©
Semiconductor Components Industries, LLC, 2012
October, 2012
−
Rev. 0
1
Publication Order Number:
EVBUM2150/D
NCP1351PRINTGEVB
NCP1351 16 V/32 V – 40 W
Printer Power Supply
Evaluation Board
User'sManual
Description
The present document describes a printer power supply
operated by the NCP1351, a fixed
t
on
/variable
off
time
controller. The board can deliver 10 W average on a 16 V
output and 30 W average on a 32 V output with a transient
peak power capability of 80 W. It however exhibits a low
standby power: below 150 mW at no load whatever the input
voltage. Let us first review the benefit of using the
NCP1351:
The NCP1351 at a Glance
Fixed t
on
, Variable t
off
Current-mode Control:
Implementing a fixed peak current mode control (hence
the more appropriate term “quasi-fixed” t
on
), the NCP1351
modulates the off time duration according to the output
power demand. In high power conditions, the switching
frequency increases until a maximum is hit. This upper limit
depends on an external capacitor selected by the designer. In
light load conditions, the off time expands and the NCP1351
operates at a lower frequency. As the frequency reduces, the
contribution of all frequency-dependent losses accordingly
goes down (driver current, drain capacitive losses, switching
losses), naturally improving the efficiency at various load
levels.
Peak Current Compression at Light Loads:
Reducing the frequency will certainly force the converter
to operate into the audible region. To prevent the transformer
mechanical resonance, the NCP1351 gradually reduces –
compresses – the peak current setpoint as the load becomes
lighter. When the current reaches 30% of the nominal value,
the compression stops and the off duration keeps expanding
towards low frequencies.
Low Standby-power:
The frequency reduction technique offers an excellent
solution for designers looking for low standby power
converters. Also, compared to the skip-cycle method, the
smooth off time expansion does not bring additional ripple
in no-load conditions: the output voltage remains quiet.
Natural Frequency Dithering:
The quasi-fixed t
on
mode of operation improves the EMI
signature since the switching frequency varies with the
natural bulk ripple voltage.
Extremely Low Start-up Current:
Built on a proprietary circuitry, the NCP1351 startup
section does not consume more than 10
m
A during the
startup sequence. The designer can thus easily combine
startup time and standby consumption.
Overload Protection Based on Fault Timer:
Every designer knows the pain of building converters
where a precise over current limit must be obtained. When
the fault detection relies on the auxiliary V
CC
, the pain even
increases. Here, the NCP1351 observes the lack of feedback
current and starts a timer to countdown. At the end of its
charge, the timer either triggers an auto-recovery sequence
(auto-restart, B and D versions) or permanently latches-off
(A and C). On C and D versions the fault timer is started at
an output power corresponding to 60% of the maximum
deliverable power; to allow transient peak power delivery.
Latch Fault Input:
A dedicated input lets the designer externally trigger the
latch to build additional protections such as over-voltage
(OVP) or over-temperature (OTP).
Figure 1. NCP1351 Evaluation Board
http://onsemi.com
EVAL BOARD USER’S MANUAL
