low efficiency in voltage mode class-d

[ssbothwell KJ6RSG]

hi. i hope you guys don't mind me making so many threads lately. i find that the most exciting part of amateur radio for me is learning about rf circuit design and i have a number of projects on my workbench right now.

i've been experimenting with voltage mode class-d amplifier design. i want to move onto current mode and then class-e at some point but want to work my way up slowly.

attached is a schematic for the amplifier i have been working with. it uses two IRF740s and an IXDD414 driver. all part selections were made based on what i have available in my parts bin. the schematic does not show the driver stage.

the design is based on W1VD's 500KHz Class-D transmitter.

the output transformer is made with a BN-43-7051 core with a 2 turns on the primary (one turn of center tapped bifilar wire) and 4 turns on the secondary. i tried out a couple different toroidal cores in single and double stacks and found that they would saturate rather quickly. the BN-43-7051 gets warm but does not saturate.

at first i did not use the ground shunted diodes and 56kohm resistors but found that the drain waveform is significantly more square with them. i have no idea if the 1n4001 diodes were an appropriate choice.

not shown in the schematic is a low pass Tee tuned to 1.8MHz (i arbitrarily chose 160m for this test circuit).

i am running the amplifier on a 12V/3A power supply.

the amplifier seems to almost be working fine, but there are a few oddities in its performance:

1. with a 1:2 ratio on the output transformer i was expecting to get 48V PkPk drain voltage but instead am getting 80V Pk-Pk and drawing over 3A. i've tried adjusting the gate voltage between 8VPkPk to 15VPkPk without a significant difference in performance.

2. the amplifier is only getting around 50% efficiency. without the ground shunted diode and 56K resistor i was getting 60-70% efficiency and lower power output but even in that situation it is still not near the ~90% i was expecting from class-d and it was still greater then the expected 48VPkPk output.

any idea whats going on? it seems like my power supply is getting overloaded but its not clear to me why the amp wants so much current or why it is putting out greater then 48VPkPk drain voltage.

[N4LTA]

If I understand correctly, you have left out the FET driver IC and are using a silcon diode rather than a high speed diode as specified. That is likely much of the problem. You have to drive the FET hard on and turn it hard off for Class D operation.

That is the principle of class D  -  you are either hard "on" with only the losses of the on resistance of the FETs and other small IR losses or you are off and have no losses.

The higher that you go in frequency, the harder this is to do.

I have build this amp and it works very well with efficiencies over 90 %.

Pat
N4LTA

[kb3ouk]

Just one question: What frequency are you trying to use the amp on? Voltage mode class D doesn't work at HF, it might be ok for 160 meters and definitely works for low MF and LF, but at HF a current mode class D amp works better. If you look at the old class D information that Jay had on his website (found here: http://w1vd.com/375wattclassDold.html ) he originally tried voltage mode for HF and ended up switching to current mode because it didn't work. Also which drain waveform were you measuring? Voltage or current? If you are in voltage mode, the drain voltage should resemble a square wave and drain current should resemble a half sine. Current mode is the opposite, voltage is half sine and current is a square wave.

[ssbothwell KJ6RSG]

n4lta,
sorry the schematic does not show the driver but it is in my actual circuit. i am feeding the FETs with 15V Pk-Pk.

wa1gfz pointed out a few problems including my transformer winding ratio and total inductance and suggested a prior shunt diode substitution. i'll post again once i make these corrections and test the circuit.

kb3ouk,

i dont intend on using this on the air but i am testing it on 160m. i'm trying to work my way through each class of amplifier operation as a learning process. i plan on tackling current mode class d once i get this amp working properly.