready-made device, configured and tested, with all the necessary certificates and guarantees in relation to assemblies and components of electronic equipment (REA)? This topic has been repeatedly raised on the pages of various publications, including in the journals of the “Components and Technologies” editorial staff. Applied to DC/DC-converters, this dilemma was considered by the author of this article in. Here are the benefits of the “buy” option for large number end-use applications using switching power supplies have been convincingly justified. However, the question remained “overboard”: if you buy, then what exactly? This article will try to fill this gap.

Weighing everything again proetcontra regarding the DC/DC converter and, as they say, having considered the arguments of the parties, we came to a clear conclusion - it is more profitable to buy it. And in order to buy exactly what we need and not make a mistake, we must, first of all, be interested in a certain range of the most important issues, most of which are discussed in great detail in.

Although there are no details here, let’s reduce the problem of choosing a DC/DC converter to a few clear points. So what we need to consider:

1. the purpose of the product for which we are choosing a DC/DC converter, and a list of associated safety standards, electromagnetic compatibility, and compliance with environmental protection requirements;
2. range operating temperatures;
3. required input voltage range;
4. current load range;
5. The required rated output voltage (voltages) over the entire load range and the acceptable accuracy of maintaining it (them), without forgetting to take into account the response to load changes (the so-called reset/surge), which, due to the peculiarities of the control loop of a particular type of converter, can cause a transient process that is unacceptable for our final product;
6. acceptable area for the converter placement for our final solution and maximum height profile, taking into account all the elements necessary for its final implementation, i.e. consider the solution as a complete device;
7. acceptable level of efficiency over the entire load range;
8. reliability of the final DC/DC converter solution in real conditions operation;
9. limit price of the final DC/DC converter solution and determination of its supplier.

Rice. 1. Example topology of a non-isolated DC/DC converter with two output voltages

Although today the market offers DC/DC converters, as they say, for every taste, color and budget, to begin with, it all comes down to the main question: do we buy an isolated or non-isolated DC/DC converter. To clearly understand what we're talking about Let us briefly explain: an isolated converter means that its output and input do not have a galvanic connection and are separated by an insulating barrier with a certain dielectric strength. This parameter indicates the resistance of the insulating barrier to the voltage applied between its input and output, and, in most cases, determines the possible field of application of the device. Accordingly, a non-isolated converter does not provide galvanic isolation between its input and output, and this fact is also decisive for its applications. All of the above are global differences that determine circuit design solutions, electrical characteristics and even the design of the converters.

It is clear that all this affects the cost of these types of DC/DC converters. The responsible developer here needs to use one useful rule. Benjamin Franklin liked to repeat it - the great American politician, whose portrait adorns the $100 bill, diplomat, encyclopedist, writer, journalist, publisher and inventor. For reference: it was he who introduced the now generally accepted designation for the electrically charged states “+” and “−” and explained the principle of operation of the Leyden jar, the ancestor of all modern capacitors, establishing that main role a dielectric plays in it, separating its conductive plates. He said: A penny saved is a penny gained (« A penny saved is a penny earned"), which very well describes his attitude to optimizing the cost of the final product.

Let's take a closer look from this position at the differences between the converters (which will help us later), weighing everything again proetcontra, and let’s dwell on the specific converter solution we need for a specific end device. That is, let’s try to get away from the typical temptation for developers like: “let’s put this here, I like it!” Sounds like a compelling argument, doesn't it?

If we turn to the price aspect, then in general terms Non-isolated converters benefit due to their nature. However, this does not apply to packaged, complete solutions with input and output filters that meet stringent EMC requirements and have high mechanical stability. Here the price differences are largely leveled out. As for the topology of non-isolated converters, we have available boost, buck, boost-buck and inverting topologies, which have already become classics and have been described many times. With some modification, using a transformer, we can obtain, on the basis of non-isolated converters, solutions with several, usually two, output voltages, and one of these voltages will be isolated, which gives whole line advantages inherent in their isolated counterparts. An example of such a solution, which is not often found in the technical literature and in practice, is shown in Fig. 1 .

This topology is effective, but for relatively small currents in the additional circuit. The author of the article used it in practice to generate two voltages of 3.3 V (3.5 A) and –12 V (0.250 mA) and used it as part of the ATX power supply system of an industrial computer board. This is all well and good, but it is not a complete solution that can be purchased in modular form. On your own, as we determined at the beginning of the article, doing anything transformative is not profitable - it is expensive, time-consuming, and the result is not guaranteed, and you need to hire specialists in this matter, which we do not have. It is worth noting that, as a rule, only the simplest options are available in the form of non-isolated converters, some of which are aimed at effectively replacing linear stabilizers.

Rice. 2. Examples of polarity conversion using an isolated DC/DC converter

As for non-isolated converters, they are acceptable and justified for building PoL (Point-of-Load) distributed power systems, that is, when it is necessary to power loads in the immediate vicinity of their location, with short lengths of intermediate power buses. Another option is the formation of power buses for boards of small form factors, where it is possible to do without land sharing and, preferably, there are no hybrid solutions, that is, where there is no combination of analog and digital stages.

What can isolated DC/DC converters give us? For an ordinary developer who does not really delve into the essence of such a “little thing” as a DC/DC converter (which can then come back to haunt him at the final stage of the project), especially in a modular design, this is a “black box” with conclusions that just does the desired function, like the same capacitor or transistor. Its main function is to create an appropriate insulating barrier and output voltage required power. However, not everything is so simple and obvious. Indeed, if the issue rests only on the insulating barrier, as, for example, this is shown by the example of the use of products from the well-known company TRACO Electronic in medical equipment in, or in the case when, due to a significant distance from the main power source, land division is required, then there is no choice, and everything is clear. A more complex issue concerns, for example, telecommunications equipment and distributed power systems with long intermediate buses, as well as equipment with biased cascades.

What can isolated DC/DC converters offer us? As Sergei Kapitsa said in the fascinating program of our youth “Obvious-Incredible,” “The question, of course, is interesting.” To reveal it, let's turn to practical examples given in . The fact is that isolated converters can, in some cases, successfully replace non-isolated ones, giving us a number of advantages, often very significant, which will simplify the design of the final product.

Because an isolated DC/DC converter has a floating output since it is not tied to the common wire or, as we often say, to ground. In the same way, we can assume that a floating entry also occurs. Therefore, any isolated DC/DC converter can be used to invert the polarity of the power bus voltage. If galvanic isolation by insulation is not required, but is available common point connection, then any output can be tied to any input, as well as to any desired reference voltage. In Fig. Figure 2 shows two possible configurations for switching an isolated DC/DC converter to produce a negative output voltage from a positive voltage at its input, and vice versa. And if you can get –15 V from +5 V with a non-isolated DC/DC converter, then getting +5 V from –48 V is no longer so easy.

Rice. 3. Simple voltage doubler

There are DC/DC applications where galvanic isolation through isolation is not required, but a higher output voltage than the input voltage is required. IN following example shown in Fig. 3 shows a voltage doubler based on a DC/DC converter that produces an output voltage that is twice the input voltage.

The advantages here are hidden in a seemingly strange fact: if a DC/DC converter is designed for a power of 15 W, then with an output voltage of 12 V, it will provide an operating current of up to 1.25 A. However, this output voltage is above the input voltage of 12 V. Therefore, the load is supplied with a voltage of 24 V with a current of 1.25 A, that is, we have a total power of 30 W.

As is known, the advantage of step-down pulsed DC/DC converters over linear ones is that they consume less current at the input than the one they supply to the load. If we need to implement internal buses from the intermediate bus as simply as possible, and with good efficiency and the inevitable decoupling across the lands, then it is better than shown in Fig. It's hard to find 4 options.

Rice. 4. Power supply with three output voltages, using DC/DC converters (Attention! The figure has been modified to fit the TRACO base!)

And in conclusion, here is another important and useful example. If you have a “hodgepodge” of analog and digital stages on the board, which, in addition, have a common 5 V power bus and ground (i.e., at first glance, this cannot be separated), then analog integrated circuits may experience problems caused by significant levels of high-frequency interference from circuits carrying digital signals. This is especially noticeable in measurement, audio or video applications. As for a common ground, it is often required where the analog and digital parts of the circuit share the same common signal source. This quite often makes their complete galvanic separation impossible.

In Fig. Figure 5 shows a seemingly meaningless circuit that converts an input voltage of 5 V into an output voltage equal to the same 5 V, and for some reason this is done by an isolated converter in a non-isolated connection. The reason why this circuit actually makes sense is because of the features and specifications of such DC/DC converters. And she helps solve the problem.

Rice. 5. Non-isolated +5V to +5V DC converter to clear the +5V bus

The essence of the solution is that the converter's input voltage range is +5 V with a certain level of unevenness from fluctuations and interference, and its output voltage is maintained at 5 V ±0.8%, so such a converter will clear not only noise and interference, but also any small voltage variations across its input, suppressing surges and transients that inevitably occur in digital stages.

A similar scheme (Fig. 5) was used by the author in one of the serial products special purpose, in which a microcontroller with digital stages, a highly sensitive amplifier and high-order analog filters were located on one extremely compact printed circuit board. The solution has shown very high efficiency when working with signals at a level of a fraction of a millivolt.

Rice. 6. X-ray a fragment of a switching stabilizer from a well-known brand (image on the left) and a replica of this product made by its competitor (image on the right)

And finally, if we use isolation separation, for example, at the level of requirements for telecommunication equipment, then is it worth pursuing and using DC/DC converters with a very high isolation barrier resistance? If you are not short on funds and the customer calmly accepts your flight of fancy, then this is your right, you can even order a converter with inlay, and, believe me, they will make it and supply it to you. Only a responsible developer should still use Benjamin Franklin's rule.

As mentioned at the beginning of the article, developers are offered a variety of DC/DC converters from a large number of manufacturers. Here you need to remember that the stingy pays twice, and for important products do not fall for dubious offers with low prices. If you think that the same name hides the same solution, then you are deeply mistaken. A replica of a famous brand can only have the same appearance and title. A clear example is shown in Fig. 6.

Rice. 7. TEL Series 8 isolated 8-W DC/DC converters from TRACO Electronic

As we were able to verify, DC/DC converters with same names, may have completely different designs. That's why at the beginning of the article among critical issues Finding a reliable supplier was mentioned. So it’s better and safer to deal with well-known brands, then you can confidently get for your money exactly the product that will meet all your requirements, and you won’t have to blush either in front of the project customer or in front of the end consumer of the product you developed.

One of these time-tested brands is the company TRACO Electronic, which in December 2016 launched a line of high-quality telecom converters TEL 8 with a power of 8 W, which are produced under the trademark (Fig. 7).

These converters are not only distinguished by their high technical characteristics and small dimensions, but also high reliability (at least 1 million hours), dielectric strength of insulation (1800 V/1 s and 1500 V/60 s), high accuracy setting the rated output voltage (±2%) and its stability (0.8% when changing the input voltage over the entire range and 1% over the entire load range - from zero to maximum), developed overload protection (150%) with self-healing (hiccup ), unacceptably low input voltage and electrostatics at the level of the requirements of the EN 61000-4-2 standard (air ±8 kV, contact ±6 kV). The TEL 8 series converters meet the requirements for electromagnetic compatibility and are resistant to external interference with field strengths up to 10 V/m (EN 61000-4-3 standard). The converters are made in metal aluminum cases and, importantly, with a built-in electromagnetic interference suppression filter according to the EN55022 Class A standard. The operating temperature range of the TEL 8 series converters is –40… +80 °C with maximum temperature housing up to +105 °C. The converters can be used in equipment operating at altitudes up to 4,000 m above sea level, have all the necessary safety certificates and comply with the requirements of the RoHS Directive. Basic electrical parameters, allowing you to do pre-selection the required converter are given in table. 1. Complete data for TRACO Electronic TEL 8 Series inverters and links to obtain certifications are available directly from the datasheet.

Table 1

Literature

1. Rentyuk V. Isolated low-power DC/DC converter: make or buy? // Electrician. 2012. No. 12.
2. Rentyuk V. New capabilities of modern DC/DC converters: features of decision-making on selection and typical applications // Electrician. 2015. No. 7–9.
3. Designing Low-cost, Multiple Output DC-DC Converters. APPLICATION NOTE, Würth Elektronik eiSos 2013-09-10.
4. Rentyuk V., Filatov V. Power supplies with high insulation breakdown voltage. Safety above all // Components and technologies. 2016. No. 3.
5. Steve Roberts. DC/DC BOOK OF KNOWLEDGE: Practical tips for the User. Second Edition, 2015.

1. DC/DC Converter TEL 8 Series, 8 Watt Rev. December 21. 2016. http://assets.tracopower.com/20170126153146/TEL8/documents/tel8-datasheet.pdf

There have already been more than a dozen reviews and improvements to the well-known kit for assembling a laboratory power supply, but with this modification I was able to make it “transformer-independent”, now it does not require alternating voltage, and works great on direct voltage!

Having purchased this kit ( great review, which was my inspiration, is here: ), I carefully read all the reviews, and did everything according to your recommendations - I changed the input diodes to Schottky ones, installed high-quality electrolytes everywhere.

But the problem of the transformer arose, we have on sale either a Soviet heavy one, which is humming and large, but ordering from China, due to the weight, is expensive. I sat down to think, thanks to Kirich’s review, I knew that a transformer is used to get a negative supply voltage for an operational amplifier without any problems. What if we get negative voltage? alternative method? I thought, and conducted an experiment by supplying power to the op-amp from another laboratory power supply. Everything worked perfectly, so I ordered these converters and decided to radically modify the circuit.

All modifications are visible from the photo; parts that are not worthwhile may not be installed right away. These are input diodes (all 4 pieces), two 1N4148 diodes, an 82 ohm resistor, a 47 uF capacitor, a 5.1V zener diode, and a 220 ohm resistor, replace it with a jumper.

Instead of the 7824 stabilizer, we solder in 7805, and in place of the fan connection we put a capacitor (I installed 330 uF 16 volts, you can put any other one with similar parameters), and next to the capacitor we drill 4 holes into which we insert the converter.

Other improvements from reverse side The boards can be seen in the photo below. I additionally soldered a 0.1 µF capacitor in parallel with the input one, just in case.

I soldered red and black wires into the holes for the legs of the input rectifier diodes - now power will be supplied to them. In the photo, the LED is also not soldered, and the adjustment resistors are directly on the board, and the 0.47 ohm is also in place. In the final version, instead of 0.47 ohm I will put 0.1 ohm, the resistors will be 10 turn, and the LED will be on the front panel.

I ordered a power supply for 24 volts 3 amperes, also from a review by the respected Kirich (). In terms of price and weight, it turns out to be cheaper than a transformer of the required size.

At your request, I am posting a scheme of improvements.

I hope this modification will be useful to you, and repeating it is not difficult. Sincerely, your Anna :)