Input voltages up to 61 V, output voltages from 0.6 V, output currents up to 4 A, the ability to externally synchronize and adjust the frequency, as well as adjust the limiting current, adjust the soft start time, comprehensive load protection, a wide operating temperature range - all these features of modern sources power supplies are achievable using the new line of DC/DC converters produced by .
Currently, the range of switching regulator microcircuits produced by STMicro (Figure 1) allows you to create power supplies (PS) with input voltages up to 61 V and output currents up to 4 A.
The task of voltage conversion is not always easy. Each specific device has its own requirements for the voltage regulator. Sometimes price (consumer electronics), size (portable electronics), efficiency (battery-powered devices), or even the speed of product development play a major role. These requirements often contradict each other. For this reason, there is no ideal and universal voltage converter.
Currently, several types of converters are used: linear (voltage stabilizers), pulsed DC/DC converters, charge transfer circuits, and even power supplies based on galvanic insulators.
However, the most common are linear voltage regulators and step-down switching DC/DC converters. The main difference in the functioning of these schemes is evident from the name. In the first case, the power switch operates in linear mode, in the second - in key mode. The main advantages, disadvantages and applications of these schemes are given below.
Features of the linear voltage regulator
The operating principle of a linear voltage regulator is well known. The classic integrated stabilizer μA723 was developed back in 1967 by R. Widlar. Despite the fact that electronics have come a long way since then, the operating principles have remained virtually unchanged.
A standard linear voltage regulator circuit consists of a number of basic elements (Figure 2): power transistor VT1, a reference voltage source (VS), and a compensation feedback circuit on an operational amplifier (OPA). Modern regulators may contain additional functional blocks: protection circuits (from overheating, from overcurrent), power management circuits, etc.
The operating principle of such stabilizers is quite simple. The feedback circuit on the op-amp compares the value of the reference voltage with the voltage of the output divider R1/R2. A mismatch is formed at the op-amp output, which determines the gate-source voltage of power transistor VT1. The transistor operates in linear mode: the higher the voltage at the output of the op-amp, the lower the gate-source voltage, and the greater the resistance of VT1.
This circuit allows you to compensate for all changes in input voltage. Indeed, suppose that the input voltage Uin has increased. This will cause the following chain of changes: Uin increased → Uout will increase → the voltage on the divider R1/R2 will increase → the output voltage of the op-amp will increase → the gate-source voltage will decrease → the resistance VT1 will increase → Uout will decrease.
As a result, when the input voltage changes, the output voltage changes slightly.
When the output voltage decreases, reverse changes in voltage values occur.
Features of operation of a step-down DC/DC converter
A simplified circuit of a classic step-down DC/DC converter (type I converter, buck-converter, step-down converter) consists of several main elements (Figure 3): power transistor VT1, control circuit (CS), filter (Lph-Cph), reverse diode VD1.
Unlike the linear regulator circuit, transistor VT1 operates in switch mode.
The operating cycle of the circuit consists of two phases: the pump phase and the discharge phase (Figures 4...5).
In the pumping phase, transistor VT1 is open and current flows through it (Figure 4). Energy is stored in the coil Lf and capacitor Cf.
During the discharge phase, the transistor is closed, no current flows through it. The Lf coil acts as a current source. VD1 is a diode that is necessary for reverse current to flow.
In both phases, a voltage equal to the voltage on the capacitor Sph is applied to the load.
The above circuit provides regulation of the output voltage when the pulse duration changes:
Uout = Uin × (ti/T)
If the inductance value is small, the discharge current through the inductance has time to reach zero. This mode is called the intermittent current mode. It is characterized by an increase in current and voltage ripple on the capacitor, which leads to a deterioration in the quality of the output voltage and an increase in circuit noise. For this reason, the intermittent current mode is rarely used.
There is a type of converter circuit in which the “inefficient” diode VD1 is replaced with a transistor. This transistor opens in antiphase with the main transistor VT1. Such a converter is called synchronous and has greater efficiency.
Advantages and disadvantages of voltage conversion circuits
If one of the above schemes had absolute superiority, then the second would be safely forgotten. However, this does not happen. This means that both schemes have advantages and disadvantages. Analysis of schemes should be carried out according to a wide range of criteria (Table 1).
Table 1. Advantages and disadvantages of voltage regulator circuits
| Characteristic | Linear regulator | Buck DC/DC converter |
| Typical input voltage range, V | up to 30 | up to 100 |
| Typical Output Current Range | hundreds of mA | units A |
| Efficiency | short | high |
| Output voltage setting accuracy | units % | units % |
| Output voltage stability | high | average |
| Generated noise | short | high |
| Circuit implementation complexity | low | high |
| Complexity of PCB topology | low | high |
| Price | low | high |
Electrical characteristics. For any converter, the main characteristics are efficiency, load current, input and output voltage range.
The efficiency value for linear regulators is low and is inversely proportional to the input voltage (Figure 6). This is due to the fact that all the “extra” voltage drops across the transistor operating in linear mode. The transistor's power is released as heat. Low efficiency leads to the fact that the range of input voltages and output currents of the linear regulator is relatively small: up to 30 V and up to 1 A.
The efficiency of a switching regulator is much higher and less dependent on the input voltage. At the same time, it is not uncommon for input voltages of more than 60 V and load currents of more than 1 A.
If a synchronous converter circuit is used, in which the inefficient freewheeling diode is replaced by a transistor, then the efficiency will be even higher.
Accuracy and stability of output voltage. Linear stabilizers can have extremely high accuracy and stability of parameters (fractions of a percent). The dependence of the output voltage on changes in the input voltage and on the load current does not exceed a few percent.
According to the principle of operation, a pulse regulator initially has the same sources of error as a linear regulator. In addition, the deviation of the output voltage can be significantly affected by the amount of current flowing.
Noise characteristics. The linear regulator has a moderate noise response. There are low-noise precision regulators used in high-precision measuring technology.
The switching stabilizer itself is a powerful source of interference, since the power transistor operates in switch mode. Generated noise is divided into conducted (transmitted through power lines) and inductive (transmitted through non-conducting media).
Conducted interference is eliminated using low-pass filters. The higher the operating frequency of the converter, the easier it is to get rid of interference. In measuring circuits, a switching regulator is often used in conjunction with a linear stabilizer. In this case, the level of interference is significantly reduced.
It is much more difficult to get rid of the harmful effects of inductive interference. This noise originates in the inductor and is transmitted through air and non-conducting media. To eliminate them, shielded inductors and coils on a toroidal core are used. When laying out the board, they use a continuous fill of earth with a polygon and/or even select a separate layer of earth in multilayer boards. In addition, the pulse converter itself is as far away from the measuring circuits as possible.
Performance characteristics. From the point of view of simplicity of circuit implementation and printed circuit board layout, linear regulators are extremely simple. In addition to the integrated stabilizer itself, only a couple of capacitors are required.
A switching converter will require at least an external L-C filter. In some cases, an external power transistor and an external freewheeling diode are required. This leads to the need for calculations and modeling, and the topology of the printed circuit board becomes significantly more complicated. Additional complexity of the board occurs due to EMC requirements.
Price. Obviously, due to the large number of external components, a pulse converter will have a high cost.
As a conclusion, the advantageous areas of application of both types of converters can be identified:
- Linear regulators can be used in low power, low voltage circuits with high accuracy, stability and low noise requirements. An example would be measurement and precision circuits. In addition, the small size and low cost of the final solution can be ideal for portable electronics and low-cost devices.
- Switching regulators are ideal for high-power low- and high-voltage circuits in automotive, industrial and consumer electronics. High efficiency often makes the use of DC/DC no alternative for portable and battery-powered devices.
Sometimes it becomes necessary to use linear regulators at high input voltages. In such cases, you can use stabilizers produced by STMicroelectronics, which have operating voltages of more than 18 V (Table 2).
Table 2. STMicroelectronics Linear Regulators with High Input Voltage
| Name | Description | Uin max, V | Uout nom, V | Iout nom, A | Own drop, B |
| 35 | 5, 6, 8, 9, 10, 12, 15 | 0.5 | 2 | ||
| 500 mA precision regulator | 40 | 24 | 0.5 | 2 | |
| 2 A regulator | 35 | 0.225 | 2 | 2 | |
| , | Adjustable regulator | 40 | – | 0.1; 0.5; 1.5 | 2 |
| 3 A regulator | 20 | – | 3 | 2 | |
| 150 mA precision regulator | 40 | – | 0.15 | 3 | |
| KFxx | 20 | 2.5: 8 | 0.5 | 0.4 | |
| Ultra-low self-drop regulator | 20 | 2.7: 12 | 0.25 | 0.4 | |
| 5 A regulator with low dropout and output voltage adjustment | 30 | – | 1.5; 3; 5 | 1.3 | |
| LExx | Ultra-low self-drop regulator | 20 | 3; 3.3; 4.5; 5; 8 | 0.1 | 0.2 |
| Ultra-low self-drop regulator | 20 | 3.3; 5 | 0.1 | 0.2 | |
| Ultra-low self-drop regulator | 40 | 3.3; 5 | 0.1 | 0.25 | |
| 85 mA regulator with low self-dropout | 24 | 2.5: 3.3 | 0.085 | 0.5 | |
| Precision Negative Voltage Regulator | -35 | -5; -8; -12; -15 | 1.5 | 1.1; 1.4 | |
| Negative voltage regulator | -35 | -5; -8; -12; -15 | 0.1 | 1.7 | |
| Adjustable Negative Voltage Regulator | -40 | – | 1.5 | 2 |
If a decision is made to build a pulsed power supply, then a suitable converter chip should be selected. The choice is made taking into account a number of basic parameters.
Main characteristics of step-down pulse DC/DC converters
Let us list the main parameters of pulse converters.
Input voltage range (V). Unfortunately, there is always a limitation not only on the maximum, but also on the minimum input voltage. The value of these parameters is always selected with some margin.
Output voltage range (V). Due to restrictions on the minimum and maximum pulse duration, the range of output voltage values is limited.
Maximum output current (A). This parameter is limited by a number of factors: the maximum permissible power dissipation, the final value of the resistance of the power switches, etc.
Converter operating frequency (kHz). The higher the conversion frequency, the easier it is to filter the output voltage. This makes it possible to combat interference and reduce the values of the external L-C filter elements, which leads to an increase in output currents and a reduction in size. However, an increase in the conversion frequency increases switching losses of power switches and increases the inductive component of interference, which is clearly undesirable.
Efficiency (%) is an integral indicator of efficiency and is given in the form of graphs for various voltages and currents.
Other parameters (channel resistance of integrated power switches (mOhm), self-current consumption (µA), thermal resistance of the case, etc.) are less important, but they should also be taken into account.
The new converters from STMicroelectronics have high input voltage and efficiency, and their parameters can be calculated using the free eDesignSuite software.
Line of pulsed DC/DC from ST Microelectronics
STMicroelectronics' DC/DC portfolio is constantly expanding. New converter microcircuits have an extended input voltage range up to 61 V ( / / ), high output currents, output voltages from 0.6 V ( / / ) (Table 3).
Table 3. New DC/DC STMicroelectronics
| Characteristics | Name | |||||||
| L7987; L7987L | ||||||||
| Frame | VFQFPN-10L | HSOP-8; VFQFPN-8L; SO8 | HSOP-8; VFQFPN-8L; SO8 | HTSSOP16 | VFQFPN-10L; HSOP 8 | VFQFPN-10L; HSOP 8 | HSOP 8 | HTSSOP 16 |
| Input voltage Uin, V | 4.0…18 | 4.0…18 | 4.0…18 | 4…38 | 4.5…38 | 4.5…38 | 4.5…38 | 4.5…61 |
| Output current, A | 4 | 3 | 4 | 2 | 2 | 3 | 3 | 2 (L7987L); 3 (L7987) |
| Output voltage range, V | 0.8…0.88×Uin | 0.8…Uin | 0.8…Uin | 0.85…Uin | 0.6…Uin | 0.6…Uin | 0.6…Uin | 0.8…Uin |
| Operating frequency, kHz | 500 | 850 | 850 | 250…2000 | 250…1000 | 250…1000 | 250…1000 | 250…1500 |
| External frequency synchronization (max), kHz | No | No | No | 2000 | 1000 | 1000 | 1000 | 1500 |
| Functions | Smooth start; overcurrent protection; overheat protection | |||||||
| Additional functions | ENABLE; PGOOD | ENABLE | LNM; LCM; INHIBIT; Overvoltage protection | ENABLE | PGOOD; protection against voltage dips; cut-off current adjustment | |||
| Crystal operating temperature range, °C | -40…150 | |||||||
All new pulse converter microcircuits have soft start, overcurrent and overheating protection functions.
A push-pull pulse generator, in which, due to proportional current control of transistors, switching losses are significantly reduced and the efficiency of the converter is increased, is assembled on transistors VT1 and VT2 (KT837K). The positive feedback current flows through windings III and IV of transformer T1 and the load connected to capacitor C2. The role of diodes that rectify the output voltage is performed by the emitter junctions of the transistors.
A special feature of the generator is the interruption of oscillations when there is no load, which automatically solves the problem of power management. Simply put, such a converter will turn on itself when you need to power something from it, and turn off when the load is disconnected. That is, the power battery can be constantly connected to the circuit and practically not be consumed when the load is off!
For given input UВx. and output UBix. voltages and the number of turns of windings I and II (w1), the required number of turns of windings III and IV (w2) can be calculated with sufficient accuracy using the formula: w2=w1 (UOut. - UBx. + 0.9)/(UBx - 0.5 ). Capacitors have the following ratings. C1: 10-100 µF, 6.3 V. C2: 10-100 µF, 16 V.
Transistors should be selected based on acceptable values base current (it should not be less than the load current!!!) And reverse voltage emitter - base (it must be greater than twice the difference between the input and output voltages!!!) .
I assembled the Chaplygin module in order to make a device for recharging my smartphone while traveling, when the smartphone cannot be charged from a 220 V outlet. But alas... The maximum that I was able to squeeze out using 8 batteries connected in parallel is about 350-375 mA charging current at 4.75 V. output voltage! Although my wife’s Nokia phone can be recharged with this device. Without load, my Chaplygin Module produces 7 V with an input voltage of 1.5 V. It is assembled using KT837K transistors.
The photo above shows the pseudo-Krona, which I use to power some of my devices that require 9 V. Inside the case from the Krona battery there is an AAA battery, a stereo connector through which it is charged, and a Chaplygin converter. It is assembled using KT209 transistors.
Transformer T1 is wound on a 2000NM ring with dimensions K7x4x2, both windings are wound simultaneously in two wires. To avoid damaging the insulation on the sharp outer and inner edges of the ring, dull them by rounding off the sharp edges with sandpaper. First, windings III and IV (see diagram) are wound, which contain 28 turns of wire with a diameter of 0.16 mm, then, also in two wires, windings I and II, which contain 4 turns of wire with a diameter of 0.25 mm.
Good luck and success to everyone who decides to replicate the converter! :)
Step-down DC-DC converters are increasingly finding their use in everyday life, households, automotive applications, and also as regulated power supplies in a home laboratory.
For example, on a heavy-duty vehicle, the voltage of the on-board cable network may be +24V, but you need to connect a car radio or other device with an input voltage of +12V, then such a step-down converter will be very useful to you.
Many people order step-down DC-DC converters from various Chinese sites, but their power is quite limited, due to the Chinese saving on the cross-section of the winding wire, semiconductor devices and inductor cores, because the more powerful the converter, the more expensive it is. Therefore, I suggest you assemble a step-down DC-DC yourself, which will surpass the Chinese analogues in power and will also be more economical. According to my photo report and the presented diagram, it is clear that assembly will not take much time.
The LM2596 chip is nothing more than a switching step-down voltage regulator. It is available in both fixed voltage (3.3V, 5V, 12V) and adjustable voltage (ADJ). Our step-down DC-DC converter will be built on the basis of an adjustable microcircuit.
Converter circuit
Basic parameters of the LM2596 regulator
Input voltage………. up to +40V
Maximum input voltage………. +45V
Output voltage………. from 1.23V to 37V ±4%
Generator frequency………. 150kHz
Output current………. up to 3A
Current consumption in Standby mode………. 80uA
Operating temperature from -45°С to +150°С
Housing type TO-220 (5 pins) or TO-263 (5 pins)
Efficiency (at Vin= 12V, Vout= 3V Iout= 3A).......... 73%
Although the efficiency can reach 94%, it depends on the input and output voltage, as well as on the quality of the winding and the correct selection of the inductor inductance.
According to the graph taken from, with an input voltage of +30V, an output voltage of +20V and a load current of 3A, the efficiency should be 94%.
Also, the LM2596 chip has current and overheat protection. I note that on non-original microcircuits these functions may not work correctly or may be completely absent. A short circuit at the output of the converter leads to failure of the microcircuit (tested on two LMs), although there is nothing surprising here; the manufacturer does not write in the datasheet about the presence of short circuit protection.
Schematic elements
All element ratings are indicated on the electrical circuit diagram. The voltage of capacitors C1 and C2 is selected depending on the input and output voltage (input (output) voltage + margin of 25%), I installed the capacitors with a margin of 50V.
Capacitor C3 is ceramic. Its denomination is selected according to the table from the datasheet. According to this table, the capacitance C3 is selected for each individual output voltage, but since the converter in my case is adjustable, I used a capacitor of average capacity 1nF.
Diode VD1 must be a Schottky diode, or another ultra-fast diode (FR, UF, SF, etc.). It must be designed for a current of 5A and a voltage of at least 40V. I installed a pulse diode FR601 (6A 50V).
Choke L1 must be rated for a current of 5A and have an inductance of 68 μH. To do this, take a core made of powdered iron (yellow-white), outer diameter 27mm, inner 14mm, width 11mm, your dimensions may vary, but the larger they are, the better. Next, we wind two wires (the diameter of each wire is 1 mm) 28 turns. I wound a single core with a diameter of 1.4 mm, but with a high output power (40W), the inductor got very hot, also due to the insufficient cross-section of the core. If you wind two wires, then you won’t be able to put the winding in one layer, so you need to wind it in two layers, without insulation between the layers (if the enamel on the wire is not damaged).
A small current flows through resistor R1, so its power is 0.25W.
Resistor R2 is tuning, but can be replaced with a constant one; for this, its resistance is calculated for each output voltage according to the formula:
Where R1 = 1kOhm (according to the datasheet), Vref = 1.23V. Then, let's calculate the resistance of resistor R2 for the output voltage Vout = 30V.
R2 = 1 kOhm * (30V/1.23V - 1) = 23.39 kOhm (reducing to the standard value, we get resistance R2 = 22 kOhm).
Also, knowing the resistance of resistor R2, you can calculate the output voltage.
Testing a step-down DC-DC converter on LM2596
During testing, a radiator with an area of ≈ 90 cm² was installed on the chip.
I carried out tests on a load with a resistance of 6.8 Ohms (a constant resistor lowered into water). Initially, I applied a voltage of +27V to the converter input, the input current was 1.85A (input power 49.95W). I set the output voltage to 15.5V, the load current was 2.5A (output power 38.75W). The efficiency was 78%, which is very good.
After 20 min. During operation of the step-down converter, diode VD1 heated up to a temperature of 50°C, inductor L1 heated up to a temperature of 70°C, and the microcircuit itself heated up to 80°C. That is, all elements have a temperature reserve, except for the throttle, 70 degrees is too much for it.
Therefore, to operate this converter at an output power of 30-40W or more, it is necessary to wind the inductor with two (three) wires and select a larger core. The diode and microcircuit can maintain a temperature of 100-120°C for a long time without any fears (except for heating everything nearby, including the case). If desired, you can install a larger radiator on the microcircuit, and you can leave long leads on the VD1 diode, then heat will be dissipated better, or attach (solder to one of the leads) a small plate (radiator). You also need to tin the tracks of the printed circuit board as best as possible, or solder a copper core along them, this will ensure less heating of the tracks during long-term operation at a high output power.
Today we will look at several circuits of simple, one might even say simple, pulsed DC-DC voltage converters (converters of direct voltage of one value to constant voltage of another value)
What are the benefits of pulse converters? Firstly, they have high efficiency, and secondly, they can operate at an input voltage lower than the output voltage. Pulse converters are divided into groups:
- - bucking, boosting, inverting;
- - stabilized, unstabilized;
- - galvanically isolated, non-insulated;
- - with a narrow and wide range of input voltages.
To make homemade pulse converters, it is best to use specialized integrated circuits - they are easier to assemble and not capricious when setting up. So, here are 14 schemes for every taste:
This converter operates at a frequency of 50 kHz, galvanic isolation is provided by transformer T1, which is wound on a K10x6x4.5 ring made of 2000NM ferrite and contains: primary winding - 2x10 turns, secondary winding - 2x70 turns of PEV-0.2 wire. Transistors can be replaced with KT501B. Almost no current is consumed from the battery when there is no load.
Transformer T1 is wound on a ferrite ring with a diameter of 7 mm, and contains two windings of 25 turns of wire PEV = 0.3.
Push-pull unstabilized converter based on a multivibrator (VT1 and VT2) and a power amplifier (VT3 and VT4). The output voltage is selected by the number of turns of the secondary winding of the pulse transformer T1.
Stabilizing type converter based on the MAX631 microcircuit from MAXIM. Generation frequency 40…50 kHz, storage element - inductor L1.
You can use one of the two chips separately, for example the second one, to multiply the voltage from two batteries.
Typical circuit for connecting a pulse boost stabilizer on the MAX1674 microcircuit from MAXIM. Operation is maintained at an input voltage of 1.1 volts. Efficiency - 94%, load current - up to 200 mA.
Allows you to obtain two different stabilized voltages with an efficiency of 50...60% and a load current of up to 150 mA in each channel. Capacitors C2 and C3 are energy storage devices.
8. Switching boost stabilizer on the MAX1724EZK33 chip from MAXIM
Typical circuit diagram for connecting a specialized microcircuit from MAXIM. It remains operational at an input voltage of 0.91 volts, has a small-sized SMD housing and provides a load current of up to 150 mA with an efficiency of 90%.
A typical circuit for connecting a pulsed step-down stabilizer on a widely available TEXAS microcircuit. Resistor R3 regulates the output voltage within +2.8…+5 volts. Resistor R1 sets the short circuit current, which is calculated by the formula: Is(A)= 0.5/R1(Ohm)
Integrated voltage inverter, efficiency - 98%.
Two isolated voltage converters DA1 and DA2, connected in a “non-isolated” circuit with a common ground.
The inductance of the primary winding of transformer T1 is 22 μH, the ratio of turns of the primary winding to each secondary is 1: 2.5.
Typical circuit of a stabilized boost converter on a MAXIM microcircuit.
