One transistor forward converter

two transistor forward converter

hello,

I have built a one transistor forward converter (10W) for 240V 50Hz mains.

It is an isolated version of an otherwise identical one which was non-isolated and worked fine.

However, this isolated one is not working well at all.

I am getting only some 2 Volts across the 11 Ohm load.

It is strange because the MOSFET gate drive pin of the MC34025 is not driving proper gate pulses......it is just giving titchy ones which dont turn the Mosfet on well enough or long enough.

When i disconnect the secondary (so its open circuit) then i can see decent proper gate drive pulses. -but only when secondary is disconnected.

I have disconnected the feedback opto and grounded the current sense to try and get it to give gate drive pulses but nothing works.

I am just wondering if the transformer is passing noise to upset the PWM controller. This would be strange since this transformer worked fine with the non isolated version.
But maybe its inter or intra-winding capacitance or leakage inductance thats causing the problem ?

For the transformer, i did not do interleaved winding but just wound ...

81 turns of primary (one complete layer)...then
71 turns of demagnetisation winding..(2nd layer) .then
14 turns of secondary winding (3rd layer)

.....is this "one on top of the other" winding technique wrong ?

i would be most grateful for replies

feedback transformer one transistor

What is this "One transistor forward converter"
I never heard about 2 transister forward converter.
Kindly post schematics

mosfet gate disconnected

hi

its like in the third post down of this



Actually, my problem i now find is ESD damage to mosfet driver in the pwm controller....

....this esd did not completely kill the pwm controller, but just damaged it such that it wouldnt work properly.

....strangely, the driver could drive a constant 200mA, but could not manage the pulse to switch a mosfet on.

on transistor forward

my problem i now find is ESD damage to mosfet driver in the pwm controller

Click to expand...

Any proofs for this assumption? I would rather expect regular behaviour of the part, e. g. triggering of the current limiter, due to inappropriate circuit design.

onetransistor

Hello FvM,

thankyou yes i changed pwm controller and it worked fine....

i didnt have any more MC34025's but i had the uc3825 which is the same so it was ok.

i was working due to lack of space on a plastic table surface...maybe that did it.

(this was a re-build of the previous post (mentiuoned above)....that didnt work cuzz of noise as stated there....in fact it was likely the poorly selected snubber components which were resonating with the primary leakage inductance.

i dont know this for sure, but changing the snubber in the rebuild made it work ok.

two transistor forward

I never heard about 2 transister forward converter.

Click to expand...

Two transistor forward converter is widely used and it's improved version of one transistor forward converter. In such converter two transistors are turned on and off at the same time. This enables to use transistors with lower Uds (2 times) voltage and also, core reset is much easier and improves efficiency. The disadvandage is that both the transistors must be driven from isolated sources.

**broken link removed**