Will this MOSFET-driver circuit work? - diyAudio
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Old 24th June 2005, 10:16 AM   #1
Danko is offline Danko  Hungary
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Question Will this MOSFET-driver circuit work?

Hello!

I have attached a schematic. Will that mosfet-driving work?
I'm designing a 35V -> 25V@5A step-down converter, with ONsemi's MC34063 IC.

Tanks in advance!
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Old 24th June 2005, 11:26 AM   #2
mflorin is offline mflorin  Romania
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Hi,

It never works.For driving the MOSFET you need an isolated voltage or a bootstrap circuit.
The base of the pre-driver transistor inside the MC34063 has a drive signal which is related to ground and not to the SW_E pin.

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Florin
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Old 24th June 2005, 11:46 AM   #3
Danko is offline Danko  Hungary
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Do I need a level shifter?
The diode "D", and the capacitor "C" isn't enough for a bootstrap-circuit?
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Old 24th June 2005, 12:52 PM   #4
mflorin is offline mflorin  Romania
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OK, if we take a closer look over the push-pull driving circuit we will see that in the off state of the internal MC34063's transistor, the pnp's base is floating.So you have a lot of chances to keep the pnp off and no signal for the gate of the mosfet.

The D and C looks indeed like a level shifter circuit, but you need an alternating signal for shifting.In the present configuration you didn't have that signal.

Watch out for the maximum gate to source voltage rating.I think you need something to limit the voltage level.

Florin
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Old 24th June 2005, 01:19 PM   #5
Danko is offline Danko  Hungary
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I draw somathing in TINA (this is a simulator-prg), it seems to work.
T4 transistor is the MC34063's internal transistor, and that R3 is also the integrated resistor, inside MC34063.

On the T4, there's 37V, which is quite high. Accourding to the datasheet of MC34063, the max. V_emitter_collector is 40V. I think, I gonna have to use a dedicated powersupply for the MC34063.

What do you think of this?
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Old 24th June 2005, 01:21 PM   #6
Danko is offline Danko  Hungary
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And the switch torned OFF
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Old 24th June 2005, 01:35 PM   #7
mflorin is offline mflorin  Romania
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You made at least one mistake in the simulation.The emitter of internal transistor isn't conected to the ground of input voltage.It is a floating point. At least this is what I see from the first schematic.

Repeate the simulation with the circuit like you draw at first time and see if it's working.
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Old 24th June 2005, 02:09 PM   #8
Danko is offline Danko  Hungary
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It's not internally connected, I connected to the gnd, the internal transistor's emitter(pin 2).
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