Green Digit Electronics

Class D amplifier

Creative Commons-Licentie

For a project at university i created this small 25W Class D amplifier. I chose for a compact and simple design, hence the TPA3123.

The schematic

TPA3123D2

The design is based around the TPA3123D2. Some properties:

The gain has been maximized tieing the GAIN pins to the supply voltage using a pull-up resistor. The imput impedance is lowered to 9kΩ.

Input Capacitance

The input capacitance and the input resistance form a High-pass filter.
Selecting a gain changes this filter so care has to be taken chosing the gain and the input capacitance. We want signals to pas that are higher then 20Hz (cut-off frequency)this is all the information we need to calculate the needed capasitance.

RC High pass filter equation

Using the above the capacitor should have a value around 0,884μF. I chose a 1μF just for the ease.

decoupling

The THD, or Total Harmonic Distorion, is a demned beast. The supply has to be able to drive the output when needed. Decoupling the supply rail near the chip is a must. not anly a 100nF capasitor but also some big capasitors that hold a lot of charge are needed. These capasitors give the chip the current it needs without straining the power supply. When a big and short burst of current is needed these capasitors can provide it. The used capasitors to provide this are the 1000μF low ESR caps. they also filter the supply rail from any low frequency noice. To filter out the High frequency noice I used a 1μF capasitor in compination with a 100nF cap.

BSN and BSP Caps

As the chip has a build in (half) H-bridge, we have a need for Bootstrac capacitors. Without these the internal Boost converters will not work (properly). the datasheet sugests 220nF ceremic capasitors.

Shutdown and mute

The TPA3123 has the ability to mute the output using the mute pin and shutdown to a power saving mode using the shutdown pin.

Cooling the chip

The high efficiency of class D amplifiers make external heatsinks unnecessary. Do note that it still needs a heatsink, but it uses the PCB groundplance to dissepate the heat.
This actualy was a problem when creating the pcb and soldering the component on. Without a reflow oven or a hot air station this sounds impossible. how else do you solder the exposed pad?
The answer is actualy quite easy. Drilling a hole where the exposed pad is supposed to be is the solution here. Doing this the groundplane on the other side of the pcb can be used to dissepate the heat.
Simple isn't it? Well not that simple ofcourse, now we have the full ground plane to heat when soldering, but it works. Would i suggest you always do it like this? No, with the prices of a hot air station as low as 100€ i see not reason not buying one if needed.

The output

The output filter, made out of an inductor and a capacitors, remove the switching noise out of the amplifier.
To calculate the output filter I used a 4Ω output load. Thanks to the cap on the output its a second order filter. Using a 22μH inductor we can calculate the needed capasitor for the output with a cut-off frequency around 40khz we get a capasitance of 680nF.

The result:



Voltage pumping

There is only one problem left: Voltage pumping. To prevent this from happening we need to shift the input in one of the channels 180°. to acomplish this we invert the signal using a simple inverting amplifier section.

The PCB