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UCC28251 Datasheet(PDF) 13 Page - Texas Instruments

Part No. UCC28251
Description  UCC28251 Advanced PWM Controller With Prebias Operation
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
Logo TI - Texas Instruments

UCC28251 Datasheet(HTML) 13 Page - Texas Instruments

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8 Detailed Description
8.1 Overview
The UCC28251 is a functional variant of the UCC28250 PWM Controller. While the same basic functionality of
the UCC28250 is largely maintained, the UCC28251 is designed to operate at a lower frequency (minimum of 27
kHz when programmed by RT pin) allowing operation in 400-V input DC-to-DC converters in off-line AC-to-DC
power supplies, where frequency-dependent high-voltage switching losses are an important concern. Various
other enhancements, (summarized in Table 1) made in UCC28251 that enhance the performance in both 400-V
input and 48-V input full-bridge and half-bridge applications in server telecom power supplies.
8.1.1 UCC28251 Enhancements Over the UCC28250
The UCC28251 is a functional variant of the UCC28250 PWM Controller. While the same basic functionality of
the UCC28250 is largely maintained, the UCC28251 is designed to enhance performance in both offline, 400-V
input DC-to-DC applications and 48-V input full-bridge or half-bridge applications. The overall improvements that
are made in UCC28251 can be summarized in the table below:
Table 1. UCC28251 Improvements
When synchronized to an external frequency
UCC28250 does not allow support frequencies
UCC28251 enables better power
source, the minimum switching frequency can be
lower than 85 kHz.
efficiency in offline (400 VIN) DC-
as low as 42 kHz. When programmed by RT
to-DC converters.
resistor, the minimum switching frequency can be
as low a 27 kHz.
PS and SP delay times are fully independent and
Whenever OUTA/B turn on time + ps delay is
UCC28251 prevents shoot-
maintain their programmed values under all
greater than ½ switching cycle time, the PS delay
through between primary and
would be clamped (identical) to SP delay.
secondary side MOSFETs
UCC28251 enables quicker shut
EN shut down delay time shortened to ~1.5 µs
UCC28250 EN shutdown time was ~6 µs.
down in offline DC-to-DC
The synchronous rectifier outputs, SRA and SRB,
The synchronous rectifier duty cycle will be equal
are ensured to follow after the primary outputs and
UCC28251 reduces MOSFET
to 0 when primary duty cycle is very narrow. This
have a minimum 50% duty cycle during startup.
stress and enables better
duty cycle loss can increased MOSFET VDS and
SRA and SRB continue to be active and will only
reverse recovery.
cause poor reverse recovery.
be disabled when UCC28251 is disabled.
No delay, immediate startup after UVLO is
UCC28251 startup is delayed by 20 µs typical after
reached. After startup, if the applied VDD happens
UCC28251 SS pin will fully
reaching VDD UVLO. This 20 µs delay provides
to drop below UVLO then rises back up above
discharge under VDD brownout
enough time for SS pin to fully discharge.
ULVO very quickly, the UCC28250’s SS pin does
not have enough time to discharge.
In UCC28250, if the OVP comparator is triggered,
When OVP is detected, the internal current source
the internal current source does not turn on for any
now turn on for a minimum of 5 µs typical. This
minimum amount of time. In some cases this may
UCC28251 allows over voltage
allows the capacitor connected to the OVP pin to
result in the external OVP capacitor not being
protection to have enough
be charged up sufficiently. (The internal current
charged properly. This could result poor voltage
voltage hysteresis.
source has been changed to 8 µA).
hysteresis for OVP protection. (The internal current
source is 11 µA).
Copyright © 2013–2014, Texas Instruments Incorporated
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