One transistor forward with high voltage transformer reset?

[cupoftea]

Many years ago, there existed a very common converter known as the "One transistor forward with high voltage transformer reset". (OTFWHVTR)

Now it is pretty much a museum piece due to the invention of high CMTI bootstrap gate drivers and the "Two Transistor Forward". (the 2 tran forward can also be done with high voltage reset and can be run at higher-than-50%-duty-cycles, just like the OTFWHVTR).
So why do you think some people are still using the OTFWHVTR?

The OTFWHVTR also need two primary FETs, hi-side FET to do the reset.

 

[Easy peasy]

The one xtor fwd often has a reset wdg with diode to put the reset energy back into the supply

this is, or was, widely used.

You can use a single transistor with no reset parts, as long as the xtor can handle the high volts, sometimes a lower power catch ckt is used ( same as flyback, RCD ) to catch the initial spike due to leakage

An higher flyback voltage lessens the reset time which may be useful in some circumstances.

[ you can do this with a two switch forward as well, and the 2 sw flyback ]

 

[Calm down]

If you're talking about a dual transistor forward converter, I think reliability is the biggest reason. Firstly, the dual transistor forward converter is connected in series with two switches, and the withstand voltage is large enough. Secondly, there is no problem of magnetic bias and simultaneous conduction of the switch in the half bridge circuit, and there is no need to worry about resonance not keeping up when the load changes rapidly and over a large range. In addition, the situation where dual transistor forward excitation can be used is medium to high power, and flyback is definitely not feasible. Of course, if the output voltage is higher than 60V, double transistor forward excitation is not suitable, and may require the use of half bridge or push-pull.

 

[cupoftea]

Thanks, with 2TF, there can arise a situation, when driven by pulse transformer for the hi side, that the gate drive duty goes suddenly to zero......then the hi side drive rings up in voltage, due to the LC in the gate drive......then, if the top FET is turned on again, during that ring-up, then the top fet cannot switch off when it should, and the transformer cannot reset.....and during this LC ring-up time, the transformer can saturate.

This is easily demo'd in simulation if you wish me to send.

Ayk, sims are just that, but certain things, they dont get wrong.

 

[Easy peasy]

" Of course, if the output voltage is higher than 60V, double transistor forward excitation is not suitable, and may require the use of half bridge or push-pull."

surely the turns ratio of the transformer allows higher that 60V for the output .... ?

--- Updated May 30, 2023 ---

@cupótea, sounds like you need better gate drive, 2 xtor fwd can be driven to 60-70% on time with the correct reset, gate drive included . . . .

 

[Calm down]

" Of course, if the output voltage is higher than 60V, double transistor forward excitation is not suitable, and may require the use of half bridge or push-pull."

surely the turns ratio of the transformer allows higher that 60V for the output .... ?

--- Updated May 30, 2023 ---

@cupótea, sounds like you need better gate drive, 2 xtor fwd can be driven to 60-70% on time with the correct reset, gate drive included . . . .

Forward excitation combined with active clamping can achieve a duty cycle of over 60, but the magnetic flux transmitted by the transformer power is unidirectional, which increases the magnetic core stress and the instantaneous current of the copper wire, reducing the ratio of price to performance. At the same time, the complex driving of active clamping can offset the simple and reliable characteristics of forward excitation.

 

[Easy peasy]

I did not mention duty cycle - transformer turns ratio - please read again

--- Updated May 30, 2023 ---

why do you quote an entirely arbitrary level of 60V ?

 

[Calm down]

I did not mention duty cycle - transformer turns ratio - please read again

--- Updated May 30, 2023 ---

why do you quote an entirely arbitrary level of 60V ?

The frequency of the commonly used AC-DC power supply is about 60KHz, and in the absence of a good magnetic reset circuit, the duty cycle needs to be less than 50%. In the absence of a PFC circuit, the rectified DC voltage of 230VAC input is approximately 320V. In order to obtain a higher output voltage, the duty cycle will be larger, which is not conducive to magnetic reset. If higher power output is required, it is necessary to increase the input voltage, increase the cost of MOS, increase frequency, increase switch losses, or increase the cost of power devices by adding transformers. The low-voltage rectifier section also needs to withstand greater voltage stress. If it is a push-pull or half bridge type, the transformer has two duty cycles in the same cycle and there is no magnetic reset problem in the opposite direction. The utilization rate of transformers and continuous current inductors is higher, and the stress of switch tubes is also lower. This is a relatively heavy load issue. Sorry, I only know so much. If you are interested, you can try using a forward converter as a power supply with an output of 60V or above.

 

[cupoftea]

@cupótea, sounds like you need better gate drive, 2 xtor fwd can be driven to 60-70% on time with the correct reset, gate drive included . . . .

Thanks...yes we appreciate this, though we dont wish to pay the extra money for a circuit that can make the primary v.s equalized during the off time.

 

[Easy peasy]

Mr. Calm seems not to appreciate the effect of transformer turns ratio in converter design ....

 

[Calm down]

Mr. Calm seems not to appreciate the effect of transformer turns ratio in converter design ....

Hi guys, there's no need to doubt that products always need to become products, and products need to be cost-effective. It's not that positive incentives cannot be used above 60V, but rather that cost-effectiveness has exceeded the optimal range. If it's not about cost-effectiveness, then the reverse excitation power supply can use sufficiently large power devices, and can also be used as a medium to high power supply.

 

[Easy peasy]

60V ? how is that a limit for cost effectiveness ? what about absolute power ?