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Date: Tue, 30 May 2017 15:36:07 +0200
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Subject: Re: LF: Iron dust toroids and heating/H bridge
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Am 30.05.2017 15:04, schrieb Graham:
> ...to question the  choice of a  half bridge , over that  of the  full 
> version?
A half bridge is easier to build because both sources of the FET are at 
GND level. So you are not forced to use a decoupling transformer for 
driving the FETs. I would recommend to use one anyway, and it works 
quite well and it is an advantage to pull the gate of the non conducting 
FET to a negative voltage.
The H bridge has an advantage when mains voltage is used directly, i.e. 
320 V DC switching voltage.
There are 1000 V FETs however, meanwhile.

Oh i just see the point, a half bridge has two FETs in series and the 
other half are capacitors. I was talking about the other circuit which 
uses the lower two FETs not the left two FETs of the H bridge :-)
I would not recommend a hlaf bridge at all.

> Its tempting to link the  harmonic content , with the high loss  rate 
> of devices ,   In your 136 vk  experiments, with the  H , did  you  
> loose fets , other  than by loss  of  tune ?
I only lost FETs in the beginning, until i discovered that i made an 
error when building up the driver. After correcting this i didn't loose 
any FETs. I passed several kWh through one pair of  FETs. No problems in 
WSPR, OP32, CW, even slow hell (which actually is for linear PAs only!). 
I have a fan blowing on the heat sink whos speed is controlled by the 
temperature of the heat sink. So i always get an impression of the 
temperature (when listening to the speed :-) ). When the antenna was not 
spot on the resonance, the fan blew a bit faster, so the losses must 
have been somewhat higher. But it never cost the FETs (IRFP460A).

> Is there a  ferrite , that  is  comfortable  in 136  and  477 service ?
Several, almost any.
>
> but worth consideration for a  driver, having  no transformer 
The standard FET drivers (ICL7667, TC4422, IXDD614CI...) are switching 
between Vss and GND. It is however saver to pull the FETs gate voltage 
to a negative value when OFF. This is given by the transformer. Since 
the voltage of the opposite winding must be negative when the other one 
is positive, it is impossible that both FETs are conducting, what ever 
the driver and transients does.

> effectively all  band from  audio up ?
 From DC up :-) Yes, but not with some high/low side drivers like 
IR2110. I built a second bridge for VLF some years ago and used it down 
to 100 Hz...

73, Stefan