Class E - H Bridge

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Class E - H Bridge

Postby NE4AM » Wed Apr 20, 2005 2:34 pm

There is another configuration for a Class-E power amplifier I'd like to mention - that is where 4 MOSFETs are configured into an H-bridge. The supply voltage is fed through a choke to the top of the bridge, and an output transformer connects to the left and right poles. Each MOSFET is shunted by a capacitor, as with conventional Class-E

There are a number of benefits to a PA of this configuration. First, the peak drain voltage of the MOSFETs is only 1.9 times the supply voltage, instead of 3.7 times, as with conventional Class-E. This allows use of lower voltage MOSFETs, with lower Rds on, for better efficiency. Further, the value of the switch-shunt C is half that of the push-pull circuit in the US5327337 patent.

The big disadvantage is that the two upper MOSFETS no longer have their sources at ground potential - requiring an isolated drive circuit.

Give it a try, gentlemen!

73 - Dave
73 - Dave NE4AM
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Postby blackplasma » Thu Jan 11, 2007 4:27 pm

I'm curious... has anyone tried this? Am I having a 'blonde moment', being under the impression that current commutates to the other rail upon switching (unless the diodes are freewheeling) a la class-D?

Synchronising the isolated drives would be troublesome above about 1-2MHz, but sounds promising for low-frequency work...
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Postby ke1gf » Tue Jan 23, 2007 6:51 pm

blackplasma wrote:I'm curious... has anyone tried this? Am I having a 'blonde moment', being under the impression that current commutates to the other rail upon switching (unless the diodes are freewheeling) a la class-D?

Synchronising the isolated drives would be troublesome above about 1-2MHz, but sounds promising for low-frequency work...


N9NEO has a ZVS (zero voltage switched) H-Bridge PWM modulator... power transformer-less btw. Also featured in QEX too.

On thing I heard about trying to make a ZVS H-bridge work on HF is that the body diode of a FET is very slow, cause H-Bridges also free-wheel
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Postby blackplasma » Tue Jan 23, 2007 9:20 pm

ZVS H-bridge PWM... therefore not class-E. Right? The highest frequency fullbridge I've done used IRFP450s and ran at 2.4MHz -- the highest frequency halfbridge I designed worked at 17MHz, and getting the synchronisation right was very, very tricky.

The intrinsic body-diode of the FETs is pretty crappy, and at more than a couple of hundred kHz, appears more like a resistor than a diode.

If slow-body-diode becomes a problem (e.g. working at < 50% duty cycle) and you need to freewheel, I put a schottky diode in series with the FET, and bypass both the schottky and FET with a proper fast diode.

I still can't see how the fullbridge would operate in class-E -- the reason I'm interested is the fullbridge would allow you to run a class-E topology straight off the mains line without needing 1200V or 1500V MOSFETs.
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Postby frank carcia » Wed Jan 24, 2007 9:01 am

Full bridge easy to say and drive is easy with transformers but the reality of it is very hard to do over 3 MHz. Might need extra diodes to protect fets from shoot through. Not worth the trouble to save a little C.

It is very cool as a modulator though. fc
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Re: Class E - H Bridge

Postby KJ8CQ » Mon Jan 14, 2008 9:46 am

K7DYY uses this (H-Bridge) configuration in his 80 meter Class D/S Senior transmitted. A couple of AM’er in this area have these transmitters and they sound great.

For those interested, schematics can be found here: http://www.k7dyy.com/srsch.htm

I’m not all that knowledgeable on Class D, but isn’t K7DYY design actually Class S since he only uses a low pass filter in the output. I thought true Class D had a tuned circuit in the output?

I guess advancements in MOSFET construction and design make Class D possible for 80 meter operation. The FET’s K7DYY used in the Senior transmitter are the 11n40 in an H-bridged configuration. The 11n40 have low input and output capacitance and he uses a transformer to drive the H-Bridge.

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