NCP1027P100G(2015) 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1027P100G
(Rev.:2015)

High-Voltage Switcher for Medium Power Offline SMPS Featuring Low Standby Power

ON Semiconductor 

NCP1027

Plugging the values in Equation 3 gives the limits within

which Rlimit shall be selected:

20−8.7

6m

v

Rlimit

v

12−8

1m

,

that

is

to say

: 1.8 kW

t 4 kW.

t

Rlimit

If we design a 65 kHz power supply delivering 12 V, then

the ratio between auxiliary and power must be: 12/20 = 0.6.

The OVP latch will activate when the clamp current

exceeds 6.0 mA. This will occur when Vauxiliary grows up

to:

1. 8.7 + 1.8 k (6 m + 1.8 m) ≈ 23 V for the first

boundary (Rlimit = 1.8 kW).

2. 8.7 + 4 k (6 m + 1.8 m) ≈ 40 V for the second

boundary (Rlimit = 4.0 kW).

Due to a 0.6 ratio between the auxiliary VCC and the

power winding, the auxiliary OVP will be seen as a lower

overshoot on the real output:

1. 23 0.6 ≈ 13.8 V

2. 40 0.6 ≈ 24 V

As one can see, tweaking the Rlimit value will allow the

selection of a given overvoltage output level. Theoretically

predicting the auxiliary drop from nominal to standby is an

almost impossible exercise since many parameters are

involved, including the converter time constants. Fine

tuning of Rlimit thus requires a few iterations and

experiments on a breadboard to check the auxiliary voltage

variations but also the output voltage excursion in fault.

Once properly adjusted, the fail−safe protection will

preclude any lethal voltage runaways in case a problem

would occur in the feedback loop.

> 30 ms

Figure 35. The burst frequency becomes so low

that it is difficult to keep an adequate level on the

auxiliary VCC.

Figure 36 describes the main signal variations when the

part operates in auto−recovery OVP.

Figure 36. If the VCC current exceeds a certain threshold, an auto−recovery protection is

activated and protects the design.

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