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Old 1st February 2006, 07:47 PM   #11
Bakmeel is offline Bakmeel  Netherlands
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Quote:
Originally posted by DaBit
Sounds a little like the output of a 'real' HF transmitter. Maybe not a bad idea, except for the quite high Q required to obtain the voltage gain wanted. Hmm...
Can you maybe elaborate a bit on the lamp characteristics? Voltage and Current required, Lamp Type, Ignition Voltage? Warm-Up time?

I know a few HF-HID Ballasts for assimilation lights (no, not for growing weed ) that operate as a series resonant tank, regulating power with frequency modulation. You will maybe need to boost your battery voltage up to, say, 300VDC.

For that you can use a full bridge driving a series resonant transformer wound 1 : 20 or so. Wind with low leakage inductance and stabilise with a small external coil. On the HV secondary you can use MKS10 capacitors for the resonant tank.
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Old 1st February 2006, 10:43 PM   #12
subwo1 is offline subwo1  United States
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Quote:
Originally posted by DaBit


Can you explain this a bit more? There is 2x Vprimary between the two end points of the primary winding when one MOSFET is on. Thus worst case I need to switch with 2x Vpri over the switch, best case 1x Vpri (when there is no current flowing through the primary before a switch closes).
I am not sure what will actually work for you. But with push-pull, when one MOSFET is switched off, the flyback voltage of the respective primary couples through the transformer to bring the other MOSFET drain down to zero, giving little or no turn-on loss if the gate is enhanced then. Problem is, Leakage inductance can lead to EMI, especially from ringing.

If you go with Bakmeel's idea of the full bridge, you do not have to worry about net dc bias on the primary windings because you can capacitor couple the transformer winding so long as the power is not very high. But a frequency of 1mhz will help let the capacitor be smaller. You still can get ZVS if the duty cycle is kept at about maximum (about 49.7%-49.7%, for example). Since the transformer primary voltage is basically clamped between 12v and ground, EMI can be lower. (I would like to experiment more in this area, though, but not with a bridge circuit and also on rectified line voltage.) Generally, the push-pull is simpler, but a ferrite toroid transformer may not be happy with an uneven duty cycle with that topology.
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Old 2nd February 2006, 04:16 PM   #13
DaBit is offline DaBit  Netherlands
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Quote:
Originally posted by Bakmeel
Can you maybe elaborate a bit on the lamp characteristics? Voltage and Current required, Lamp Type, Ignition Voltage? Warm-Up time?
Sure. The lamp used is a Philips CDM-T MasterColour 70W bulb. This a ceramic discharge tube metal halide lamp. Nominal lamp input power is 70W. Nominal lamp current is 0.95A, nominal lamp voltage is 85V. Obviously the crest factor is not 1 . Voltage increases with age. Ignition voltage is 2kV minimum, startup/arc establish voltage is 275V minimum. The warmup time is 3 minutes.

Quote:
I know a few HF-HID Ballasts for assimilation lights (no, not for growing weed ) that operate as a series resonant tank, regulating power with frequency modulation. You will maybe need to boost your battery voltage up to, say, 300VDC.
Sounds not too difficult.

Quote:
Originally posted by subwo1
I am not sure what will actually work for you. But with push-pull, when one MOSFET is switched off, the flyback voltage of the respective primary couples through the transformer to bring the other MOSFET drain down to zero, giving little or no turn-on loss if the gate is enhanced then. Problem is, Leakage inductance can lead to EMI, especially from ringing.
Depending on leake inductance flyback sounds like a fairly picky way of controlling the switch turn-on. Resonant switching seems more controllable.
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