The schematics for a carrier based switching power amplifier for audio will be presented here. The document contains some photos about the built unit. You will also find the references for further reading. The design is based on book "Introduction to Electroacoustics and Audio Amplifier Design" by W. Marshall Leach, Jr.[see it]. All PWM circuit component values (for generator and feedback) are calculated by this book. The switching frequency was chosen 300kHz which is somewhat sufficent for a full-range audio amplifier.
The unit diagram shows various logical building blocks of the amplifier. The arrows show imaginary flow of signal or power, of course, they have nothing to do with the real currents :-). This drawing will not describe how and why pulse amplifiers work, you probably know it already.
As you can see there are many power supplies involved. My first switching amplifier had 3-4 supplies attached. The problem is not with different voltages. The trouble is how they are referenced to each other. However, all voltages can be generated from a single +-40V rails. The +-40V rails was chosen as an example, the actual rail voltage could range from 20V to 80V or more. In the schematics below I reference the main rail as +-E. For a multi-channel amplifier you can reuse the triangle generator and the power supplies.
The integrator is used to provide negative feedback for the amplifier. IC2B on the schematics is the balanced amplifier that converts the symmetric signals from the both halves of the output stage into one PWM signal that is referenced to ground. The PWM signal is then feed into an integrator IC2A. The integrator pole frequency is chosen between the highest audio frequency (20kHz) and switching frequency (300kHz). In this design it's chosen 60kHz. Since IC2A has infinite (or very large) DC gain it will also minimize the DC voltage at the output.
Triangle generator provides triangular electrical signal. The signal is used to generate PWM signal at the comparator.
The comparator will compare two signals: 1) audio together with feedback signal from the integrator; 2) the triangular signal from the triangle generator. This results PWM signal at the comparator output. That signal is then feed through the level shifter into the output stage that amplifies it.
Since the front end is referenced to ground but the output driver circuit is referenced to -E rail, we need a level shifter. In the circuit below, T1 acts as a level translator. The logic inverters are used for pulse shaping and inverting.
| PWM | Output A | Output B |
|---|---|---|
| 0 | 1 | 0 |
| 1 | 0 | 1 |
The output stage consists of two half bridges forming a H-bridge. Instead of IR2110, IR2010 can be used. The MOSFETs must match the power supply voltages and the maximum current. RD networks at gates generate additional deadtime and help against shootthrough which could destroy our transistors. Too big deadtime will introduce distoration. The deadtime can be adjusted by selecting R1-R4. Suitable values can be detected by experimenting. Bigger resistor values result in longer deadtime. The MOSFETs should go only slightly warm without any load.
28uH, must stand output current without saturating. Recommended design is to use toroidal core. The wire is easy to wound on these. Micrometals has some suitable cores (metarial -2). I used the core T-106-2 without any problems.
The PWM controller requires a bipolar 15V power supply. The needed current is less than 50mA (depends on op amps used). Therefore T1 and T2 might need small heatsinks.
The driver circuit (IR2010) will need 12V and the inverter circuit (HC04) requires 5V. Although IR2010 can sink high pulse current (up to 2A) the average current is quite small, less than 10mA.
Some photos about the amplifier.
The prototype has some differences in the schematics. Firstly, instead of full H-bridge, I used half-bridge because I had only one IR2110 in hand at building time. Secondly, some of the decoupling capacitors are missing from the board.
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Output stage MOSFETs, 2 in here since I built the prototype as half-bridge. The heatsink does not even go slightly warm and thus might be completely unnecessary. You can also see the output filter (toroid and capacitor). On the right to the inductor there is 12V regulator with MJE340 for local 12V to supply the driver circuit. |
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The triangle generator, modulator and the feedback loop. The smaller module is the level shifter. Two transistors in TO126 (namely MJE340/350) are for bi-polar 15V power supply to feed the generator. |
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The generator block. From left to right the IC's are: triangle generator (NE5532), comparator (LM319N) and feedback amplifier/integrator (NE5532). |
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The bottom side of the prototype amplifier. The brown capacitor on the right is a part of the low-pass filter (I use the prototype as a subwoofer amplifier). The schematics above does not show the filter. |
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The closeup photo of MOSFET driver circuit. You can see the bootstrap capacitor (blue, on right) and the diode (1N4148). |
, 18.08.06, Index