[Amps] OK, here's a home brewing question. Class D amp, FET dead, time understanding?

[Manfred Mornhinweg]

Hi Chris,

> OK, it's not actually a SMPS, but similar, a 136kW LF amp. I built it
> some time ago.

I hope you mean 136kHz, not kW! :-)

> I had been looking at gate and drain
> wave forms and saw nothing horrible to my eyes. 

Well, your drain waveforms look quite horrible to me! You must be 
getting strong harmonics on the output.

> I then read up on
> "dead time" the time neither device should be on to stop short
> circuits or over current, using two channels and two probes. This
> gives the waveform attached (I hope...) and to me there doesn't seem
> to be any real dead time. Is it my measurement inabilities, my
> misunderstanding of dead time concepts, or have I found an issue? 

Well... indeed your driver circuit, using the complimentary outputs of a 
flipflop passed through simple gate drivers, provides no deadtime. But 
with the power FETs arranged as they are, you don't need any dead time 
at all! So that's no bug.

Deadtime is needed whenever you have two transistors in series, such as 
in a half bridge circuit, and when also the transistors take longer to 
switch off than on (which is typical). But your transmitter is a 
push-pull arrangement powered through a choke, CH1. If both FETs are 
driven on at the same time, all that will happen is that they will share 
the current flowing in that choke, and that this current will slowly 
increase, at a rate defined by the choke's inductance and the supply 
voltage. So the choke limits the current that can flow while the two 
FETs conduct at the same time, and thus nothing bad happens if their 
conduction cycles overlap slightly.

On the contrary, if there were a dead time, then the choke would force 
the current to flow even while both FETs are off, and that would drive 
both drains to a higher voltage, limited only by the capacitance to 
ground, which in your circuit is given by the two 4n7 capacitors. 
Basically these capacitances, together with the operating current, 
define how fast the drain voltages would rise if both FETs turned off. 
This determines after how much deadtime the drain voltage would exceed 
the maximum allowed voltage for those FETs.

So, in a circuit like this, dead time isn't necessary at all, and too 
much dead time is not allowed.

 > Even on reduced
> voltage to the PA FET's I get one popping quite often, and I can see
> no other issues, like bad antenna matching etcetera.

I can see at least one issue, most certainly!

One of them is your TX enabling method: When you switch off TX that way, 
you are cutting the 12V supply to the driver circuit, but you have a 
100µF capacitor on that supply! And your driver circuit consumes very 
little current. So the voltage on that capacitor will drop slowly, and 
since those driver chips work down to 4.5V or even less, there will be a 
significant time in which they aren't driving your FETs on and off, but 
half on and then off! When they are half on, they dissipate a huge lot 
of power. It's perfectly possible that they exceed their safe operating 
area every time you disable the TX line, and that over time that damages 
the FETs until they blow up.

Instead you should find a better way to make the TX switching. Such as 
switching the 100V supply. Or if the FETs used have enough avalanche 
power capability to absorb the power stored in CH1, then you can safely 
disable TX by resetting the flipflop through the safety circuit input.

Also make sure that you are correctly handling the thermal side of 
things. At 1kW output, these FETs are probably dissipating a significant 
power, and need to be heatsinked well enough.  Many FETs die in the 
hands of hams simply from totally banal overheating!

> Where does "dead time" come from? Is it from the architecture of the
> driver chips(s) itself / themselves? Or is external circuitry needed?

Some driver chips have built-in fixed dead time, and in others it can be 
programmed by means of an external resistor, or less commonly a 
capacitor. Typically driver chips intended for bridges or half bridges 
have this function.

In other circuits it's implemented before the driver chips. For example, 
many pulse width modulator ICs, such as the very common TL494, allow 
programming the dead time. Those that don't have a dedicated dead time 
control pin, control it by the clock capacitor value.

> I see propagation delay figures cited, but I don't think this is the
> same as dead time.

No. When the propagation delay is the same for high-to-low and 
low-to-high transitions, then there is no dead time. When a driver chip 
implements deadtime, the output low-to-high transition has a much longer 
"propagation delay" then the high-to-low transition.

> On the drain waveform capture I am not sure what
> the blip is before the drain voltage rises. Is that some dead time, or
> an attempt for both devices to conduct together?

My interpretation: The blip starts when the FET gets out of conduction, 
and ends when its opponent starts conducting and this propagates from 
one drain to the other, via the delay of the 4n7 capacitors and the 
coil. Only after some while does the voltage then really go up. This, 
together with the very bad drain voltage waveform, makes me think that 
your tank circuit is totally out of tune! This looks like a class E 
amplifier (not class D), but one that isn't tuned correctly!

> I changed from a single dual output inverting gate driver chip type
> TC4426 to two single output inverting chips type TC4452 in order to be
> able to drive more powerful, higher gate capacitance MOSFET's in the
> future. Maybe these have caused an issue, I had nothing like the same
> failure rate before my mods.

Hey, the TC4452 is non-inverting, while the TC4426 is inverting! And 
when the 4013 flipflop gets a RESET signal in your circuit, both of its 
outputs go high. So, with the original chip, if the protection circuit 
activates, the transmitter is turned off by switching off both FETs. But 
with the chip you selected, the protection cirucit turns both FETs _on_ 
at the same time, and that's sure to blow them up!!!

You should have used the TC4451 instead, that's the inverting version!

Manfred

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