I took the shortcut on the HID-bulb-on-the-motorbike project by using the existing 220V ballast preceded by a 12VDC -> 260VDC converter. That works now:
But I am still not satisfied. The electronics are still big and bulky. Next try will be a regular hard-switched converter, preferrably pushpull. But to avoid acoustic resonance in the arc discharge, I need to feed the bulb AC with a frequency of <1kHz or >1MHz. Most ballast choose the <1kHz option by doign a DC/DC stepup and a H-bridge inverter. To cut on components and reduce the size, I still find the >1MHz route much more attractive.
Are there special problems that needs to be adressed? Or is it a fairly simple pick the right ferrite, pick the right controller and beef up the gate drivers (and place the power stage very close to the MOSFET's) kind of job?
Is there any chance that a regular power ferrite such as Ferroxcube 3C90 or Epcos N87 material can be used at these frequencies without exceptional core losses? I am not sure I can obtain suitable cores in Ferroxcube 3F4, Epcos N49 or similar ferrite.
But I am still not satisfied. The electronics are still big and bulky. Next try will be a regular hard-switched converter, preferrably pushpull. But to avoid acoustic resonance in the arc discharge, I need to feed the bulb AC with a frequency of <1kHz or >1MHz. Most ballast choose the <1kHz option by doign a DC/DC stepup and a H-bridge inverter. To cut on components and reduce the size, I still find the >1MHz route much more attractive.
Are there special problems that needs to be adressed? Or is it a fairly simple pick the right ferrite, pick the right controller and beef up the gate drivers (and place the power stage very close to the MOSFET's) kind of job?
Is there any chance that a regular power ferrite such as Ferroxcube 3C90 or Epcos N87 material can be used at these frequencies without exceptional core losses? I am not sure I can obtain suitable cores in Ferroxcube 3F4, Epcos N49 or similar ferrite.
I'm struggling with that. ZVS control and component choice is not exactly trivial. And the benefits of ZVS are not that high at 12V primary voltage. I'm not interested in extremely high efficiencies; somewhere in the neighborhood of 70% would be nice. But I am interested in a small form factor. Plenty of power to burn, but no space to mount the electronics which burns the power 
subwo1 said:Oh, I forgot you were using 12v as the power source. I forget, do you need to regulate the output, or will current limiting be enough?
I need to regulate lamp current, but I intend to use a ballasting inductor before the lamp. I need that anyway to keep the HV ignition pulses away from the power source.
Eva said:
I have two choices:
1) Use a flyback or boost converter, or eventually a pushpull to create a constant current source, and use a H-bridge to create a low-frequency AC square-wave. Control is quite difficult due to the parabolic voltage vs. current characteristic of the lamp.
2) Use a HF converter, and feed the lamp directly, eventually through a ballasting reactor to make the impedance of the lamp appear positive, which simplifies the control loop?
I intend to use the casing as the heatsink. That is also what I did now in the 12V->260V push-pull converter; I mounted the MOSFET's on a copper strip, and bolted that to the aluminium case. The copper distributes heat well, and the aluminium box provides a surface large enough to get rid of the 20-40W.
Do you think option 2 would result in a larger PCB? One or two push-pull gate drivers are smaller than a H-bridge with high Vds MOSFET's and gate drivers. But what about the magnetics? Would those be much smaller? I doubt it; higher frequency also means higher core and AC resistance losses. But comparing a 200kHz and 1MHz case is a bit of homework I can do myself.
You could consider a series resonant tank circuit fed by a half bridge. If you design your inductor such, that the resonant tank will have an appropriate Q, you can control power with frequency modulation.
Use an above-resonant starting circuit to generate the high ignition voltage.
You will have to make careful considerations preventing too much EMI. At these frequencies everything becomes an antenna
Use an above-resonant starting circuit to generate the high ignition voltage.
You will have to make careful considerations preventing too much EMI. At these frequencies everything becomes an antenna
DaBit said:
Maybe you can operate a push-pull, then, near full duty cycle to ease switching problems with the MOSFETs. It can give ZVS characteristics to push-pull with a constant switching frequency. You would have to watch for flux imbalance on the core, though, since it is not AC coupled to its drive waveforms.
subwo1 said:
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).
DaBit said:
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.
DaBit said:
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.
Bakmeel said:
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. I know a few HF-HID Ballasts for assimilation lights (no, not for growing weed
Sounds not too difficult.
subwo1 said:
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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