 Single transistor flyback driver
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Sulaiman
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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
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Driving bipolar transistors in a flyback with efficient duty-cycles (>>50%)
requires some thought for the transformer drive;
e.g. A typical 12V logic/control/drive power supply, 90% duty,
If the primary of the transformer is ON for 90% of the time
then during the OFF period the voltage across the primary must flyback to
AT LEAST 9x 12V or the primary will saturate after a few cycles.
The switch transistor will 'see' this 9x 12V plus 12V supply = 120 Volts MINIMUM
So fairly a high voltage transistor is required to drive the primary of the driver transformer.
This flyback pulse will also appear across the secondary of the driver transformer
and will be more reverse volts than a reverse biased base-emitter junction can withstand.
In addition there are the usual problems associated with transformer leakage 'spikes'
so snubbing is required also.
I think that bipolars are very good for flybacks
especially for high volume manufcturing
or when we 'recycle' used equipment.
For a one-off hobby project it's probably best to accept a little inefficiency
and use an IGBT, which can be driven directly (with protection) by a 555 etc.
I accept the even greater inefficiency of MOSFETs for my hobby stuff because
they're easy to use AND I have several.
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