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Old 22nd March 2005, 06:21 PM   #1
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Question H-Bridge using Bipolar Transisitors

Im currently doing my electronics project at uni which is a class D amp, certain limitations where placed on this design . Firstly i was to use a PIC Microcontroller as my A/D convertor and then using this to determine the duty cycle of my PWM output. Due to limitations on the PIC I get a maximum switching frequency of 50KHz. Because this is a low frequency to that using an individual comparator and 555 Timer I can use BJT's in an H- bridge to operate in a push pull operation. So far Ive found it extremely difficult trying to get information on this using discrete componenets. Anyone have any suggestions?? Its really annoying as I am slowly running out of time.
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Old 22nd March 2005, 07:36 PM   #2
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Here is a white paper on H-Class
Attached Files
File Type: pdf h-bridge_2.pdf (80.0 KB, 155 views)
Free Schematic and Service Manual downloads www.audio-circuit.dk, Spare time company (just for fun): www.dupont-audio.com
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Old 24th March 2005, 02:05 AM   #3
Alme is offline Alme  Ukraine
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andy_mcm, it's mainly not problem about switching speed but about power required to drive four bases of bipolar H-bridge. I think you can make switching off 200V type BJTs in about 200-300ns or so wich is not so bad (comparing for example with popular integrated gate drivers which feature typical 500-700ns dead-time/delay) but you will need to feed about 2A into each base and if you have multiple parallel devices than even higher current - and this may be your case because H-bridge is usually made for output power over 1 kilowatt, and you cannot typically handle this power with single BJT because bipolar current ratings are lower comparing to FETs.
BTW what is intended output power?
So consider making two additional well-isolated floating supplies plus one common for negative rail to be able to drive all bases (you cannot drive them for example with decoupling transformer in D-class amp as it is sometimes in SMPS).
I would really recommend to make H-bridge with FETs or IGBTs - much simpler way especially if you don't have much time.
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