Class D amplifier driven by 555 output

[darryl_co]

Hi,
I am building a electronic descaler.I need to drive a power stage using MOSFETS from the 555 output.Previously I used a 3 transistor amplifier but the problem was that the NPN (Q4) transistor heats up much more than the PNP (Q5) transistor.My questions are
1) Is the MOSFETs biasing right
2) How do limit the current flowing through the MOSFETs without using resistors in the drain or source

 

[Nigel Goodwin]

No idea on the biasing, but what makes you think it's a class D amplifier?.

It's just a high powered switch.

 

[ronsimpson]

Your top MOSFET (M1) will never turn on. (not enough to drive a load)

 

[MrAl]

No idea on the biasing, but what makes you think it's a class D amplifier?.

It's just a high powered switch.

Hi Nigel and darryl,


Nigel:
He he, i almost didnt catch that one Would have got it faster if you put a little smilie after your last sentence though.

darryl:
To operate efficiently in class D you need to introduce some dead time. Your circuits (both of them) dont seem to have any at all. That could easily cause one or both transistors to heat up much more than it should and of course waste power.

To introduce some dead time you need to modify the drive circuit a little so that there is some delay for the turn on but as little as possible for the turn off. When the drive signal tries to turn off a transistor it takes a finite time for that transistor to actually turn off, and if during this time the other transistor turns on then we end up with a near short right across the power supply buss. This causes a large current spike through both transistors and could cause excessive heating and even destruction. With a small delay for the turn on, the opposite transistor gets time to turn off completely and this prevents them both from being on at the same time.

The usual way to achieve this is to use an asymmetrical driver. You can use a little delay circuit which actually delays the turn on signal by a small amount, or you can use asymmetrical impedance where the gate resistance for turn on is larger than the resistance for turn off. The delay is probably the best, but the asymmetrical resistance path is simpler requiring a diode to steer more current to the gate when it is time to turn off than when it is time to turn on. For example:

  Drive o---R1---+--R2---+---o Gate (N MOSFET)
                 |       |
                 +--*D1--+

                      * is the cathode

In the above, the diode steers more current during turn off than turn on (assuming N MOSFET).

 

[alec_t]

The buffer amps A1-A4 output only 1V in the sim. Is the amp model using a default value?

 

[darryl_co]

Your top MOSFET (M1) will never turn on. (not enough to drive a load)

what is not enough ? If you are looking at simulation of the circuit than the drawing is only for representation purpose as unfortunately in simulation the output swing of the inverter gates A1 to A4 is 0 to 1 Volt instead of 0 to 9 Volts

 

[darryl_co]

The buffer amps A1-A4 output only 1V in the sim. Is the amp model using a default value?

In simulation I do not know how to achieve 9 volt swing for the A1-A4.It would be helpful if you can help with the settings.

 

[darryl_co]

The use of additional inverter gates output of A1 coupled to A3 than A$ would produce a slight delay, maybe its too less.According to your illustration in using the diode should I also used a resistor between gate and drain?

 

[Nigel Goodwin]

Nigel:
He he, i almost didnt catch that one Would have got it faster if you put a little smilie after your last sentence though.

Nothing to have a smilie for - it's not a class D amplifier, it's just a class C switch (or would be if the biasing etc. was corrected).

 

[alec_t]

In simulation I do not know how to achieve 9 volt swing for the A1-A4. It would be helpful if you can help with the settings.

Right click on the A1 etc symbol, then enter 'Vhigh=9' in the first Value field. Attached is a mod of your circuit. I have replaced the 555 with a 10kHz standard pulse generator so that the sim runs faster.

 

[ronv]

Driver

The top FET needs to be a P Channel for it to work properly. You can use the network proposed by Mr. Al to eliminat the "shoot thru". If you use a CD40106 you can run it off of the 16 volts so the FET will turn off.
You can set the voltage levels in the attribute window for the IC. I used a CD40106. You can check the datasheet for the threshod and hysterisis values.

 

[darryl_co]

The top FET needs to be a P Channel for it to work properly. You can use the network proposed by Mr. Al to eliminat the "shoot thru". If you use a CD40106 you can run it off of the 16 volts so the FET will turn off.
You can set the voltage levels in the attribute window for the IC. I used a CD40106. You can check the datasheet for the threshod and hysterisis values.

Do I have to use a P Channel MOSFET? Can I not use both N channel?
The 555 is shown to run on 9 volts as it would be run through a 7809 regulator, for frequency stabilization, powered by a 12 Volt ac source rectified and filtered generating about 16 VDC for the power stage

 

[ronv]

You need a different driver to use a NFET for the top FET as for it to turn all the way on the gate needs to be 8 to10 volts above the source. The way it is now the source will only go to 9 volts and the other 7 volts will be dropped across the FET making the power disapation very high so it will get very hot. They make drivers called high side drivers so you can use a NFET there but that would require another IC.

 

[darryl_co]

You need a different driver to use a NFET for the top FET as for it to turn all the way on the gate needs to be 8 to10 volts above the source. The way it is now the source will only go to 9 volts and the other 7 volts will be dropped across the FET making the power disapation very high so it will get very hot. They make drivers called high side drivers so you can use a NFET there but that would require another IC.

Are you referring to MIC4426 or MIC4428 as high side drivers? Then could replace the inverter with the MIC4428

 

[darryl_co]

Is there some modification that can be done to the 3 transistor amplifier so that the NPN and PNP power output get equally Hot?

 

[ronv]

Take a look at the IRF2110. It would work for your application.
Let me look at the transistors.

 

[ronv]

I'm not sure I know what a descaler is. Can you explain how it is susposed to work? I'm guessing it is a series resonant circuit with the inductor and capacitor?
I'll be back later.

 

[MrAl]

Nothing to have a smilie for - it's not a class D amplifier, it's just a class C switch (or would be if the biasing etc. was corrected).

Hi Nigel,


Yes good point! It's not 'really' a class D until it modulates, etc.

[Edit]
Oh wait, he's using a 555 to drive it, so all it would take to modulate is an input voltage to the cv input. That would certainly modulate the output as a class D would do...perhaps without the perfection of better feedback.
Now i have to wonder what this will be used for: (for a descaler, not audio)

 

[alec_t]

Now i have to wonder what this will be used for: (for a descaler, not audio)

For descaling (allegedly) lime scale in water pipes. There are doubts as to the efficiency/function of these devices.

 

[ronv]

Push Pull

Try this.