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What's the difference between soft switching and hard switching?

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diego.fan

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In PA design, I often find these two words. But from google I cannot find clear and easy to understood answer.
Please give me some hint.
 

Re: what's the difference between soft switching and hard switching?

In PA design, I often find these two words.

In which context? What is switched, when?
 

Re: what's the difference between soft switching and hard switching?

When you are talking Class E PAs, you are talking an
output that is switching (nonlinear) and using an output
filter to regain purity and also do an impedance transform.
So topologically you are very like a buck DC-DC converter
(minus the gross shunt-C of the output filter). Concepts
from DC-DC world have bled over to the RF cousin.

Soft switching means you're using the inductor's
"momentum" to pull the output low, when you open the
high side switch. The low side switch is engaged after
a "break before make" period that allows the output to
swing to ground under its own power, just in time to
prevent the current from taking parasitic diodes
forward (D-B diode, ESD diode, either is dissipative).

This can be tuned / "ballistic" or it can be actively
managed by a special control loop to optimize the
turnoff pedestal voltage where a slug of inefficiency
awaits.

At RF you probably don't have a good opportunity
to layer on a bunch of falling edge endpoint feedback
and most of what you will see is hard switched (and
besides, RF likes controlled impedances and changing
from low to high to low is probably not so clean,
distortion-wise). If the output stage looks like a
big fat CMOS inverter, and no obvious designed skew
in the high side, low side drive taper chains (always
some, to minimize shoot-through, of course, but you
would be talking delays approaching quarter-period
for soft switched RF if RF to you means GHz) then
call it hard switched.

In simulation you can make the call by just looking
at output port current on the HL edge, and see if
it's coming from the matching network (soft) or
the low side switch (hard).

A Class A, Class AB, etc. PA are not switching styles,
hard nor soft.
 

Re: what's the difference between soft switching and hard switching?

Are you talking about switching an input signal on and off?
Hard switching is abrupt on and off.
Soft switching ramps up the volume then ramps down the volume so it is smooth, not abrupt.
 

Re: what's the difference between soft switching and hard switching?

Sorry that I don't explain clearly. here is the source.

It is not so easy to integrate the Class D amplifier in CMOS at higher frequencies.
A first drawback is the hard switching property of the amplifier, since
the switch will close while the voltage across the switch in not equal to zero. In
CMOS, the switch will have a large parasitic drain-source capacitance. If the
transistor is turned on, the charge on the parasitic capacitance will correspond to an energy.and this energy is dissipated in the switch

This is the big difference compared to a hard-switching Class D, where a
parasitic switch capacitance will always lead to power dissipation and a reduction
of the efficiency. In Class E, the switch is closed when the voltage
becomes zero and therefore no switching losses occur. This is also referred to
as soft switching or zero voltage switching, (ZVS) .
 

Re: what's the difference between soft switching and hard switching?

All the class-D amplifier ICs available have very high efficiency and work well. Then why does anybody a class-E amplifier?
Oh, is this useless school homework?
 

Re: what's the difference between soft switching and hard switching?

I assume the OP is referring to class D RFPAs, not class D audio PAs (shame on whoever decided to apply the same term to audio PAs).

For class D RFPAs, there are two variants, on being voltage mode (VMCD) and the other is current mode (CMCD). VMCD switches are zero current, while CMCD switches at zero voltage. Since voltage switching is generally more of a problem as frequency increases, the CMCD is more suitable for high frequency operation.
 

Re: what's the difference between soft switching and hard switching?

Why do you talk with letters instead of with words?
PA is Public Address with speakers. RF is Radio frequency. What is RFPA??
You explained that CM is Common Mode but what is CD, a shiny audio disc?? Maybe CD is Current Deficiency?
 

PA: power amplifier
RFPA: RF power amplifier
VMCD: Voltage mode class D
CMCD: Current mode class D
 

Re: what's the difference between soft switching and hard switching?

All the class-D amplifier ICs available have very high efficiency and work well. Then why does anybody a class-E amplifier?
Oh, is this useless school homework?


I'm not 100% sure. But from theoretical point, in ClassE the switch is closed when the voltage becomes zero. So there is no dynamic power loss. But class D, it does have voltage when switch is closed. The dynamic power loss is proportional to fCV^2. It means if f increases-->the loss increases-->efficiency decreases. So class D can only be used in low frequency, i.e. audio.

But class E don't have this problem, so class E can have better efficiency in high frequency. That'a an advantage.
 

I'm not sure I believe you when you say "the" switch
closes when voltage = 0. Because then how do you
add power to the load / match network? Who's doing
the lifting?

Or do you mean the low side switch only?

Many CMOS Class E PAs out there whose vendors ought
to have put out much cheerleading and some applications
and architectural info. Was all the rage at my last company
5 years back when I left, they were all over that Hawaii
IEEE RF conference with papers and so on. Not my thing,
but I got some by osmosis.
 

I'm not sure I believe you when you say "the" switch
closes when voltage = 0. Because then how do you add power to the load / match network? Who's doing the lifting?

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I'm not sure I can answer your question. But I think if simplfying the sch of class E PA, we can get a switch+cap+Res in parallel, and then in series with ind. When opening switching, we can get V-t because of LC. When fine tuning Q, we could create a point when V exaclty equals to 0(It means Vds=0). At that time we close switch, if assuming Rds close to 0, then Vds continues to be 0. Finally we can get @t=closing point, Vds=0,Ids=0; @t=open point,Vds=0. It means there is no Ids-Vds overlap which means no power loss. This should be soft switching.
 

You still haven't clarified what class D amplifier circuit you're referring to...
 

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