Output filter class D amp
Classic, the output filter is a second order LC (L1 C13). R13 and C14 dampen oscillating surge due to resonance between L1 and C13 if no speaker is connected. The filter is optimized for a loudspeaker 8 Ohm but also supports a speaker of 4 ohms.
The inductor must support at least the peak output current without saturating. Inductors with iron powder ( iron powder ) strong heat (100 ° C at rest) because of losses in the core (magnetization cycles / demagnetization) at about 250kHz. Here, the inductance is iron powder: besides warming up while the amp works fine with.
The output filter is not part of the feedback loop against. Its output voltage is not controlled by the integrator. This allows a simpler installation, more stable, but slightly reduces the damping factor of the amplifier.
With a loudspeaker 4 ohm (or two 8 Ohm parallel) means in practice that there is a lack of treble. The filter is not quite suitable to 4 Ohms, as shown in the simulation (simulation made with the free LTSpice).
Autostart class D amp
We have seen that if the input "IN" of the IR2184 is high statically, T1 and T2 are off (C12 being unloaded and locked D4).
You have to get a signal of a few millivolts (a little bit of music) to the input of the amplifier class D for self oscillation starts. If we add a few megohms resistance in parallel with C5 to avoid saturation of the integrator, it happens that unsuccessful starting attempts. Nothing like a little music to start and establish oscillation. This phenomenon is due to the blocking of T1 and T2 in both established static regime (integrator output locked to + 7.4V, and IN high, around 5.2V relative to -Vcc).
The amp starts spontaneously if Vcc is established after -Vdc but this start is a little twisted (achievable with dual power supply stabilized lab).
In normal use of an audio amplifier, this poses no problem. This Class D amp is simply "sleep" before the first tiny musical solicitation.
Power Supply in the Class D amp
+ Vcc side, R20 biases the zener DZ1. It takes minimum 4 mA to power U1 operating. LED can be inserted (choice of color!) in series with R20.
-Vdc Side, R21 creates two power supplies with DZ2 and DZ3. While it takes only 4mA for U1, U2 takes about 30mA. That is why R21 does "only" 1kOhm. You can choose 680 Ohms or 820 Ohms 5W version with no problem, which allows the amp still operate at lower voltages (+/- 35VDC).
First model class D amp IR2184
The first achievement of this class D amp allowed to adjust the values of each component, and prove that a class D amp is quite easy to achieve with a half bridge control IC (IR2184).
- Two capacitors 4700uF 63V filtering
- Integrated Circuits TL072 and IR2184
- 5W Resistance 680 Ohms
- Output filter
- Blue capacitor between Vcc and -Vdc (470nF 250VDC)
- Power Transistors IRFB4620
- SMD components for the rest
Power supply: +/- 40V +/- 60V ...
Power output: 4 ohms 300Wrms about
No fan: natural cooling at full power sufficient to 4 Ohms (resistive dummy load)!
The Class D amp is easily achieved through the control of the output stage. It's amazing how the power transistors little heat, which can significantly reduce the size of the radiator and therefore the amp to achieve.