UC3825 - Minimum pulse width

Korbinian Gleis39

Other Parts Discussed in Thread: UC3825

Hi to everyone,

i am using the uc3825 for a halfbridge converter. i want to know what's the minimum pulse width is?

I am using a switching frequency of 75kHz. Is ther a dependency between the choice of Rt or Ct with the minimum pulse width?

thanks

Korbinian

over 8 years ago

0 John Stevens over 8 years ago

TI__Expert 7395 points

Korbinian,
I've contacted the appropriate product group. You should see a response soon.
Regards,
John

0 Korbinian Gleis39 over 8 years ago in reply to John Stevens

Prodigy 30 points

Hi John,

in my measuerments i got 40ns minimum pulse width. 20ns rise/fall time with no capacitive load at OUTA or OUTB.
Regards,
Korbinian

0 Oliver Pan91 over 8 years ago in reply to Korbinian Gleis39

TI__Intellectual 2230 points

Hi Korbinian,

If open loop, the duty cycle is fixed at its maximum, so the minimum on time is depend on the configuration of Rt and Ct, according to the Figure 2 & 3 in page 6 of datasheet, when Rt=1k & Ct=470pF it can achieve 280ns minimum pulse width. If close-loop, the duty cycle is depend on the error amplifier output and the RAMP signal, the minimum duty cycle can achieve 0% at this time.

Thanks
Oliver

0 Korbinian Gleis39 over 8 years ago in reply to Oliver Pan91

Prodigy 30 points

Hi Oliver,

i am using Ct=1nF and RT about 20kOhms (Pot) to achieve 75kHz. I shorted the Pin 1 (INV) and 3 (EA Out) and on Pin 2 (NI) i set the voltage for the duty cycle.

With this settings i can achieve about 280ns, but if i set a 100Ohms resistor in series to Ct to ground, i can achieve the 40ns.

Rise an fall times are about 20ns, so this is not really a right angular pulse anymore.

Regards,

Korbinian

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UC3825 - Minimum pulse width