BIPOLAR ALTERNATIVE TO H BRIDGE

[Altra]

Is there a way to modify the circuit to make the ramp less steep?

If I understand your question right. You can add more parallel capacitance across the coil. This will widen the half sine, slowing the change in current. This will also lower the peak flyback voltage. Tfly = 3.14*sqrt(LC).

[green]

If I understand your question right. You can add more parallel capacitance across the coil. This will widen the half sine, slowing the change in current. This will also lower the peak flyback voltage. Tfly = 3.14*sqrt(LC).

I tried stepping the capacitance C1, 100p 200p 400p in spice polar2D(reply #30). Appears ramp is steeper with increasing capacitance.

[moodz]

I tried stepping the capacitance C1, 100p 200p 400p in spice polar2D(reply #30). Appears ramp is steeper with increasing capacitance.

The relatively high resistance of your coil is causing the ramp ... increasing the capacitance only makes the ramping worse in accordance with an RC constant.

You are losing more than half a watt in your coil due to I2R losses.

[green]

The relatively high resistance of your coil is causing the ramp ... increasing the capacitance only makes the ramping worse in accordance with an RC constant.

You are losing more than half a watt in your coil due to I2R losses.

Thanks, could you describe your coil to get 300uH, .2 ohms and 100p. Diameter, number of turns, wire size, winding method(bundle, spiral, ?)and cable length.

[green]

The polar drive circuit is interesting, would like to try it. My Tx coil resistance is closer to 3 ohms. Tried a spice circuit with Tx=3 ohms and all MOSFETS STW11MN80. Get about 18A/sec ramp at switch point. Is 18A/sec acceptable? Is there a way to modify the circuit to make the ramp less steep?

Circuit change that reduces ramp with spice. Don't know if would with real circuit.

[moodz]

Thanks, could you describe your coil to get 300uH, .2 ohms and 100p. Diameter, number of turns, wire size, winding method(bundle, spiral, ?)and cable length.

The coil I use for bench testing is "old faithfull" .. which admittedly has seen better days. The resistance is actually 0.29 ohms. It has 17 turns of very expensive .. at the time I bought it several years ago ... litz wire ... might be 2 mm x section area .. and the diameter is 18 inches.

Its unsheilded ... and the winds are flat spiral basket weave for minimal capacitance. 296 uH last time I measured it.

But that is not the point ... a coil is a current operated device .. you dont want series resistances if you can avoid ... just adds to charging times if reactances are involved and power loss.





Probably worth considering also that the I would probably add capacitance ( eg 1000pf ) across the coil to limit peak flyback as once the current gets up to +/- 1 amp level the peak flyback is exceeding 1 KV. ( using SIC 1.4 KV mosfets for the output devices ).

[lucifer]

Hi moodz,
Thanks for sharing your circuit! Can you please explain how it is performing better than the standard H-bridge circuit with diodes instead of your isolation switches? Is there any benefit in using the MOSFET body diodes instead of normal diodes? In terms of efficiency your additional switches add a bit more resistance in the current path when turned on, though that would be negligible if low on-resistance MOSFET is used.
Is a single gate driver capable of driving all 3 the transistors at once?

[Carl-NC]

Hi moodz,
Thanks for sharing your circuit! Can you please explain how it is performing better than the standard H-bridge circuit with diodes instead of your isolation switches? Is there any benefit in using the MOSFET body diodes instead of normal diodes? In terms of efficiency your additional switches add a bit more resistance in the current path when turned on, though that would be negligible if low on-resistance MOSFET is used.
Is a single gate driver capable of driving all 3 the transistors at once?
[ATTACH]55539[/ATTACH]

The standard H-bridge drive like you show is not capable of square wave current drive. That is the purpose of Moodz' circuit, it is a completely different class of operation.

[moodz]

Hi moodz,
Thanks for sharing your circuit! Can you please explain how it is performing better than the standard H-bridge circuit with diodes instead of your isolation switches? Is there any benefit in using the MOSFET body diodes instead of normal diodes? In terms of efficiency your additional switches add a bit more resistance in the current path when turned on, though that would be negligible if low on-resistance MOSFET is used.
Is a single gate driver capable of driving all 3 the transistors at once?
[ATTACH]55539[/ATTACH]

Thanks Lucifer ... I found that the circuit I posted had twice the current magnitude and twice the speed and ran off lower voltages than an equivalent H bridge.

I used 500 volt mosfets for the 2 isolating switches and 4 milliohm low voltage mosfets for the rest.

I used a garden variety dual o/p inverting mosfet driver to drive the common gate drives nothing special .... still achieved 500 ns current transitions. no point using external diodes to increase this speed as the flyback voltage gets very high ... too high and the mosfets avalanche.

The driver was a TCC4428 1.5 amp mosfet driver actually.

[lucifer]

Thanks Lucifer ... I found that the circuit I posted had twice the current magnitude and twice the speed and ran off lower voltages than an equivalent H bridge.

I used 500 volt mosfets for the 2 isolating switches and 4 milliohm low voltage mosfets for the rest.

I used a garden variety dual o/p inverting mosfet driver to drive the common gate drives nothing special .... still achieved 500 ns current transitions. no point using external diodes to increase this speed as the flyback voltage gets very high ... too high and the mosfets avalanche.

The driver was a TCC4428 1.5 amp mosfet driver actually.

Good job, Moodz! I've been playing with your circuit in LTSpice and I'm eager to try it in real. I understand that only 2 high voltage mosfets are required since the rest don't see the flyback voltage. It's amazing how well it works with such a low power supply voltage.

[green]

Wanting to try the circuit. Need to order some parts, capacitors to add across coil to prevent avalanche if needed being one of them. Thinking 100p 1000V NPO ceramics. Good choice or not?

[eclipse]

I would prefer polypropylene film. Obtain few caps so you adjust the capacitance to obtain desired voltage.

[green]

I would prefer polypropylene film. Obtain few caps so you adjust the capacitance to obtain desired voltage.

Searched polypropylene capacitors. They have an AC and DC rating. If peak volts=500V, p-p=1000V. What rating should I purchase? I'm thinking (I squared L=E squared C). With .5A, 300uH and 500p, peak volts calculates about 387V. Why is simulation volts(355V) lower than calculated?

[Altra]

Hi Green,

I've been using this style of metal film.

This seller has various values rated at 2KV. You pay shipping on first and the rest are free

https://www.ebay.com/itm/16363239905...QAAOSw-9NcpSIR

[eclipse]

CBB such as these is just a regular polypropylene film capacitor