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  • Contents

I Description

This blog uses the universal integrated chip TL494 to convert analog signals into PWM (pulse width modulation) signals. In the output part, N-channel MOSFET and P-channel MOSFET are used to form a switching power amplifier.

tl434

Catalog

I Description

II Introduction

2.1 Switching Power Amplifier Overview

2.2 TL494 Introduction

III Scheme Design

3.1 Duty Cycle Adjustment Circuit

3.2 Input Signal Compression Circuit

3.3 MOSFET Drive Circuit

3.4 Working Principle of Output Part

IV Experimental Results

V Conclusion

FAQ

Ordering & Quantity

 

II Introduction

2.1 Switching Power Amplifier Overview

With the rapid development of high-speed power MOSFET production technology, the operating frequency of MOSFET is getting higher and higher, the driving method is getting safer and the price is getting lower.

Therefore, a large number of switching power amplifiers applied to various household appliances and industrial alarms have appeared on the market in recent years.

Compared with the linear power amplifier, although the circuit of the switching power amplifier is slightly more complicated. But it is very efficient and can reduce the size of the heat sink, even without using the heat sink. Therefore, the volume of the product can be greatly reduced.

2.2 TL494 Introduction

TL494 is a switching power supply pulse width modulation (PWM) control chip. For many years, as the cheapest double-ended PWM chip, TL494 has been widely used in double-ended topologies such as push-pull and half-bridge. Because of its lower operating frequency and single-ended output port characteristics. It is often used with power bipolar transistors (BJT). If used with power MOSFET, an external circuit is required. TL494 works in a wide voltage range from 7V to 40V, with a maximum operating frequency of 200kHz, with the internal sawtooth generator, PWM generator, and lag time adjustment functions.

III Scheme Design

Figure 1 is a block diagram of a TL494-based switching power amplifier. The key to the circuit design is the duty cycle adjustment circuit, input signal compression circuit, and MOSFET drive circuit.

tl494 switching power amplifierFigure 1.  TL494 Switching Power Amplifier

3.1 Duty Cycle Adjustment Circuit

The duty cycle is the key to improving voltage utilization during PWM signal modulation. Because TL494 is an integrated chip for switching power supply. Therefore, the minimum lag time is set to 0.1V internally. The maximum duty cycle is approximately 96% at the output of the transmitter stage. Figure 2 shows the input part and part of the circuit for PWM signal modulation.

signal input section and pwm generator

Figure 2. Signal Input section and PWM Generator

In Figure 2, when C4=1000pF and R4=24k, the operating frequency is about 78kHz.

If there is no duty cycle adjustment circuit D8, D17, R23, because the comparison point of the internal delay time comparator is 0.1V. So the minimum on-time is about 1.52μs, and the minimum duty cycle is D=1.52/13≈12%. Therefore, the voltage utilization rate will decrease during PWM.

If D8, D17, and R23 are used, a 0.82V bias voltage will be generated at the point E of the capacitor C4 for the sawtooth wave generation, and the starting point of the sawtooth wave will be increased from 0V to 0.82V. Therefore, the on-time is reduced to 0.64μs, and the minimum duty cycle is reduced to D=0.64/13≈4.9%. This can significantly improve the voltage utilization.

Figure 3 is the output waveform when there is no duty cycle adjustment circuit.

Figure 4 is the output waveform when there is a duty cycle adjustment circuit.

output waveform without duty cycle adjustment circuit

Figure 3. Output Waveform without Duty Cycle Adjustment Circuit

output waveform with duty cycle adjustment circuitFigure 4. Output Waveform with Duty Cycle Adjustment Circuit

3.2 Input Signal Compression Circuit

Because the input signal of the alarm has a large variation range, it is necessary to compress the signal with a large amplitude according to a certain ratio. In Figure 2, R6, R16, D10, D11 constitute the input signal compression circuit, and its key is to use the input characteristics of the diode. Figure 5 shows its input characteristics. Among them, D10 and D11 are connected in parallel to compress signals in both positive and negative directions.

output characteristics of input signal compression circuitFigure 5. Output Characteristics of Input Signal Compression Circuit

The compression ratio depends on the values of R6 and R16. The larger the value, the larger the compression ratio. By adjusting the values of R6 and R16, the change range of the compressed signal is set to -0.82V~0.82V. The amount of change is 1.64V. We can see Figure 4, the sawtooth voltage variation range is 0.82V~3.25V. So the output signal variation range of the TL494 internal error amplifier is 2.43V.

The gain of the internal error amplifier depends on R7 and R20. By adjusting their values, when the amount of change of the compressed signal is 1.64V, the output signal change range of the internal error amplifier can be set to 2.43V.

Since most alarms use tweeters, the bass with a large amplitude can be greatly reduced.

3.3 MOSFET Drive Circuit

P-channel MOSFET uses IRF9540. It has the characteristics of the maximum operating voltage of 100V, the maximum operating current of 18A, and saturation when VGS voltage is 5V~15V.

N-channel MOSFET uses IRF540. It has the characteristics of the maximum operating voltage of 100V, the maximum operating current of 27A, and saturation when VGS voltage is 5V~15V.

The driving transistor Q3 adopts NPN type C8050, and Q7 adopts PNP type C8550. Both of these two drive transistors have the characteristics of a maximum operating voltage of 30V, a maximum operating current of 1A, and a VBE of 12V.

Figure 6 shows the MOSFET drive circuit.

MOSFET drive circuit

Figure 6. MOSFET Drive Circuit

Figure 7 shows the MOSFET driving principle waveform.

  • When the pulse voltage at point A is low, the current flows through the reverse bias of the Zener diode D7 and the transistor Q3 to form a VGS voltage, and QH is turned on.
  • When the pulse voltage at point A is high, the current flows through the reverse bias of the Zener diode D9 and the transistor Q7 to form a VGS voltage, and QL is turned on.

In addition, Figure 7 also shows detailed driving waveforms.

  • l When the pulse voltage is low, the voltage is lower than VL to make QH turn on.
  • l When the pulse voltage is high, its voltage is higher than VH to make QL turn on.

It takes a certain amount of time to change from VL to VH. At this time, QH and QL will be cut off at the same time. Therefore, the pulse change process is very safe.

MOSFET driving principle waveformFigure 7. MOSFET Driving Principle Waveform

The VGS of QH and QL is determined by the following formula:

Where:

  • VGS is the driving voltage of MOSFET;
  • VC is the power supply voltage;
  • VD is the regulated voltage of Zener tubes D7 and D9 (usually the same Zener tube is used);
  • VBE is the counter breakdown voltage of C8050 and C8550.

Figure 8 is the measured drive waveform. When the pulse voltage changes from low to high, the time for QH and QL to cut off at the same time is about 100~300ns.

measured drive waveform

Figure 8. Measured Drive Waveform

3.4 Working Principle of Output Part

As shown in Figure 6, the output part consists of QH, QL and L3, C8, C5, and C7. The output voltage is transmitted to the load after filtering high frequency waves through L3 and C8. Generally, an electrolytic capacitor is used at the output end, but this circuit uses C5 and C7 to form a half bridge, and then connect the midpoint to the load. The advantage of this connection method is that the two capacitors are not only the transmission path of the output signal (the capacitance value is the parallel value of the two capacitors), but also has a filtering effect on the power supply (the capacitance value is the series value of the two capacitors at this time), and reduce the internal pressure of the capacitor by half.

IV Experimental Results

Table 1 shows the quiescent current when the input voltage is 35V and the operating frequency is 78kHz when using different voltage regulator diodes.

It can be seen from Table 1:

When the voltage regulation value of the voltage stabilizing diode is 0V, 5V, the distance between the conduction points of VL and VH is too close, and the conduction time is too long, and there is a larger static current. Although the current is relatively small at 20V, the MOSFET generates severe heat. As can be seen from Table 1, when the operating voltage is 35V, the selection range of the Zener diode is 7.5V~15V.

V Conclusion

The experimental results show that the PWM signal of TL494 is used for N-channel MOSFET and P-channel MOSFET to form a switching power amplifier with a unique driving mode to overcome the shortcomings of simultaneous conduction of two power MOSFETs.

Not only that, it also has ideal drive waveforms, efficiency greater than 95%, good bandwidth and low price, which fully meets the requirements of industrial alarms. And under 18W output power, compared with the power amplifier composed of TDA7481, there is not much difference, and there is basically no heating phenomenon, and the heat sink can be removed.

If you want to get more output power, you only need to increase the working voltage to more than 35V and fit a proper Zener diode.


FAQ

  • What is TL494?

TL494 is a PWM controller IC used for power electronics circuits. It comprises of on-chip two error amplifiers an oscillator with adjustable frequency feature, an output flip-flop having pulse steering control, and an output control circuit with feedback.

  • What is the detailed description of TL494?

The TL494 device incorporates all the functions required in the construction of a pulse-width-modulation (PWM) control circuit on a single chip. Designed primarily for power-supply control, this device offers the flexibility to tailor the power-supply control circuitry to a specific application. The TL494 device contains two error amplifiers, an on-chip adjustable oscillator, a dead-time control (DTC) comparator, a pulse-steering control flip-flop, a 5-V, 5%-precision regulator, and output-control circuits. The error amplifiers exhibit a common-mode voltage range from –0.3 V to VCC – 2 V. The dead-time control comparator has a fixed offset that provides approximately 5% dead time. The on-chip oscillator can be bypassed by terminating RT to the reference output and providing a sawtooth input to CT, or it can drive the common circuits in synchronous multiple-rail power supplies. The uncommitted output transistors provide either common-emitter or emitter-follower output capability. The TL494 device provides for push-pull or single-ended output operation, which can be selected through the output-control function. The architecture of this device prohibits the possibility of either output being pulsed twice during push-pull operation.

  • What are TL494 product features?
    • 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
  • What is PWM IC?

The TL494 fixed frequency PWM Controller can be used for DC to DC conversion regardless of buck or boost topology. ... This IC feature an output control circuit, a flip flop, a dead time comparator, two different error amplifiers, a 5V reference voltage, an oscillator, and a PWM comparator.

  • How does PWM IC work?

As its name suggests, pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.

  • Which IC is better for a buck converter, TL494 or UC3843?

They mainly differ in type of control…
TL494 => voltage mode control (One loop control) ….
while UC3843 uses current mode control (Nested loop control, with a inner/fast current loop and another outer/slower voltage loop)…
Typically voltage mode are used in multiple output converters with good cross-regulation. Current mode when you want to parallel multiple converters to make a single converter with higher current rating…
TL494 is a very popular IC. If you have simple requirements… TL494 is recommended…

  • How do I properly set the feedback pin on a TL494 SMPS IC?

The feedback pin is the output of both error amplifiers, used in comparing and adjusting the output pulse width to the DC control voltage.
On various circuits I have looked up, the op-amp connected to pins 2 & 3 are used to set the gain of the feedback loop, using 2 resistors with one resistor connecting to 2.5V potential divider on 5V reference voltage. With the other connecting to the output (via suitable isolation)
The gain appears to be set at 101, using a 51k feedback with 510 ohms to the 2.5V reference. It is used to control the gain of the feedback voltage. No literature I have yet found, gives an indication on how this gain be set, except a graph showing an open loop gain of 1000, presumably the gain is set for the best stability, although there will also be a time constant.

  • Why is there no frequency compensation required in TI's TL494 example buck regulator design (operational amplifier, buck phase, shift phase, margin, TL494, electronics)?

It's a fixed frequency PWM controller with internal dead time timer. Frequency compensation is not required. Take a look at the datasheet.

  • How to use TL494?

 

 

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