NCP1072STAT3G 查看數據表（PDF） - ON Semiconductor

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NCP1072STAT3G

High-Voltage Switcher for Low Power Offline SMPS

ON Semiconductor 

NCP1070, NCP1071, NCP1072, NCP1075, NCP1076, NCP1077

Vnom * VCC(clamp)

Vstby * VCC(min)

Itrip

v Rlimit v

ICCskip

(eq. 2)

Where:

Vnom is the auxiliary voltage at nominal load

Vstby is the auxiliary voltage when standby is entered

Itrip is the current corresponding to the nominal operation. It

thus must be selected to avoid false tripping in overshoot

conditions. Always use the minimum of the specification for

a robust design, i.e. Itrip < IOVP.

ICCskip is the controller consumption during skip mode.

This number decreases compared to normal operation since

the part in standby does almost not switch. It is around

0.36 mA for the NCP1072 65 kHz version.

VCC(min) is the level above which the auxiliary voltage must

be maintained to keep the controller away from the dynamic

self supply mode (DSS mode), which is not a problem in

itself if low standby power does not matter.

If a further improvement on standby efficiency is

concerned, it is good to obtain VCC around 8 V at no load

condition in order not to re−activate the internal clamp

circuit.

Iclamp > IOVP

Figure 27. A More Detailed View of the NCP107x Offers Better Insight on How to Properly Wire an Auxiliary Winding

Since Rlimit shall not bother the controller in standby, e.g.

keep VCC to above VCC(min) (7.2 V maximum), we

purposely select a Vnom well above this value. As explained

before, experience shows that a 40% decrease can be seen on

auxiliary windings from nominal operation down to standby

mode. Let’s select a nominal auxiliary winding of 13 V to

offer sufficient margin regarding 7.2 V when in standby

(Rlimit also drops voltage in standby...). Plugging the values

in Equation 2 gives the limits within which Rlimit shall be

selected:

13

* 8.4

6m

v

Rlimit

v

8 * 7.2

0.36m

that is to say: 0.77 kW < Rlimit < 2.2 kW.

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

the ratio between auxiliary and power must be: 13 / 12 =

1.08. The OVP latch will activate when the clamp current

exceeds 6 mA. This will occur when Vauxiliary grows−up

to:

1. 8.4 + 0.77k x (6m + 0.8m) ≈ 13.6 V for the first

boundary (Rlimit = 0.77 kW)

2. 8.4 + 2.2k x (6m +0.8m) ≈ 23.4 V for the second

boundary (Rlimit = 2.2 kW)

Due to a 1.08 ratio between the auxiliary VCC and the

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

on the real output:

1. 13.6 / 1.08 ≈ 12.6 V

2. 23.4 / 1.08 ≈ 21.7 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

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