High Power High Efficiency Buck Converter Circuit using TL494

A buck converter circuit (step-down converter) is a DC-to-DC switching converter that steps down voltage while maintaining a constant power balance. The main feature of a buck converter is efficiency, which means that with a buck converter on board, we can expect extended battery life, reduced heat, smaller size, and improved efficiency. We previously made a few simple Buck converter circuits and explained their basics and design efficiency.

So, in this article, we are going to design, calculate and test a high-efficiency buck converter circuit based on the popular TL494 IC, and at last, there will be a detailed video showing the working & testing part of the circuit. So without further ado, let's get started. This comprehensive guide covers the complete buck converter circuit diagram with explanation, calculations, and real-world testing results, achieving over 70W output power.

What is a Buck Converter and How Does It Work?

A buck converter circuit (step-down converter) is a DC-to-DC switching power supply that reduces input voltage while maintaining constant power balance.

Buck Converter Block Diagram Components

A buck converter usually comprises these elements:

Switching Element: Will perform a switching operation between ON and OFF as per the control signal.
Inductor (L1): Stores energy in its magnetic field and returns it as required. 
Freewheeling Diode (D1): Allows continuous current to flow when the switch is turned OFF. 
Output Capacitor (C1): Works to reduce and smooth the output voltage ripple. 
Control Circuit: Works to regulate the switching frequency and the duty cycle.

The above figure shows a very basic buck converter circuit. 

Buck Converter Working Principle

To know how a buck converter works. The buck converter working mechanism operates through two distinct phases: The first condition is when the transistor is ON; the next condition is when the transistor is OFF. 

Transistor On state

In this scenario, we can see that the diode is in open open-circuit condition because it's in the reverse-biased state. In this situation, some initial current will start flow through the load, but the current is restricted by the inductor, thus the inductor also starts to charge up gradually. Therefore, during the on-time of the circuit, the capacitor builds up the charge cycle by cycle, and this voltage reflects across the load.

Transistor Off state

When the transistor is in an off state, the energy stored in the inductor L1 collapses and flows back through the diode D1, as shown in the circuit with the arrows. In this situation, the voltage across the inductor is in reverse polarity, and so the diode is in forward-bias condition. Now, due to the collapsing magnetic field of the inductor, the current continues to flow through the load until the inductor runs out of charge. All this happens while the transistor is in the off condition.

After a certain period, when the inductor is almost out of stored energy, the load voltage starts to fall again. In this situation, the capacitor C1 becomes the main source of current; the capacitor is there to keep the current flowing until the next cycle begins again.

Now, by varying the switching frequency and switching time, we can get any output from 0 to Vin from a buck converter.

 

IC TL494

Now, before building a TL494 buck converter, let's learn how the PWM controller TL494 works.

The TL494 IC has 8 functional blocks, which are shown and described below.

 

TL494 IC Functional Block Analysis

The TL494 serves as the heart of this TL494 buck converter, integrating eight critical functional blocks:

Function Block	Pin Numbers	Purpose	Key Specifications
5V Reference	Pin 14	Stable voltage reference	±5% accuracy, 7-40V input
Oscillator	Pins 5,6	PWM timing generation	f = 1/(RT × CT)
Error Amplifiers	Pins 1,2,15,16	Feedback control	-0.3V to VI-2V common mode
Dead Time Control	Pin 4	Minimum off-time	3% to 100% range
Output Control	Pin 13	Push-pull/parallel mode	Ground = parallel, REF = push-pull
Output Transistors	Pins 8,11	Drive switching elements	200mA sink/source capability

1. 5-V Reference Regulator

The 5V internal reference regulator output is the REF pin, which is pin 14 of the IC. The reference regulator is there to provide a stable supply for internal circuitry like the pulse-steering flip-flop, oscillator, dead-time control comparator, and PWM comparator. The regulator is also used to drive the error amplifiers, which are responsible for controlling the output.

Note! The reference is internally programmed to an initial accuracy of ±5% and maintains stability over an input voltage range of 7V to 40V. For input voltages less than 7V, the regulator saturates within 1V of the input and tracks it. 

2. Oscillator

The oscillator generates and provides a sawtooth wave to the dead time controller and the PWM comparators for various control signals.

The frequency of the oscillator can be set by selecting timing components RT and CT.

The frequency of the oscillator can be calculated by the formula below

Fosc = 1/(RT * CT )

For simplicity, I have made a spreadsheet, by which you can calculate the frequency very easily.

Note! The oscillator frequency is equal to the output frequency only for single-ended applications. For push-pull applications, the output frequency is one-half of the oscillator frequency.

3. Dead-time Control Comparator

The dead time, or simply to say off-time control, provides the minimum dead time or off-time. The output of the dead time comparator blocks switches transistors when the voltage at the input is greater than the ramp voltage of the oscillator. Applying a voltage to the DTC pin can impose additional dead time, thus providing additional dead time from its minimum of 3% to 100% as the input voltage varies from 0 to 3V. In simple terms, we can change the Duty cycle of the output wave without tweaking the error amplifiers.

Note! An internal offset of 110 mV ensures a minimum dead time of 3% with the dead-time control input grounded.

4. Error Amplifiers 

Both high-gain error amplifiers receive their bias from the VI supply rail. This permits a common-mode input voltage range from –0.3 V to 2 V less than VI. Both amplifiers behave characteristically of a single-ended single-supply amplifier, in that each output is active high only.

5. Output-Control Input 

The output-control input determines whether the output transistors operate in parallel or push-pull mode. Connecting the output control pin, which is pin-13, to ground sets the output transistors in parallel operation mode. But connecting this pin to the 5V-REF pin sets the output transistors in push-pull mode.

6. Output Transistors

The IC has two internal output transistors, which are in open-collector and open-emitter configurations, by which it can source or sink a maximum current of 200mA.

Note! The transistors have a saturation voltage of less than 1.3 V in the common-emitter configuration and less than 2.5 V in the emitter-follower configuration. 

TL494 Key Features and Specifications

• Complete PWM Power-Control Circuitry
• Uncommitted Outputs for 200-mA Sink or Source Current
• Output Control Selects Single-Ended or Push-Pull Operation
• Internal Circuitry Prohibits Double Pulse at Either Output
• Variable Dead Time Provides Control Over Total Range
• Internal Regulator Provides a Stable 5-V
• Reference Supply With 5% Tolerance
• Circuit Architecture Allows Easy Synchronisation 

Note! Most of the internal schematic and operations description is taken from the datasheet and modified to some extent for better understanding. 

Components Required

1. TL494 IC - 1
2. TIP2955 Transistor - 1
3. Screw Terminal 5mmx2 - 2
4. 1000uF,60V Capacitor - 1
5. 470uF,60V Capacitor - 1
6. 50K,1% Resistor - 1
7. 560R Resistor - 1
8. 10K,1% Resistor - 4
9. 3.3K,1% Resistor - 2
10. 330R Resistor - 1
11. 0.22uF Capacitor - 1
12. 5.6K,1W Resistor - 1
13. 12.1V Zener Diode - 1
14. MBR20100CT Schottky Diode - 1
15. 70uH (27 x 11 x 14 ) mm Inductor - 1
16. Potentiometer (10K)   Trim-Pot - 1
17. 0.22R Current Sense Resistor - 2
18. Clad Board Generic 50x 50mm - 1
19. PSU Heat Sink Generic - 1
20. Jumper Wires Generic - 15

Buck Converter Schematic Diagram

The buck converter schematic diagram for the High-Efficiency Buck Converter is given below. The buck converter schematic diagram incorporates several critical design elements for optimal performance:

Circuit Construction and PCB Design

For this demonstration of this high current buck converter, the circuit is constructed in handmade PCB, with the help of the schematic and PCB design files [Gerber file]; please note that if you are connecting a big load to the output buck converter then a huge amount of current will flow through the PCB traces, and there's a chance that the traces will burn out. So, to prevent the PCB traces from burning out, I have included some jumpers which help to increase the current flow. Also, I have reinforced the PCB traces with a thick layer of solder to lower the trace resistance.

The inductor is constructed with 3 strands of parallel 0.45 sq mm enamelled copper wire.

Buck Converter Design Calculations

To properly calculate the values of the inductor and capacitor, I have used a document from Texas Instruments.

After that, I have made a Google spreadsheet to make the calculation easier

Testing High Voltage Buck Regulator

To test the circuit, the following setup is used. As shown in the above image, the input voltage is 41.17 V and the no-load current is .015 A, which makes the no-load power draw less than 0.6W.

Before any of you jump and say what a bowl of the resistor is doing on my testing table.

Let me tell you, the resistors get very, very hot during the time of testing the circuit with full load conditions, so I have prepared a bowl of water to prevent my working table from burning

Tools used to test the circuit

1. 12V lead-acid battery.
2. A transformer which has a 6-0-6 tap and a 12-0-12 tap
3. 5 10W 10r Resistance in parallel as a load
4. Meco 108B+TRMS Multimeter
5. Meco 450B+TRMS Multimeter
6. Hantek 6022BE Oscilloscope

Input Power for High Power Buck Converter

As you can see from the above image, the input voltage drops to 27.45V in a load condition, and the input current is 3.022 A, which is equal to an input power of 82.9539 W.

Output Power

As you can see from the above image, the output voltage is 12.78V and the output current draw of 5.614A, which is equivalent to a power draw of 71.6958 W.  

So the efficiency of the circuit becomes (71.6958 / 82.9539) x 100 % = 86.42 %

The loss in the circuit is due to the resistors for powering the TL494 IC and

Absolute maximum current draw in my testing table

From the above image, it can be seen that the maximum current drawn from the circuit is 6.96 A, it is almost

In this situation, the main bottleneck of the system is my transformer, which is why I cannot increase the load current, but with this design and with a good heat sink, you can easily draw more than 10A of current from this circuit.

Note! Any of you wondering why I have attached a massive heat sink to the circuit, let me tell you that at the moment I do not have any smaller heat sink in my stockpile.

Further Enhancements

This TL494 buck converter circuit is for demonstration purposes only; hence, there is no protection circuit added in the output section of the circuit

1. An output protection circuit must be added to protect the load circuit.
2. The inductor needs to be dipped into varnish; otherwise, it will generate audible noise.
3. A good quality PCB with a proper design is mandatory
4. The switching transistor can be modified to increase the load current

FAQ: Buck Converter Circuit

⇥ What's the biggest benefit of using a buck converter over a linear regulator?
Buck converters will function at 85-95% efficiency compared to the 30-60% efficiency of linear regulators. Therefore, buck converters produce less heat, which allows for improved battery life and heatsinks to be smaller and thus buck converters are an excellent choice for applications where efficient voltage regulation could be critical while consuming high levels of power.

⇥ What is the source of output voltage ripple in buck converters?
Output voltage ripple in the buck converter is primarily due to inductor current variation and the equivalent series resistance (ESR) of the output capacitors. You can reduce ripple by increasing switching frequency, using a greater inductance factor or using additional low ESR-type capacitors. Output voltage ripple should typically be <1% of the output voltage.

⇥ What is the best method to determine switching frequency in a buck converter?
You will need to evaluate and balance the component size against efficiency. Higher than 100kHz will typically minimise inductor and capacitor sizes, although switching losses can increase. Frequencies lower than 20kHz will lead to the best efficiencies for the converter in your application, but can cause larger component sizes. For most applications, 20-100kHz is a set number.

⇥ What is the function of dead time control in TL494-based buck converters?
Dead time control is used to eliminate shoot-through when the switching transistor is in an on-off state. The TL494 dead time control circuit has 3 to 100% dead time with dead time control to ensure that TL494 achieves the full extent of the switching cycle to avoid damage in push-pull configurations.

⇥ How do you obtain a constant current output in a buck converter?
Include current sense resistors in series with the output and input this signal to TL494's error amplifier. If the current goes above the set point, the control loop lowers the duty cycle so that constant current is maintained despite load changes.

I hope you liked this article and learned something new from it. If you have any doubts, you can ask in the comments below or use our forum for a detailed discussion.

Practical Projects and Circuits Built Around TL494

The TL494 has been a key part of many of our projects. We’ve applied the TL494 in different practical circuits, and you can explore those projects through the links provided.