Boost Converter Mosfet Problem

Thread Starter

jdavids1

Joined Mar 26, 2010
1
For school we are designing a boost converter with PI control.


The control part and switching works fine up to the mosfet, when testing the mosfet with a purely resistive load (approximately 200ohms VDD-Drain) the mosfet output followed exactly our gate signal (Approx 40Khz). However once i connect the actual boost circuit (150mH inductor, 470uF cap, 25Ohm load, fast recovery diode). The mosfet switching then stops following the gating signal, instead switching at a rate closer to 150Hz giving us a very large ripple, taxing our supply greatly, etc.


Does anyone have any ideas as to why this could be happening? Our gating signal is still fine even when this is happening. However it could be seen as being a bit more noisy.

We are using an optocoupler between the switching and the gate driver, however the optocoupler common reference point is the other circuits common point.

Any insight would be appreciated.

James
 

beenthere

Joined Apr 20, 2004
15,819
You need to post the schematic for commentary.
We are using an optocoupler between the switching and the gate driver, however the optocoupler common reference point is the other circuits common point.
Sounds like a problem, though. If both the IRLED and phototransistor share the same ground, no isolation is being provided. Plus, if the driving signal ground is not referenced to the load ground, then the gate signal is floating.
 

SgtWookie

Joined Jul 17, 2007
22,230
Optocouplers can be very slow.

When you post your schematic, be certain that the part numbers of all semiconductors are included, especially the MOSFET and the optocoupler.

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