NCP1351PRINTGEVB: 32V @ 30W + 16V @ 10W, 85 ~ 264V in

The present document describes a printer power supply operated by the NCP1351, a fixed ton/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.

Fixed t-on, Variable t-off Current-mode Control: Implementing a fixed peak current mode control (hence the more appropriate term “quasi-fixed” ton), 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 ton 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 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 VCC, 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).