Mc34063 drop inverted. Three heroes - pulse converters on the MC34063. Oscillograms of operation at various points in the inverter circuit

This calculator allows you to calculate the parameters of a pulsed DC-DC converter on the MC34063A. The calculator can calculate boost, step-down and inverting converters using the widely available microcircuit mc33063 (aka mc34063). The data of the frequency-setting capacitor, maximum current, coil inductance, and resistor resistance are displayed on the screen. Resistors are selected from the nearest standard values ​​so that the output voltage most closely matches the required value.

Ct- capacity of the frequency-setting capacitor of the converter.
Ipk- peak current through the inductance. The inductance must be designed for this current.
Rsc- a resistor that will turn off the microcircuit if the current is exceeded.
Lmin- minimum coil inductance. You cannot take less than this denomination.
Co- filter capacitor. The larger it is, the less ripple, it should be LOW ESR type.
R1, R2- a voltage divider that sets the output voltage.

The diode must be an ultrafast or Schottky diode with a permissible reverse voltage of at least 2 times the output.

IC supply voltage 3 - 40 volts, and the current Ipk should not exceed 1.5A

Some time ago I already published a review where I showed how to make a PWM stabilizer using KREN5. Then I mentioned one of the most common and probably the cheapest DC-DC converter controllers. Microcircuit MC34063.
Today I will try to complement the previous review.

In general, this microcircuit can be considered outdated, but nevertheless it enjoys well-deserved popularity. Mainly due to the low price. I still use them sometimes in my various crafts.
That’s actually why I decided to buy myself a hundred of these little things. They cost me 4 dollars, now from the same seller they cost 3.7 dollars per hundred, that’s only 3.7 cents apiece.
You can find them cheaper, but I ordered them as a kit with other parts (reviews of a charger for a lithium battery and a current stabilizer for a flashlight). There is also a fourth component, which I ordered there, but more on that another time.

Well, I’ve probably already bored you with the long introduction, so I’ll move on to the review.
Let me warn you right away, there will be a lot of photos.
It all came in bags, wrapped in bubble wrap. Such a bunch :)

The microcircuits themselves are neatly packed in a bag with a latch, and a piece of paper with the name is pasted onto it. It was written by hand, but I don’t think there will be any problems recognizing the inscription.

These microcircuits are produced by different manufacturers and are also labeled differently.
MC34063
KA34063
UCC34063
Etc.
As you can see, only the first letters change, the numbers remain unchanged, which is why it is usually called simply 34063.
I got the first ones, MC34063.

The photo is next to the same mikruha, but from a different manufacturer.
The one under review stands out with clearer markings.

I don’t know what else can be seen, so I’ll move on to the second part of the review, the educational one.
DC-DC converters are used in many places; now it is probably difficult to find an electronic device that does not have them.

There are three main conversion schemes, all of them are described in 34063, as well as in its application, and in one more.
All the described circuits do not have galvanic isolation. Also, if you look closely at all three circuits, you will notice that they are very similar and differ in the interchange of three components, the inductor, the diode and the power switch.

First, the most common one.
Step-down or step-down PWM converter.
It is used where it is necessary to reduce the voltage, and to do this with maximum efficiency.
The input voltage is always greater than the output voltage, usually at least 2-3 Volts; the greater the difference, the better (within reasonable limits).
In this case, the current at the input is less than at the output.
This circuit design is often used on motherboards, although the converters there are usually multi-phase and with synchronous rectification, but the essence remains the same, Step-Down.

In this circuit, the inductor accumulates energy when the key is open, and after the key is closed, the voltage across the inductor (due to self-induction) charges the output capacitor

The next scheme is used a little less frequently than the first.
It can often be found in Power-banks, where a battery voltage of 3-4.2 Volts produces a stabilized 5 Volts.
Using such a circuit, you can get more than 5 Volts, but it must be taken into account that the greater the voltage difference, the harder it is for the converter to work.
There is also one not very pleasant feature of this solution: the output cannot be disabled “software”. Those. The battery is always connected to the output via a diode. Also, in the case of a short circuit, the current will be limited only by the internal resistance of the load and battery.
To protect against this, either fuses or an additional power switch are used.

Just like last time, when the power switch is open, energy is first accumulated in the inductor; after the key is closed, the current in the inductor changes its polarity and, summed with the battery voltage, goes to the output through the diode.
The output voltage of such a circuit cannot be lower than the input voltage minus the diode drop.
The current at the input is greater than at the output (sometimes significantly).

The third scheme is used quite rarely, but it would be wrong not to consider it.
This circuit has an output voltage of opposite polarity than the input.
It's called an inverting converter.
In principle, this circuit can either increase or decrease the voltage relative to the input, but due to the peculiarities of the circuit design, it is often used only for voltages greater than or equal to the input.
The advantage of this circuit design is the ability to turn off the output voltage by closing the power switch. The first scheme can do this as well.
As in previous schemes, energy is accumulated in the inductor, and after closing the power switch it is supplied to the load through a reverse-connected diode.

When I conceived this review, I didn’t know what would be better to choose as an example.
There were options to make a step-down converter for PoE or a step-up converter to power an LED, but somehow all this was uninteresting and completely boring.
But a few days ago a friend called and asked me to help him solve a problem.
It was necessary to obtain a stabilized output voltage regardless of whether the input was greater or less than the output.
Those. I needed a buck-boost converter.
The topology of these converters is called (Single-ended primary-inductor converter).
A couple more good documents on this topology. , .
The circuit of this type of converter is noticeably more complex and contains an additional capacitor and inductor.

This is how I decided to do it

For example, I decided to make a converter capable of producing stabilized 12 Volts when the input fluctuates from 9 to 16 Volts. True, the power of the converter is small, since the built-in key of the microcircuit is used, but the solution is quite workable.
If you make the circuit more powerful, install an additional field-effect transistor, chokes for higher current, etc. then such a circuit can help solve the problem of powering a 3.5-inch hard drive in a car.
Also, such converters can help solve the problem of obtaining, which has already become popular, a voltage of 3.3 Volts from one lithium battery in the range of 3-4.2 Volts.

But first, let's turn the conditional diagram into a principle one.

After that, we’ll turn it into a trace; we won’t sculpt everything on the circuit board.

Well, next I will skip the steps described in one of my tutorials, where I showed how to make a printed circuit board.
The result was a small board, the dimensions of the board were 28x22.5, the thickness after sealing the parts was 8mm.

I dug up all sorts of different parts around the house.
I had chokes in one of the reviews.
There are always resistors.
The capacitors were partially present and partially removed from various devices.
The 10 µF ceramic one was removed from an old hard drive (they are also found on monitor boards), the aluminum SMD one was taken from an old CD-ROM.

I soldered the scarf and it turned out neat. I should have taken a photo on some matchbox, but I forgot. The dimensions of the board are approximately 2.5 times smaller than a matchbox.

The board is closer, I tried to arrange the board more tightly, there is not a lot of free space.
A 0.25 Ohm resistor is formed into four 1 Ohm resistors in parallel on 2 levels.

There are a lot of photos, so I put them under a spoiler

I checked in four ranges, but by chance it turned out to be in five, I didn’t resist this, but simply took another photo.
I didn’t have a 13K resistor, I had to solder it to 12, so the output voltage is somewhat underestimated.
But since I made the board simply to test the microcircuit (that is, this board itself no longer has any value for me) and write a review, I didn’t bother.
The load was an incandescent lamp, the load current was about 225mA

Input 9 Volts, output 11.45

The input is 11 Volts, the output is 11.44.

The input is 13 volts, the output is still the same 11.44

The input is 15 Volts, the output is again 11.44. :)

After that I thought about finishing it, but since the diagram indicated a range of up to 16 Volts, I decided to check at 16.
At the entrance 16.28, at the exit 11.44

Since I got hold of a digital oscilloscope, I decided to take oscillograms.

I also hid them under the spoiler, since there are quite a lot of them

This is of course a toy, the power of the converter is ridiculous, although useful.
But I picked up a few more for a friend on Aliexpress.
Perhaps it will be useful for someone.

This opus will be about 3 heroes. Why heroes?))) Since ancient times, heroes are the defenders of the Motherland, people who “stole”, that is, saved, and not, as now, “stole”, wealth.. Our drives are pulse converters, 3 types (step-down, step-up, inverter ). Moreover, all three are on one MC34063 chip and on one type of DO5022 coil with an inductance of 150 μH. They are used as part of a microwave signal switch using pin diodes, the circuit and board of which are given at the end of this article.

Calculation of a DC-DC step-down converter (step-down, buck) on the MC34063 chip

The calculation is carried out using the standard “AN920/D” method from ON Semiconductor. The electrical circuit diagram of the converter is shown in Figure 1. The numbers of the circuit elements correspond to the latest version of the circuit (from the file “Driver of MC34063 3in1 – ver 08.SCH”).

Fig. 1 Electrical circuit diagram of a step-down driver.

IC outputs:

Conclusion 1 - SWC(switch collector) - output transistor collector

Conclusion 2 - S.W.E.(switch emitter) - emitter of the output transistor

Conclusion 3 - TS(timing capacitor) - input for connecting a timing capacitor

Conclusion 4 - GND– ground (connects to the common wire of the step-down DC-DC)

Conclusion 5 - CII(FB) (comparator inverting input) - inverting input of the comparator

Conclusion 6 - VCC- nutrition

Conclusion 7 - Ipk— input of the maximum current limiting circuit

Conclusion 8 - DRC(driver collector) - the collector of the output transistor driver (a bipolar transistor connected according to a Darlington circuit located inside the microcircuit is also used as an output transistor driver).

Elements:

L 3- throttle. Better use the throttle open type(not completely closed with ferrite) - DO5022T series from Coilkraft or RLB from Bourns, since such a choke enters saturation at a higher current than the common closed-type chokes CDRH Sumida. It is better to use chokes with higher inductance than the calculated value obtained.

From 11- timing capacitor, it determines the conversion frequency. The maximum conversion frequency for 34063 chips is about 100 kHz.

R 24, R 21— voltage divider for the comparator circuit. The non-inverting input of the comparator is supplied with a voltage of 1.25V from the internal regulator, and the inverting input is supplied from the voltage divider. When the voltage from the divider becomes equal to the voltage from the internal regulator, the comparator switches the output transistor.

C 2, C 5, C 8 and C 17, C 18— output and input filters, respectively. The output filter capacitance determines the amount of output voltage ripple. If in the process of calculations it turns out that for given value ripple requires a very large capacitance, you can do the calculation for large ripples, and then use an additional LC filter. The input capacitance is usually taken 100 ... 470 μF (TI recommendation is at least 470 μF), the output capacitance is also taken 100 ... 470 μF (taken 220 μF).

R 11-12-13 (Rsc)- current-sensing resistor. It is needed for the current limiting circuit. Maximum output transistor current for MC34063 = 1.5A, for AP34063 = 1.6A. If the peak switching current exceeds these values, the microcircuit may burn out. If it is known for sure that the peak current does not even come close to the maximum values, then this resistor can not be installed. The calculation is carried out specifically for the peak current (of the internal transistor). When using an external transistor, the peak current flows through it, while a smaller (control) current flows through the internal transistor.

VT 4 – an external bipolar transistor is placed in the circuit when the calculated peak current exceeds 1.5A (at a large output current). Otherwise, overheating of the microcircuit can lead to its failure. Operating mode (transistor base current) R 26 , R 28 .

VD 2 – Schottky diode or ultrafast diode for voltage (forward and reverse) of at least 2U output

Calculation procedure:

• Select rated input and output voltages: V in, V out and maximum

output current I out.

In our scheme V in =24V, V out =5V, I out =500mA(maximum 750 mA)

• Select the minimum input voltage V in(min) and minimum operating frequency fmin with selected V in And I out.

In our scheme V in(min) =20V (according to technical specifications), choose f min =50 kHz

3) Calculate the value (t on +t off) max according to the formula (t on +t off) max =1/f min, t on(max)- maximum time when the output transistor is open, toff(max)— maximum time when the output transistor is closed.

(t on +t off) max =1/f min =1/50kHz=0.02 MS=20 μS

Calculate ratio t on/t off according to the formula t on /t off =(V out +V F)/(V in(min) -V sat -V out), Where V F- voltage drop across the diode (forward - forward voltage drop), V sat- the voltage drop across the output transistor when it is in a fully open state (saturation - saturation voltage) at a given current. V sat determined from the graphs or tables given in the documentation. From the formula it is clear that the more V in, V out and the more they differ from each other, the less influence they have on the final result V F And V sat.

(t on /t off) max =(V out +V F)/(V in(min) -V sat -V out)=(5+0.8)/(20-0.8-5)=5.8/14.2=0.408

4) Knowing t on/t off And (t on +t off) max solve the system of equations and find t on(max).

t off = (t on +t off) max / ((t on /t off) max +1) =20μS/(0.408+1)=14.2 μS

t on (max) =20- t off=20-14.2 µS=5.8 µS

5) Find the capacitance of the timing capacitor From 11 (Ct) according to the formula:

C 11 = 4.5*10 -5 *t on(max).

C 11 = 4.5*10 -5 * t on (max) =4.5*10 - 5*5.8 µS=261pF(this is the min value), take 680pF

The smaller the capacitance, the higher the frequency. Capacitance 680pF corresponds to frequency 14KHz

6) Find the peak current through the output transistor: I PK(switch) =2*I out. If it turns out to be greater than the maximum current of the output transistor (1.5 ... 1.6 A), then a converter with such parameters is impossible. It is necessary to either recalculate the circuit for a lower output current ( I out), or use a circuit with an external transistor.

I PK(switch) =2*I out =2*0.5=1A(for maximum output current 750mA I PK(switch) = 1.4A)

7) Calculate Rsc according to the formula: R sc =0.3/I PK(switch).

R sc =0.3/I PK(switch) =0.3/1=0.3 Ohm, We connect 3 resistors in parallel ( R 11-12-13) 1 ohm

8) Calculate the minimum capacitance of the output filter capacitor: C 17 =I PK(switch) *(t on +t off) max /8V ripple(p-p), Where V ripple(p-p)— maximum value of output voltage ripple. The maximum capacity is taken from the standard values ​​closest to the calculated one.

From 17 =I PK (switch) *(t on+ t off) max/8 V ripple (p — p) =1*14.2 µS/8*50 mV=50 µF, take 220 µF

9) Calculate the minimum inductance of the inductor:

L 1(min) = t on (max) *(V in (min) — V sat— V out)/ I PK (switch) . If C 17 and L 1 are too large, you can try to increase the conversion frequency and repeat the calculation. The higher the conversion frequency, the lower the minimum capacitance of the output capacitor and the minimum inductance of the inductor.

L 1(min) =t on(max) *(V in(min) -V sat -V out)/I PK(switch) =5.8μS *(20-0.8-5)/1=82.3 µH

This is the minimum inductance. For the MC34063 microcircuit, the inductor should be selected with a deliberately larger inductance value than the calculated value. We choose L=150μH from CoilKraft DO5022.

10) Divider resistances are calculated from the ratio V out =1.25*(1+R 24 /R 21). These resistors must be at least 30 ohms.

For V out = 5V we take R 24 = 3.6K, thenR 21 =1.2K

Online calculation http://uiut.org/master/mc34063/ shows the correctness of the calculated values ​​(except Ct=C11):

There is also another online calculation http://radiohlam.ru/teory/stepdown34063.htm, which also shows the correctness of the calculated values.

12) According to the calculation conditions in paragraph 7, the peak current of 1A (Max 1.4A) is near the maximum current of the transistor (1.5 ... 1.6 A). It is advisable to install an external transistor already at a peak current of 1A, in order to avoid overheating of the microcircuit. This is done. We select transistor VT4 MJD45 (PNP type) with a current transfer coefficient of 40 (it is advisable to take h21e as high as possible, since the transistor operates in saturation mode and the voltage drops across it is about = 0.8V). Some transistor manufacturers indicate in the datasheet title that the saturation voltage Usat is low, about 1V, which is what you should be guided by.

Let's calculate the resistance of resistors R26 and R28 in the circuits of the selected transistor VT4.

Base current of transistor VT4: I b= I PK (switch) / h 21 uh . I b=1/40=25mA

Resistor in the BE circuit: R 26 =10*h21e/ I PK (switch) . R 26 =10*40/1=400 Ohm (take R 26 =160 Ohm)

Current through resistor R 26: I RBE =V BE /R 26 =0.8/160=5mA

Resistor in the base circuit: R 28 =(Vin(min)-Vsat(driver)-V RSC -V BEQ 1)/(I B +I RBE)

R 28 =(20-0.8-0.1-0.8)/(25+5)=610 Ohm, you can take less than 160 Ohm (same as R 26, since the built-in Darlington transistor can provide more current for a smaller resistor.

13) Calculate the snubber elements R 32, C 16. (see the calculation of the boost circuit and the diagram below).

14) Let's calculate the elements of the output filter L 5 , R 37, C 24 (G. Ott “Methods for suppressing noise and interference in electronic systems” p.120-121).

I chose - coil L5 = 150 µH (same type choke with active resistive resistance Rdross = 0.25 ohm) and C24 = 47 µF (the circuit indicates a larger value of 100 µF)

Let's calculate the filter attenuation decrement xi =((R+Rdross)/2)* root(C/L)

R=R37 is set when the attenuation decrement is less than 0.6, in order to remove the overshoot of the relative frequency response of the filter (filter resonance). Otherwise, the filter at this cutoff frequency will amplify the oscillations rather than attenuate them.

Without R37: Ksi=0.25/2*(root 47/150)=0.07 - the frequency response will rise to +20dB, which is bad, so we set R=R37=2.2 Ohm, then:

C R37: Xi = (1+2.2)/2*(root 47/150) = 0.646 - with Xi 0.5 or more, the frequency response decreases (there is no resonance).

The resonant frequency of the filter (cutoff frequency) Fср=1/(2*pi*L*C) must lie below the conversion frequencies of the microcircuit (thus filtering these high frequencies 10-100 kHz). For the indicated values ​​of L and C, we obtain Faver = 1896 Hz, which is less than the operating frequency of the converter 10-100 kHz. Resistance R37 cannot be increased by more than a few Ohms, as the voltage across it will drop (with a load current of 500mA and R37=2.2 Ohms, the voltage drop will be Ur37=I*R=0.5*2.2=1.1V).

All circuit elements are selected for surface mounting

Oscillograms of operation at various points in the buck converter circuit:

15) a) Oscillograms without load ( Uin=24V, Uout=+5V):

Voltage +5V at the output of the converter (on capacitor C18) without load

The signal at the collector of transistor VT4 has a frequency of 30-40Hz, since without load, the circuit consumes about 4 mA without load

Control signals on pin 1 of the microcircuit (lower) and based on transistor VT4 (upper) without load

b) Oscillograms under load(Uin=24V, Uout=+5V), with frequency-setting capacitance c11=680pF. We change the load by decreasing the resistance of the resistor (3 oscillograms below). The output current of the stabilizer increases, as does the input.

Load - 3 68 ohm resistors in parallel ( 221 mA) Input current – ​​70mA

Yellow beam - transistor-based signal (control) Blue beam - signal at the collector of the transistor (output) Load - 5 68 ohm resistors in parallel ( 367 mA) Input current – ​​110mA

Yellow beam - transistor-based signal (control) Blue beam - signal at the collector of the transistor (output) Load - 1 resistor 10 ohm ( 500 mA) Input current – ​​150mA

Conclusion: depending on the load, the pulse repetition frequency changes, with a higher load the frequency increases, then the pauses (+5V) between the accumulation and release phases disappear, only rectangular pulses remain - the stabilizer works “at the limit” of its capabilities. This can also be seen in the oscillogram below, when the “saw” voltage has surges - the stabilizer enters current limiting mode.

c) Voltage at the frequency-setting capacitance c11=680pF at a maximum load of 500mA

Yellow beam - capacitance signal (control saw) Blue beam - signal at the collector of the transistor (output) Load - 1 resistor 10 ohm ( 500 mA) Input current – ​​150mA

d) Voltage ripple at the output of the stabilizer (c18) at a maximum load of 500 mA

Yellow beam - pulsation signal at the output (s18) Load - 1 resistor 10 ohm ( 500 mA)

Voltage ripple at the output of the LC(R) filter (c24) at a maximum load of 500 mA

Yellow beam - ripple signal at the output of the LC(R) filter (c24) Load - 1 resistor 10 ohm ( 500 mA)

Conclusion: the peak-to-peak ripple voltage range decreased from 300mV to 150mV.

e) Oscillogram of damped oscillations without a snubber:

Blue beam - on a diode without a snubber (insertion of a pulse over time is visible not equal to the period, since this is not PWM, but PFM)

Oscillogram of damped oscillations without snubber (enlarged):

Calculation of a step-up, boost DC-DC converter on the MC34063 chip

http://uiut.org/master/mc34063/. For the boost driver, it is basically the same as the buck driver calculation, so it can be trusted. During online calculation, the scheme automatically changes to the standard scheme from “AN920/D”. Input data, calculation results and the standard scheme itself are presented below.

— field-effect N-channel transistor VT7 IRFR220N. Increases the load capacity of the microcircuit and allows for quick switching. Selected by: The electrical circuit of the boost converter is shown in Figure 2. The numbers of circuit elements correspond to the latest version of the circuit (from the file “Driver of MC34063 3in1 – ver 08.SCH”). There are elements in the diagram that are not on standard scheme online calculation. These are the following elements:

• Maximum drain-source voltage V DSS =200V, because the output voltage is high +94V
• Low channel voltage drop RDS(on)max =0.6Om. The lower the channel resistance, the lower the heating losses and the higher the efficiency.
• Small capacitance (input), which determines the gate charge Qg (Total Gate Charge) and low gate input current. For a given transistor I=Qg*FSW=15nC*50 KHz=750uA.
• Maximum drain current Id=5A, since pulse current Ipk=812 mA at output current 100 mA

- voltage divider elements R30, R31 and R33 (reduces the voltage for the VT7 gate, which should be no more than V GS = 20V)

- discharge elements of the input capacitance VT7 - R34, VD3, VT6 when switching the transistor VT7 to the closed state. Reduces the decay time of the VT7 gate from 400nS (not shown) to 50nS (waveform with a decay time of 50nS). Log 0 on pin 2 of the microcircuit opens the PNP transistor VT6 and the input gate capacitance is discharged through the CE junction VT6 (faster than simply through resistor R33, R34).

— the coil L turns out to be very large when calculating, a lower nominal value L = L4 (Fig. 2) = 150 μH is selected

— snubber elements C21, R36.

Snubber calculation:

Hence L=1/(4*3.14^2*(1.2*10^6)^2*26*10^-12)=6.772*10^4 Rsn=√(6.772*10^4 /26*10^- 12)=5.1KOhm

The size of the snubber capacity is usually a compromise solution, since, on the one hand, the larger the capacity, the better the smoothing ( less number oscillations), on the other hand, each cycle the capacitance is recharged and dissipates part of the useful energy through the resistor, which affects the efficiency (usually, a normally designed snubber reduces the efficiency very slightly, within a couple of percent).

By installing a variable resistor, we determined the resistance more accurately R=1 K

Fig.2 Electrical circuit diagram of a step-up, boost driver.

Oscillograms of operation at various points in the boost converter circuit:

a) Voltage at various points without load:

Output voltage - 94V without load

Gate voltage without load

Drain voltage without load

b) voltage at the gate (yellow beam) and at the drain (blue beam) of transistor VT7:

on the gate and drain under load the frequency changes from 11 kHz (90 µs) to 20 kHz (50 µs) - this is not PWM, but PFM

on the gate and drain under load without a snubber (stretched - 1 oscillation period)

on gate and drain under load with snubber

c) leading and trailing edge voltage pin 2 (yellow beam) and on the gate (blue beam) VT7, saw pin 3:

blue - 450 ns rise time on VT7 gate

Yellow - rise time 50 ns per pin 2 chips blue - 50 ns rise time on VT7 gate

saw on Ct (pin 3 of IC) with control release F=11k

Calculation of DC-DC inverter (step-up/step-down, inverter) on the MC34063 chip

The calculation is also carried out using the standard “AN920/D” method from ON Semiconductor.

The calculation can be done immediately “online” http://uiut.org/master/mc34063/. For an inverting driver, it is basically the same as the calculation for a buck driver, so it can be trusted. During online calculation, the scheme automatically changes to the standard scheme from “AN920/D”. Input data, calculation results and the standard scheme itself are presented below.

— bipolar PNP transistor VT7 (increases load capacity) The electrical circuit of the inverting converter is shown in Figure 3. The numbers of circuit elements correspond to the latest version of the circuit (from the file “Driver of MC34063 3in1 – ver 08.SCH”). The scheme contains elements that are not included in the standard online calculation scheme. These are the following elements:

— voltage divider elements R27, R29 (sets the base current and operating mode of VT7),

— snubber elements C15, R35 (suppresses unwanted vibrations from the throttle)

Some components differ from those calculated:

• coil L is taken less than the calculated value L = L2 (Fig. 3) = 150 μH (all coils are of the same type)
• output capacitance is taken less than the calculated one C0=C19=220μF
• The frequency-setting capacitor is taken C13=680pF, corresponding to a frequency of 14KHz
• divider resistors R2=R22=3.6K, R1=R25=1.2K (taken first for output voltage -5V) and final resistors R2=R22=5.1K, R1=R25=1.2K (output voltage -6.5V)

The current limiting resistor is taken Rsc - 3 resistors in parallel, 1 Ohm each (resulting resistance 0.3 Ohm)

Fig.3 Electrical circuit diagram of the inverter (step-up/step-down, inverter).

Oscillograms of operation at various points of the inverter circuit:

a) with input voltage +24V without load:

output -6.5V without load

on the collector – accumulation and release of energy without load

on pin 1 and the base of the transistor without load

on the base and collector of the transistor without load

output ripple without load

When the developer of any device is faced with the question “How to get the required voltage?”, the answer is usually simple - a linear stabilizer. Their undoubted advantage is their low cost and minimal wiring. But besides these advantages, they have a drawback - strong heating. Linear stabilizers convert a lot of precious energy into heat. Therefore, the use of such stabilizers in battery-powered devices is not advisable. Are more economical DC-DC converters. That's what we'll talk about.

Back view:

Everything has already been said about the operating principles before me, so I won’t dwell on it. Let me just say that such converters come in Step-UP (step-up) and Step-Down (step-down) converters. Of course, I was interested in the latter. You can see what happened in the picture above. The converter circuits were carefully redrawn by me from the datasheet :-) Let's start with the Step-Down converter:

As you can see, nothing tricky. Resistors R3 and R2 form a divider from which the voltage is removed and supplied to the leg feedback microcircuits MC34063. Accordingly, by changing the values ​​of these resistors, you can change the voltage at the output of the converter. Resistor R1 serves to protect the microcircuit from failure in the event of a short circuit. If you solder a jumper instead, the protection will be disabled and the circuit may emit a magic smoke on which all electronics operate. :-) The greater the resistance of this resistor, the less current the converter can deliver. With its resistance of 0.3 ohms, the current will not exceed half an ampere. By the way, all these resistors can be calculated by mine. I took the choke ready-made, but no one forbids me to wind it myself. The main thing is that it has the required current. The diode is also any Schottky and also for the required current. As a last resort, you can parallel two low-power diodes. The capacitor voltages are not indicated on the diagram; they must be selected based on the input and output voltage. It's better to take it with double reserve.
The Step-UP converter has minor differences in its circuit:

Requirements for parts are the same as for Step-Down. As for the quality of the resulting output voltage, it is quite stable and the ripples are, as they say, small. (I can’t say about ripples myself since I don’t have an oscilloscope yet). Questions, suggestions in the comments.

The MC34063 is a fairly common type of microcontroller for building both low-to-high and high-to-low voltage converters. The features of the microcircuit lie in its technical characteristics and performance indicators. The device can handle loads well with a switching current of up to 1.5 A, which indicates a wide range of its use in various pulse converters with high practical characteristics.

Description of the chip

Voltage stabilization and conversion- This is an important function that is used in many devices. These are all kinds of regulated power supplies, conversion circuits and high-quality built-in power supplies. Most consumer electronics are designed specifically on this MS, because it has high performance characteristics and switches a fairly large current without problems.

The MC34063 has a built-in oscillator, so to operate the device and start converting voltage to different levels, it is enough to provide an initial bias by connecting a 470pF capacitor. This controller is very popular among a large number of radio amateurs. The chip works well in many circuits. And having a simple topology and simple technical device, you can easily understand the principle of its operation.

A typical connection circuit consists of the following components:

• 3 resistors;
• diode;
• 3 capacitors;
• inductance.

Considering the circuit for reducing voltage or stabilizing it, you can see that it is equipped with deep feedback and a fairly powerful output transistor, which passes voltage through itself in a direct current.

Switching circuit for voltage reduction and stabilization

It can be seen from the diagram that the current in the output transistor is limited by resistor R1, and the timing component for setting the required conversion frequency is capacitor C2. Inductance L1 accumulates energy when the transistor is open, and when it is closed, it is discharged through the diode to the output capacitor. The conversion coefficient depends on the ratio of the resistances of resistors R3 and R2.

The PWM stabilizer operates in pulse mode:

When a bipolar transistor turns on, the inductance gains energy, which then accumulates in the output capacitance. This cycle is repeated continuously, ensuring a stable output level. Provided that there is a voltage of 25V at the input of the microcircuit, at its output it will be 5V with a maximum output current of up to 500mA.

Voltage can be increased by changing the type of resistance ratio in the feedback circuit connected to the input. It is also used as a discharge diode during the action of the back EMF accumulated in the coil at the time of its charging with the transistor open.

Using this scheme in practice, it is possible to produce highly efficient buck converter. In this case, the microcircuit does not consume excess power, which is released when the voltage drops to 5 or 3.3 V. The diode is designed to provide reverse discharge of the inductance to the output capacitor.

Pulse reduction mode voltage allows you to significantly save battery power when connecting low-power devices. For example, when using a conventional parametric stabilizer, heating it during operation required at least 50% of the power. What then can we say if an output voltage of 3.3 V is required? Such a step-down source with a load of 1 W will consume all 4 W, which is important when developing high-quality and reliable devices.

As the practice of using MC34063 shows, the average power loss is reduced to at least 13%, which became the most important incentive for its practical implementation to power all low-voltage consumers. And taking into account the pulse-width control principle, the microcircuit will heat up insignificantly. Therefore, no radiators are required to cool it. The average efficiency of such a conversion circuit is at least 87%.

Voltage regulation at the output of the microcircuit is carried out due to a resistive divider. When it exceeds the nominal value by 1.25V, the comporator switches the trigger and closes the transistor. This description describes a voltage reduction circuit with an output level of 5V. To change it, increase or decrease it, you will need to change the parameters of the input divider.

An input resistor is used to limit the current of the switching switch. Calculated as the ratio of the input voltage to the resistance of resistor R1. To organize an adjustable voltage stabilizer, the middle point of a variable resistor is connected to pin 5 of the microcircuit. One output is to the common wire, and the second is to the power supply. The conversion system operates in a frequency band of 100 kHz; if the inductance changes, it can be changed. As the inductance decreases, the conversion frequency increases.

Other operating modes

In addition to the reduction and stabilization operating modes, boost modes are also quite often used. differs in that the inductance is not at the output. Current flows through it into the load when the key is closed, which, when unlocked, supplies a negative voltage to the lower terminal of the inductance.

The diode, in turn, provides inductance discharge to the load in one direction. Therefore, when the switch is open, 12 V from the power source and the maximum current are generated at the load, and when it is closed at the output capacitor, it rises to 28 V. The efficiency of the boost circuit is at least 83%. Circuit feature when operating in this mode, the output transistor switches on smoothly, which is ensured by limiting the base current through an additional resistor connected to pin 8 of the MS. The clock frequency of the converter is set by a small capacitor, mainly 470 pF, while it is 100 kHz.

The output voltage is determined by the following formula:

Uout=1.25*R3 *(R2+R3)

Using the above circuit for connecting the MC34063A microcircuit, you can make a step-up voltage converter powered from USB to 9, 12 or more volts, depending on the parameters of resistor R3. To carry out a detailed calculation of the characteristics of the device, you can use a special calculator. If R2 is 2.4k ohms and R3 is 15k ohms, then the circuit will convert 5V to 12V.

MC34063A voltage boost circuit with external transistor

The presented circuit uses a field-effect transistor. But there was a mistake in it. On the bipolar transistor it is necessary to change in some places K-E. Below is a diagram from the description. The external transistor is selected based on the switching current and output power.

Quite often, to power LED light sources, this particular microcircuit is used to build a step-down or step-up converter. High efficiency, low consumption and high stability of the output voltage are the main advantages of the circuit implementation. There are many LED driver circuits with different features.

As one of the many examples practical application You can consider the following diagram below.

The scheme works as follows:

When a control signal is applied, the internal trigger of the MS is blocked and the transistor is closed. And the charging current of the field-effect transistor flows through the diode. When the control pulse is removed, the trigger goes into the second state and opens the transistor, which leads to the discharge of gate VT2. This connection of two transistors Provides quick on and off VT1, which reduces the likelihood of heating due to the almost complete absence of a variable component. To calculate the current flowing through the LEDs, you can use: I=1.25V/R2.

Charger for MC34063

The MC34063 controller is universal. In addition to power supplies, it can be used to design a charger for phones with an output voltage of 5V. Below is a diagram of the device implementation. Her principle of operation is explained as in the case of a regular downward conversion. The output battery charging current is up to 1A with a margin of 30%. To increase it, you need to use an external transistor, for example, KT817 or any other.